Crustal movement investigations

Abstract

Studies of horizontal crustal movement using conventional geodetic methods have been considerably expanded in the quadrennium 1971–1974. The basic fault monitor Geodimeter network now covers most of the major faults in California as well as the zone of faulting that extends into Nevada. Isolated Geodimeter networks in seismic areas of Montana, New Mexico, Utah, and Washington are also monitored. Part of the Geodimeter network along the San Andreas fault has been monitored for over 15 yr, and there appears to be a systematic deviation from a linear trend for most of the lines [ Greensfelder and Bennett , 1973; Savage et al. , 1973]. However, this deviation may be an artifact of a change in survey procedures in mid‐1969. The conclusions that can be drawn from the Geodimeter observations at present are the following: (1) The present apparent interplate motion across the San Andreas fault in central California is only 30–40 mm/yr [ Savage and Burford , 1973], in contrast to the average of 50–60 mm/yr as estimated from magnetic anomalies at the mouth of the Gulf of California. (2) Measurements of fault creep on the creeping segment of the San Andreas fault agree reasonably well with the plate movement indicated by geodetic measurements, this agreement suggesting that fault creep on this segment is the principal mode of accommodation [ Savage and Burford , 1971]. (3) The rate of strain accumulation along the San Andreas fault is not well measured anywhere, but the overall tensor strain rate appears to be about 0.3 microstrain/yr or less [ Savage et al. , 1973]. A considerable effort has also been expended reanalyzing old triangulation data for networks that cross the San Andreas fault. In such a study, Thatcher [1974] deduced evidence for several meters of afterslip on the San Andreas fault at depths greater than 10 km following the 1906 earthquake. This appears at the surface as a very rapid accumulation of strain in the years immediately following the earthquake. Meade [1974] has called attention to a remarkably uniform change of the astronomic azimuth with time for a line that crosses the San Andreas fault about 35 km south of the southern terminus of the 1906 rupture. This change of azimuth implies a uniform right lateral motion across the fault amounting to 32 mm/yr in the period 1885–1962. By comparing a 1942 triangulation survey and a 1970 Geodimeter survey, Page [1972] found neither lateral slip nor shear strain accumulation consistent with lateral slip across the Denali fault in Alaska. He did observe a north‐south extension that was possibly an effect of strain release at the time of the 1964 Alaska earthquake.