Continuous strain measurements from 3 three-component invar wire strainmeters installed 1200, 1500, and 1700 m from the San Andreas fault indicate no observable strain change at the instrument resolution (< 10 -8) during 10 episodic creep events on the fault. These strain observations indicate that the slip area responsible for the creep observations is near surface and of quite limited extent. The episodic creep character probably results from the failure properties of near-surface materials rather than general fault behavior, which is better indicated perhaps by averaged creep. Deeper slower slip apparently loads the surface material. Longer-term strain changes (410-*) do occur, but the form of the signal is not what would be expected from simple models, nor is it consistent, for successive events. The amplitude does not increase with creep event amplitude, and similar changes occur without creep events. Deeper slip on the San Andreas fault apparently is smoother than would be inferred from the duration of episodic creep observations. Unfortunately, signal discrimination capability gets worse at longer periods and needs improvements if slow deformation waves are to be detected at strain levels below 10 -7.
Three years of continuous records of surface tilt preceding a moderate earthquake (ML = 4.3) on January 8, 1977, have been obtained at a point 5.5 km from the earthquake epicenter. A possible short-term percursive tilt to the southwest started December 18, 1976 and reached a maximum amplitude of 2 μradians relative to the tilt trend at this time. Other changes of this amplitude are evident, however, in the 3-year record. The sense of tilt changed abruptly following the earthquake, gradually returning to the general tilt trend. A substantial postseismic tilt of 10 μradians is consistent with aseismic slip of the Hayward fault, or on any of several other faults local to the tiltmeter in this region. The data are insufficient to discriminate between these possibilities, and accompanying surface displacements are apparently too small to be detected in the geodetic records. Short-term accelerated tilting just preceding the seismic events, as proposed by Wood and Allen (1971), are not apparent in these data. An observed coseismic tilt step of 0.14 μradians does not agree with that expected from current fault-failure models.
Four mercury-tube type tiltmeters, operated by the U.S. Geological Survey, have been in continuous operation since 1969 in the San Francisco Bay area. This paper contains a brief description and history of these tiltmeters, and a summary of data recorded during the years 1974 to 1983. Two of these tiltmeters are located in the University of California, Berkeley, Byerley seismic vault just east of the Hayward fault, lat. 37*51.6', and long. 122°14.9' at an elevation of 276 meters, as shown in Fig. 1. The tunnel for the Byerley vault is drilled into the steeply folded beds of the Clairemont formation. This formation consists of thin bedded shales and cherts. The vault is about 2 km from the Hayward fault. The Wildcat, University, and the Strawberry Canyon faults are also located in the area. The University and Strawberry Canyon faults run perpendicular to the Hayward and Wildcat faults. The other two instruments are located in a vault in the Presidio of San Francisco, lat. 37.79, and long. 122.47. The vault was originally constructed as an ammunition bunker during the Spanish-American war. This vault has been used as a test facility by the USGS since 1968. It is Duilt on Franciscan chert and sandstone on top of a hill, overlooking the Pacific Ocean near the Golden Gate bridge. The walls are reinforced concrete approximately 1/2 meter thick, and the roof is heavily reinforced with the steel girders. The interior rooms can be isolated from one another. A thick (5 meter) overburden of sand covers the bunkers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.