The influence of the Presumpscot Formation on seismic hazard in southern coastal Maine

As ice retreated from southern coastal Maine from 15,000 to 13,000 years ago, the ocean inundated coastal areas where the earth’s crust had been temporarily depressed by the weight of glacial ice. A thick veneer of glacial-marine clay and silt – the Presumpscot Formation – was deposited in coastal Maine lowlands. Due to its fine-grained character, shear waves from seismic events travel slowly through the Presumpscot Formation in comparison to other surficial sedimentary units. Low shear-wave velocities result in amplification of seismic waves, potentially increasing local seismic hazard. Prior study demonstrated that careful assignment of National Earthquake Hazards Reduction Program (NEHRP) site classifications to surficial geologic units based on shear velocities, can greatly improve earthquake loss estimations using programs like HAZUS-MH, particularly when compared with estimates using default and proxy values. The October 16, 2012 magnitude 4.0 earthquake in Hollis, Maine, provided another opportunity to assess the influence of the Presumpscot Formation on seismic hazard. Occurring in early evening, this strongest Maine earthquake in nearly 40 years was widely felt across the region. The Hollis area is near the transition between the western mountains which are mostly underlain by till, and the lowlands, mostly underlain by marine sand and mud. Using the USGS database of more than 2,000 “Did you feel it?” geolocated responses in southern Maine, we tested whether respondents experienced different intensities of ground shaking depending on substrate. When normalized by population density within each surficial unit, we found no statistical difference in respondents’ experiences. However, when normalized for areal extent of each unit, we found that more people responded in areas underlain by NEHRP class ‘E’ materials (including the Presumpscot Formation) than for other classes. Our results suggest potentially greater intensity of ground shaking and seismic hazard in areas underlain with sediment of the Presumpscot Formation.