In April 1906, an earthquake struck San Francisco, killing more than 3,000 people and leaving 300,000 homeless. 83 years later, another thing happened, although not so terrible in terms of consequences. Catastrophists predict: sooner or later there will be a big earthquake that will level San Francisco to the ground, and the city will disappear into huge gaps in the earth's crust. And the reason for this is a crack in the ground called the St. Andreas Fault. Can a terrible earthquake be caused artificially? Where the continents are rushing and what forces pushed Africa away from South America - The New Times was looking for answers to these questions
Yuri Panchul, Sunnyvale, California
During the Cold War, there was a story that there was a Soviet nuclear missile aimed at a certain point (“water tower”) in California, which would cause the state’s crust to split into two pieces. The western portion would then be flooded by the Pacific Ocean, killing most of the 30 million Californians, including residents of Los Angeles and San Francisco. Of course, this story was not born in the USSR Ministry of Defense, but was a distorted account of the 1978 Hollywood film “Superman”.
1300 km of fear
But is there a grain of reality in this story? Along the coast of California there really is a 1,300-kilometer-long San Andreas fault, separating the Pacific and North American tectonic plates. The San Andreas (together with the adjacent Hayward, Calaveras and other faults) is a source of large earthquakes.
In some places the San Andreas is visible as a ravine, in other places it is almost invisible. The eastern and western sides of the fault move parallel to each other: the western - to the north, and the eastern - to the south. The movement of the plates occurs approximately at the rate of growth of human nails - 3–4 centimeters per year. This movement can be seen on the roads that cross San Andreas: displaced road markings and signs of regular road repairs are visible at the fault site. The most visible manifestation of the “work” of the fault is the ancient volcano Ninah, which was formed 23 million years ago, after which it was neatly, like a cake, “cut” by the San Andreas fault into two halves, and the left half “went” along the fault over millions of years 314 kilometers north and became Pinnacles National Monument.
Where are the continents heading?
What forces move thousands of kilometers of pieces of the earth's surface? Until the 20th century, the answer to this question was unknown. More precisely, there was not even a question: geological science believed that the continents were motionless, and sections of the earth’s crust moved only down and up, according to the theory of geosynclines accepted in the mid-19th century.
But since the 16th century, cartographers have noticed that the coasts of Africa and South America may be superimposed on each other, like two pieces of a broken plate, after which some researchers have periodically put forward the idea that the continents are moving. The German scientist Alfred Wegener gave the most arguments. In 1915, Wegener showed that the coasts of different continents not only coincide in contour, but also contain the same types of rocks, as well as fossils of similar animal species. Wegener suggested that 200 million years ago there was a single supercontinent Pangea, which subsequently split into parts that became modern Eurasia, America, Australia and Antarctica. For 50 years, Wegener's theory was considered a bunch of random coincidences, since geophysicists believed it was impossible that a continent (a mass of rock) could move on another mass of rock (the solid floor of the oceans) without being destroyed by friction. The situation changed only after World War II, when the US military, using sonar, mapped the oceans and discovered in the middle of them long chains of seamounts, clearly of volcanic origin. Researcher Harry Hess showed that the floor of the Atlantic Ocean is moving in two directions from a mountain range running in the middle of the Atlantic. The spreading ocean floor carries continents like a subway escalator carries passengers.
And who moves them...
As a result of the research of Hess and other scientists in the 1960s, a revolution occurred in geology comparable to the Copernican revolution in astronomy. It turned out that the earth's crust consists of several large plates (African, North American, Pacific, Eurasian and others), as well as a large number of small plates that move at a speed of several centimeters per year, colliding with each other. Each plate is about 100 kilometers thick. Beneath the plates that form the “lithosphere” is a hot, viscous layer about 200–400 kilometers thick called the asthenosphere. Tectonic plates “float” on it, carrying continents.
When plates collide, depending on the nature of the collision, mountains (for example, the Himalayas), island chains (for example, the Japanese islands), depressions and volcanoes are formed. When the oceanic and continental plates collide, the oceanic plate moves down. This is due to the fact that the ocean crust has a different chemical composition and greater density. Gerry Hess called the process a “conveyor belt”: new crust is born from solidified lava in the middle of the ocean, moves slowly for millions of years, after which it sinks back into the depths and melts.
Why do plates on the San Andreas Fault move sideways and not towards each other? The fact is that for 40 million years, a complex “dance” of three tectonic plates (Pacific, Farallon and North American) took place in the region, the boundaries between which passed at an angle to each other. The Farallon plate was “pushed” under the North American plate, after which the Pacific plate began to slide sideways along the former boundary of the Farallon and North American plates.
Tectonic plates are like froths driven by the convection currents of boiling soup. In the 19th century, scientists did not understand how this “soup” could continue to “boil” at all. According to the calculations of the famous physicist William Thomson (Lord Kelvin), according to the laws of thermodynamics, the Earth should have cooled in just 20 million years. This contradicted geologists' estimates of the age of the Earth. Thomson did not take into account the heating of the Earth by the decay of radioactive elements, which were discovered only at the beginning of the 20th century. Because of this heating, the Earth continues to be hot after four and a half billion years of existence. We live on a huge nuclear reactor - planet Earth!
Earth shaking
Well, okay, continents are moving, but how does this affect our lives, besides the need to periodically repair several small roads crossing the San Andreas Fault? The point is that the movement is not continuous. Each shift begins with an accumulation of stress, which is “discharged” by a jerk during a large or small earthquake. In the central part, the fault “creeps” due to thousands of microearthquakes that are not felt by humans. But sometimes the tension is not discharged for a long time, after which the movement occurs in a jump.
