EARTHQUAKE CYCLICITY IN THE NEW YORK METROPOLITAN AREA

Earthquake activity in the New York metropolitan area (NY, NJ, PA, CT) appears to be cyclical both temporally and spatially. Identification of such cycles may provide an important first step in predicting earthquakes particularly in intraplate settings. Since instrumental monitoring began in 1975, earthquake activity has exhibited an approximate 40-year cycle following the equation: y = -3E-05x4 + 0.0034x3 - 0.1167x2 + 0.9518x + 11.394 where X equals time and Y equals frequency of earthquakes and with an R²=0.49. This cycle ranges from a peak of about 25 events per year to a low of about 3 per year. Based upon historical records, this cycle appears to have persisted for at least ~125 years with higher magnitude events during the active periods. Based upon this cycle, increased earthquake activity was predicted for the next several years and in 2016 the area had the most events (16) in 30 years. Earthquakes occur in clusters for short periods of time. These clusters occur both in short duration periods (12 months) and longer term periods (4-8 years). In 2016, 12 of the 16 earthquakes occurred within a 10 km area. Longer term clusters include multiple epicenter trends. Trends of 3 or more epicenters in short duration periods define apparent active faults with a primary orientation maximum of 270° and secondary 323° and 358° maxima (n=60) which sharply contrasts the generally NE-trending major faults in the area. These apparent E-W and NW-trending faults are visible on satellite imagery and are especially prominent in the crystalline Highlands Province. There are a few examples of surface expression of these faults and may indicate surface rupture. Earthquake activity varies across the area by geologic province but also within provinces. Although rocks are the same, the northern NJ Highlands province is more than twice as active as the adjacent southern NJ and western Hudson Highlands likely as the result of density of optimally oriented faults. It is one of the most densely active areas in the eastern US.