The Montana earthquake of June 28, 1925, G.M.C.T.
Authors: Byerly Perry 
Published: May 2021

1. The epicenter of the great Montana earthquake of June 28, 1925, G.M.C.T., is located at:

ϕ = 46 ° 24 ′ ± 05 ′ N . λ = 111 ° 14 ′ .5 ± 06 ′ W .

The time of occurrence is placed at:

O = 1 h 21 m 05 s ± 01 s . G.M.C.T.

2. The travel-time curve for the first preliminary wave is drawn and compared with previous curves. Two abrupt changes of slope are present at epicentral distances, corresponding to wave paths penetrating to depths of 400 m and 1,700 km. The only possibilities of an early first preliminary type, P1, are at Cartuja, where the arrival is five seconds before the time indicated by the new curve, and at Tacubaya, from which only a bulletin was received.

3. Examination of the nature of the first wave indicates that the first motion was a compression within a sector of angular magnitude of between 61° and 105° toward the north, and a dilatation in other directions. The earth amplitude of the first motion appears to change very little with Δ, the second amplitude showing greater loss with increased Δ.

4. The periods of the preliminary groups do not appear to depend on epicentral distance. The periods of the reflections P do not differ greatly from those of PR1, but there seems to be a possible tendency for the periods of SR as registered to exceed those of S.

5. The position of the maximum in the group is measured, where possible, for P and S. In the P group, there appears no tendency for the position of the maximum to vary regularly with epicentral distance. But in the S group, the maximum tends to advance toward the beginning of the group. There appears to be a tendency for the S group to vibrate in the plane of propagation.

6. There appear to be four groups of surface waves registered, (1) the long wave of Gutenberg, v = 4.35 km/sec. (ca.), but with a period of less than one minute; (2) the normal L, v=3.8 to 3.9 (ca.); (3) a new phase called X, which carries the maximum amplitude on Pacific paths, and less energy on other paths, v=3.6 (ca.); (4) the normal M, which carries the waves of maximum amplitude and regular character over non-Pacific paths, v=3.3 (ca.).

7. The periods of the first six minutes of the maximum group are investigated. It is found that for the Pacific Coast stations, 10° &lt;Δ&lt;15°, the average period is nine to twelve seconds, regardless of the natural period of the seismograph. For eastern America, the period registered was a function of the natural period of the instrument. These periods were five to seven, nine to eleven, fifteen to twenty. At stations of great epicentral distances, periods of fifteen to twenty-four seconds were registered regardless of the instrumental period. It appears that the long period is present in the first minute at nearer stations, but does not dominate because of the presence of shorter periods. At the more distant stations, the short periods have been lost.

8. The periods of the L group were measured. The average period for southwestern America is twenty seconds, for eastern America, eight seconds; for Europe, thirty seconds; for Honolulu and Apia, ten seconds. It is concluded that there is evidence of a thinner surface layer in eastern America than in western America.

9. The heavy aftershock, which took place about forty-five minutes after the main shock, originated at a focus probably to the south and west of that of the first shock.