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Seismicity and focal mechanisms associated with the subduction of the Juan de Fuca plate beneath the Olympic Peninsula, Washington

GEOPHYSICS PROGRAM UNIVERSITY OF WASHINGTON, SEATTLE, WASHINGTON 98195
LAMONT-DOHERTY GEOLOGICAL OBSERVATORY, PALISADES, NEW YORK 10964

Abstract

Two and a half years of data from an expanded, telemetered seismic network on the Olympic Peninsula reinforce the conclusion that the Juan de Fuca plate is subducting beneath Washington. However, the rate of seismicity is presently much lower than typical rates at other convergent margins.

The located earthquakes can be divided into two groups on the basis of depth. At the coast, a shallow zone extends to a depth of approximately 20 km while a deep zone begins at 25 km. On the eastern edge of the Peninsula, the shallow zone has thickened to 30 km and the depth to the top of the deep zone has increased to 45 km. The rate of crustal seismicity is much lower on the Peninsula than in the adjacent Puget Sound region. The deep zone is 5 to 10 km thick and dips to the east at an angle of about 11°. The deep events appear to be occurring in the upper part of the subducting oceanic lithosphere.

Focal mechanisms were constructed for 16 events in the suite of deep events with magnitudes ranging from 1.6 to 4.3. The solutions show primarily normal faulting with the P axes normal to the slab and the T axes displaying a range of angles within the slab with the average T axis in the downdip direction. Small events with down-dip tension in the downgoing slab are consistent with the slab sinking under its own weight. Several strike-slip events may be caused by a shearing within the slab. These features are consistent with ongoing subduction of the Juan de Fuca plate beneath North America.

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