Decrease in deformation rate observed by two-color laser ranging in long valley caldera.

After the January 1983 earthquake swarm, the last period of notable seismicity, the rapid rate of deformation of the south moat and resurgent dome of the Long Valley caldera diminished. Frequently repeated two-color laser ranging measurements made within a geodetic network in the caldera during the interval June 1983 to November 1984 reveal that, although the deformation accumulated smoothly in time, the rate of extension of many of the baselines decreased by factors of 2 to 3 from mid-1983 to mid-1984. Areal dilatation was the dominant signal during this period, with rates of extension of several baselines reaching as high as 5 parts per million per annum during the summer of 1983. Within the south moat, shear deformation also was apparent. The cumulative deformation can be modeled as the result of injection of material into two points located beneath the resurgent dome in addition to shallow right lateral slip on a vertical fault in the south moat.

Observations of strain accumulation across the san andreas fault near palmdale, california, with a two-color geodimeter.

Two-color laser ranging measurements during a 15-month period over a geodetic network spanning the San Andreas fault near Palmdale, California, indicate that the crust expands and contracts aseismically in episodes as short as 2 weeks. Shear strain parallel to the fault has accumulated monotonically since November 1980, but at a variable rate. Improvements in measurement precision and temporal resolution over those of previous geodetic studies near Palmdale have resulted in the definition of a time history of crustal deformation that is much more complex than formerly realized.

Earthquake Prediction: Absence of a Precursive Change in Seismic Velocities before a Tremor of Magnitude 3frac34.

P-wave velocities in the region near the source of a tremor of magnitude 3(3/4) were constant to within 2 percent for 41 days before the event; no evidence of a precursive change in velocity was found. Observations of S-wave velocities and the ratio of P-wave to S-wave velocities also showed no precursive changes. In recent studies, premonitory changes in body-wave velocities of about 10 percent and having a duration of 2 to 3 weeks have been reported for crustal earthquakes of this size.