But unfortunately for principled writers, gaping faults exist only in movies and novels. The ground on the two sides of the fault slide past each other, they do not pull apart. If the fault could open, there would be no friction.
Without friction, there would be no earthquake. Shallow crevasses can form during earthquake induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not gape open during an earthquake. The ocean is not a great hole into which California can fall, but it is itself land at a somewhat lower elevation with water above it. Instead, southwestern California is moving horizontally northward towards Alaska as it slides past central and eastern California.
The dividing point is the San Andreas fault system, which extends from the Salton Sea in the south to Cape Mendocino in the north. The Pacific Plate is moving to the northwest with respect to the North American Plate at approximately 46 millimeters two inches per year the rate your fingernails grow. At this rate, Los Angeles and San Francisco will one day about 15 million years from now be next-door neighbors, and in an additional 70 million years, Los Angeles residents will find themselves with an Alaska zip code!
The San Andreas fault cannot create a big tsunami like the ones that happened in Sumatra in or Japan in Those earthquakes happened on subduction zone faults, on which fault slip caused vertical uplift of the sea floor. While a part of the San Andreas fault near and north of San Francisco is offshore, the motion is mostly horizontal, so it will not cause large vertical motions of the ocean floor that would generate a tsunami.
Earthquakes on other faults offshore California as well as underwater landslides triggered by strong shaking can create local tsunamis, some of which may be locally damaging. They mostly occur within fault lengths of the mainshock. For the largest earthquakes, this is a long distance; it is thought that the San Francisco earthquake triggered events in southern California, western Nevada, southern central Oregon, and western Arizona, all within 2 days of the mainshock.
As a general rule, aftershocks represent readjustments in the vicinity of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. If an aftershock is larger than the first earthquake then we call it the mainshock and the previous earthquakes in a sequence become foreshocks. It is possible to have two earthquakes of about the same size in a sequence. Given that very large earthquakes are rare to begin with, it is not surprising that we have not yet observed two very large earthquakes so close together in time in California.
Often, people wonder if an earthquake in Alaska may have triggered an earthquake in California; or if an earthquake in Chile is related to an earthquake that occurred a week later in Mexico. Over long distances, the answer is no. Even the Earth's rocky crust is not rigid enough to transfer stress efficiently over thousands of miles. There is evidence to suggest that earthquakes in one area can trigger seismic activity within a few hundred miles, including aftershocks clustered near the main shock.
There is also evidence that some major earthquakes manage to trigger seismicity over much greater distances thousands of miles , but these triggered quakes are small and very short lived. Earthquakes induced by human activity have been documented in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the filling of large reservoirs for water supplies. Most of these earthquakes were minor. Deep mining can cause small to moderate quakes and nuclear testing has caused small earthquakes in the immediate area surrounding the test site, but other human activities have not been shown to trigger subsequent earthquakes.
Within the central and eastern United States, the number of earthquakes has increased dramatically over the past few years. Between the years , there was an average of 21 earthquakes of magnitude three and larger in the central and eastern United States. In , alone, there were M3 and larger earthquakes. Most of these earthquakes are in the magnitude 3? There were reports of damage from some of the larger events, including the M5.
The increase in seismicity has been found to coincide with the injection of wastewater in deep disposal wells in several locations, including Colorado, Texas, Arkansas, Oklahoma and Ohio. Much of this wastewater is a byproduct of oil and gas production and is routinely disposed of by injection into wells specifically designed and approved for this purpose. However, we can significantly mitigate their effects by characterizing the hazard e. There are many things being done now by the USGS and other agencies to protect people and property in the United States in the event of a major earthquake.
Scientists agree that even large nuclear explosions have little effect on seismicity outside the area of the blast itself. The largest underground thermonuclear tests conducted by the United States were detonated in Amchitka at the western end of the Aleutian Islands, and the largest of these was the 5 megaton test code-named Cannikin that occurred on November 6, that did not trigger any earthquakes in the seismically active Aleutian Islands.
On January 19, , a thermonuclear test, code-named Faultless, took place in central Nevada. The code-name turned out to be a poor choice because a fresh fault rupture some 4, feet long was produced.
Seismograph records showed that the seismic waves produced by the fault movement were much less energetic than those produced directly by the nuclear explosion. Locally, there were some minor earthquakes surrounding the blasts that released small amounts of energy.
Scientists looked at the rate of earthquake occurrence in northern California, not far from the test site, at the times of the tests and found nothing to connect the testing with earthquakes in the area. Seismologists have observed that for every magnitude 6 earthquake there are about 10 of magnitude 5, of magnitude 4, 1, of magnitude 3, and so forth as the events get smaller and smaller.
This sounds like a lot of small earthquakes, but there are never enough small ones to eliminate the occasional large event. It would take 32 magnitude 5's, magnitude 4's, OR 32, magnitude 3's to equal the energy of one magnitude 6 event. So, even though we always record many more small events than large ones, there are far too few to eliminate the need for the occasional large earthquake.
