Abstract

On Nov. 23, 1980, a small (magnitude 2.9) earthquake occurred on the Chelmsford-Lowell, Massachusetts border and was felt by most residents of the area. Adequate station coverage by the M.I.T. and Weston Observatory Seismic Networks insured a precise determination of the hypocentral location at lat. 42.63, long. −71.36, depth 1.5 km, O.T. 00:39:32.0 UCT. Rarely does an earthquake occur close enough to a station in New England for good depth control. The fault plane solution shows either strike-slip or dipslip faulting with a NE-SW trending P-axis. This is consistent with overcoring measurements in a nearby granite quarry. This earthquake was heard as well as felt, and the maximum intensity was IV (M.M.). We also re-examined the data for another earthquake which occurred near the present event on June 23, 1938. Our re-computed epicenter was at lat. 42.60, long. −71.42, O.T. 03:57:55.9 UCT with the depth constrained at the surface, and the magnitude was 2.7. The P-wave first motions for this event are consistent with the solution for the 1980 event. The maximum intensity was V (M.M.). The occurrence of these two earthquakes demonstrates for the first time that focal depths in New England can be very shallow (≤ 2 km), which accounts for the relatively high epicentral intensities observed for some small earthquakes.

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