Using Aftershocks to Help Locate Historical Earthquakes

Abstract

Abstract For historical earthquakes, the spatial distributions of macroseismic intensity reports are commonly used to estimate the event locations. The methods to locate historical earthquakes assume that the highest seismic intensity shows the best estimate of the location of the earthquake. Uncertainties in the locations estimated from macroseismic data can be due to an uneven geographic distribution of sites with intensity reports, variations in intensities due to local soil conditions, ambiguous historical reports, and earthquake directivity effects. Additional constraint on the location of a historical earthquake can come from places where most aftershocks were felt, because these localities may have been closest to the fault on which the mainshock took place. Examples of estimated earthquake locations based on aftershocks are those of the 1727 MLg 5.6 earthquake in northeastern Massachusetts, the MLg 5.7 earthquake in Maine, and the 1755 MLg 6.2 earthquake offshore of Cape Ann, Massachusetts. In all of these cases, the earthquake locations based on the aftershock data are somewhat different from previous locations derived from the macroseismic intensities alone. Uncertainties with this method include identifying aftershocks in historical accounts and the possibility that smaller events that are reported following a strong earthquake are not on or near the mainshock rupture. Even so, evidence of possible aftershock activity may help constrain the location of that mainshock. Because aftershocks of strong earthquakes (M≥7) can last months to years, archival research for aftershocks must be carried out with a somewhat different mindset than that for a mainshock.