San Andreas Fault in California. Faults in the USA: seismologists predict a catastrophe Movement along the fault in the Paleogene-Neogene and pre-Paleogene time

The San Andreas Fault first attracted the attention of Californian geologists in 1890. It has been argued that the name "San Andreas Fault" was introduced in 1895 (Lawson's article; Crowell, 1962). This happened about 10 years after the discovery of the Median longitudinal fault in Japan.

However, it was not until the 1906 San Francisco earthquake that the fault quickly became widely known. Along the fault line running through the western outskirts of the city, displacements of up to 7 m appeared at a distance of about 430 km. The appearance of this seismic fault proved for the first time that the displacement continues north of San Francisco. Prior to 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 fault movement. However, in 1911, Reid, on the basis of accurate measurements taken in the fault zone, proposed the theory of elastic recoil to explain the mechanism of the origin of an earthquake and movements along the fault. The model of a pair of forces proposed by him was adopted as the source mechanism, which was replaced in the 60s by the model of a double pair of forces. However, Reid's theory of elastic recoil is still used to explain the mechanism of seismic fault formation.

The seismic event of 1906, during which movements occurred along an ordinary fault, led to the emergence of the concept and term "active fault". Geomorphologists still come to inspect the distinct topographical features observed along the fault in order to study the relief formed by the active shift.

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 along it on both sides of the fault. In 1953, Hill and Dibbly found that since the Cretaceous, this displacement has exceeded 500 km. Almost simultaneously, a hypothesis was put forward that the rocks on either side of the Alpine Fault in New Zealand experienced a horizontal displacement of about 450 km. In the 1950s, geologists everywhere began to pay attention to such large strike-slip faults or lateral faults. Moody's article, which argues that shifts underlie all known geological structures in the world, is typical of this time. In the 1960s, the San Andreas began to be seen as an example of transform faults (Wilson, 1965). It became the touchstone for the concept of plate tectonics.

The name "active" given to the San Andreas fault did not mean that there were minor movements along it every day. Rather, it means the likelihood that one day movement may occur along it, as happened in 1906. However, subsequently, in the southern part of San Francisco, an area was discovered in which the fault is literally active, and movement along it is continuous . On the floor and in the walls of the winery, located directly above the fault, cracks appeared even when no particular seismic activity was observed. In 1960, these unusual phenomena were found to reflect fault movement and were reported in academic circles. It was with the example of the San Andreas Fault that geologists learned that continuous motion could actually exist as a type of fault activity. This phenomenon has been called "tectonic creep" (tectonic creep). Later, it was also observed in the North Anatolian fault zone in Turkey.

Thus, the San Andreas fault and its activity have had a significant impact on the development of earth sciences. In this chapter we are going to focus mainly on its geological features.

Fault distribution and structure

On fig. 2.II.1 shows the general layout of the San Andreas Fault. From Point Arena, 100 miles north of San Francisco, it runs in an almost straight line southeast past San Francisco. Further, it cuts through the Coast 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 into the sea, and in the Shelter Cove area, south of Cape Mendocino, it changes direction to sublatitudinal, passing into a large crushing zone (Mendocino fracture zone) at the bottom of the Pacific Ocean. The southern end of the fault extends into Mexico, where it joins the East 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 were found, which extend in the direction of the northeast - southwest. The geological structure and topography of the main fault become more complicated here. This segment is called Big Bend (Big Bend). To the north and south of it, not only the general strike of the fault is different, but to the south it branches into several large faults. The 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 intermountain basin. Several excellent examples of fault-related landforms have been found along its northern margin. Further north, the fault appears in the lowlands located around the 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 curved by tectonic slumping. North of Hollister, the fault crosses the hills that bound the western edge of the San Francisco Bay Lowland, extending further north along the seabed for a distance of about 10 km west of the Golden Gate. The San Francisco International Airport is located only a few kilometers east of the San Andreas Fault. During landing or takeoff, one can observe spectacular linear near-fault landforms and lakes. San Andreas, lying on the fault and giving it its name.

