Earthquake-Engineering-Module-1.pdf

DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Mai...

DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] Module 1: Introduction to Earthquakes and Seismology The introduction to earthquakes and seismology (Module 1) is about earthquakes – its origin, how it propagate (seismic waves), how it is describe and how it can be located. Before designing structures against earthquake, understanding the physical nature of earthquake and plate tectonics is important. Objectives: 1. Be able to understand the physical nature of earthquakes; how and why earthquakes occur and how earthquakes propagate. 2. Differentiate between magnitude and intensity. 3. Be able to locate the epicentre of an earthquake and obtain its magnitude. Outline: Introduction to Earthquakes and Seismology 1. Earthquakes and Urbanization 2. Elements of Seismology 3. Earthquake Instruments 4. Seismic Propagation 5. Earthquake Descriptors 6. How to Locate the Epicenter of an Earthquake Content: EARTHQUAKES AND URBANIZATION INTRODUCTION Earthquakes have been an integral component of the geologic evolution of planet earth. Since the dawn of history, mankind has been continually reminded of their ruinous power, usually without warning. Although the first attempt to fully document a seismic event and its effects probably occurred in 1755 following the great earthquake in Lisbon, Portugal, scientific earthquake research is mainly a product of the 20th century. Because of the complex nature of earthquake effects, current investigations encompass many disciplines, including those of both the physical and social sciences. Research activities center on such diversified topics as earthquake mechanics, earthquake prediction and control, the prompt and accurate detection of tsunamis (seismic sea waves), earthquake-resistant construction, seismic building code improvements, land use zoning, earthquake risk and hazard perception, disaster preparedness, plus the study of the concerns and fears of people who have experienced the effects of an earthquake. Data from these investigations help to form an integrated picture of a most complex field of study termed urban seismology. This chapter attempts to amalgamate recent research input comprising the vivifying components or urban seismology at a level useful to those having an interest in the earthquake and its effects upon an urban environment. However, because some of those interested in the earthquake-urban problem may not have a strong background in the physical sciences, the succeeding chapters will be devoted to an examination of major earthquake parameters. SEVERITY OF THE PROBLEM Earthquake Engineering Page 1  DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] There are several dreaded characteristics of an earthquake. Unlike other rapidly occurring natural hazards, earthquakes usually strike without warning or regard to time of day or season of the year and are characterized by numerous direct effects that is, ground shaking and permanent crustal movements and induced effects, such as landslides, avalanches, ground subsidence, liquefaction, ground fissuring, tsunamis, seiches, and fire. Earthquakes can kill, injure, and cause property damage thousands of kilometers from their point of origin. Earthquakes are often perceived, although incorrectly, as a force capable of destroying the very foundation of the planet, which helps to explain the feeling of fear and helplessness that transgresses all elements of society. The 1991 Luzon earthquake is a typical in the respect that it did not result in the unusually high death count that can result from seismic events. Other areas have not been as fortunate as Luzon. Recorded history has repeatedly been witness to the devastation of cities and the killing of millions. As a conservative estimate, the death count for all seismic events most probably exceeds 5,000,000 and injuries would be in the tens of millions. China has lost more than 2,100,000 of its citizens; Japan more than 500,000; Italy more than 370,000; and India more than 350,000. Table 1 lists major earthquakes and death counts from 526 through 2003. The largest loss of life was associated with the Shaanxi, China (now People’s Republic of China) earthquake of 1556, in which approximately 830,000 lives were lost. This count compares to some 600,000 American deaths incurred in all wars and rank as the third worst natural disaster in the history of humanity. It is preceded only by the 1931 Yellow River, China flood (3,700,00 deaths) and the 1970 Ganges Delta and Bangladesh cyclonic storm (more than 1,000,000 deaths). The second most disastrous earthquake also occurred in the People’s Republic of China. The July 27, 1976 Hopei Province events reportedly killed approximately 655,000 people and injured more than 700,000. The third most catastrophic seismic event was the 1737 Calcutta, India earthquake that killed more than 300,000 people. Several earthquakes have been responsible for 100,000 or more deaths. TABLE 1: RECORDED INTERNATIONAL MAJOR EARTHQUAKES August 13-15, 1868 Peru, Ecuador 40,000 N.A. DEATH MAGNITUD DATE LOCATION Venezuela, S E May 16, 1875 16,000 N.A. May 20, 526 Antioch, Syria 250,000 N.A. Colombia 856 Corinth, Greece 45,000 N.A. Charleston, SC, August 31, 1886 60 6.6 1057 Chihli, China 25,000 N.A. USA Near Mt. Etna, June 15, 1896 Japan, (sea wave) 27,120 N.A. February 11, 1169 15,0001 N.A. Sicily April 4, 1905 Kangra, India 19,000 8.6 1268 Cilicia, Asia Minor 60,000 N.A. San Francisco, April 18-19, 1906 5032 8.3 September 27, USA Chilhli, China 100,000 N.A. 1290 August 17, 1906 Valparaiso, Chile 20,000 8.6 May 20, 1293 Kamakura, Japan 30,000 N.A. October 21, 1907 Central Asia 12,000 8.1 January 26, 1531 Lisbon, Portugal 30,000 N.A. December 28, 1908 Messina, Italy 83,000 7.5 January 24, 1556 Shaanxi, China 830,000 N.A. January 13, 1915 Avezzano, Italy 29,980 7.5 November, 1667 Shemaka, Cucasia 80,000 N.A. Mona Passage, P. October, 11, 1918 116 7.5 January 11, 1693 Catania, Italy 60,000 N.A. Rico December 30, 1730 Hokkaido, Japan 137,000 N.A. December 16, 1920 Gansu, China 200,000 8.6 October 1, 1737 Calcutta, India 300,000 N.A. September 1, 1923 Yokohama, Japan 143,000 8.3 June 7, 1755 Northern Persia 40,000 N.A. March 16, 1925 Yunnan, China 5,000 7.1 November 1, 1755 Lisbon, Portugal 60,000 8.75* May 22, 1927 Nan-Shan, China 200,000 8.3 February 4, 1783 Calabria, Italy 30,000 N.A. December 25, 1932 Gansu, China 70,000 7.6 February 4, 1797 Quito, Ecuador 41,000 N.A. March 2, 1933 Japan 2,990 8.9 1811-12 New Madrid, MO N.A. 8.7* March 10, 1933 Long Beach, USA 115 6.2 Aleppo, Asia January 15, 1934 Bihar-Nepal, India 10,700 8.4 September 5, 1822 22,000 N.A. Minor April 21, 1935 Taiwan (Formosa) 3,276 7.4 December 28, 1828 Echigo, Japan 30,000 N.A. My 30, 1935 Quetta, India 50,000 7.5 Earthquake Engineering Page 2 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] January 25, 1939 Chillan, Chile 28,000 8.3 March 3, 1985 Chile 146 7.8 December 26, 1939 Erzincan, Turkey 30,000 8.0 Michoacan, September 19,1985 9,500 8.1 December 20, 1946 Honshu, Japan 1,330 8.4 Mexico June 28, 1948 Fukui, Japan 5,390 7.3 October 10, 1986 El Salvador 1,000+ 5.5 August 5, 1949 Pelileo, Ecuador 6,000 6.8 Colombia- March 6, 1987 4,000+ 7.0 August 15, 1950 Assam, India 1,530 8.7 Ecuador March 18, 1953 Northwest Turkey 1,200 7.2 India-Nepal August 20, 1988 1,450 6.6 June 10-17, 1956 North Afghanistan 2,000 7.7 border July 2, 1957 North Iran 1,200 7.4 China-Burma November 6, 1988 1,000 7.3 December 13, 