In March, Elizabeth Cottrell, a geologist at the Smithsonian’s National Museum of Natural History and director of the Institution’s Global Volcanism Program, created a helpful video explanation of Japan’s devastating Sendai Earthquake. So when a magnitude 5.8 quake occurred in Mineral, Virginia, yesterday, just 84 miles southwest of Washington, D.C., we went to her with our questions.
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Why was the earthquake felt over such a broad geographic range?
The East Coast has much more contiguous bedrock that is less broken up by faults and other kinds of tectonic boundaries. On the West Coast, there are just a lot more faults, which damp the energy. On the East Coast, energy can pass more efficiently and travel farther.
What other factors play into how an earthquake is felt?
It has to do with the nature of the material you are sitting on. If you have sediment that sloshes around you can get a lot more damage or a lot more ground motion than you feel on solid ground for the same magnitude earthquake. This was also a shallow earthquake, so it went through less material.
What is known about the faults in that area of Virginia?
The USGS has a very good write-up and very nice map about this fault system and the history of the faults. It definitely is a very large earthquake, the largest we have on record, especially recorded with modern instruments, for this system. I think people on the West Coast are somewhat poking fun of the East Coast. But this earthquake is certainly nothing to sneeze at.
What do they know so far about this fault?
It’s a reverse fault, part of the Central Virginia Seismic Zone. A reverse fault is a thrust fault—one piece of rock sliding over another along a plane—with a high angle, meaning the fault goes into the earth more steeply. They are the opposite of normal faults. It is the kind of thing that is easy to diagram but difficult to explain.
What damage happened at the museum and your lab?
The Natural History Museum is a 100-year-old building, and things aren’t as earthquake ready here because earthquakes are not as common. The doors in our department are no longer square. Some doors don’t close. Some doors stick. So the doors are out of true, so there is clearly some shifting in the building. There was a lot more damage at the Museum Support Center [in Suitland, Maryland], which is still closed today. We had damage in the Minerals Hall. A lot of minerals fell out of their specimen holders, and that hall is closed today. In my own lab, I run really high temperature furnaces for doing experiments on rocks and I have to get in right now and assess that damage.
What was your personal reaction?
When we had the first shocks, I totally clapped my hands and smiled and I was like, “Oh, boy, an earthquake!” I knew immediately it was an earthquake and was really excited. Then it really started shaking and my reaction was one of fear and moving to the interior of the building and getting into a door jam. It was at least another five minutes before the fire alarms went off, but I knew we were going to have to evacuate. You could immediately see that there could be structural damage, so I was preparing to evacuate.
As a geologist, what were some of the questions running through your mind?
What was the magnitude? And, actually my guess was a 5.5. So I did pretty well there. And where is the epicenter? Because if it is really far away, then wherever the epicenter was could have a lot of damage. Until you know the magnitude and where the epicenter is, it is hard to gauge what you felt where you are. For example, people felt the earthquake in New York, but it felt like a very small earthquake to them.
Do you have any big unanswered questions that are maybe on the minds of other geologists and seismologists right now?
Right after it happened you are also wondering about aftershocks and how big the aftershocks might be. We did have aftershocks but not big ones.
Does the occurrence of an earthquake increase chances for more earthquakes in the area?
No, not necessarily.
What do geologists and seismologists do now? Do they heavily instrument that area to monitor it more?
I don’t think that will be an outcome. We are very well instrumented already in the United States for that. This was recorded by lots of modern seismographs.