Rupture Directions

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Science  27 Oct 2000:
Vol. 290, Issue 5492, pp. 675
DOI: 10.1126/science.290.5492.675a

If you apply stress to a rock, a rupture may propagate along a zone of weakness, for example along a small fracture or a boundary between different minerals. While it is relatively easy to study the growth and direction of rupture in rocks in a laboratory, it is more difficult to study ruptures in Earth's crust.

Rubin and Gillard have been able to measure the direction of rupture along the San Andreas Fault by analysis of precisely located earthquakes. Using a waveform cross-correlation technique, they determined the location of about 4300 earthquakes along 50 kilometers of the San Andreas Fault, just south of the San Francisco Bay area. As the earthquake sites could be located within meters whereas the ruptures extend for hundreds of meters, the direction of rupture could be measured. Ruptures tended to travel more easily in the direction of the weaker material. Along the San Andreas Fault, the weaker material is either the North American plate rocks, which have a lower velocity than the Pacific plate rocks, or fault gouge, which is finely crushed and fractured rock squeezed between the two plates. — LR

J. Geophys. Res. 105, 19095 (2000).

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