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News Link • Geology

Sumatra earthquake mysteries examined

• http://phys.org, by Lin Edwards
 A strike-slip is one in which there is an almost vertical rupture, leading to two plates sliding horizontally past each other. The San Andreas Fault in California produces strike-slip earthquakes, and the most powerful earthquake along this fault was the San Francisco quake of 1906, which measured 7.8.

Events as powerful as the earthquake are usually caused by ruptures in subduction zones, in which the edge of one tectonic plate slips underneath the edge of the adjoining plate. An earthquake of this type on the sea bed can cause massive tsunamis because of vertical displacement of water above the point of slippage. A recent example was the 9 earthquake off Japan last year.

The Sumatra earthquake’s magnitude surprised scientists because it was the most powerful strike-slip earthquake ever recorded. The quake was also unusual in that the rupture was in the middle of an oceanic plate rather than at the boundary between two plates. The slippage distance was surprisingly large; the 1906 San Francisco earthquake produced a slippage of 4.5 meters, while in Sumatra the distance was 21.3 meters.

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