Information about earthquakes

Category: Education/Science >> Science and Technology

Analysis:

How do seismic waves propagate? The graphic below illustrates this visually. Take the Northridge earthquake in California as an example. On January 17, 1994, the magnitude was 6.8. Northridge, a community in the San Fernando Valley not far north of Los Angeles, was hit by a major earthquake at 4:31 AM local time on January 17, 1994. impact. About 60 people died and property damage was estimated at $30 billion. Because the earthquake occurred on Martin Luther King Jr. Day, there were not as many people on the highway that morning as there are on a typical Monday morning. This fact likely contributed to the lower death toll. Engineers were both delighted and surprised by the earthquake's impact. After the 1971 San Fernando earthquake (not far north of the earthquake's epicenter), many of the area's highway bridges were reinforced. None of these reinforced bridges collapsed. However, several bridges that had been planned for reinforcement collapsed. Many steel buildings are breaking at the seams.

When an earthquake occurs, seismic waves travel through the Earth's interior and surface. Speed ??up time and you can see it all happen. The image to the right shows how surface waves propagate outward from where an earthquake occurs. The cutaway diagram shows body waves propagating through the interior of the Earth and changing when encountering internal obstacles. The yellow bar on the surface marks the propagation range of surface waves.

This graphic shows actual seismograms collected

from seismic stations around the world. When each seismic phase (P wave, S wave, etc.)

reaches the earth's surface and a certain station on the cross-section map, you can

see the changes in the seismic waveform. Following P waves and S waves are surface waves. They are the seismic waves that cause the main damage in earthquakes. There are two types of surface waves: one is Love wave, in which material particles move horizontally back and forth in a direction perpendicular to the direction of wave propagation; the other is Rayleigh wave, in which material particles move in the same direction as the wave propagation direction. Make vertical forward and backward movements. Seismologists use the arrival times of these seismic waves to determine the Earth's internal structure.

Occurrence and types of earthquakes

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Earthquakes are divided into two categories: natural earthquakes and artificial earthquakes. Natural earthquakes are mainly tectonic earthquakes. They are caused by the rupture and dislocation of rocks deep underground, which sharply release the energy accumulated over a long period of time and spread in all directions in the form of seismic waves, causing shaking and ground shaking on the ground. Tectonic earthquakes account for more than 90% of the total number of earthquakes. Followed by earthquakes caused by volcanic eruptions, called volcanic earthquakes, accounting for about 7% of the total number of earthquakes. In addition, earthquakes may also occur under certain special circumstances, such as cave collapse (collapse earthquakes), large meteorites impacting the ground (meteor impact earthquakes), etc.

Artificial earthquakes are earthquakes caused by human activities. For example, vibrations caused by industrial blasting and underground nuclear explosions; high-pressure water injection in deep wells and water storage in large reservoirs increase the pressure on the earth's crust and sometimes induce earthquakes.

The place where seismic waves originate is called the earthquake source. The vertical projection of the earthquake source on the ground is called the epicenter. The depth from the epicenter to the source is called the focal depth. Usually, earthquakes with a focal depth less than 70 kilometers are called shallow earthquakes, earthquakes with a depth of 70-300 kilometers are called intermediate earthquakes, and earthquakes with a depth greater than 300 kilometers are called deep earthquakes. Destructive earthquakes are generally shallow earthquakes. For example, the focal depth of the 1976 Tangshan earthquake was 12 kilometers.

Seismic zone

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Earthquakes are mainly distributed in the Pacific Rim Belt, Albis-Himalayan Belt, Mid-Atlantic Ridge and Mid-India Ocean Ridge. Generally speaking, earthquakes mainly occur in tectonic activity zones such as ocean ridges and rift valleys, trenches, transform faults, and ancient plate margins within continents.

Focus: It is the place within the earth where an earthquake occurs.

Focal depth: The distance from the earthquake source vertically upward to the surface is the focal depth. We call earthquakes that occur within 60 kilometers as shallow earthquakes; 60-300 kilometers as intermediate earthquakes; and more than 300 kilometers as deep earthquakes. The deepest earthquake recorded so far reaches 720 kilometers.

