Microearthquake data from a downhole seismometer network on the San Andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the Parkfield characteristic earthquakes. The source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most. Furthermore, the magnitude-frequency statistics suggest that at local magnitude 0.6, the cumulative number of small events begins to fall off the usual Gutenberg-Richter (b = -1) relation, in which the number of events increases exponentially with decreasing magnitude. The downhole seismometer data establish a baseline from which the evolution of the earthquake process at Parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical Gutenberg-Richter relation may be operating for the smallest of Parkfield microearthquakes.

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We have developed a technique to measure seismic attenuation within an active fault‐zone at seismogenic depths. Utilizing a pair of stations and pairs of earthquakes, spectral ratios are performed to isolate attenuation produced by wave‐propagation within the fault‐zone. This empirical approach eliminates common source, propagation, instrument and near‐surface site effects. The technique was applied to a cluster of 19 earthquakes recorded by a pair of downhole instruments located within the San Andreas fault‐zone, at Parkfield California. Over the 1‐40 Hz bandwidth used in this analysis, amplitudes are found to decrease exponentially with frequency. Furthermore, the fault‐zone propagation distance correlates with the severity of attenuation. Assuming a constant Q attenuation operator, the S‐wave quality factor within the fault‐zone at a depth of 5‐6 kilometers is 31 (+7,−5). If fault‐zones are low‐Q environments, then near‐source attenuation of high‐frequency seismic waves may help to explain phenomenon such as fmax. Fault‐zone Q may prove to be a valuable indicator of the mechanical behavior and rheology of fault‐zones. Specific asperities can be monitored for precursory changes associated with the evolving stress‐field within the fault‐zone. The spatial and temporal resolution of the technique is fundamentally limited by the uncertainty in earthquake location and the interval time between earthquakes.

The coda waves of 21 Parkfield earthquakes were recorded simultaneously at the surface and at a depth of 198 m. We characterize these codas by computing (1) the time dependence of the integral of squared particle velocity, (2) their frequency content, (3) time‐frequency coda decay planes, (4) the frequency dependence of root mean square ellipticity, and (5) the difference in their normalized decay rates. These five measures reveal significant differences in the early portion of the uphole versus downhole codas. Our analysis solves for coda Q as an explicit function of both time and frequency. Average apparent coda Q for the 21 events observed both at the surface and downhole is Qa(f, τ) = 39(f/f0)0.43 (τ/τ0)0.37, with the reference frequency and time defined at 1 Hz and 1 s, respectively. Coda Q exhibits a trend in its value over the 1‐month period of the study. The trend correlates with a spatial variation in source locations and is not produced by temporal variations of the Q of the medium. The spectral ratio between the uphole signal and the downhole signal varies with time through the coda. The early portion of the uphole coda experiences a pulse of low‐frequency energy not seen in the downhole coda. To explain this observation, we propose a model of the coda that includes body wave to surface wave scattering near the site. In this model, the near‐surface acts as both a filter and a scattering waveguide. The coda at the surface is the sum of a coda produced by scattering deep in the lithosphere and a coda produced by scattering in the near‐surface. The scattering in the near‐surface is made up of near‐vertically traveling multiples and horizontally travelling surface waves. The guided waves are recorded in the surface coda but are absent from the coda recorded at depth. The time‐frequency structure of the surface waves in the uphole coda is determined by the velocity, scattering, and attenuation characteristics of the site. By fitting the model to our data, we have found that the fractional conversion factor of body waves to surface waves travelling toward the receiver is 0.03 at the Vineyard Canyon site. The relatively rapid damping of these trapped waves can be modelled by assuming a quality factor of QT = 50. The principles of this model are general enough to be of use at other locations.

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