abstract

An earthquake of magnitude ML 4.5 occurred on June 20, 1976 in an area of complex faulting in northeastern California, near the intersection of the Sierra Nevada, Modoc Plateau, Cascade Range, and Basin and Range geological provinces. P-wave first motion plots for larger aftershocks of this earthquake indicate maximum and minimum compressive stress, respectively, in north-south and east-west directions, with predominantly strike-slip motion. Focal depths for these events ranged from 7 to 15 km, consistent with other earthquake sequences in the region. Origin times of more than 4,700 aftershocks for the period between June 20 and July 1 are compared with the phase of solid-earth tidal components appropriate for normal and shear stress on northeast- and northwest-trending fault planes. Based on this comparison, approximately 20 per cent more earthquakes occurred at times when the normal compressive stress on the fault plane was decreasing, and the shear stress was increasing in the sense of slip on the fault plane. This correlation may be explained by two large bursts of aftershocks that occurred at times when tidal stresses were favorable for motion on the fault plane, rather than continuous triggering of small events during the entire sequence.

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