This research continues work aimed at understanding the low- frequency radiation from large earthquakes in the frequency band from 1 to 15 millihertz, a frequency range previously unrecorded by high-gain seismographs. Preliminary analysis of 30 large earthquakes has shown that most earthquakes are ordinary in the sense that their low-frequency source spectra are consistent with the moment, origin time, centroid time, and duration estimated from higher-frequency data and standard scaling relations. Some earthquakes, however appear to be compound sources; that is, in addition to a fast, "jerky" rupture with a characteristic duration of tens of seconds, they comprise an episode of slow, smooth deformation with a duration of hundreds of seconds or longer. In addition, for some of these events, the slow component must involve substantial moment release prior to the beginning of the fast component. The assertion that slow precursors exist for some large earthquakes has significant implications for models of earthquake nucleation and strategies for short-term earthquake prediction. This research is a component of the National Earthquake Hazard Reduction Program.
