PROBABILITIES OF FELT EARTHQUAKES FOR THE NEW ENGLAND REGION

EBEL, JOHN E. and ALAN L. KAFKA, Weston Observatory, Boston College, 381 Concord Rd., Weston, MA 02493, USA, ebel@bc.edu, kafka@bc.edu

An analysis of the seismicity of New England and nearby areas from 1975 to 1998 indicates that some aspects of the network seismicity of the region behave in a non-Poissonian manner. Specifically, the rate of occurrence of two felt earthquakes within 10 days of each other somewhere in the New England region is higher than expected from a Poisson distribution based on the mean rate of earthquakes of magnitude 2.7 and above. Furthermore, chances of a felt earthquake somewhere in New England within a week or so of an M*3.5 event in the region is enhanced still further. Analyses suggest that this increased activity rate within a week or so of an M*2.7 event is probably not due to aftershocks in the earthquake catalog. For anywhere within New England, the Poisson probability of a felt earthquake of M*2.7 is 11% during any 7-day period based on the rate of earthquakes observed by the network since 1975. However, in the network catalog we have observed that 22% of the M*2.7 events are followed by another M*2.7 event somewhere in New England within 7 days. Furthermore, 35% of the M*3.5 events in the network catalog are followed within 7 days by an M*2.7 event somewhere in New England. In the 1975-1998 dataset there are observed 13 temporal "clusters" of seismicity, defined as 4 or more events that all occur at less than the mean earthquake repeat time from one event to the next. The typical cluster averages 2-3 months in duration. While these clusters encompass only 16% of the time duration of the catalog analyzed, 60% of the M*3.5 earthquakes in the dataset took place in the clusters. Long periods of quiescence with no felt earthquakes are followed by almost no M*3.5 events. These observations suggest that the chances of a more widely felt earthquake in the New England region may be enhanced when the region as a whole is more seismically active.