1957 West Iran 1,300 7.3 border May 21-30, 1960 South Chile 5,000 9.5 December 7, 1988 Soviet Armenia 55,000 7.0 September 1, 1962 Northwest Iran 12,230 7.3 San Francisco Bay October 17, 1989 62 7.1 July 26, 1963 Skopje, Yugoslavia 1,100 6.0 Area March 27, 1964 Alaska 131 9.2 May 30, 1990 North Peru 115 6.3 August 19, 1966 East Turkey 2,520 7.1 June 20, 1990 West Iran 40,000+ 7.7 August 31, 1968 Northeast Iran 12,000 7.3 July 16, 1990 Luzon, Philippines 1,621 7.8 Yunnan Prov., Pakistan, Afgh, January 4, 1970 10,000 7.5 February 1, 1991 1,200 6.8 China border March 28, 1970 West Turkey 1,100 7.3 October 19, 1991 North India 2,000 7.0 May 31, 1970 North Peru 66,000 7.8 March 13, 15, 1992 East Turkey 4,000 6.2/6.0 San Fernando June 28, 1992 South California 1 7.5/6.6 February 9, 1971 65 6.6 Val., CA Flores, Isl., December 12, 1992 2,500 7.5 April 10, 1972 South Iran 5,054 7.1 Indonesia Managua, Off Hokkaido, December 23, 1972 5,000 6.2 July 12, 1993 200+ 7.7 Nicaragua Japan Pakistan (9 Southwest December 28, 1974 5,200 6.3 September 1, 1992 116 7.0 Towns) Nicaragua September 6, 1975 Turkey (Lice, etc) 2,300 6.7 October 12, 1992 Cairo, Egypt 450 5.9 February 4, 1976 Guatemala 23,000 7.5 September 29, Maharashtra 9,7483 6.3 May 6, 1976 Northeast Italy 1,000 6.5 1993 South India Irian Jaya, New January 17, 1994 Northridge, CA 61 6.8 June 25, 1976 422 7.1 Guinea South Sumatra, February 15, 1994 215 7.0 July 27,1976 Tangshan, China 655,000 8.0 Indon Mindanao, Cauca, Southwest August 16, 1976 8,000 7.8 June 6, 1994 1,000 6.8 Philippines Colombia Northwest Iran- August 19, 1994 North Algeria 164 6.0 November 24, 1976 5,000 7.3 USSR border January 16, 1995 Kobe, Japan 5,502 6.9 March 4, 1977 Romania 1,500 7.2 Sakhalin Isl. May 27, 1995 1,989 7.5 August 19, 1977 Indonesia 200 8.0 Russian Northwest October 1, 1995 Southwest Turkey 73 6.0 November 23, 1977 100 8.2 Argentina West coast October 9, 1995 40+ 7.6 September 16, Mexico Northeast Iran 15,000 7.8 1978 February 3, 1996 Southwest China 200+ 7.0 September 12, Iran Jaya, Indonesia 100 8.1 February 17, 1996 53 7.5 1979 Indonesia Colombia, Turkmen-Iran December 12, 1979 800 7.9 February 4, 1997 79 6.9 Ecuador border October 10, 1980 Northwest Algeria 3,500 7.7 February 27, 1997 West Pakistan 100+ 7.3 November 23, 1980 South Italy 3,000 7.2 February 28, 1997 Northwest Iran 1,000+ 6.1 June 11, 1981 South Iran 3,000 6.9 May 10, 1997 North Iran 1,560 7.5 July 28,1981 South Iran 1,500 7.3 Madhya Pradesh, May 21, 1997 40+ 6.1 West Arabian India December 13, 1982 2,800 6.0 Peninsula Northeast July 9, 1997 82 6.9 North Honshu, Venezuela May 26, 1983 81 7.7 Japan September 26, Central Italy 11 5.5/5.7 October 30, 1983 East Turkey 1,342 6.9 1997 Earthquake Engineering Page 3 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] September 28, Sulawesi, 17+ 5.9 Source: World Almanac 2005 1997 Indonesia October 15, 1997 Illapel, Chile 8.12 6.8 January 10, 1998 Zhangbel, China 50 6.2 Takhar, Northeast February 4,8, 1998 2,323 6.1 Afghanistan March 25, 1998 BallenyIsalnds --- 8.2 Sumatra, April 1, 1998 --- 7.0 Indonesia Irian Jaya, April 27, 1998 --- 7.4 Indonesia Ryukyu Is., May 3, 1998 --- 7.4 Taiwan May 22, 1998 Central Bolivia 105 6.5 Northeast May 30, 1998 4,700+ 6.9 Afghanistan Kamchatka, June 1, 1998 --- 6.5 Russia June 27, 1998 Adana, Turkey 144 6.3 July 9, 1998 Azores, Portugal 10 5.8 Source: World Almanac 2020 November 9, 1998 Banda Sea --- 6.6/7.0 November 29, East Indonesia 34 7.8 1998 December 29, Fiji Islands --- 6.9 1998 Papua New January 19, 1999 --- 7.0 Guinea Armenia, January 25, 1999 1,185+ 6.0 Colombia January 28, 1999 Aleutan Islands --- 6.6 February 6, 1999 Sta. Cruz Islands --- 7.3 Central February 11, 1999 60 6.0 Afghanistan Uttar Pradesh, March 28, 1999 87 6.8 India New Britain, April 5, 1999 Papua New --- 7.4 Guinea East Rusia, April 8, 1999 --- 7.1 Northeast China May 7, 1999 Southern Iran 26+ 6.2 New Britain, May 10, 16, 1999 Papua New --- 7.1 Guinea June 16, 1999 Puebia Mexico 16 6.7 August 17, 1999 Western Turkey 16,965+4 7.4 September 7, 1999 Athens, Greece 143 5.9 September 21, Taichung, Taiwan 2,321 7.6 1999 November 12, 1999 Duzce, Turkey 675+ 7.2 Sumatra, June 4, 2000 103 7.9 Indonesia San Vicente, El January 13, 2001 800+ 7.6 Salvador January 26, 2001 Gujarat, India 20,000+ 7.9 May 21, 2003 N. Algeria 2,200+ 6.8 December 26, 2003 Bam. SE Iran 26,271 6.6 Earthquake Engineering Page 4 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] The Philippines has been very fortunate in terms of lives lost as compared to other countries with an earthquake hazard. Our worst seismic disaster was the 1991 Luzon earthquake through which at least 1,600 lives were lost. The death count for all destructive Philippine earthquake hazards with property damage total of about 509 billion pesos. It is probable, however, that out worst seismic disasters are ahead of us. In certain years, the greatest loss of life from natural hazards is attributable to the earthquake. However, on the average, approximately 10,000 lives are lost each year to this hazard. For the period from 1974 to 1999, earthquake casualties ranked third behind flood and hurricane deaths. Approximately 56,000 people were killed by earthquakes during this 20-year period. The urban development of the Philippines is a very recent phenomenon when compared to other countries, which have seismic risks; this helps explain why so many countries have a long history of great loss of life caused by devastating earthquakes. Countless cities in these countries have occupied unsafe sites for centuries, and periodically they have been partially or totally. For example, Managua, Nicaragua was hit by destructive earthquakes in 1844, 1858, 1881, 1898, 1913, 1918, 1928, 1931, 1968, and 1972. The site of the city has never been abandoned, and after each quake, a great number of seismically unsafe structures rise from the ruins to await a similar fate sometimes in the future. The situation in the Philippines, as well as in other countries, is rapidly changing, as the earth becomes an overpopulated and urban planet. As these urban areas rapidly expand, a greater percentage of the world’s population encroaches upon active seismic zone, and earthquakes are becoming one of the most awesome geologic hazards to life and property. 5 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] ELEMENTS OF SEISMOLOGY The word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquake – is a series of elastic wave in the earth’s crust caused by sudden relaxation of strains accumulated by along geologic faults and volcanic actions and resulting in movements of the earth’s surface. It consists of ground vibration principally the horizontal and vertical vibration, although the ground motion is in any considerable direction. The acceleration, which goes with the vibration, is the one that induces the earthquake force. Earthquake – a vibratory shaking of the ground caused by some sudden disturbance of natural origin within the earth. The vibrations are elastic waves traveling at high speed through the earth. An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. An earthquake originates at a considerable depth below the surface of the earth at a point on the fault plane where the stress that produces the slippage is a maximum. This point is called the focus or hypocenter. While the point on the earth’s surface directly above it is called the epicenter. Faults – fracture on the earth’s crust or breaks in rocks along which there is displacement of rocks on one side relative to the other. Focus/ hypocenter – the point on a fault at which the first movement or break occurs during an earthquake. Figure 1: Earthquake Fault Seismology -The study of causes, propagation and affects of the movements of the earth’s crust that result from an earthquake is known as seismology. A. Types of Earthquakes I. Types of earthquakes as to its origin (natural): 1. Tectonic earthquakes – are caused by the sudden slippage along a fault or line of dislocation in the outer part of the earth; are earthquakes or waves of distortion resulting from ruptures or a sudden movement along existing fault in the earth’s crust. An intraplate earthquake is an earthquake that occurs in the interior of a tectonic plate, whereas an interplate earthquake is one that occurs at a plate boundary. Intraplate earthquakes are rare compared to earthquakes at plate boundaries. Nonetheless, very large intraplate earthquakes can inflict heavy damage. Notable examples of damaging intraplate earthquake are the 1811-1812 earthquakes in New Madrid, Missouri, and the 1886 earthquake in Charleston, South Carolina. 