CE021 Introduction to Earthquake Engineering (2024-2025) PDF

TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES DEPARTMENT OF CIVIL ENGINEERING FIRST SEMESTER 2024 - 2025 CE021 EARTHQUAKE ENGINEERING CE021 INTRODUCTION TO EARTHQ...

TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES DEPARTMENT OF CIVIL ENGINEERING FIRST SEMESTER 2024 - 2025 CE021 EARTHQUAKE ENGINEERING CE021 INTRODUCTION TO EARTHQUAKE ENGINEERING PART 1 CHRISTIAN Y. IBONIA, RCE Course Instructor, CE021 FIRST SEMESTER 2024 - 2025  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTENDED LEARNING OUTCOMES At the of the course, the students shall be able to: 1. Describe earthquakes, their worldwide distribution, what causes them, their likely damage mechanisms, measuring scales, and current efforts on the prediction of strong seismic ground motions. 2. Discuss the Philippine Fault System and how it differs from the United States (US) Fault System.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTRODUCTION TO EARTHQUAKE ENGINEERING EARTHQUAKE An earthquake is a sudden release of energy in the Earth's crust that creates seismic waves. Most earthquakes are produced when stress builds up along a fault over a long time, eventually causing the fault to slip. Most earthquakes occur due to the movement of tectonic plates, which are large pieces of the Earth's crust that fit together like a jigsaw puzzle.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTRODUCTION TO EARTHQUAKE ENGINEERING EARTHQUAKE Similar kinds of energy are released by: volcanic eruptions, the underground movement of magma, catastrophic landslides, and natural and human caused explosions. Some earthquakes are caused by other human activities, such as: the filling of reservoirs behind dams and the disposal of wastewaters associated with the exploration for oil and gas  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTRODUCTION TO EARTHQUAKE ENGINEERING HOW DO CONTINENTS DIFFER FROM OCEAN BASINS? Our planet is divided into continents and oceans. Continents and oceans differ in the types and thicknesses of the rocks they contain and, as we will learn later, form in very different ways. Within the oceans are major variations in the depth and character of the seafloor from place to place.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTRODUCTION TO EARTHQUAKE ENGINEERING HOW DO CONTINENTS DIFFER FROM OCEAN BASINS? The land also varies in elevation and character, such as higher, vegetation- covered mountains in eastern Australia than in the rest of the continent.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTRODUCTION TO EARTHQUAKE ENGINEERING HOW DO CONTINENTS DIFFER FROM OCEAN BASINS? Each region, whether on land or beneath the ocean, has its own geologic history, and the landscape and rocks contain clues as to the geologic events that affected each place.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTRODUCTION TO EARTHQUAKE ENGINEERING WHAT IS INSIDE THE EARTH? Earth consists of concentric layers that have different compositions. The outermost layer is the crust, which includes continental crust and oceanic crust. Beneath the crust is the mantle, Earth’s most voluminous layer. The molten outer core and the solid inner core are at Earth’s center.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT IS INSIDE THE EARTH? 1 Continental crust, the thin that averages 35 to 40 km (20–25 mi) in thickness.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT IS INSIDE THE EARTH? 2 Oceanic crust has an average thickness of about 7 km (4 mi).  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT IS INSIDE THE EARTH? 3 The mantle extends from the base of the crust down 2,900 km (1,800 mi).  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT IS INSIDE THE EARTH? 4 The lower mantle has a composition similar to the upper mantle, but it contains minerals formed at very high pressures. Nearly all of the mantle is solid, not molten.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT IS INSIDE THE EARTH? 5 The outer core is molten, but the inner core is solid.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING INTRODUCTION TO EARTHQUAKE ENGINEERING WHAT IS INSIDE THE EARTH? 1. The crust and uppermost mantle together form an upper, rigid layer called the lithosphere (lithos means “stone” in Greek).  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE DESCRIBE AN EARTHQUAKE? It releases mechanical energy, some of which is transmitted through rocks as vibrations called seismic waves. These waves spread out from the site of the disturbance and travel through the interior or along the surface of Earth. Scientists record the waves using scientific instruments at seismic stations.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE DESCRIBE AN EARTHQUAKE? The place where the earthquake is generated is called the hypocenter or focus.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE DESCRIBE AN EARTHQUAKE? The epicenter is the point on Earth’s surface directly above where the earthquake occurs.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE DESCRIBE AN EARTHQUAKE? The curved bands show the peaks of waves radiating from the hypocenter. The intensity and duration of waves are measured by seismic stations (locations 1 and 2).  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES MOST EARTHQUAKES? When rocks on opposite sides of a fault slip past one another abruptly, the movement generates seismic waves, while materials near the fault are pushed, pulled, and sheared. Slip along any type of fault can generate an earthquake.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES MOST EARTHQUAKES? NORMAL FAULTS The rocks above the fault (the hanging wall) move down with respect to rocks below the fault (the footwall). The crust is stretched horizontally, so earthquakes related to normal faults are most common along divergent plate boundaries.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES MOST EARTHQUAKES? NORMAL FAULTS continental rifts OCEANICIC SPREADING CENTERS CONTINENTAL RIFTING  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES MOST EARTHQUAKES? REVERSE AND THRUST FAULTS Many large earthquakes are generated along reverse faults, especially the gently dipping variety called thrust faults. The hanging wall moves up with respect to the footwall. Such faults are formed by compressional forces, like those associated with subduction zones and continental collisions.