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December 14, 1998
By Tim Stephens
New results from sites around the world show a consistent trend in the rupture duration of earthquakes occurring in subduction zones: Rupture duration varies with depth, from shallow, slow events (including the kind that trigger tsunamis) to faster, more typical ruptures at greater depths.
Graduate student Susan Bilek and professor and chair of earth sciences Thorne Lay, both of UC Santa Cruz, first observed this depth-dependent relationship in the northern Japan subduction zone (Bilek and Lay, Science 281:1175-1178, 1998). They have now extended their observations to subduction zones in South America, Central America, Mexico, and Alaska.
Major earthquakes are common along subduction zones, where one plate of the Earth's crust dives down below another plate. Typically an oceanic plate slides under a continental plate, generating earthquakes along the interface between the two plates. Some of the largest and most destructive earthquakes occur on subduction zone faults.
Bilek set out to analyze how the rupture process varies with depth, in part because previous observations indicated that large tsunamis may be generated by shallow earthquakes with abnormally long rupture durations. In addition, rupture duration is an indicator of the rigidity of sediments along the fault and provides information about friction between the plates and other properties of the subduction zone.
Bilek examined earthquake records for each subduction zone, determined the source depth for each earthquake, and calculated the duration of the first large pulse of energy generated by the rupture process. She found the same general trend of decreasing duration with depth in each subduction zone, as well as some interesting differences. For example, the northern Japan subduction zone shows an abrupt change in behavior at a certain depth, whereas the Alaska-Aleutian subduction zone shows an almost exponential decay of rupture duration with increasing depth.
The researchers are now looking at certain properties of the subduction zone interfaces that might explain these differences in behavior. Bilek says the most important factors are probably the amount of sediment being subducted and the roughness of the subducting plate's surface.
Ultimately, this work may lead to a new understanding of the processes involved in subduction zone earthquakes, which include some of the most destructive earthquakes in the world.