Low frequency characteristics of twelve large earthquakes

A global survey of the low-frequency (1-21 mHz) source characteristics of large events are studied. We are particularly interested in events unusually enriched in low-frequency and in events with a short-term precursor. We model the source time function of 12 large earthquakes using teleseismic data at low frequency. For each event we retrieve the source amplitude spectrum in the frequency range between 1 and 21 mHz with the Silver and Jordan method and the phase-shift spectrum in the frequency range between 1 and 11 mHz with the Riedesel and Jordan method. We then model the source time function by fitting the two spectra. Two of these events, the 1980 Irpinia, Italy, and the 1983 Akita-Oki, Japan, are shallow-depth complex events that took place on multiple faults. In both cases the source time function has a length of about 100 seconds. By comparison Westaway and Jackson find 45 seconds for the Irpinia event and Houston and Kanamori about 50 seconds for the Akita-Oki earthquake. The three deep events and four of the seven intermediate-depth events are fast rupturing earthquakes. A single pulse is sufficient to model the source spectra in the frequency range of our interest. Two other intermediate-depth events have slower rupturing processes, characterized by a continuous energy release lasting for about 40 seconds. The last event is the intermediate-depth 1983 Peru-Ecuador earthquake. It was first recognized as a precursive event by Jordan. We model it with a smooth rupturing process starting about 2 minutes before the high frequency origin time superimposed to an impulsive source.