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Small earthquakes, structure, and tectonics of the Puget Sound region

GEOPHYSICS PROGRAM UNIVERSITY OF WASHINGTON, SEATTLE, WASHINGTON 98195

Abstract

The Puget Sound region of Washington lies along the tectonically active margin of western North America, a region which is important to the understanding of large scale plate interactions in the eastern Pacific. A six-station high-sensitivity telemetry network has been operating for more than a year providing data for an initial study of seismicity, structure, and tectonics. The network covers an area of about 200 km N-S by 150 km E-W in a zone of concentrated seismic activity. Earthquakes ranging in magnitude 1 to 4 are located within the network at a rate of about one event every 2 days.

Analysis of regional earthquakes lying outside the network yields an apparent P_n velocity of 7.79 km/sec. Lacking reliable refraction data, a crustal model is generated by minimization of P residuals. Using this model, the average depth of all located events is 21 km; however, earthquakes are reliably located as deep as 50 km and a few isolated events may be deeper. No strong lineations are observed in the pattern of hypocenters, although there are some general zones of high activity within the array. Magnitude estimates are successfully based on a coda-length scheme. Recurrence rates approximately obey the equation log N = 3.92 – 0.96 m where N is the cumulative number of events above magnitude m per year.

Composite focal mechanisms for three groups of events in the region are suggestive of N-S compression and taken together with absence of a distinct Benioff zone indicate that crustal consumption may have ceased in the area. The N-S compressive tectonics reflect the regional stress field associated with the movement of the American plate with respect to the Pacific plate as defined by slip on the San Andreas and Queen Charlotte Islands fault systems. The Puget Sound-Olympic Mountains region may have occupied a position at the northeast corner of the Juan de Fuca plate during a latter phase of active underthrusting with the localization of seismicity caused by stress inhomogeneity at the plate corner.

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