NORTHERN EXPOSURE: SEISMOTECTONICS OF THE ISB IN NORTHERN MONTANA

At least two, and possibly as many as four, late Quaternary surface rupturing earthquakes have occurred on the Mission fault in northern Montana near the north end of the ISB. Further north, several large potentially active normal faults, including the Swan and South Fork faults, lack clear evidence of late Quaternary surface rupture. Nonlinear hypocenter uncertainty calculations and 3D velocity-hypocenter inversions confirm Stickney and Lageson's (2004) findings that earthquakes near the southern Swan and South Fork fault trends occur at depths > 20 km and may extend to depths of at least 25 km. However, the Montana Regional Seismographic Network (MRSN) does not encompass the northern Swan and South Fork faults and currently cannot currently constrain focal depths of earthquakes occurring near these northern faults. The Bureau of Reclamation operated four temporary seismographic stations from Dec. 2006 to Sep. 2007 to supplement the MRSN in the northern Montana to study seismicity near the Swan and South Fork faults. The temporary network recorded 35 local earthquakes in the region encompassed by the northern Swan fault and South Fork faults. All these earthquakes occurred at depths < 15 km. In contrast, in the same eleven month period the MRSN located at least 10 earthquakes in the 15-25 km depth range near the southern Swan fault, the same region that has had persistent earthquake activity at depths > 15 km since monitoring by the MRSN commenced in 1980. The 3D velocity inversion indicates that the deep earthquakes along the southern Swan fault occur on the east flank of a region of thickened > 7 km/s lower crust that extends about 50 km south of the Swan fault. The temporary seismographic network confirmed that ISB seismicity extends well north of the Mission fault to the northern end of the Swan and South Fork faults, but that the unusually deep earthquakes along the southern Swan and South Fork faults do not extend further north. A swarm of earthquakes was located close to the downdip projection of the northern Swan fault near Columbia Falls and several small earthquakes occurred close to the downdip projection of the South Fork fault. The seismicity and focal mechanisms suggest that active ISB normal and strike-slip faulting extends at least to the northern ends of the Swan and South Fork faults.