STABILITY OF CODA-Q IN THE REGION OF PARKFIELD, CALIFORNIA - VIEW FROM THE US-GEOLOGICAL-SURVEY-PARKFIELD DENSE SEISMOGRAPH ARRAY

Many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate-to-larg e. earthquakes. Parkfield, California, on the San Andreas Fault, is a promising location for studying premonitory changes in coda Q, Q(c), b ecause a large earthquake is likely to occur there. We have investigat ed Q(c) using recordings from the U.S. Geological Survey Parkfield Den se Seismograph Array, which is a digital array with 14 triaxial sensor s and an aperture of about 1 km. For each earthquake we can measure Q( c) from up to 42 recordings. Their average is more stable than the mea surement from a single station. Using clustered seismicity, we have de veloped criteria for selecting events and reducing scatter in the meas urement. The Q(c) value determined from a seismogram depends on the po sition and length of the analysis window. Thus Q(c) should always be m easured from the same length window starting at the same lapse time re gardless of the source location. In addition, the band-limited signal- to-noise ratio at the end of the analysis window is important. Q(c) de termined in two frequency bands, 4-8 Hz and 8-16 Hz, from a tight clus ter of 26 events which occurred between December 1989 and January 1994 has not changed, despite M 4.7 and M 4.6 events in October 1992 and N ovember 1993. Q(c) measured from local events (Delta < 60 km) in three frequency bands shows larger scatter but has also not changed during this period. For monitoring Q(c), observations should include array av eraged measurements from a single lapse time. Because Q(c) measurement s made using an analysis window that starts at a constant multiple of the S wave lapse time depend on epicentral distance, a procedure combi ning the evaluation of the time and distance dependences of Q(c) also gives stable observations.