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES MOST EARTHQUAKES? REVERSE FAULTS continental rifts  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES MOST EARTHQUAKES? STRIKE-SLIP FAULTS The two sides of the fault slip horizontally past each other. This can generate large earthquakes. Most strike-slip faults are near vertical, but some have moderate dips. The largest strike-slip faults are transform plate boundaries, like the San Andreas fault in California.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES MOST EARTHQUAKES? STRIKE-SLIP FAULTS continental rifts SAN ANDREAS FAULT  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO VOLCANOES AND MAGMA CAUSES EARTHQUAKES? 1. An explosive volcanic eruption causes compression, transmitting energy as seismic waves.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO VOLCANOES AND MAGMA CAUSES EARTHQUAKES? 2. Volcanoes add tremendous weight to the crust, and this loading can lead to faulting and earthquakes.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO VOLCANOES AND MAGMA CAUSES EARTHQUAKES? 3. The flanks of such volcanoes can fall apart catastrophically, causing landslides that shake the ground as they break away and travel down the flank of the volcano.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO VOLCANOES AND MAGMA CAUSES EARTHQUAKES? 4. The emplacement of magma can cause a series of small and distinctive earthquakes, called volcanic tremors.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT ARE SOME OTHER CAUSES OF SEISMIC WAVES? LANDSLIDE Catastrophic landslides, whether on land or beneath water, cause ground shaking.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT ARE SOME OTHER CAUSES OF SEISMIC WAVES? EXPLOSIONS Mine blasts and nuclear explosions compress Earth’s surface, producing seismic waves measurable by distant seismic instruments.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING EARTHQUAKES CAUSED BY HUMANS Reservoirs built to store water fill rapidly and load the crust, which responds by faulting and flexing. Humans have also caused earthquakes by injecting wastewater underground into a deep well (drill hole) at the Rocky Mountain Arsenal northwest of Denver.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DOES FAULTING CAUSE EARTHQUAKES? What Processes Precede and Follow Faulting? Before faulting, rocks change shape (i.e., they strain) slightly as they are squeezed, pulled, and sheared. Once stress builds up to a certain level, slippage along a fault generally happens in a sudden, discrete jump. Faulting reduces the stress on the rocks, allowing some of the strained rocks to return to their original shapes. This type of response, where rocks return to their original shape after being strained, is called elastic behavior.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING PRE-SLIP AND ELASTIC STRAIN An active strike-slip fault has modified the appearance of a landscape for hundreds of thousands of years, causing a linear trough along the fault. Some segments of streams follow the fault.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING PRE-SLIP AND ELASTIC STRAIN With time, stress increases along the fault as depicted by the upward-sloping line, which plots stress as a function of time. In response, the rocks may deform elastically, changing shape slightly without breaking.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING SLIP AND EARTHQUAKE Over time, stress along the fault becomes so great that it exceeds the fault’s ability to resist it. As a result, the fault slips and the rocks on opposite sides of the fault rapidly move past each other.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING SLIP AND EARTHQUAKE In the stress-versus-time graph, the point at which the earthquake occurred is shown as an orange dot. At this point, the rocks were no longer strong enough and there was not sufficient friction along the fault surface to prevent movement.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING POST-SLIP With the stress partially relieved, the rocks next to the fault relax by elastic processes and largely return to their original, unstrained shape. The movement that has occurred along the fault, however, is permanent.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING POST-SLIP In the stress-versus-time graph , the release of stress after the earthquake is only temporary. The black dot at the end of the line is the current state of stress, and the cycle of stress buildup and release will continue. In this way, the rock strains elastically before the earthquake, ruptures during the earthquake, and mostly returns to its original shape afterwards. This sequence is called stick-slip behavior because the fault sticks and then slips.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO EARTHQUAKE RUPTURES GROW? Most earthquakes occur by slip on a preexisting fault, but the entire fault does not begin to slip at once. Instead, the earthquake rupture starts in a small area (the hypocenter) and expands over time.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO EARTHQUAKE RUPTURES GROW? A rupture starts on a small section of the fault below Earth’s surface and begins to expand along the preexisting fault plane. Some rocks break adjacent to the fault, but most slip occurs on the actual fault surface, which is weaker than intact rock.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO EARTHQUAKE RUPTURES GROW? As the edge of the rupture migrates outward, it may eventually reach Earth’s surface, causing a break called a fault scarp. The rupture migrates in both directions, but it may expand farther in one direction than in the other.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO EARTHQUAKE RUPTURES GROW? The rupture continues to grow along the fault plane and the fault scarp lengthens. The faulting relieves some of the stress, and rupturing will stop when the remaining stress can no longer overcome friction along the fault surface. At that point, the earthquake stops.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING EARTHQUAKE RUPTURE IN THE FIELD In this photo, the scarp is cutting through granite. The fault had mostly strike-slip movement, with some vertical movement. The fault is part of a zone of strike-slip faults that are related to the San Andreas transform boundary, but farther into the continent. The zone is called the East California Shear Zone and poses a significant risk.