Earthquakes in the Adirondack Region

The Adirondack region of northern New York State is one of the more seismically active parts of the northeastern U.S. The three largest known earthquakes in the region caused about $20 million of damage (in 2002 dollars) to Cornwall, Ontario, and to Massena, New York in 1944 (magnitude 5.8), caused slight damage in a sparsely settled part of the southern Adirondack Mountains in 1983 (magnitude 4.9), and damaged the vicinity of Plattsburg, New York, on April 20, 2002 (magnitude 5.0). Moderately damaging earthquakes strike somewhere in the region every few decades, and smaller earthquakes are felt about once every three or four years.

Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

Faults

Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most of the Adirondack region's bedrock was formed as several generations of mountains rose and were eroded down again over the last billion or so years.

At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast, east of the Rocky Mountains this is rarely the case. The Adirondack region is far from the nearest plate boundaries, which are in the center of the Atlantic Ocean and in the Caribbean Sea. The region is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even the known faults are poorly located at earthquake depths. Accordingly, few Adirondack earthquakes can be linked to named faults. It is difficult to determine if a known fault is still active and could slip and cause an earthquake. As in most other areas east of the Rockies, the best guide to earthquake hazards in the Adirondack region is the earthquakes themselves.