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November Public Lecture: Did You Feel It? The Virginia Earthquake of August 23, 2011

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November Public Lecture: Did You Feel It? The Virginia Earthquake of August 23, 2011

 

Dave Applegate: Good evening. We're starting a few minutes later to allow the last folks to get through our marvelous security system. So, thank you all for weathering that particular storm. Good evening. Thank you very much for coming. My name is Dave Applegate, I'm the USGS Associate Director for Natural Hazards.

And I have the pleasure of getting to introduce tonight's two speakers. These lecture series is intended to showcase some of the exciting science that's done here at USGS. And hopefully give you a better understanding of the science behind some of the issues that affect our daily lives.

In the case of tonight's speakers -we're going to be talking about one particular day specifically the event on August 23rd. An earthquake that was felt by more people than any earthquake before in U.S. history.

01:00

Our first speaker is going to be Mike Blanpied. And Mike is the Associate Coordinator for the USGS Earthquake Hazards where he oversees our hazard assessment and research activities. He also oversees the National Earthquake Prediction Evaluation Council, which is an expert group that advises the USGS in earthquake prediction forecasting methods.

He coordinates our earthquake disaster assistance's team which is a joint effort with the US Agency for International Development. Sends team to hot spots like Haiti, and Turkey, Indonesia, other areas that have been impacted by earthquakes. Mike joined USGS as a researcher in Menlo Park, California in 1989 after getting his PhD from Brown University.

Our second speaker Mark Carter is a research geologist with the USGS Appalachian Blue Ridge Project. Mark is a geologic mapper with 16 years of experience. He came to the USGS in 2009 after spending six years with the Virginia Division of Geology in Mineral Resources. Their geological survey.

02:08

And eight years before that with the North Carolina Geological Survey. He's completed the geologic mapping in four states, eight geologic provinces. And has nearly 35 published in manuscript geologic maps, bulletins, other materials. His graduate degree is from the University of Tennessee at Knoxville.

And relevant to tonight's topic -Mark is one of two USGS geologist who live within five miles the epicenter of the magnitude 5.8 Mineral, Virginia earthquake. And I actually see the other USGS geologist Chris Wills is sitting there in the back as well.

He and his family had felt more than 100 aftershocks since August 23rd. Now, before I passed it over to the Mike and Mark show. I get the distinct pleasure of asking you to silence your cell phones. So, with that I would like to introduce Mike Blanpied.

03:21

Mike Blanpied: Good evening. Thank you all for coming out. Appreciate the wait you had outside and for coming out on evening. It's been an exciting few months here at USGS following these earthquake and as it was an exciting day when it occurred. How is this? Can you hear me now?

Audience: Yes. No.

Mike Blanpied: Hello. So, back on August 23rd, where I was -was in my office on the third floor of the USGS here. I was actually talking to a reporter about something completely unrelated to earthquakes in Virginia when the earth started shaking. Building started shaking.

04:03

I spent several years in California and in fact I joined the USGS just before the Loma Prieta earthquake in1989 -the world series earthquake that did several amount of damage out there. That was my first and biggest quake. This was actually the second strongest earthquake that I felt.

The shaking was not severe where I was sitting, but it was certainly significant. I recognized that it was an earthquake. I was slightly taken a back that I was feeling a big earthquake here in Virginia. But having knew intellectually, yes we do have earthquakes in these area.

And I'll be talking a little bit about that. But of course the experience that I've had since moving here back in 2003. And you've experienced that most of you have had. You'll look in these area is not one of earthquakes as a daily hazard. But in fact earthquakes more than hazard in our area. Perhaps not the biggest one, but a significant one.

05:06

OK. How are we doing now? Are we doing any better?

Audience: Good.

Mike Blanpied: OK. Good. Thank you. So, yes earthquake certainly are a hazard here. We also have many other natural hazard. And a point that I made, try to remember to come back to at the end of my remarks at the beginning of these two part lectures. We can take some steps to prepare ourselves so the next time an earthquake happens -one we won't be as surprised because we've looked through one.

