Earthquake Expert, U.S. Geological Survey(USGS)
Friday, July 16, 2010; 11:00 AM
The U.S. Geological Survey reported a 3.6 magnitude earthquake centered in Montgomery County at 5:04 a.m. Friday. The epicenter was in Gaithersburg near I-270 and Route 124 (39.145 degrees N, 77.222 degrees W), USGS reported in a preliminary finding. Its depth was 3.1 miles.
Authorities in the District and Montgomery and Arlington counties said there were no early reports of damage. Still, many residents were dialing 911 to report the rumbling.
Mike Blanpied, a geologist with the U.S. Geological Survey, was online Friday, July 16, at 11 a.m. ET to discuss the earthquake.
Mike Blanpied: Good morning. Lots of people in the DC metro area were awoken (or startled, if already awake) by this morning's M3.6 earthquake, just after 5 am. I'll answer your questions about the earthquake, the history of earthquakes in the area, and any other questions you may have about earthquakes and earthquake hazards, over the next hour. I appreciate your interest.
Mike Blanpied: For additional information about this and other earthquake topics, please see our web site http://earthquake.usgs.gov.
While there, I encourage you to fill out our "Did you feel it?" form about the quake. 12,000 people have already submitted their observations, helping to build a map, color-coded by zip code, showing the intensity of shaking in the area.
Leesburg, Va.: The USGS Web site classifies this morning's event as a "significant earthquake." When was the last time the Washington, D.C., region experienced a significant earthquake event prior to this morning?
Mike Blanpied: This may be largest earthquake to have shaken the greater DC metro area since we've been measuring earthquakes here. It turns out that our area does not experience many quakes big enough to feel, although there have been a smattering of M2 earthquakes over the years. Farther afield, there have been larger earthquakes in southern PA, in DE, and in central and southwestern VA, and some of those have been large enough to do damage.
Riverdale, Md.: Realistically, with the earthquake centered where it was, could I have felt it/would it have woken me up in College Park/Riverdale, Md.?
I know I woke up at some point early this morning because my windows rattled and I wondered if my son had come in and slammed the door, but the rattle didn't seem to be right for that.
Mike Blanpied: Yes, there are certainly many people in your area who reported feeling the earthquake. See our "Did you feel it?" map to see the intensity of shaking all over the region.
Fairfax, Va.: I woke up around 5 a.m. this morning and thought I was still dreaming because although I felt the bed and walls shake, there was no sound of the quake itself. I thought I was going crazy.
Anyway, how often do earthquakes occur in the DC/VA/MD area, how many earthquakes have been recorded for this area, and how strong was the strongest earthquake to hit the area?
Mike Blanpied: There are not many earthquakes recorded right around here, but the area is sometimes shaken by ones in the region. Our USGS National Earthquake Information Center (NEIC) has prepared this summary of the historical seismicity of the region:
Earthquakes in Maryland and Northern Virginia are uncommon but not unprecedented. In the past four decades, there have been about a dozen magnitude 2.0 and higher earthquakes within 100 km of this recent earthquake. The largest earthquake on record in Virginia is a magnitude 5.6 in Giles County on May 31, 1897. More recently in the broader area, a magnitude 4.1 earthquake struck Lancaster County, Pennsylvania on April 22, 1984. This Pennsylvania earthquake caused slight damage at Conestoga, Lampeter, Mt Nebo and New Providence and was felt from West Virginia to Connecticut. A series of small, felt earthquakes spanning March to July 1993 occurred near Columbia, Maryland, within 33 km (20 miles) of the July 16, 2010 earthquake. The largest in this series was a magnitude 2.7 in March, 1993. On May 5, 2003 a magnitude 3.9 event struck near Cartersville, Virginia about midway between Charlottesville and Richmond. That earthquake was felt widely in central Virginia and in parts of Maryland. The most recent earthquake in this region was a magnitude 2.0 event on May 6, 2008 near Annandale, Virginia.
Bethesda, Md.: How come my cats and dog did not react prior to or during the earthquake?
Mike Blanpied: Many animals responded to the shaking of the earthquake, because they are sensitive to even slight shaking.
However, the earth did not give any indication that the earthquake was about to occur--although from time to time there have been claims of "animal earthquake prediction," it has never been demonstrated that animals have a special ability in this area. In fact, the earth may not give any warning at all!
