Stability of coda Q in the region of Parkfield, California: View from the U.S. Geological Survey Parkfield Dense Seismograph Array

Abstract

Many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate‐to‐large earthquakes. Parkfield, California, on the San Andreas Fault, is a promising location for studying premonitory changes in coda Q, Qc, because a large earthquake is likely to occur there. We have investigated Qc using recordings from the U.S. Geological Survey Parkfield Dense Seismograph Array, which is a digital array with 14 triaxial sensors and an aperture of about 1 km. For each earthquake we can measure Qc from up to 42 recordings. Their average is more stable than the measurement from a single station. Using clustered seismicity, we have developed criteria for selecting events and reducing scatter in the measurement. The Qc value determined from a seismogram depends on the position and length of the analysis window. Thus Qc should always be measured from the same length window starting at the same lapse time regardless of the source location. In addition, the band‐limited signal‐to‐noise ratio at the end of the analysis window is important. Qc determined in two frequency bands, 4–8 Hz and 8–16 Hz, from a tight cluster 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 November 1993. Qc measured from local events (Δ < 60 km) in three frequency bands shows larger scatter but has also not changed during this period. For monitoring Qc, observations should include array averaged measurements from a single lapse time. Because Qc measurements made using an analysis window that starts at a constant multiple of the S wave lapse time depend on epicentral distance, a procedure combining the evaluation of the time and distance dependences of Qc also gives stable observations.