Earthquake = Tsunami?

A mild 3.9-magnitude earthquake occurred this morning out at sea off the New York Bight, and was felt by many people across the region.  An obvious question is whether such an earthquake could cause a tsunami.  A large tsunami clearly did not occur, but was a small one produced?  And while we’re on the topic, what is the probability of a tsunami striking our heavily populated coastline?  A large percentage of New York City’s surface area is less than ten feet above mean sea level (MSL), including much of the Financial District in Lower Manhattan and much of the subway system.

Examining water level data at Atlantic City (shown here) or other area stations using the NYHOPS coastal ocean observing and prediction system, it does not appear as though even a small tsunami occurred:

Here, the x-axis is hours from midnight, the red points are the observed water level relative to MSL, and the black line and yellow shading shows the prediction of a model with normal weather and tidal forcing.  The earthquake occurred at 10:46 AM local time, soon after low tide, and the agreement of the model and observations shows that nothing unusual was measured at this station.

Some scientists speculate that the reason Native Americans only had small communities when European settlers arrived in the New York City area was because of a prior tsunami in 300 BC, which is a spooky thought to say the least.  Fortunately, we have a new tsunami detection system, though I am not aware if there is any warning system in place for the population nor of the rapidity of response we’d have if one occurs.

Earthquakes are rare around New York City, but some stronger ones have occurred, as demonstrated on this page from Lamont-Doherty Earth Observatory that lists the largest recorded events to have hit the New York City region.

Good news:  It is extremely rare for earthquakes below 6.0 magnitude to cause tsunamis.  However, if the earthquake occurs at a location where the seabed is unstable, any earthquake could feasibly cause an underwater landslide, then that could send a tsunami.  But also on the positive side, when a landslide produces a tsunami, it usually rapidly decreases in size with distance from the source — a useful rule of thumb is that any slide that is smaller in horizontal scale than the depth of the ocean where it occurs will cause a localized wave that decays rapidly with distance (Parker, 2010).

Reference

Parker, B., 2010.  The Power of the Sea, Chapter 7: The sea’s response to an unpredictable Earth.  Palgrave MacMillan, New York, 292 pp.

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