Earthquake Facts & Earthquake Fantasy
SUMMARY. Scaling relationships between seismic moment, rupture length, and rupture width have of possible rupture widths in geological dip-slip environments. (e.g. Scholz nearly events, large enough for a good statistical analysis. logarithmic nature of (1), a similar relationship is variation of in obtaining stable results (in other words, a good the predicted variations of b with earthquake. The magnitude is the most often cited measure of an earthquake's size, but it is not it became possible to accurately locate earthquakes and measure the ground earthquake magnitude scale (a logarithmic relationship between earthquake the same value if the earthquake is recorded locally or from a great distance.
The deadliest earthquake in recorded history struck Shensi province in China inkilling aboutpeople. The magnitude 7. Inthe magnitude 6. The earthquake in Chile on May 22,is the strongest to be recorded in the world with magnitude 9. For the record, the largest U.
It was a magnitude 9. California has the most earthquakes in the United States. Alaska registers the most earthquakes in a given year, with California placing second, until when a sudden increase in seismicity in Oklahoma pushed it well past California as the second most active in terms of magnitude M 3.
In there were M3 and greater earthquakes in Oklahoma and about in California. As of April Oklahoma events is still well ahead of California 29 events.
California, however, has the most damaging earthquakes, including a M6. Florida and North Dakota have the fewest earthquakes each year. Earthquakes can occur near the surface or deep below the surface. But the very deepest earthquakes only occur at subduction zones where cold crustal rock is being pushed deep into the earth. In California, earthquakes are almost all in the top 15 miles of the crust, except in northern California along the Cascadia Subduction Zone, which extends into Oregon, Washington, and British Columbia.
Seismologists use earthquakes to study the interior of the earth and to pinpoint faults and geologic structures such as the core-mantle boundary, subduction zones, and the subsurface extent of the San Andreas Fault. The ground can open up during an earthquake. A popular cinematic and literary device is a fault that opens during an earthquake to swallow up an inconvenient character.
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.
California will eventually fall into the ocean. 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.
Frequently Asked Questions about Earthquakes (FAQ)
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!
An earthquake on the San Andreas fault can cause a large tsunami. 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.
Frequently Asked Questions about Earthquakes (FAQ)
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.
Two major earthquakes occurred on the same day, so they must be related. 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 milesbut these triggered quakes are small and very short lived.
People can cause earthquakes. 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 yearsthere was an average of 21 earthquakes of magnitude three and larger in the central and eastern United States.
Inalone, 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.
The top waveform shows the broad-band displacement, the lower trace shows the corresponding ground motion that would register on a Wood-Anderson seismograph.
Earthquake weather - Wikipedia
Richter also developed a distance correction to account for the variation in maximum ground motion with distance from an earthquake the dashed curves shown in the above diagram show his relationship for southern California. The precise rate that the peak ground motions decrease with distance depends on the regional geology and thus the magnitude scale for different regions is slightly dependent on the "distance correction curve".
Thus originally, Richter's scale was specifically designed for application in southern California. Richter's method became widely used because it was simple, required only the location of the earthquake to get the distance and a quick measure of the peak ground motion, was more reliable than older measures such as intensity. It became widely used, well established, and forms the basis for many of the measures that we continue to use today.
Generally the magnitude is computed from seismographs from as many seismic recording stations as are available and the average value is used as our estimate of an earthquake's size.
We call the Richter's original magnitude scale ML for "local magnitude"but the press usually reports all magnitudes as Richter magnitudes. Teleseismic Magnitude Scales To study earthquakes outside southern California, Richter extended the concepts of his local magnitude scale for global application.
In the 's through the 's together with Beno Gutenberg, Richter constructed magnitude scales to compare the size of earthquakes outside of California. Ideally they wanted a magnitude scale that gave the same value if the earthquake is recorded locally or from a great distance. That way you could compare the seismicity of earthquakes in different parts of Earth.
But the extension of methods to estimate the local magnitude is complicated because the type of wave generating the largest vibrations and the period of the largest vibrations recorded at different distances from an earthquake varies. Near the earthquake the largest wave is a short-period S-wave, at greater distances longer-period surface waves become dominant.
To exploit the best recorded signal the largest magnitude scales were developed for "teleseismic" distant observations using P waves or Rayleigh waves. Eventually the teleseismic P-wave scale became known as "body-wave magnitude" and the Rayleigh wave based measure came to be called "surface-wave magnitude".
The surface-wave magnitude is usually measured from 20s period Rayleigh waves, which are very well transmitted along Earth's surface and thus usually well observed. Gutenberg and Richter developed two magnitudes for application to distant earthquakes: Problems with Magnitude Scales There are several problems associated with using magnitude to quantify earthquakes, and all are a direct consequence of trying to summarize a process as complex as an earthquake in a single number.
First, since the distance corrections depend on geology each region must have a slightly different definition of local magnitude. Also, since at different distances we rely on different waves to measure the magnitude, the estimates of earthquake size don't always precisely agree. Also, deep earthquakes do not generate surface waves as well as shallow earthquakes and magnitude estimates based on surface waves are biased low for deep earthquakes.
Also, measures of earthquake size based on the maximum ground shaking do not account for another important characteristic of large earthquakes - they shake the ground longer. A seismologist immediately recognizes the nature of such an event by the single frequency contained in the record. Are there certain months of the year that are more seismically active than others?
No, there are no months that have more earthquakes than others. Examining the list of Canadian or global earthquakes, there isn't a season that stands out as having an increased number of earthquakes.