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING EARTHQUAKE RUPTURE IN THE FIELD Movement along a normal fault ruptured the land surface during the 1983 Borah Peak earthquake, forming a fault scarp along the mountain front.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING EARTHQUAKE RUPTURE IN THE FIELD The 1959 Hebgen Lake earthquake in southern Montana just outside Yellowstone National Park formed a several-meter-high fault scarp. The earthquake and fault scarp were generated by slip along a normal fault.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING EARTHQUAKE RUPTURE IN THE FIELD Scarps are most obvious soon after they form, but they become more obscure over time. Erosion rounds off the top edge of the scarp, and sediment accumulates at the base of the scarp, producing a rounded step in the topography.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING BUILDUP AND RELEASE OF STRESS The figure below shows a conceptual model of how the amount of stress changes over time.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING BUILDUP AND RELEASE OF STRESS When the amount of stress equals the strength of the fault, the fault slips, and the stress immediately decreases to the original level.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING BUILDUP AND RELEASE OF STRESS It increases gradually (sloping line), and then decreases abruptly (vertical line) when an earthquake occurs.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING BUILDUP AND RELEASE OF STRESS It increases gradually (sloping line), and then decreases abruptly (vertical line) when an earthquake occurs. This process is called the earthquake cycle.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING BUILDUP AND RELEASE OF STRESS The average time between repeating earthquakes is called the recurrence interval.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHERE DO MOST EARTHQUAKES OCCUR? Where do earthquakes occur, and how can we explain the distribution of earthquakes across the planet?  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHERE DO MOST EARTHQUAKES OCCUR? Most earthquakes occur along plate boundaries, and maps of earthquake locations outline Earth’s main tectonic plates. There are some regions, however, where seismicity (earthquake activity) is more widespread, extending far away from plate boundaries and into the middle of continents.  CE021 EARTHQUAKE ENGINEERING Where Do Earthquakes Occur in the Eastern Hemisphere?  CE021 EARTHQUAKE ENGINEERING Where Do Earthquakes Occur in the Western Hemisphere and Atlantic Ocean?  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES EARTHQUAKES ALONG PLATE BOUNDARIES AND WITHIN PLATES? Seafloor spreading forms new oceanic lithosphere that is very hot and thin. Stress levels increase downward in Earth, but in mid-ocean ridges the rocks in the lithosphere get very hot at a shallow depth, too hot to fracture (they flow instead). As a result, earthquakes along mid-ocean ridges are relatively small and shallow, with hypocenters less than about 20 km (12 mi) deep.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES EARTHQUAKES ALONG PLATE BOUNDARIES AND WITHIN PLATES? Many earthquakes occur along the axis of the mid-ocean ridge, where spreading and slip along normal faults down drop blocks along the narrow rift. Numerous small earthquakes also occur when magma intrudes into fissures.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES EARTHQUAKES ALONG PLATE BOUNDARIES AND WITHIN PLATES? As the newly created plate moves away from the ridge, it cools, subsides, and bends. The stress caused by the bending forms steep faults, which are associated with relatively small earthquakes.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAUSES EARTHQUAKES ALONG PLATE BOUNDARIES AND WITHIN PLATES? Strike-slip earthquakes occur along transform faults that link adjacent segments of the spreading center. Largely because of the typically thin lithosphere, earthquakes along these oceanic transform faults are small and shallow.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? A subduction zone, where an oceanic plate underthrusts beneath another oceanic plate or a continental plate, undergoes compression and shearing along the plate boundary. It can produce very large earthquakes.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? As the oceanic plate moves toward the trench, it is bent and stressed, causing earthquakes in front of the trench.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? Larger earthquakes occur in the accretionary prism as material is scraped off the down going plate. Shearing within the prism causes the rocks to slip and produce earthquakes along numerous thrust faults.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? Large earthquakes occur along the entire contact between the subducting plate and the overriding plate. The plate boundary is a huge thrust fault called a megathrust.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? Earthquakes along megathrusts are among the most damaging and deadly of all earthquakes. During large earthquakes, the megathrust can rupture upward all the way to the seafloor, displacing the seafloor and unleashing destructive waves in a tsunami.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? Earthquakes can also occur within the overriding plate due to movement of magma and from volcanic eruptions. Compressive stresses associated with plate convergence can cause thrust faulting behind the magmatic arc.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? The downgoing oceanic plate is relatively cold, and so it continues to produce earthquakes from shearing along the boundary, from downward-pulling forces on the sinking slab, and from abrupt changes in mineralogy.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO SUBDUCTION ZONES? Subduction zones are typically the only place in the world producing deep earthquakes, as deep as 700 km (430 mi). Below 700 km, the plate is too hot to behave brittlely or to cause earthquakes.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO CONTINENTAL COLLISIONS? During continental collisions, one continental plate underthrusts beneath another. Collisions can be extremely complex, as different parts collide at different times and rates.