But also we may be better prepared. And if the services do well and so forth, we'll be prepared to write that out as a mere discomfort. Many of those preparation steps are ones that would always also take to ride out a snowstorm or a hurricane, or anything else that disrupts transportation and services. So, let's get started. And looks like Mark has my pointer.

06:03

Mark Carter: Checked if we can talk. There it is.

Mike Blanpied: All right. So, let's start with just talking about the earthquake. As you know, as you've heard or know, or already did hear the earthquake was the largest one in Virginia in over a century. Prior to that a magnitude 5.8 or so back in 1897 down in the Southwest corner of the state.

As we know, it was centered in that low population area between Richmond and Charlottesville. Of course folks like Mark who live in the area, they don't consider that to be significant population. It certainly shook a lot of people very hard.

Give us a lucky earthquake it was felt very roughly and did a considerable amount of damage, but there are no fatalities. In my world, that's a big thumbs up. However with considerable amount of damage, the estimates that we have are still of course being accumulated but are we in excess of $100 million in direct damage. That's damage caused by the shaking.

07:09

And then indirect damages sort of the downstream effects of business interruption and so forth. That may go beyond that. The remarkable thing about this earthquake is illustrated in this map here, which is from our field service. Each of these zip codes is colored based on the inputs from our website from people who have logged in or have logged their experience by filling out a form.

If you have not, you can go to the website usgs.gov and enter your experiences. All the way from Northern Florida up into Ontario, out west of Mississippi this was felt. It's hundreds and hundreds of miles. It's really remarkable. Also remarkable, 150,000 people have come to our site and told us of their experience here. It is by far the most people who have responded to an earthquake on our field service.

08:04

Because of the strong shaking that cause evacuations, and business interruption and so forth across the Central Virginia and the D.C. area. As you know, it damaged some of the structures downtown. As they've mentioned the folks who live in the Central area have been subjected to hundreds of aftershocks that are ongoing.

And at any point following here we will continue to keep information about the earthquake at our Website, earthquake.usgs.gov. What I'm going to do is present a little bit of a context about this earthquake and broader context of Central and Eastern U.S..

And then what Mark is going to do is to tell you about what the USGS and its partners do in response to this earthquake. What kind of science was done and is going to be done in Central Virginia to try to get a handle on what happened and what may happen in the future.

09:05

As we know the half of the Central area is quite heavily damaged. Mark is going to walk in through some of that in much more detail. One of the significant stories coming out and continuing to play out is the very strong shaking that was experienced at the North Anna power plant.

The power plant is shut down now well, continue inspection to go on to see whether the strong shaking that was experienced there caused any damage and to make preparations to eventually restart their reactor. Significant that amount of cause have to do with the fact that those reactor are still off.

Now, in comparison we will have a lot of other earthquakes around the world that have been of moderate to large magnitude all the way up to the magnitude 9 Tohoku earthquake. Not the ones that really tossed us is this magnitude 6 earthquake that struck right underneath the city of Christchurch some several months ago.

10:06

This earthquake was a direct hit on an urban area. And these is really are our biggest fear. We have big earthquakes that are often of distance. This earthquake in Virginia did some damage but we're going to survive that. Christchurch is really in bad shape following this earthquake and killed 300 or more people, very large buildings toppled, crushed and so forth.

Very widespread devastation and effect on the business and the livelihoods of folks there that continue. We think of the Virginia earthquake as a lucky strike. We certainly do have the potential for large earthquakes on the East happened and surpassed.

This one struck far away from urban areas, had this earthquake struck directly underneath Richmond, it might have been a very different story. So, we did have an earthquake, but in some ways we got lucky.

11:05

Here in Reston we have a recording of the earthquake. This is up to my seismometer, located at the Reston fire station on Wiehle Avenue. And what you see here is a horizontal motion. Two components of horizontal motion and vertical motion. And this is measured in acceleration.

Here is the velocity of the ground, and here is the displacement or the motion of the ground. And you can see that especially the west direction out there is very high amount of acceleration. A really remarkable for an earthquake at this distance.

Outwest is what we'll talk about in a few minutes. We wouldn't expect to see this kinds of heavy acts of shaking at this distance of several tens of miles or from the epicenter. Let's return back now to this digit film of map. And this one is zoomed in somewhat to the central Virginia area and the Potomac and the city area here.

