INTRODUCTION TO EARTHQUAKES An earthquake is the vibration of the earth produced by a release of energy as a sudden movement of rock mass occurs along faults. Earthquakes are very common geologic phenomena: -over 800,000 earthquakes occur every year -nearly all of these are very minor, too small to be felt by people -but about 20 earthquakes of M=7.0 (the size of the 1989 Loma Prieta earthquake) or larger occur around the world annually. Large earthquakes are among the most destructive of natural phenomena. -during historic times, earthquakes in populated areas have caused 50,000 or more deaths at least 16 times. The most destructive historic earthquake occurred in China in 1556 -it is estimated that 830,000 people died -one reason for the huge death toll was that people in the region lived in cave dwellings excavated from bluffs of soft sediment, which collapsed during the earthquake. More recently, another large earthquake in China in 1976 killed at least 250,000 people -some estimates as high as 700,000. Because of their destructive power, scientists have studied earthquakes very intently, with the hope of understanding their exact causes and mechanisms of occurrence -and to use this knowledge to find ways of predicting the location and time of occurrence of large earthquakes. Earthquakes have also provided earth scientists with a powerful tool for studying the properties and structure of the earth's interior -vibrations set up by large earthquakes propagate throughout the earth -studies of the travel times and travel paths of these vibrations allow us to determine the density and other physical properties of rocks below the surface -and how these properties vary with depth -revealed the layered structure of the earth, with divisions into core, mantle, and crust. ORIGIN OF EARTHQUAKES: ELASTIC REBOUND The way in which earthquake vibrations are generated was not understood by geologists when the large 1906 earthquake struck San Francisco and northern California. During this earthquake, much of northern part of the San Andreas fault ruptured: from Hollister northward to Cape Mendocino. -the shaking lasted in San Francisco for 48 seconds. -during this time, the crust west of the fault shifted northwestward -horizontal offset at the fault reached as much as 6 m (20 ft) in Marin County. After the earthquake, studies revealed that strain had been building up in the region surrounding the fault for a long time - the evidence was found in land surveys made during the 50 years preceding the quake -the surveys showed that distant points on either side of the fault were slowly being displaced during the decades prior to the quake -the displacement was horizontal, parallel to the fault, as much as 3 m (10 ft) -but no movement occurred on the San Andreas fault itself during this time: the fault was locked by friction -displacement resulted from gradual deformation spread through the rocks on either side of the fault, across a zone tens of km wide. Survey lines crossing the fault were being bent by the build-up of strain in the rocks surrounding the fault: On the basis of this evidence, the Elastic Rebound mechanism of earthquake generation was proposed. This theory is based on the fact that when rocks are subjected to small, temporary stress, they behave like elastic materials: -they change shape temporarily -but when stress is removed, they spring back to their original shape -in effect, when elastic materials deform, they store up elastic energy, which is released when stress is removed, and enables them to return to their original shape. The bending of rocks adjacent to a locked fault stores up elastic energy and increases the stress along the fault -eventually, stresses acting parallel to the fault exceed the friction that prevents movement -the fault suddenly slips, with movements as rapid as several meters in a few seconds -as the fault slips, the rocks adjacent to the fault suddenly "unbend", and spring back to their original shape, releasing the stored elastic energy -the sudden motion of the unbending rocks generates the vibrations that we feel as an earthquake. The potential for earthquakes is greatest along active faults that are temporarily locked -the size of the earthquake is related to how long the fault has been locked, how much elastic strain has built up, and how large a portion of the fault slips. The slip that occurs during a major earthquake may not relieve all of the built-up stress: -after a major quake, additional smaller offsets commonly occur along faults -these generate smaller earthquakes called aftershocks -despite their smaller magnitude, aftershocks can do significant damage because many structures may be badly weakened by the main quake. Major earthquakes may also be preceded by smaller quakes called foreshocks -monitoring faults for foreshocks is one strategy for earthquake prediction. SEISMIC WAVES The vibrations produced by an earthquake radiate outward from the earthquake focus in all directions -the study of earthquake vibrations is called seismology -and the instruments used to measure and record seismic vibrations are called seismographs. -the record of these vibrations is a