Epicenter: The ground directly above the earthquake source is called the epicenter. The epicenter and its vicinity are called the epicenter area, also known as the epicenter area. The distance from the epicenter to any point on the ground is called the epicentral distance (referred to as epicentral distance). Those with epicenter distances within 100 kilometers are called local earthquakes; those with epicenter distances within 1,000 kilometers are called near earthquakes; those with epicenter distances greater than 1,000 kilometers are called telequakes.

Seismic waves: During an earthquake, the elastic waves that appear inside the earth are called seismic waves. It's like throwing a pebble into water and the waves will spread in all directions.

Seismic waves mainly include longitudinal waves and transverse waves. Waves whose vibration direction is consistent with the propagation direction are longitudinal waves (P waves).

Longitudinal waves from the underground cause the ground to vibrate up and down. Waves whose vibration direction is perpendicular to the propagation direction are transverse waves (S waves). Transverse waves from underground can cause horizontal shaking of the ground. Shear waves are the main cause of building damage during earthquakes.

Since longitudinal waves propagate faster inside the earth than transverse waves, during an earthquake, longitudinal waves always reach the surface first, while transverse waves always lag behind. In this way, when a large near-earth earthquake occurs, people generally feel up and down bumps first, and then feel strong horizontal shaking after a few to more than ten seconds. This is very important, because longitudinal waves give us a warning, telling us that transverse waves causing damage to buildings are coming soon, and we need to take precautions quickly.

During the 1976 Tangshan earthquake, a cadre living in a building was suddenly awakened by the earthquake. Since this cadre usually had some knowledge about earthquakes, when he felt the earthquake jolts, he quickly got under the table. Five or six seconds later, the roof collapsed. Until noon, after he was rescued, he deeply felt that if he had not decisively ducked under the table, he would have died long ago. He said knowledge of earthquakes saved his life.

One of the great achievements of seismology is the complete understanding of the mechanism by which seismic waves are excited. At the end of the last century, a seismologist commented on the earthquake and wrote: "The cause of the earthquake is still hidden in the hazy, and may be an eternal mystery, because the places where these strong vibrations occur are below the range of distant human observation." Many of his contemporaries believed that volcanism was the primary cause of earthquakes, while others favored that earthquakes originated from the huge gravity differences caused by tall mountains.

After the establishment of the seismic network in the early 20th century, global monitoring of seismic activity was completed, and it was discovered that many major earthquakes occurred far away from volcanoes and mountains. More and more geologists are making field investigations of destructive earthquakes their mission. They are often shocked by the magnitude of the ground's faults, which can be identified by deforming the terrain along linear systems. It became clear to scientists at the end of the last century that earthquakes in general are closely related to tectonic processes that cause widespread deformation of the Earth's surface, creating mountains, rift valleys, ocean ridges and trenches. Geologists speculate that large-scale and rapid dislocation of surface rocks is the cause of strong earthquakes. Their inferences quickly developed into confident statements, and the mechanisms by which most earthquakes occur have been discovered.

Today it is believed that almost all natural shallow earthquakes have the same cause. Large-scale deformation of the Earth's outer layer caused by plutonic tectonic forces is the root cause of earthquakes. Sudden slippage along geological faults is the direct cause of seismic wave energy radiation.

4.1 Geological faults

In the laboratory, pressure on rocks can cause them to "crack" and "destroy" in different ways. In some sudden ruptures, the fracture cuts the rock, the rocks on both sides slide relative to each other, and multiple cracks break the rock into pieces. If the broken pieces of rock can be put back together again, this type of failure is called brittle failure. In another type of rock failure, the two sides of the specimen do not slip suddenly, but grind slowly and remain bonded together along an inclined section. Failure of this rock cannot release the stored elastic energy as quickly as brittle failure.

In nature, large-scale rupture surfaces are called geological faults. The two sides of a fault can slide past each other gradually and imperceptibly, as seen in the laboratory, or they can rupture suddenly, releasing energy in the form of earthquakes. In the latter case, the two sides of the fault shift in opposite directions, so that rocks that once lined the fault become displaced. Many of the fractures are very long, some traceable for thousands of meters at the surface.

Fracture displays a variety of characteristics ***. They may be clear crack surfaces with only small visible dislocations (Figure 4.1), or they may be extended fracture zones in the rock, tens or hundreds of meters wide, which are along the fracture zone The result of repeated movements from time to time. Once a fault is formed, it often becomes a place where continuous displacement occurs under the action of sustained stress. This can be confirmed by the fragmented rock argillaceous material near the fault section. Most of the rock masses on the fault section contain abundant fractures caused by rock displacement. The rock in the fault zone can be very finely crushed and sheared during several earthquakes, turning it into a plastic clay material called fault gouge. This material is so weak that elastic energy cannot be stored as it is in deeper, brittle elastic rocks.