12:04

And you can see these colors show widespread shaking that was experienced by people who reported on the digi field site. On the left we have a computation of the shaking. This is our shake map product and it's all ready to get that out on the Web and global to folks who within two five minutes of the earthquake at U.S. and a few more less than that anywhere in the world.

It's a calculation based on the epicenter and location magnitude and what we know about geology. And the key point is based on what we know about our seismic energy falls off with distance as it travels north of the epicenter.

Now, in the ideal world these map should look approximately the same. You'll see that they do not -the shaking these yellow colors, the moderate shaking were much farther here than they do in shake map. Which suggests that our calculation about shaking falls off the distance may not be precise.

13:03

Of course we don't have a lot of big earthquakes on the east, and so we have to graph these key opportunities to the science necessary to predict this better. And one of the key reasons we need to know that is for building codes and so forth, or for planning for the future. It's not enough to know what earthquakes might occur and how often.

We need to know what that shaking is going to be -how it's distributed. So, we know how to build our buildings, not more strong than but not too weak either. So, we'll be learning quite a bit from this earthquake. Another thing we put out immediately after large earthquakes is a property called pager.

Here's the shake map beginning it contours, can be laid out on top of a population map in the grays. And then count the number of people who were shaken. This color scale amount, you may not be able to read it, but it has the number of people shaken at various levels of intensity. From that we can take another step. So, what does that imply?

14:02

As the first guess about the economic advantage of casualties that they have been just experienced by the region. And here's a calculation that's made on that basis and the estimate the most likely economic damage was over a $100 million between $100 million and a billion Dollars. That turned out probably be true.

The most likely casualty count was essentially none, with the possibility of a few if people got lucky. And that turned out to be true as well. So, this is a very nice almost immediate snapshot of the earthquake. Goes out, but it comes to us in a response mode, it goes to FEMA, it goes to White House and so forth.

And it allows people to get a snapshot of what just happened and then understand about what sort of response is going to be needed so that that response kick in here. Moving ahead to seismic hazard, what are the implications of this earthquake for our understanding of the hazards of the region?

15:03

This is a map of USGS National Science and Hazard Map, it's encompassing of what we know about likely shaking throughout the country. We update this map every six years or so like you'll notice is there's a lot of reds out here in the west. We know this is a big playground, these are plates out here the San Andreas and so forth.

We know there is the earthquakes here. We know about the earthquakes in the new map and seismic so on where 200 years since that sequence making of 1912. It did a lot of shaking there. We know that Charleston has had big earthquake. What you may have not known is that central Virginia has some legion of elevated hazards as well.

Now, we compute this map by putting together pretty much everything we know. We put together information about what kind of ongoing seismic there is. There are small earthquakes, where the faults are? How compacted they are? What geomecies are telling us, GPS is telling us about how the earth is deforming which maybe leading to earthquakes.

16:04

And then again how that shake and falls off with distance which we call continuation. And we put that all together to put this map. Zooming in on Virginia, you can see that we have these areas that have elevated hazard. And these are mostly related to the fact that we've had these history of ongoing earthquakes in the central Virginia seismic saw in here.

We had an earthquake back in December 2003, that's why we felt and others in the Giles. The Giles County seismic zone. We've had that earthquake in 1897 and ongoing earthquakes. And then even much more active area in the Tennessee seismic zone here in Tennessee.

And then it would be off here as the screen as the new map area the most active of these of all. But you can see this earthquake back in August 23rd, fell square in there. So, seismologically this was not a surprise.

17:01

Now seismologically it was not all that well recorded either. We don't have a very dense seismic network in eastern U.S. We've got a smattering of seismometers, it was recorded but not very precisely. Ones an earthquake occurred for seismologist gathers as much information as possible especially when aftershocks are going on it can give us more information.

These white squares in here show various seismometers that put in place over the days following the earthquake. Mark Carter and Steve Nicks had a very key role. He coordinated many different institutions that worked together to get these seismometers out there to capture those aftershocks.

One of the very fast work. So, we have a very rich aftershock sequence that's still being recorded. The poor folks living in the area really got all. This shows each line here is 15 minutes, 10 minutes of seismometers record and that's 24 hours from top to bottom.

