Abstract

An earthquake of Mc = 4.7 occurred near Gaza in the township of Sanbornton in central New Hampshire on January 19, 1982. The main shock was followed by a sequence of aftershocks which was monitored by a network of portable seismographs installed in the area from January 19 thru February 17. Over fifty aftershocks were recorded by the portable network, including two which were of a sufficient magnitude to be detected by permanent stations of the Northeastern United States Seismic Network. These two events allowed the absolute calculation of locations for the main shock and the two Mc = 2.6 aftershocks of January 19 that are calibrated to the later aftershock locations using a relative hypocentral location algorithm. Arrival time data for these events indicate a northwestward progression of epicentral location with time. In addition, a cluster of generally shallow, small magnitude events occurred in an area to the north-northwest of the January 19 activity. This northwesterly migration of seismicity is consistent with geological and geophysical studies conducted in the area which suggest a potential for northwest-trending basement structures in the region along with the presence of shallow, multidirectional fracture patterns.

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