Module 1-Specialized 413a-Earthquake Engineering.pdf

DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] SPECIALIZED...

DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] SPECIALIZED 413a – Earthquake Engineering 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. I. Course 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. II. Course Outline: Introduction to Earthquakes and Seismology 1. 2. 3. 4. 5. 6. III. Earthquakes and Urbanization Elements of Seismology Earthquake Instruments Seismic Propagation Earthquake Descriptors How to Locate the Epicenter of an Earthquake Learning 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. P a g e 1 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] SEVERITY OF THE PROBLEM 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. DATE TABLE 1: RECORDED INTERNATIONAL MAJOR EARTHQUAKES 1811-12 New Madrid, MO N.A. DEATH MAGNITUD LOCATION Aleppo, Asia S E September 5, 1822 22,000 Minor 8.7* N.A. May 20, 526 Antioch, Syria 250,000 N.A. 856 Corinth, Greece 45,000 N.A. December 28, 1828 Echigo, Japan 30,000 N.A. 1057 Chihli, China 25,000 N.A. August 13-15, 1868 Peru, Ecuador 40,000 N.A. 15,0001 N.A. May 16, 1875 16,000 N.A. 60,000 N.A. 60 6.6 February 11, 1169 1268 September 27, 1290 May 20, 1293 Near Mt. Etna, Sicily Cilicia, Asia Minor Chilhli, China 100,000 N.A. Kamakura, Japan 30,000 N.A. August 31, 1886 Venezuela, Colombia Charleston, SC, USA June 15, 1896 Japan, (sea wave) 27,120 N.A. April 4, 1905 Kangra, India 19,000 8.6 5032 8.3 San Francisco, January 26, 1531 Lisbon, Portugal 30,000 N.A. January 24, 1556 Shaanxi, China 830,000 N.A. November, 1667 Shemaka, Cucasia 80,000 N.A. August 17, 1906 Valparaiso, Chile 20,000 8.6 January 11, 1693 Catania, Italy 60,000 N.A. October 21, 1907 Central Asia 12,000 8.1 Messina, Italy 83,000 7.5 Avezzano, Italy 29,980 7.5 116 7.5 April 18-19, 1906 USA December 30, 1730 Hokkaido, Japan 137,000 N.A. December 28, 1908 October 1, 1737 Calcutta, India 300,000 N.A. January 13, 1915 June 7, 1755 Northern Persia 40,000 N.A. November 1, 1755 Lisbon, Portugal 60,000 8.75* February 4, 1783 Calabria, Italy 30,000 N.A. December 16, 1920 Gansu, China 200,000 8.6 N.A. September 1, 1923 Yokohama, Japan 143,000 8.3 February 4, 1797 Quito, Ecuador 41,000 October, 11, 1918 Mona Passage, P. Rico P a g e 2 | 34 COLLEGE OF ENGINEERING AND DON HONORIO VENTURA STATE UNIVERSITY DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] March 16, 1925 Yunnan, China 5,000 7.1 October 10, 1980 Northwest Algeria 3,500 7.7 May 22, 1927 Nan-Shan, China 200,000 8.3 November 23, 1980 South Italy 3,000 7.2 December 25, 1932 Gansu, China 70,000 7.6 June 11, 1981 South Iran 3,000 6.9 2,990 8.9 July 28,1981 South Iran 1,500 7.3 2,800 6.0 81 7.7 1,342 6.9 146 7.8 9,500 8.1 1,000+ 5.5 4,000+ 7.0 1,450 6.6 1,000 7.3 55,000 7.0 62 7.1 March 2, 1933 Japan March 10, 1933 Long Beach, USA 115 6.2 January 15, 1934 Bihar-Nepal, India 10,700 8.4 April 21, 1935 Taiwan (Formosa) 3,276 7.4 My 30, 1935 Quetta, India 50,000 7.5 January 25, 1939 Chillan, Chile 28,000 8.3 October 30, 1983 East Turkey December 26, 1939 Erzincan, Turkey 30,000 8.0 March 3, 1985 Chile December 20, 1946 Honshu, Japan 1,330 8.4 June 28, 1948 Fukui, Japan 5,390 7.3 August 5, 1949 Pelileo, Ecuador 6,000 6.8 August 15, 1950 Assam, India 1,530 8.7 March 18, 1953 Northwest Turkey 1,200 7.2 June 10-17, 1956 North Afghanistan 2,000 7.7 July 2, 1957 North Iran 1,200 7.4 December 13, 1957 West Iran 1,300 7.3 May 21-30, 1960 South Chile 5,000 9.5 September 1, 1962 Northwest Iran 12,230 7.3 July 26, 1963 Skopje, Yugoslavia March 27, 1964 Alaska August 19, 1966 East Turkey August 31, 1968 January 4, 1970 Northeast Iran Yunnan Prov., China 1,100 6.0 131 9.2 2,520 7.1 12,000 7.3 10,000 7.5 West Turkey 1,100 7.3 May 31, 1970 North Peru 66,000 7.8 65 6.6 5,054 7.1 April 10, 1972 December 23, 1972 December 28, 1974 San Fernando Val., CA South Iran Managua, Nicaragua Pakistan (9 Towns) 5,000 5,200 February 4, 1976 Guatemala 23,000 7.5 May 6, 1976 Northeast Italy 1,000 6.5 August 16, 1976 November 24, 1976 Tangshan, China Mindanao, Philippines Northwest IranUSSR border Indonesia 200 8.0 1978 September 12, 1979 December 12, 1979 Northeast Iran Ecuador Ecuador India-Nepal border China-Burma border Soviet Armenia San Francisco Bay Area July 16, 1990 Luzon, Philippines 1,621 7.8 1,200 6.8 Pakistan, Afgh, border October 19, 1991 North India 2,000 7.0 March 13, 15, 1992 East Turkey 4,000 6.2/6.0 June 28, 1992 South California 1 7.5/6.6 2,500 7.5 200+ 7.7 116 7.0 450 5.9 9,7483 6.3 61 6.8 215 7.0 1,000 6.8 December 12, 1992 Flores, Isl., Indonesia Off Hokkaido, Japan Southwest Nicaragua October 12, 1992 Cairo, Egypt September 