This happened during the 1906 earthquake in San Francisco, when in the area of the epicenter the “left” part of California shifted relative to the “right” by almost 7 meters. The shift began 10 kilometers under the ocean floor in the San Francisco area, after which, within 4 minutes, the shear pulse spread across 430 kilometers of the San Andreas Fault - from the village of Mendocino to the town of San Juan Bautista.
The main villain's plan
Thus, it is impossible to flood coastal California with a targeted nuclear explosion on the San Andreas fault. The plates in the fault area do not move towards each other, but to the sides (along the north-south line), so pushing the Pacific plate under the North American plate is less realistic than sinking an aircraft carrier with a kick. But is it possible to cause serious destruction with an artificial earthquake? Oddly enough, this idea was not only tested in Hollywood films. In 1966, geologists from the US Geological Survey (USGS) noticed an unexpected sequence of earthquakes in the area of the Rocky Flats military arsenal in Colorado. The timing of the earthquakes coincided exactly with the moments when the military got rid of liquid waste by pumping it under pressure deep into the ground. Geologists conducted an experiment by pumping water into an abandoned oil field near the town of Rangeley in Colorado. For the first time in history, people artificially caused an earthquake.
After this, the USGS briefly discussed the idea of preventing large earthquakes along the San Andreas by releasing fault stress using a large number of microquakes. However, the USGS decided not to experiment, since it is clear that they would not have enough money to pay in case of an error for the complete destruction of Los Angeles or San Francisco.
It could be worse
Despite the earthquakes, California is one of the nicest places to live on Earth. Most of the state's residents live in one or two-story houses and know the safety precautions. Therefore, the significant earthquake in San Francisco in 1989 did not cause much destruction. After all, there are problems in other places on the planet - hurricanes, tsunamis or unfavorable political conditions. And the San Andreas Fault is not the most dangerous geological feature in the United States. For example, there is the Yellowstone supervolcano, which about two million years ago covered the entire western half of the modern United States with ash. A huge number of animals died even thousands of kilometers from the eruption - due to dust that entered the lungs and contaminated drinking water. Such eruptions change the climate of the entire planet for years, causing a “volcanic winter.” But the topic of volcanoes and supervolcanoes deserves a separate article.
Information sources:
1. Michael Collier. A Land in Motion – California’s San Andreas Fault. Golden Gate National Parks Conservancy. University of California Press, 1999.
2. Allan A. Schoenherr. A Natural History of California. University of California Press, 1995
3. Sandra L. Keith. Pinnacles National Monument. Western National Parks Association. 2004.
4. Bill Bryson. A Short History of Nearly Everything. Broadway Books, 2005.
5. Wikipedia – Plate Tectonics, San Andreas Fault, Supervulcano, etc.
6. Man-made earthquake – http://www.usgs.gov/newsroom/article.asp?ID=343
At first glance, the streets of Taft, in central California, are no different from the streets of any other city in North America. Houses and gardens along wide avenues, parking lots, street lights every few steps. However, a closer look reveals that the line of the same lamps is not entirely straight, and the street seems to twist, as if it were taken by the ends and pulled in different directions.
The reason for these oddities is that Taft, like many large urban centers in California, is built along the San Andreas Fault - a crack in the earth's crust, 1050 km of which runs through the United States.
The strip, which stretches from the coast north of San Francisco to the Gulf of California and extends approximately 16 km inland, represents the line between two of the 12 tectonic plates on which the Earth's oceans and continents are located.
Let's find out more about him...
Photo 2.
The average thickness of these plates is about 100 km, they are in constant motion, drifting on the surface of the liquid inner mantle and colliding with each other with monstrous force as their location changes. If they creep on top of each other, huge mountain ranges such as the Alps and Himalayas rise into the sky. However, the circumstances that gave rise to the San Andreas Fault are completely different.
Here, the edges of the North American (on which much of this continent rests) and Pacific (which supports most of the Californian coast) tectonic plates are like ill-fitting gear teeth that do not fit one another, but do not fit neatly into the grooves intended for them. The plates rub against one another, and the friction energy generated along their boundaries has no outlet. Where such energy accumulates in the fault determines where the next earthquake will occur and how strong it will be.
Photo 3.
In the so-called “floating zones,” where plate movement occurs relatively freely, the accumulated energy is released in thousands of small tremors, causing virtually no damage and recorded only by the most sensitive seismographs. Other sections of the fault - they are called “lock zones” - seem completely motionless, where the plates are pressed against one another so tightly that no movement occurs for hundreds of years. The tension gradually increases until finally both plates move, releasing all the accumulated energy in a powerful jerk. Then earthquakes occur with a magnitude of at least 7 on the Richter scale, similar to the devastating San Francisco earthquake of 1906.
Photo 4.
Between the two described above lie intermediate zones, whose activity, although not as destructive as in the castle zones, is nevertheless significant. The city of Parkfield, located between San Francisco and Los Angeles, lies in this intermediate zone. Earthquakes with a magnitude of up to 6 on the Richter scale can be expected here every 20-30 years; the last one happened in Parkfield in 1966. The phenomenon of earthquake cyclicity is unique to this region.
Since 200 AD e. There have been 12 major earthquakes in California, but it was the 1906 disaster that brought the San Andreas Fault to the attention of the whole world. This earthquake, with its epicenter in San Francisco, caused destruction over a colossal area stretching from north to south for 640 km. Along the fault line, the soil shifted 6 m in a matter of minutes - fences and trees were toppled, roads and communication systems were destroyed, the water supply stopped, and fires that followed the earthquake raged throughout the city.
Photo 5.
As geological science has developed, more advanced measuring instruments have appeared that can constantly monitor the movements and pressure of water masses under the earth's surface. For a number of years before a major earthquake, seismic activity increases slightly, so it is quite possible that they can be predicted many hours or even days in advance.
Architects and civil engineers take into account the possibility of earthquakes and design buildings and bridges that can withstand a certain amount of ground vibration. Thanks to these measures, the 1989 San Francisco earthquake destroyed mostly older structures without causing damage to modern skyscrapers.