18:05

And you can see if you can just try to count the earthquakes there. Many of them are quite small, but those will go on day and night underneath the houses, the bedrooms, the businesses, the schools. It's why we felt all of those. And with an occasional one like this, this will.

So, this was a week after the earthquake, a week after that still going on. This took a while to lie down and it was always still in the process of dying down. Which doesn't remove the possibility that we could have another large energy earthquake.

That possibility has tapered off but it is still there. Now, let's compare what happened here to what goes on out west where we have much more experience. This compares our digifield map that I showed to you on the first slide with one from earthquake 6.5 in Central California a number of years ago. And when I showed to you like this, say one of those, those kind of looked similar, but the map scale is completely different.

19:05

Now, rescale the map so that they are the same, now they look like that. That's the entire area of California that felt that 6.5 earthquake this nice. Quite inside the amount area that felt the small earthquake in the east. Why is that? Well, earthquakes in the east are different.

And the reason they're different is because of the geology. Our fear underlying by the middle of the tectonic plate. Hundreds of millions of years ago was very active area, amount of steam built the Atlantic close enough and being ripped apart again.

Very active geologically. But since then it's been quiet. Those rocks have cooled, they've densified. Fractures in those rocks that are absorbed that are seismic shaking have sealed up. Now, the earthquake should size the energy shaking into the ground and it rings like a bell of those quakes travel, and travel, and travel very nicely all through these steps.

20:04

Out west the rocks are young and active, warm, broken up, full of fractures trying to shove seismic waves through that is tough. And seismic waves die off and they don't make it very far. So, that's a key difference and so we don't get as many earthquakes lease, but when we do they affect a lot of people.

So, here are some of the reasons that we are very concern about central and eastern U.S earthquakes even though we may not have that very often. We do have very large faults that lure, but that we don't have them, they're not very active and so it's hard to know where they are.

Mark will talk about efforts to try and understand the faults. When earthquakes do occur in those hard rocks that radiate bottom energy that travels long distances. We have fix soils, basins the seismic waves caused the soils to jiggle like jello. And in some cases they will liquify. Mark will talk about that as well. This turning to jello effect that house a lot of damage.

21:07

And then the other event, other factors is that because earthquakes don't happen much, communities around are again prepared the structures which can be older are just well prepared. So, even though we don't have a lot of earthquakes here, they have very large impact. So, the hazard actually is buried on.

Just as a reminder I mentioned that a matter earthquake sequence back in 1811-1812, we had a surge of large earthquakes in these area along the Mississippi between Tennessee, Arkansas, Missouri, Kentucky which is having ongoing earthquakes today.

Back in December of 1811 there was a large earthquake that shook quite broadly followed by aftershock until the dawn the next day. In January, there was another earthquake. And then the biggest one of all came in February. And this one was so large estimated somewhere above magnitude 7, I'm not really quite sure.

It was so strong and waves traveled so far, it was very significantly felt all the way to the east coast and maybe it caused damages in the east.

22:14

So, these earthquakes are once significant. One of the things we worry about is where it's going to happen? When it might happen in the future? And what are they going to do? And again we have this national map.

And so, first you need to know over our national issue. Here are some of the earthquakes of significant size that have happened over the last decades. And you can see that they're not all in the west. They grow and populate the high in the various country. Very nice and they will continue to do so.

And so, we're doing our best to understand where, when, and how big. And hopefully you can be doing, taking some steps to make sure if they happen here again, that you're very safe.

I'm going to leave this up just for a moment here. Do take a look sources for more information and we're going to switch and Mark is going to tell you about more details about Central Virginia.

[Applause]

23:43 Mark Carter: My name is Mark Carter. I am a research geologist of the Eastern Geology Paleoclimate Science Center here in Reston. As mentioned earlier I was also one of the two USGS geologists who lived within 10 miles of the epicenter of the magnitude 5.8 earthquake that had shook.
24:10

So, Mike and I would like to do is share with you my story, my account of USGS involvement in response to this earthquake, particularly in the three days after the earthquake shook. So, here 1:51PM, 04PM, August 23rd, 2011 as reported by the seismograph in Lynchburg, Virginia which is about 175 miles south of Washington D.C. About 75 miles southwest of the epicenter in Mineral. Where were you? Did you feel it? Well, my story actually starts right here in Reston. I was stuck here for.

