Variations in fault slip and strain accumulation at Parkfield, California: Initial results using two-color geodimeter measurements 1984-1988
Repeated length measurements of several geodetic baselines near Parkfield, California, have revealed significant variations in the local rates of shallow fault slip and strain. This network of baselines, surveyed several times each week, straddles the San Andreas fault in the transition zone between the creeping section to the northwest and the locked section to the southeast. The length measurements, characterized by a precision approaching 0.1 ppm, reveal large fluctuations in the rates of baseline extension. Principal mode analysis of the length change data indicates that the two largest components of the signal are (1) secular extensions and contractions consistent with surface slip on the main strand of the San Andreas fault, and (2) a large seasonal oscillation with no obvious spatial coherence. On most of these baselines, the second component appears to be in phase with seasonal rainfall. When data from the baselines with the largest amplitude of the seasonal signal are excluded, the remaining data can be modeled in terms of both spatial and temporal variations in surface slip, variations in the components of the changes in uniform strain, and the possible displacement of the central monument in this radial network. In parameterizing this model, the spatial variation of slip beneath the near surface is reflected by changes in shear strain. Although the computed secular shear is highly dependent upon the specified parameterization of surface slip, the data are consistent with the hypothesis that slip at intermediate depths lags behind the surface slip rate. However, the range in models that fit the data does not necessarily imply that there is a deficit in slip at depth relative to the surface. Comparison of the inferred values of surface slip from the model with the observed fault slip measured by very short baseline creep meters indicates close agreement in secular rates, but the short-term variations observed with the creep meters are either highly attenuated or nonexistent in the modeled slip since the modeled slip is a spatial average which smooths out possible short-wavelength variations in the surface slip for which the creep instruments are most sensitive. An interesting conclusion from the two-color data is that surface slip on the San Andreas fault appears to be spread over a 2-km-wide zone on the south flank of Middle Mountain but is confined to a very narrow zone to the south as the fault passes through the center of the network. This conclusion is dependent upon the assumption that a few critical monuments are stable and track tectonic displacements in the long term. Finally, the largest observed strain change is an extensional strain coincident with the Kettleman Hills earthquake M 5.5 in August 1985.