Photo 6.
Then 63 people died - most due to the collapse of a huge section of the double-decker Bay Bridge. According to scientists, California is facing a serious disaster in the next 50 years. An earthquake with a magnitude of 7 on the Richter scale is expected to occur in southern California, in the Los Angeles area. It could cause billions of dollars in damage and claim 17,000-20,000 lives, with smoke and fires potentially killing an additional 11.5 million people. And because frictional energy along a fault line tends to accumulate, each year that gets us closer to an earthquake increases its likely severity.
Photo 7.
Lithospheric plates move very slowly, but not constantly. The movement of the plates occurs approximately at the rate of growth of human nails - 3-4 centimeters per year. This movement can be seen on roads that cross the San Andreas Fault: displaced road markings and signs of regular road repairs are visible at the fault site.
Photo 8.
In the San Gabriel Mountains north of Los Angeles, the asphalt of streets sometimes bulges as forces accumulating along a fault line put pressure on the mountain range. As a result, on the western side, rocks compress and crumble, annually forming up to 7 tons of fragments, which are getting closer and closer to Los Angeles.
Photo 9.
If the tension of the layers is not discharged for a long time, then the movement occurs suddenly, with a sharp jerk. This happened during the 1906 earthquake in San Francisco, when in the area of the epicenter the “left” part of California shifted relative to the “right” by almost 7 meters
The shift began 10 kilometers under the ocean floor in the San Francisco area, after which, within 4 minutes, the shear impulse spread along 430 kilometers of the San Andreas fault - from the village of Mendocino to the town of San Juan Bautista. The earthquake measured 7.8 on the Richter scale. The whole city was flooded.
By the time the fires broke out, more than 75% of the city had already been destroyed, with 400 city blocks in ruins, including the center.
Photo 10.
Two years after the devastating earthquake in 1908, geological research began, which continues to this day. Research has shown that over the past 1,500 years, major earthquakes have occurred along the San Andreas Fault approximately every 150 years.
Photo 11.
Plate tectonics is the main process that largely shapes the appearance of the Earth. The word “tectonics” comes from the Greek “tekton” - “builder” or “carpenter”; in tectonics, plates are called pieces of the lithosphere. According to this theory, the Earth's lithosphere is formed by giant plates that give our planet a mosaic structure. It is not continents that move across the surface of the earth, but lithospheric plates. Moving slowly, they carry continents and the ocean floor with them. The plates collide with each other, squeezing out the earth's surface in the form of mountain ranges and mountain systems, or are pushed inwards, creating ultra-deep depressions in the ocean. Their mighty activity is interrupted only by brief catastrophic events - earthquakes and volcanic eruptions. Almost all geological activity is concentrated along plate boundaries.
Photo 12.
San Andreas Fault The thick line running down from the center of the picture is a perspective view of California's famous San Andreas Fault. The image created using data collected by SRTM (Radar Topographic Imaging) will be used by geologists to study the dynamics of faults and the shapes of the Earth's surface resulting from active tectonic processes. This segment of the fault is located west of Palmdale, California, about 100 km northwest of Los Angeles. The fault represents an active tectonic boundary between the North American Plate on the right and the Pacific Plate on the left. In relation to each other, the Pacific platform is away from the viewer, and the North American platform is towards the viewer. Two large mountain ranges are also visible: the San Gabriel Mountains on the left and the Tehachapi Mountains on the upper right. Another fault, the Garlock, lies at the foot of the Tehachapi ridge. The San Andreas and Garlock faults meet in the center of the image near the town of Gorman. In the distance, above the Tehachapi Mountains, lies California's Central Valley. Antelope Valley can be seen along the base of the hills on the right side of the image.
Photo 13.
Photo 14.
The San Andreas Fault runs along the line of contact between two tectonic plates - the North American and Pacific. The plates move relative to each other by about 5 cm per year. This creates severe stresses in the crust and regularly causes large earthquakes centered on the fault line. Well, small tremors happen here all the time. Until now, despite the most careful observations, it has not been possible to identify signs of an upcoming large earthquake in the data on weak tremors.
The San Andreas Fault, which cuts across the west coast of North America, is a transform fault, that is, one where two plates slide along each other. Near transform faults, earthquake foci are shallow, usually less than 30 km below the Earth's surface. The two tectonic plates in the San Andreas system move relative to each other at a rate of 1 cm per year. The stresses caused by the movement of the plates are absorbed and accumulated, gradually reaching a critical point. Then, instantly, the rocks crack, the plates shift and an earthquake occurs.
Photo 15.
Photo 16.
Photo 17.
Photo 18.
Photo 19.
Photo 20.
This is not a still from the filming of another disaster movie, or even computer graphics.
Here we looked at this earthquake in the USA in detail - FILM DISASTER IN REALITY
Some of the world's largest megacities are located right in the area of the most dangerous faults in the earth's crust. Californians living along the San Andreas fault line are constantly threatened by devastating earthquakes.
At first glance, the streets of Taft, in central California, are no different from the streets of any other city in North America. Houses and gardens along wide avenues, parking lots, street lights every few steps. However, a closer look reveals that the line of the same lamps is not entirely straight, and the street seems to twist, as if it were taken by the ends and pulled in different directions. The reason for these oddities is that Taft, like many of California's major urban centers, is built along the San Andreas Fault, a crack in the Earth's crust that runs 1,050 km across the United States.
The strip, which stretches from the coast north of San Francisco to the Gulf of California and extends approximately 16 km inland, represents the line between two of the 12 tectonic plates on which the Earth's oceans and continents are located.
