360 YEARS OF NORTHEASTERN U.S. EARTHQUAKES

A new wall map* shows the locations and sizes of more than 1000 known earthquakes that were large enough to have been felt (magnitudes 3.0-7.0) in the northeastern U.S. or nearby parts of Canada. The map is a visually striking educational tool that was made in collaboration with northeastern seismologists and emergency managers. The map shows aspects of northeastern U.S. seismic hazard that are known by specialists but probably not by many others. (1) Damaging earthquakes occur anywhere, anytime. Most of those shown on the map occurred in areas with scattered smaller earthquakes before or since. However, the most recent damaging shock struck rural northwestern Pennsylvania just 3 years ago, in a seismically quiet region. (2) Earthquakes occur at highly irregular intervals. Intervals between mainshock earthquakes of magnitude 5.0 or larger within the map area averaged 8 years since 1638. However, individual intervals between mainshocks ranged from less than a year to 114 years. (3) Cities are not immune. New York was damaged in 1737 and 1884. The Boston-Newbury area was damaged three times within 28 years in the middle 1700’s, although not again since 1755. (4) Moderate earthquakes of magnitudes 5-6 occur much more frequently than the larger earthquakes that damage much larger areas. Therefore, it is worth noting the calculations and maps by S. Harmsen (URL http://geohazards.cr.usgs.gov/eq/html/deagg.html, for 2% probability of exceedance in 50 years) showing that future short-period (0.2 s) ground motions in the northeastern U.S. would be caused mostly by moderate local earthquakes of magnitude 5-6, instead of larger, less frequent earthquakes in seismic zones outside the northeastern U.S. (New Madrid, Missouri; Charleston, South Carolina; Charlevoix, Quebec). Short-period shaking poses the largest threat to the numerous 1-2 story buildings throughout the northeastern U.S.

* Wheeler and others, 2001, USGS Geol. Inv. Map I-2737, 1 sheet, scale 1:1,500,000; URL http://greenwood.cr.usgs.gov/pub/i-maps/i-2737/