Earthquakes in New York City

Most earthquakes occur near active plate boundaries, such as subduction zones, mid ocean ridges, or transform faults. The state of New York lies on a passive margin, thousand of miles form the nearest tectonic activity. Yet there are concerns about a hazardous earthquake occurring in the state of New York, even in New York City. But why?

The state of New York lies in an area containing numerous faults that are relatively inactive compared to the well-known fault systems of the San Andreas and Pacific regions. However, these faults are believed to still produce earthquakes. Since 1730 there have been over 400 earthquakes recorded in the state of New York, making it the state third highest in earthquake activity east if the Mississippi River. The list of recent earthquakes includes the magnitude 5.1 earthquake on April 20, 2002 in Plattsburgh, NY and the magnitude 2.4 earthquake on Jan 17 2001 in Manhattan, NY.

Suites of earthquakes occur near the Great Lakes region, though the northern part of the state, and around the city of New York. (see USGS map http://neic.usgs.gov/neis/states/new_york/ ) One fault system, the 125^th Street Fault Zone, runs through New York City from 125^th Street, through Central Park, and on to East 96^th Street.

But why does this make New York a place vulnerable to a dangerous earthquake? Because no one has considered New York a prime earthquake target, it wasn’t until 1996 that New York City added a seismic provision to its building code. This means that homes, office buildings, roadways and bridges constructed before this time have not been built to withstand a sizable earthquake.

The largest recorded earthquake in the state was a magnitude of about 5.8 occurring in 1944 in Massena, NY, and scientists theorize that earthquakes of magnitude 6 or 7 are possible within the state. Earthquakes of magnitude this high create a particular danger for the city of New York. Because a large part of the city is not underlain by bedrock but by loosely compacted glacial till, an earthquake of magnitude 6 or higher could result in the liquification of the ground. Liquification results when ground saturated with water looses its structural stability and acts as a fluid during ground shaking. This could cause the complete destruction of structures as was seen in the 1906 San Francisco earthquake.

This information does not imply however, that New York is in immediate danger of a major earthquake. The likelihood of a damaging earthquake occurring in the state of New York is still believed to be small, with potentials increasing in the northern part of the state. Research as to just what causes these earthquakes and the possibilities if their occurrence is still ongoing. However, citizens should be aware that the possibility still exists, and that precautionary measures are not outside of reason.

http://www.nysm.nysed.gov/geosige.html

http://neic.usgs.gov/neis/states/new_york/new_york_history.html

http://manhattan.about.com/?once=true&

Seismology of September 11

The attacks on the Twin Tower in New York City have been recorded in many ways. One of those ways was on the seismograms located in five Northeaster states. The closest of these stations was the PAL station at Palisades, NY, operated by the Lamont-Doherty Earth Observatory, while the farthest was located in Lisbon, New Hampshire.

The PAL station, at a distance of 21 miles, recorded the impacts of the planes at magnitudes of 0.7 and 0.9, an event that was barely detectable. The waves traveled as Love waves, moving horizontally along the surface of the ground. The collapses of the towers were recorded at magnitudes of 2.1 and 2.3. These magnitudes are comparable to a small earthquake, much like the one that occurred in Manhattan in January of 2001. The seismic waves that resulted from the vertical force of the collapses were recorded as Raleigh waves, also traveling along the surface of the ground. The duration of the ground shaking lasted about 3 seconds, corresponding to the time it took for the building to collapse. Recording of smaller events later on in the day may have corresponded to partial collapses of the remains of buildings.

The seismic waves resulting form the attacks on the Twin towers were not strong enough to cause damage to any nearby buildings, however. It is still thought that much of the damage was done by debris.

-Figures from:

http://www.geo.arizona.edu/geophysics/faculty/wallace/WTC/

-Also cited:

http://www.ldeo.columbia.edu/LCSN/Eq/20010911_WTC/fact_sheet.htm

http://www.geotimes.org/dec01/WebExtra123.html