Module 1 - Introduction-1.pdf

14/08/2024 MODULE 1: INTRODUCTION CEPE 2S | Professional Course – Specialized 2 (Earthquake Engineering) Prepared by: EN...

14/08/2024 MODULE 1: INTRODUCTION CEPE 2S | Professional Course – Specialized 2 (Earthquake Engineering) Prepared by: ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University INTERIOR OF THE EARTH Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 1  14/08/2024 INTERIOR OF THE EARTH Geospheres INTERIOR OF THE EARTH Convection Current The high pressure and temperature gradients between the crust and the core cause convection currents to develop in the viscous mantle as shown in the figures. These convection currents cause the earth’s mass to circulate. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 2  14/08/2024 INTERIOR OF THE EARTH Tectonic Plates The convective flows of mantle cause the crust and some portion of the mantle to slide on the hot molten outer core. The earth’s crust, therefore, is not static but subjected to motion. It consists of several gigantic rigid rock plates (called tectonic plates) of about 80km in thickness that float in slow motion in a viscous (partially plastic) mantle. The surface of the earth consist of seven major tectonic plates and many smaller ones (Figure 3). INTERIOR OF THE EARTH Tectonic Plates These plates are presumed to move laterally and grind together at their margins. These plates move in difference directions and at difference speeds relative to each other at a rate of 5 to 10 cm per year on the plastic mantle. This movement is called plate tectonics. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 3  14/08/2024 INTERIOR OF THE EARTH Tectonic Plates Tectonic plates are made of either oceanic or continental crust. These oceanic plates are pushed against and subduct under the continental plates, resulting in continental drift. INTERIOR OF THE EARTH Tectonic Plates When an oceanic plate collides with a continental plate, it slides beneath the continental plate forming a deep oceanic trench. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 4  14/08/2024 INTERIOR OF THE EARTH Tectonic Plates Pacific Ring of Fire is a region around much of the rim of the Pacific Ocean where many volcanic eruptions and earthquakes occur. CAUSES OF EARTHQUAKE Earthquakes are vibrations or oscillations of the ground surface caused by a transient disturbance of the elastic or gravitational equilibrium of the rocks at or beneath the surface of the earth. The disturbance and the consequent movements give rise to elastic impulses or waves. The origin or causes of earthquakes can be explained by the following theories. 1. Elastic Bound Theory 2. Plate Tectonic Theory Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 5  14/08/2024 CAUSES OF EARTHQUAKE 1. Elastic Bound Theory First proposed by M.F. Reid in 1906, attributes the occurrence of tectonic earthquakes to the gradual accumulation of strain in a given zone and subsequent gradual increase in the elastic forces stored. Where the oceanic and continental plates collide, they may be locked in place, that is, these may be prevented from moving because of the frictional resistance along the plate boundaries. This causes building up of stresses along the plate edges until sudden slippage due to elastic rebound or fracture of the rock occurs, resulting in sudden release of strain energy that may cause the upper crust of the earth to fracture along a certain direction and form a fault. The gradual accumulation and subsequent release of stress and strain is described as elastic rebound. CAUSES OF EARTHQUAKE 1. Elastic Bound Theory The upper parts of the earth’s crust and lithosphere are very strong and brittle. When this rock is subjected to deformation, it actually bends slightly (Fig. 1.3). However, it is able to withstand very light stress with only slight bending or strain. The elastic rebound theory requires the strain to build up rapidly up to the elastic limit of the rock. Beyond this point, the earth’s crust ruptures due to the formation of a fault and the bent rock snaps back to regain its original shape, releasing the stored energy in the form of rebounding and violent vibrations (elastic waves). These vibrations shake the ground; the maximum shaking effect is felt along the fault. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 6  14/08/2024 CAUSES OF EARTHQUAKE 1. Elastic Bound Theory Most earthquakes occur along the boundaries of the tectonic plates and are called interplate earthquakes (Great Assam earthquake, 1950). The others occurring within the plates themselves, away from the plate boundaries (Latur earthquake, 1993), are called intraplate earthquakes. In both types, slips are generated during the earthquake at the fault along both horizontal and vertical directions, known as dip slip [Fig. 1.4(a)], and the lateral direction, known as strike slip [Fig. 1.4(b)]. CAUSES OF EARTHQUAKE 1. Elastic Bound Theory The moment of each couple is known as the earthquake moment or seismic moment. Recently, the seismic moment has been used as a measure of earth-quake size. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 7  14/08/2024 CAUSES OF EARTHQUAKE 1. Elastic Bound Theory The elastic rebound theory implies that an earthquake relieves the accumulated stresses along the portion of the fault on which rupture occurs. Further, this segment will not rupture again until the stresses build up again which, of course, will take its own time. Therefore, earthquakes can reoccur only after some period of time and that, perhaps, depends on the amount of energy released in the earthquake. CAUSES OF EARTHQUAKE 2. Plate Tectonic Theory Studies related to continental drifts, volcanic eruptions, and ridges on ocean floors have led to development of the theory of plate tectonics. According to that, the earth’s crust consists of a number of large rigid blocks called crustal plates. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 8  14/08/2024 CAUSES OF EARTHQUAKE 2. Plate Tectonic Theory The plates may also move side by side along the same direction or in opposite directions. The relative motion of crustal plates gives rise to three kinds of plate boundaries or marginal zones. These types are described as divergent (constructive margin), convergent (destructive margin), and transform (conservative margin) or parallel plate boundaries, as shown in figure. CAUSES OF EARTHQUAKE 2. Plate Tectonic Theory The Earth’s Tectonic Plates, with convergent and divergent boundaries indicated with red arrows. Credit: msnucleus.org Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 9  14/08/2024 CAUSES OF EARTHQUAKE 2. Plate Tectonic Theory Convergent boundary occurs when two plates from opposite directions come together and collide. When, upon collision, the two plates are pushed upwards against each other, they form major mountain systems such as the Himalayas. CAUSES OF EARTHQUAKE 2. Plate Tectonic Theory Divergent boundary occurs when two tectonic plates move away from each other. A well-known divergent boundary is the Mid-Atlantic Ridge. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 10  14/08/2024 CAUSES OF EARTHQUAKE 2. Plate Tectonic Theory Transform boundaries are places where plates slide sideways past each other. At transform boundaries lithosphere is neither created nor destroyed. Many transform boundaries are found on the sea floor, where they connect segments of diverging mid-ocean ridges. California's San Andreas fault is a transform boundary with a length of 1,200-km. California's San Andreas Fault CAUSES OF EARTHQUAKE Faults According to PHILVOLCS, a fault is a break, fracture, fissure or zone of weakness where movement or displacement had occurred or may occur again. It may extend hundreds of kilometers across the earth’s surface and tens of kilometers downward. Additionally, an active fault is a fault that has moved within the last 10,000 years. It shows evidence or has documented history of its recent movement. An earthquake is a weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth’s surface. Earthquakes occur along tectonic plate boundaries and active faults. Three Types of Fault according to PHIVOLCS Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 11  14/08/2024 CAUSES OF EARTHQUAKE Faults The Marikina Valley Fault System, also known as the Valley Fault System (VFS), is a dominantly right-lateral strike- slip fault system in Luzon and is the most seismically active fault in the Philippines, according to PHIVOLCS. CAUSES OF EARTHQUAKE Faults According to PHILVOLCS, most of the inland earthquakes are caused by the movement along the Philippine Fault, a 1,300km-long fault, that traverses from Ilocos Region in the north to eastern Mindanao in the south. Movements along other active faults are also responsible for the present-day high seismicity of the Philippines. PHILIPPINE FAULT Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 12  14/08/2024 ACTIVE FAULTS, TRENCHES, AND VOLCANOES IN THE PHILIPPINES PHIVOLCS FaultFinder The PHIVOLCS FaultFinder is an application capable to do proximity searches to active faults. It may be used to determine the location of active faults in an area and to measure the shortest distance between an active fault and a user’s current location, which is determined by the gadget’s tracking device. It may also be used to measure the shortest distance between an active fault and a specific site, which is identified by a user. