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Update: Seismological Research Letters article on preliminary results from Dover, DE deployment accepted.
At shortly before 5 pm on Thursday, November 30, 2017, there was a magnitude 4.1 earthquake east of Dover, DE. This event has caused little damage and no casualties. It is of significant scientific interest because earthquakes of this size on the east coast of the United States are rare in general, and earthquakes in Delaware, even more so.
Here's a map of all of the earthquakes (orange dots) bigger than a M=2 that have happened in the eastern US since Jan 1, 2011. They're scaled by magnitude. Notice that the circle in Delaware (the one with the red arrow pointing at it) is kind of off on its own. That's the earthquake that just happened. There are other clear clusters of earthquakes. There's one in eastern Tennessee/western North Carolina. There's one around the big 2011 M=5.8 earthquake in northern Virginia. But there really hasn't been much going on seismically in Delaware. So why did this one happen, and why was it so big?
The answer, I'm afraid, is that we have no satisfying answer (at least not that I, your humble narrator, am aware of). We understand much less about earthquakes in plate interiors (like the eastern United States) than we do earthquakes along plate boundaries (like California, Alaska, or Mexico). We do know that there are a lot of faults along the eastern margin of the US. Some of them date back to the days when Pangea broke up (about 180 million years ago).
When you break up a tectonic plate, it's a little like breaking a snickers bar. It doesn't just break neatly into two pieces. The crunchy outer bit (in our case, the Earth's crust) breaks in a few different places before the whole bar settles on one final place to come apart. These other cracks in the chocolate... I mean, crust... are known as "failed rifts".
The east coast has lots of failed rifts. These failed rifts tend to fill with sediments and become what we call "rift basins" (grey shaded regions on the map here). Notice that one of these basins ends pretty close to the location of our Delaware earthquake. Also note that in general, the earthquake locations above don't really consistently line up with the locations of these basins.
So, did a failed rift from the breakup of Pangea play a role in our most recent earthquake? Maybe. But there's clearly a whole lot more to the story than we currently understand.
One way we can study these types of earthquakes is to map out the locations of the aftershocks that follow them. Aftershocks tend to be very small, so if we want to find and locate them accurately, we need seismic stations pretty much right on top of them. To that end, a group of scientists (yours truly included) went out to Delaware the very next morning to install seismometers across the study area. The team included researchers from Carnegie, but also scientists and students from the United States Geological Survey (USGS), the University of Maryland, Columbia University, Lehigh University, and the Defense Nuclear Facilities Safety Board. We got a lot of help from the Delaware Geologic Survey, and the Delaware Division of Fish and Wildlife in finding good spots for our stations. We're also very grateful to the generous land owners who opened their doors to us and allowed us to install our equipment on their property.
All told, we got 14 stations up and running on the Delaware side, and another four on the New Jersey side within 24 hours of the earthquake (shown in red). We'll be looking at these data over the next weeks and months to try to understand the structures and forces responsible. We won't be able to completely answer the question "what causes these earthquakes" but we will have a new piece to add to the puzzle. One of these days, we'll see what the big picture looks like.
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