Faults were once classified based on their geometry and relative slip directions. As shown in Figure 4.2, the positioning of a fault in three-dimensional coordinates is given by two angles: the first is the inclination of the fault, that is, the angle formed between the section and the horizontal plane. The second is the direction of the fault, that is, the angle of the fault line exposed on the surface relative to the due north direction.

Figure 4.1 A small, clear normal fault cutting through the rock formation near Kanab, Utah

Figure 4.2 Types of geological faults

Oblique faults (Fig. (right) have the characteristics of both horizontal movement (strike-slip fault) and vertical movement (normal fault and reverse fault).

Faults can be classified according to their movement directions along the dip and along the strike. Strike-slip faults, sometimes called transverse faults, can cause two sides of a fault to slip horizontally relative to each other. The rock moves relatively parallel to the strike. If we stand on one side of this fault and see the movement on the other side from left to right, this fault movement is called right-lateral strike-slip. Likewise, left-lateral strike-slip faults can be identified.

The movement of a fault can occur entirely along the dip, which is called a dip-slip fault.

At this time, one side of the fault moves up and down relative to the other side, and the fault movement is basically parallel to the fault tendency. The rock is dislocated vertically, sometimes forming a small and visible rock wall, which is called a fault cliff. This type of fault can be divided into two subcategories: one is a normal fault, in which the rock above the inclined section moves downward relative to the rock below the fault in a dip-slip fault; conversely, a reverse fault means the rock above the inclined section moves upward. A thrust fault is a reverse fault with a very small dip angle. Faults are rarely purely strike-slip or dip-slip; usually they have horizontal and vertical components of motion. This type of fracture is called an oblique fracture. Some fracture surfaces failed to penetrate the overlying soil from the bedrock because the near-surface soil absorbed the differential slip. At this time, the fault can only be detected by digging trenches or cutting through hidden cliffs.

4.2 Earthquakes from Other Sources

Most destructive earthquakes—such as the 1906 San Francisco earthquake, the 1988 Armenian earthquake, and the 1992 California Landers earthquake—are caused by Occurs from the sudden rupture of fault rock. Although earthquakes are usually referred to these so-called tectonic earthquakes, strong ground shaking can also be the result of many other sources.

The second well-known type of earthquakes are those that occur with volcanic eruptions. Many people, like early Greek philosophers, imagined earthquakes to be associated with volcanic activity. Indeed, earthquakes occur impressively in conjunction with volcanoes in many parts of the world. We now know that although volcanic eruptions and earthquakes are both the result of tectonic forces in rocks, they do not necessarily occur at the same time. Today we call earthquakes that occur associated with volcanic activity volcanic earthquakes.

In large volcanic earthquakes, the focal mechanism determined from seismic waves may be the same as for tectonic earthquakes. Near an erupting volcano, elastic strain energy accumulates in the rock as the rock deforms due to the accumulation and movement of magma. These strains cause fault ruptures like tectonic earthquakes, but are not directly related to volcanoes. However, due to the rapid movement of erupting magma in underground volcanic channels and the stimulation of superheated steam and gas, the surrounding rocks can tremble, which is called volcanic tremor.

Another type of earthquake occurs when an underground cave or mine collapses, causing a small "collapse" earthquake. This phenomenon is a variant of what is commonly known as a mine explosion. When a mine explosion occurs, the stress induced in the mining site causes a large amount of rock to explode and fly out of the mining face, generating seismic waves.

A spectacular landslide along the Mantaro River in Peru on April 23, 1974 caused seismic waves equivalent to a magnitude 4.5 earthquake. About 1.6 cubic kilometers of rock slid 7 kilometers, killing about 450 people. This landslide was not driven by an adjacent tectonic earthquake but by the instability of the mountain. Part of the gravitational potential energy is converted into seismic waves during the rapid downward movement of soil and rock, and is clearly recorded by seismic stations hundreds of kilometers away. A seismometer 80 kilometers away recorded a 3-minute earthquake. The duration of this shaking is consistent with the speed and extent of the ground slip, which was operating at about 140 kilometers per hour within the 7 kilometers of observed slip.