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO CONTINENTAL COLLISIONS? Large thrust faults form near the plate boundary in both the overriding plate and underthrusting plate (not shown), causing large but shallow earthquakes.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO CONTINENTAL COLLISIONS? Large, deadly earthquakes are produced along the plate boundary, or megathrust, between the two continental plates.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO CONTINENTAL COLLISIONS? Thrust faults also form within both continental plates, causing moderately large earthquakes. The immense stresses associated with a collision can reactivate older faults within the interior of either continent, as is now occurring in Tibet and China.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO CONTINENTAL COLLISIONS? Strike-slip faults and normal faults may be generated as entire regions are stressed by the collision zone or are shoved or sheared out of the way.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES RELATED TO CONTINENTAL COLLISIONS? Any oceanic plate material that was subducted prior to the collision is detached, so actual subduction and associated earthquakes stop. A few deep earthquakes have resulted from the sinking motion of such detached slabs.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES GENERATED WITHIN CONTINENTS? In addition to continental collisions, earthquakes occur in other tectonic settings within continents. These settings include: 1. continental rifts, 2. continental strike-slip faults, 3. magmatic areas, and 4. Reactivated preexisting faults.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES GENERATED WITHIN CONTINENTS? Continental rifts generally produce normal faults, whether the rift is a plate boundary or is within a continental plate. The normal faults downdrop fault blocks into the rift, causing normal-fault earthquakes. Such earthquakes are typically moderate in size.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES GENERATED WITHIN CONTINENTS? A transform fault can cut through a continent, moving one piece of crust past another. The strike-slip motion causes earthquakes that are mostly shallower than 20 to 30 km (10 to 20 mi), but some of these strike- slip earthquakes can be quite large.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES GENERATED WITHIN CONTINENTS? Intrusion of magma (shown here in red) within a plate can cause small earthquakes as the magma moves and creates openings in the rock. Moving magma can produce distinctive earthquakes, which are unlike those produced by movement along faults.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE EARTHQUAKES GENERATED WITHIN CONTINENTS? Preexisting faults in the crust can readjust and move as the continental plate becomes older and is subjected to new stresses, such as from distant plate boundaries. Reactivation of these structures can occur in the interior of a plate and produce large earthquakes  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO EARTHQUAKE WAVES TRAVEL? EARTHQUAKES GENERATE VIBRATIONS that travel through rocks as seismic waves. The word seismic comes from the Greek word for earthquake. Scientists who study earthquakes are seismologists. Geophysical instruments record and process information on seismic waves, and these data allow seismologists and geologists to understand where and how earthquakes occur.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? Earthquakes generate several different types of seismic waves. Seismologists study body waves, which are waves that travel inside Earth, and surface waves, which travel on the surface of Earth.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? BODY WAVES There are two main types of body waves, P-waves and S-waves, which propagate in different ways.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? P-WAVES One type of body wave is called a primary wave, or simply a P-wave. It is like a sound wave that compresses the air through which it travels. The P-wave is the fastest seismic wave, traveling through rocks at 6 to 14 km/s depending on the properties of the rock. For comparison, sound waves in air travel at an average of 0.3 km/s; P-waves are more than 20 times faster.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? P-WAVES P-waves can travel through solids and liquids because these materials can be compressed and then released.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? S-WAVES Secondary waves, also called S-waves, shear the rock side to side or up and down. This movement is perpendicular to the direction of travel. S-waves are slower (3.5 km/s) than P-waves and cannot travel through liquids, such as magma. When S-waves fail to pass through some part of the Earth, like the outer core, we infer that this region is mostly molten, rather than a solid.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? SURFACE WAVES There are two main kinds of surface waves: Rayleigh waves and Love waves. Surface waves cause the damage during an earthquake.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? RAYLEIGH WAVES One type of surface wave is a Rayleigh wave, also called a vertical surface wave because it displaces the surface in a vertical (up and down) direction. A Rayleigh wave is similar to an ocean wave, in that material moves up and down in an elliptical path.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT KINDS OF SEISMIC WAVES DO EARTHQUAKES GENERATE? LOVE WAVES The second type of surface wave is a Love wave, which is also described as a horizontal surface wave for the way material vibrates horizontally and shuffles side to side. Like an S-wave, the motion of material in a Love wave is perpendicular to the direction in which the wave travels.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW SEISMIC WAVES ARE RECORDED? Sensitive digital instruments called seismometers are able to precisely detect a wide range of earthquakes. The recorded seismic data are uploaded to computers that process signals from hundreds of instruments registering the same earthquake. These computers calculate the location of the hypocenter and the magnitude or strength of the earthquake.