August 2, 1999

Earthquake analysis sheds new light on the nature of destructive earthquakes

By Tim Stephens

The world's largest earthquakes occur in subduction zones, where one plate of the earth's crust dives down below another plate. A new analysis by UCSC researchers of subduction zone earthquakes indicates that key properties of the fault zones change systematically with depth, resulting in very different types of earthquakes depending on the depth at which the fault ruptures.

Generalized diagram of a subduction zone, with an expanded-scale view of the seismogenic zone at the interface between the two plates of the earth's crust (Larger illustration).

The new findings represent a significant advance in understanding some of the most destructive types of earthquakes, including those that cause tsunamis, said professor of earth sciences Thorne Lay. Lay and graduate student Susan Bilek conducted the study and published their findings in the July 29 issue of the scientific journal Nature.

Bilek and Lay analyzed the records of hundreds of earthquakes that occurred along subduction zones in Japan, Alaska, Mexico, Central America, Peru, and Chile. These are all locations where an oceanic plate is sliding under a continental plate, generating earthquakes along the interface between the two plates. The researchers found that the rigidity of the rock and sediments in the area of contact between the two plates increased steadily with depth in all six subduction zones.

"Rigidity plays a role in determining the amount and duration of shaking that occurs in an earthquake," Bilek said. The rigidity of the material where a fault ruptures affects both the duration of the rupture and the speed of the resulting seismic waves, she explained.

Bilek found that earthquakes in subduction zones vary from shallow events that rupture slowly to faster ruptures at greater depths. This is particularly important in light of previous observations by other researchers indicating that large tsunamis may be generated by shallow earthquakes with abnormally long rupture durations. Some tsunamis occur when an earthquake generates a submarine landslide, but that mechanism can be ruled out in many cases. The new results support the hypothesis that tsunami-causing earthquakes occur in regions of low rigidity at shallow depths, Bilek said.

In addition, according to Lay, the results suggest that tsunami earthquakes can occur in many more places than previously expected, because the properties that characterize them were found in all of the subduction zones studied.

"The consistency between the different subduction zones surprised us," Lay said. "The relationship between rigidity and depth appears to be a common attribute of subduction zones in regions with otherwise very different characteristics."

The recognition of this systematic variation provides a powerful tool for seismologists, Lay said. By incorporating an analysis of rigidity variations into their models of earthquake mechanisms, seimologists may improve the accuracy of their earthquake probability calculations, he noted.

The mechanism behind the systematic increase in rigidity with depth is uncertain, but one possibility is the effect of increasing pressure on sediments carried down into the subduction zone beneath the overriding plate, Bilek said. As pressure and temperature increase with depth beneath the surface, sediments become compacted, water is squeezed out, and minerals undergo major alterations, all of which can increase the rigidity of the subducted materials.

Other factors may also be involved, such as a systematic change in the way stress is released through the rupture process at increasing depths, Lay said. "It is even possible that the actual frictional mechanics of earthquakes change with depth, perhaps as a function of the amount of water present, but this is more speculative," Lay said.

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