GRAVITY SURVEY OF THE EPICENTRAL REGION OF THE AU SABLE FORKS, NEW YORK EARTHQUAKE OF APRIL 20, 2002

On April 20, 2002 at 6:50 A.M. Daylight Savings Time, an earthquake of 5.3 magnitude was detected by the Potsdam Seismic Network in the Adirondack Region of northern New York. This earthquake of maximum Modified Mercalli Intensity VII in the epicentral region was felt over Northeastern United States and Southeastern Canada. Over $16,000,000 in damage, due to slope failure along Route 9N, damaged chimneys and structural damage in alluvial valleys, was caused by the earthquake. The location of the hypocenter was 7 Km north of Au Sable Forks at 44.50^oN and 73.68^oW at a depth of 11 Km. The epicenter is located in a belt of leucogranitic gneiss known as the Lyon Mt. Gneiss. Gravity mapping was conducted in the epicentral region of the earthquake and a computer-contoured gravity map was constructed. The gravity map indicates the epicenter of the earthquake lies along a north-south trending gravity gradient separating a gravity low in the west from a gravity high in the east. The gravity gradient and anomalies lie within the Lyon Mt. Gneiss. Forward potential modeling of an interpolated gravity traverse across the gradient suggests the existence of a high angle north-south fault structure characterized by low density possibly due to highly fractured rock that extends to within a few hundred meters of the surface. A fault plane solution by a Lamont-Doherty scientist based on the mainshock and aftershocks indicates a north-south trending reverse fault dipping about 43^o westward which is consistent with our interpretation of the gravity data.