Abstract

Earthquakes recorded by stations of the Lamont-Doherty seismic network in the greater New York City area are analyzed to determine magnitudes and the relationship between seismicity and geologic structures. Between 1974 and 1983, the configuration of stations in this region remained relatively stationary and the type of recording devices (visual drum recorders and 16-mm photographic recorders) did not change. This distribution of stations and recording devices allows for a uniform measurement of magnitudes and seismicity. Magnitudes of these earthquakes are determined by comparing amplitudes and signal duration measured from high-frequency (5 to 10 Hz) data recorded by the local network with mbLg and ML determined from data at frequencies near 1 Hz. During the period of time studied (nearly 10 yr), 61 earthquakes were located in this region, but none of these earthquakes exceeded 3.0 on the mbLg scale. The largest event (mbLg = 3.0) occurred in the Coastal Plain province of northern New Jersey.

The magnitude threshold for uniform detection of events throughout this region during the period of time studied is estimated to be mbLg = 1.6. With events below this threshold removed from the catalog of network seismicity, we find that about half of the earthquakes studied occurred within 10 km of the Ramapo fault system. This fault system lies about 30 km northwest of New York City and has been interpreted by several investigators to be the most active fault system in the greater New York City area. However, earthquakes at least as large as those recorded near the Ramapo fault were located as far as 50 km from this fault (and within 20 km of New York City), in geologic structures that surround the Newark basin. While the Ramapo fault can by no means be ruled out as a possible source zone for earthquakes in the greater New York City area, the geologic structures associated with most (if not all) earthquakes in this region are still unknown. Thus, the cause of earthquakes in this region remains an enigma.

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