Time-dependent gravity in southern California, May 1974 to April 1979.

The Southern California gravity monitoring project, begun in May 1974, is intended to coordinate gravity measurements with the long-baseline three-dimensional geodetic measurements of the Astronomical Radio Interferometric Earth Surveying (Aries) project, which uses radio interferometry with extragalactic radio sources. Gravity data from 28 of the stations, monitored on an approximately 1- to 2-month basis, have a single-reading standard deviation of 11 μGal that gives a relative single determination between stations a standard deviation of 16 μGal. The averaging of data reduces the uncertainty, and if gravity does not change during the averaging time, it appears that gravity at a station relative to the base can be determined with a standard error of 2 to 3 μGal. Where stations could not be placed on lowporosity bedrock, the effects of variable groundwater levels must be considered. The largest gravity variation observed, 80 μGal, correlates with nearby water well variations and with smoothed rainfall. Smoothed rainfall data appear to be a good indicator of the qualitative response of gravity to changing groundwater levels at other suprasediment stations, but frequent measurement of gravity at a station is essential until the quantitative calibration of the station's response to groundwater variations is accomplished. The largest earthquake to occur during the survey time near the gravity network was the August 13, 1978, Santa Barbara Channel event (ML = 5.1–5.7, MS = 5.6). The closest gravity station to this earthquake, 67 km east of the epicenter, also exhibits the network's largest gravity change that cannot be related to factors other than tectonic distortion. This change is a 50-μGal low occurring from mid-1975 to mid-1977. It is not known if the occurrence of the gravity anomaly is coincidental or related to the process of preparation for the earthquake.