|Themes > Science > Earth Sciences > Geology > Earthquakes > Types of Earthquakes|
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
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.
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.
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 Javan Trench.
Sometimes continental sections of plates
collide; both are too light for subduction to 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 Iranian plate.
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
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