[1] Starting early in 2005, the positions of GPS stations in the San Gabriel valley region of southern California showed statistically significant departures from their previous behavior. Station LONG moved up by about 47 mm, and nearby stations moved away from LONG by about 10 mm. These changes began during an extremely rainy season in southern California and coincided with a 16-m increase in water level at a nearby well in Baldwin Park and a regional uplift detected by interferometric synthetic aperture radar. No equivalent signals were seen in GPS station position time series elsewhere in southern California. Our preferred explanation, supported by the timing and by a hydrologic simulation, is deformation due to recharging of aquifers after near-record rainfall in [2004][2005]. We cannot rule out an aseismic slip event, but we consider such an event unlikely because it requires slip on multiple faults and predicts other signals that are not observed.

Frequent surveys of seven trilateration networks in southern California over the interval 1973–1980 suggest that a regional increment in strain may have occurred in 1978–1979. Prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north‐south compression. This secular trend was interrupted sometime in 1978–1979 by an increment in both north‐south and east‐west extension in five of the seven networks. The onset of this change appears to have occurred first in the networks farthest south. The changes occurred without any unusual seismicity within the networks, but the overall seismicity in southern California was unusually low prior to and has been unusually high since the occurrence. The average principal strain rates for the seven networks in the 1973–1980 interval are 0.17 μstrain/yr north‐south contraction and 0.08 μstrain/yr east‐west extension. Although the observed increment in strain could be related to unidentified systematic error in the measuring system, a careful review of the measurements and comparisons with three other measuring systems reveal no appreciable cumulative systematic error.

Rapid field deployment of a new type of continuously operating Global Positioning System (GPS) network and data from Southern California Integrated GPS Network (SCIGN) stations that had recently begun operating in the area allow unique observations of the postseismic deformation associated with the 1999 Hector Mine earthquake. Innovative solutions in fieldcraft, devised for the 11 new GPS stations, provide high-quality observations with 1-year time histories on stable monuments at remote sites. We report on our results from processing the postseismic GPS data available from these sites, as well as 8 other SCIGN stations within 80 km of the event (a total of 19 sites).

A 24‐station trilateration network spanning the San Andreas and Calaveras faults near Hollister, California, has been surveyed each year between 1971 and 1978, inclusive. Two moderate (ML = 5) earthquakes have occurred within the network during the interval. No convincing preseismic or coseismic anomalies associated with those earthquakes have been identified. The deformation of the network can be described roughly by rigid body motion of the three blocks bounded by the two faults with accommodation occurring by right‐lateral strike slip on the San Andreas (13±2 mm/a) and Calaveras (17±2 mm/a) faults. The required slip rates are within the range of the observed fault creep on those faults. A more detailed analysis of the deformation indicates appreciable strain accumulation (0.4 μstrain/a tensor shear) within the block lying between the San Andreas and Calaveras faults. Many of the features of the observed deformation can be produced by an elementary dislocation model, indicating that most of the deformation is associated directly with slip on the major faults. The network is not extensive enough to define uniquely the relative motion across the San Andreas fault system, but the data are consistent with a value of about 38 mm/a. The rate of deformation in 1971–1978 was not uniform but rather appears to have been higher than normal in 1973–1974 and lower than normal in 1975–1976. Supplement is available with entire article on microfiche. Order from American Geophysical Union', 2000 Florida Avenue, N.W., Washington, D.C. 20006. Document J79‐008;$01.00. Payment must accompany order.

In September 1991, the U. S. Geological Survey began continuous operation of two permanent Global Positioning System (GPS) sites near the Hayward fault. We use two and one half years of data from an 8‐km baseline to investigate GPS processing strategies, errors in the time and frequency domains, and the uncertainties of rates of change calculated from such data. Experiments with session lengths show that at least 6 hours of data should be used to obtain a precision of 2 to 4 mm. Experiments with broadcast and improved orbits show that the broadcast orbit is sufficient for this short baseline. Single‐frequency solutions have larger rms scatter; for scenarios made mostly in daylight, ionospheric delay systematically shortens the baseline length by 2.4 parts per million for L1 and 4.0 parts per million for L2. For the dual‐frequency results, the rms scatter about the best‐fitting straight line is 2.1 mm for baseline length, 2.2 mm for north, 2.9 mm for east, and 11.2 mm for vertical. For Winton relative to Chabot, the rates of change are −0.6±0.1 mm/yr in length, 2.8±0.1 mm/yr in north, −8.1±0.2 mm/yr in east, and −8.1±0.6 mm/yr in vertical. The baseline rate of change is consistent with right‐lateral shear across the Hayward fault but is not consistent with the −3.5 mm/yr predicted by the model of Lienkaemper et al. (1991). The large westward and downward motion of Winton relative to Chabot may be due to monument instability. Power spectra appear white at high frequencies; the estimated standard deviations for periods shorter than 5 days are 1.2 mm for length and north, 1.9 mm for east, and 5.5 mm for vertical. Power spectral density increases only slightly as frequency decreases from 0.2 to 0.03 cycle per day, and there is no distinct corner. In particular, no characteristic 1/ƒ2 signature of random walk monument noise emerges from the white noise. Estimated autocorrelations for length, north, east, and vertical fall to about 0.1 for lag times shorter than 10 days and fluctuate about zero for lag times longer than about 25 days. Because the time series is short, it is possible that random walk monument noise exists but is undetectable in the estimated power spectral density and autocorrelation functions. We use the estimated full covariance matrix to calculate the standard deviations of the baseline rate of change for various sampling schemes. The theoretical standard deviation of dL/dt determined from 1 year of daily observations is 0.28 times that determined from 1 year of annual observations. We can obtain a similar uncertainty from 2 years of measurements made every 30 days and better results from 5 years of annual measurements. However, daily measurements allow the detection and correction of offsets that sometimes occur with equipment or firmware changes.