Because earthquakes often cause ground slips, sometimes on a large scale, it is difficult to separate cause and effect. The largest landslide in modern history may have occurred in Uso in the Pamir Mountains of Russia in 1911. Galitzin, a pioneer of modern seismology, recorded seismic waves caused by the Uso landslide on his seismometer near St. Petersburg. Therefore, the seismic waves emitted by the landslide traveled 3,000 kilometers. He initially thought he had recorded a normal tectonic earthquake. It was not until 1915 that he sent a survey team to study the Uso landslide and discovered that the landslide swept away 2.5 cubic kilometers of rock!

Figure 4.3 New Zealand Mount Cook after 14 million cubic meters of rock, ice and snow collapsed on December 15, 1991

Scenario (a) and the Mount Cook avalanche seismogram recorded 75 kilometers away, which is equivalent to one 3.9 magnitude earthquake (b)

It is a rare occurrence for a large meteorite to collide with the atmosphere or the earth's surface to cause a collision earthquake. A miraculous example is that the Tunguska meteorite entered the earth's atmosphere in a remote area of ??Siberia on June 30, 1908. Under the stress and heat caused by the rapid slowdown of the atmosphere, the meteorite exploded at a height of less than 10 kilometers above the earth's surface. Large areas of forest were leveled. Many seismic stations in Russia and Europe, some as far away as 5,000 kilometers away, recorded seismic waves. At first people thought it was a major tectonic earthquake.

There are some records of earthquakes induced by fluid injection into deep wells or impoundment in large reservoirs, although the mechanism is still thought to be the release of strain energy by fault rupture. These examples raise the question: To what extent can water from a well or reservoir induce earthquakes that would otherwise occur many years later?

A well-documented case is the Lake Mead event, It occurred at the Hoover Dam on the Colorado River after the reservoir was filled in 1935. There was no historical record of seismic activity in the area before the lake was formed, but small earthquakes occurred frequently after the water was impounded. When the reservoir was filled with water, a local seismic station was established, and records showed that there was a close correspondence between the number of earthquakes and changes in the reservoir's water storage.

This effect is most obvious for large reservoirs with water depths exceeding 100 meters and a volume of 1 cubic kilometer.

However, most of these large reservoirs are earthquake-free, with only five of the world's 26 largest reservoirs experiencing unquestionably induced earthquakes, including the Kariba Dam in Zambia and the Aswan High Dam in Egypt. The most reasonable explanation may be that the vicinity of the well or reservoir has been strained by tectonic forces so that the fracture is almost ready to slide. The water head increases the pressure, thereby increasing the stress in the rock and driving slip; water can also weaken the rock, Reduce rock strength.

Finally, humans explode chemical explosives and nuclear devices causing explosive earthquakes. In a near-surface explosion, the seismic waves generated in the broken area propagate in all directions. When the initial P wave reaches the ground, the ground will bulge outward. If the energy is large enough, the rock and soil will be thrown around, just like in a quarry.

Of course, humans and beasts sometimes cause earthquakes, although usually very small ones, such as mechanical knocks on the ground.

4.3 Slow accumulation of elastic energy

Let us further discuss the causes of tectonic earthquakes. The forces deep in the earth cause rocks in seismic activity areas to deform, and the deformation gradually increases over time. This deformation is largely elastic, at least over millennia or so. The so-called elastic deformation refers to the volume and shape changes of rocks when force is applied, and when the force is removed, they will spring back to their original shape, just like a squeezed rubber ball. This elastic rock motion can be detected through sophisticated systematic geodetic measurements to distinguish between elastic and inelastic (i.e. irreversible) deformations.

To achieve this purpose, there are three main geodetic methods. Two ways to determine horizontal movement size. The first type uses a small telescope to measure the angles between marks on the ground. This process is called triangulation. The second type is called trilateration, which measures the distance between ground marks. In modern trilateration techniques, light (sometimes a laser beam) is reflected from a mirror at a vantage point at a certain distance, and a photoelectric rangefinder is used to measure the time it takes for the light to travel back and forth in both directions (Figure 4.4). Over long paths, the speed of light varies with atmospheric conditions. Therefore, in precision measurements, an airplane or helicopter is flown along the line of sight and the air temperature and pressure are measured so that corrections can be made. These measurements are accurate to about 1.0 centimeters at a distance of 20 kilometers.

