Epicenters of earthquakes occurring between 1627 and 2003 in the New York City metropolitan region. Historical earthquakes and other events prior to 1970 are plotted with "hexagons", whereas earthquakes that have occurred since 1970 – when systematic earthquake monitoring began in the region – are plotted with "circles". The symbol size is proportional to magnitude. The largest known earthquake is this region occurred on 10 August 1884, offshore near Far Rockaway. Seismographic stations are plotted with "black triangles" for short-period stations and "red triangles" for modern broadband seismographic stations. These stations are part of ANSS (Advanced National Seismic System) and are used to monitor earthquakes and ground motion near important buildings and critical infrastructure. The Ramapo Fault System is shown as a red line. Note how the earthquakes tend to be associated with the Ramapo Fault System.

The Ramapo Fault System The Ramapo Fault is part of a system of north-east striking, southeast-dipping faults, which are mapped from southeastern New York to eastern Pennsylvania and beyond. These faults were active at different times during the evolution of the Appalachians, especially in the Mesozoic when they served as border faults to the Newark Basin and other extensional basins formed by the opening of the Atlantic Ocean approximately 200 million years ago.

Seismicity Earthquakes, including damaging historic events with magnitudes as high as m_b = 5.2, occur throughout the lower Hudson region in southern New York and northern New Jersey (See Figure). For example, notable earthquakes in the region around Peekskill include: June 7, 1974, Wappingers Falls sequence (m_b(Lg)=3.3); January 17, 1980, Annsville, near Peekskill, New York (m_b(Lg)=2.9). Earthquake epicenters in the region, although scattered, generally follow a southwest-to-northeast trend from eastern Pennsylvania, through New Jersey, and into the Hudson Highlands in Rockland, Westchester and Putnam Counties. Although the epicenters are scattered, the seismicity patterns appear to be concentrated along several geologic features, including the Ramapo Fault System. Seismologists can detect active faults when the locations of individual events can be resolved accurately. Interestingly, many of these active faults appear to cross the Ramapo Fault System, indicating a complex relationship between present-day stress in the Earth’s crust and pre-existing geological features. This is an area of frontier seismological research.

For example, seismologists from the Lamont-Doherty Earth Observatory of Columbia University, working with equipment supplied by the U.S. Geological Survey, are currently monitoring a swarm of earthquakes near Milford, New Jersey, within one kilometer of the mapped Ramapo Fault. This study suggests that the levels of crustal stress in the Northeastern U.S. are sufficient to activate fractures associated with the Ramapo Fault System. An open question is whether these active structures will fracture in a large damaging earthquake. Damaging earthquakes have occurred in the Northeast several times in the last three centuries, indicating that this is a distinct possibility. Unfortunately, the ability to monitor potential earthquake sources with sufficient accuracy, as well as the ground motion near critical buildings and infrastructure, is compromised by the limited availability of modern instrumentation.

Earthquake Monitoring and its Applications:

In areas such as the Northeast, where background seismicity levels are low, and the implications of a destructive earthquake are high, earthquake monitoring is essential to mapping presently active geologic faults. The same instruments are also used to calibrate the regional application of national earthquake hazard mapping procedures, which are used in the development and implementation of appropriate building codes, and the regulation of critical facilities such as bridges and power plants. Lamont-Doherty, through the Advanced National Seismic System, is a partner with the U.S. Geological Survey in developing the operational capability and scientific background needed for earthquake hazard reduction in the Northeast.

This briefing document was prepared by: Klaus Jacob, Won-Young Kim, Arthur Lerner-Lam*, and Leonardo Seeber (all at: Lamont-Doherty Earth Observatory of Columbia University). Funding for regional earthquake monitoring is provided to Columbia by the U.S. Geological Survey under the National Earthquake Hazard Reduction Program andthe Advanced National Seismic System.

*to whom correspondence should be addressed.

Version dated 22 March 2004