2. Volcanic earthquakes – are associated with volcanic eruption or subterranean movement of magma; result from sudden movements of liquid lava below the surface or from fracturing of the rocks as a result of lava movement. Ex. Mount St. Helens eruption of 1980 6 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] II. Types of earthquakes as to focal depth: 1. Shallow earthquakes – earthquakes with focal depths from the surface to about 70 km. Majority of the earthquakes are of shallow origin. Most of the tectonic earthquakes detected thus far have originated at depths of no more than sixty (60) kilometers. They are considered as surface earthquakes. Their origin is associated with deformations of the Earth’s crust, which is in a state of constant deformations owing to the convection currents in the plastic zones of the mantle. 2. Intermediate earthquakes - earthquakes with focal depths from 70 km – 300 km. (12%) 3. Deep earthquakes - earthquakes with focal depths greater than 300 km. (depths ranging 300-700 km). They commonly occur in patterns called Benioff zones like Japan, Vanuatu, Tonga and Alaska, and are normally associated with deep ocean trenches. (6%) III. Artificially induced earthquakes (human activity): 1. collapse of caverns or mine workings 2. injection of fluids into deep wells 3. detonation of large underground nuclear explosions 4. excavations of mines 5. quarry blasting 6. filling of large reservoirs B. THEORIES RELATED TO EARTHQUAKES 1. Theory of Plate Tectonics. According to this theory, the earth’s outer shell consists of huge plates up to 60 miles in thickness that floats on a partially plastic layer of the upper mantle (asthenosphere). These plates are assumed to move laterally and grind together of their margins, thus producing earthquake faults. The theory states that Earth's outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates ( about 14) also exist, including the Arabian, Nazca, and Philippines plates. 2. Continental Drift Theory. The theory, first advanced by Alfred Wegener, that the earth's continents were originally one land mass called Pangaea. About 200 million years ago Pangaea split off and the pieces migrated (drifted) to form the present-day continents. The predecessor of plate tectonics. 3. Elastic Rebound Theory. When friction between rocks on either side of a fault is such as to prevent the rocks from slipping easily, or when the rock under stress is not already fractured, some elastic deformation occurs before failure. When the stress at last exceeds the rupture strength of the rock (or friction between rocks along an existing fault), sudden movement occurs along the fault: an earthquake. The stressed rocks, released by the rupture, snap back elastically to their previous dimensions, a phenomenon known as elastic rebound. As formulated by the American geologists Harry Fielding Reid, the theory explains that a tectonic earthquake occurs when stresses in rock masses have accumulated to a point where they exceed the strength of the rocks, leading to rapid fracture. These rock fractures usually tend in the same direction and may extend over many kilometers along the zone of weakness. 7 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] PLATE BOUNDARIES o At the boundaries between these huge plates of rock and soil, the plates sometimes move apart, and magma, or molten rock, comes to the surface, where it's called lava. It cools and forms new parts of the crust. The line where this happens is called a divergent plate boundary. o The plates also can push against each other. Sometimes, one of the plates will sink underneath the other into the hot layer of magma beneath it and partially melt. Other times, the edges of the two plates will push against each other and rise upward, forming mountains. This area is called a convergent plate boundary. o But in other instances, plates will slide by and brush against each other -- a little like drivers on the highway sideswiping each other, but very, very slowly. At the region between the two plates, called a transform boundary, pent-up energy builds in the rock. A fault line, a break in the Earth's crust where blocks of crust are moving in different directions, will form. Most, though not all, earthquakes happen along transform boundary fault lines. Megathrust earthquakes occur at subduction zones at destructive plate boundaries (convergent boundaries), where one tectonic plate is forced (or subducts) under another. Due to the shallow dip of the plate boundary, which causes large sections to get stuck, these earthquakes are among the world's largest, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all five earthquakes of magnitude 9.0 or greater have been megathrust earthquakes; in fact, no other type of known tectonic activity can produce earthquakes of this scale. Examples of megathrust earthquakes are listed in the following table. Estimated Moment Event Tectonic Plates Involved Other Details/Notes Magnitude (Mw)  The quake generated a large tsunami in African Platesubducting the eastern Mediterranean Sea and 365 Crete 8.0+ beneath the Eurasian caused significant vertical displacement in earthquake Plate the island of Crete. 1575 Nazca Platesubducting Valdivia 8.5 beneath the South earthquake American Plate Juan de Fuca 1700 Platesubducting  Slip length: 1000 km (625 mi) Cascadia 8.7–9.2 beneath the North earthquake American Plate 8 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] 1737 Pacific Platesubducting  Duration: 15 minutes Kamchatka 9.0–9.3 beneath the Okhotsk  Depth: 40 km earthquake Plate Hypothesized to be part 1755 Lisbon of a young subduction 9.0 earthquake zone but origin still debated Nazca Platesubducting 1868 Arica 9.0 beneath the South earthquake American Plate 1877 Antofagasta Nazca Platesubducting (Northern 8.8 beneath the South Chile) American Plate earthquake Philippine Sea 1946 Platesubducting Nankaidō 8.1 beneath the Eurasian earthquake Plate 1950 Nicoya Cocos Platesubducting Peninsula 7.7 beneath the Caribbean (Costa Rica) Plate earthquake 1952 Pacific Platesubducting  Depth: 30 km Kamchatka 9.0 beneath the Okhotsk earthquake Plate 1957 Pacific Plate subducting Andreanof 8.6–9.1 beneath the North Islands American Plate earthquake  Depth: 33 km 1960 Great Nazca Plate subducting  Slip length: 1000 km (625 mi) Chilean 9.5 beneath the South  Slip width: 200 km (125 mi) Earthquake American Plate  Slip motion: 20 m (60 ft) 1964 Alaska  Duration: 4–5 minutes earthquake Pacific Plate subducting  Depth: 25 km ("Good 9.2 beneath the North  Slip length: 800 km (500 mi) Friday" American Plate  Slip motion: 23 m (69 ft) earthquake)  The total vertical displacement measured by sonar survey is about 40 m in the vicinity of the epicenter and occurred as two separate movements which created 2004 Indian India Platesubducting two large, steep, almost vertical cliffs, one Ocean 9.3 beneath the Burma above the other. earthquake Plate  Duration: 8–10 minutes  Depth: 30 km  Slip length: 1300 km (1000 mi)  Slip motion: 33 m Nazca Platesubducting 2010 Chile beneath the South 8.8 earthquake American Plate 9 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] C. TYPES OF FAULTS (Slip Fault Direction) The two sides of a fault are called the hangingwall and footwall.  