Statistical aspects of New England seismicity from 1975 to 1982 and implications for past and future earthquake activity

Abstract The seismicity of New England from 1 October 1975 to 30 November 1982 has been studied to determine how well it represents past seismicity and what it might imply about future earthquakes. Just over 100 earthquakes with magnitudes between 2.0 and 4.7 were recorded during this modern time period, while the historic earthquake catalog of Chiburis (1981) lists 760 earthquakes in New England from 1568 to 1975, the largest having maximum Modified Mercalli intensity VII shaking. The spatial distribution of the recent seismicity reproduces that for the historical seismicity patterns quite well, with the larger earthquakes in the modern record having preferentially occurred along coastal and lowland New England or in central New Hampshire. Space-time plots show that the seismicity displays considerable spatial variation, and that there are only vague hints of migrations of epicenters. The number of earthquakes as well as the amount of seismic energy release appear essentially random in time, and the recurrence curve for the modern time period (log number of events versus magnitude) is a straight line for events between magnitudes 2 and 4.5 with a b value of 0.84 (±0.03). The sizes of the largest earthquakes in the historic record are those which would be expected by extrapolating the present seismicity rate backward in time using a Gumbel type I distribution. One major discrepancy between the statistics of the recent and historic catalogs is that the latter does not have as many moderate (magnitudes 5 to 5.8) earthquakes as would be expected from the recent data. This discrepancy could be due to inaccuracies in the historic catalog, to unusually high recent seismicity or to a real historical shortage of such moderate magnitude earthquakes. Large earthquakes may be more likely to happen in areas of past and present high seismicity, especially in central and southern New Hampshire, along coastal Maine, and along the south coast to New England.