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Create your free OpenLearn profile. Course content Course content. Another reason is that the locations of faults, and thus earthquakes, in continental rift zones are affected by pre-existing geological structures within continental crust.
In the case of the Lake Baikal rift, the strong, ancient crust of the Siberian Craton influences the orientation of the faults forming the rift. Faults run parallel to the craton near Lake Baikal. As rifting extends to the east, the part of the craton in the upper right of Figure Along convergent plate margins with subduction zones, earthquakes range from shallow to depths of up to km. Earthquakes occur where the two plates are in contact, as well as in zones of deformation on the overriding plate, and along the subducting slab deeper within the mantle.
The result is that epicentres of earthquakes farther to the interior of the overriding plate will correspond to increasingly deep earthquakes. Where the Pacific plate subducts beneath the North American plate, forming the Aleutian volcanic arc Figure Earthquakes between 0 and 33 km deep red circles occur closest to the subduction zone red line; teeth point in the direction of the subducting slab.
While there is some overlap, earthquakes between 33 and 70 km deep white circles occur in a band that reaches farther the north. Farthest north are the epicentres for earthquakes between 70 and km deep green dots. The deepest earthquake during the seven year interval shown in Figure It occurred at a depth of km. Earthquakes occur in subduction zones for a variety of reasons. Stresses associated with the collision of two plates cause deformation in the overriding plate, and thus shallow earthquakes.
Shallow earthquakes also happen on the subducting slab when a locked zone orange line, Figure The locked zone is where the largest earthquakes on Earth, called megathrust earthquakes , occur. There is the potential for a wider rupture zone on a gently dipping subduction zone boundary compared to other boundaries.
If subduction is rapid, the subducting plate will bend more as it enters the mantle slab A in Figure Stress from bending can cause shallow to intermediate earthquakes on these plates. Even without bending, the subducting slab can become stretched by its own weight as it falls into the mantle. The km and km discontinuities in Figure When the subducting slab reaches the km discontinuity the top of the lower mantle , the increase in density in the surrounding mantle may slow down the leading edge of the sinking slab.
Earthquakes can be generated when the slab is compressed by the lower mantle resisting its motion at the same time that the upper part of the slab continues to fall. Slower rates of subduction mean that the subducting slab will enter the mantle at a lower angle slab B in Figure These slabs might not have earthquakes from being bent downward into the mantle, as with slab A, but earthquakes may be triggered by changes in stress if the plate relaxes and unbends. The bar chart on the right of Figure Earthquakes are most abundant at the surface, and then fall to a minimum at km.
The number of earthquakes remains low until almost km depth, and reaches a second peak around km depth. The second peak might be explained by interactions between the subducting plate and the dense mantle beneath the km discontinuity, but another hypothesis is that it is related to delayed mineral transformations.
The subducting slab warms as it goes deeper into the mantle, but the warming is not uniform. The outer edges of the slab will warm before the interior does. The km discontinuity is where olivine is transformed into the minerals wadsleyite and ringwoodite under normal mantle pressure and temperature conditions.
However, if the interior of the subducting slab is still too cool at that depth, olivine will be retained to depths below km.
Olivine weakens prior to transforming into the high pressure minerals, and the weakening may make it easier for the slab to rupture. Where continents collide, earthquakes are scattered over a much wider area compared to earthquakes along mid-ocean ridges, transform margins, or subduction zones.
An example is where the Indian plate collides with the Eurasian plate Figure At one time, India was a separate continent, and ocean crust separated India from the Eurasian plate.
For a time, a subduction zone existed where ocean lithosphere from the Indian plate subducted beneath the Eurasian plate. But when the two land masses finally met, they became locked together and the subduction zone was closed off. Today the Indian plate is still pushing against the Eurasian plate in the regions indicated by the red arrows in Figure The collision is accommodated by transform boundaries along the Indian plate.
Regions of overall transform motion are indicated in Figure The majority of earthquakes in Figure Deeper earthquakes may be caused by continued northwestward subduction of part of the Indian plate beneath the Eurasian plate in this area. Even though the area is no longer a subduction zone, the subducted slab still remains, and is subject to stresses that can trigger earthquakes. Some of the earthquakes in Figure That squeezing has caused the Eurasian plate to be thrust over the Indian plate, building the Himalayas and the Tibet Plateau to enormous heights.
Most of the earthquakes of Figure The southernmost thrust fault in Figure Intraplate earthquakes within-plate earthquakes are those that occur away from plate boundaries.
Some intraplate earthquakes are related to human activities. When humans trigger earthquakes it is referred to as induced seismicity. In Saskatchewan there have been 20 earthquakes since all less than magnitude 4 , and the majority occurred near potash mines. Excavation changes the stress in surrounding rocks, so earthquakes may occur in the rocks above excavated parts of the mine. In Alberta, induced seismicity is triggered by hydraulic fracturing operations when water pressure increases along existing faults, causing them to slip.
Apply Filter. What is the difference between aftershocks and swarms? Aftershocks are a sequence of earthquakes that happen after a larger mainshock on a fault. Aftershocks become less frequent with time, although they can continue for days, weeks, months, or even What does it mean that the earthquake occurred at a depth of 0 km?
What is the geoid, and what does it have to do with earthquake depth? An earthquake cannot physically occur at a depth of 0 km or -1km above the surface of the earth. In order for an earthquake to occur, two blocks of crust must slip past one another, and it is impossible for this to happen at or above the surface of the earth.
So why do we report that the earthquake occurred at a depth of 0 km or event as a Why do so many earthquakes occur at a depth of 10km? Ten kilometers is a "fixed depth".
Sometimes data are too poor to compute a reliable depth for an earthquake. In such cases, the depth is assigned to be 10 km.
Why that number? In many areas around the world, reliable depths tend to average 10 km or close to it. For example, if we made a histogram of the reliable depths in such an area, we'd Where can I find earthquake educational materials? Start with our Earthquake Hazards Education site. Can we cause earthquakes? Is there any way to prevent earthquakes? Earthquakes induced by human activity have been documented at many locations in the United States and in many other countries around the world.
Earthquakes can be induced by a wide range of causes including impoundment of reservoirs, surface and underground mining, withdrawal of fluids and gas from the subsurface, and injection of fluids into What is surface faulting or surface rupture in an earthquake? Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. What is an earthquake and what causes them to happen?
An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel. In California there Foreshocks, aftershocks - what's the difference? Foreshocks are earthquakes that precede larger earthquakes in the same location.
An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs. Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a Can the position of the moon or the planets affect seismicity? Earthquakes are equally as likely to occur in the morning or the evening. Many studies in the past have shown no significant correlations between the rate of earthquake occurrence and the semi-diurnal tides when using large earthquake catalogs.
Several recent studies, however, have found a correlation between earth tides caused by the position of
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