SOURCE AND CHARACTER OF EARTHQUAKES IN THE NEW JERSEY/NEW YORK CITY REGION

Small, instrumentally recorded earthquakes from New Jersey and southeastern New York occur in sequentially active northwest trending fault zones in ~4 year cycles, within an estimated, larger ~40 year cycle. From 1975 to 2015, x328 tectonic events were recorded within a 2° x 2° area, around the tristate region. Magnitudes ranged from 0.2 to 3.8, with x27 magnitude 0.9 or less, x169 between 1.0 to 1.9, x121 between 2.0 to 2.9, and x11 between 3.0 to 3.9, with an average and mode of 1.8 and 1.5 respectively. A greater frequency occurred between 1976 – 1980 with an average of ~15 earthquakes/year but steadily decreased to an average of ~3/year from 1999 – 2003 before recovering ever since. Focal depth is independent of magnitude, with an average and mode of ~5.4 and ~5 km respectively, and range of ~1 to ~20 km.

There is a concentration of activity along the eastern edge of the Highland Province which has led researchers to conclude that the Ramapo Fault was responsible for the seismicity. However, patterns of aftershocks and earthquake clusters shows that small northwest trending faults are primarily responsible. Multiple related earthquakes commonly occur in groups of 3 to 4 and define northwest fault source trends. Further, clustering of earthquakes occur in localized areas for several years before subsiding and shifting to another area and they too define northwest trends. The active faults have been mapped locally and are visible on remote sensing images of the region. The clustering of the earthquakes along the eastern edge of the Highlands may result from the rheological contrast of the crystalline rocks with the sedimentary rocks of the Newark basin. Based upon current stress orientations, these northwest trending faults should be undergoing reverse motion, though no fault plane solutions have been determined.