Izmit earthquake Faults focus Seismic waves earthquake effects

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1 Earthquakes Introduction Faults & Earthquakes Seismic Waves Effects of Earthquakes Measurement of Earthquakes Distribution of Earthquakes Earthquake Prediction Summary Diseased nature oftentimes breaks forth In strange eruptions; oft the teeming earth Is with a kind of colic pinch'd and vex'd By the imprisoning of unruly wind Within her womb; which, for enlargement striving, Shakes the old beldame earth and topples down Steeples and moss-grown towers. William Shakespeare

2 Introduction Earthquakes represent the vibration of Earth because of movements on faults. The focus is the point on the fault surface where motion begins. The epicenter is the point on Earth's surface directly above the focus. The deadly Izmit earthquake struck northwest Turkey on August 17, 1999, at 3 a.m. Over 14,000 residents of the region were killed as poorly constructed apartment complexes pancaked to the ground, each floor collapsing on the one below (Fig. 1). The death toll from this single event was greater than the average annual loss of life from all earthquakes worldwide. An earthquake occurs when Earth s surface shakes because of the release of seismic energy following the rapid movement of large blocks of the crust along a fault. Faults are breaks in the crust that may be hundreds of kilometers long and extend downward 10 to 20 km (6-12 miles) into the crust. The 1,200 km (750 miles) long San Andreas Fault that separates the North American and Pacific Plates in California is the most active fault system in the contiguous U.S. The Izmit earthquake occurred on the North Anatolian Fault, a fault that is of similar length and sense of movement as the San Andreas Fault. Unraveling the movement history of large faults that produce devastating but infrequent earthquakes can help predict the potential threat from similar faults elsewhere. The point on the fault surface where movement begins, the earthquake source, is termed the focus. Seismic waves radiate outward from the focus. Earthquake foci (plural of focus) occur at a range of depths. The majority of earthquakes occur at shallow depths that range from the surface down to 70 km (44 miles). Less frequent intermediate ( km; miles) and deep ( km; miles) earthquakes are generally associated with subduction zones where plates descend into the mantle. Damage is greatest from shallow earthquakes because the seismic waves travel a shorter distance before reaching the surface. The earthquake effects, the type of damage associated with earthquakes, include changes in the natural environment such as landslides but most attention is focused on Figure 1. Collapsed structures destroyed by the shallow 1999 Izmit, Turkey (top), and 1994 Northridge, California (bottom), earthquakes. Images courtesy of USGS Expedition to Turkey and USGS Open-File Report (Northridge). 2

3 the impact on constructed structures. Building codes are in place in most earthquake-prone areas but they are of little use if enforcement is lax, as was the case in Turkey. Following the earthquake it was discovered that some contractors had cut corners in the construction of multistory apartment complexes. The poorly built structures were left as piles of rubble amongst other apartments that remained standing. Figure 2. A smashed car buried under fallen bricks resulting from the 2001 Nisqually earthquake near Seattle, Washington. The Northridge (see Fig. 1) and Nisqually earthquakes were of similar magnitudes but the latter occurred further below the surface, reducing the scale of the damages resulting from the shaking. Image courtesy of FEMA News Photos. Figure 3. The focus is the source of the earthquake and the epicenter is the point on the surface directly above the focus. In contrast, on February 28, 2001, the strong Nisqually earthquake (Fig. 2) occurred below western Washington 56 km (35 miles) south of Seattle. Buildings in Seattle and the surrounding communities sustained relatively little structural damage, no one was killed, and only a handful of people received anything more than minor injuries. Seattle has enforced a stringent building code over the last 30 years that requires new structures to be able to withstand large earthquakes. In addition, over the last decade, many older buildings and bridges were retrofitted to ensure that they could endure the big earthquake predicted for the region. Residents in western Washington were doubly fortunate, not only did they have well-built structures but the earthquake occurred much further below the surface than the Izmit quake, further reducing the resulting ground shaking. Seismic waves are captured by a recorder known as a seismograph. The relative arrival times of different types of seismic waves is used to determine the distance of the seismograph station from the origin of the earthquake. Three or more records can be used to pinpoint the earthquake's epicenter, the geographic location of the point on the earth s surface directly above the focus (Fig. 3). Earthquakes are named for the epicenter location, for example, the Nisqually earthquake occurred 53 km (33 miles) below the mouth of the Nisqually River in western Washington. Loss of life in the Turkish earthquake was greatest in the city of Izmit, located close to the earthquake's epicenter. Earthquake distribution is far from random. Earthquakes occur on faults that are preferentially located along plate boundaries. The largest earthquakes along convergent plate boundaries. One method of earthquake measurement is to determine the level of destruction following an earthquake. However, as the Turkish earthquake so vividly illustrates, the degree of damage is often related more to human activity than the earthquake itself. Consequently, more quantitative measures have been adopted that measure the magnitude and timing of the vibration 3

4 of sensitive instruments (seismographs) and the distance from the earthquake to calculate a value for the event. This information is often combined with data on the geology surrounding the fault to generate an even more accurate measure. Over two million people were killed during the previous century by earthquakes and associated phenomena. The threat of future earthquakes in heavily populated regions like California (and Turkey) has spurred efforts in earthquake prediction. Analysis of past earthquakes allows the determination of the potential size and location of future events, the problem is determining when such events will occur with any degree of accuracy. The principal difficulty is that large, dangerous earthquakes occur at intervals measured in decades or centuries yet our record of past earthquake activity stretches back only a few hundred years, making it inadequate for rigorous predictions. Earthquakes are expensive. Even the relatively minor damages from the Nisqually earthquake cost $2 billion to repair and the substantial damages resulting from the 1994 Northridge quake have been estimated to cost $30 billion, making it one of the costliest disasters in U.S. history. People living in areas with the potential for damaging earthquakes may seek to purchase earthquake insurance to provide some financial protection for their property. Approximately 40% of residents in Northridge had insurance and their insurance companies endured significant losses in paying an estimated $15 billion in claims. The Northridge event caused insurers to drastically rethink earthquake coverage. Deductibles rose to 10 to 15% and policies now exclude loss of building contents and reductions in other costs. These less generous insurance policies would have trimmed claims from the Northridge earthquake to about $4 billion. Estimated annual cost of earthquake insurance for a $100,000 home in California: $280 Think about it... How frequently do earthquakes occur near where you live? Where do earthquakes occur in your state? Go to the USGS National Earthquake Information Center s website ( and answer these basic questions. 4