In southern California, south of Big Bend, the San Andreas Fault west of Los Angeles forks into the Banning Fault and Mission Creek Fault. Further west, other faults (San Gabriel and San Jaquinto) run almost parallel. The Salton Sea, east of which is crossed by the San Andreas Fault, is a long, narrow strip below sea level; it has many fault-related features, such as shallow volcanic cones and hot springs. This lowland continues southward into the Gulf of California.

As already mentioned, the San Andreas fault is accompanied by a series of similar faults that run almost parallel. They are usually considered together and referred to as the "San Andreas Fault System".

Although the small-scale diagrams (see Fig. 2.II.1) show the San Andreas fault as a single line, more detailed maps (at a scale of 1:250,000 or 1:50,000) show that it is 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 lenticular scales were found within the fault zone (Fig. 2.II.2). The substance of which they are composed often differs from that of the surrounding rocks. Their formation is associated with movement along the fault, which causes the separation and movement of rocks on both sides of it. It is believed that the development of this type of fault zones is due to the fact that the slip surface (fault plane) formed in the rock for some reason turns out to be inactive, and that new slip planes are formed nearby. In general, the strike of a fault at an early stage of activity will not be exactly parallel to the general strike and may be highly curved. In contrast, the fault lines active in the Quaternary are relatively straight. Based on these facts, there is an idea that the ancient faults developed en echelon, at a later stage of the movement they are connected and at the last stage a flat 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 fracture line, then to an increase in frictional resistance in the curved section, and finally, to the formation of a new and straight fracture line with relatively low frictional resistance. In addition, there may be some collapse and collapse of sedimentary layers deposited in the fault zone as a result of their vertical displacement accompanying the shear. In any case, the San Andreas fault has a well-developed wide fault zone, indicating a complex history of development.

The rocks in the immediate vicinity of the fault plane under the action of movements along it are often intensely schistose, crushed and broken by cracks, which can be seen 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 action of high confining pressure (confining pressure), the rocks remain externally undisturbed, but microscopic examination reveals that they have experienced internal crushing. Under conditions of low geostatic pressure, fractured rocks become progressively clayey and “fault gouges” or “fault pugs” appear. It is known that such friction clay is often established along fault lines active in the Quaternary in the San Andreas fault zone.

Based on the observations of the fault planes within the fault zone and from its linear distribution, it can be concluded that the dip of the San Andreas fault is subvertical in fact. Detailed seismic studies have shown that underground micro-earthquakes propagate in a plane, following a fault zone, and that this plane is subvertical. The origin of these micro-earthquakes is limited to depths of 10-20 km or less. Deeper, 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 the Paleogene-Neogene and pre-Paleogene time

In 1953, Hill and Dibbly published an important scientific paper on the San Andreas Fault. Using the experience of Dibbly, who carried out geological surveys, and the data available at that time, they came to the conclusion that the older the layers along the fault, the greater should be their right-hand displacement, and its value for Cretaceous sedimentary strata reaches 500 km. Information on the age and degree of displacement of the various layers subsequently became more accurate, and now virtually no one disputes the existence of a right-handed displacement of 300 km or more, which occurred from the Miocene to the present.

Much work has been done to study the displacement of layers of the Paleogene-Neogene and Cretaceous age (Fig. 2.II.4). The most numerous and reliable data are on displacement in Miocene rocks. Marine and continental deposits of various phases of the Miocene are widespread on both sides of the fault. All ancient geographic features of these layers, such as the shape of sedimentation basins, thickness and distribution of sediments, sedimentary facies, especially the distribution of marine and continental layers, which gives an idea of the ancient coastline, as well as the distribution of fossil fauna, typical pebbles or sands contained in sediments , are unnaturally interrupted along the fault line (Addicott, 1968; Huffman, 1972). If we move these rocks back along the fault line and combine them, then the Miocene volcanic rocks east of Big Bend will 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 petrological characteristics and stratigraphic succession, they are also found to be identical in radiometric age and trace elements. This study made it possible to establish with complete certainty that at the turn of 23.5 million years ago there was a right-sided shift to a distance of about 310 km, 22 million years ago - about 295 km, and 8-12 million years ago - 240 km.

