EXTENSION DIRECTIONS IN SOUTHWESTERN MONTANA INFERRED FROM FAULT PLANE SOLUTIONS

Seismicity in southwestern Montana, between 44.5°-46.5°N and 111°-113.5°W, is produced by slip on a complicated array of reactivated normal, normal oblique, and strike slip faults. Fault plane solutions indicate that the area is experiencing and overall northeast-southwest extension. However, it is recognized that the least principal stress (T-axis) rotates from approximately north-south to east-west along a south to north transect through the study area. This rotation has been explained by a Northern Rocky Mountain sub plate boundary to the south of the Centennial Valley (around 44.5°-45°N). An alternate interpretation is that the T-axis pattern is “arcuate” and is related to the Yellowstone Hot Spot. To examine this rotation of extensional stress further, T-axes from over 450 fault plane solutions were plotted as rose diagrams in 13, 30’ by 30’ segments. T-axes throughout the region are nearly horizontal. Presentation of T-axis data in this way suggests that the primary change in extension direction occurs between 45°N and 46°N and 111.5°W and 112.5°W. The mean T-axis azimuth between 45°N and 45.5°N trends 42°, while the mean T-axis azimuth between 45.5°N and 46°N trends 70°. T-axes are consistent with an approximate north-south extension direction south of 45°N, which includes the 15° trend of the T-axis from the 1959 Hebgen Lake earthquake (M_w 7.3), and an approximate east-west direction north of 46°N. At the resolution of this analysis, the actual pattern is therefore not strictly radial or arcuate and the primary T axis trend change occurs significantly north of the hypothesized sub plate boundary. The N-S trends of the T-axes in the southern part of the region are roughly parallel to the trends of range front faults bounding the Madison and Paradise valleys. This may explain why comparatively few earthquakes occur along these faults.