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW SEISMIC WAVES ARE RECORDED? The mass hangs from a frame that in turn is attached to the ground. When the ground shakes, the frame shakes too, but the suspended mass resists moving because of inertia.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW SEISMIC WAVES ARE RECORDED? This device only records ground movement parallel to the red arrows, so it only records a single direction or single component of motion.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW SEISMIC WAVES ARE RECORDED? A modern seismic detector, called a seismograph, contains three seismometers oriented 90° from each other to record three components of motion (north-south, east- west, and up-down). From these three components, seismologists can determine the source and strength of the seismic signal.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW SEISMIC WAVES ARE RECORDED? Seismologists place seismographs away from human noise and vibration and bury them to reduce wind noise. Seismic waves (in yellow) can come from any direction and still be recorded.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC RECORDS VIEWED? Prior to the 1990s, seismic waveforms were mostly represented as curves on a paper seismogram, which is a graphic plot of the waves recorded by a seismometer.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC RECORDS VIEWED? SEISMOGRAM It plots vibrations versus time. On seismometers, time is marked at regular intervals so that we can determine the time of the arrival of the first P- and S-waves.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC RECORDS VIEWED? Background vibrations, unrelated to the earthquake, commonly look like small, somewhat random squiggles on seismograms.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC RECORDS VIEWED? After an earthquake, P-waves arrive first, marked by the larger squiggles. From other information, we know that this earthquake occurred at 8:00 a.m. The time of the P- wave’s arrival was 2.5 minutes later in this example.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC RECORDS VIEWED? The S-wave arrives later. The delay between the P-wave and the S-wave depends primarily on how far away the earthquake occurred. The longer the distance from the earthquake, the greater the delay.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC RECORDS VIEWED? Surface waves arrive last and cause intense ground shaking, as recorded by the higher amplitude squiggles on the seismogram.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING AMPLITUDE AND PERIOD Seismic waves are characterized by how much the ground moves (wave amplitude) and the time it takes for a complete wave to pass ( period).  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING AMPLITUDE AND PERIOD Period is related to the wavelength and velocity of the wave. Both amplitude and wavelength can be measured from a seismogram. Amplitude is critical when estimating the strength and damage potential of an earthquake.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING AMPLITUDE AND PERIOD The period can also be a critical component in assessing potential damage, because buildings vibrate when shaken by earthquakes. Every building has a natural period that can match, or resonate with, the earthquake wave. Resonance can cause intensified shaking and increased damage.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE DETERMINE THE LOCATION AND SIZE OF AN EARTHQUAKE EARTHQUAKES OCCUR DAILY AROUND THE WORLD, and a network of seismic instruments records these events. Using the combined seismic data from several instruments, seismologists calculate where an earthquake occurred and its magnitude (how large it was). This can be done automatically using computers.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? Seismologists maintain thousands of seismic stations that sense and record ground motions. When an earthquake occurs, parts of this network can record it. Large earthquakes generate seismic waves that can be detected around the world. Smaller earthquakes are detected only locally.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? Seismometers in the U.S. National Seismic Network represent a fraction of all seismometers.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? On October 1, 2005, a moderate earthquake is felt in Colorado. Three seismic stations (labeled DUG, WUAZ, and ISCO) record wave arrivals and are chosen to locate the epicenter.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? Records from at least three stations are compared when calculating an earthquake location. Ordinarily, records from many stations are used in an automated computer- based process.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? P-waves travel faster than S- waves, and so they reach a seismic station some time before the S-wave arrives. The time interval between the arrival of the P-wave and S- wave is called the P-S interval. The farther a station is from the earthquake, the longer the P-S interval will be.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? The three seismograms show differences in the P-S interval. Based on the P-S intervals, ISCO, which has the shortest P-S interval, is the closest station to the earthquake, followed by DUG and WUAZ.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? The P-S interval is proportional to the distance from the epicenter to the seismic station, although slightly affected by the types of materials through which the waves pass. This relationship is shown on a graph as a time-travel plot.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? P-S intervals are measured from the seismograms shown in part 2 and then plotted on the graph. This gives the distance from each station to the earthquake’s epicenter.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? From the graph, the distance from each station to the epicenter is now known, but not the direction.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? A circle is drawn around each station, with a radius equal to the distance calculated from plotting the P-S interval on the graph.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? The intersection of three circles is the epicenter of the earthquake. If more circles were plotted, they should intersect at the same point.