In addition, attempts were made to reconstruct the paleogeographic settings for the Eocene and Cretaceous layers. It has been established that at the turn of 44-49 million years ago, a right-sided shift occurred at a distance of about 305 km (Clark and Nilson, 1973), and since the deposition of the Cretaceous layers - at a distance of about 500 km. It was noted that the magnitude of the shift, which amounted to approximately 305 km over the time period of 44-49 Ma, within a possible error, is almost equal to the magnitude of the shift, which was approximately 310 km over 23.5 Ma. Pre-Cretaceous shear distances have been determined from apparent displacements of pre-Cretaceous granitic basement rocks (Saline 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 Saline 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, the results of recent studies of Sr isotope ratios in the Salinian blocks indicate a shift of approximately 510 km, which is fully consistent with the calculations performed so far.

On fig. 2.II.5 shows the displacement of rocks in different periods of time. From the graph it follows that in the periods between 50 and 20 million years (Eocene - early Miocene) there was almost no activity along the San Andreas Fault. It revived between 20 and 10 million years ago and continues to the present, with an increasing displacement rate.

In fact, all the data considered earlier were obtained from the area located north of Big Bend. South of the bend, investigations are severely hampered by the development of parallel or even left-sided faults almost at 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, a right-handed shift of about 300 km was established only from the time: Miocene formations and no evidence of an earlier shift could be obtained. In southern California, the Miocene formations found southwest of Big Bend (near Tejon), together with pre-Tertiary basement rocks along the San Andreas and San Gabriel faults, which run parallel to the west (Crowell, 1962, 1973), are shifted to south for a distance of about 260 km (to the mountains of Orokopia). Since pre-Tertiary basement rocks containing Precambrian rocks are comparable in both areas, activity along these faults probably began during or after the deposition of the Miocene formations (about 12 Ma ago).

Summing up the above, it should be noted that the San Andreas fault in southern California, apparently, appeared relatively recently, and the total displacement along it is only half that observed north of Big Bend (500-600 km). Therefore, many researchers believe that other faults were once active in southern California, and not the current San Andreas fault, and that this explains the absence of 200-300 km in the magnitude of the displacement. For example, Sappé 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 Big Bend, and the missing displacement of 300 km occurred there. Sappé called it the "proto-San Andreas Fault" and built a reconstruction in which he moved the western pre-Cretaceous Saline blocks south of the eastern wall along this fault (see Section VI, Fig. 2.VI.2).

Quaternary movements along the fault

We mentioned earlier that part of the San Andreas is currently experiencing continuous movement. Careful measurements indicate an average annual velocity of a few centimeters (5 cm or less), varying by location and time. Over the past 60 years, the average speed of traffic in the southern part of Hollister, as can be inferred from the horizontal displacement 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 Big Bend. However, ample topographical evidence, namely crooked valley outlines, shifting rivers, and shifting during the great earthquake of 1857 (right-handed shift of about 10 m), suggests that fault shifting in these areas occurs only during large earthquakes, such as in 1857, which happen once every few hundred years. If such a rare large displacement associated with an earthquake is averaged over time, then the shear rate along the fault still turns out to be 2-4 cm per year, which is very similar to the displacement rate in areas of tectonic slip.

These shear rates are less than the horizontal slip rate (about 5 cm/yr) expected from the horizontal strain rates in the fault zone as determined by geodetic measurements. They are also less than the relative spreading rate of the Pacific and American plates, which was calculated from the spreading rate of the ocean floor in the Gulf of California (about 6 cm/year). As we will show below, this is probably because the San Andreas is affected by only a fraction 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 in a vast territory that has captured the western margins of the American continent from Western California through the Sierra Nevada mountains to the province of Basins and Ranges in the east. If the geological survey reveals the combination of strata of different ages along the fault, then it is easier for us to assume that this is due to the displacement of the basement blocks up and down on both sides of the fault. However, such a position can occur without any displacement up or down at all, since the layers are not infinite, in the horizontal direction, and, moreover, not horizontal. It is quite possible that they will take a position against layers of a different age simply as a result of displacement along the strike. The "horizontalists" pointed this out in connection with the history of the San Andreas Fault (Hill and Dibbly, 1953; Crowell, 1962).

In the relief developed along the San Andreas fault, there are reliable signs that in some areas, at least in the Quaternary, a vertical displacement occurred. However, it can be said that this fault is an almost perfect macroscopic example of a long-lived shear. Despite the vast periods of geologic time that have passed since then, it appears that layers that formed under nearly identical depositional conditions at the same time are now located at about the same height, even if they are horizontally displaced by a distance of 300 km or more.

