Continuous Monitoring of Seismic Energy Release Associated with the 1994 Northridge Earthquake and the 1992 Landers Earthquake
We have developed a method to detect long-period precursors for large earthquakes observed in southern California, if they occur. The method allows us to continuously monitor seismic energy radiation over a wide frequency band to investigate slow deformation in the crust (e.g., slow earthquakes), especially before large earthquakes. We used the long-period records (1 sample/sec) from TERRAscope, a broadband seismic network in southern California. The method consists of dividing the record into a series of overlapping 30-min-long windows, computing the spectra over a frequency band of 0.00055 to 0.1 Hz, and plotting them in the form of a time-frequency diagram called spectrogram. This procedure is repeated daily over a day-long record. We have analyzed the 17 January 1994 Northridge earthquake (M_w = 6.7), and the 28 June 1992 Landers earthquake (M_w = 7.3). No slow precursor with spectral amplitude measured over a duration of 30 min larger than that of a magnitude 3.7 was detected prior to either event. In other words, there was no precursor whose moment was larger than ∼0.003% of the mainshock.
© 1996, by the Seismological Society of America. Manuscript received 3 July 1995. The authors wish to thank Robert Clayton from Caltech for his invaluable advice in constructing the spectrogram. We thank Frank Wyatt from UCSD and Malcolm Johnston from the USGS for sharing their tilt-meter records of the 1992 Landers earthquake. This study was supported by USGS Grant 1434-93-G-2305 and USGS Grant 1434-95-G-2554. Contribution Number 5561 of the Division of Geology and Planetary Sciences, California Institute of Technology.
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