25:01

I was at the basement of this building cutting rocks and I didn't feel that.

[Laughter]

Mark Carter: It wasn't until I was evacuating this building sometime about right here that I actually found out what was happening. We were evacuated and in fact a lot of people, somebody walked by with his cell phone and it just kind of make a general state to the crowd that said it was an earthquake.

And it was a magnitude 5.9, and it was in Mineral. And turned me and professed by ignorance or what tonight but I leaned toward the next said, "you know that stinks. I come all the way from Reston and we had an earthquake about that."

[Laughter]

Mark Carter: I haven't made the connection yet, but I was evacuating a building 100 miles from the epicenter of an earthquake that did occur in my backyard.

26:05

So, we all are as well as the rest of the country with the damage that brought up here in the DC area and Washington Monument and the Smithsonian cancel then showed the National Cathedral the way some of the damages incurred in the suburbs.

For instance, in Park Maryland, this is another USGS Geology Center Inner Science Center, lost a chimney in their house. And this is a wall collapsed just down the road here at Tyson's Quarter to visit something, I think it's a photography business.

What I want to talk to you tonight about was what was happening in my home county of Louisa. Louisa is located about 100 miles southwest of Washington DC. It's located about halfway between Charlottesville and Richmond. Just a little sort of geographically located here. I see forms the southern boundary of Louisa County in the Northeast boundary of the county.

27:07

So, just to continue my story here. My wife wasn't at home when the earthquake struck either. She was actually alone in the car. She didn't for schedules. So, neither one of us really knew what was happening back home until about 3 o'clock where she thought we were finally hit in contact and she called our neighbors who lived next door which about 200 yards from our house.

And the conversation went something like this: She called up and our neighbor asked, "Are you and the girls all right?" And my wife said, "Well, yeah. We're OK. Well, where are you? Well, we're at mom's and dad's house here in Richmond. OK. You need to leave the girls there and you need to come home. And she said she can't.

And she said, "Well, geez why, we're watching the TV here and they just reported some broken chimneys and a little bit of damage in Mineral, but that's about it."

28:11

"No. It's a little bit worse than that." We'll just leave the girls and come back as soon as you can." We have earthquakes in the Central Virginia south, well not say all the time. We have one every year and a half or so. They happen at about magnitude 3.3.

So, somebody here in the audience, somebody else here felt shaken. It's something we talked about awhile at the post office the next day. And the wide or church till Sunday. This was an unprecedented event in Louisa county.

Error post office was damaged or EMV was damaged. At a grocery store the bins were damaged. Some of the churches were severely damaged.

29:04

Two of their schools had been severely damaged. One will probably need to be raised and rebuilt. It's just completely unusable now. Unprecedented event we've had. Just was just feels few hours after the earthquake I think we were all beginning to realize that.

So, my wife gets home about 5 o'clock I believe she opens the front door to a vase in the floor. Cracks in our foundation, cracks in some of our sheep rock. We were blessed though, our neighbor the folks that she had called, their house were near the earthquake looks like it actually shifted about three inches towards their house and shifted back.

So, the problem was that it had an attached garage and the attached garage didn't go back towards the house. So, that's about a two inch gap in their foundation.

30:02

I mentioned earlier about the other USGS geology sitting back Chris Wills. Chris and his wife lost both chimneys to their 18 turning is it Chris? Vintage house. One of which actually came to the roof where it came down. So, what I want to do now is talk to you more about my response as a USGS geologist in the dates what after the event.

About 7 o'clock that night I received a call Tuesday night of the 23rd I received a call from Steve Shimmer Associate Director of the Science Center firstly asked if me and my wife were OK.

Then he asked if I will be willing to help to coordinate USGS efforts in the county. Community emergency response team members line ups, connections with the county. And probably more important than my house wasn't as damaged as it hurts a lot had a little bit free time if you will, if you call OK. I agreed to.

