The Ghost of an Earthquake

摘要

Unlike the shadowy remains of de-parted souls, past earthquakes can leave traces that are detectable today with modern geophysical instrumentation. If large enough, a seismic event in the upper crust (less than about 15-km depth) can change the state of stress in the Earth to mantle depths (depths greater than about 30 km. These stresses subsequently relax over time scales of weeks to decades. The length of time needed to fully equilibrate depends on the material properties of the lower crust and upper mantle, and the size and style of the earthquake. On page 1473 of this issue, Gourmelen and Amelung report evidence that relaxation following large earthquakes in the early to mid-20th century is presently observable in central Nevada. Because it is transitory, the presence of this signal has profound implications for our interpretation of geodetic data with respect to crustal deformation in the western United States.