Strike-Slip Fault: This occurs on an approximate vertical fault plane as the rock on one side of the fault slides horizontally past the other. Two types of a strike-slip fault are the left lateral faults and the right lateral faults as shown in Figure 2a. If the block on the far side of the fault moves to the left, the fault is called left- lateral. If the block on the far side moves to the right, the fault is called right-lateral. The fault motion is caused by shearing forces; occurs along transform plate boundaries. Examples of right lateral/slip fault are the San Andreas Fault in California and the North Anatolian Fault.  Dip-Slip Fault: This occurs when the fault is at an angle to the surface of the Earth and the movement of the rock is up or down. (Figure 2b) The types of faults on this category are: 1. Normal Fault – the hanging wall moves down and the foot wall up; when the relative movement is in an upward and downward direction on a nearly vertical fault plane. The block above the fault moves down relative to the block below the fault. It is caused by tensional forces and results in extension; also called normal-slip fault, tensional fault or gravity fault. This type of faulting is often observed in the Western United States Basin and Range Province and along oceanic ridge systems along divergent plate boundaries. 2. Reverse Fault – the reverse happen; the hanging wall moves up and the foot wall moves down; occurs along convergent boundaries. Thrust Fault – This is a special category of the reverse fault. The fault plane lies at a low angle to the Earth’s surface; when the earth is under compressive stress across the fault and slippage is in an upward and downward direction along an inclined fault plane; or when the block above the fault moves up relative to the block below the fault. This fault is caused by compressional forces and results in shortening. Blind Thrust – the thrust fault that does not extend all the way the Earth’s surface. The ground above the blind thrust bends instead of breaking so that the surface has only rolling hills.  Oblique Slip Fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. Oblique slip fault is caused by a combination of shearing and tension or compressional forces. 10 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] EARTHQUAKE RECORDS AND MEASURING INSTRUMENTS SEISMOGRAPHS Special instruments known as seismometers or seismographs can measure ground movements during an earthquake. They can measure the epicenters, focal depths, and magnitudes, and are used to measure and record seismic trains. The ground movements are graphically recorded in a seismogram. By an analysis of seismograms, it is possible to determine the velocity of propagation of the various types of seismic waves for each particular earthquake, and a systematic study of such records has made possible a fairly exact analysis of the physical characteristics of the earth. Seismographs also make it possible to locate the focus of the earthquake by determining the difference between the times of arrival of the seismic waves at different seismological stations. Seismographs were first installed at the University of California, Berkeley, in 1887. At present, many types of seismographs are currently available, and most incorporate similar principles. Unless otherwise indicated, the following descriptions summarizes a typical seismograph: 1. Use is made of a pendulum (free mass) attached to a rigid frame, which in a permanent seismological station is anchored to a concrete platform. When the ground vibrates during an earthquake, the pendulum’s inertia tends to delay its motion. This results in a measure of differential motion. A seismometer pendulum can be designed to have a high or low period sensitivity. Many seismograph pendulums take 10 or even 20 seconds to make a complete swing. The reason for this is that most waves from distant earthquakes have long periods and long wavelengths. It requires a long period pendulum to obtain a measurable distance between the moving pendulum and the moving ground. In nearby earthquakes, the ground vibrations are of short period, and almost any simple short period pendulum would suffice. 2. Seismological stations often house six seismometers for obtaining a complete description of ground motion as shown in Figure 4: Figure 4: Typical Seismological Station  Short-period (wave periods in the 0.05- to 0.20-second range), north-south (N) motion.  Short-period, east-west (E) motion.  Short-period, vertical (Z) motion.  Long-period (wave periods in the 15- to 100-second range), N motion.  Long-period, E motion.  Long-period, Z motion. 3. A braking device to prevent its own free-period swinging and to show the arrival separation of various wave phases must damp the pendulum. Free—period swaying can be especially pronounced for long-period seismometers if not properly damped. A pendulum damped to best reflect ground motion is termed critical. 11 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] 4. The motion of a seismograph pendulum must be magnified relative to the ground motion, allowing very small wave amplitudes as small as 10-10 centimeters. Magnification is accomplished by mechanical, optical, or electromagnetic procedures. Figure 5: Simple Pendulum Seismographs Figure 5 shows simple pendulum seismographs recording three directions of ground motion. If three instruments are arrayed at right angles to each other (horizontal vectors do not have north-south, east-west) the system is called a triaxial seismograph. Figure 6: Recorded Microseisms. Seismic background noises recorded on seismograms as shown in Figure 6 are termed microseisms. The most prominent microseisms have wave periods ranging from 4 – 7 seconds. ACCELEROGRAPHS Since the vibratory motion of the ground is manifested in structures in the form of inertial forces directly related to the acceleration of the ground, scientists have designed instruments called “strong-motion seismographs” which make it possible to record in a graph called an accelerogram the motions of the ground during an earthquake. Unlike seismographs, these instruments do not operate continuously but have a special actuator that turns them on when the acceleration of the ground exceeds a certain threshold, so that they can record the most important portion for the accelerogram. Accelerographs do not run continuously but is activated only by strong earth motions. A typical example of an acceleroghraph is shown in Figure 7. 