Seismological studies at Parkfield. IV. Variations in controlled-source waveform parameters and their correlation with seismic activity

work of three-component sensors has been illuminated 52 times using a shear-wave vibrator in three orientations at up to eight source points, in a search for temporal changes in elastic wave P and S velocities, anisotropy, or attenuation. The monitoring interval includes the beginning and end of a severe 3-yr drought and four earthquake sequences, two of which produced the only A-level alerts to date in the Parkfield Prediction Experiment. A comprehensive study of the entire data set reveals a pro-gressive travel-time advance in the coda of S waves propagating in a localized region southeast of Middle Mountain. The anomalous wave field exhibits high apparent velocities, suggesting deep penetration of the fault zone, although similar changes are not seen in waveforms from repeating similar microearthquakes. Accompanying the changes in travel time were systematic variations in spectral content and polar-ization of the same segments of the wave field. These variations correlate well in time and space with significant features of seismicity, fault creep, and water levels at Parkfield. A preferred mechanism for the phenomenon is changing hydrologic conditions along the affected stretch of the fault zone, possibly deformation-induced, that perturb the shallow-propagating S coda in the upper few hundred meters of section. Background One element of the Parkfield prediction experiment (PPE) has been the regular excitation of seismic P and S waves by a shear-wave Vibroseis at several source points (VPs) throughout the 10-station borehole network in a search for evidence of changes associated with the nucleation pro-cess of the anticipated M6 earthquake. Three source orien-tations are employed at each VP to monitor possible changes in fault-zone anisotropy. Measurements were made at roughly 6-week intervals from the 1987 experiment onset through 1990 and have since been repeated every 3 to 4 months, with special recordings during and after the M4 + earthquake sequences and their associated alerts from Oc