5 Faults & Earthquakes Earthquakes represent the vibration of Earth because of movements on faults. Faults can be identified by the offset of rock layers on either side of the fault surface. Normal and reverse faults are types of dip-slip faults. Left-slip and right-slip faults are types of strike-slip faults. An earthquake occurs when Earth shakes because of the release of seismic energy following the rapid movement of large blocks of the crust along a fault. A fault is a fracture in the crust. During the Izmit earthquake, the crust broke along the North Anatolian Fault in northern Turkey (Fig. 4). When fault movement occurs it may be slow and gradual and generate only small earthquakes, or it may be rapid and catastrophic causing widespread destruction. Ground shaking associated with most earthquakes is over in a matter of seconds but it involves such large regions of Earth s crust that tremendous amounts of energy are released. The ground shook for 45 seconds in the Izmit earthquake and affected a region of approximately 100,000 square kilometers. Faults may be hundreds of kilometers in length but only part of longer faults typically break during an earthquake (Fig. 4). Fault segments that have not experienced a recent earthquake are termed seismic gaps and are considered potential sites for future events. The Izmit earthquake occurred in a 150 km (94 mile) long gap at the western end of the North Anatolian Fault. Figure 4. Earthquake sequence along the North Anatolian fault, Turkey, A series of large earthquakes have occurred on the fault system; each resulting from only one segment of the fault breaking at a time. Image courtesy of USGS. 5

6 Two adjacent fault segments to the east and west broke during large earthquakes in 1963 and Eleven earthquakes of magnitude 6.7 or greater occurred along segments of the fault over the previous 60 years. Even though scientists can identify potential seismic gaps, the faults may not cooperate to generate an earthquake. Earthquake specialists predicted an earthquake would strike the region around the small California town of Parkfield before the end of 1993 but the quake still hasn't shown up, despite the fact that there are millions of dollars worth of instruments in the ground waiting for the big day to arrive. Even the largest earthquakes require relatively small fault movements because such large volumes of rock are involved. Offsets on faults for the largest of earthquakes are less than 10 meters, and typically less than 5 meters per quake. The accumulated movement from hundreds of thousands of earthquakes over millions of years results in the formation of mountains in association with plate boundaries. Fault Classification Faults are distinguished as dip-slip or strike-slip faults. Two types of dip-slip faults are identified on the basis of the relative motion of rocks across an inclined fault surface (Fig.5). The block of rocks above a fault is termed the hanging wall; the footwall lies below the fault. Miners working in shafts that crossed faults were able to hang their lanterns from the hanging wall while their feet remained below the fault in the footwall. Inclined faults can be identified by the offset of rock layers on either side of the fault surface. The hanging wall moves down relative to the footwall in a normal fault. In contrast, the hanging wall moves up relative to the footwall in a reverse fault. Movement on a dip-slip fault often results in a break or Figure 5. Top: The hanging wall (hw) lies above an inclined fault; the footwall (fw) lies below the fault. Bottom: Normal fault. Figure 6. Fault scarp formed during the Hebgen Lake earthquake, Montana, Person in foreground is approximately 2 meters tall. Surface at bottom of slope was at same elevation as upper surface prior to movement on the normal fault. 6

7 offset at the land surface. This break in slope is known as a fault scarp (Fig. 6). Figure 7. An example of a left-slip strike-slip fault. Two types of strike-slip faults, left-slip and right-slip faults, are identified on the basis of the motion on vertical fault surfaces (Fig. 7). An observer, standing on one side of the fault, sees objects on the other side of the fault move to the right for a right-slip fault or to the left for a left-slip fault. The 1,200 km long San Andreas Fault, California, is a famous example of a right-slip fault. Areas of frequent earthquake activity are laced with faults. Maps of California show that several faults make up the San Andreas Fault system. The North Anatolian Fault that broke during the Izmit earthquake is also a right-slip fault (Fig. 8) and is of similar length as the San Andreas fault. Figure 8. Offset in a fence as a result of the Izmit earthquake. Note the relatively small movement on the fault, even though the earthquake was large. Can you classify the fault? Image courtesy of USGS Expedition to Turkey. Figure 9. Plate tectonic setting for the Izmit earthquake. The small Anatolian Plate is moving westward as it is wedged between the converging Arabian and Eurasian plates. A subduction zone in the eastern Mediterranean Sea marks the boundary with the African Plate to the south. Faults and Plate Boundaries Earthquake distribution is far from random. Earthquakes occur on active faults and active faults are preferentially located along plate boundaries (Fig. 9). Although, both dip-slip and strike-slip faults are associated with all types of plate boundary, each type of boundary is characterized by a specific fault style. Strike-slip faults are common at transform plate boundaries 7

