RECENT SEISMICITY AROUND LITTLETON, MA

Since early colonial times the area around Littleton, MA has experienced regular small earthquakes. For example, since the modern, telemetered regional seismic network became operational in New England in the mid-1970s, 34 earthquakes with M ≤ 3.0 have been detected from this area. To more precisely locate the earthquakes in this seismic zone, we have been operating 4 portable seismic stations, called the ALSA array, within a few kilometers each of Littleton, MA since 2017. During this time 10 local earthquakes have taken place in the Littleton area and have been recorded by ALSA, which has allowed these events to be located with an epicentral precision of about 1 km and a depth precision of about 2 km. The well-located earthquakes distribute along a N-S trend that is about 15 km in length. The focal depths range from less 1 km to 7.7 km, with most events having focal depths of less than 4 km. Some of the ALSA stations detected strong Rg waves for some of the events, confirming focal depths less than 4 km. The events in this Littleton seismic zone do not occur on any mapped bedrock fault but rather span the Nashoba Terrane between the Paleozoic Bloody-Bluff and Clinton-Newbury Faults. If the events in the Littleton area are late aftershocks of a strong prehistoric earthquake, the 15-km length of the seismic zone indicates that the prehistoric mainshock had Mw ~ 6.2. The Littleton seismic zone is in the NNW suburbs of Boston, suggesting that the seismic hazard of Boston might be greater than current seismic hazard maps indicate.