New York, N.Y.: The Post reports that the quake was felt in West Va., and in Pennsylvania -- why was such a small quake (3s often are described like a large truck going by) felt over such a large distance? is there something special about the geology of the region that allows the shock waves to spread?
Mike Blanpied: Good question.... It turns out that the old, cold rocks of the central and eastern US are very good at transmitting seismic waves over large distances. Even a modest earthquake like this one can be felt a hundred miles or more away. Out west, where the rocks are younger and more broken up by recent geologic activity, they have more earthquakes but each one shakes a smaller area.
Recall the M5.0 earthquake in southern Quebec a couple of weeks ago? It was felt in 19 US States and many Canadian Provinces!
Kensington, Maryland: How long did today's quake last?
Mike Blanpied: The slip on a fault under ground probably only lasted a second or two. But that slip sent a cascade of seismic waves traveling out in all directions. People living some miles away felt shaking for five to ten seconds or so. (And while that's happening, ten seconds can feel like a very long time!)
washingtonpost.com: Did you feel it? -- Tell us! (USGS)
Nokesville, VA: I'm confused. I thought DC sat in the middle of a tectonic plate. Are there other events besides plate movements that cause earthquakes... and if so, do you know what caused this one?
Mike Blanpied: You're right--we are in the interior of the North American tectonic plate, which stretches from the San Andreas fault and Cascadia coast in the west, to the middle of the Atlantic ocean to the east. However, the entire plate is under stress, and there are earthquakes that cause there to be hazard in the majority of our states.
The USGS publishes a seismic hazard map for the US, that serves as the basis for seismic building code standards adopted by states and towns across the country. For a copy of that map, see the Fact Sheet here: http://pubs.usgs.gov/fs/2008/3018/, or click the "Hazards" tab on our web site.
D.C.: I've seen some smatterings (mostly in comments sections) around the Web that this may be connected to the cap finally being successful in the Gulf. Of course, anything is possible, but how likely do you think that is?
Mike Blanpied: Not possible. The forces acting in the crust around the gulf well are local to that area, and the faults in our area respond to stresses near them. The gulf is far too removed from the DC area for their to be a link.
washingtonpost.com: 2008 United States National Seismic Hazard Maps (USGS)
Arlington, VA: What do you mean by M2 and what is the probable cause of this earthquake?
Mike Blanpied: The M stands for Magnitude, and is a measure of the energy of the earthquake. We estimate the energy using seismic recordings from seismometers arrayed throughout the region. The "Learn" tab on our web site http://earthquake.usgs.gov has more information about magnitude scales. The cause of the earthquake is the large stresses that act on our continent. In this case the squeezing stresses caused one of the many faults in our area to slip.
Atlanta, Ga.: Which area of the United States is the least susceptible to earthquakes, even of a small magnitude? Would it be the Great Plains?
Mike Blanpied: As you'll see on our seismic hazard map (see above), the north central states and farthest southeast have very low seismic hazard. (They do have their share of weather hazards, however!)
Rockille, Md.: I heard a loud, low rumbling noise during the quake. What was this sound?
Mike Blanpied: Lots of people reported hearing sounds during the earthquake, which is quite typical. Some of that comes from the shaking of the ground itself, and some comes from the rattling and slipping and sliding of joints and connections within the building. Some buildings make very distinctive sounds that may be heard even when the shaking is too light to be felt!
Rockville MD: What is the probability of further earthquakes after this one? I know they come in clusters. When are the conditions "back to normal"?
Mike Blanpied: After any earthquake there is a chance of aftershocks, and occasionally those aftershocks may themselves be large enough to cause damage. With the passage of time, that hazard dies away and we return to the usual level of hazard in this area, which remains modest but not zero.
The best thing to do is to let this earthquake be a reminder that we do have a hazard here, and to consider taking a few simple steps to be sure that you're safe at work and home if a larger one strikes. One of the most widespread inconveniences caused by earthquakes is a power outage that also affects communications; so it's good to have a plan in place for power outages, which may be handy for blackouts caused by any other cause as well.