8 where two plates move in opposite directions. Reverse faults occur most frequently at convergent boundaries where plates collide; and normal faults are most common at divergent boundaries such as oceanic ridges, where plates break apart. Think about it... Finish the partially completed concept map for faults and earthquakes found at the end of the chapter. Print the page and fill in the blanks with appropriate terms. Seismic Waves Seismic waves can be divided into surface waves that travel on Earth's surface and body waves that travel through Earth. Body waves are further divided into S waves and P waves. Seismic waves are recorded on a seismogram at a seismograph station. The distance of an earthquake epicenter from a seismograph station is determined by the difference in the arrival times of P and S waves at a seismograph station. Earthquake magnitude is calculated using the amplitude (height) of the S wave recorded on a seismogram. Seismic waves represent the energy released from the earthquake focus. There are two types of seismic waves: Surface waves travel on Earth s surface and cause much of the destruction associated with earthquakes. Undulations of the land surface during an earthquake are a representation of surface waves (Fig. 10). Surface waves may result in vertical motions (Rayleigh waves), much like waves traveling through water, or sideways motions (Love waves) with no vertical component of movement. Body waves travel through Earth s interior. These are further subdivided into P (primary) waves and S (secondary or shear) waves based upon their vibration direction and velocity. Variations in seismic wave velocity are used to infer the properties of Earth s interior. Figure 10. Rayleigh (top) and Love waves (bottom) are surface waves with contrasting motion directions generated during an earthquake. 8

9 P waves vibrate parallel to their travel direction in the same way a vibration passes along a slinky toy (Fig. 11). P waves travel at speeds of 4 to 6 km per second (2.5-4 miles per second) in the uppermost part of the crust. S waves vibrate perpendicular to their travel direction, like the wave that passes along a rope when it is given a sharp jerk (Fig. 11). S wave velocity is 3 to 4 km per second (2-2.5 miles per second) in the shallow crust. Figure 11. Analogs of P wave (left) and S wave motion (right). P waves are similar to the passage of a vibration through a slinky. The vibration occurs in the same direction as the wave travels. S wave motion is analogous to a vibration moving along a rope. The vibration occurs perpendicular to the direction in which the wave travels. The velocity of seismic waves is lower in loose, unconsolidated materials (sand, partially melted rock) and higher in solid materials (rock). Both P and S waves are generated at an earthquake focus as a result of movement on a fault. P waves will arrive at a recording station (seismograph station) first because of their greater velocity. Surface waves are the last to arrive because P and S waves travel a more direct route through the earth (Fig. 12). Figure 12. Contrasting travel paths for surface waves and body waves following an earthquake. The record of an earthquake at a seismograph station is a seismogram (Fig. 13). The principal elements of a seismogram that interest seismologists (scientists who study earthquakes) are the relative size of the recorded waves and the difference in time that the first P and S waves were recorded. The amplitude of the recorded wave is proportional to the magnitude of shaking associated with the earthquake but shaking may vary with the character of the material underlying the seismograph station. Loose, unconsolidated materials (e.g., mud, sediment) may exaggerate the shaking whereas solid bedrock may result in smaller vibrations. 9

10 Figure 13. An idealized seismogram illustrating the sequential arrival of seismic waves. Determination of the distance from an epicenter require calculating the difference in arrival time of P and S waves (~14 seconds). Earthquake magnitude is related to the amplitude of the recorded S wave. The difference in arrival time between P and S waves on a seismogram can be used to determine the distance of the station from the earthquake source and the amplitude (height) of the S wave recorded at the station can be used to determine earthquake magnitude (see Measurement of Earthquakes). The time interval between the recorded arrival of P and S waves increases the further the seismograph station is located from the epicenter. Seismologists match the time difference with standard curves (Fig. 14) to determine distance from the earthquake. Figure 14. Graph of distance from the epicenter and time for seismic waves to reach seismograph station. The time interval between the arrival of P and S waves increases with increasing distance from the epicenter. 10

11 Data at a single seismograph station are insufficient to pinpoint the earthquake epicenter because a seismogram yields only the distance from the earthquake source. The epicenter could be located anywhere along a circular arc of the calculated distance from the seismograph station. Seismologists must use data from multiple recording stations to learn the location of the event. The common intersection point for several circles plotted relative to different stations represents the point on the surface above the earthquake source (Fig. 15). Figure 15. An earthquake originating in the Pacific Northwest would be recorded at seismograph stations in Denver, Quebec, and Lima (Peru). The difference in arrival times between P and S waves would be least at Denver and greatest at Lima. Circles plotted at each station reflect the distance from the epicenter but the direction can only be determined by identifying the intersection point for three or more circles. Think about it... Try the Virtual Earthquake exercise that guides users through the determination of the location of an earthquake epicenter and earthquake magnitude using records of seismic waves recorded at three seismograph stations. Print the "Virtual Seismologist" certificate upon completion of the exercise. vcourseware4.calstatela.edu/virtualearthquake/vquakeintro.html 11

12 Effects of Earthquakes A major earthquake under a heavily populated area in the U.S. could result in thousands of deaths. Several effects of earthquakes could result in extensive damages. Ground shaking can collapse buildings. Uplift may raise or lower large areas of Earth's surface. Liquefaction in water-saturated sediment can result in the collapse and subsidence of the ground surface. Landslides are a potential hazard on steep slopes in seismic zones. Tsunamis, giant sea waves, are dangerous to coastal communities, especially around the Pacific Ocean. A magnitude 6.7 earthquake struck the Northridge suburb of Los Angeles on January 17, The earthquake resulted in the deaths of 57 people and injured over 9,000 more. There are about 150 earthquakes of this magnitude worldwide each year but this was the first time a quake of this size occurred in a heavily developed area of the U.S. An earthquake of similar size killed over 50,000 people in Iran in The Elysian Park fault was recently discovered below downtown Los Angeles and may produce substantial future earthquakes. Movement on the 55 km (35 miles) long fault could result in up to 5,000 deaths, leave 750,000 homeless, and cause $100 billion in damages in Los Angeles. A comparable earthquake in Kobe, Japan (exactly one year after the Northridge quake), killed over 6,000 people. The images in the following figures show damage from the largest recorded U.S. earthquake (Alaska, 1964) and illustrate the effects of earthquakes. All images taken from USGS National Earthquake Information Center (NEIC). Sudden changes on or near the earth s surface result from earthquakes and may include: Figure 16. Part of a railroad bridge over the Copper River was shaken loose by the 1964 Anchorage earthquake. Image courtesy of USGS. Ground Shaking: Rapid horizontal movements associated with earthquakes may shift homes off their foundations and cause tall buildings to collapse or "pancake" as floors collapse down onto one another. Shaking is exaggerated in areas where the underlying sediment is weak or saturated with water (Figs. 16, 17). 12

