Abstract

The magnitude MD = 5.0 Robinson Point earthquake occurred on 29 January 1995 at 03:11 UTC approximately 25 km south of Seattle, Washington. We investigate the single foreshock, the mainshock, and the weak aftershock sequence using sport-period and broadband data recorded by the Pacific Northwest Seismograph Network. P-wave first motions from the mainshock indicate reverse faulting with east-west trending nodal planes. Aftershock epicenters cluster about the main-shock with aftershock hypocenters exhibiting a nearly vertical distribution, possibly an effect of depth error. The mainshock, at a depth of 19.6 km, is the deepest event in the sequence. An empirical Green's function analysis indicates that the mainshock source time function (STF) is impulsive with a 0.3- to 0.4-sec duration. A minimum stress drop of 430 bars is inferred, identifying this as a high-stress-drop event. Comparison with five other moderate-sized Pacific Northwest mainshock-aftershock sequences suggests that mainshocks with depths greater than about 18 km have few, relatively small aftershocks that are confined to small source regions.

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