So we had an Earthquake!
Restored from the Empower Network archive (2011–2017), lightly edited to meet our current advertising standards. Views are the original author’s.
How could we have had an Earthquake?
We in the northeast don’t give much thought to earthquakes or earthquake prevention. Most people that I’ve asked seem surprised that we had one that was felt over such a wide area. Some don’t remember other earthquake activity that we have had.
As you can see from this map, earthquake activity is happening all the time here in the northeast. It is just that the magnitude is relatively small and intensity also low. By the way, you may recall that magnitude is a measure of the seismic power or strength of the shock wave, that is, its amplitude. Intensity is a measurement of the damage caused. One way to remember it is that the damage a powerful earthquake causes results in people living “in tent cities”.
I was in karate class at the time, and none of us there felt it. Our world was already rocking due to the partner drills we were doing. My wife who was sitting with a group of friends did feel it. For them it occurred as a “train rumble” or “big truck passing”, but since it was neither of those they came up with the correct answer, earthquake.
Many people think that an earthquake can only occur on the boundary of tectonic plates. Some recall from an Earth Science class that certain boundaries are more likely than others to produce a larger magnitude earthquake. New England doesn’t lie on a tectonic plate boundary, so what the heck is giving us an earthquake?
Well, there are other causes for an earthquake.
One of the perspective shifts that a student of science has to deal with is that of scale. In this case that of geologic time and the size of forces acting on the Earth. Recall that New England and comparable latitudes around the earth were covered in an ice sheet a mile thick or more. The incredible weight of all that frozen water pressed down on the land already stressed by other earlier geologic forces. Much of the East has fewer fractures than elsewhere in the U.S. and so these smaller quakes can be felt over large areas. The faults are poorly mapped and there is a lower degree of seismic monitoring here than in places like California and other active earthquake zones.
During the latest advance of the Laurentide ice sheet, ice extended to cover most of Canada and the northern United States.
In New England, the ice reached its furthest extent at Long Island around 21,000 years ago. During the last glacial maximum (LGM), global sea level was greatly reduced due to the large volume of water trapped in continental ice sheets in North America and other parts of the world. Ice in New England extended out onto the continental shelf, which at that time was not covered by water due to low sea level. Even as recently as 10,000 years ago what is current New England had ice. Our lakes and healthy crop of rocks and boulders are a gift from that glacial period.
Germain to today’s discussion however is what is known as rebound (or unloading). That sounds rather hectic and quick for a geologic process. Nonetheless, as the earth in our area recovers from all that pressure, we will continue to get an earthquake now and then.
Any time deep rock is broken or shifts position, etc. that energy is transmitted through the surrounding rock in all directions. Depending on the density and composition of the rock through which it travels, its speed and power are altered. This has been used to map the interior of the Earth as well as monitor and identify underground nuclear testing worldwide.
I can see that I’m going on a bit longer than usual today. Suffice it to say that there are many causes, types, and magnitudes of earthquake activity. I have only scratched the surface here.
Still looking for themes and topics of interest to you my gentle readers.