13 Figure 17. This map shows in color those parts of the contiguous 48 states that have a 10% chance of experiencing an earthquake strong enough to cause appreciable damage in a 50-year period. In the yellow areas, maximum ground shaking would be strong enough to damage unreinforced masonry buildings, even those built on bedrock. Darker colors are at the same risk for more intense shaking, and areas left blank would have less intense shaking. Image courtesy of USGS. Fault Rupture and Uplift: Break of the ground surface by the fault plane may form a step in the surface known as a fault scarp (Fig. 6). Large sections of Earth s surface may change elevation as a result of uplift on an earthquake fault (Fig. 18). Mountains east of Los Angeles were uplifted 0.3 meters (1 foot) by the 1994 Northridge earthquake. Liquefaction: Liquefaction occurs when water-saturated sediment is reorganized because of violent shaking. The sediment collapses, expelling the water, and causing the ground surface to subside. Figure 18. Top: Uplifted sea floor in Prince William Sound, Alaska. The 400-meter-wide white surface was raised above sea level. Bottom: Diagonal crack represents the upper part of a landslide in an Anchorage residential district associated with the 1964 earthquake. Images courtesy of USGS. Landslides: Earthquakes are often associated with mountains formed along convergent plate boundaries. The steep slopes present in these environments are prone to landslides when shaken (Fig. 18). Landslides are common following earthquakes in California. Tsunamis: Giant sea waves are generated by submarine earthquakes, especially noted from the Pacific Ocean. Tsunamis caused by earthquakes around the ocean s perimeter may travel thousands of miles to destroy coastal property in Hawaii. Tsunami waves may reach heights of 15 meters (50 feet) near shore and travel at speeds up to 960 km/hr (600 mph). Many casualties associated with the 1964 Alaska earthquake resulted from tsunamis. 13

14 The Pacific Tsunami Warning System (PTWS) is a network of stations that attempt to identify potentially damaging tsunamis from earthquakes in or around the Pacific Ocean. The PTWS issues warnings or watches that predict tsunami arrival times for coastal areas. Think about it Review the possible effects of earthquakes and examine a description of the 1989 Loma Prieta earthquake and/or the 1906 San Francisco earthquake and suggest what could be done to diminish the potential for damages and loss of life resulting from earthquakes. 2. Use the Venn diagram located at the end of the chapter to compare and contrast the characteristics and effects of the 1989 Loma Prieta and 1906 San Francisco earthquakes. Loma Prieta information available here: San Francisco earthquake information available here: Measurement of Earthquakes There are three methods used for measuring earthquakes. The Modified Mercalli scale measures intensity and is often used to rank the cultural effects of historical earthquakes. Mercalli scale values vary with distance from epicenter, building materials used, and population density. The Richter scale is the most well known and measures earthquake magnitude using the amplitude (height) of the S wave recorded on a seismogram. Each division in the Richter scale represents a 10-fold increase in amplitude and an approximate 30-times increase in energy released. The moment-magnitude scale has recently found favor as a method that more accurately measures energy release on large faults. There are three principal methods of measuring the effects of earthquakes. 10 largest U.S. Earthquakes (with momentmagnitude values) 1. Prince William Sound, Alaska 1964 (9.2) 2. Andreanof Islands, Alaska 1957 (8.8) 3. Rat Islands, Alaska 1965 (8.7) 4. Shumagin Islands, Alaska 1938 (8.3) 5. Lituya Bay, Alaska 1958 (8.3) 6. Yakutat Bay, Alaska 1899 (8.2) 7. Cape Yakataga, Alaska 1899 (8.2) 8. Andreanof Islands, Alaska, 1986, (8.0) 9. New Madrid, Missouri, 1812 (7.9) 10. Fort Tejon, California, 1857 (7.9) 14

15 Five most destructive historical earthquakes (number of deaths) 1556 Shansi, China (830,000) 1737 Tangshan, China (255,000) Modified Mercalli scale is used to measure damage and human perception of an earthquake. Richter scale is the most familiar and measures the size of the seismic waves recorded at a seismogram. Moment-magnitude scale has replaced the Richter scale in popularity among geophysicists because it gives a more accurate interpretation of the amount of energy released by an earthquake. Modified Mercalli Scale The Mercalli scale measures earthquake intensity: the level of destruction of the earthquake (higher values) and the effect of the event on people (lower values). The scale ranks intensity from I to XII (1-12) using Roman numerals. The table below summarizes the characteristics of the Mercalli scale Aleppo, Syria (230,000) 1927 Xining, China (200,000) 856 Damghan, Iran (200,000) Index I II III IV V VI VII VIII IX X XI XII Effects of Earthquake on People and Structures Not felt by people. Felt by people at rest on upper floors of buildings. May be felt by people indoors. Vibrations similar to the passing of a truck. Felt indoors by many, outdoors by few. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Felt by all; many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. Slight to moderate damage in ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Slight damage in buildings designed to withstand earthquakes; heavy damage in poorly constructed structures. Chimneys, columns, walls may fall. Considerable damage in specially designed structures. Damage great in substantial buildings, partial collapse. Buildings shifted off foundations. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. Total damage, objects thrown into air. 15

16 Figure 19. Locations of U.S. earthquakes causing damage , Mercalli intensity VI to XII. Large red squares represent locations of largest earthquakes (intensity XII). Note squares in southeast Missouri from New Madrid ( ) earthquakes. Source NEIC. The Mercalli scale is relatively easy to use but it is not widely applicable to modern earthquakes because its interpretation is dependent upon: Variations in population density: earthquake intensity would be underestimated in sparsely populated areas. Building materials and methods: earthquakes of similar size could give different values depending upon building codes. Distance from the epicenter: values decrease with increasing distance from the epicenter. Each earthquake has several different intensity values making it difficult to compare individual events. Some of the largest historical U.S. earthquakes occurred in the eastern half of the country (Fig. 19). Three major earthquakes were centered in southeastern Missouri (New Madrid) over a three-month period from December 1811 to February The Mercalli scale is useful in ranking historical earthquakes that occurred before the widespread use of seismographs (after World War II). Notice that the map above contains many historical earthquakes in the eastern half of the U.S., some equally severe as those in California. Isoseismal maps can be created that show areas of equal earthquake intensity (Fig. 20). A comparison of isoseismal maps for earthquakes of similar size from the eastern (New Madrid, Missouri) and western (San Francisco, California) U.S. illustrates that the eastern event was felt over a much larger area. Figure 20. Isoseismal map of 1964 Alaska earthquake showing areas with equivalent damages following the largest recorded U.S. earthquake. 16

17 Richter Scale The Richter scale measures earthquake magnitude, the amplitude of seismic waves recorded on a seismograph following an earthquake. (See the Virtual Earthquake exercise on page 11). Charles Richter developed the scale in the 1930s to measure shallow earthquakes in California. These early measurements of magnitude (M L - local magnitude) simply relied on using two factors (the difference in P- and S-wave arrival times and S- wave amplitude). The measured earthquakes were less than 600 km (375 miles) from the seismograph stations and occurred at similar depths in the crust. M b = log 10 (A/T) + Q Formula to determine magnitude from body waves (M b ) where A is the amplitude of ground motion (microns); T is time taken for motion (seconds); and Q is a correction for distance from the epicenter and the focal depth (kilometers). More complex formulae to determine magnitude from seismic body waves or surface waves were developed as the number of seismograph stations increased and it was recognized that earthquakes occurred at a range of depths. The Richter scale is logarithmic, each division represents a 10- fold increase in the ground motion associated with the earthquake, and ~30-times increase in energy released. For example, a magnitude 7 earthquake has ten times as much ground motion (and releases over 30-times the energy) as a magnitude 6, 100 times as much motion (900 times the energy) as a magnitude 5, 1,000 times the motion of a magnitude 4, etc. Magnitude Ground Motion Energy ,000 27, , , ,000 24,300, ,000, ,000, ,000,000 21,870,000,000 Unlike the Mercalli scale, the Richter scale does not have a maximum value; it is open-ended. The largest earthquakes measured with the Richter scale have magnitudes between 8 and 9. It is probable that rocks in Earth s crust are unable to withstand stresses necessary to generate earthquakes of magnitude 9 or more. 17

18 The terminology used to describe earthquakes is dependent upon their magnitude. Description Magnitude Equivalent Intensity Number per Year Great 8+ XI-XII 1 Major IX-X 18 Strong VII-VIII 120 Moderate VI-VII 800 Moment-Magnitude Scale The moment-magnitude (M w ) scale measures the energy released by the earthquake more accurately than the Richter scale. The amount of energy released is related to rock properties such as the rock rigidity, area of the fault surface and amount of movement on the fault. It provides the most accurate means of comparison of large earthquakes. M w = 2/3 log 10 (M o )-10.7 Formula to determine magnitude where M o = msd, where m is shear strength of the faulted rock, S is the area of the fault, and d is fault displacement. Think about it... Answer the conceptest question below. Three sites (L1, L2, L3) record earthquake intensity and earthquake magnitude for the same earthquake. L1 is located closest to the earthquake focus and L3 is farthest away. The intensity values are greatest at and the earthquake magnitude (calculated using seismograms). a) L1; is the same at each site. b) L3; is the same for each site. c) L1; decreases with distance from the focus. d) L3; decreases with distance from the focus. Distribution of Earthquakes Earthquakes are most frequent along plate boundaries. The largest earthquakes are associated with convergent plate boundaries. Oceanic ridges are characterized by shallow earthquakes. Deep earthquakes (to depths of ~700 km) occur within subduction zones along convergent plate boundaries. 18

19 The most devastating earthquakes are typically shallow earthquakes (0-33 km depth). Alaska has the largest U.S. earthquakes but California has the most damages because of a larger population. Some large historical earthquakes have been identified in the eastern U.S., in particular a swarm of three major quakes which occurred at New Madrid, Missouri, Where Do Earthquakes Occur? Earthquakes occur at many sites around the world but seismicity is concentrated in specific locations. A map of the Pacific Ocean basin showing the location of large earthquakes over a 20-year period ( ) is presented below. Compare the map with a map of plate boundaries for the same area (Fig. 21). Earthquake distributions have several characteristics: There is a strong correlation between earthquake foci and plate boundaries (Fig. 21). Swarms of earthquakes resulting from collisions of Figure 21. Top: Distribution of earthquake focal depths around the Pacific Ocean. Orange and yellow dots represent shallow focal depths (0-70 km); green and blue focal depths are 71 to 300 km; purple and red dots represent focal depths of 301 km or greater. Bottom: Plate boundaries (yellow lines) within and around the Pacific Ocean. Notice the correlation between plate boundaries and the distribution of earthquake foci. Images courtesy of USGS NEIC. 19

20 continental plates form a belt across central Asia, through the Middle East and southern Europe. Continental interiors that are far removed from plate boundaries (e.g., Canada) have few earthquakes. A belt of shallow earthquakes can be traced along the global oceanic ridge system from the center of the Atlantic ocean, through the Indian Ocean, around the southern Pacific Ocean, and into the East Pacific. Earthquakes are present under hot spots such as the Hawaiian Islands in the central Pacific Ocean. The largest earthquakes are associated with convergent plate boundaries (Fig. 22). Figure 22. Locations and focal depths of earthquakes of magnitude 7 and greater from Colors correspond to focal depths (see scale). Note that the majority of large earthquakes are located around the rim of the Pacific Ocean, an area characterized by convergent plate boundaries. Source: USGS NEIC. How Does Focal Depth Vary with Location? Epicenter locations on the maps above are colored based upon the depths of the earthquake foci. Several patterns are obvious from the map: Deep earthquakes (focal depth >300 km) are present in association with subduction zones along convergent plate boundaries such as western South America (Nazca/South American Plates), southern Alaska (Pacific/North American Plates), and the southwest Pacific (Pacific/Australian Plates). The focal depths increase below overriding plates at convergent boundaries in the direction of inclination of 20

21 subduction zones. For example, the Nazca Plate descends below South America and foci increase in depth toward the interior of the continent. The only area where deep earthquakes are not present along the Pacific Rim is in the western U.S. where a transform plate boundary exists. The largest earthquakes are typically shallow earthquakes where seismic energy is released closer to Earth's surface. Divergent plate boundaries such as the oceanic ridge systems in the north Atlantic Ocean and continental rift valleys (East Africa) are characterized by earthquake focal depths of less than 33 km. Where Are the Most Seismically Active Areas in North America? The most seismically active states are along the western margin of the continent (Fig. 23). In the U.S., Alaska and California, in that order, experience the most earthquakes. Damage caused by earthquake activity is greatest in California because of its larger population. Most of the largest earthquakes in U.S. history occurred on the southern coast of Alaska, along the convergent boundary between the Pacific and North American plates. The effects of earthquakes in eastern North America are felt further from their sources because the crust is less fractured (more rigid) than in the west. Earthquakes of comparable size Figure 23. Seismicity in the conterminous U.S. reflected by earthquakes between

22 in California affected a much smaller area (compare isoseismal maps for the San Francisco and New Madrid earthquakes). Some of the largest historical earthquakes occurred in the eastern half of the continent. For example, three major earthquakes were centered in southeastern Missouri (New Madrid) over a three-month period from December 1811 to February States in the northern Great Plains of the U.S., such as North Dakota, and adjacent provinces in central Canada (Manitoba, Saskatchewan) have experienced the fewest significant earthquakes. Think about it... Examine the world map at the end of the chapter and predict which locations are most likely to have experienced recent earthquake activity then go to online maps (URL below) of current seismicity to check your predictions. Earthquake Prediction Earthquakes represent the deadliest of natural hazards. Earthquakes typically occur in areas of active faults, especially along plate boundaries. Earthquake magnitude increases with fault length. Various instruments and satellite observations can be used to measure the buildup of strain in rocks. Scientists predict the long-term probability of earthquakes for specific locations on the basis of information about strain accumulations and recurrence interval. Short-term prediction, days or weeks before an earthquake, is still a long way off. Earthquakes represent the most deadly natural hazard. Over two million people have been killed this century alone by earthquakes and associated phenomena. The threat of future earthquakes in heavily populated regions like California has spurred efforts to discover ways to predict future earthquake 22

23 Average annual losses from floods: $5.2 billion Average annual losses from hurricanes: $5.4 billion Figure 24. The FEMA report on the potential damages from earthquakes identified West Coast states as having the combination of active faults and large population centers that may result in the greatest damages from earthquakes. The smallest risk occurs in North Dakota and Minnesota where earthquake damages would account for less than $10,000 annually. activity. The basic questions in earthquake prediction are When? Where? and How big? A recent report by the Federal Emergency Management Agency (FEMA) estimated that the average annual property damage from U.S. earthquakes totaled $4.4 billion. California alone accounted for 75% of this total. Several years may pass with few large events and little associated damage but a single big earthquake in a large city can have a price tag of as much as $30 billion. Population density and active seismicity have the greatest influence over estimates of potential damages. When averaged over several decades, the potential cost of earthquake damages for the populous eastern U.S. ranks alongside that of the more seismically active Rocky Mountain states where population density is much lower (Fig. 24). The upper Midwest and Great Plains states have the least risk for significant earthquake-related damages. Where? How big? Answers to the Where? and How big? questions are already known in regions of frequent seismic activity. The answers to these questions depend on an understanding of the earthquake mechanism. We have already discussed the fact that earthquakes occur on faults. Many active faults have already been discovered but some questions remain about the potential size of earthquakes on faults that have no associated historical earthquakes. For such faults, scientists attempt to estimate future earthquake magnitudes from fault size. Earthquake magnitude is directly related to fault length - the longer the fault the bigger the earthquake (Fig. 25). The 1906 San Francisco earthquake (M w 7.7) was caused by rupture of 400 km (250 miles) of the San Andreas Fault and shaking lasted for 23