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE LOCATE EARTHQUAKE? We calculate the depth of the earthquake’s hypocenter in a similar way, using the interval between the P-wave and another compressional wave that forms when the P-wave reflects off Earth’s surface near the epicenter.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE MEASURE THE SIZE OF AN EARTHQUAKE? The magnitude of an earthquake is a measure of the released energy and is used to compare the sizes of earthquakes. There are several ways to calculate magnitude, depending on the earthquake’s depth. The most commonly mentioned scale, called the “Richter” or “Local” magnitude (Ml) scale.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE MEASURE THE SIZE OF AN EARTHQUAKE? MEASURING AMPLITUDE Seismometers are calibrated so that the measurements made by two different instruments are comparable. The maximum height (amplitude) of the S-wave is measured on the seismogram. It is proportional to the earthquake energy. This measure is used for shallow earthquakes.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE MEASURE THE SIZE OF AN EARTHQUAKE? MAGNITUDE The amplitude of S-waves decreases as a wave propagates. We plot the relationship between distance and S-wave amplitude on a graph called a nomograph.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE MEASURE THE SIZE OF AN EARTHQUAKE? MAGNITUDE For each seismic station, we draw a line connecting the distance and amplitude of the S-wave.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO WE MEASURE THE SIZE OF AN EARTHQUAKE? MAGNITUDE The earthquake’s magnitude is where each line crosses the center column. These three lines for the 2005 Colorado earthquake all agree, and they yield a 4.1 local magnitude (Ml). Magnitude is a logarithmic scale, so a one-unit increase in magnitude represents tenfold increase in ground motion.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAN THE INTENSITY OF GROUND SHAKING TELL US ABOUT AN EARTHQUAKE? Some of the most damaging earthquakes occurred before seismometers were invented. For such events, we rely on reports of damage and shaking intensity as a way to classify the relative sizes of earthquakes.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAN THE INTENSITY OF GROUND SHAKING TELL US ABOUT AN EARTHQUAKE? The Modified Mercalli Intensity Scale, abbreviated as MMI, describes the effects of shaking in everyday terms. A value of “I” or “II” reflects a barely felt earthquake. A value of “XII” indicates complete destruction of buildings, with visible surface waves throwing objects into the air and destroying structures.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT CAN THE INTENSITY OF GROUND SHAKING TELL US ABOUT AN EARTHQUAKE?  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING ENERGY OF EARTHQUAKES The Richter magnitude describes the amount of ground motion, but the scale is logarithmic. The ground motion increases by a factor of 10 from a magnitude 4 to a 5, from a 5 to a 6, and so on.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING ENERGY OF EARTHQUAKES Another common measure of earthquake energy is moment magnitude or Mw, which is calculated from the amount of slip (displacement) on the fault and the size of fault area that slipped. Moment magnitude is applicable for both large and small earthquakes and is widely used by scientists.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO EARTHQUAKES CAUSE DAMAGES? “earthquakes don’t kill people, buildings or tsunamis do”  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT WERE SOME MAJOR NORTH AMERICAN EARTHQUAKES? ALASKA, 1964 A magnitude 9.2 (Mw) earthquake, one of the three or four largest earthquakes ever recorded, struck southern Alaska in 1964. It killed 128 people, triggered landslides, and collapsed parts of downtown Anchorage and nearby neighborhoods.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT WERE SOME MAJOR NORTH AMERICAN EARTHQUAKES? SAN FRANCISCO, 1906 A huge earthquake occurred when the San Andreas fault ruptured near San Francisco. The earthquake was likely a magnitude 7.7 to 7.8 (Mw) although not directly measured on seismometers. The earthquake ruptured the surface, leaving behind a series of cracks and open fissures. More than 3,000 people were killed and much of the city was devastated by fires that broke out after the earthquake.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT WERE SOME MAJOR NORTH AMERICAN EARTHQUAKES? NORTHRIDGE, LOS ANGELES AREA, 1994 This magnitude 6.7 (Mw) earthquake was generated by a thrust fault northwest of Los Angeles. The earthquake killed 60 people and caused $20 billion in damage. A section of freeway buckled, crushing the steel-reinforced concrete slabs.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT WERE SOME MAJOR NORTH AMERICAN EARTHQUAKES? MEXICO CITY, 1985 A magnitude 8.0 (Mw) earthquake occurred at a subduction zone along the southwestern coast of Mexico, well west of Mexico City. It damaged or destroyed many buildings in Mexico City and killed at least 9,500 people. Destruction was extensive in part because Mexico City is built on lake sediments deposited in a bowl-shaped basin.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT WERE SOME MAJOR NORTH AMERICAN EARTHQUAKES? HEBGEN LAKE, YELLOWSTONE AREA, 1959 This magnitude 7.3 (Mw) event was generated by slip along a normal fault northwest of Yellowstone National Park. Ground shaking set loose the massive Madison Canyon slide, which buried 28 campers and formed a new lake, aptly named Earthquake Lake.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT WERE SOME MAJOR NORTH AMERICAN EARTHQUAKES? NEW MADRID, 1811-1812 New Madrid, Missouri, experienced a series of large (Mw 7.8–8.1) earthquakes generated over an ancient fault zone in the crust. The 1811–1812 earthquake death toll was relatively low because of the sparse population at the time. The New Madrid zone has a high earthquake risk and, as shown on the earthquake-hazard map below, is one of two areas in the eastern United States that are predicted to experience strong earthquakes in the future.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING WHAT WERE SOME MAJOR NORTH AMERICAN EARTHQUAKES? CHARLSTONE, 1886 This earthquake occurred at the highest risk area along the East Coast, near Charleston, South Carolina. It had an estimated magnitude of 7.3 (Mw), the largest ever recorded in the southeastern United States. Buildings incurred some damage, and 60 people died. The tectonic cause for this earthquake is still debated by geologists.