East Coast earthquake: Seismologist discusses

Aug 23, 2011

Dr. Graham Kent chatted about what happened to cause the 5.9 earthquake that recently hit the United States East Coast, and about earthquakes in general. Ask now!

Hello from Lake Tahoe Basin! Let's begin.

Is this just a "pre-earthquake" to a larger earthquake?

That tends to be the case about five percent of the time with normal earthquakes.  Those are called foreshocks.  That's something we're always aware of and why it's always important for people to plan for that or even just large aftershocks.  Either way, people need to be aware.  So it's not super common, but about five percent.  More common in Nevada.

Would the quake in Colorado this morning have triggered or be related to the Virginia quake?

No.  Very, very, very unlikely.

What is the likelyhood of an aftershock?

I'm sure there will be more instruments put in the area, and one will see hundreds if not thousands of aftershocks.  Most will be very small quakes.  But a small handful of those will be large enough to be felt.  Some of the larger aftershocks could potentially be the straw that breaks the camel's back.  People need to be aware of that.  They can be damaging. 

Has there been an earthquake in DC before? I've lived here 24 years and never experienced one in this area.

I'm not the most familiar with that area, but there was a very large earthquake down in Charleston over 100 years ago that did considerable damage along the East Coast.  So, you occasionally get earthquakes like this East Coast even though there's not "active fault line" there.

Does it matter how the ground moves as an indication of intensity or possibility of aftershocks?

I'll turn this around: It does matter what type of ground your house is built upon.  If it is very weak soil you'll get amplificiation and it'll seem like a bigger earthquake.  If you're built on stable ground, it will minimize the shaking for that location.

5.9 is a moderate quake but there a lot of historic buildings in the DC area - could this quake or aftershocks affect the buildings' structural integrity?

I think certainly as you move closer to the epicenter, that's certainly true.  There will be people inspecting various builidngs.  I think the most important point in that regard, is that the Earth's crust is a lot colder there, so energy travels much further without being dissipated.  Even though it's a 5.9, it's a lot bigger deal than a 5.9 would be in California or Nevada.  You might see damage further away from the epicenter than you might expect.

Should the Metro really be running its trains? Are Metro tunnels and bridges safe without being checked first and are the safe in the event of an aftershock?

I woudl assume that even though most readers would not think this, there are contingency plans for these events.  I'm sure that throughout the East Coast there are people doing those types of inspections depending on how much damage there is in a given area.  DC may be far enough away from the epicenter it's not an issue.  That's what I would hope, and in the nation's capital I assume there are contingency plans.  If you're not seeing major structural damage in DC, I can understand them moving forward. 

If you were on a long bridge expance like the Chesapeake Bay Bridge would you be in any more or less danger than on the ground beside it? Do you think that people on the bridge felt more or less of the earthquake activity?

So the quesion is: Are you safer on a bridge or not?  Answer: I'm never going to pass on being on terra firma in the middle of an open field. 

But bridges and buildings are designed to absorb ground motion, not to resist it.  In some cases, if you're in San Fran for example, and the tower is swaying, that's a good thing.  The energy turns into motion.  So motion isn't a bad thing.

Why was the earthquake felt all the way from VA to Ohio? Is it typical for all earthquakes to have such a strong shake?

The whole East Coast used to be a plate boundry where Africa and Europe collided with North America.  At a later time, it obviously pulled apart.  It still has a lot of stresses in that collison, and then as it erodes and cools down over time it relives stresses - and those are along those old fault lines from years ago. So they are kind of reactivated fault zones. 

These events are relativly rare as opposed to being on an active boundry like in California and Nevada.

Where can I go to find how much shaking there was around my home?

Very soon the USGS will take the seismometers in the area and create what is called a shakemap.  Once this earthquake has a name, type the name of the quake along with "shakemap" into Google Earth and USGS should have it up. 

FEMA is using a shakemap right now to determine what areas have the most damage.  If you're really adventerous, you can go on the USGS website and find that.

I'm an elementary school teacher. What is the best thing for students to do when they feel an earthquake. When I lived in California, students got under their desks and put their arms over their heads. What about getting outside if there's a field clear of buildings instead?

The students need to drop, cover and hold on.  You do not try to make it to the door or outside.  There's a program called The Great Shakeout, which started in California.  Several states have been adopting it.  California and Nevada are doing The Great Shakeout on Oct. 20.  You can get info from www.shakeout.org.  This is really important.  Everyone in the U.S. needs to be prepared to drop, cover and hold on.

What can you tell us about the connections between earthquakes and hydrofracking for natural gas? and also mountaintop removal mining?

All three things you just mentioned all have to do with release of stress. There's a lot of controversy right now about hydrofracking - essentially overpressuring a reservoir to result in failure.  That's an effect of you can add water to a reservoir, take material off a mountain top, push fluid into a confined space - that's how we can induce failure. 

Obviously some of the concern is whether we can produce a man-made earthquake that then triggers a larger earthquake.  That said, mother nature and her forces are ultimately going to rule the day.   They are much larger, as we've seen throughout the U.S. and the world in the last decade.

Could the impending hurricane expected to hit the East Coast have caused the Earthquake?

Absolutely not.  But, if there's a lot of damage, which I'm assuming there are in certain areas, and some of the buildings aren't doing so hot, the wind forces on those buildings may cause a problem.  So I guess that's a conern, or at least it would be a concern of mine.

Thanks to the Washington Post producer for transcribing for me from this 9,000 ft. high elevation!

Those concerned about aftershocks can go to the Shake Out website and learn how to secure their space.

In This Chat
Dr. Graham Kent
Dr. Graham M. Kent has recently been appointed Director of the Nevada Seismological Laboratory and Professor in the Department of Geological Sciences and Engineering at the University of Nevada, Reno. Previous to this past July (2009), Graham was a Research Geophysicist at Scripps Institution of Oceanography and had been Director of the Visualization Center at Scripps from 2001-2009. Dr. Kent is a native of Lake Tahoe, California, where he graduated from South Tahoe High School in 1980. He attended San Diego State University, where he studied Geophysics and graduated Valedictorian of the Class of 1985. Sooner thereafter, he entered graduate school at Scripps Institution of Oceanography receiving his PhD in 1992. After a 4-year-long appointment at Woods Hole Oceanographic Institution, Graham returned to Scripps to continue his work in geophysics, with an emphasis toward seismic studies of extensional tectonics, ranging from magma chambers beneath mid-ocean ridges to fault hazards at Lake Tahoe. More recently, he led an effort to use advanced visualization techniques to study faulting and volcanic systems. He is married to Stephanie Kent, a Tahoe native, and has two children, Matthew and Christine. Graham was also a NASA Astronaut Finalist, Class of 1996.
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