24 nearly two minutes. In contrast the magnitude 6.7 Northridge earthquake was caused by displacement on a 14 km (9 miles) long fault segment and the duration of shaking was just 7 seconds. Figure 25. Relationship between earthquake magnitude and fault size for a series of California earthquakes. Earthquake magnitude increases with fault length. When? Displacement on faults is related to crustal deformation associated with plate tectonics and is concentrated in relatively narrow zones along plate margins. Stresses build up in rocks where plates interact. Faults exhibit movement when stresses reach sufficient levels. Rocks adjacent to the fault may be deformed prior to fault movement. Stresses cause deformation of rocks (strain) and geologists can measure the accumulation of strain in deforming rocks in an effort to predict the timing of future earthquakes. Strain can be measured in the vicinity of active faults using a variety of instruments including creepmeters, strainmeters, and satellite positioning systems. Creepmeters survey displacement between two points on opposite sides of a fault. As strain increases the distance between points increases. Strainmeters measure the distortion of the originally circular profile of cylindrical boreholes as a result of deformation. Boreholes are distorted to an increasingly elliptical shape in section as strain accumulates. Satellites of the Global Positioning System (GPS) can be used to continually monitor the location of receivers on the ground on either side of a fault. Distances between stations distributed over an area of hundreds of square kilometers can be determined to within a few centimeters. Monitoring of stations over months or years reveals changes in the relative positions of receivers related to the buildup of strain along the fault. 24

25 Scientists can establish an average recurrence interval - the time between earthquakes of similar magnitude - for individual faults by determining the ages of offset layers of rocks and/or sediment. Analysis of how much time has elapsed since the last earthquake and the amount of energy that was released (magnitude) help reveal which faults may be storing up sufficient strain for earthquakes in the relatively near future. Probability Theory Researchers have used statistical methods to predict the probability of future damaging earthquakes on particular faults with sufficient record of seismicity. Faults with a high probability of an earthquake exhibit a lot of stored strain and a long time interval without fault movement. Figure 26. The probability of fault movement varies along the San Andreas Fault. Segments along the southern half of the fault system are most likely to break, especially at Parkfield. In 1990 a panel of experts convened by the National Earthquake Prediction Evaluation Council estimated a 67% probability for a major earthquake on one of four segments of the San Andreas fault in the San Francisco Bay area between 1990 and 2020 (Fig. 26). Scientists predicted the near certainty (95% probability) of an earthquake at Parkfield, California, between Parkfield, located on the San Andreas Fault, averaged a magnitude 6 earthquake every 22 years since Geophysicists distributed an array of monitoring instruments around Parkfield in the 1980s hoping to pick up a signal that would aid in predicting future earthquakes. 25

26 However, the earthquake has still not occurred illustrating a potential pitfall of prediction by probability. Probability theory assumes a random occurrence of earthquakes but recent analyses suggest that earthquakes cluster together in groups of events. For example, one magnitude 6 or larger earthquake occurred every four years on average between 1836 and 1911 in and around San Francisco. There were no more earthquakes of that magnitude in the 68 years that followed. However, since 1979 there have been four more magnitude 6 events. Scientists are now concerned that the release of strain on one fault may increase the potential for movement on an adjacent fault in ways that cannot be accounted for in traditional probability theory. Even if it becomes possible to accurately predict earthquake activity to within a specific year, it is unlikely that individual events can be pinpointed to within a few months, let alone weeks or days. Furthermore, it is unlikely that we would be able to collect sufficient data to predict earthquakes in areas of infrequent seismic activity. Given the difficulty in predicting the timing of future earthquakes we would be well advised to focus instead on engineering solutions that attempt to earthquake-proof key structures. Think about it... Following graduation you get a job working for a county planning task force in California. The task force must examine the setting of several different cities and identify which is at greatest risk for future earthquake damages from movement on known faults. You are given the assignment to create an evaluation rubric to rank the relative dangers for different cities. Go to the evaluation rubric frame at the end of the chapter to complete the exercise. Summary 1. What is an earthquake? Vibration of Earth due to a rapid release of energy. Energy is released because of rapid movement on a fault. 26

27 2. What is a fault? A fracture on which movement has occurred. Rapid movement of 1 to 10 meters is typically necessary to generate a significant earthquake. Faults are distinguished as dip-slip or strike-slip faults. 3. What is the earthquake focus? The focus is the point on the fault surface where movement begins, the earthquake source. Seismic waves radiate outward from the focus. Earthquake foci occur at a range of depths; shallow (0-70 km), intermediate ( km), and deep ( km). Shallow earthquakes are the most common. 4. What is the earthquake epicenter? The epicenter is the geographic location of the point on Earth s surface directly above the focus. Earthquakes are named for the epicenter location, for example the 1994 Northridge earthquake occurred several kilometers below the city of Northridge in metropolitan Los Angeles. 5. What are the differences between body waves and surface waves? Seismic waves represent the energy released from the earthquake focus. There are two types of seismic waves. Surface waves travel on Earth s surface. Undulations of the land surface during an earthquake are a representation of surface waves. Body waves travel through Earth s interior. These are further subdivided into P (primary) waves and S (secondary or shear) waves on the basis of their vibration direction and velocity. 6. How do P and S waves differ? P waves vibrate parallel to their travel direction in the same way a vibration passes along a slinky toy. P waves travel at speeds of 4 to 6 km per second. S waves vibrate perpendicular to their travel direction, like the wave that passes along a rope when it is given a sharp jerk at one end. S wave velocity is 3 to 4 km per second. 7. What is a seismogram? The record of an earthquake at a seismograph station is a seismogram. The difference in arrival time between P and S waves on a seismogram can be used to determine the distance of the station from the earthquake source. Furthermore, the amplitude (height) of the S wave recorded at the station can be used to determine earthquake magnitude. 27