31:14

There were a number of organizations who were mobilized at this time to get in to Louisa County to start or basically your task to rebuild to three aspects through tasks. First and foremost damage assessment. what's destroyed, what's left, what do we need to do about that.

Essentially the seismograph placement. And the third is recording natural features that were produced by the earthquake. We'll talk about each of these here in just a few minute.

The important thing though is we have three days because all of us who will come in to Louisa County knew that major hurricane was coming up the east coast and by Saturday morning we would have a hurricane sitting right on top of us.

32:06

These two will be severely impacted by hurricane winds and forces anything that was produced by the earthquake in any natural feature most likely it will be washed away by Saturday morning. So, this particularly was very, very, very time critical for us.

I say up here in Reston on that Tuesday evening, just because traffic. I'm sure you all know where the traffic and all these buildings were evacuated. Also, got a couple of hours sleep. We got up earlier in the morning it was actually back in Louisa County at first flight.

And the first thing of course that I started doing as well was damage assessment. In fact before we even got back to our house I stopped and took a look at the bridge we got in the damage by the earthquake.

33:00

By about nine o'clock in the morning I'm getting up with two of my former colleagues with the Virginia Division of Geology and Mineral Resources Matt Heller and Dave Spears, and we began a systematic damage assessment sweep of Southern Louisa County in the vicinity of the epicenter.

And what we initially found was just what we expected to find, just what the news being able to report. The farther away from the epicenter we were seeing skewed chimneys, skewed weather veins across where we've got. We've seen taco chimneys of steel. We were actually seen a chimneys that are completely pressed.

What we were not prepared for was the amount of damage right in the epicenter area. And I don't think too many people were very aware of the amount of damage here. Houses completely off of their foundation.

34:01

Houses that had been shortened by an inches or more. These houses has been short by eight inches and a quarter. And this upper floor actually collapsed into the lower floor. We saw many examples of this, This is why we saw house chimneys that came down and as they came down they actually came down into other parts of the house and destroyed those other parts of the house.

I've got a neighbor three blocks down from Louisa who has just been able to move back into part of their house with that they had the chimney come down and basically took about took about half of their house. Few examples of these as well which you will notice is it had a rip right here.

This isn't a brick veneer, this is a structural wall buried wall here. The force of matter from the energy from that earthquake hit this house so hard, it's like hitting a piece of plate glass with a sledge hammer. Just completely smashed the side of this house.

35:06

We were blessed, we were fortunate, it was a miracle, whatever you want to call it. It's amazing there were no casualties from this event. Given what we saw right there in the epicenter area. So, what came from all of these in the days and weeks after the earthquake. Matt Heller with the Virginia division of Geology and Mineral Resources put together this damage assessment map that shows the areas with the most damage.

But then there were two reasons number one really to assist Louisa County Lake relief efforts, but secondly this is what I'm using the map for -I hope people produce the map for to see if there's a correlation between the underlying geology and where there most damage occur. There's a couple of things here. Here's houses before that I mentioned earlier like will be up here.

36:04

I lived right in the with the quake. Chris Wills lives about two miles north of me in towards Louisa. We're both right here in the very, very edge of this zone of maximum structural damage. The second task in the task that I was most involved with as a field geologist which recording, locating, and recording natural features produced by the earthquake.

What we were really looking for was surface rupture. The magnitude 5.8 earthquake is right on the very edge with having produced actual cracks in the ground from the earthquake. we haven't found anything yet, that's definitive. We're still working, that's two and a half months out. We're still working despite of the rain.

37:04

What we did find in those three days that right after the earthquake were features that were very indicative of magnitude 5.8 earthquake in the ground shaking that we associate with that. We have samples of large rocks falling off, bed rocks and river banks slump in right in the epicenter area.

What we also found which was fascinating and again very, very indicative of the magnitude 5.8 earthquake we found liquefaction features. But basically the liquefaction these are sand moles you can think of liquefaction features if you have a saturated set and you start shaking it from the earthquake.

The springs in that sediment will compact and as they compact it will be expelled and produces what's basically sand volcano if you know.