Baltimore, Md.: For tall bookcases, people in regions with stronger quakes often attach an anti-tipping strap from the bookcase to the wall. In our area, do knowledgeable people such as specialists in earthquakes consider doing that in their own houses? What is the peak acceleration needed to tip a tall shelf, or knock items off?
Mike Blanpied: It really does not take much shaking to cause tall furniture to topple, or for pictures to fall off a wall and scatter broken glass. Speaking for myself, I would take the few minutes needed to secure the most vulnerable of those objects, especially if they are in sleeping areas or areas that kids or pets may occupy. For information on these and other simple preparedness steps, see the Prepare tab on our web site.
Silver Spring, MD: What is the name of the fault responsible for this morning's earthquake?
Mike Blanpied: We don't know that, and may never know. The earth is riddled with faults, and even quite a small one could have hosted this morning's earthquake. It's possible that it occurred on a fault that does not reach the earth's surface, in which case it would not even have a name (even if we know it exists).
Bethesda, MD: I, like many out there, was woken up by this morning's quake, and the sound and movement was quite evident in the early morning stillness of my bedroom. But I wonder, if this same quake had occurred later in the day once folks were up and about their routines, if the reactions would have been the same. My question is, assuming ideal conditions (i.e. quiet, still surroundings) what is the lowest magnitude earthquake that humans could feel? Thanks.
Mike Blanpied: It is common for a magnitude 3 earthquake to be widely felt in the eastern US, and earthquakes even as small as M2.0 can be felt locally (assuming there are people living in the area). Once we get down to that magnitude or below, some earthquakes will not be felt at all.
washingtonpost.com: Earthquake Hazards Program: Prepare (USGS)
Hartford, Conn.: Is there any link between the massive amount of oil leaking from the earth in the gulf and earthquakes?
What displaces the oil? What causes such high pressure to push all that oil up?
Mike Blanpied: Again, there is no link between the gulf oil and our earthquake up here in MD/VA/DC. The reason that oil is so highly pressurized at depth is because of the tremendous weight of the overlying rock--everything is very tightly squeezed that deep underground, so when one provides an escape route for oil, it can be happy to escape.
washingtonpost.com: Latest Earthquakes (USGS)
Gaithersburg, Md.: With the quake being three miles deep and a spread so quickly.... is there a back-up plan for checking utility lines that sometimes crack but don't fully break till weeks down the road?
Mike Blanpied: Utility companies have good knowledge of their routes and the age and relative vulnerabilities that may exist. The USGS does provide a tool called ShakeCast that can assist utility companies or others to identify potential trouble spots and to prioritize inspections. ShakeCast pushes out a ShakeMap to the user, that being a rapidly calculated map showing the likely shaking in the region--this can be sent within five minutes or less. At the user end, the shaking is compared against a list of locations and vulnerabilities, and potential trouble spots are identified and notifications sent. ShakeCast was developed by USGS in cooperation with the California Department of Transportation (CalTrans), which has tens of thousands of bridges and overpasses to keep track of.
Gaithersburg, Md.: There are several rental communities in this area, one of which I reside in. Do you have specific safety recommendations for those of us living on higher floors of buildings? We're on the third floor, accessible by staircase.
Mike Blanpied: It's unlikely that you will be faced with severe building damage from earthquakes in our area, but it's worth thinking about emergency escape routes and plans, even if fire is the more likely reason you might use it. Depending on the construction style, shaking can be greater on higher floors, so there may be more propensity for furniture to fall, etc. For multi-family structures with a many-car garage serving as the first floor, there can be some chance that the first floor, with its open construction, to be damaged, or even for that part of the structure to collapse. This was widely seen following the 1989 "world series" earthquake near San Jose, CA.
Chevy Chase, MD: Were there aftershocks? I thought I awakened to one at 8:05 PM, or was I just dreaming of the initial one that woke me at 5:05 AM?
Thanks Ann Chevy Chase
Mike Blanpied: We have recorded one aftershock so far, a magnitude 2.0 quake just a few minutes after the first, at 5:16am. There may have been other aftershocks approaching that size, and large enough to be felt, that were not detected by the seismometers in our area.
Great Falls, Va.: Do you think this is an indication that we'll see an increase in earthquake activity?
Mike Blanpied: I doubt it, as there's no indication that we're seeing a trend toward more frequent earthquakes. Earthquakes may happen at any time, and this one just serves as a reminder of that fact.