seismogram. Some earthquake energy sets up a wave-like disturbance of the earth's surface: surface waves -like the ripples on the surface of water But most earthquake energy moves as body waves: -waves that move through the rocks in the interior of the earth. There are two types of seismic body waves: -Primary or P-waves, also called compressional waves -Secondary or S-waves, also called transverse waves. P-waves As P-waves travel through rock, the rocks oscillate back and forth in the direction the wave is travelling -rocks are alternately pushed and pulled in the direction of wave motion -or rocks are alternately compressed and expanded. A P-wave can be simulated with a large spring -if the spring is stretched, and you bunch the coils at one end -when released, the zone of compression travels longitudinally down the spring, followed by a zone of expansion. Sound traveling through the air moves in this same way, as alternate compressions and expansions of the air -because solids, liquids, and gasses can all be compressed elastically, P-waves can travel through all three phase states of matter. S-waves S-waves are what physicists call transverse waves: -rocks oscillate back and forth at right angles to the direction of wave travel S-waves can be illustrated by tying one end of an elastic rope to something and shaking the other end -an S-wave travels down the rope and reflects back from the tied end S-waves produce temporary changes in shape of the rocks which are recovered as the waves pass -S-waves propagate readily through rocks, but cannot be transmitted through a liquid or gas (unlike P-waves) -if you try to push a liquid sideways, it simply flows -it does not spring back when the stress is removed This difference between P-waves and S-waves allows us to determine which parts of the earth's interior are solid and which parts are liquid. Surface waves The motion of surface waves is similar to that of S-waves, but the motion dies out downward below the surface -the ground surface oscillates back and forth at right angles to the direction of wave travel -both up-and-down and horizontal side-to-side motions occur in different types of surface waves. -when large shallow earthquakes occur, observers near the epicenter can sometimes see the ground surface undulating up and down by a meter or more as the surface waves pass. Seismic Waves and Seismograms How can we recognize different seismic waves on a seismogram? -for earthquakes some distance away from a recording station, this turns out to be fairly easy -the different waves arrive at the seismograph at different times -because the different wave types travel at different velocities. The first wave to arrive is the P-wave: which is the source of the name. -the P-wave is the fastest type of seismic wave -the exact velocity depends upon rock type, rock density, and depth -for granite at shallow depths in the crust, Vp = 6 km/sec The S-wave is the second wave to arrive: -it is significantly slower than the P-wave -for granite, Vs = 3.5 km/sec -for any rock type, the P-wave travels 1.7 times as fast as the S-wave. The S-waves also usually have a higher amplitude than the P-waves -the wiggles on the seismograph trace are more pronounced. Surface waves are slightly slower than S-waves: -travel at 0.9 times the velocity of S-waves -so they are the last to arrive -usually have much higher amplitudes than the body waves. LOCATING EARTHQUAKES The source of an earthquake is the point where a fault first begins to rupture -this nearly always occurs some distance below the surface -for many deeper earthquakes, the zone of rupture never reaches the surface, and there may be no surface offset. This point of origin of the earthquake waves is called the earthquake focus. -the epicenter of an earthquake is the point on the earth's surface directly above the focus. The difference in velocity and arrival times of P-waves and S-waves provides the data that allow earthquake epicenters to be located. When an earthquake is recorded at a seismograph station, we don't initially know the exact time at which the earthquake began So we can't use the arrival time of the P-wave alone, for example, to determine how far away the epicenter is. But for any seismic station more than a few miles from the earthquake epicenter, there will be a time lag between the arrival of the P and S waves. -as the waves radiate away from the epicenter, the slower S-wave lags farther and farther behind the P-wave -so the difference in arrival times increases with distance. If both P and S-waves are recorded at our station, we can easily measure the time lag between their first arrivals -that time difference will correspond to a specific distance from the epicenter. Unfortunately, with a single seismograph station, we don't know the direction from which the seismic waves were traveling. But the epicenter must lie on a circle centered on the seismograph station, with a radius equal to the calculated distance In order to find the exact location of the epicenter, we must have distance measurements from at least three seismograph stations -the circles drawn around each station should all intersect in a single common