Only lithosphere has the strength and the brittle behavior to fracture in an earthquake.
The map below locates earthquakes around the globe. They are not evenly
distributed; the boundaries between the plates grind against each other,
producing most earthquakes. So the lines of earthquakes help define the
(from the USGS)
In cross section, the Earth releases its internal heat by convecting, or
boiling much like a pot of pudding on the stove. Hot asthenospheric mantle
rises to the surface and spreads laterally, transporting oceans and
continents as on a slow conveyor belt. The speed of this motion is a
few centimeters per year, about as fast as your fingernails grow.
The new lithosphere, created at the ocean spreading centers, cools as
it ages and eventually becomes dense enough to sink back into the mantle.
The subducted crust releases water to form volcanic island chains above,
and after a few hundred million years will be heated and recycled back to
the spreading centers.
There are three main plate tectonic environments: extensional, transform,
and compressional. Plate boundaries in different localities are subject
to different inter-plate stresses, producing these three types of earthquakes.
Each type has its own special hazards.
At spreading ridges, or similar extensional boundaries, earthquakes are shallow, aligned strictly along the axis of spreading, and show an extensional mechanism. Earthquakes in extensional environments tend to be smaller than magnitude 8. (Click here for an explanation of earthquake magnitude).
A close-up topographic picture of the Juan de Fuca spreading ridge, offshore
of the Pacific Northwest, shows the turned-up edges of the spreading center.
As crust moves away from the ridge it cools and sinks. The lateral offsets
in the ridge are joined by transform faults.
(from RIDGE, LDEO/Columbia Univ.)
A satellite view of the Sinai shows two arms of the Red Sea spreading
ridge, exposed on land.
Extensional ridges exist elsewhere in the solar system, although they
never attain the globe-encircling extent the oceanic ridges have on Earth.
This synthetic perspective of a large volcano on Venus is looking up the
large rift on its flank.
(from the USGS)
At transforms, earthquakes are shallow, running as deep as 25 km; mechanisms indicate strike-slip motion. Transforms tend to have earthquakes smaller than magnitude 8.5.
The San Andreas fault in California is a nearby example of a transform,
separating the Pacific from the North American plate. At transforms the
plates mostly slide past each other laterally, producing less sinking or
lifing of the ground than extensional or compressional environments.
The yellow dots below locate earthquakes along strands of this fault
system in the San Francisco Bay area.
(from NASA/JSC; topography from NOAA)
At compressional boundaries, earthquakes are found in several settings ranging from the very near surface to several hundred kilometers depth, since the coldness of the subducting plate permits brittle failure down to as much as 700 km. Compressional boundaries host Earth's largest quakes, with some events on subduction zones in Alaska and Chile having exceeded magnitude 9.
This oblique orbital view looking east over Indonesia shows the clouded
tops of the chain of large volcanoes. The topography below shows the Indian
plate, streaked by hotspot traces and healed transforms, subducting at the
Sometimes continental sections of plates collide; both are too light for
occur. The satellite image below shows the bent and rippled rock layers of
the Zagros Mountains in southern Iran, where the Arabian plate is impacting the
Nevada has a complex plate-tectonic environment, dominated by a
combination of extensional and transform motions. The Great Basin shares
some features with the great Tibetan and Anatolian plateaus. All three
have large areas of high elevation, and show varying amounts of rifting
and extension distributed across the regions. This is unlike oceanic
spreading centers, where rifting is concentrated narrowly along the
plate boundary. The numerous north-south mountain ranges that dominate
the landscape from Reno to Salt Lake City are the consequence of
substantial east-west extension, in which the total extension may be as
much as a factor of two over the past 20 million years.
(Topo map from the Lamont-Doherty Earth Observatory of Columbia Univ.; motions added from published GPS results.)
The extension seems to be most active at the eastern and western margins of the region, i.e. the mountain fronts running near Salt Lake City and Reno. The western Great Basin also has a significant component of shearing motion superimposed on this rifting. This is part of the Pacific - North America plate motion. The total motion is about 5 cm/year. Of this, about 4 cm/yr takes place on the San Andreas fault system near the California coast, and the remainder, about 1 cm/year, occurs east of the Sierra Nevada mountains, in a zone geologists know as the Walker Lane.
As a result,
Nevada hosts hundreds of active extensional faults, and several significant
transform fault zones as well. While not as actively or rapidly deforming
as the plate boundary in California, Nevada has earthquakes over much
larger areas. While some regions in California, such as the western Sierra
Nevada, appear to be isolated from earthquake activity, earthquakes have
occurred everywhere in Nevada.
J. Louie, 11 May 2001 (with contributions from J. Anderson)
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