Earthquakes and Stress

Earthquakes occur because some geological movement, such as plate motions or magma moving about, has created stress (or force) in the lithosphere (crust and upper mantle). Each earthquake tells us something about these forces because it shows us which way the rocks next to the fault were pushed. By using many of these earthquakes together we can then determine what the forces are like. A major goal is to determine how the stress field in the lithosphere is changed by large earthquakes. To do this we need excellent recordings of many small earthquakes. Data like this can be obtained from local seismic networks such as operated in California. Perhaps the most exciting result from this work was the demonstration that the 1989 Loma Prieta earthquake removed almost all of the stress from the rocks around the fault. This pattern has also been found after the 1992 Landers earthquake in southern California and may be typical of the largest earthquakes. After somewhat smaller earthquakes the stress field seems to rotate. This was first seen after the 1983 Coalinga earthquake and has also been seen after the 1986 Oceanside and 1994 Northridge earthquakes.