point: the epicenter. For seismograph stations close to the epicenter, the depth of the earthquake focus also affects the travel time of seismic waves -seismic waves from a deep earthquake will take longer to reach a nearby station than waves from a shallower earthquake -travel-time data from a number of nearby seismic stations can be used to establish the focal depth. Earthquake foci vary in depth from 5 km to 700 km below the surface. -seismologists have divided earthquakes into three depth ranges (which are somewhat arbitrary) -shallow earthquakes are less than 70 km deep. -intermediate depth earthquakes are between 70 and 300 km deep. -deep earthquakes are deeper than 300 km. About 90% of all earthquakes are shallow-focus earthquakes. -intermediate and deep earthquakes are exceptional, and played an important role in working out the plate-tectonic theory. EARTHQUAKE MAGNITUDE AND INTENSITY Earthquakes vary greatly in strength and in the effects they produce at the surface. Two different ways to compare the relative strength of earthquakes are the magnitude and intensity. Magnitude The magnitude of an earthquake is a fairly objective measure of the strength of an earthquake. -is determined from the ground motions measured by seismic instruments. The basic assumption in magnitude measurement is that the amplitude of seismic waves recorded on seismograms is related to the amount of energy released in the earthquake -the larger the earthquake, the greater the amplitude of the vibrations. The technique for determining earthquake magnitude was developed by Charles Richter and associates at Cal Tech in 1935 -the magnitude scale is now called the Richter Scale Magnitude is determined by measuring the amplitude of the largest wave recorded by a standard type of seismograph: this is usually the surface wave. -but seismic waves spread out and weaken as they move away from the source -the wave amplitude diminishes at more distant seismic stations -so observed amplitudes are corrected for these variations due to distance from the source Because the size of earthquakes varies enormously, the amplitudes of ground motions measured by seismographs differs by factors of thousands from earthquake to earthquake. -so to obtain a scale which expresses variations in amplitude in small numbers, the Richter Scale is logarithmic This means that an increase in Richter magnitude of 1.0 between two earthquakes is equal to a ten-fold increase in wave amplitude -a magnitude 5.0 earthquake has a wave amplitude 10 times that of a magnitude 4.0 quake -a magnitude 6.0 quake has an amplitude 100 times higher -a magnitude 7.0 quake has an amplitude 1,000 times higher. The smallest earthquakes that can be felt by people under ideal circumstances are about magnitude 1.5 -damage to structures may occur at magnitudes greater than 4.5. The Richter scale has no inherent upper limit -but the largest earthquake magnitude that has been measured this century is 8.9, in two earthquakes. -this appears to represent a natural upper limit, related to the strength of rocks in the crust. Seismic wave amplitude correlates in a crude way with the amount of energy released by an earthquake -an increase in magnitude of 1.0 increases the amount of seismic energy released by a factor of 30, not just 10. -so a magnitude 6.0 earthquake releases 900 times more energy than a magnitude 4.0 quake. The largest earthquakes (over Magnitude 8) release millions of times more energy than the smallest earthquakes felt by people. -as a result, they are much rarer: Richter Magnitude Number per year 7.5) occurred within seismic gaps that had been previously identified by seismologists. Recognizing seismic gaps on active faults helps to predict where we can expect to experience major earthquakes -but much more detailed studies are required to try to predict when we can expect such a quake to occur. Recurrence Intervals Some indications of the timing of quakes can be revealed by the past behavior of faults -for many faults, there is evidence that earthquakes of similar magnitude repeatedly break the same segment of the fault, and produce similar offsets. -the time between these repeated quakes is called the recurrence interval. One of the best examples of such regular behavior is part of the San Andreas fault near Parkfield in central California -Parkfield is the southern termination of the "creeping" section of the fault -a few miles NW of town, the fault takes a slight bend, which serves to isolate the Parkfield section from the creeping section. Earthquakes have occurred on the Parkfield section in 1857, 1881, 1901, 1922, 1934, and 1966 -the average recurrence interval has been about 22 years +-2, with the 1934 quake coming 10 years early. The 1857 Parkfield quake occurred several hours before the Ft. Tejon quake -seismologists speculate that the displacement on the Parkfield segment may have triggered the larger offset on the segment of the fault to the south, which must have been already strained to the breaking point. The last three Parkfield quakes have been recorded by seismic instruments -all had magnitudes of 5.6, with a M=5.1 foreshock occurring 17 minutes before the main