The average thickness of these plates is about 100 km, they are in constant motion, drifting on the surface of the liquid inner mantle and colliding with each other with monstrous force as their location changes. If they creep on top of each other, huge mountain ranges such as the Alps and Himalayas rise into the sky. However, the circumstances that gave rise to the San Andreas Fault are completely different.
Here, the edges of the North American (on which much of this continent rests) and Pacific (which supports most of the Californian coast) tectonic plates are like ill-fitting gear teeth that do not fit one another, but do not fit neatly into the grooves intended for them. The plates rub against one another, and the friction energy generated along their boundaries has no outlet. Where such energy accumulates in the fault determines where the next earthquake will occur and how strong it will be.
In the so-called “floating zones,” where plate movement occurs relatively freely, the accumulated energy is released in thousands of small tremors, causing virtually no damage and recorded only by the most sensitive seismographs. Other sections of the fault - they are called “lock zones” - seem completely motionless, where the plates are pressed against one another so tightly that no movement occurs for hundreds of years. The tension gradually increases until finally both plates move, releasing all the accumulated energy in a powerful jerk. Then earthquakes occur with a magnitude of at least 7 on the Richter scale, similar to the devastating San Francisco earthquake of 1906.
Between the two described above lie intermediate zones, whose activity, although not as destructive as in the castle zones, is nevertheless significant. The city of Parkfield, located between San Francisco and Los Angeles, lies in this intermediate zone. Earthquakes with a magnitude of up to b on the Richter scale can be expected here every 20-30 years; the last one happened in Parkfield in 1966. The phenomenon of earthquake cyclicity is unique to this region.
Since 200 AD e. There have been 12 major earthquakes in California, but it was the 1906 disaster that brought the San Andreas Fault to the attention of the whole world. This earthquake, with its epicenter in San Francisco, caused destruction over a colossal area stretching from north to south for 640 km. Along the fault line, the soil shifted 6 m in a matter of minutes - fences and trees were toppled, roads and communication systems were destroyed, the water supply stopped, and fires that followed the earthquake raged throughout the city.
As geological science has developed, more advanced measuring instruments have appeared that can constantly monitor the movements and pressure of water masses under the earth's surface. For a number of years before a major earthquake, seismic activity increases slightly, so it is quite possible that they can be predicted many hours or even days in advance.
Architects and civil engineers take into account the possibility of earthquakes and design buildings and bridges that can withstand a certain amount of ground vibration. Thanks to these measures, the 1989 San Francisco earthquake destroyed mostly older structures without causing damage to modern skyscrapers.
Then 63 people died - most due to the collapse of a huge section of the double-decker Bay Bridge. According to scientists, California is facing a serious disaster in the next 50 years. An earthquake with a magnitude of 7 on the Richter scale is expected to occur in southern California, in the Los Angeles area. It could cause billions of dollars in damage and claim 17,000-20,000 lives, with smoke and fires potentially killing an additional 11.5 million people. And because frictional energy along a fault line tends to accumulate, each year that gets us closer to an earthquake increases its likely severity.
The San Andreas Fault first came to the attention of California geologists in 1890. It is believed that the name "San Andreas Fault" was coined in 1895 (Lawson's paper; Crowell, 1962). This occurred approximately 10 years after the discovery of the longitudinal Median fault in Japan.
However, it was only after the 1906 San Francisco earthquake that the fault quickly became widely known. Along a fault line running through the western outskirts of the city, displacements of up to 7 m appeared at a distance of approximately 430 km. The appearance of this seismic fault proved for the first time that displacement continues north of San Francisco. Before this, it was traced only to the south of the city, at a distance of about 600 km.
Given the fact that the movement was sudden, it was widely believed that the 1906 earthquake was caused by movement along the fault. However, in 1911, Reid, based on precise measurements taken in the fault zone, proposed the theory of elastic recoil to explain the mechanism of earthquake generation and movement along the fault. The model of a pair of forces he proposed was adopted as the source mechanism, which was replaced in the 60s by the model of a double pair of forces. However, Reid's elastic recoil theory is still used to explain the mechanism of seismic fault formation.
The seismic event of 1906, during which movement occurred along a normal fault, gave rise to the concept and term “active fault.” Geomorphologists still come to inspect the distinct topographic features observed along the fault in order to study the topography created by active shear.
The attention of geologists was attracted by the fact that the displacements along the fault during the earthquake were horizontal. Further studies showed that over the course of geological time, horizontal displacements of several kilometers occurred on both sides of the fault. In 1953, Hill and Dibbley found that since the Cretaceous period, the magnitude of this displacement exceeded 500 km. Almost simultaneously, a hypothesis was put forward that the rocks on both sides of the Alpine Fault in New Zealand experienced horizontal displacement over a distance of about 450 km. In the 1950s, geologists everywhere began to pay attention to such large strike-slip faults or lateral faults. Moody's paper, which argues that shifts underlie all known geological structures in the world, is typical of this time. In the 1960s, the San Andreas Fault began to be viewed as an example of transform faults (Wilson, 1965). It became a touchstone for the concept of plate tectonics.
The name "active" given to the San Andreas Fault did not mean that minor movements occurred on it every day. Rather, it means the possibility that it might one day move, as happened in 1906. However, an area was subsequently discovered in the southern part of San Francisco in which the fault is literally active, and movement along it is continuous . Cracks appeared in the floor and walls of the winery located directly above the fault even when no particular seismic activity was observed. In 1960, these unusual phenomena were determined to reflect movement along the fault and were reported in academic circles. It was from the San Andreas Fault that geologists learned that continuous movement could actually exist as a type of fault activity. This phenomenon was called tectonic creep. Later it was also observed in the North Anatolian Rift Zone in Turkey.
Thus, the San Andreas Fault and its activity have had a significant impact on the development of geosciences. In this chapter we are going to focus mainly on its geological features.