Figure 4.4 The laser beam used for geodetic surveying in Parkfield, California, pointed at a distant mirror

The third type of measurement is to determine the vertical direction by establishing leveling lines in the field Size of movement. This type of leveling simply determines the elevation of datum points placed at different locations on the ground. Repeated measurements reveal changes between measurements. The national survey network is to set up national benchmark survey piles at fixed locations on the land. Wherever possible, the horizon line will be extended to the edge of the continent so that mean sea level can be used as a reference point for determining absolute changes in land elevation. In recent years, synchronous satellites have also been used as known reference points, using fixed points on the earth's surface to transmit radio waves to satellites for travel time ranging.

Different measurement methods show that in seismically active areas, such as California and Japan, both horizontal and vertical ground motions reach observable magnitudes. They also show that stable areas of the continents, such as the ancient blocks of Canada and Australia, have changed little, at least in the recent past. Perhaps the most important results from regional deformation measurements related to earthquakes come from California. There they began taking measurements as early as 1850 and regularly after the 1906 San Francisco earthquake. Its results play a key role in modern theories of earthquake occurrence. Measurements along the San Andreas fault system have been further improved over the past decade with an eye toward earthquake prediction. Surveyors used optical and laser beam photoelectric rangefinders to measure the distance between datum points on mountaintops on either side of the San Andreas Break. The trend changes in strain are particularly clear, with measurements indicating right-lateral deformation of the faults and little change in the length of lines that do not cross the main fault zone.

4.4 Elastic Rebound Principle

In scientific discovery, it is often not the first description of an event or the first proposal of a hypothesis that is remembered, but the events that convinced the scientific community. An event where something new was actually discovered. The now widely accepted physical principles of rupture mechanisms responsible for earthquakes were established by convincing studies of the 1906 San Andreas earthquake. Two sets of triangulation surveys were made before 1906 across the area cut by the San Andreas Fault, one in 1851-1865 and the other in 1874-1892. American engineer Reid noticed that the far point opposite the fault moved 3.2 meters in the 50 years to 1906, with the west side moving in a north-northeast direction. When these measurements were compared with a third set of data measured after the earthquake, it was found that significant horizontal shear occurred parallel to the rupture of the San Andreas fault both before and after the earthquake (see Figure 8.4 in Chapter 8) .

Since Reed's work, the seismological community has generally believed that natural earthquakes are caused by sudden sliding of the upper part of the Earth along a geological fault. This slip propagates along the fracture surface, and the speed at which this slip rupture propagates is less than the seismic shear wave speed in the surrounding rock. The stored elastic strain can cause the rocks on both sides of the fracture to rebound to a roughly unstrained position. Thus, at least in most cases, the longer and wider the deformed region, the more energy will be released and the greater the magnitude of the tectonic earthquake will be. Figure 4.5 shows the relationship between seismic moment and fault length.

Figure 4.5 The relationship between the seismic moment of a large intraplate earthquake and the length of the fault rupture zone

As shown in Figure 4.6, the forces that caused the 1906 earthquake are plotted in the diagram. Think of this illustration as an aerial view of a row of fences running vertically across the San Andreas. The fence cuts vertically across the fault, extending many meters on either side. Tectonic forces, represented by empty arrows, act to strain elastic rocks. As they slowly work, the line (the fence) bends, shifting left side relative to right side. This strain cannot continue indefinitely, and sooner or later those weak rocks, or those at the point of maximum strain, will fail. This rupture will be followed by rebound, or rebound on both sides of the rupture. In this way, D in the rocks on both sides of the fracture in Figure 4.6 jumps back to D1 and D2. Figure 4.7 shows the dislocation of a fence across a fault after the fault ruptured in the 1906 earthquake.

Figure 4.6 The result of the elastic rebound of the fence across the fault

(a) The fence across the fault bends under the action of tectonic force, and points A and B move toward Move in the opposite direction;

(b) A rupture occurs at point D, and the strained rock on both sides of the rupture springs back to D1 and D2

Figure 4.7 Cross San Andreas in the seaside area The broken fence moved 2.6 meters during the 1906 San Francisco earthquake

The land in the distance moved to the right

Since the 1906 earthquake, elastic rebound has been confirmed as a tectonic direct cause of earthquakes. Just like the tighter the clockwork is wound, the greater the elastic strain of the rock, the greater the energy stored. When the fracture breaks, the stored elastic energy is quickly released, partly becoming heat and partly becoming elastic waves. These waves constitute an earthquake.