28 8. What are the principal effects of an earthquake? Ground Shaking: Rapid horizontal movements associated with earthquakes. Shaking is exaggerated in areas where the underlying sediment is weak or saturated with water. Fault Uplift: Large sections of the earth s surface (thousands of square kilometers) may change elevation as a result of uplift on an earthquake fault. Liquefaction occurs when water-saturated sediment is collapses due to violent shaking. Landslides: Earthquakes are often associated with mountains formed along convergent plate boundaries. The steep slopes present in these environments are prone to landslides when shaken. Tsunamis are giant sea waves generated by submarine earthquakes, especially noted from the Pacific Ocean. 9. What methods can be used to measure an earthquake? There are three methods used for measuring earthquakes. The Modified Mercalli scale measures earthquake intensity represented by damages associated with earthquakes. The Richter scale is the most well known and measures earthquake magnitude using the amplitude (height) of the S-wave recorded on a seismogram. The moment-magnitude scale has recently found favor as a method that more accurately measures energy release on large faults. 10. How is the Modified Mercalli scale used? The Mercalli scale measures earthquake intensity: the level of destruction of the earthquake (higher values) and the effect of the event on people (lower values). The scale ranks intensity from I to XII (1-12) using Roman numerals. Values of I to VI represent increasing awareness of people; VII to XII involve increasing damages associated with the event. The Mercalli scale is not widely used for modern earthquakes because it is inaccurate in areas of low population density and in cities which lack stringent building codes, and has a variety of values with distance from the epicenter. 11. What regions of the U.S. have a history of earthquake activity? Earthquakes are common in states along present-day plate boundaries (California, Alaska) and are least common in the continental interior (North Dakota, Minnesota). However, some ancient fault zones in Missouri (New Madrid) and South Carolina (Charleston) have experienced major infrequent earthquake events. 28

29 12. What is the difference between great, major, and strong earthquakes? Great, major, and strong earthquakes are differentiated by Richter magnitude. Great earthquakes (magnitude 8+) are rare (average 1 per year); an average of 18 major earthquakes occur annually with a magnitude of 7 to 7.9; strong earthquakes are more common (120 per year) with a magnitude of 6 to How is the Richter scale used? The Richter scale measures earthquake magnitude, the amplitude of S waves recorded on a seismograph following an earthquake. The Richter scale is logarithmic, each division represents a ten-fold increase in the ground motion associated with the earthquake, and ~30-times increase in energy released. For example, a magnitude 7 earthquake has 10-times as much ground motion (and releases over 30-times the energy) as a magnitude 6, 100 times as much motion (900 times the energy) as a magnitude 5, 1,000 times the motion of a magnitude 4, etc. 14. What controls the distribution of earthquakes? Earthquakes are concentrated in narrow seismic belts along plate boundaries. The largest earthquakes are typically associated with convergent boundaries. 15. Is there a difference in the distribution of deep and shallow earthquakes? Deep earthquakes (to depths of 800 km) occur only in association with subduction zones along convergent plate boundaries. Shallow earthquakes occur along all plate boundaries. 16. Are all U.S. earthquakes confined to the active plate boundary along the western U.S.? Most U.S. earthquakes occur in Alaska and California but several smaller quakes occur along old fault zones in the continental interior. A swarm of major earthquakes of magnitude 7 to 8 occurred near New Madrid, Missouri, in a three-month span from December 1811 to February What factors control the size of future earthquakes? Earthquake magnitude is related to fault length. Longer faults yield larger earthquakes that shake the ground for longer periods. Future large earthquakes are anticipated where strain has accumulated along faults that have not experienced recent 29

30 seismic activity. Scientists predict the long-term probability of earthquakes for specific locations on the basis of information about strain accumulation and recurrence interval. 18. How do scientists determine the time between earthquakes? Scientists estimate the recurrence interval - time between earthquakes of similar magnitude - for individual faults by determining the ages of offset layers of rocks and/or sediment. Analysis of how much time has elapsed since the last earthquake and the amount of energy that was released (magnitude) help reveal which faults may be storing up sufficient strain for earthquakes in the relatively near future. 30

31 Concept Map: Faults and Earthquakes Finish the partially completed concept map for faults and earthquakes below. Print the page and fill in the blanks with appropriate terms. Try to complete the map after reading the section on faults in the this chapter. If you need some help, use some of the terms in the list below to complete the concept map. There are more terms than spaces available. strike-slip 1-10 meters fault scarp dip-slip New Madrid horizontally hot spots California plate boundaries volcanoes 1-10 kilometers stream valleys San Andreas fault 1,000 meters faults Alaska 1,000 kilometers segments 31

32 Venn Diagram: Loma Prieta vs. San Francisco Earthquakes Use the Venn diagram, below, to compare and contrast the similarities and differences between the 1989 Loma Prieta and 1906 San Francisco earthquakes. Both events occurred in the same region. Print this page and write features unique to either group in the larger areas of the left and right circles; note features that they share in the overlap area in the center of the image. Loma Prieta San Francisco 32

33 Earthquake Locations Examine the map below and answer the following questions. 1. Which location is likely to have experienced the largest number of recent earthquakes? a) A b) B c) C d) D e) E 2. Which location is likely to have experienced the deepest recent earthquake? a) A b) B c) C d) D e) E Go to St. Louis University s (SLU) site to examine the distribution of earthquakes over the last 14 days or view maps of current seismicity of the world from the USGS National Earthquake Information Center (NEIC) to check your predictions. SLU: NEIC: 33

34 Earthquake Risk Evaluation Rubric Following graduation you get a job working for a county planning task force in California. The task force must examine the setting of several different cities and identify which is at greatest risk for future earthquake damages from movements on known faults. You are given the assignment to create an evaluation rubric to assess factors that will influence the risk of potential damage from a future earthquake. The city that scores the highest using the rubric will receive additional county funds to protect key structures from earthquake damage. One factor is included as an example in the table below, identify four more. Consider the relationship between faults and earthquakes, the geologic properties of the location, and cultural factors when developing your rubric. Factors Low Risk (1 point) Moderate Risk (2 points) High Risk (3 points) Proximity to fault Far (more than 100 km) Intermediate ( km) Close (less than 20 km) Reviewing your evaluation rubric you realize that some factors are more significant than others. Your team decides to double the score of the most important factor. Which do they choose? Why? 34

35 A map of the county showing the locations and characteristics of four cities is provided below. Use your rubric to decide which site will receive funding to retrofit key buildings and other structures. 35