Astudy of original records and bulletin readings for two strong earthquakes in the Ossipee Mountains district of southeastern New Hampshire in December, 1940, gave for both
S started from a depth of approximately 35 km., and P at the same time from a depth of 15 km. The second shock was multiple. Times at the focus were:
The velocities observed, in km/sec., were:
Mechanism at the focus probably differed from that of earthquakes which involve surface faulting, but is not discussed in detail in this paper. First motion from the second earthquake was reversed from that of the first at near-by stations. No foreshocks preceded the first earthquake; there was a small one just before the second; and there were only ten after-shocks, the last one occurring on February 12, 1941.
The Richter magnitude was at least 6 for the first and slightly greater for the second main shock. From this the energy can be computed as 1020 ergs or 7.4 × 1012 foot-pounds. This ranks both shocks as potentially destructive. They were felt to a distance of 350 miles, which means an area of potential perceptibility of the order of 385,000 square miles.