A nine-station portable seismic net, 50 km in diameter, was operated for 50 days in late 1971 near Flathead Lake, Montana at the juncture of the southern extension of the Rocky Mountain trench and the northern section of the Intermountain Seismic Belt. During the time the net was in operation, 259 locatable earthquakes ranging in coda magnitude from 0.1 to 3.3 were recorded. The earthquakes fall in two source regions approximately 15 km apart. In the northern region, the focal depths average 5 km, while those in the southern region range from 5 to 12 km. During part of the recording period, the activity in the southern region defined a planar zone striking north-northwest with a dip of 70° to the east-northeast. The better-defined fault-plane solutions in the region suggest northsouth compression. A procedure for automatically picking the first arrivals with a computer was used in analyzing the data.

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