38:01

And so we found several examples of this right along south end where you're looking for these in the streams and will reflect quite some. So we found some by the time the south end of river and some of the other. Again right there in that epicenter area. So, the last task that was being performed in those days after the earthquake.

And I'm not going to spend too much time for the actual test, because I wasn't that involved with that actually other than sending a few teams to some property over that I knew would be amenable to having seismographs place to their backyard.

This was the seismograph placement. We're now will say here though we have seven groups out placing seismographs, straight from the USGS. The first seismograph teams as Mike mentioned earlier. The first seismograph team who were actually in Louisa County on Tuesday night.

39:03

And the first portable seismographs were in the ground at least mid day. They started collecting data on what essentially they call a very robust aftershock sequence that we'll talk about that in just a minute.

By the first week of September there's actually over 250 portable seismographs in Louisa County areas. The 40 plus were three parts and we had an additional 200 in one component of geofoam set up. So, all over the county recording these aftershocks.

I will now do few here for a minute. Let's talk about the data collected from those seismographs and how it relates to the geology in the area. Let's start with the main shock here. The data from the main struck suggest that the fault that ruptured was originated by 10 degrees east. And it had a northwest damage the very steep getting to the northwest.

40:11

It was what we geologist call a, "reverse fault,' so that one body of rock will shook up and over another body of rock. Similar to these diagrams here. The north 10 degrees east is a too odd for this area, but the northwest it is kind of odd for a regional fault that we see in this area.

The main shock again there's a little bit of question about the location and depth. But it was about six kilometers down or which will be about three and a half miles or so. What aftershocks though and I say that the less aftershocks we've actually had over 600 aftershocks.

This number right here is actually from about mid October. Jeff Mudsy with TVA is compiling a list of aftershocks and it just continues to grow that we had one just a couple of months ago.

41:12

Fortunately I slept through it so I guess I tell you this where is the one you after a while. Well, the aftershocks we're showing something just a little different though. The aftershocks we're showing a fault still reverse fault, but it's a lower angle reverse fault. The geologist call that reverse fault because right about 30 degrees here we're about 45 degrees from southeast.

Very much in line with original geology. What's interesting here is if you take what the aftershocks were showing it projects that fault up to the surface. And actually cross out right behind my house.

[Laughter]

42:03

It runs off right through Chris' property. And then he ends up into Louisa. So, you know, Chris and I are right on top of this thing. And it just won't show up it continues to go, go, go. I'm going to point out another thing before I sit the animation here. I want you to take a look at this line right here which is just showing this line of aftershocks because what we want to admit the sort of a three mutual animation of what's going on beneath our feet here.

The data is from Jeff Mudsy this is most of the 600 aftershocks. This was actually done by my wife she is a researcher out with we did this to explore.

Just geographic information here. There's Cuckoo, here's Mineral, here's Louisa. The two samples here are two schools that were so severely damaged of all.

43:06

This line of aftershock right here to line what I just pointed out previously. This is the fault that goes around my house and Chris' house. I want to explain really quickly though for those who are not familiar with the difference between an earthquake focus and an earthquake epicenter.

The focus is the actual point in xyz space right to launch and depth of where the earthquake actually curbs along the fault. If you project that earthquake focus up to the surface then that becomes the epicenter of that earthquake.

OK. So, this is the plain view or a map view. OK. So, all these little you'll see right here are epicenters. This has just been crossed around by Chris and I this is why it's out there formed to the east southeast, and the middle.

44:02

So, to start this animation we're going to go from plan view into rotating down from down from plan to the horizontal. OK. Here we go. There it is. The orange dots now are the actual earthquake foci. These is were the earthquakes have formed.

Let's say about three and a half miles. So, some of it are very shallow. And some of it are very deep. So, rotating this thing around faults will be down that plain, that fault that continues to rupture right there. Three dimensions that's the structure that continues to fall between or beneath a person at the house. That's north 30 degrees east depth in about 45-50 degrees to the southeast.

45:00

But what I also want to point out is this guy over here. If you look at that and rotate that around with you can look right down that plain. Now, that's not the north 10 degrees east. Right what the main shock showed that that does give you that northwest deep.