Vertical strains in rocks are also common. In this case, elastic rebound occurs along the inclined section, causing the ground level to collapse vertically and forming a fault scarp. Fault scarps caused by major earthquakes can reach several meters high and sometimes extend for tens or hundreds of kilometers along the direction of the fault.

Experiments in rock mechanics laboratories have illuminated how strain changes in Earth's rocks in the early stages of earthquakes. In these experiments, water-saturated rock specimens are compressed in a fluid medium at high temperatures. Such studies indicate that the crust slowly strains under the action of local tectonic forces, causing a concentration of microfractures in the rock adjacent to tectonic faults. The water slowly spreads and fills the cracks and pores in the rock. The volume of the highly strained zone along the fracture increases due to the development of microcracks, and this expansion process further weakens the fracture zone. At the same time, the water in the cracks reduces the rock's binding force and reduces friction across the underlying fault plane, allowing the rock to loosen and eventually slide along a major fracture plane. Fractures deformed in this way produce elastic rebound and propagate expansion.

Earthquake foreshocks and aftershocks can also be understood by studying the crack development process near the main slip. Foreshocks are the result of strain along the fracture and microscopic fractures in the ruptured material at a time when the main fracture has not developed because physical conditions are not yet mature. Limited slip in the foreshock slightly changes the force pattern. The movement of water and the distribution of micro-cracks finally caused a larger rupture to begin, causing a main shock. The throwing and severe shaking of rock blocks along the main rupture and local heating caused the physical conditions along the rupture to be very different from those before the main earthquake. As a result, additional small fractures occur, causing aftershocks. Afterwards, the strain energy in the region gradually decreases, like a dull clock, and may return to stability after many months.

4.5 The largest earthquake in the United States in 40 years

We imagine that because a strong earthquake relieves the strain on a fault, once the aftershocks end in an area, calm will follow. But major faults are often just one of a complex grid of faults that threaten an area. Catastrophic release of strain energy on one fracture may increase stress on adjacent fractures. The largest earthquakes to hit the continental United States in recent years illustrate how unpredictable a major earthquake can be on a region's seismicity and seismic hazards.

At 4:58 a.m. on Sunday, June 28, 1992, a strong earthquake struck the town of Landers in the remote Mojave Desert in California (see Figure 4.10). The surface wave magnitude of its mainshock was 7.5. Later, it was discovered that the large trunk of the elastic rebound was broken. It was precisely because of its dislocation that it caused strong shaking in Southern California, which was felt as far away as Denver, Colorado.

The epicenter was located between the town of Landers and the Yucca Valley, about 30 kilometers northeast of the San Andreas Fault Zone. The sparsely populated settlement suffered high-intensity shaking. Mr Gobrogge described the damage to his bowling lane wall in the Yucca Valley: "It was horrific, it was horrific, it just wouldn't calm down, it just kept rocking and never stopped. "This earthquake, officially known as the Landers earthquake, occurred in the same area as the oft-cited 1952 Kern earthquake. However, because it is located in the desert, only one person died and five others were seriously injured. The earthquake destroyed more than 77 homes and damaged 4,300 households, with property damage estimated at approximately US$50 million.

In the days that followed, hundreds of seismologists and geologists came to collect data and witnessed clear evidence of rupture.

The spectacular right-moving surface dislocation forms a series of strike-slip faults, arranged in a "flying geese" shape. Each fault is adjacent to the other fault in front and is located on the right or left side in front, like a series of steps. This series of faults is connected to the main fault and is mapped on the geological map of California, but because they are separated by 10 kilometers at their ends, they were once considered separate faults. As a continuous segment of deep faults, individual faults are thought to have slipped 12,000 years ago but have not been active since. Accordingly, it is not expected that an earthquake with a magnitude of 7.5 will occur, covering all 80 kilometers of faults.

The surface slip measured along the fault reaches 2 meters near Landers, as shown in Figures 4.8 and 4.9, and the offset is approximately 5.5 meters along the northwest part of the rupture. There are also amazing 1-meter-high seismic cliffs that appear in some sections along the main fault bend.

