Observed times of the Montana earthquakes, 1935*

Unusual interest attaches to seismograms of the earthquakes destructive at , 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 be recorded at distances up to about 90°. The similarity of the seismograms, the constancy of time differences between corresponding phases of the two , show that they must have had nearly the same epicenter and focal . This epicenter must have been very close to the city of Helena; the damage, and the great number of small associated shocks reported felt and even as strong, are good noninstrumental evidence of this. addition, part of the second large shock, and a large number of small shocks, recorded at Helena by the United States Coast and Geodetic Survey. The -P interval on these records is about one or two seconds.