As a result of the shifts that took place during the Quaternary, numerous large and small depressions and uplands formed along the fault line. By tracing these landforms along the fault line, it is easy to see that the direction of the vertical displacement changes within a short distance. For example, in the Carrizo Valley, long narrow hills along the fault line, formed as a result of the relative uplift of the southwest fault line, gradually decrease over several hundred meters with a significant strike gradient, while the northeast wall, on the contrary, becomes uplifted. At the foot of such hills, on the fault line, graben-like depressions are often located, but at a short distance they become shallow, narrow and disappear among the hills. The origin of such sign-changing landforms along an almost perfect shear is believed to be explained by the fact that in the case of a shear along a fault plane that is not ideally even in the geometric sense, localized extensions and compressions occur in curved areas of the earth's crust, causing the formation of lowered and raised surface forms. terrain, respectively. In New Zealand, the fact that the location of such vertical offsets 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.

The San Andreas Fault as a Plate Boundary

On maps of the world depicting lithospheric plates, the San Andreas Fault is shown as the boundary between the Pacific and American plates. The striped pattern of magnetic anomalies on the Pacific Ocean floor off the coast of California south of the Mendocino crush zone indicates that the age of the ocean floor is decreasing as one 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 submarine ridges of Gorda and Juan de Fuca off the coast of northern California and the East Pacific Rise, which extends 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 oceanic ridges (Wilson, 1965; Atwater, 1970).

The age of the ocean floor bordering the American continent off the coast of California is the greatest (29 Ma) at Cape Mendocino in the zone of the northern part 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 trough off the coast of California near Cape Mendocino about 29 million years ago, was absorbed by this trough 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 sank from the north. As a result, the trench turned into a transform fault (the San Andreas fault). (In the geometry of plate tectonics, this should happen in the situation shown in Fig. 2.II.6). Thus, the transform fault propagated to the south, replacing the ocean trench, and reached the Gulf of California about 4 Ma ago.

These conclusions, obtained from the study of the oceanic plate, mean that the San Andreas Fault was born and the displacement along it began about 29 million years ago. The southwestern edge of the fault was also likely an oceanic plate. However, none of the considerations are consistent with the geologic evidence for the continent that we reviewed above. How can they be explained? The explanation provided by Atwater and Garfunkel is as follows. The transform fault that 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 until that time, and the displacement along it was right-handed. 29 million years ago, the land block (dotted areas in Fig. 2II.6, c and d) between the aforementioned newly formed transform fault (slip 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. Starting from the Miocene and later, the rate of right-sided displacement along the San Andreas increased (see Fig. 2.II.5) due to the fact that the degree of adhesion of the transform fault to the eastern margin of the continental block increased with time. Since the time of transformation of the oceanic trench into a transform fault came 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, and the movement became so difficult that the displacement began to occur mainly along the existing weakening on the continent, namely along the San Andreas Fault.

Thus, the overall picture of movement along the San Andreas fault, at least after the middle of the Tertiary period, is similar to the picture of the relative displacement of two plates, the American and the Pacific, which form part of the world plate system.

Several other major San Andreas fault class slips (1000 km) are known on other continents. Most of them are active and are well recorded topographically on satellite images. The main examples of the Pacific Ring Belt are the Denali fault system in Alaska (about 2000 km long, with a right-handed shift of 400-700 km), the Median longitudinal fault in Japan (about 1000 km, right-handed shift), the Philippine fault zone (about 1300 km long, with a left-hand offset), the Great Sumatran Fault Zone on about. Sumatra (about 800 km, dextral displacement), Alpine Fault in New Zealand (about 1000 km, dextral displacement approximately 450 km), Atacama Fault in Chile (approximately 800 km long, dextral displacement), etc. In Eurasia, the Altyntag Fault can be noted (about 1500 km long, left-hand offset) on the territory of the PRC, along with the Talas-Fergana fault in the Kirghiz-Kazakhstan region of the USSR (900 km long, with a right-hand offset of 250 km); Herat Faults (1100 km or more, dextral), Chamen (800 km long, 500 km levon), and the North Anatolian Fault in Turkey (900 km, dextral).

