NYC Earthquakes: Can It Happen There?

Thursday 28 September 2017 (7:00 PM) - Dr. Charles Merguerian

Ulster Community College, Stone Ridge, NY

Principal - Duke Geological Laboratory, Stone Ridge, NY 12484

Professor Emeritus - Hofstra University, NY

All geologists and seismologists agree that earthquakes produce dislocations known as faults and that preexisting faults and joints tend to localize new, recurrent earthquakes.  The bedrock beneath New York City has always considered solid and impervious to seismic activity but it is cut by a great number of NW-trending brittle faults that cut diagonally across Manhattan.  One of these, the "famous" 14th Street fault underlies a deep buried valley and controls the lower-than-average height of buildings of the New York skyline in Manhattan between 23rd and Canal streets.  The Manhattanville fault along 125th Street underlies a broad, NW-trending U-shaped valley, and the Mosholu fault in the Bronx shows similar trend, geomorphic expression and exhibits offset of surface features.  These faults underlie valleys in the region and hold the key to understanding NYC seismic risk.

Modern seismicity along NW-trending faults in NYC is a matter of scientific record.  The epicenter of a small earthquake (~2.4 Richter) localized in NYC on 17 January 2001 plots adjacent to the trace of the 125th Street fault near 102nd Street and Park Avenue in Manhattan .  Later that year, on 27 October 2001, another similar earthquake (~2.6 Richter) struck NYC with an epicenter near 55th Street and Eighth Avenue .  Their epicenters are spatially coincident with known NW-trending faults.  North of NYC, seismicity along the NW-trending Dobbs Ferry fault in late October 1985 included two small (~4.0) tremors and many aftershocks.  More robust earthquakes in and around the vicinity of NYC were recorded in 1884 (~5.0-5.5), 1783 (~4.9), and 1737 (~5.2). More recently, the August 1884 magnitude ~5.0 NYC earthquake produced 4 m long by 3 m deep soil openings, cracked buildings and chimneys in Brooklyn . Clearly, the dynamic relationship between earthquakes and movement on reactivated faults mandates assessment of the potential seismic risk for populated urban areas underlain by deeply eroded crystalline rocks.

Dr. Merguerian will provide evidence that the NW-trending faults of NYC should be considered “active faults”.  Unequivocal post-glacial ground rupture is difficult to demonstrate in NYC where most bedrock faults are deemed to have formed at depth and then later elevated and eroded up to the surface.  Yet, the Bronx River, which formerly flowed SSW in an open valley underlain by the Inwood Marble, shows diversion away from its "pirated" marble valley by exhibiting displacement of course acrossthe NW-trending right-lateral Mosholu fault.  South of the fault, the river now flows in a non-glaciated south-trending V-shaped gorge through more-competent gneiss and schist of the Hartland Formation. 

Image above right shows obvious offset of orange grove planted in former straight rows across the San Andreas fault in California, the result of protracted fualt creep. (Image by John W. Shelton, Geology Illustrated, 1976.)