SEARCHING FOR SLOW AND SILENT EARTHQUAKES USING FREE OSCILLATIONS

Our knowledge of Earth structure can be used to distinguish far-field seismic signals from near-field noise. In particular, earthquakes excite ground motion at the eigenfrequencies of the Earth's normal modes. By observing the spectral levels in small bands centered on the known frequencies of the normal mode resonance peaks and comparing them to the spectral levels of ambient noise, it is possible to detect earthquakes at low frequencies without knowing anything about them at high frequencies. We apply the technique to the Earth's fundamental spheroidal modes 0S8-0S43 as recorded by the IDA network for the 2-year interval 1978-1979. We calculate the level of excitation at 3-hour intervals using a time domain integration technique designed to optimize the signal-to-noise ratio for each mode. The algorithm detects 50% of all teleseismic earthquakes having seismic moments M0 ≥ 1 × 1018 N m (Mw ≥ 6.0), and it detects all events having M0 ≥ 3 × 1018 N m (Mw ≥ 6.3). Slow earthquakes, ie, those having anomalously large characteristic durations, are observed to be enriched in low-frequency mode excitation relative to other events of comparable magnitude. -from Authors