Majestic clear straight lines cut into the surface of the Earth - these are the faults that appear on satellite photographs. One of the tasks of the earth sciences should be to explain the origin of these shifts with a horizontal displacement of hundreds of kilometers.

The legendary San Andreas Fault was formed as a result of the collision of the Pacific and North American lithospheric plates. As their border, the fault originates in Mexico, crosses the state from south to north, passing Los Angeles through San Bernardino, and goes into the ocean right under San Francisco

The fault is at least 16 km deep and 1,280 km long (from east to south of California). All earthquakes occur along this boundary.

"St. Andreas Fault. Will San Francisco disappear into the earth's crust?"
Author Yuri Panchul, Sunnyvale, California

The Russian magazine "New Times" (The New Times) published my popular science article on geology, plate tectonics and experiments on artificial induction of earthquakes.

http://newtimes.ru/magazine/2008/issue063/doc-47647.html

In April 1906, an earthquake hit San Francisco, killing more than 3,000 people and leaving 300,000 homeless. After 83 years, another thing happened, although not so terrible in terms of consequences. Catastrophists predict: sooner or later there will be a big earthquake that will raze San Francisco to the ground, and the city will disappear into huge breaches 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 are the continents rushing and what forces pushed Africa away from South America - The New Times was looking for answers to these questions

During the Cold War, there was a story that there was a Soviet nuclear missile aimed at a certain point (“water tower”) in California, hitting which would cause the earth’s crust of the state to split into two pieces. After that, the western chunk would be inundated by the Pacific Ocean, which would cause the death of most of the 30 million Californians, including residents of Los Angeles and San Francisco. Of course, this tale was not born in the USSR Ministry of Defense, but was a distorted presentation of the 1978 Hollywood movie Superman.

1300 km of fear

But is there a grain of reality in this bike? A 1,300-kilometer-long San Andreas fault indeed runs along the coast of California, separating the Pacific and North American tectonic plates. San Andreas (together with the adjacent Hayward, Calaveras and other faults) is a source of large earthquakes.

The most visible manifestation of the “work” of the fault is the ancient volcano Ninakh, which formed 23 million years ago, after which it was neatly, like a cake, “cut” into two halves by the San Andreas fault, and the left half “left” along the fault for millions of years. 314 kilometers to the north and became the Pinnacles National Monument.

Where are the continents going?

What forces move thousand-kilometer 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 only moved up and down, according to the theory of geosynclines adopted in the middle of the 19th century.

But since the 16th century, cartographers have noticed that the coasts of Africa and South America can be superimposed on each other, like two pieces of a broken plate, after which some researchers periodically put forward the idea that the continents are moving. Most of the arguments were given by the German scientist Alfred Wegener. In 1915, Wegener showed that the coasts of different continents not only coincided in contour, but also contained the same types of stones, 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 thought to be a collection of coincidences, as geophysicists thought it improbable that a continent (a mass of rock) could move across another mass of rock (the hard bottom of the oceans) without being eroded by friction. The situation changed only after World War II, when the US military, using sonar, mapped the oceans and found in the middle of them long chains of seamounts, clearly of volcanic origin. Researcher Harry Hess showed that the bottom of the Atlantic Ocean is moving apart in two directions from the mountain range passing in the middle of the Atlantic. The expanding ocean floor carries the continents like an escalator in a subway carries passengers.

And who drives them...

As a result of the research of Hess and others 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 slab 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 the continents.

When plates collide, depending on the nature of the collision, mountains are formed (for example, the Himalayas), chains of islands (for example, the Japanese islands), depressions and volcanoes. When the oceanic and continental plates collide, the oceanic plate goes down. This is due to the fact that the oceanic crust has a different chemical composition and greater density. Gerry Hess called the ongoing process a “conveyor belt”: a new crust is born from hardened lava in the middle of the ocean, slowly moves for millions of years, after which it sinks back into the bowels and melts.

Why are the plates on the San Andreas fault moving sideways and not towards each other? The fact is that for 40 million years in the region there was a complex "dance" of three tectonic plates (Pacific, Farallon (Farallon) and North American), the boundaries between which were at an angle to each other. The Farallon Plate was "pushed" under the North American, after which the Pacific began to slide sideways along the former border of the Farallon and North American plates.

