By definition, areas where earthquakes occur are areas of active tectonics. Earthquakes represent the sudden release of slowly accumulated strain energy and hence provide direct evidence of active tectonic processes. Understanding the seismic hazard in a region can be enhanced by comparing earthquake mechanism data with geodetic observations of strain accumulation and geologic information on recent tectonics [ Ward, 1994]. For this reason the systematic accumulation of earthquake mechanisms represents an important contribution to ongoing studies of seismic hazard. In addition to this steady increase in catalogs of earthquake mechanisms, there were several earthquakes that occurred during the last quadrennium that were particularly important due to their implications for seismic hazards.
During the last 4 years there were a number of earthquakes that might be termed ``unexpected'' These events provide direct evidence for active tectonic processes in areas that previously may have been considered inactive. Continuing studies of the Coalinga earthquake sequence [ Stein and Ekström, 1992]; Ekström et al., 1992] led to a greater appreciation for the seismic hazard posed by blind thrust faults . Hwang and Kanamori [1992] studied a sequence of three major strike-slip, intra-plate earthquakes in the Gulf of Alaska. Abers et al. [1993] studied the Bering Shelf earthquake of 1991 and concluded that it indicated ongoing deformation of the Zemchung Basin on the Bering Shelf.
Several recent earthquakes are of particular interest because of their
tectonic implications. The 1987 Whittier Narrows and 1994 Northridge
earthquake demonstrated that buried thrust faults are a major hazard for
the Los Angeles Basin. The 1991 Costa Rica (
=7.7) earthquake
[ Goes et al, 1993] was a major thrust earthquake that
probably represents the northern extension of the Panama deformed belt.
The 1992 Cape Mendocino earthquake [ Oppenheimer et
al., 1993] represents the first recorded large earthquake on the Cascadia
subduction zone interface. The two large aftershocks of this event
(
=6.6,6.6), like the sequence of earthquakes studied by
Hwang and Kanamori [1992], represent internal
deformation of an oceanic plate as it approaches a subduction zone.