P wave crustal velocity structure in the greater Mount Rainier area from local earthquake tomography

We present results from a local earthquake tomographic imaging experiment i n the greater Mount Rainier area. We inverted P wave arrival times from loc al earthquakes recorded at permanent and temporary Pacific Northwest Seismo graph Network seismographs between 1980 and 1996. We used a method similar to that described by Lees and Crosson [1989], modified to incorporate the p arameter separation method for decoupling the hypocenter and velocity probl ems. In the upper 7 km of the resulting model there is good correlation bet ween velocity anomalies and surface geology. Many focal mechanisms within t he St. Helens seismic zone have nodal planes parallel to the epicentral tre nd as well as to a north-south trending low-velocity trough, leading us to speculate that the trough represents a zone of structural weakness in which a moderate (M 6.5-7.0) earthquake could occur. In contrast, the western Ra inier seismic zone does not correlate in any simple way with anomaly patter ns or focal mechanism fault planes, leading us to infer that it is less lik ely to experience a moderate earthquake. A similar to 10 km-wide low-veloci ty anomaly occurs 5 to 18 km beneath the summit of Mount Rainier, which we interpret to be a signal of a region composed of hot, fractured rock with p ossible small amounts of melt or fluid. No systematic velocity pattern is o bserved in association with the southern Washington Cascades conductor. A m idcrustal anomaly parallels the Olympic-Wallowa lineament as well as severa l other geophysical trends, indicating that it may play an important role i n regional tectonics.