Tectonic plates are like foams driven by 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 down 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 its existence. We live on a huge nuclear reactor - planet Earth!

earth shaking

Okay, the continents are moving, but how does this affect our lives, in addition to the need to periodically repair a few small roads that cross the San Andreas fault? The problem is that the movement is not continuous. Each shift begins with a build-up of stress, which is “discharged” with a jerk during a large or small earthquake. In the central part, the fault “creeps” due to thousands of micro-earthquakes that are not felt by humans. But sometimes the voltage 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 the “left” part of California moved relative to the “right” by almost 7 meters near the epicenter.

The shift began 10 kilometers under the ocean floor in the San Francisco area, after which, within 4 minutes, the shift impulse spread to 430 kilometers of the San Andreas Fault - from the village of Mendocino to the town of San Juan Batista.

By the time the fires broke out, more than 75% of the city had already been destroyed, with 400 city blocks lying in ruins, including the center.

Two years after the devastating earthquake in 1908, geological research began, which continues to this day. Studies have shown that over the past 1500 years, major earthquakes have occurred in the San Andreas fault region, approximately every 150 years.

The villain's plan

Thus, it is impossible to flood coastal California with a point 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 one is less realistic than flooding an aircraft carrier with a kick. But is it possible to cause serious damage with an artificial earthquake? Oddly enough, this idea was tested not only in Hollywood films. In 1966, geologists from the United States Geological Survey (USGS) noticed an unexpected sequence of earthquakes near the Rocky Flats military arsenal in Colorado. The timing of the earthquakes exactly coincided with the moments when the military disposed of liquid waste by pumping it under pressure deep underground. Geologists set up an experiment by pumping water into an abandoned oil field near the town of Rangely in Colorado. For the first time in history, humans artificially caused an earthquake.

After that, the USGS discussed for some time the idea of preventing large earthquakes along the San Andreas by relieving fault stress with a large number of micro-quakes. However, the USGS decided not to experiment, since it is clear that they would not have enough money to pay in case of a mistake for the complete destruction of Los Angeles or San Francisco.

It gets worse

Earthquakes notwithstanding, California is one of the nicest places to live on Earth. Most residents of the state live in one-two-story houses and know the precautions. Therefore, the significant San Francisco earthquake in 1989 did not cause much destruction. After all, there are problems elsewhere on the planet - hurricanes, tsunamis or an unfavorable political situation. And the San Andreas fault is not the most dangerous geological object 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 got into the lungs and polluted 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. Artificial earthquake - http://www.usgs.gov/newsroom/article.asp?ID=343

Used sources.

A new disaster movie called "San Andreas" from Warner Bros. hits theaters in May this year. Dwayne Johnson stars as a rescue pilot during a magnitude 9 earthquake that hits California. It is surprising that many do not even suspect why the film was named that way. Many even think that the name was chosen in honor of the popular game "GTA San Andreas". That is why I decided to tell you about the geological miracle - the San Andreas Fault in California.

Bird's eye view of the San Andreas Fault:

The San Andreas Fault is the boundary where two tectonic plates collide - the Plate and the North American. The fault divides California into two parts and stretches to the Mexican border. San Diego, Los Angeles, and Big Sur are on the Pacific Plate, while San Francisco, Sacramento, and the Sierra Nevada are on the North American. The fault is 810 miles long and extends to depths of at least 15 kilometers.

The plates slide along each other just along this fault. The Pacific moves northwest relative to the North American, and it is this movement that causes earthquakes. They move along each other at about 1.5 inches per year, but the movement is rather erratic. For many years the plates were locked without moving, pressed against each other. At the same time, colossal tension accumulated, looking for a way out in earthquakes. During the 1906 San Francisco earthquake, roads, fences, and trees along the fault were displaced by several yards.

The San Andreas Fault is perfectly visible from the air for most of its length. California experiences thousands of small earthquakes each year, but the big ones only occur after long periods of silence. The last major earthquake along the San Andreas Fault was in 1906, a 7.8 magnitude San Francisco earthquake. It's hard to predict when the next big hit will be, but it's quite likely in the near future. A new USGS study suggests that California will experience an 8-magnitude impact in the next 30 years.