12 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] Kinemetrics FBA-23 accelerograph. Figure 7: Accelerograph Accelerographs are placed in buildings at various heights and on different ground types. The instruments are powered by solar cells. Accelerograms provide important data on the responses of different structures and geologic units to near-field strong earthquake motions (see Figure 8). Through mathematical integration procedures, acceleration curves can be processed to produce ground velocity (first integration) and ground displacement (second integration) curves. Such data are extremely valuable in earthquake engineering and hazard reduction programs. Figure 8: Accelerograms. EPISENSOR – new version of FBA 23, it records ground acceleration much more accurately and with greater sensitivity. SEISMOSCOPES The Chinese philosopher Chang Hêng [Chang Hêng is also referred to as Choko and Tyoko, modifications of the Japanese form of his name.] invented the earliest known seismoscope in 132 A.D. The instrument was said to resemble a wine jar of diameter six feet. On the outside of the vessel there were eight dragonheads, facing the eight principal directions of the compass. Below each of the dragonheads was a toad, with its mouth opened toward the dragon. The mouth of each dragon held a ball. At the occurrence of an earthquake, one of the eight dragon-mouths would release a ball into the open mouth of the toad situated below. The direction of the shaking determined which of the dragons released its ball. The instrument is reported to have detected a four-hundred-mile distant earthquake that was not felt at the location of the seismoscope Seismoscopescomplement accelerographs for obtaining strong-motion data near epicenters. These are magnetically damped conical pendulum devices that are free to move in any horizontal direction. Unlike seismographs, they have no timing capability. A metal stylus writes a 13 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] record of ground motions, via angular deflections of the pendulum on a smoked glassplate.Figure 10 shows some samples of the responses of three standardized seismoscopes. Mallet's seismoscope The image of a cross-hairs in C is reflected from the surface of mercury in the basin B and viewed through a magnifier, D. Figure 9: Seismoscope Figure 10: Sample Responses from a Seismoscope. CREEPMETERS Creepmetersmeasures fault slip by recording the displacement between 2 piers or monuments located on opposite sides of the fault. Typically, an invar wire is anchored to one pier and is stretched across the fault. Its displacement relative to the second pier is measured electronically and checked periodically with a mechanical measurement. Using the angle of the wire from the strike of the fault, the change in distance between the two piers is directly proportionally to fault slip. Because the piers are anchored to about 2 meters depth, they are subject to the influence of seasonal (winter) rainfall. Many of the creepmeters show an annual cycle due to the wetting and drying of the near-surface materials within the fault zone. In addition, creep is influenced by large rainfall events and nearby earthquakes. MAGNETOMETERS Magnetometers measure changes in local magnetic fields resulting from a combination of mean crustal stress change, fluid flow associated with earthquakes, fault slip, and a number of processes related to volcanic activity. To isolate these local magnetic fields, the magnetic data must be corrected for normal geomagnetic field variations, magnetic storms and other disturbances including those generated by cultural activity. Ultra-precise, absolute instruments with a precision of 0.2 nanotesla are used. Higher frequency disturbances are monitored using fluxgate or coil magnetometers. The monitoring sensors are carefully located in regions where the local magnetic field gradient is less than 1 nanotesla/meter to avoid spurious signals being generated during ground movement generated by earthquake shaking and volcanic eruptions. Because proton magnetometer measurements are absolute, data from any particular year can be compared with that from yesterday or 20 years ago or longer. 14 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] PORE PRESSURE MONITORS Pore Pressure Monitor is an instrument record fluid pressure changes in deep boreholes that may be driven by fault activity. Measurements can be made to better than 0.1 millibar. STRAINMETERS Strainmeters for continuous crustal strain monitoring are highly sensitive instruments with precision of less than 1 part per billion (i.e. less than 1 inch in 16,000 miles). They are usually installed in boreholes where surface noise is greatly reduced. These instruments monitor the change in crustal strain near active faults and volcanoes associated with fault slip, earthquakes, and volcanic activity. Currently, numerous instruments have been installed by the USGS along the San Andreas fault, in the Long Valley Caldera, and by other institutions near active faults and volcanoes in the US, Japan, China, Iceland, Italy, and Taiwan. TILTMETERS Tiltmeters are highly sensitive instruments used to measure ground tilt (rotation) near faults and volcanoes caused by fault slip and volcanic uplift. The precision to which tilt can be measured is less than 1 part per billion (i.e. less than 1 inch in 16,000 miles). For crustal monitoring applications, these instruments are mostly installed in boreholes to avoid spurious ground tilts produced by differential thermal expansion in near-surface materials, rainfall and pumping effects. HYDROPHONE is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Such piezoelectric materials or transducers can convert a sound signal into an electrical signal since sound is a pressure wave. Some transducers can also serve as a projector, but not all have this capability, and may be destroyed if used in such a manner. GALVANOMETER – use for detecting/measuring a small electric current by movements of a magnetic needle or of a coil in magnetic field. EARTHQUAKE NETWORKS Obtaining systematic earthquake data from different stations is a prime requisite for various types of seismological research and for surveillance activity programs. To accomplish this, identical instruments or instruments from which the data can be reduced to standard formats must be used. When this is accomplished, the resulting seismograph array is often called as a network. Seismic networks can be worldwide, regional, or local, and permanent or temporary, examples are: Global Seismographic Network, Pacific Northwest Seismic Network (PNSN), International Federation of Digital Seismograph Networks (FDSN) and Lamont-Doherty Cooperative Seismographic Network (LCSN). 15 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] PROPAGATION OF SEISMIC DISTURBANCES SEISMIC WAVES Seismic waves are the vibrations from earthquakes that travel through the earth. They are the waves of energy suddenly created by rock fracture within the earth or an explosion. The propagation velocity of the waves depends on density and elasticity of the medium. Seismic wave fields are recorded by a seismometer, hydrophone (in water), or accelerometer. Earthquakes generate two general classes of elastic waves. They are termed as elastic in the sense that rock units return to their original positions or shapes once the seismic waves have passed. 1. Body waves – waves that travel through the earth’s interior. They follow raypaths refracted by the varying density and modulus (stiffness) of the Earth's interior. The density and modulus, in turn, vary according to temperature, composition, and phase. This effect is similar to the refraction of light waves. S. D. Poisson discovered these waves in 1830 while investigating wave propagation through elastic media. 