A Detailed Study of the Aftershocks of the 1979 Earthquake Near Bath, Maine*

Abstract

An earthquake of M C = 4.0 occurred north of the town of Bath in coastal Maine on April 18, 1979. This earthquake was immediately followed by a number of aftershocks that were recorded by the New England Seismic Network stations operated by Weston Observatory of Boston College and by M.I.T. From late May until July 1979, Weston Observatory operated a portable seismic network in the epicentral area and recorded a number additional microaftershocks. The aftershocks recorded by the permanent stations suggest that the main shock ruptured predominantly toward the southwest or west. All but one of the microaftershocks recorded by the portable network were located at between 3 and 7 km depth, possibly on the Cape Elizabeth fault. The data analysis is consistent with any of the following hypotheses: that the main shock and most of the aftershocks occurred on an ancient fault feature, the Cape Elizabeth fault; that the main shock occurred southeast of the Cape Elizabeth fault but caused microaftershocks to occur on the fault; or that the main shock coincidentally caused microaftershocks to occur near but not on the Cape Elizabeth fault. The last hypothesis notwithstanding, the microaftershock locations provide the strongest evidence to date of some modern seismicity being associated with an ancient fault in New England.