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 wid e frequency band to investigate slow deformation in the crust (e.g., s low earthquakes), especially before large earthquakes. We used the lon g-period records (1 sample/sec) from TERRAscope, a broadband seismic n etwork in southern California. The method consists of dividing the rec ord into a series of overlapping 30-min-long windows, computing the sp ectra 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 procedu re is repeated daily over a day-long record. We have analyzed the 17 J anuary 1994 Northridge earthquake (M(w) = 6.7), and the 28 June 1992 L anders earthquake (M(w) = 7.3). No slow precursor with spectral amplit ude 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 similar to 0.003% of the mainsh ock.