Figure 4.8 A pair of satellite images of a 256-kilometer-wide area along the Emerson Fault in the Mojave Desert

This fault is a fault that was interrupted during the Landers earthquake. One of the strips breaks. The image on the left was taken on July 27, 1991, 11 months before the earthquake; the image on the right was taken just 27 days after the earthquake. The cracks in the ground caused by rupture during the earthquake

are clearly visible, extending from the upper left corner to the lower right corner. The displacement across the fault at this location is approximately 4 meters

Figure 4.9 Fresh section of the Emerson fault scarp showing slip (called a scratch) after the 1992 Landers earthquake

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The Randers earthquake was followed by a most unusual earthquake chain reaction. The mainshock was followed by a series of aftershocks along the sliding fault (Figure 4.10). As a rule, large shallow earthquakes are followed by a sudden and dramatic increase in seismicity over a larger area in the days that follow. Three hours after the main earthquake, another strong earthquake (MS=6.5) occurred near Big Bear Lake, and the ground was shaken again. This earthquake was caused by the slip of another fault about 45 kilometers west of the source of the first fault. Computational simulations were used to examine the stress changes in the regional fault system. The results showed that the fault slip of the Landers earthquake caused an increase in stress in some parts of the fault, and the Big Bear Lake earthquake occurred because of this. Calculations also show that the Landers earthquake may have enhanced the stress on the South San Andreas Fault, strengthening the trend of strike-slip shear, while reducing the pressure around the San Andreas against the surroundings. This force is Invisible and continuous. The concentration of these effects may increase the probability of major earthquakes in this area in the future.

Figure 4.10 Distribution map of aftershocks and faults within 25 days after the Landers earthquake in Southern California

The main shock is represented by an asterisk, and the change in color indicates that the regional earthquakes caused by 1979 to 1992 Stress changes,

The stress increases east of the Cajon Pass of the San Andreas Fault and decreases west of it

Within 24 hours immediately after the Landers mainshock, in The regional network within 600 kilometers from the epicenter detected 11 earthquakes with magnitudes greater than 3.4. According to the normal probability of earthquakes in California and Nevada, the chance of such two major events occurring consecutively is only one in a billion. Such simultaneous earthquakes rarely occur in geological history! We therefore speculate that the Landers earthquake caused this seismicity*** by directly adding elastic strains in the rocks, or by its seismic waves passing through individual fractures and causing varying stresses on them** *.

What is most difficult to understand is the significant increase in the frequency of small earthquakes along the east side of the Sierra Nevada, from south of Owen Valley to north to Long Valley Crater, 400 kilometers from Landers. The Mona Basin, the Lassen Mountains, and the northernmost Mount Shasta in Northern California, 800 kilometers away from the main rupture, also experienced significant increases in background seismic activity.

Many accelerometers were triggered by the Landers earthquake, and they plotted strong wobbling signals. Observations at many locations around the rupture source indicate that the epicenter rupture of the Landers earthquake may have started in the south and propagated northward. The ground movement at the northern end of the fault is much stronger than at the southern end of the fault. Listeners can experience the same effect, like an increase in sound intensity as a loudspeaker is moved closer. The technical term is directional focusing, which describes the concentration of energy in one direction caused by the movement of the wave source. Because the direction of the rupture varies, the movement can be larger or smaller than average, so the intensity of the ground motion depends on the direction of the rupture.

4.6 Earthquake Moment

The most useful measure of the overall size of an earthquake is derived from a mechanical model of sudden slip on a fault surface due to tectonic stresses. This measure, mentioned in Chapter 3, is called seismic moment. It was proposed by American seismologist Aki in 1966. It is now popular among seismologists because it is directly linked to the physical nature of the fault rupture process. It can be used to infer the geological characteristics of active fault zones.

The mechanical concept of moment can be described by a simple experiment. Place your hands on either side of the heavy square table and push with one hand and pull with the other in a horizontal direction. The wider the hands are apart, the easier it is for the table to turn. In other words, the force required to rotate the table decreases as the leverage of the two arms increases. These two forces with the same magnitude and opposite directions are called a couple. The magnitude of this couple is called the moment, and its magnitude is determined by multiplying the value of one of the two forces by the distance between them.

This concept can be extended to the system of forces that cause slippage on geological faults.

In this case, ground