2. Surface waves – waves that travel along outer layer of the earth along the surface. Body waves are further divided into two types: 1. The P (or pressure/ primary) waves travel through the body of the earth at the highest speeds. They are longitudinal or compressional waves that can be transmitted by both solid and liquid materials in the earth’s interior. With P waves, the particles of the medium vibrate in a manner similar to sound waves, and the transmitting rocks are alternately compressed and expanded; push (compress) and pull (expand) with rocks in the direction the wave is traveling. A P wave moves between 4 – 7 km/sec depending on the density of the rock it’s moving through. In P wave, rocky material on its direction of travel compresses then expand at the way it passes. A P wave is similar to a wave traveling through a spring. The coil compresses and expands in the direction the wave it’s traveling. 2. The S (or secondary/shear) wave travels only through solid material within the earth. With s waves, the particle motion is traverse to the direction of travel and involves the shearing of the transmitting rock. S waves produced an up-and-down and side-to-side motion of the earth that shakes the ground both vertically and horizontally at right angles to the direction of wave travel and produces the major damage to the structures. S wave travel at about 2 – 4 km/sec through the rock about 60% of the speed of the P wave. It is similar to wave traveling along a piece of rope; the wave move along the rope by moving a section of the rope up and down. In solid rock P-waves travel at about 6 to 7 km per second; the velocity increases within the deep mantle to ~13 km/s. The velocity of S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth's crust up to 7 km/s in the deep mantle. As a consequence, the first waves of a distant earth quake arrive at an observatory via the Earth's mantle. 16 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] Surface waves are analogous to water waves and travel along the Earth's surface. They travel slower than body waves. Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave. Two types of surface waves: a. Love wave – produces horizontal earth movement similar to that produced by S waves. It causes side to side motion perpendicular for it’s direction of travel. It can cause damage by breaking roads as well as pipes; surface waves that cause circular shearing of the ground. They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity. They are the slowest and have the largest amplitude. b. Rayleigh wave – produces vertical earth movement. It moves at the surface of the earth up, forward, down and back in a circle. It can cause damage by knocking buildings off their foundations. Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water (note, however, that the associated particle motion at shallow depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves, roughly 90% of the velocity of S waves for typical homogeneous elastic media. When earthquake waves arrive at a surface of discontinuity either in the interior of the earth or at its surface, they undergo multiple reflection and refraction, giving rise to new types of waves. Figure 1 is a magnitude trace of a seismogram showing the arrival of P, S, and L waves from a seismic event. The total sequence of seismic waves is termed the wave train. The beginning of each new burst of energy is called a phase, but note the amplitude (one-half the trace height) differences each phase. Total elapsed time between the first wave arrival and the drop-off to background noise is referred to as the earthquake duration or coda length. Figure 1: P-, S-, L Waves The trajectories of seismic waves in the interior of the earth are not straight lines but curves that are concave on the side towards the surface. Owing to the fact that the medium through which the waves are propagated is heterogeneous and increases in density towards the center of the planet. The following is a listing of major distinguishing parameters for each wave type. P waves: 1. Possess longitudinal motion analogous to sound waves. 2. Propagate through all material types (solid, liquid, gas). 3. Are the first types in the total wave train to arrive at a seismological station. 4. Velocity is determined by √ Where: = velocity = bulk modulus (resistance to volume changes) 17 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected]  = rigidity modulus (resistance to change in shape)  = density 5. Create a push (compressional) – pull (dilatation or relaxation) effect on materials, which they pass in a vector parallel to the wave path. (see Figure 2). 6. Can be perceived, if the earthquake is large enough, as a sudden shock emanating from blow the surface. 7. Characterized by longitudinal motion. They should be able to produce audible sounds upon passing from the solid earth to the atmosphere if they possess the correct frequency. Figure 2: Seismic Wave Motions S waves: 1. Are similar to electromagnetic waves (light, heat, radio waves) in that there are vertical (SV) and horizontal (SH) components, each at right angles to the direction of wave propagation (the vertical component is somewhat weakened by the opposing gravitational force). 2. Propagate only through solid substances because gases and liquids cannot be sheared. Since  = 0 in liquids and gases, = 0. 3. Are usually the second wave types in the total seismic wave train to arrive at a recording station (see Figure 1). 4. Velocity is determined by √ 5. Shear and twist crustal material as they move through it (Figurer 2); if unconsolidated, particle motion is at right angles to the direction of wave propagation. 6. Begin a new series of vibrations, often more damaging to the works of construction than the P phase. Figure 3 demonstrates the motion characteristics of P and S waves with a Slinky toy, which has a high visual impact. Stages include A – Stationary. B – Sharp slap to the top initiates P waves. C – Compressed and dilated coil zones move up and don the Slinky. D – Sharp sideways slap generates S waves. E – Transverse motion of S waves. 18 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] Figure 3: Slinky Toy Characteristics of P- and S-waves Surface (L) waves: 1. Originate at the epicenter, moving outward in all directions. Surface waves can circle the earth several times and generate one-millimeter ground amplitudes over the earth’s entire surface in large earthquakes. However, such ground motions are not perceived by humans because of low L wave accelerations and long wavelengths (tens of kilometers). 2. Are generally characterized by 10- to 20-second wave periods, 20- to 80-km wavelengths, and 3 km/sec velocities. Extremely long period surface waves (8- to 10- minute periods, wavelengths exceeding 2,000 km) are termed as mantle waves. 3. Even though L waves contain substantial amounts of energy, their long periods smooth their imparted ground motion, thereby greatly reducing their damage potential. 4. Wave amplitudes are largest at the surface, decreasing with depth, and the largest amplitudes are associated with shallow-focus earthquakes. When the focus is shallow, L wave amplitudes are the largest for the wave train (see Figure 1). 5. Are reflected or refracted from the original wave paths when geological contacts are encountered. Long waves: 1. Are generated from “trapped” S waves. They are similar to SH waves. (see Figure 2). 