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES TRAVEL THROUGH MATERIALS? An earthquake or other source of seismic energy generates seismic waves, which radiate out from the source in all directions. The path that any part of the wave travels is a seismic ray. If the physical properties of the material do not change from place to place, then a seismic ray travels in a straight line. In this case, a family of straight rays diverges outward from the source.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES TRAVEL THROUGH MATERIALS? Most seismic waves encounter boundaries between materials with different physical properties, causing the waves to reflect, speed up, or slow down. Some of the energy is reflected off the interface as a reflected wave. Some of the energy is bent as it crosses the boundary. This process of bending is known as refraction.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES REFRACT THROUGH DIFFERENT MATERIALS? 1. If a seismic wave passes into a material that causes it to slow down, it will be refracted away from the interface at a steeper angle.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES REFRACT THROUGH DIFFERENT MATERIALS? 2. If a descending seismic ray passes from a slow material to a faster one, it will be refracted to a shallower angle.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES REFRACT THROUGH DIFFERENT MATERIALS? 3. If a rising seismic ray passes from a fast material to a slower one, it will be refracted upward toward the surface.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES TRAVEL THROUGH EARTH’S CRUST AND MANTLE? 1. Refraction causes seismic waves to take curved paths through the Earth. Because rocks get denser deeper in Earth, steeply descending rays will first be refracted to shallower angles as they encounter faster and faster material at depth. Subsequently, the waves will then be bent back toward the surface as they pass back through slower, less dense material.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES TRAVEL THROUGH EARTH’S CRUST AND MANTLE? 2. In the figures below, an earthquake sends seismic waves into the crust and mantle. Both waves are refracted back toward the surface. Waves in the mantle travel faster than those in the crust, resulting in an interesting and useful phenomenon.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES TRAVEL THROUGH EARTH’S CRUST AND MANTLE? 3. Close to the earthquake, waves that travel through the crust arrive sooner than those from the mantle because the crustal waves travel a shorter distance.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES TRAVEL THROUGH EARTH’S CRUST AND MANTLE? 4. Farther from the earthquake, waves that travel through the mantle arrive at the surface first because the faster velocity lets them overtake the crustal waves.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW DO SEISMIC WAVES TRAVEL THROUGH EARTH’S CRUST AND MANTLE? 5. Seismologists observe at what distance from the hypocenter the mantle waves begin to arrive first. They then use simple computer models of velocities, crustal thicknesses, and ray paths to calculate the depth to the crust-mantle boundary.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC WAVES USED TO EXAMINE THE EARTH’S DEEP INTERIOR? Seismologists recognize distinct boundaries within Earth, largely based on changes in seismic velocities and other behaviors. Such changes reflect the physical and chemical properties of the rock layers through which the seismic waves pass. Not all seismic waves make it through every part of Earth. Observing where particular kinds of waves are blocked helps determine which parts of Earth are molten.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC WAVES USED TO EXAMINE THE EARTH’S DEEP INTERIOR? As P-waves travel through Earth, they speed up and slow down as they pass through different kinds of material. Their velocity depends upon three factors: 1. how easily the rocks are compressed; 2. how rigid the material is; and 3. the density of the material. “faster velocities indicate denser rocks”  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC WAVES USED TO EXAMINE THE EARTH’S DEEP INTERIOR? As P-waves and S-waves travel through Earth, they are refracted and follow curved paths that return them to the surface.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC WAVES USED TO EXAMINE THE EARTH’S DEEP INTERIOR? Along the core-mantle boundary, some P-waves are refracted inward because the outer core has a slower velocity than the adjacent mantle. These P- waves pass through the core and out toward the other side of Earth.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC WAVES USED TO EXAMINE THE EARTH’S DEEP INTERIOR? There is a zone, called the P-wave shadow zone, that receives no direct P-waves. This is because the P-waves are either refracted upward before they reach this area or are refracted inward through the core.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC WAVES USED TO EXAMINE THE EARTH’S DEEP INTERIOR? On the opposite side of Earth from the seismic source, there is also an S-wave shadow zone that receives no direct S-waves. This implies that S- waves cannot pass through the core.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING HOW ARE SEISMIC WAVES USED TO EXAMINE THE EARTH’S DEEP INTERIOR? Seismologists also learn about Earth’s interior by studying indirect waves. These are waves that have reflected off boundaries or have changed wave type as they crossed a boundary (e.g., mantle to core).  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE MOHO The boundary between the crust and mantle is named the Mohorovicic Discontinuity after the last name of the Croatian seismologist who discovered it. Most geoscientists simply call it the Moho.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE MOHO The boundary between the crust and mantle is named the Mohorovicic Discontinuity after the last name of the Croatian seismologist who discovered it. Most geoscientists simply call it the Moho.