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When an earthquake happens, when does it stop?
Question Date: 2012-03-14
Answer 1:

An earthquake occurs when rocks break along faults and release seismic energy. The actually breaking of the rock that causes earthquakes happens very quickly. Usually a fault only takes a few seconds to break. Sometimes movement on one fault will cause other faults to break, so a sort of chain reaction of faults breaking may last up to a few tens of seconds (probably less than a minute).

The seismic energy released from an earthquake lasts a lot longer than the fault rupture that causes the earthquake. Intense shaking (from surface waves) can last in an area for several minutes. Seismic waves travel through the earth until their energy is absorbed surrounding rocks. If seismic energy was not absorbed by surrounding rock, seismic waves would travel around the earth forever. Seismic waves from big earthquakes can travel around the world several times. For example surface waves (the type of seismic wave that causes most of the ground motion that we feel) from the 2004 Sumatra earthquake (9.0 magnitude really big) were detected by seismometers after traveling around the earth at least twice. This took over 300 minutes (5 hours) [1].

[1] Aster, R. Sumatra- Andaman Islands Earthquake (Mw=9.0): Global displacement wavefield from the global seismograph network. Data from IRIS/USGS.

Answer 2:

An earthquake stops when one of two things happens: either (1) the force that has built up in the ground that caused the earthquake in the first place runs out, or (2) the blocks of earth sliding past each-other catch on each-other again, causing them to grind to a halt.

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