2. Are usually faster than Rayleigh waves, with velocity Ldetermined by the relation 1 L2 Rayleigh waves: 1. Introduce a retrograde elliptical motion similar to ocean waves (see Figure 2). 2. Have a velocity Rdetermined by the relation R 0.92 Although body and surface waves lose energy by geometrical spreading, significant differences exist between them because of their different propagation patterns. Body waves are propagated in approximately spherical wave forms with amplitudes diminishing as a function of distance traveled. Surface waves travel outward in the form of expanding cylinderswherein the amplitudes decline as the square root of the distance traveled. Furthermore, internal damping in rocks and surficial materials absorbs wave energy. Rule of thumb: On the average, the kilometer distance to the earthquake is the number of seconds between the P and S wave times 8. Slight deviations are caused by inhomogenities of subsurface structure. By such analyses of seismograms the Earth's core was located in 1913 by Beno Gutenberg. Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is divided into 754 Flinn-Engdahl regions (F-E regions), which are based on 19 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions. EARTHQUAKE DESCRIPTORS Earthquakes are described in terms of magnitude and intensity. Magnitude is a quantitative, based on an instrument. It is the energy released at the source of an earthquake, while intensity describes an earthquake in qualitative terms, based upon personal observations. It is defined as the strength ofseismic shaking at a given location. Magnitude is expressed in Arabic numbers and intensity in Roman numerals. A. Magnitude – a measure of the total energy released during an earthquake; determine from the amount of materials, which slides along the fault and the distance it is displaced. Types of Magnitude Scales 1. Richter Magnitude Scale or Local Magnitude Scale. Earthquake magnitude is reported using the Richter scale from the motions measured by seismic instruments. The Richter scale/ Richter magnitude scale (ML)was named after Charles Francis Richter who developed it in 1935. Richter showed that, the larger the intrinsic energy of the earthquake, the larger the amplitude of ground motion at a given distance. He calibrated his scale of magnitudes using measured maximum amplitudes of shear waves on seismometers particularly sensitive to shear waves with periods of about one second. The records had to be obtained from a specific kind of instrument, called a Wood-Anderson seismograph. Table 1 Earthquake magnitudes and expected world incidence Richter Earthquake Estimated Number per Magnitudes Effects/ Description Year 2.0 – 2.9 Generally not felt, but recorded. 300,000 3.0 – 3.9 Smallest usually felt 49,000 4.0 – 4.9 Minor earthquake; minor damage 6,200 detected 5.0 – 5.9 Damaging earthquake; slight 800 damage to structures 6.0 – 6.9 Destructive earthquake; can be 120 destructive in populous regions 7.0 – 7.9 Major earthquake; inflict serious 18 damage >8.0 Great earthquakes. Produce total 1 to 2 destruction to communities near epicenter. It is a base-10logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude (shaking amplitude) of the largest displacement from zero on a particular type of seismometer (Wood–Anderson torsion). So, for example, an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0. The effective limit of measurement for local magnitude ML is about 6.8. 2. Moment Magnitude Scale.The moment magnitude scale (MW) was introduced in 1979 by Thomas C. Hanks and Hiroo Kanamori as a successor to the Richter scale, and it comes from the seismic moment. To get an idea of the seismic moment, we go back to the elementary physics concept of torque. A torque is a force that changes the angular momentum of a system. It is defined as the force times the distance from the center of rotation. Earthquakes are caused by internal torques,from the interactions of different blocks of the earth on opposite sides of faults. After some rather 20 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] complicated mathematics, it can be shown that the moment of an earthquake is simply expressed by where Mo = seismic moment or moment of an earthquake, dyne-cm.  = rock rigidity = 3 * 1011 dyne/cm2 or 30 GPa A = fault area, cm2. d = slip distance, cm. A standard relation to convert a seismic moment to a moment magnitude is Mw = 2/3[log10 Mo – 16.0] (Mo in dyne-cm) Mw = 2/3[log10 Mo– 9.81] (Mo in N-m) One advantage of the moment magnitude scale is that unlike other magnitude scales, it does not saturate at the upper end. There is no particular value beyond which all large earthquakes have about the same magnitude. For this reason, moment magnitude is now most often used estimate of large earthquake magnitudes. The subscript w in the moment magnitude scale M w means mechanical work accomplished. 3. Surface wave magnitude (Ms) – is based on surface waves which primarily travel along the uppermost layers of the earth and is defined in terms of the logarithm of the maximum amplitude of the ground motion for the surface waves with a wave period of 20 seconds. 4. P-wave magnitude (Mb) is defined in terms of the amplitude of the P wave recorded on a standard seismograph. This scale is intended for deep focus earthquakes. 5. Duration magnitude – The concept of Earthquake Duration Magnitude - originally proposed by Bisztricsanyin 1958 using surface waves only - is based on the realization that on a recorded earthquake seismogram the total length of the seismic wavetrain - sometimes referred to as the CODA - reflects its size. Thus larger earthquakes give longer seismograms [as well as stronger seismic waves] than small ones. The seismic wave interval measured on the time axis of an earthquake record - starting with the first seismic wave onset until the wavetrainamplitude diminishes to at least 10% of its maximum recorded value - is referred to as "earthquake duration". It is this concept that Bisztricsany first used to develop his Earthquake Duration Magnitude Scale employing surface wave durations. Magnitude Summary The symbols used to represent the different magnitudes are Magnitude Symbol Wave Period Local (Richter) ML S or Surface Wave* 0.8 s Body-Wave mb P 1s Surface-Wave Ms Rayleigh 20 s Moment Mw Rupture Area, Slip > 100 s Duration Md Surface wave duration (CODA) *at the distances appropriate for local magnitude, either the S-wave or the surface waves generally produce the largest vibrations. SEISMIC ENERGY Empirical studies using available data on a number of recorded earthquakes have made it possible to devise a formula that establishes the relation between the energy released by an 21 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] earthquake and its magnitude. The equations and the years in which they were introduced are presented in Table 2. The energy released by an earthquake activity is not the total ``intrinsic'' energy of the earthquake, transferred from sources such as gravitational energy or to sinks such as heat energy. It is only the amount radiated from the earthquake as seismic waves, which ought to be a small fraction of the total energy transferred during the earthquake process. Energy Equation Year of Introduction Energy Increase for One Unit of (log10 E) Magnitude Change 8.0 + 2.0 ML 1936 100.00 * 11.3 + 1.8 ML 1942 63.12 * 12.0 + 1.8 ML 1949 63.11 * 11.0 + 1.6 ML 1954 39.94 * 11.8 + 1.5 ML 1956 31.62 * Table 2: Relation Between Historical Equations and Energy Release for One Unit of Magnitude Change From Table 2, the equation devised by Gutenberg-Richter is thought