How deep is an earthquake?

For over a half a century, seismologists have known that some earthquakes originate well below the earth's outer crust. Most of these deep quakes dot the world map at collision boundaries between plates; the quakes are thought to result from the build-up of stresses in one plate as it is subducted or dragged down under another.

Several hundred earthquakes orginating deeper than 650 kilometers, including some at 800 km, have been reported since the 1930s. However, according to two geophysicists who recently evaluated the earthquake catalogs, the evidence backing up almost all of the reported very deep events is weak, inconsistent with other data or nonexistent. The deepest reliable depths, they conclude in the Feb. 10 JOURNAL OF GEOPHYSICAL RESEARCH, are at 670 to 680 km.

Philip B. Stark at Scripps Institution of Oceanography in La Jolla, Calif., and Cliff Frohlich at the University of Texas at Austin also found that the number of large quakes drops quite abruptly below 650 km. But the researches can't tell if seismic activity completely stops at 680 km or if it continues deeper, only with dimisnishing vigor.

Knowing the maximum depth of earthquakes is key to understanding the structure of the mantle, says Stark. An abrupt cessation of earthquakes at one depth means that some property of the mantle, perhaps its composition, has changed there. One model consistent with an abrupt cessation holds that the lower of two convecting layers in the mantle blocks the downward plunge of the subducted plate. In contrast, Stark and Frohlich believe, on the basis of other data, that the subducting slab continues much farther. They think the change in seismicity at 670 km is due to phase change, or rearrangement of the slab crystal structure, which might strengthen or relieve stresses in the slab, or anneal cracks.