The explanation for this can be found by considering that the mechanisms that cause earthquakes are independent of seasonal temperature changes see effects of cold temperatures on earthquakesand independent of the changes in position of the Earth in the solar system at different times of the year.
It is internal geological forces that play the most important role in generating earthquakes.
Most large earthquakes are as a result of immense continental plates, called tectonic plates, that move, one with respect to another. The driving force for this movement is found in the Earth's mantle in the form of convective currents. These currents carry the tectonic plates around the Earth generating earthquakes and volcanic eruptions. The movement of the plates creates strain which is then accumulated in faulted areas causing earthquakes. Both the movement of the plates and the accumulation of strain along faults are continual processes independent of the time of year.
Since the distance between the Earth and Sun changes throughout the year due to the elliptical trajectory of the Earth around the Sun, it seems possible that the attractive gravitational forces between the two bodies might cause extra strain in the Earth's crust. However, strain models have shown that this extra force is insignificant compared to the tectonic force present.
Since the temperature and gravitational forces are the only forces changing with the seasons, seasonal effects can be eliminated as a factor in influencing the frequency of earthquakes. What is the intensity of an earthquake? See the Modified Mercalli Intensity Scale.
Can people cause earthquakes? Minor earthquakes have been triggered by human activities such as mining rockbursts and cavity collapsethe filling of reservoirs behind large dams, and the injection of fluids into wells for oil recovery or waste disposal. Large dams hold back enormous quantities of water. Some of this water may penetrate into cracks in the underlying rock, and sometimes this may trigger small earthquakes under or very near the reservoir.
Following an underground nuclear explosion, small earthquakes have often been recorded near the test site. These are due to the collapse of the cavity created by the explosion.
Man-made earthquakes always occur close to the site of the activity. There is no link between human activities like these and earthquakes occurring hundreds or thousands of kilometres away.
Does a small earthquake mean that a larger earthquake is coming? No, except for very rare exceptions. Every year, hundreds of earthquakes occur in Canada. Only a very tiny minority of these precede a larger earthquake. Although a large earthquake may be preceded by a foreshock the Saguenay earthquake of November is an examplethe occurrence of a small earthquake is not in itself a typical sign.
Hundreds of small earthquakes occur every year in Canada, whereas major earthquakes have occurred only a few times in this century. A small earthquake, however, provides an ideal opportunity to offer reminders about safety measures to take before, during and after an earthquake.
Magnitude What is the "magnitude" of an earthquake? Magnitude is a measure of the amount of energy released during an earthquake. It is frequently described using the Richter scale. To calculate magnitude, the amplitude of waves on a seismogram is measured, correcting for the distance between the recording instrument and the earthquake epicentre.
Since magnitude is representative of the earthquake itself, there is only one magnitude per earthquake. Taking the Saguenay QU earthquake of November 25, as an example, one could not therefore speak of magnitude 6 at Quebec City and magnitude 4 to 5 at Montreal.
The effects or intensities experienced at different places were different, but the magnitude of the earthquake is unique; in this example, it was 6 on the Richter scale. Magnitude thus has more to do with the effects of the earthquake overall. The magnitude scale is logarithmic. This means that, at the same distance, an earthquake of magnitude 6 produces vibrations with amplitudes 10 times greater than those from a magnitude 5 earthquake and times greater than those from a magnitude 4 earthquake.
In terms of energy, an earthquake of magnitude 6 releases about 30 times more energy than an earthquake of magnitude 5 and about times more energy than an earthquake of magnitude 4.
Upsurge in big earthquakes predicted for 2018 as Earth rotation slows
It is very unlikely that an earthquake of magnitude less than 5 could cause any damage. What is the difference between the "magnitude" and the "intensity" of an earthquake? The Intensity scale is designed to describe the effects of an earthquake, at a given place, on natural features, on industrial installations and on human beings.
The intensity differs from the magnitude which is related to the energy released by an earthquake. What it is the difference between magnitudes ML and mN?
Without going into the seismological details, the magnitude defined by Charles Richter is the source of all magnitude scales. Over the years however, it was realized that the magnitude that Richter had defined for California ML means local magnitudedid not apply to Eastern North America where the seismic waves attenuate differently.
- Earthquake Facts & Earthquake Fantasy
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Otto Nuttli, a seismologist at the University of Saint-Louis in the United States, developed a magnitude formula which corresponded better to the reality of Eastern America. One of the formulas which Nuttli derived is used to measure the seisms of Eastern Canada.
The formulation used is called Magnitude Nuttli or mN. In order to simplify communication with the public, Canadian seismologists will often refer to the Richter magnitude whereas strictly speaking the seisms that occur in Eastern Canada are measured according to the Nuttli magnitude.
An exception exists for the very small earthquakes of the Charlevoix Region, where the Richter scale is used. Around the world other scales of magnitude exist according to the source conditions of the earthquakes depththe conditions of attenuation, the type of measured wave, etc.
More and more, seismologists describe earthquakes according to the magnitude of the moment scale MW or M. Certain earthquakes have a negative magnitude, is this an error? No, it is not an error. As magnitude calculations are based on a logarithmic scale, a ten-fold drop in amplitude decreases the magnitude by 1.
Let us assume that on a seismogram: Naturally, a negative magnitude is found only for very small events, which are not felt by humans. Is there a maximum magnitude for an earthquake? Though theoretically there is no mathematical limit with the magnitude calculation, physically there is a limit.