abstract

On June 7, 1974, at 19:45:37 GMT, an earthquake of magnitude 3.3 (mb, Nuttli) occurred at Wappingers Falls, New York (41°37.75′N, 73°56.5′W). The maximum intensity was Modified Mercalli V, with a radius of perceptibility of 10 km. This high intensity and rapid fall-off of intensity with distance are presumably associated with a shallow focal depth. Within 10 hr of the event, a portable array of microearthquake recording instruments was installed in the epicentral area. During the following 6-day period, over 100 aftershocks were recorded. Accurate locations could be made for 42. The aftershocks occurred within or beneath an allochthonous block of dolomitic limestone (Wappinger Group) which has been extensively quarried. Focal depths varied from zero to 112 km. A composite fault-plane solution for the main shock and the well-located aftershocks indicates a thrusting mechanism with nodal planes striking N40°W and dipping 60° to the SW and 30° to the NE. This mechanism supports the north-northeast-trending maximum compressive stress in eastern North America hypothesized by Sbar and Sykes (1973). The available evidence, including previous seismic history, locations, and the composite fault-plane solution indicate that this earthquake sequence and possibly past earthquakes in the same area may have been triggered by crustal unloading associated with quarrying operations in the presence of high horizontal compressive stress.

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