THE BLUE MOUNTAIN LAKE, NY STATE EARTHQUAKE SWARM OF 1971-1973

If you were living in the Blue Mountain Lake area in the Adirondacks from 1971 through 1973, you may have heard sounds like distant thunder or sonic booms. These sounds were due to thousands of earthquakes that occurred from May 1971 to April 1972 and July 1973. An earthquake swarm is a long series of small earthquakes with no principal event. The largest events recorded in the swarm were of magnitudes 3.6 and 3.4 on May 23 1971.

No published record of previous earthquake activity was known in Blue Mountain Lake except for a few reports in 1944. Several residents reported sounds similar to those of the 1971-1972 swarm but not recognized as quakes. It is likely that previous earthquakes occurred but were undetected.

The focal depths of the earthquakes were all shallow ranging form .5 to 4 km. A satisfactory explanation has never been put forth to explain these earthquake swarms. To determine the cause of the swarm, magnetic and gravity measurements were made in the epicentral region of the swarm. Gravity mapping indicated a steep gravity gradient along the northern shore of the lake where the epicenters are located. Magnetic measurements indicate magnetic anomalies in the area possibly related to structures or fractures causing the earthquakes.

Heavy rainfall occurred in the Blue Mountain lake area prior to the earthquakes resulting in the lake rising 25 cm (10 inches). It is believed that numerous fractures at shallow depth as indicated by the gravity and magnetic surveying provided conduits for water infiltration from the lake to trigger the swarm. A gravity low in the epicentral region is attributed to the fractures at shallow depth.