quake. -each ruptured the same 20-mile segment of the fault, with the rupture beginning at the north end. -seismograms of these three earthquakes recorded at distant stations are nearly identical in character and amplitude, suggesting similar amount of energy was released. Based on the average recurrence interval of 22 years, the next Parkfield quake was expected between 1988 and 1993, and is now overdue -just as the 1934 quake came early, the expected quake appears to be late -even for this most well-behaved of fault segments, earthquakes are not as predictably periodic as we would like. Unfortunately, the U.S.G.S. bet heavily on the predicted arrival time -they hoped to catch an earthquake in action -installed hundreds of monitoring devices to record changes in the crust around the fault before and after the earthquake -hoping to get data that would help define the mechanism of triggering of the earthquake, and reveal possible precursor phenomena that might be used to help provide short-term earthquake predictions. For larger-magnitude earthquakes, the recurrence interval is considerably longer than 22 years -may be over 200 years -too long to accurately determine it with our short recorded history in California. However, if we know the amount of offset that occurs in each major quake, and the long-term rate at which strain accumulates along the fault, we can calculate the expected average interval between quakes. For example, assume that we determine the long-term rate of movement on a fault averages out to 5 cm/year -but this occurs in abrupt offsets of 5 m (500 cm) during large earthquakes -the earthquake offset divided by the long-term rate gives the interval between earthquakes -in this case the recurrence interval is 100 years. This is the time it takes for elastic strain to build up along the locked fault to the level at which friction along the fault is finally overcome, and fault slip can occur. Both the characteristic offsets during major earthquakes and the long-term strain rate must be determined by painstaking geologic studies of active faults -and recent techniques allow direct estimates of recurrence intervals of major earthquakes. -these studies of ancient earthquake effects are sometimes called paleoseismology. PALEOSEISMOLOGY Paleoseismology involves studies of offset landforms such as stream channels -and studies of young sediment layers that accumulate in stream channels, ponds, and marshes along faults. Stream channels that cross a strike-slip fault like the San Andreas fault have their downstream portion progressively offset from the upstream portion -for a while, the stream will flow along the fault, connecting upper and lower portions -eventually the distance becomes too great -a new lower valley is cut straight across the fault, but it will be offset in turn. One such offset stream is Wallace Creek, along the San Andreas fault north of Los Angeles -the young, active channel has been offset about 130 m by the fault -an older, abandoned channel is also preserved, offset an even greater distance. Studies of the abandoned channel showed that it is filled up by sediment deposited by a single debris flow -a flowing mixture of water, mud, and gravel triggered by heavy rains, that had flowed down the channel and plugged it. -fragments of fossil wood in the channel fill deposit were dated by the Carbon-14 technique: 3,700 years old. This plugging of the old channel was the event that triggered the cutting of the present downstream channel segment -this segment therefore began to form 3,700 years ago -has been offset about 130 m in that time -dividing the total offset by the time yields a long-term slip rate of 3.4 cm/year for the fault. The Wallace Creek area is part of the San Andreas fault that was last broken by the 1857 Ft. Tejon earthquake -amount of offset in this region was about 9 meters. -some small streams in the area have several abandoned and offset downstream channels -the regular spacing between the offset channels suggests that 9 meters is the typical offset that occurs during major earthquakes in this area. If the 130 meters of offset of Wallace Creek is due to successive 9-m offsets during large quakes -this would require about 14 such quakes (130/9) If the 14 earthquakes occurred at regular intervals during the 3700 years -the average interval between earthquakes would be 3700/14, or about 265 years. -since the last major quake was in 1857, the most likely time for the next great earthquake on this part of the fault would be in 2120. Other studies of the San Andreas fault have looked in more detail at sediment layers accumulating in ponds and marshes along the fault -during the long intervals between earthquakes, sediment gradually accumulates at the surface, burying the last fault break -but during the next earthquake, this layer may be broken by the fault movement -after the earthquake, sediment will again accumulate to form a new, younger layer that covers up the break. Sediment sequences such as this are studied by digging trenches across the active fault trace -for the San Andreas fault, the fault motion is dominantly horizontal -but when the horizontal offset is several