29, Maharashtra 1993 South India Northridge, CA South Sumatra, Indon Cauca, Southwest Colombia August 19, 1994 North Algeria 164 6.0 January 16, 1995 Kobe, Japan 5,502 6.9 1,989 7.5 73 6.0 40+ 7.6 200+ 7.0 53 7.5 79 6.9 May 27, 1995 8.2 October 1, 1995 15,000 7.8 October 9, 1995 100 8.1 7.9 Colombia- 7.7 100 800 El Salvador 6.3 February 17, 1996 Colombia, Mexico 115 February 3, 1996 Indonesia Michoacan, 40,000+ June 6, 1994 August 19, 1977 Japan West Iran 7.8 7.3 North Honshu, North Peru February 15, 1994 5,000 Peninsula June 20, 1990 8.0 8,000 West Arabian May 30, 1990 655,000 7.2 September 16, October 17, 1989 January 17, 1994 1,500 Argentina December 7, 1988 7.1 Romania November 23, 1977 November 6, 1988 422 March 4, 1977 Northwest August 20, 1988 September 1, 1992 6.7 July 27,1976 March 6, 1987 6.3 2,300 Guinea October 10, 1986 July 12, 1993 Turkey (Lice, etc) June 25, 1976 September 19,1985 6.2 September 6, 1975 Irian Jaya, New May 26, 1983 February 1, 1991 March 28, 1970 February 9, 1971 December 13, 1982 February 4, 1997 Sakhalin Isl. Russian Southwest Turkey West coast Mexico Southwest China Iran Jaya, Indonesia Turkmen-Iran P a g e 3 | 34 COLLEGE OF ENGINEERING AND DON HONORIO VENTURA STATE UNIVERSITY DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph border E-Mail Address: [email protected] August 17, 1999 Western Turkey 16,965+4 7.4 February 27, 1997 West Pakistan 100+ 7.3 September 7, 1999 Athens, Greece 143 5.9 February 28, 1997 Northwest Iran 1,000+ 6.1 September 21, 2,321 7.6 North Iran 1,560 7.5 1999 Taichung, Taiwan May 10, 1997 Duzce, Turkey 675+ 7.2 40+ 6.1 103 7.9 82 6.9 800+ 7.6 May 21, 1997 July 9, 1997 September 26, 1997 Madhya Pradesh, India Northeast Venezuela Central Italy September 28, Sulawesi, 1997 Indonesia October 15, 1997 Illapel, Chile January 10, 1998 Zhangbel, China February 4,8, 1998 March 25, 1998 April 1, 1998 April 27, 1998 May 3, 1998 May 22, 1998 May 30, 1998 June 1, 1998 Takhar, Northeast Afghanistan BallenyIsalnds Sumatra, Indonesia Irian Jaya, Indonesia Ryukyu Is., Taiwan Central Bolivia Northeast Afghanistan Kamchatka, Russia November 12, 1999 June 4, 2000 January 13, 2001 11 5.5/5.7 17+ 5.9 8.12 6.8 50 6.2 2,323 6.1 --- 8.2 --- 7.0 --- 7.4 --- 7.4 105 6.5 4,700+ 6.9 --- 6.5 6.3 June 27, 1998 Adana, Turkey 144 July 9, 1998 Azores, Portugal 10 5.8 November 9, 1998 Banda Sea --- 6.6/7.0 East Indonesia 34 7.8 Fiji Islands --- 6.9 --- 7.0 1,185+ 6.0 November 29, 1998 December 29, 1998 January 19, 1999 January 25, 1999 Papua New Guinea Armenia, Colombia January 28, 1999 Aleutan Islands --- 6.6 February 6, 1999 Sta. Cruz Islands --- 7.3 60 6.0 87 6.8 --- 7.4 --- 7.1 26+ 6.2 --- 7.1 16 6.7 February 11, 1999 March 28, 1999 Central Afghanistan Uttar Pradesh, India Sumatra, Indonesia San Vicente, El 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 Source: World Almanac 2005 Source: World Almanac 2020 New Britain, April 5, 1999 Papua New Guinea April 8, 1999 May 7, 1999 East Rusia, Northeast China Southern Iran New Britain, May 10, 16, 1999 Papua New Guinea June 16, 1999 Puebia Mexico P a g e 4 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph 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. P a g e 5 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph 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. P a g e 6 | 34 COLLEGE OF ENGINEERING AND DON HONORIO VENTURA STATE UNIVERSITY DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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 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. P a g e 7 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph 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. P a g e 8 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] Examples of megathrust earthquakes are listed in the following table. Event Estimated Moment Magnitude (Mw) 365 Crete 8.0+ earthquake 1575 Valdivia 8.5 earthquake 1700 Cascadia 8.7–9.2 earthquake Tectonic Plates Involved  The quake generated a large tsunami in the eastern Mediterranean Sea and caused significant vertical displacement in the island of Crete.