Fault distribution and structure
In Fig. 2.II.1 shows the general layout of the San Andreas fault. From Point Arena, 160 km north of San Francisco, it stretches almost in a straight line to the southeast, past San Francisco. Then it cuts through the Coastal Ranges and, crossing the Transverse Ranges, reaches the depression in which the lake is located. Salton Sea. In the north, near Point Arena, it goes out to sea, and in the Shelter Cove area, south of Cape Mendocino, it changes direction to sublatitudinal, moving into a large fragmentation zone (Mendocino fracturing zone) on the bottom of the Pacific Ocean. The southern end of the fault extends into Mexico, where it connects with the Eastern Pacific Rise in the southern Gulf of California. The length of the fault only on land (from Shelter Cove to the northern shores of the Gulf of California) is about 1300 km. Its direction on the map is generally northwest to southeast, but in the north of the Transverse Ranges, north of Los Angeles, it becomes almost exactly latitudinal, and the fault line forms a noticeable bend. In this area, in addition, several other large faults have been discovered that extend in the northeast-southwest direction. The geological structure and topography of the main fault here becomes more complex. This segment is called the Big Bend. To the north and south of it, not only is the general extent of the fault different, but to the south it branches into several large faults. The amount of displacement of geological complexes along the fault in the south is definitely less than in the north.Directly northwest of Big Bend lies the famous Carrizo Plain, a semi-desert intermontane basin. Several fine examples of fault-related landforms have been discovered along its northern edge. Even further north, the fault appears in the lowlands located around San Francisco Bay, stretching across the plains between the Diablo and Gabilan ridges. Here the Calaveras and Hayward faults branch off to the north. Not far from this place is the town of Hollister, on the streets of which the stone walls of houses are distorted by tectonic sliding. North of Hollister, the fault crosses the hills that border the western edge of the San Francisco Bay lowlands, extending further north along the seafloor for a distance of about 10 km west of the Golden Gate. San Francisco International Airport is located only a few kilometers east of the San Andreas Fault. During landing or takeoff, you can observe spectacular linear near-fault landforms and lakes. San Andreas, which lies on the fault and gives it its name.
In southern California, south of the Big Bend, the San Andreas Fault, west of Los Angeles, branches into the Banning and Mission Creek faults. Further west, other faults (San Gabriel and San Jaquinto) run almost parallel. The Salton Sea, with its eastern portion crossed by the San Andreas Fault, is a long, narrow strip below sea level; it has many features associated with the fault, such as shallow volcanic cones and hot springs. This lowland continues south into the Gulf of California.
As already mentioned, the San Andreas Fault is accompanied by a number of similar faults that run almost parallel. They are usually considered together and called the "San Andreas Fault System".
Although small-scale diagrams (see Fig. 2.II.1) depict the San Andreas Fault as a single line, more detailed maps (scale 1:250,000 or 1:50,000) show that it consists of several lines. In general, they form a fault zone a few kilometers wide (the fault system described earlier is a combination of fault zones). A number of lens-shaped scales were discovered within the fault zone (Fig. 2.II.2). The substance of which they are composed often differs from the substance of the surrounding rocks. Their formation is associated with movement along a fault, which causes the separation and movement of rocks on either side of it. It is believed that the development of this type of fault zone is due to the fact that the slip surface (fault plane) formed in the rock, for some reason, becomes inactive, and that new slip planes form nearby. In general, the strike of a fault at an early stage of activity will not be exactly parallel to the overall strike and may be highly curved. In contrast, fault lines active in the Quaternary are relatively straight. Based on these facts, there is an idea that ancient faults developed en echelon, at a later stage of movement they are connected and at the last stage a smooth fault line appears. However, there is another hypothesis that attributes these differences to mechanical heterogeneity in the rocks adjacent to the fault, as shown in Fig. 2.II.3 (Rogers, 1973). This hypothesis considers the sequence in which localized plastic deformation of rocks occurs as a result of their different properties. Initially this leads to bending of the primary fault line, subsequently to an increase in frictional resistance in the curved section and finally to the formation of a new and straight fault line with relatively low frictional resistance. In addition, some collapse and collapse of sedimentary layers deposited in the fault zone may occur as a result of their vertical displacement accompanying the strike-slip fault. In any case, the San Andreas Fault has a well-developed wide fault zone, indicating a complex history of development.
Rocks in the immediate vicinity of the fault plane, under the influence of movements along it, are often intensely schistose, crushed and cracked, which is visible both with the naked eye and under a microscope. Such rocks are considered under the general name “cataclastic rocks”. When shear movements along a fault occur relatively deep, under the influence of high confining pressure, the rocks remain externally undisturbed, but microscopic examination reveals that they have experienced internal fragmentation. Under conditions of low geostatic pressure, crushed rocks become increasingly clayey and “fault gouge” or “fault pug” appear. It is known that such friction clay is often established along fault lines active in the Quaternary period in the San Andreas fault zone.
Based on observations of fault planes within the fault zone and its linear distribution, it can be concluded that the dip of the San Andreas fault is subvertical. Detailed seismic studies have shown that underground microearthquakes propagate along a plane, following the fault zone, and that this plane is subvertical. The origin of these microearthquakes is limited to depths of 10-20 km or less. Deeper down, no earthquakes occur, and it is likely that the relative displacement of the two sides of the fault at depth is replaced by plastic deformation.