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 13  14/08/2024 NATURE AND OCCURRENCE OF EARTHQUAKE When there is a sudden localized disturbance in rocks, waves similar to those caused by a stone thrown into a pool spread out through the earth. An earthquake generates a similar disturbance. The maximum effect of an earthquake is felt near its source, diminishing with distance from the source (earthquakes shake the ground even hundreds of kilometers away) NATURE AND OCCURRENCE OF EARTHQUAKE The vibrations felt in the bedrock are called shocks. Some earthquakes are preceded by smaller foreshocks and larger earthquakes are always followed by aftershocks. Aftershocks are usually due to fresh ruptures or readjustment of fractured rocks Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 14  14/08/2024 NATURE AND OCCURRENCE OF EARTHQUAKE The point of generation of an earthquake is known as the focus or center. The point on the earth’s surface directly above the focus is known as epicenter. The depth of the focus from the epicenter is known as the focal depth. The distance from the epicenter to any point of interest is known as the focal distance or epicentral distance. SEISMIC WAVES The large strain energy released during an earthquake travels in the form of seismic waves in all directions (Fig. 1.8), with accompanying reflections from earth’s surface as well as reflections and refractions as they traverse the earth’s interior (Fig. 1.9 ). These waves can be classified as body waves travelling through the interior of the earth consisting of P-waves (primary waves) and S –waves (secondary waves), and surface waves resulting from interaction between body waves and surface layers of earth—consisting of L -waves (love waves) and Rayleigh waves (Fig.1.10). Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 15  14/08/2024 SEISMIC WAVES a. Body Waves b. Surface Waves SEISMOGRAPH A seismograph is an instrument used to measure the vibration of the earth. Seismographs are used to measure relatively weak ground motions. The records produced are called seismograms. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 16  14/08/2024 SEISMOGRAPH The record produced by the seismograph is called a seismogram. Philippine Seismic Stations and Volcano Observatories Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 17  14/08/2024 LOCATING EARTHQUAKE The location of an earthquake implies the location of its epicenter. In order to determine the location of an earthquake, we need to have recorded a seismogram of the earthquake from at least three seismograph stations at different epicentral distances. The preliminary location of the earthquake at each station is based on the relative arrival times of P- and S-waves taken from the seismograph. The common point of intersection of the three circles will be the likely location of the epicenter. 3 2 1 LOCATING EARTHQUAKE The location of an earthquake implies the location of its epicenter. In order to determine the location of an earthquake, we need to have recorded a seismogram of the earthquake from at least three seismograph stations at different epicentral distances. The preliminary location of the earthquake at each station is based on the relative arrival times of P- and S-waves taken from the seismograph. The common point of intersection of the three circles will be the likely location of the epicenter. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 18  14/08/2024 MEASUREMENTS OF EARTHQUAKES Structural engineers are concerned with the effect of earthquake ground motions on structures, that is, the amount of damage inflicted on the structures. This damage (stress and deformation) potential depends to a large extent on the size (severity) of the earthquake. The severity of an earthquake can be assessed in the following ways: a) quantifying its magnitude in terms of the energy released—measuring the amplitude, frequency, and location of the seismic waves; b) evaluating the intensity—considering the destructive effect of shaking ground on people, structures, and natural features. In short, there are two ways by which we can measure the strength of an earthquake: magnitude and intensity. It is easier to measure the magnitude because, unlike the intensity, which can vary with location and has no mathematical backing, the magnitude of a particular earthquake remains constant. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 19  14/08/2024 MEASUREMENTS OF EARTHQUAKES A. Intensity Intensity is a non-instrumental perceptibility or qualitative measure of damage to structures, ground surface effects, and human reactions to earthquake shaking. It refers to the degree of destruction caused by it and is assigned as Roman Capital Numerals. Some of the most common intensity scales are: MEASUREMENTS OF EARTHQUAKES Modified Mercalli Intensity (MMI) The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above. https://www.usgs.gov/programs/earthquake-hazards/modified-mercalli-intensity-scale Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 20  14/08/2024 MEASUREMENTS OF EARTHQUAKES PHIVOLCS Earthquake Intensity Scale (PEIS) The PHIVOLCS Earthquake Intensity Scale (PEIS) is a seismic scale used and developed by the Philippine Institute of Volcanology and Seismology (PHILVOLCS) to measure the intensity of an earthquake. It was developed as a response to the 1990 Luzon earthquake. PHILVOLCS cites seismic scale specifically developed for the Philippine setting, the different geography of each country and other "geological considerations" led to the development of PEIS. The scale measures the effect of an earthquake on a given area based on its relative effect to people, structures and objects in the surroundings. The PEIS was adopted in the Philippines in 1996 replacing the Rossi-Forel scale. MEASUREMENTS OF EARTHQUAKES Comparison between seismic intensity scales Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 21  14/08/2024 MEASUREMENTS OF EARTHQUAKES The distribution of intensity at different places during an earthquake is shown graphically using isoseismals, lines joining places with equal seismic intensity. MEASUREMENTS OF EARTHQUAKES B. Magnitude Magnitude of an earthquake is a measure of the amount of energy released at the focus. It is a quantitative measure of the actual size or strength of the earthquake and is a much more precise measure than intensity. It is represented by Arabic Numbers (e.g. 4.8, 9.0). Earthquake magnitudes are based on the direct measurements of the size (amplitude) of seismic waves, made with recording instruments, rather than on subjective observant of the destruction caused. The total energy release by an earthquake can be calculated from the amplitude of the waves and the distance from the epicenter. A magnitude number is assigned to an earthquake on the basis of the amount of ground displacement or vibration it produces. The reading at a given station is adjusted for the distance of the instrument from the earthquake’s epicenter, because ground vibration naturally decreases with increasing distance from the site of the earthquake, as the energy is dissipated. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 22  14/08/2024 MEASUREMENTS OF EARTHQUAKES The most common magnitude scales are: a) Richter or Local Magnitude (𝑀 ) b) Body Wave Magnitude (𝑚 ) c) Surface Wave Magnitude (𝑀 ) d) Moment Magnitude (𝑀 ) The Richter magnitude is based on the energy release of the earthquake, which is closely related to the length of the fault on which the slippage occurs. Earthquake magnitudes are most commonly measured using this scale – Richter magnitude. Earthquakes with 𝑀 greater than 5.5 cause significant damage, while an earthquake of 𝑀 = 2 is the smallest event normally felt by people. However, this scale cannot measure magnitude above 𝑀 = 7, because the high frequency waves recorded locally have wavelength shorter than the rupture length of large earthquakes. Richter magnitude exhibits several limitations. It is applicable only to small and shallow earthquakes. Later, to express the size of earthquakes around the planet, Gutenberg and Richter developed a surface wave magnitude (𝑚 ) and body wave magnitude (𝑀 ). The two scales were adjusted such that they were consistent with the Ritcher scale (𝑀 ). MEASUREMENTS OF EARTHQUAKES In 1970s, the researchers developed the Moment Magnitude (𝑀 ). This is the moment released during an earthquake rupture. Moment magnitude has proved to be the best measure, since the seismic moment (on which moment magnitude is based) is a measure of the whole dimension of the fault, which, for great earthquakes, may extend to hundred of kilometers. It is the most suitable magnitude scale for describing the size of very large earthquakes. Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 23  14/08/2024 MEASUREMENTS OF EARTHQUAKES Richter Magnitude (𝑴𝑳 ) MEASUREMENTS OF EARTHQUAKES Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 24  14/08/2024 MEASUREMENTS OF EARTHQUAKES SEISMIC HAZARDS Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 25  14/08/2024 SEISMIC HAZARDS SEISMIC HAZARDS Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 26  14/08/2024 SEISMIC HAZARDS SEISMIC HAZARDS Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 27  14/08/2024 SEISMIC HAZARDS SEISMIC HAZARDS Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 28  14/08/2024 SEISMIC HAZARDS SEISMIC HAZARDS Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 29  14/08/2024 END OF PRESENTATION Lecture of ENGR. RYAN M. LAYUG CE Department, College of Engineering Tarlac State University 30