[4]  Slip length: 1000 km (625 mi)   Duration: 15 minutes Depth: 40 km  Depth: 30 km    Depth: 33 km Slip length: 1000 km (625 mi) Slip width: 200 km (125 mi) African Platesubducting beneath the Eurasian Plate Nazca Platesubducting beneath the South American Plate Juan de Fuca Platesubducting beneath the North American Plate 1737 Kamchatka 9.0–9.3 earthquake Pacific Platesubducting beneath the Okhotsk Plate 1755 Lisbon 9.0 earthquake Hypothesized to be part of a young subduction zone but origin still debated 1868 Arica 9.0 earthquake Nazca Platesubducting beneath the South American Plate 1877 Antofagasta (Northern 8.8 Chile) earthquake Nazca Platesubducting beneath the South American Plate 1946 Nankaidō 8.1 earthquake Philippine Sea Platesubducting beneath the Eurasian Plate 1950 Nicoya Peninsula (Costa Rica) earthquake 1952 Kamchatka earthquake 1957 Andreanof Islands earthquake 1960 Great Chilean Earthquake Other Details/Notes 7.7 Cocos Platesubducting beneath the Caribbean Plate[5] 9.0 Pacific Platesubducting beneath the Okhotsk Plate 8.6–9.1 Pacific Plate subducting beneath the North American Plate 9.5 Nazca Plate subducting beneath the South American Plate P a g e 9 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph 1964 Alaska earthquake ("Good 9.2 Friday" earthquake) Pacific Plate subducting beneath the North American Plate 2004 Indian Ocean 9.3 earthquake India Platesubducting beneath the Burma Plate 2010 Chile 8.8 earthquake Nazca Platesubducting beneath the South American Plate E-Mail Address: [email protected]  Slip motion: 20 m (60 ft)     Duration: 4–5 minutes Depth: 25 km Slip length: 800 km (500 mi) Slip motion: 23 m (69 ft)  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 two large, steep, almost vertical cliffs, one above the other. Duration: 8–10 minutes Depth: 30 km Slip length: 1300 km (1000 mi) Slip motion: 33 m     C. TYPES OF FAULTS The two sides of a fault are called the hanging wall 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 P a g e 10 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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. 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 P a g e 11 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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. 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. P a g e 12 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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. 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. P a g e 13 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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 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. Figure 9: Mallet's seismoscope Figure 10: Sample Responses from a Seismoscope. P a g e 14 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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. CREEPMETERS Creepmeters measures 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. Ultraprecise, 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. 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. P a g e 15 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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). P a g e 16 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph 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 P a g e 17 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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. 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. 2. 3. 4. Possess longitudinal motion analogous to sound waves. Propagate through all material types (solid, liquid, gas). Are the first types in the total wave train to arrive at a seismological station. Velocity is determined by P a g e 18 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 4 𝑣1 = √( + 𝜇)/𝜌 3 Where:  = velocity  = bulk modulus (resistance to volume changes)  = 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 𝒗𝟐 = √𝝁/𝝆 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. P a g e 19 | 34 COLLEGE OF ENGINEERING AND DON HONORIO VENTURA STATE UNIVERSITY DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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. 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 10minute 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 P a g e 20 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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 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. P a g e 21 | 34 COLLEGE OF ENGINEERING AND DON HONORIO VENTURA STATE UNIVERSITY DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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 complicated mathematics, it can be shown that the moment of an earthquake is simply expressed by 𝑴𝒐 = 𝝁𝑨𝒅 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] Mw = 2/3[log10 Mo– 9.81] (Mo in dyne-cm) (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. P a g e 22 | 34 DON HONORIO VENTURA STATE UNIVERSITY COLLEGE OF ENGINEERING AND DHVSU Main Campus, Villa de Bacolor, Pampanga Cabambangan, Villa de Bacolor 2001, Pampanga, Philippines Tel. No. (6345) 458 0021; Fax (6345) 458 0021 Local 211 URL: http://dhvsu.edu.ph E-Mail Address: [email protected] 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)