Observations and Tectonic Setting of Historic and Instrumentally Located
Earthquakes in the Greater New York CityPhiladelphia Area
by Lynn R. Sykes, John G. Armbruster, Won-Young Kim, and Leonardo Seeber
Abstract A catalog of 383 earthquakes in southeastern New York, southwestern
Connecticut, northern New Jersey, and eastern Pennsylvania, including metropolitan
New York City and Philadelphia, is compiled from historical and instrumental data
from 1677 through 2006. A magnitude-felt area relationship is used to calculate the
equivalent magnitude mbLg prior to the advent of abundant instrumental data in 1974.
Revised locations are computed for a number of historic earthquakes. Most hypo-
centers are concentrated in older terranes bordering the Mesozoic Newark basin in
the Reading, Manhattan, and Trenton prongs and in similar rocks found at a shallow
depth beneath the coastal plain from south of New York City across central New
Jersey. Historic shocks of mbLg 3 and larger were most numerous in the latter zone.
The largest known event, mbLg 5.25, occurred just offshore of New York City in 1884.
Many earthquakes have occurred beneath the 12-km wide Ramapo seismic zone (RSZ)
in the eastern part of the Reading prong, where station coverage was the most ex-
tensive since 1974. The southeastern boundary of the RSZ, which is nearly vertical,
extends from near the surface trace of the Mesozoic Ramapo fault to depths of
1215 km. Because the Mesozoic border fault dips about 50°60° southeast, earth-