Movements along the fault in Paleogene-Neogene and pre-Paleogene times
In 1953, Hill and Dibbley published an important scientific paper on the San Andreas Fault. Using Dibbley's geological surveying experience and the data available at the time, they concluded that the older the strata along a fault, the greater the dextral displacement, with values as high as 500 km for Cretaceous sediments. Information about the age and degree of displacement of the various layers has subsequently become more accurate, and virtually no one now disputes the existence of dextral displacement of 300 km or more that occurred from the Miocene to the present.Much work has been done to study the displacement of layers of Paleogene-Neogene and Cretaceous age (Fig. 2.II.4). The most numerous and reliable data on displacement are in Miocene rocks. Marine and continental sediments of various Miocene phases are widespread on both sides of the fault. All ancient geographical features of these strata, such as depositional basin shapes, thickness and distribution of sediments, sedimentary facies, especially the distribution of marine and continental strata, which gives an idea of the ancient coastline, and the distribution of fossil fauna, typical pebbles or sands contained in sediments , are unnaturally interrupted along the fault line (Addicott, 1968; Huffman, 1972). If these rocks are moved back along the fault line and combined, the Miocene volcanic rocks east of Big Bend coincide with the development of similar Miocene volcanic rocks in the Gabilan Range, south of San Francisco. Not only do these volcanic rocks resemble each other in petrology and stratigraphic sequence, they are also found to be identical in radiometric and trace element ages. This study made it possible to establish with complete certainty that at the turn of 23.5 million years ago there was a right-lateral displacement of about 310 km, 22 million years ago - about 295 km, and 8-12 million years ago - 240 km.
In addition, attempts have been made to restore the paleogeographic settings for the Eocene and Cretaceous layers. It was established that at the turn of 44-49 million years ago there was a dextral displacement of about 305 km (Clark and Nilsson, 1973), and since the deposition of the Cretaceous layers - a distance of about 500 km. It was noted that the magnitude of the shift, which was approximately 305 km over a period of time of 44-49 million years, is, within the possible error, almost equal to the magnitude of the shift, which was approximately 310 km over 23.5 million years. Displacement distances for pre-Cretaceous periods have been determined from the apparent displacements of pre-Cretaceous granitic basement rocks (Salinian blocks) developed on the western side of the fault relative to similar basement rocks on the eastern side (approximately 500 km), but exact figures are not clear. This is due to the fact that the northern boundaries of the Salinian blocks, west of Bogueda Head, 70 km north of San Francisco, have not yet been precisely established. The same is true of the situation on the eastern side, from where they migrated. However, recent studies of Sr isotope ratios in the Salinian blocks indicate a displacement of approximately 510 km, which is fully consistent with calculations carried out so far.
In Fig. 2.II.5 shows rock displacements during different periods of time. The graph shows that during the periods between 50 and 20 million years (Eocene - early Miocene) there was almost no activity along the San Andreas fault. It was revived between 20 and 10 million years ago and continues to the present day, and the rate of displacement is increasing.
Virtually all of the data discussed previously was obtained from the area located north of Big Bend. South of the bend, research is greatly hampered by the development of parallel or even left-lateral strike-slip faults at almost right angles to the main fault, each with its own history of development (Crowell, 1973). However, it should be noted that south of Big Bend, dextral displacement of about 300 km has only been established since the deposition of the Miocene formations and no evidence of earlier displacement has been obtained. In southern California, Miocene formations found southwest of the Big Bend (near Tejon), together with pre-Tertiary basement rocks along the San Andreas and San Gabriel faults, which strike parallel to the west (Crowell, 1962, 1973), are offset toward south for a distance of about 260 km (to the Orocopia mountains). Since pre-Tertiary basement rocks containing Precambrian rocks are comparable in both areas, activity along these faults probably began during or after deposition of the Miocene formations (ca. 12 Ma).
To summarize the above, it should be noted that the San Andreas fault in southern California appears to be relatively recent, and the total displacement along it is only half that observed north of the Big Bend (500-600 km). Therefore, many researchers believe that faults other than the current San Andreas Fault were once active in southern California, and that this explains the lack of 200-300 km in displacement. For example, Sappe believed that the Newport-Inglewood fault near Los Angeles (see Fig. 2.II.1) in the Paleogene was a continuation of the San Andreas fault, located north of the Big Bend, and the missing displacement of 300 km occurred there. Sappe called it the “proto-San Andreas fault” and constructed a reconstruction in which he moved the western pre-Cretaceous Salinian blocks along this fault south of the eastern side (see section VI, Fig. 2.VI.2).
Quaternary movements along the fault
We mentioned earlier that part of the San Andreas Fault is currently experiencing continuous movement. Careful measurements indicate an average annual speed of a few centimeters (5 cm or less), varying depending on location and time. Over the past 60 years the average rate of traffic in the southern part of Hollister, as can be inferred from the horizontal movement of old fences on farms, etc., has been no more than 2 cm/year. This type of fault creep is not found at all further south in the Carrizo Lowlands or around the Big Bend. However, extensive topographic evidence, namely curved valley contours, displaced rivers, and displacement during the great earthquake of 1857 (a right-lateral slip of approximately 10 m), suggests that fault displacement in these areas occurs only during large earthquakes, such as in 1857, which occur once every few hundred years. If such a rare large displacement associated with an earthquake is averaged over time, the rate of displacement along the fault is still equal to 2-4 cm per year, which is very similar to the rate of displacement in areas of tectonic sliding.These shear rates are less than the horizontal slip rate (about 5 cm/yr) expected from the horizontal deformation rates in the fault zone as determined by geodetic measurements. They are also less than the relative rate of separation of the Pacific and American plates, which was calculated from the rate of ocean floor spreading in the Gulf of California (about 6 cm/year). As we will show below, this is likely because the San Andreas Fault is affected by only a portion of the relative displacement of the two plates. The missing part of the displacement is realized through displacements along other faults and turns into deformation of the earth's crust over a vast area that covers the western margins of the American continent from Western California through the Sierra Nevada mountains to the Basin and Range province in the east. If a geological survey reveals the juxtaposition of strata of different ages along a fault, then it is easier for us to assume that this is due to the displacement of the foundation blocks up and down on both sides of the fault. However, such a position can occur without any upward or downward displacement at all, since the layers are not infinite in the horizontal direction and, moreover, are not horizontal. It is quite possible that they will take a position opposite layers of a different age simply as a result of displacement along strike. "Horizontalists" pointed this out in connection with the history of the San Andreas Fault (Hill and Dibbley, 1953; Crowell, 1962).