meters or more, there is usually at least a little vertical offset that shows up in the trench walls. In looking at such a sequence in a trench wall, we use principles of relative age dating to determine the sequence of events -the principle of cross-cutting relationships tells us that the fault is younger than any layer that it cuts -the principle of superposition tells us that the overlying, unbroken layer must be younger than the fault motion. In ponds and marshes, the sediments often contain fragments of charcoal form fires, or pieces of wood, leaves, or marsh vegetation -new laboratory techniques allow even small fragments to be dated using the Carbon-14 technique -if many layers can be dated, then the age of each faulting event (earthquake) can be bracketed by these absolute ages. One of the first studies of this kind was done at Pallett Creek northeast of Los Angeles -the southernmost part of the fault that ruptured in 1857 At Pallett Creek, swamp deposits about 6 meters thick were studied -contained about 80 recognizable sediment layers, deposited over the last 2,000 years. -the sequence showed the evidence of a number of large earthquakes, whose ages could be determined with a possible error of a few decades: 1720, 1550, 1350, 1080 -in fact, there was evidence for 12 major quakes in the past 1700 years. The average recurrence interval for major earthquakes in the Pallett Creek area was calculated to be about 150 years -but the interval varied from a high of 300 years to a low of 55 years. Paleoseismic studies such as this have been undertaken along many parts of the San Andreas fault system -mostly sponsored by the U.S.G.S. -they suggest that most parts of the San Andreas fault system are less regular than the Parkfield segment -recurrence intervals are at best only a guide to enable geologists to focus attention on faults most likely to rupture in the near future. EARTHQUAKE PRECURSORS The main hope for development of short-term earthquake predictions in the 1970's lay in studies of various types of physical phenomena that have been seen to occur in the months or days prior to some major earthquakes -these short-term phenomena can be called earthquake precursors Changes in earthquake frequency Large earthquakes generally occur in seismic gaps -for many large historic quakes, small earthquakes have begun occurring in the seismic gap in the months prior to the major earthquake -these sometimes increase in magnitude, and culminate in strong foreshocks in the final days before the main quake. So monitoring of lock faults for signs of increased earthquake frequency is one major component of efforts toward predicting earthquakes -although a sudden increase in activity suggests an increased likelihood of a major earthquake on a locked fault, it does not tell us when such an earthquake will occur. Ground uplift and tilting Locallized changes in the level of the ground surface sometimes occur in the area of an impending earthquake -these can be detected by repeated ground surveys -and by sensitive instruments called tiltmeters which can detect minute changes in the tilt of the land surface -imagine that we could pick up the United States by one edge like a rigid dinner plate -if we picked up the eastern U.S. and raised it by 2 inches, tiltmeters elsewhere in the country could detect the very slight resulting tilt. A major earthquake occurred on the west coast of Japan in 1964 (M = 7.5) -this is the quake that caused the tilting of apartment buildings due to liquefaction. Much of the coastal area around the epicenter underwent slow uplift in the early part of the century -but this uplift accelerated about 10 years prior to the quake, then leveled off -and the ground began to subside again several years before the quake -unfortunately, this pattern in the survey data was not discovered until after the earthquake -but if it occurred again in the future it might provide some warning. Water levels in wells The level of underground water has been seen to rise and fall in wells before large earthquakes -presumably this occurs because of deformation of the rocks surrounding the fault, which changes the pressure on the water contained in fractures and pores in the rock. Electrical resistivity of the earth If electrodes are placed in the ground several kilometers apart, a weak electrical current will flow between them -this current flows mostly through the water that saturates the rock -the resistance to current flow can be measured, and is the electrical resistivity -the electrical resistivity of rocks along active faults has been monitored in some areas in the U.S. and the Soviet Union -some changes in resistivity have been seen before earthquakes -related to changes in volume of cracks and pores that contain water. Data from Soviet studies suggest that resistivity first increases, then decreases just before an earthquake. Radon content of ground water Radon is a gas produced by the radioactive decay of uranium in rocks -it diffuses out of the rocks and dissolves in groundwaters But radon itself is radioactive -has a half-life of only 3.8 days, so it does not remain in groundwater for very long -any change in