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE NEED FOR A PRELIMINARY GEOHAZARD ASSESSMENT The Philippine government proceeded to issue DENR AO2000-28 as its long-term response to the urgent need of protecting lives and property from destruction brought about by such geologic hazards. The Engineering Geology and GeoHazard Assessment (EGGA) process requires a land development project proponent to request the appropriate MGB office for a site geological scoping survey.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE NEED FOR A PRELIMINARY GEOHAZARD ASSESSMENT CONDUCTED BY: Practicing Geologist Qualified Engineer EGGAR  Technical review by an MGB panel  Revisions  Endorsement to EMB  Required for Issuance of ECC  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING NATURAL AND MAN-MADE INFLUENCES EARTHQUAKE natural phenomena, but which can be triggered by large dam construction; FLOODING a natural event but which can be caused or exacerbated by man, as a result of deforestation, building on floodplains and so on. CONTAMINATED LAND man-made but can be from naturally occurring substances such as arsenic, methane gas or radon gas.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING SEISMICITY In case of such a major earthquake structures, slopes and foundations will be subjected to seismic loading. Earthquakes can trigger other seismic hazards such as landslides, liquefaction, lateral spreading, differential settlement, tsunamis, seiches and even fires.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING SEISMICITY A Collapsed Building during the July 16, 1990 Northern Luzon Earthquake is shown in the figure. The Philippine Mobile Belt is sandwiched by trenches on both sides and traversed along its entire length by the Philippine Fault. Palawan and Zamboanga are in the Eurasian margin.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING NEARFAULT AND FARFAULT NEARFAULT A fault within the 5 km distance is a nearfault. FARFAULT A fault beyond that distance is a far-fault, located in the far field.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING ACTIVE FAULT An active fault is one that has moved during the last 10,000 years. Faulting, whether through a seismic fault creep or through a catastrophic ground rupture, refers to actual displacement or dislocation along a fault.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING ACTIVE FAULT The Philippine Mobile Belt is therefore tectonically, seismically and volcanically active. Palawan and Zamboanga, on the other hand, are part of the Eurasian Margin and are therefore tectonically and seismically inactive. There are no earthquake generators within the margin, although it can experience earthquakes generated by bounding structures such as the Sindangan Fault or Cotabato Trench.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING ACTIVE FAULT The Philippine Mobile Belt is therefore tectonically, seismically and volcanically active. Palawan and Zamboanga, on the other hand, are part of the Eurasian Margin and are therefore tectonically and seismically inactive. There are no earthquake generators within the margin, although it can experience earthquakes generated by bounding structures such as the Sindangan Fault or Cotabato Trench.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE PRELIMINARY GEOHAZARD ASESSMENT For seismic design of vertical buildings, ASEP National Structural Code of the Philippines (2010) requires input of: 1. Distance from active fault 2. Soil type  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE PRELIMINARY GEOHAZARD ASESSMENT For seismic design of vertical buildings, ASEP National Structural Code of the Philippines (2010) requires input of: 1. Distance from active fault 2. Soil type  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE PRELIMINARY GEOHAZARD ASESSMENT For bridges, requirements of DWPH Bridges Seismic Design Specifications (DPWH-BSDS): 1. December 2013 JICA Study and the DGCS Volume 5 – Bridge Design.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE PRELIMINARY GEOHAZARD ASESSMENT For earth retaining structures and earthworks: 1. DGCS V olume 4 – Highway Design recommends designing using the quasi-static method, which will require a peak ground acceleration and then a reduction factor. 2. At this stage the requirements are to identify the distance from active fault and peak ground acceleration at the site.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE PRELIMINARY GEOHAZARD ASESSMENT The peak ground acceleration can also be obtained from the PHIVOLCS web site maps, and an example is shown in Figure. An estimate of the ground motion specific to a site can be calculated. In order to determine the peak ground acceleration that a site can experience in the case of a major earthquake, the attenuation model of Fukushima and Tanaka (1990) is applied.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING THE PRELIMINARY GEOHAZARD ASESSMENT Correction factors are applied to the mean peak acceleration depending on the type of foundation material: rock =0.6 hard soil = 0.87 medium soil = 1.07 soft soil = 1.39.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING TYPES OF EARTHQUAKE HAZARDS GROUND RUPTURE Deformation on the ground that marks, the intersection of the fault with the earth’s surface. It has the following effects: fissuring and displacement of the ground due to movement of the fault.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING TYPES OF EARTHQUAKE HAZARDS GROUND SHAKING Disruptive up, down and sideways vibration of the ground during an earthquake. It has the following effects: ground shaking may cause damage or collapse of structure; may consequently cause hazards such as liquefaction and landslide.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING TYPES OF EARTHQUAKE HAZARDS LIQUEFACTION Phenomenon wherein sediments, especially near bodies of water, behave like liquid similar to a quicksand. It has the following effects: sinking and/ or tilting of structure above it; sandboil; fissuring.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING TYPES OF EARTHQUAKE HAZARDS EARTHQUAKE-INDUCED LANDSLIDE Down slope movement of rocks, solid and other debris commonly triggered by strong shaking. It has the following effects: erosion; burial and blockage of roads and rivers.  CE021 EARTHQUAKE ENGINEERING CE021 EARTHQUAKE ENGINEERING TYPES OF EARTHQUAKE HAZARDS TSUNAMI Series of waves caused commonly by an earthquake under the sea. It has the following effects: flooding; coastal erosion; drowning of people and damage to properties.