Earthquakes in the Area Surrounding the Quabbin Reservoir in Central Massachusetts

Abstract

On October 2, 1994 a magnitude 3.4 earthquake occurred near the town of Hardwick, Massachusetts. Near the epicenter and in surrounding towns, this earthquake generated ground motion at the level of intensity IV on the Modified Mercalli scale. The rate of seismic activity is quite low in this area, even when compared to the generally low rate of activity in some of the more “active” parts of the northeastern United States. Thus, the occurrence of the 1994 earthquake motivated an investigation of the history of seismic activity in central Massachusetts. The record of seismicity in central Massachusetts extends back to the early 1800's, and during that period of time there have been about a dozen small earthquakes in the area (magnitudes ranging from 1.6 to 3.4). Earthquake-generated ground motion at the level of about intensity IV has been reported for five earthquakes in this area. Between 1939 and 1946, the area near the 1994 epicenter was evacuated and flooded to create the Quabbin Reservoir (with a capacity of 1.6 km 3 of water). Because of the presence of this reservoir in the epicentral area, we were curious as to whether or not the 1994 earthquake, or any of the other earthquakes in this area, might have been reservoir-induced. Based on our initial analysis of locations and magnitudes of earthquakes in the study area, there appeared to have been an increase in the level of earthquake activity in the vicinity of the reservoir, beginning at about the time that the reservoir was filled. A more detailed analysis of earthquake catalogs and felt effects of earthquakes that occurred in 1854, 1931, 1951, and 1994, however, revealed that there is no strong evidence of such an increase in seismicity near the reservoir.