THE SEISMIC HAZARD IN THE NORTHEASTERN U.S. DUE TO OFFSHORE EARTHQUAKES

While most U.S. East Coast earthquakes take place on-shore, there are also some off-shore earthquakes that have been recorded. For example, a number of earthquakes have been detected from the Gulf of Maine and from south of Long Island. Many of these earthquakes seem to be located in the areas of Meoszoic rift structures, suggesting that these rift structures are being reactivated in the modern plate tectonic stress field. Further offshore, there have been several earthquakes detected from the continental slope region. Some of these earthquakes have occurred east of Maine and south of Nova Scotia in an area where a submarine canyon runs into the deep ocean. Other such events have been detected from the Atlantic coast continental slope east of New Jersey. These continental slope earthquakes have taken place in the same seismotectonic setting as the Grand Banks earthquake of 1929. The 1929 earthquake was M 7.3, and it caused a damaging tsunami that affected the south coast of Newfoundland. A large submarine slump associated with the 1929 earthquake severed a number of transatlantic telephone cables, the timing of which demonstrates that the slump took place at the time of the earthquake. The similarity of the tectonic settings of the 1929 earthquake and the continental slope events off the U.S. northeastern coast suggests that major earthquakes along the U.S. northeastern coast that could trigger a damaging tsunami may be possible. The data are currently insufficient to estimate the probabilities of such events.