Tilt has been measured continuously at tidal sensitivity in borehole and observatory sites in the San Francisco Bay region, California. These sites, within a few kilometres of a m ajor fault and the Pacific Ocean, are part of an extensive network measuring strain and microseismicity. Static response of the Presidio site to ocean loading at the M 2 frequency best fits a finite-element model with lower shear modulus in the San Andreas fault zone than in adjacent material at the same depth. The Presidio tilt data exhibit a secular trend less than 3 (rrad/year superim posed on local earthquake and meteorological effects. On two occasions earthquakes (M„ > 4.3) occurred within 55 km of a station and were preceded by anom alous tilt accumulating to 1 ^rad over several days with an accelerated rate of tilt a few hours before the events. The root-mean-square (r.m.s.) difference for two stations of 25 km apart for 700 h before and after one of these events was 5 x 10-8 and 2 x 10 ^ respectively . A similar r.m.s. difference was observed before and after a larger ( M b> 5) but more distant (180 km) earthquake from the same two stations. This latter event did not, however, exhibit the extreme linear slope (10~9 rad/h) of the two earlier earthquakes. Although such anomalies can be correlated with meteorological activity over short periods of time, they do not correlate for periods approaching one month. A transfer function derived during a period w hen there were no local earthquakes can be used for calculating tilt response to surface loading from telemetered meteorological and tilt data. These results can then be input for a prediction beyond the data, and the error in prediction monitored as a final output for instrument performance and potential earthquake hazard.
Ocean loading tides corrections of GPS stations in Antarctica