For a map showing the earthquakes recorded in our area, see: http://earthquake.usgs.gov/earthquakes/states/virginia/seismicity.php
Only our cat noticed: We've worked hard to get our cat to "request" breakfast at a somewhat reasonable hour, like 7 or 8 in the morning, but had a real back-sliding today, with loud meowing between 5-ish and 5:30 until, of course, breakfast was served.
This morning's news reports may explain what woke her up!
Mike Blanpied: I think you're right!
washingtonpost.com: Earthquake video: D.C. area awakens to rumble (AP)
Germantown, Md.: Is it possible to predict an earthquake before it happens? If so, how soon before? Are earthquakes of higher magnitudes easier to detect? Is there any monitoring in this area?
Mike Blanpied: Scientists have been trying to find the "holy grail" of earthquake predicting for centuries, and using modern methods, data and earth science for decades, without success. It may well be that the earth simply does not provide any indication that a fault is about to slip. Or, it may be that there are some subtle signs that we might eventually learn to detect; however, if those signs are the same for small and large earthquakes, we may still remain unable to know that a large earthquake is about to strike.
What we DO know is that reasonable attention to building design and planning can take the sting out of even the largest quakes. What we'd love is for our country to be well enough prepared that we ride out an earthquake with hardly a blink of the eye!
Silver Spring, Md.: Are there ever 'preshocks'? I was awakened by noise and shaking of my bed a little after 2 a.m. How could I find out if this was measured by USGS?
Mike Blanpied: If there was a small foreshock at that time, it was too small to be detected by our seismic network. All earthquakes large enough to measure are immediately put onto our web site.
Arlington, Va.: Did the earthquake feel stronger because it was so shallow? If it were 10 km deep rather than 3 km deep would we have felt the same intensity? Thank you.
Mike Blanpied: For deeper earthquakes, the seismic waves travel through more rock before reaching the surface, and are therefore more attenuated. They may still shake a large area, however, and even a very deep earthquake can cause damage if it's big enough.
Bethesda, Md.: Can you comment on the amount of slippage? Vertical or horizontal? Compass direction? Does this event cause you to recalculate the risk estimates for our area? How long to worry about aftershocks?
Mike Blanpied: The earthquake had a "thrust" sense--i.e., it occurred due to one piece of earth sliding up a fault "ramp" relative to another. In this case it occurred due to east-west compression, and slip was on a fault dipping either to the east or the west (one can't tell which one without more information than we have at this time). I have not calculated the likely slip, but it would probably measure in inches.
Falls Church, Va.: I woke up this morning at 5 a.m. with my bed shaking and my things rattling in the shelves. I came downstairs to my mother who didn't feel a thing! Even though we were in the same house and she was awake at the time. She thought I just had a bad dream. I can't think of a reason she wouldn't feel the quake downstairs, but is there an explanation? I also notice there have been different accounts from residents who have felt it or did not feel it even though they were in the same area, why is that?
Mike Blanpied: That may have been because the higher story of the house shook more than the lower one, due to the construction of the house and the fact that the higher story is not as constrained by the foundation. Or it could be that she's a heavier sleeper!
Rockville, Md.: I live rather close to the epicenter -- less than 5 miles as a crow flies. I didn't feel the earthquake a bit. My co-worker in NW D.C. felt it. Was I too close to feel it?
Mike Blanpied: No, many people located essentially right over the earthquake felt it. It's likely that you happened to be located on a somewhat stronger piece of ground, though that's just a guess.
Mike Blanpied: We've reached the end of the hour (and a bit beyond). I appreciate all the good questions, and also your experiences from the earthquake, and I apologize if I was unable to get to yours.
We're fortunate to live in a place in which the political earthquakes are usually more resounding than the natural ones. However, this is a good time to talk with your family, your colleagues, or your landlord about earthquake safety, just to make sure that nobody will be hurt or unduly inconvenienced should a bigger earthquake strike our area. Kids should be taught to "duck, cover and hold on" when the earth shakes, and families should have a plan for how to get in touch and how to get home should the power fail (or worse) due to an earthquake or other natural hazard. Once it happens, it's too late to start planning, so have that conversation today!
Thanks again, and have a good rest of the day.
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