Causes Earthquakes

Causes

Most earthquakes are caused by the sudden slip along geologic faults. The faults slip because of movement of the Earth’s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the Earth’s crust.

A Elastic Rebound Theory

In 1911 American seismologist Harry Fielding Reid studied the effects of the April 1906 California earthquake. He proposed the elastic rebound theory to explain the generation of certain earthquakes that scientists now know occur in tectonic areas, usually near plate boundaries. This theory states that during an earthquake, the rocks under strain suddenly break, creating a fracture along a fault. When a fault slips, movement in the crustal rock causes vibrations. 

The slip changes the local strain out into the surrounding rock. The change in strain leads to aftershocks (smaller earthquakes that occur after the initial earthquake), which are produced by further slips of the main fault or adjacent faults in the strained region. The slip begins at the focus and travels along the plane of the fault, radiating waves out along the rupture surface. 

On each side of the fault, the rock shifts in opposite directions. The fault rupture travels in irregular steps along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations that propagate as seismic waves. After the earthquake, strain begins to build again until it is greater than the forces holding the rocks together, then the fault snaps again and causes another earthquake.

B Human Activities

Fault rupture is not the only cause of earthquakes; human activities can also be the direct or indirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal, filling reservoirs with water, and firing underground nuclear test blasts can, in limited circumstances, lead to earthquakes. These activities increase the strain within the rock near the location of the activity so that rock slips and slides along pre-existing faults more easily. 

While earthquakes caused by human activities may be harmful, they can also provide useful information. Prior to the Nuclear Test Ban treaty, scientists were able to analyze the travel and arrival times of P waves from known earthquakes caused by underground nuclear test blasts. Scientists used this information to study earthquake waves and determine the interior structure of the Earth.

Scientists have determined that as water level in a reservoir increases, water pressure in pores inside the rocks along local faults also increases. The increased pressure may cause the rocks to slip, generating earthquakes. Beginning in 1935, the first detailed evidence of reservoir-induced earthquakes came from the filling of Lake Mead behind Hoover Dam on the Nevada-Arizona state border. Earthquakes were rare in the area prior to construction of the dam, but seismographs registered at least 600 shallow-focus earthquakes between 1936 and 1946. Most reservoirs, however, do not cause earthquakes.