Geodetic data along the San Andreas fault between Parkfield and San Francisco, California (latitudes 36°N and 38°N, respectively), have been re‐examined to estimate the current relative movement between the American and Pacific plates across the San Andreas fault system. The average relative right lateral motion is estimated to be 32 ± 5 mm/yr for the period 1907–1971. Between 36°N and 37°N it appears that most, if not all, of the plate motion is accommodated by fault creep. Although strain is presumably accumulating north of 37°N (San Francisco Bay area), the geodetic evidence for accumulation is not conclusive.

The U.S. Geological Survey (USGS) has been monitoring aseismic fault slip in central California for more than 10 years as part of an earthquake prediction experiment. Since 1968, the USGS creep network has grown from one creep meter at the Cienega Winery south of Hollister to a 44‐station network that stretches from Hayward, east of San Francisco Bay, to Palmdale in southern California. In general, the long‐term slip pattern is most variable on sections of the faults where several magnitude 4 and larger earthquakes occurred during the recording period (e.g., Calaveras fault near Hollister and San Andreas fault between San Juan Bautista and Bear Valley). These fault sections are the most difficult to characterize with a single long‐term slip rate. In contrast, sections of the faults that are seismically relatively quiet (e.g., San Andreas fault between Bear Valley and Parkfield) produce the steadiest creep records and are easiest to fit with a single long‐term slip rate. Appendix is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐004; $1.00. Payment must accompany order.

Regularity of seismic slip along a 9 km segment of the Calaveras fault zone is believed to result from steady‐state loading of a creeping fault to generate stresses on an isolated stuck patch which moves in a stick‐slip event in the magnitude range 3 to 4 whenever a critical threshold is reached. The patch behavior can be described by a simple model similar to the spring‐driven frictional models used in laboratory simulations of stick‐slip. The (M ≥ 3) recurrence time for this model is directly proportional to the seismic slip (computed from magnitudes) since the last time the threshold was reached. If the model is correct, an (3 ≤ M ≤ 4) earthquake should occur at 37° 17′ ± 2′ N, 121° 39′ ± 2′ W within 48 days of January 1, 1977.

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Repeated length measurements of several geodetic baselines near Parkfield, California, have revealed significant variations in the local rates of shallow fault shp and strain. This network of baselines, surveyed several times eadt 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 basehues, the second component appears to be in phase with seasonal rainfall. When data from the baselines with the largest asnplitude 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 shp 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 shp, the data are consistent with the hypothesis that shp at intermediate depths lags behind the surface shp 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 shp from the model with the observed fault shp 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 shp since the modeled shp is a spatial average which smooths out possible shortwavelength 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/i//5.5 in August 1985. INTRODUCTIONSince mid-1984, the U.S. Geological Survey (USGS) has been monitoring length changes on a number of geodetic baselines near Parkfield, California. These measurements are made several times weekly and are one component in a program of measurements that were initiated soon after a prediction of a...