CHANGES IN SOURCE AND SITE EFFECTS COMPARED TO CODA Q-1 TEMPORAL VARIATIONS USING MICROEARTHQUAKES DOUBLETS IN CALIFORNIA

We analyse spectral ratio of the coda of doublets of microearthquakes. Our purpose is to find evidence for temporal changes of the attenuation in the crust before a large magnitude earthquake. A Moving Window Cross Spectral analysis of the coda of doublets gives a plot of the spectral ratio as a function of lapse time along the seismogram, for several frequency bands (SR(T, f) plot). From a certain pattern in the SR(T, f) plot, we should infer a temporal change in coda attenuation. Several doublets recorded in Central California by the USGS network are analysed. Using events very close in time from one another, we show that the radiation pattern can be different enough to induce important variations in the spectral ratio of the first arrivals and of the coda. Another doublet exhibits a strong variation of the low frequencies for stations in the region of Hollister (California), where Phillips and Aki (1986) have noted a strong amplification of low frequencies, that they attribute to site effects on unconsolidated sediments in the fault zone. These variations could be related to slight changes in local conditions (creep, or water table). On the other hand, some doublets, in the vicinity and close in time to the August 1979 Coyote Lake Earthquake (M=5.9), show no variation in SR(T, f) related to this earthquake: this proves that there was no major change in attenuation in the crust preceding this large shock. If a change occurred, it should have been confined to a very limited region, which was not sampled by the many paths we studied. The employed method probably provides today the most accurate estimation of spectral ratio in the coda of microearthquakes. It shows that there are numerous kinds of variations and that it is not straightforward to relate them to coda attenuation changes only. Also, we must be circumspect when dealing with the coda of microearthquakes in relation to forecasting earthquakes. © 1990 Birkhäuser Verlag.