Observations and Tectonic Setting of Historic and Instrumentally Located Earthquakes in the Greater New York City–Philadelphia Area

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 magnitudembLg prior to the advent of abundant instrumental data in 1974. Revised locations are computed for a number of historic earthquakes. Most hypocenters 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 extensive 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 12–15 km. Because the Mesozoic border fault dips about 50°–60° southeast, earthquakes of the RSZ are occurring within middle Proterozoic through early Paleozoic rocks. Which faults within the RSZ are active is unclear. Well-located activity in the Manhattan prong since 1974 extends to similar depths but cuts off abruptly at all depths along a northwest-striking boundary extending from Peekskill, New York, to Stamford, Connecticut. That boundary, which is subparallel to brittle faults farther south, is inferred to be a similar fault or fault zone. Those brittle features may have formed between the Newark, Hartford, and New York bight basins to accommodate Mesozoic extension. The Great Valley in the northwestern part of the study region is nearly devoid of known earthquakes. While few focal mechanism solutions and in situ stress measurements of high quality are available, the maximum compressive stress is nearly horizontal and is oriented about N64°E, similar to that in adjacent areas. The catalog is likely complete for events of mbLg >5 since 1737, ≥3:5 since 1840, and ≥3:0 since 1928. Extrapolation of the frequency-magnitude relationship indicates that an event of mbLg ≥6:0 is expected about once per 670 yr. Online Material: Observations and tectonic setting of historic and instrumentally located earthquakes in the greater New York City–Philadelphia area.