THE AUSABLE FORKS, NY EARTHQUAKE OF APRIL 20, 2002

On April 20, 2002 an earthquake of magnitude 5.3 occurred near AuSable Forks, NY in the Adirondacks. This earthquake was detected by four short period vertical seismic field stations located in the St. Lawrence Valley and Northwest Adirondacks. The epicenter location was in the Peasleeville Quadrangle 10 km north of AuSable Forks. The bedrock in the area is Precambrian granite gneiss overlain by Potsdam sandstone. The focal depth of 11 kms puts the hypocenter well within the Precambrian gneiss. Extending east and northeastward across the area is a topographic lineament interpreted as a high angle thrust fault however, no relation is known to exist between the lineament and the earthquake. A fault plane solution by a Lamont-Doherty scientist indicates a north-south trending fault with an intermediate dip to the west caused the earthquake.

During the past 10 years, students at SUNY Potsdam have been conducting gravity surveys in northern New York. A computer contoured gravity map of the epicentral region of the AuSable Forks area indicates the epicenter lies along a north-south trending gravity gradient separating a gravity low in the west from a gravity high in the east. Forward potential modeling of the gravity data across the gradient provides support of the existence of a narrow high angle fault structure characterized by low density. The low density is possibly due to highly fractured rock extending to within a few hundred meters of the surface. The gravity modeling supports the fault plane solution of a north-south trending thrust fault of Lamont-Doherty scientist.