Tomography and 3D imaging may crack quake prediction

17 Jun 2002

The energy-- or magnitude-- of an earthquake is dependent on three factors: how hard the rock is (sheer rigidity), how fast the fault ruptures, and the size of the fault's rupture area (length times depth). Although the length of many faults is measurable at the surface, the depth of the seismically active part of a fault is not directly observable and is assumed to be the maximum depth of previous quakes. In areas where there are few records of earthquakes, the depth is often assumed to be constant.

University of Houston associate professor of geology Hua-Wei Zhou has found these assumptions about depth to be incorrect. Using a tomographic technique he developed to analyze seismic data from 37,000 earthquakes in southern California since 1981, Zhou has produced 3-D images of the subsurface which show that the maximum depth of the quakes varies widely both along and across the faults.

According to Zhou, the depth of an earthquake depends on the type of underlying rock structure. In areas with minerals that become plastic at lower temperatures, the maximum depth of quakes is shallower than in areas with harder basement rocks. Because the other factors which determine seismic depth (temperature, strain rate, and fluid pressure) are constant at a given depth, it is this lateral variation in rock formation which controls the depth, and thus the magnitude, of potential earthquakes. The composition of faults can therefore be used to estimate the potential magnitude of quakes, Zhou reported in the recent issue of Science.