The topography along the San Andreas Fault shows strong evidence that vertical displacement occurred in some areas at least during the Quaternary. However, it can be said that this fault is an almost perfect macroscopic example of long-lived slip. Despite the vast periods of geological time that have passed since then, it turns out that layers formed under almost identical depositional conditions at the same time are still located at approximately the same height, even if horizontally they are displaced by a distance of 300 km or more.
As a result of movements that occurred during the Quaternary period, numerous large and small depressions and hills formed along the fault line. By tracing these landforms along the fault line, it is easy to notice that the direction of vertical displacement varies within a short distance. For example, in the Carrizo Valley, long narrow hills located along the fault line and formed as a result of the relative uplift of the southwestern flank of the fault gradually decrease over several hundred meters with a significant gradient along strike, while the northeastern flank, in contrast, becomes uplifted. At the foot of such hills, graben-shaped depressions are often located on the fault line, but over a short distance they become shallow, narrow and disappear among the hills. The origin of such alternating relief forms along an almost ideal shear is believed to be explained by the fact that in the case of a shear along a fault plane that is not perfectly flat in the geometric sense, localized stretching and compression occur in curved areas of the earth's crust, causing the formation of lowered and elevated surface forms relief, respectively. In New Zealand, the fact that the distribution of such vertical displacements along the shear line is not uniform in either space or time has been seriously studied; this is considered one of the characteristic features of shifts.
San Andreas Fault as a plate boundary
World plate maps show the San Andreas Fault as the boundary between the Pacific and American plates. The banded pattern of magnetic anomalies on the Pacific Ocean floor off the coast of California south of the Mendocino Fracture Zone indicates that the age of the ocean floor decreases as it approaches California. Therefore, the oceanic ridge in which this ocean floor was formed has probably already disappeared under the American continent. It can be assumed that the Gorda and Juan de Fuca underwater ridges off the coast of northern California and the East Pacific Rise, which extends all the way to the Gulf of California from the south, are the remnants of this oceanic ridge. In this sense, the San Andreas Fault is a transform fault connecting two northern and southern ocean ridges (Wilson, 1965; Atwater, 1970).The age of the ocean floor bordering the American continent off the coast of California is greatest (29 million years) at Cape Mendocino in the zone of the northern section of the San Andreas Fault. It gradually becomes younger towards the south, and in the Gulf of California in Mexico it is only about 4 million years old. Thus, it is believed that the oceanic ridge from which this bottom was formed, moving from the west, came into contact with the subduction zone along a deep-sea trench off the coast of California near Cape Mendocino about 29 million years ago, was absorbed by this trench and disappeared, under the American continent. At that time, the direction of the ridge (submeridional) and the trench (northwest - southeast) were not parallel (Fig. 2.II.6), and therefore the ridge was plunging from the north. As a result, the trench turned into a transform fault (San Andreas Fault). (In the geometry of plate tectonics, this would occur in the situation shown in Fig. 2.II.6). Thus, the transform fault propagated southward, replacing the oceanic trench, and reached the Gulf of California about 4 million years ago.
These conclusions, obtained from the study of the oceanic plate, mean that the San Andreas Fault originated and displacement along it began about 29 million years ago. The southwestern side of the fault was also likely an oceanic plate. However, no considerations are consistent with the geological data for the continent that we reviewed above. How can you explain them? The explanation presented by Atwater and Garfunkel is as follows. The transform fault, which began to develop off the coast of California 29 million years ago, was not the San Andreas fault itself. The fault that preceded the modern one existed on the American continent before this time, and the displacement along it was right-sided. 29 million years ago, the land block (areas covered with dots in Fig. 2II.6, c and d) between the above-mentioned newly formed transform fault (slip fault in Fig. 2.II.6, c and d) and the existing San Andreas fault gradually connected with the coastal transform fault and began to move along with the Pacific plate. The relative displacement of the American plate at that time mainly occurred along the eastern margins of this block, namely along the modern San Andreas fault. Beginning in the Miocene and later, the rate of dextral displacement along the San Andreas fault increased (see Fig. 2.II.5) due to the fact that over time the degree of coupling of the transform fault to the eastern margin of the continental block increased. Since the time of transformation of the oceanic trench into a transform fault occurred immediately after the absorption of the ridge, the plate boundary was still hot and soft and slid along the axis of the trench. Over time, however, it cooled and hardened, making movement so difficult that displacement occurred primarily along an existing weakness in the continent, namely the San Andreas Fault.
Thus, the general pattern of movement along the San Andreas Fault, at least after the mid-Tertiary period, is similar to the pattern of relative movement of the two plates, American and Pacific, which form part of the world plate system.
Several other major strike-slip faults of the San Andreas Fault class (1000 km) are known on other continents. Most of them are active and are well recorded topographically in images from space. The main examples of the Pacific Ring Belt are the Denali fault system in Alaska (about 2000 km long, with a dextral displacement of 400-700 km), the longitudinal Median fault in Japan (approximately 1000 km, dextral displacement), the Philippine fault zone (about 1300 km long, with left-sided displacement), the Great Sumatran fault zone on the island. Sumatra (about 800 km, right-lateral displacement), Alpine Fault in New Zealand (about 1000 km, right-lateral displacement approximately 450 km), Atacama Fault in Chile (approximately 800 km long, with right-lateral displacement), etc. In Eurasia, the Altyntag fault can be noted (about 1500 km long, left-sided displacement) on the territory of the PRC, along with the Talas-Fergana fault in the Kyrgyz-Kazakhstan region of the USSR (900 km long, with a right-sided displacement of 250 km); the Herat faults (1100 km or more long, dextral displacement), Chamen (800 km long, 500 km sinistral displacement) and the North Anatolian fault in Turkey (900 km long, dextral displacement).
Majestic, clear straight lines cut into the surface of the Earth - this is how these faults appear on space photographs. One of the goals of the Earth sciences must be to explain the origin of these shifts with horizontal displacements of hundreds of kilometers.
Highway workers in the Californian city of Hayward repaired a displaced curb that was a clear example of the activity of the Hayward Fault. Seismologists have been monitoring this curb for more than 45 years. /website/
The famous curb was located at the intersection of Rose and Prospect streets. It gradually shifted relative to the other curb, which aroused great interest among scientists. From 1974 to 1979, the curb, along with part of the road, moved about two centimeters. Over time, the border slabs lying nearby stopped touching completely.
However, local authorities decided to repair the road and installed a wheelchair ramp at the site of the shift. It turned out that the city administration simply did not know about the importance of this place. “If we had known about the shift, we probably would have looked at the curb differently and even tried to help scientists document it,” said Assistant City Manager Kelly McAdoo.
"It is sad. It was a real disappointment. It was indeed unusual to have such evidence of a fault right here. Now all scientists are left with are photographs that document the pavement slipping over the years,” Auckland science journalist Andrew Alden wrote on his blog.
This shift most clearly demonstrated the movement of the Pacific and North American tectonic plates, but there was other evidence of underground activity. In addition to the displacement of curbs, seismologists observed cracks in the asphalt, divergence of columns in the colonnade of a sports stadium, and other signs. Scientists collected more accurate data using high-precision sensors installed at the fault boundaries.
Studying the displacement of the curb was very important for scientists, since the movement of the Pacific and North American tectonic plates could cause a strong earthquake in the near future. In addition, the displacement of these plates caused a much more dangerous fault - the San Andreas Fault. While the Hayward Fault accounts for some of the overall movement between the plates, the San Andreas Fault is the major transformation of the boundary between the Pacific and North American plates.
San Andreas Fault
This fault extends 1,300 kilometers along the coast of the state of California, mostly on land. The fault goes deep about 16 kilometers. The thickness of the lithospheric plates is approximately 100 kilometers. They drift along the liquid lava, creeping one on top of the other, causing earthquakes and other disasters.
The edges of two plates on the San Andreas Fault resemble poorly fitting teeth on a gear. They rub against each other, and the friction energy generated along their boundaries finds no outlet. In places where plate movement occurs relatively freely, the accumulated energy is released in thousands of small shocks. They cause almost no damage and are recorded only by sensitive instruments.
In other places, the plates are pressed quite tightly, and when they move, they immediately release powerful energy. Then earthquakes with a magnitude of at least 7 on the Richter scale occur. Such an earthquake could occur in the next 50 years, seismologists say. It could cause billions of dollars in damage and claim up to 20,000 lives.
Double break
San Andreas is considered the most likely location for the next seismic event in the next few decades. However, the disaster could be more devastating if San Andreas activity impacts the San Jacinto Fault, which runs through San Bernardino, Riverside, San Diego and Imperial County in Southern California.
Juliana S. Lozos, an assistant professor of geological sciences at the University of California, Riverside, determined that a similar event occurred about 200 years ago. It caused a powerful shock that was felt over a large area from Los Angeles to San Diego. The magnitude 7.5 San Juan Capistrano earthquake that occurred on December 8, 1812 was the result of two faults rupturing simultaneously, Lozos said.
It was previously believed that the earthquake was caused by the San Andreas Fault. However, computer modeling showed that the earthquake began further south - in the San Jacinto area, and then involved San Andreas in the disaster. The simultaneous activity of two faults can be very dangerous for California. The state's infrastructure was built to withstand tremors caused by a single fault. The consequences of a simultaneous rupture can be unpredictable.
Cascadia Fault
The Cascadia Fault, which extends 900 kilometers from Vancouver Island to Northern California, also poses a serious danger to the United States. Cascadia lies at the junction of the oceanic plate and the North American continental plate. The ocean plate compresses the continental plate, as a result of which it shrinks by 30–40 mm annually.
According to seismologists, sooner or later the pressure between the plates will accumulate to the limit, after which a strong shock will occur, leading to a megaearthquake with a magnitude of 8.7 to 9.2. The shock will cause a giant wave, some of which will even reach Japan. The wave can rise to a height of up to 30 meters, seismologists believe. According to the US Emergency Management Agency (FEMA), Cascadia could cause the death of 13 thousand people.
Seismologists consider Cascadia more dangerous than San Andreas, since the movement of Cascadia will entail not only an earthquake, but also a giant tsunami. Moreover, 45 years ago scientists did not know about the existence of this fault. Therefore, the United States is not prepared for such destructive events. The country's authorities have begun conducting large-scale exercises in case of a disaster in the Cascadia subduction zone. FEMA plans to conduct them in the future.
New Madrid Fault
The north of the American state of Alabama is located in the zone of influence of the New Madrid Fault. This fault is about 20 times larger than the San Andreas. The last earthquake in this seismic zone occurred in 1812. However, recently activity along the fault line has begun to increase.
"I think most people know an earthquake can happen here, but they just can't remember the last time they were shaken," said Gary Patterson, a geologist at the Memphis-based Earthquake Research and Information Center. Earthquakes that previously occurred in this region were felt at a distance of 1,000 to 1,200 kilometers from the epicenter, the scientist noted.
According to FEMA's scenario, more than 900 people in Alabama could be affected by a magnitude 7.7 earthquake. In the United States as a whole, 86 thousand residents may be affected. Computer modeling based on the 1812 earthquake showed that a repeat of the same seismic event is possible in the next 50 years.