And so, what this had shown you that's and I'm going to rotate this all through from the rest of it. What this is showing us is there is probably not one fault in action here, but there are multiple faults that are interacting with each other as we are seeing this continued aftershock of sequence.

So, we put all of these together, and put the aftershocks together is up what data sets together. What does it tell us about the regional geology or what the regional geology tell us about what we've seen from the aftershocks. This is the point the presentation where next slides will be detailed geologic map with smoking gun faults in the whole world.

[Laughter]

46:05

I can't. the point is that we have no detailed geologic mapping for area or from immediate descend of the aftershock. This is a huge question mark. In my opinion as a field geologist, these is the most important task that needs to be done now to understand if you have any help at all of understanding what went on, on August 23rd was actually continued to go on between I believe.

What we do have here though is fairly good regional geologic mapping. But there are two problems with this, we've got good mapping off here to the northeast of regional mapping of lower piece of Mineral. We have geomapping down towards the south and off there to the west.

The problem though is is that you'd ran the geology from the northeast down here, the rock units and the structures and the fault down to the epicenter, then drag the rock units and the faults and the structures up from the southwest. You get the epicenters off and then they don't quite match up.

47:17

Again the need for geologic mapping. The second problem is is that the main shock in the aftershock don't seem to really correlate with any one particular structure. What it does look like, is the main shock which they store right here few in the aftershocks if you take these and put them in depths and get their earthquake foca from these shocks.

They looked like they are occurring along the contact between what we call the particles salvation with area is that is a microsheets and so factor to the west what we call a formation which is form a sedimentary volcanic scene back of May.

48:02

Looks like it's occurring into that contact. But before that contact is called law down south -that contact isn't considered to be a fault and so there's an issue there. That large area of aftershocks just to the east of Reston. Those aftershocks don't seem to correlate with anything.

They just float down here in the middle of the formation. What I think is actually the least, we take those to depth they may be corresponding to the contact point of what we call the surrounded up here just north of Louisa. The contact formation.

So, yeah you take a look at the three image. What I think is going on is that the main crack actually occurred at that contact formations. But there are small branch of the wall with that main shock occurred right there.

49:09

Actually put stress on the rocks beneath it of there to the left what we call the foot wall of that fault in contact with the next stress was applied from main 5.8 magnitude earthquake. The next series of aftershocks went along that context.

So, what we're looking at right there is either a new fault or a previously unrecognized fault between the and just long formation. That's what has been producing the bulk of the aftershocks that first. That rupture there at that contact that's too huge to earthquake on the structure with the...

50:01

So, I want to leave you tonight at the Gilboa Church which is within Cuckoo, Virginia, which is a small about two miles southeast of Mineral, 1:52 PM, August 23rd, 2011, Gilboa Christian Church is an old structure. It was built in the mid 1800s. Serves as a here every Sunday.

At 1:51 all four walls of this church were compromised. In fact, the back wall of the church here former back wall actually collapsed into the sanctuary and it covered the whole pulpit first three rows of pews.

The point is we're very fortunate that there were no casualties from this event. Had this earthquake occurred two days, in a few hours previous though there would have been casualties right here at the church.

51:03

As Mike said earlier, we dodged a bullet from this one. Will we be so fortunate next time there is large magnitude earthquake in the east coast? Thanks. And I think it's time for questions.

[Applause]

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Title: November Public Lecture: Did You Feel It? The Virginia Earthquake of August 23, 2011

Description:

The magnitude 5.8 earthquake that struck Louisa County was among the largest to occur along the eastern seaboard of the United States. It caused extensive damage in central Virginia and was likely felt by more people than any other earthquake in U.S. history. Join USGS scientists Mike Blanpied and Mark Carter on November 2nd to discuss the seismology of the earthquake, its effects, and its context in the geology of Virginia. Mike Blanpied is the associate program coordinator for the Earthquake Hazards Program at USGS, and Mark Carter is a research geologist with USGS who lives in Mineral, VA, near the epicenter of the earthquake.

Location: Louisa County, VA, USA

Date Recorded: 11/2/2011

Audio Producer: Alex Demas , U.S. Geological Survey


Usage: This audio file is public domain/of free use unless otherwise stated. Please refer to the USGS Copyright section for how to credit this audio.
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