An introduction to earthquakes in the U.S.

The U.S. Geological Survey (USGS) introduces what an earthquake is and what causes them to happen, plus the depth at which they occur

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 less 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 are two plates – the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coastline. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. The primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone.

The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant “creep” resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is not constant, strain can build up for hundreds of years, producing great earthquakes when it finally releases.

At what depth do earthquakes occur? What is the significance of the depth?

Earthquakes occur in the crust or upper mantle, which ranges from the earth’s surface to about 800 kilometres deep (about 500 miles).

The strength of shaking from an earthquake diminishes with increasing distance from the earthquake’s source so the strength of shaking at the surface from an earthquake that occurs at 500 km deep in considerably less than if the same earthquake had occurred at 20 km depth.

Also, the depths of earthquakes give us important information about the Earth’s structure and the tectonic setting where the earthquakes are occurring. The most prominent example of this is in subduction zones, where plates are colliding and one plate is being subducted beneath another. By carefully plotting the location and depth of earthquakes associated with a subduction zone, we can see details of the zone’s structure, such as how steeply it is dipping and if the down-going plate is planar or is bending. These details are important because they give us insight into the mechanics and characteristics of the deformation in the subduction zone.

The deepest earthquakes occur within the core of subducting slabs – oceanic plates that descend into the Earth’s mantle from convergent plate boundaries, where a dense oceanic plate collides with a less dense continental plate and the former sinks beneath the latter. The plate boundary contact between two such plates generates very large, shallow subduction zone earthquakes such as the Sumatra 2004 M9.1 event and the 2011 M9.0 Japan earthquake and is only active to relatively shallow depths – approximately 60 km. However, because oceanic slabs are relatively cold with respect to the surrounding mantle in deeper subduction zone environments, faults within the core of these slabs remain brittle and can generate earthquakes to depths of as much as 700 km (e.g., Pacific Plate beneath Japan and Kamchatka and beneath Tonga).

As the slab descends into the mantle, rheology changes (viscosity characteristics) cause the plate to bend and deform and generates these earthquakes. The trend of such events can be seen in cross-sections of subduction zones and are known as “Wadati-Benioff Zones”.

Within continents and along continental plate boundary transform faults such as the San Andreas, faults are only active in the shallow crust – perhaps to depths of approximately 20 km.

Accurately determining the depth of an earthquake is typically more challenging than determining its location, unless there happens to be a seismic station close and above the epicentre. So generally, errors on depth determinations are somewhat greater than on location determinations.

Are earthquakes associated with variations in the geomagnetic field?

Electromagnetic variations have been observed after earthquakes, but despite decades of work, there is no convincing evidence of electromagnetic precursors to earthquakes. It is worth acknowledging that geophysicists would actually love to demonstrate the reality of such precursors, especially if they could be used for reliably predicting earthquakes!

Learn more: USGS Geomagnetism Program

How is hydraulic fracturing related to earthquakes and tremors?

Reports of hydraulic fracturing causing felt earthquakes are extremely rare. However, wastewater produced by wells that were hydraulically fractured can cause “induced” earthquakes when it is injected into deep wastewater wells.

Wastewater disposal wells operate for longer durations and inject much more fluid than the hydraulic fracturing operations. Wastewater injection can raise pressure levels in the rock formation over much longer periods of time and over larger areas than hydraulic fracturing does. Hence, wastewater injection is much more likely to induce earthquakes than hydraulic fracturing.

Most wastewater injection wells are not associated with felt earthquakes. A combination of many factors is necessary for injection to induce felt earthquakes.

Learn more at the USGS Induced Earthquakes website

Do earthquakes large enough to collapse buildings and roads accompany volcanic eruptions?

Not usually. Earthquakes associated with eruptions rarely exceed magnitude 5 and these moderate earthquakes are not big enough to destroy buildings and roads.

The largest earthquakes at Mount St. Helens in 1980 were magnitude 5, large enough to sway trees and damage buildings, but not destroy them. During the huge eruption of Mount Pinatubo in the Philippines in 1991, dozens of light to moderate earthquakes (magnitude 3 to 5) were felt by several hundred thousand people. Many houses collapsed, but not primarily because of the shaking. Heavy ash from the eruption (made heavier by rain from a hurricane) accumulated on roofs and crushed them.

Stronger earthquakes sometimes DO occur near volcanoes as a result of tectonic faulting. For example, four magnitude 6 earthquakes struck Long Valley caldera, California, in 1980 and a magnitude 7.2 earthquake struck Kilauea Volcano, Hawaii, in 1975. Both volcanoes were quiet at the time. The Hawaii earthquake triggered a small eruption at the summit of Kilauea. No eruption occurred at Long Valley.

Learn more about how the USGS monitors volcano seismicity

 

Earthquake and Geologic Hazards

U.S. Geological Survey (USGS)

Tel: +1 703 648 6786

wleith@usgs.gov

www.usgs.gov

www.twitter.com/usgs  

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