DIFFERENTIAL ANALYSIS OF CODA-Q USING SIMILAR MICROEARTHQUAKES IN SEISMIC GAPS .2. APPLICATION TO SEISMOGRAMS RECORDED BY THE PARKFIELD HIGH-RESOLUTION SEISMIC NETWORK

Nearly two-thirds of the microseismicity occurring at Parkfield, Calif ornia, consists of repeating clusters of closely located events exhibi ting waveform cross-correlation values exceeding 0.95. This observatio n, coupled with a high probability for an M approximate to earthquake within the next decade (Working Group on California Earthquake Probabi lities, 1988), make this segment of the San Andreas fault system a pri me location to search for temporal variations in coda Q associated wit h crustal changes that may occur during the earthquake cycle. The coda comparison analysis method described in Part 1 of this study (Aster e t al., 1996) is applied to data from four borehole stations of the Par kfield High Resolution Seismic Network (HRSN) for the period January 1 987 through February 1994. We examine 21 of the nearly 300 repeating c lusters observed to date. The restriction of the data set to only the most highly similar sources is observed to be critical; small differen ces in the source process, even for event pairs with centroid location s within approximately 20 m, significantly increase the spurious noise level in the parameter estimates. Absolute 68% confidence bound estim ates obtained on the repeatability of coda Q for all microearthquake p airs are +/- 10% for frequencies below 20 Hz. Tighter constraints are obtainable by reconstructing the temporal history of coda Q from the f irst difference measurements between microearthquake pairs. These func tional reconstructions show that the mean value of coda Q has not vari ed by more than approximately 5% from 3 to 30 Hz at the 1 sigma level during the study period. However, other observables, such as an increa sed level of microseismicity, suggest that a preparatory stage for the next Parkfield earthquake has begun, indicating that coda Q is so far showing no sign of sensitivity to preparatory processes at Parkfield.