to be the most accurate, and is used widely. log10 E = 11.80 + 1.50ML where ML= magnitude measured on the Richter scale. E = the energy released, ergs. erg = unit of work equal to 1 dyne acting through a distance of 1 cm. erg = 1 dyne-cm. erg = 3.722 * 10-4 horsepower – hrs. erg = 1 * 10-7 joules. joule = 1 Nm. dyne = force needed to accelerate a freestanding mass 1 cm/sec. dyne = 1 gm-cm/sec2 dyne = 10-5 N. 1 horsepower = 746 watts. More recently, Dr. HirooKanamori(1993) came up with a relationship between seismic moment and seismic wave energy. E = Mo/20,000 Table 3 shows the relation of an earthquake magnitude and its corresponding energy released. Richter Magnitude TNT for Seismic Energy Yield Example (kN) (Approximate) -1.5 0.00164 Breaking rock on table 1.0 0.13 Large blast on construction site 1.5 1.40 2.0 9.810.00 Large quarry or mine blast 2.5 45.13 3.0 284.49 3.5 716.13 4.0 9,810.00 Small nuclear weapon 4.5 50,031.00 Average tornado 5.0 313,920.00 5.5 784,800.00 6.0 9,810,000.00 6.5 49,050,000.00 8 7.0 3.14392 * 10 Largest thermonuclear weapon 9 7.5 1.5696 * 10 9 8.0 9.8100 * 10 10 8.5 4.9050 * 10 11 9.0 3.1392 * 10 12 10.0 9.810 * 10 San Andreas-type fault circling Earth 22 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] 15 12.0 1.5696 * 10 Fault Earth in half through center or Earth’s daily receipt of solar energy Table 3: Relationship Between Magnitude and Energy Released Two general statements can be made reference to faults and magnitude. 1. As the length of the fault trace increases, the potential for larger magnitude earthquakes increases. 2. If the depth is held constant for shocks of shallow foci, the length of surface rupturing and crustal offset increases with magnitude. B. Intensity– is a measure of the degree of shaking at a specific place; it is a measure of the earthquake’s effects on humans and on surface features. It is measured in terms of the damage done to man-made structures and the changes done to man-made structures and the changes wrought in the earth’s surface. Seismic intensity represents the direct or macroseismic effects of an earthquake on humans, their products, and the features of the earth’s surface at some locale as determined by direct observation. It is, therefore, an attempt to assess the severity of a seismic activity. Earthquake intensity is highly variable due to many factors including magnitude, epicentral distance, focal depth, geologic/soil conditions, type of construction (including age and workmanship), and the expertise of the observer. Intensity varies over the geographic region, whereas the magnitude for the same earthquake would ideally be the same, regardless of locale. While scales of seismic intensity are coming into general use, they relate, for the most part, to the effects of earthquakes on structures typical of certain specific locations. It is, therefore, very difficult to make a comparative evaluation of earthquakes occurring in different places. The Rossi-Forel Scale In 1878, Michele Stefano De Rossi and Francois-Alphonse Forel introduced the first scale to gain wide acceptance. It is still used in some parts of Europe. The Rossi-Forel scale is comprised of ten effect descriptions, each designated by a Roman numeral as shown in Table 5. Intensity Description Microseismic shock. Recorded by a single seismograph or by seismographs of the I same model, but not by several seismographs of different kinds; the shock felt by an experienced observer. Extremely feeble shock. Recorded by several seismographs of different kinds; felt II by small number of persons at rest. Very feeble shock. Felt by several persons at rest; strong enough for the direction III or duration to be appreciable. Feeble shock. Felt by several persons in motion; disturbance of movable objects, IV doors windows, cracking ceilings. Shock of moderate intensity. Felt generally by everyone; disturbance of furniture, V beds, etc.; ringing of some bells. Fairly strong shock. General awakening of those asleep; general ringing of bells; VI oscillation of chandeliers; stopping of pendulum clocks; visible agitation of tree and shrubs; some startled persons leaving their dwellings. Strong Shock. Overthrow of movable objects; fall of plaster; ringing of church of VII bells; general panic, without damage to buildings. VIII Very strong shock. Fall of chimneys; cracks in the walls or ceilings. IX Extremely strong shock. Partial or total destruction of some buildings. Shock of extreme intensity. Great disaster; ruins; disturbances X of the strata; fissures in the ground; rock falls from mountains. Table 5: Rossi-Forel Intensity Scale of 1883 The Modified Mercalli (MM) Intensity Scale 23 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines ARCHITECTURE Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph DHVSU Main Campus, Villa de Bacolor, Pampanga E-Mail Address: [email protected] The seismic intensity scale most widely used today is the Modified Mercalli (MM) Intensity Scale (1931), as modified in 1956. The abridged version is presented in Table 6. The Mercalli Scale was devised by Giuseppe Mercalli in 1902 and was modified by Harry O. wood and frank Neumann in 1931, in which intensity is considered to be more uniformly graded. The 1931 MM scale was revised in 1965 to conform to construction and cultural practices in New Zealand. Four categories of masonry construction are defined, with effects included at appropriate levels. Regarding to cultural practices, an item receiving special attention is the domestic water tank (cylindrical corrugated iron, soldered, or riveted seams), which is compliance in rural areas. Intensity Description I Not felt. Marginal and long-period effects of large earthquakes. II Felt by persons at rest, on upper floors, or favorably placed. Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration III estimated. May not be recognized as an earthquake. Hanging objects swing. Vibrations like passing of heavy trucks; or sensation of a jolt like a heavy ball striking the walls. Standing motor cars rock. Windows, dishes, IV doors rattle. Glasses clink. Crockery slashes. In the upper range of IV, wooden walls and frame crack. Felt outdoors; direction estimated. Sleepers awakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. V Shutters, pictures move. Pendulum clocks stop, start, change rate. Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, etc., off shelves. Pictures off walls. VI Furniture moved or overturned. Weak plaster and masonry D cracked. Small bells ring (church, school). Trees, bushes shaken (visibly, or heard to rustle). Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices (also unbraced VII parapets and architectural ornaments—CFR). Some cracks in masonry C. Waves on ponds; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged. Steering of motor vehicles affected. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. VIII Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes. General panic. Masonry D destroyed; masonry heavily damaged, sometimes with complete collapse; masonry B seriously damaged. (General damage to foundations-CFR). Frame structures, if not bolted, shifted off foundations. Frames IX cracked. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in the ground. In alleviated areas sand and mud ejected, earthquake fountains, sand craters. Most masonry structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, X embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly. XI Rails bent greatly. Underground pipelines completely out of service. XII Damag