Triggering of microearthquakes by earth tides, and other features of the Truckee, California, earthquake sequence of September, 1966

Abstract

During an earthquake sequence near Truckee, California, in September 1966, several thousand aftershocks were tape-recorded on a small, tripartite seismic array. Using playbacks of these recordings, together with seismograms from three nearby stations, focal coordinates were calculated for 79 of the larger aftershocks, and epicenters were determined for an additional 29 shocks. The focal region in which these earthquakes were located had approximate dimensions 3 × 6 × 10 km, with its long axis trending N22°E and plunging about 45°NE. Focal depths ranged from 0.8 to 10.3 km, and there was a gradual migration of aftershocks from the vicinity of the main shock at the deeper, northeast end of the zone, to the shallower, southwest end. Variations in the hourly rate of occurrence of aftershocks for the period September 13-29 were cross-correlated with an earth-tide record for the same period, and a statistically significant positive correlation was found between these two sets of data. The earthquake frequency trace was also subjected to a power-spectrum calculation, and was found to contain periodic components identical to those of the principal components of the solid-earth tides. A P-wave fault-plane solution for the main shock agreed both with the line of surface breakage and with a plane fitted to the zone of aftershock foci. Of 245 observations of first-motion for well-located aftershocks, all but 11 were explained by a single type of mechanism, with slight variations in orientation. The determination of 494 magnitudes for earthquakes of this sequence provided the basis for a discussion of problems connected with the interpretation of frequency-magnitude curves. The velocities of waves P_g and S_g were determined for a line extending southeast of Truckee, along the axis of the Sierra Nevada.