HR: 0830h
AN: S31E-0803    [PDF]
TI: Updated Western Montana Velocity Model With Regional Model Comparison
AU: * Zeiler, C P
EM: cpzeiler@mtech.edu
AF: Montana Tech, 1300 W. Park Street, Butte, MT 59701 United States
AU: Stickney, M C
EM: mstickney@mtech.edu
AF: Montana Tech, 1300 W. Park Street, Butte, MT 59701 United States
AU: Speece, M A
EM: mspeece@mtech.edu
AF: Montana Tech, 1300 W. Park Street, Butte, MT 59701 United States
AB: We determined a new crust and upper mantle velocity model for western Montana using a 1-D layered earth approach. P-wave arrival times recorded on 358 stations from 1,432 well recorded earthquakes provided input for a sparse damped least squares sensitivity matrix. We solved for 1-D velocity structure, depth boundary (refractor) positions, station corrections, and hypocentral positions from calculations of travel-time residuals. The final model showed the first layer having a velocity of 5.7 km/s and a depth of 7.0 km, the second layer having a velocity of 6.1 km/s and a depth of 19.8 km, the third layer having a velocity of 6.5 km/s and a depth of 39.7 km, and an upper mantel velocity of 8.0 km/s. We split the study area into subregions and separately determined 1-D velocity models for each subregion to observe lateral velocity variations. Velocity models determined for subregions showed a maximum difference of 0.25 km/s velocity and 3.5 km layer-thickness variations from the model obtained for the entire study area. Station corrections computed for the subregions and the entire study area were correlated to local travel-time anomalies rather than anomalies of wider extents associated with multiple stations. The new model and station corrections improve resolution and accuracy of hypocenter locations for western Montana earthquakes. A comparison of published velocity models for the Intermountain Seismic Belt of western Montana shows significant trends of the crustal thickness changes. 1-D velocity models from eastern Washington, the Yellowstone Caldera, the Snake River Plain, and eastern Montana suggest that the crust gradually thickens from NW to SE across the region. The subregions velocity model analysis showed a thicker high velocity layer for the central part of the study area. A 3-D velocity model in development for our study area should refine this interpretation.
DE: 1734 Seismology
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 7215 Earthquake parameters
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting