Observed times of the Montana earthquakes, 1935

Unusual interest attaches to seismograms of the earthquakes destructive at Helena, Montana, in October, 1935. The two principal shocks, at 4 h on October 19 and 18 h on October 31 (G.C.T.), were large enough for the P phases to be recorded at distances up to about 90°. The similarity of the seismograms, and the constancy of time differences between corresponding phases of the two shocks, show that they must have had nearly the same epicenter and focal depth. This epicenter must have been very close to the city of Helena; the considerable damage, and the great number of small associated shocks reported as felt and even as strong, are good noninstrumental evidence of this. In addition, part of the second large shock, and a large number of small shocks, were recorded at Helena by the United States Coast and Geodetic Survey. The S-P interval on these records is about one or two seconds.