St. Andreas Fracture. Will San Francisco disappear into the earth's crust?

http://newtimes.ru/magazine/2008/issue063/doc-47647.html

Yuri Panchul, Sunnyvale, CA

1300 km of fear

In some places, 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 one - to the north, and the eastern one - 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 traverse the San Andreas, showing shifted road markings and signs of regular pavement repair at the fault. The most visible manifestation of the “work” of the fault is the ancient volcano Ninakh, which formed 23 million years ago, after which it was neatly, like a cake, “cut” into two halves by the San Andreas fault, and the left half “left” along the fault for millions of years. 314 kilometers to the north and became the Pinnacles National Monument.

Where are the continents going?

And who drives them...

earth shaking

This happened during the 1906 earthquake in San Francisco, when the “left” part of California moved relative to the “right” by almost 7 meters near the epicenter. The shift began 10 kilometers under the ocean floor in the San Francisco area, after which, within 4 minutes, the shift impulse spread to 430 kilometers of the San Andreas Fault - from the village of Mendocino to the town of San Juan Batista.

The villain's plan

It gets worse

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. Artificial earthquake - http://www.usgs.gov/newsroom/article.asp?ID=343

Some of the world's largest megacities are located just in the area of the most dangerous faults in the earth's crust. Californians living on 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, car parks, street lights every few steps. However, a closer look reveals that the line of the same lamps is not quite straight, and the street seems to be twisted, as if it was taken by the ends and pulled in different directions. The reason for these oddities is that Taft, like many Californian large urban centers, is built along the San Andreas Fault - cracks in the earth's crust, 1050 km of which run through the United States.

The strip, stretching from the coast north of San Francisco to the Gulf of California and extending into the depths of the earth for about 16 km, is a line connecting two of the 12 tectonic plates on which the oceans and continents of the Earth 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 when their location changes. If they crawl one on top of the other, huge mountain ranges rise into the sky, such as the Alps and the Himalayas. However, the circumstances that gave rise to the San Andreas fault are completely different.

Here, the edges of the North American (on which most of this continent rests) and Pacific (supporting most of the California coast) tectonic plates are like ill-fitting gear teeth that do not fit one over the other, but do not fit neatly into the grooves intended for them. The plates rub against each other, and the friction energy formed along their boundaries does not find an outlet. It depends on which part of the fault such energy accumulates, where the next earthquake will occur and what strength will be.

In the so-called "floating zones", where the movement of the plates is relatively free, the accumulated energy is released in thousands of small shocks, which do almost no damage and are recorded only by the most sensitive seismographs. Other sections of the fault - they are called "castle zones" - seem to be completely immovable, where the plates are pressed against each other so tightly that there has been no movement for hundreds of years. The tension gradually builds up 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, is nevertheless significant. The city of Parkfield, located between San Francisco and Los Angeles, is in such an intermediate zone. Earthquakes with magnitude 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 for this region.

From 200 AD e. 12 major earthquakes occurred in California, but it was the disaster of 1906 that attracted the attention of the whole world to the San Andreas Fault. This earthquake, with its epicenter in San Francisco, caused destruction in a colossal area stretching from north to south for 640 km. Along the fault line, in a matter of minutes, the soil shifted 6 m - fences and trees were knocked down, roads and communication systems were destroyed, the water supply stopped, and the fires that followed the earthquake raged throughout the city.

As the science of geology has developed, more advanced measuring instruments have appeared that can constantly monitor the movements and pressure of water masses under the earth's surface. During 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 force of the earth's surface vibrations. Thanks to these measures, the 1989 San Francisco earthquake destroyed most of the buildings of the old structure, without harming modern skyscrapers.

Then 63 people died - most due to the collapse of a huge section of the two-tier Bay Bridge. According to scientists, in the next 50 years, California faces a serious disaster. It is assumed that an earthquake with a magnitude of 7 on the Richter scale will occur in southern California, in the Los Angeles area. It could cause billions of dollars in damage and cause 17,000 to 20,000 deaths, and another 11.5 million people could die from smoke and fires. And since the energy of friction along the fault line tends to accumulate, each year that brings us closer to an earthquake increases its likely strength.