Revised Velocity Structure of Western Montana

Abstract

Using a one-dimensional (1D) layered earth approach, we determined a new crust and upper-mantle velocity model for western Montana to improve earthquake hypocenter locations. P -wave arrival times recorded on 280 stations from 1432 well-recorded earthquakes provided input for a sparse damped least-squares sensitivity matrix. We solved for 1D velocity structure, refractor depths, station corrections, and hypocentral positions by minimizing travel-time residuals. The new model has three layers with P -wave velocities of 5.70, 6.12, and 6.53 km/sec, with corresponding interface depths of 7.0, 19.8, and 39.7 km below the surface. The upper-mantle velocity is 8.00 km/sec. We determined station corrections and show that the Yellowstone caldera is the only regional geologic feature not adequately modeled by our new 1D velocity model. Use of the new model and station corrections reduced hypocenter location uncertainties and travel-time residuals, implying improved hypocenter locations for western Montana earthquakes. Online material : Table of seismographic stations.