Injecting high-pressure fluids deep into the ground is known to be able to trigger earthquakes—to cause them to occur sooner than would have been the case without the injection. This would be a dangerous pursuit in any populated area, as one might trigger a damaging earthquake. The faults that cause earthquakes come in several different forms. The two sides of a strike-slip or transform fault move parallel to each other in opposite directions.
One side of a thrust fault moves over the other which plunges deep into the planet. Normal faults occur where two plates pull away from each other, causing sections of one or both to fall or molten lava to well up in between and form new crust. Earthquakes can also occur in the middle of tectonic plates for a variety of reasons.
Most of these are small, but a few, such as the and earthquakes along the New Madrid fault in the Mississippi Valley, are strong enough to cause significant and widespread damage. Volcanoes tremble as magma moves through subterranean chambers and channels, the weight of water in reservoirs as they fill often cause small tremors to occur, earthquakes on faults in the middle of continents occasionally remind people that places like Kansas, Siberia, and the Sahara Desert are also a part of our active and ever-changing planet.
Most earthquakes, however, occur at plate boundaries and many of those are in the crust beneath the ocean. The type of fault and the "stickiness" of a particular section of fault can influence the size of an earthquake, or its magnitude.
The best-known scale for measuring magnitude is the Richter scale, but that yardstick is a relative measure of earthquake size and is estimated from instrument readings of ground shake.
The Richter scale is gradually being replaced by the moment magnitude scale, or seismic moment, which is a calculated measure of the energy released by an earthquake that takes into account such factors as the area of fault ruptured, the amount that the plates moved relative to one another, and the rigidity of rock that broke.
Like the Richter scale, the moment magnitude is a logarithmic value, so that a magnitude 6 earthquake releases twice as much seismic energy as a magnitude 5 quake. Extremely sensitive instruments called seismometers measure and record seismic waves given off by earthquakes and other events, many of which are too slight to feel.
Networks of seismometers, located around the world enable scientists to determine the location of an earthquake on the surface of the Earth—its epicenter—above its hypocenter, or its precise origin below the ground.
The shaking motion we feel as an earthquake is actually made up of different types of seismic waves that can be separated into two different groups: body waves, which travel through the depths of the planet, and surface waves, which shake the outermost crust.
Body waves make up the largest of an earthquake and include primary or P waves and secondary or S waves. P waves compress and decompress the rocks in the direction the wave is traveling as it passes through the Earth as if the rocks were a giant spring. S waves move the rocks up and down or side-to-side perpendicular to the direction they travel.
These waves travel slower, but can be the most damaging, shaking buildings and other infrastructure on the surface. Surface waves travel through the uppermost layers of Earth's crust and are similar to ripples in water. They are divided into two types: Love waves and Rayleigh waves. Love waves are similar to S waves in that they move perpendicular to the direction of the wave, but they only move side-to-side.
The horizontal shaking of Love waves is particularly damaging to the foundations of structures. Rayleigh waves produce motion similar to rolling ocean waves in the ground. They move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling. Surface waves travel more slowly than body waves P and S ; and of the two surface waves, Love waves generally travel faster than Rayleigh waves.
Earthquakes come in many sizes and happen all across the planet every day. Of course, large earthquakes often cause more damage than small ones, but when small or medium-size earthquakes occur in densely populated regions or in regions with poorly designed or in adequate infrastructure, the results can be just as destructive.
On the morning of January 12, , a magnitude While not as large as some other recent earthquakes that did less damage, the shaking demolished everything from shacks in shantytowns to the Presidential Palace in Port-au-Prince.
Nearly a quarter of a million people died and more than a year later many people were still living in temporary shelters or in badly damaged buildings. Since then, the Haitian people and government, as well as aid agencies working in Haiti have also had to deal with increased crime and outbreaks of serious communicable diseases such as cholera.
Such conditions arising in the aftermath of earthquakes can be almost as devastating as the seismic event itself—sometimes more so—and can take many different forms. Landslides and avalanches : Earthquakes can dislodge or weaken soil and rock on hillsides that may not actually slide for decades. Floods : Earthquakes may cause damage to dams or trigger landslides into lakes and rivers that in turn overflow their banks. Fires : Following an earthquake, damage to power and gas lines can spark fires; if water mains have been ruptured, it may be difficult to extinguish fires.
Soil liquefaction: Shaking water-saturated, granular soil transforms it from a solid to a liquid, causing buildings or bridges built on it to tilt or sink. However, its height is usually less than a meter 3. Seismic waves from an earthquake can emerge from the seafloor as an acoustic sound wave that travels through the ocean toward the surface and can strike a ship.
If the sound is strong enough, the ship will be violently rocked. These are called seaquakes. Seaquakes can be so strong that sometimes those onboard think they have hit ground. This summer, 27 college students
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