radon content in the water must result from current changes in the rate of addition of radon to the water from the surrounding rock. Increases in the radon content of groundwater have been measured before some major earthquakes -these increases might result from fracturing of the rock, that would enable radon to escape from the rock into the water more rapidly. Animal Behavior For centuries, unusual animal behavior has been reported to have occurred before earthquakes -birds becoming agitated and flying away -dogs running outside of houses -horses and cows running around wildly -snakes and frogs emerging from hibernation in the middle of winter It is certainly possible that some animals can sense phenomena that humans cannot -such as variations in electrical fields or magnetic fields -related to the physical changes going on in the rocks before a quake For example, some biologists believe the homing sense in birds such as geese may lie in their ability to sense the earth's magnetic field. On the other hand, most reports of unusual animal behavior have been gathered second and third-hand, and after the earthquake -it is human nature to see cause-and-effect connections between unusual events happening close together in time -similar odd behavior occurring at other times may be ignored. The U.S.G.S. has funded several experiments and surveys to test animal behavior during earthquakes -one was a survey of 50 households in Willits CA after a M = 4.7 quake in 1977 -17 out of the 50 households reported unusual behavior -for a larger quake in Montana in 1978, only 1 person out of 35 interviewed reported any unusual behavior -from this the researchers concluded that some individual animals within some species may be able to sense an impending quake. Another 4-year project in the Bay Area set up a phone hotline that people could call anytime they saw a pet or other animal behaving in strange ways. -over this time, 13 tremors of M 4 or higher occurred in the area -by looking at phone records of the place and time of the calls, researchers were able to see if there had been an increased incidence of unusual behavior before the quakes -about half of the quakes showed significant increases in the number of calls in the preceding month -but the other half showed nothing unusual. As with other precursors, it appears that unusual animal behavior is also a very hit-or-miss thing -cannot be relied upon in earthquake prediction efforts EXAMPLES OF EARTHQUAKE PREDICTIONS The Chinese have achieved the only success with short-term earthquake predictions in a populated area -the most famous one for an earthquake which occurred in 1975 near the city of Haicheng in northeast China, inhabited by 100,00 people. As early as 1970, the State Seismological Bureau had identified the Haicheng area as the likely location of future large earthquakes -in 1974, surveys revealed an increase in the rate of uplift of the surface -the authorities expanded their network of instruments in the area, and enlisted thousands of amateur observers to assist the seismologists By the end of 1974, farmers had reported that water levels were changing in wells, and scientists detected increases in radon levels. On December 20, residents were warned that a large quake was imminent -the people slept outside in the snow for two nights, but no quake occurred. On February 1, 1975, a tiny quake occurred near Haicheng (M < 1), followed the next day by several more, in an area where no quakes had occurred previously -by this time 70% of the water wells being monitored showed changes in water levels, and a number were spouting water at the surface During February 3, the local earthquakes began to get larger and to occur more rapidly -increased to M = 4 by the next morning, then dropped off At 10:00 AM on Feb. 4, the seismologists issued a prediction that a major quake would strike in the next day or two -in Haicheng, the people were evacuated from buildings into tents and other temporary shelters -disaster relief facilities were mobilized -shops and businesses closed -foreshocks continued throughout the day, and at 7:36 in the evening the main quake struck, with M = 7.6. 90% of the buildings in Haicheng were severely damaged or destroyed -but only a few people were killed, because everyone was outside and well away from the buildings. During the following year, 1976, three other damaging earthquakes were successfully predicted in China. But in July of 1976, a M = 8.0 earthquake struck Tangshan, a city of 1 million people east of Beijing, -the quake struck entirely without warning -there had been no foreshocks, and no other precursor phenomena observed -the death toll was 240,000 people, making this one of the most tragic natural disasters in history. The following month, in August 1976, the Chinese authorities issued an earthquake warning for parts of southeastern China -many people slept outdoors in tents for almost 2 months -but no earthquake occurred. Unfortunately, events such as these indicate that precursor phenomena do not always occur before large earthquakes -or they may occur without being followed by a quake.
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