Magnitude 6.4s in Pakistan
Early this morning, October 29, 2008, a magnitude 6.4 earthquake struck near Quetta, Pakistan. Twelve hours later, a second 6.4 struck in the same area. Dr. Harley Benz, Scientist-in-Charge at the USGS National Earthquake Information Center, fills us in with the details.
Scott Horvath: Welcome. Thanks for listening to the USGS CoreCast. I’m Scott Horvath. Around 11:09 P.M. Eastern Time, 4:09 A.M. at the epicenter, there was a 6.4 magnitude earthquake that struck 35 miles or 60 kilometers north-northeast of Quetta, Pakistan. And I just had found out speaking with our guest today that there was another 6.4 earthquake that just occurred as well within the same region, approximately about 12 hours later.
So joining me by the phone today is Dr. Harley Benz, a Scientist-in-Charge with National Earthquake Information Center with the USGS to talk about the details of this seismic event and the most recent one.
Harley, thanks for joining me on the phone this morning.
Harley Benz: You’re welcome.
Scott Horvath: So can you tell us a little bit about the first earthquake that occurred earlier this morning in Pakistan and possibly elaborate on the second one as well?
Harley Benz: Yes, there were two magnitude 6.4 earthquakes in an area that is known for earthquakes in the magnitude 6 and 7 range that have done damage in the past. The most recent earthquake to have done significant damage was an earthquake in 1997, about 100 kilometers away. It was magnitude 7.1. In that case, the earthquake was farther into the mountain range in the smaller population and it resulted in 57 deaths, thousands of homeless and about 500 houses damaged.
But the biggest earthquake in this area was a magnitude 7.5 earthquake that occurred in 1935 near Quetta and that one had an estimated death hold of about 60,000 people. So it is an area that is known for large damaging earthquakes.
Scott Horvath: I was reading on the earthquake website this morning that the depth of the epicenter was approximately 9.3 miles below the surface. Is there any way to determine with what we know about the geology and the population of that area of Pakistan if the earthquake originating at that depth would possibly create major damage within the surrounding areas of that region?
Harley Benz: This is an area that has some pretty large towns like Quetta that are densely populated, but it’s also in this fold-and-thrust belt so it goes from large population centers like Quetta to smaller villages. And I think what’s important about these two earthquakes were that they were shallow and that they were relatively large, magnitude 6.4. And because of that, towns and villages close to the epicenter are going to be likely significantly damaged. Our models based on our pager results showed that there were about 100,000 people exposed to very strong ground shaking and in particular, the structures that were vulnerable or fragile were likely to be heavily damaged.
Scott Horvath: Do we know the current depth of the second one that just occurred? Is that around the same depth as the first one?
Harley Benz: Yes, these two earthquakes were essentially in the same area that we have located about four miles or five miles apart. They’re in the same depth range. The style of faulting is exactly the same so the structure that produced the first fault likely produced the second earthquake.
Scott Horvath: Okay. Do you know if this particular earthquake occurred on a known fault or a known fault structure with the specific geology of the area and the plates?
Harley Benz: Well, I’m not an expert on this area, but there are several things we do know about this area from a broad-scale geology. This is in the margin between where the Indian plate is moving past the Eurasian plate and they are moving past each other at about 40 millimeters per year, roughly 1-1/2 inches per year. This is a large fold-and-thrust belt and it is quite complex geologically. There is an area where you have significant thrusting that’s producing these ranges, but you also have a lot of motion and deformation has taken up on what are called left lateral strike-slip faults.
In the case of these two earthquakes, they were both left lateral strike-slip faults within the Sulaiman Range or near the boundary between the Quetta block and the Quetta basin in the Sulaiman Range. It is an area that geologists have studied for years and the style of faulting that we see in the seismology is very consistent with the geology.
Scott Horvath: So where can the public go to get more information about these particular quakes that just occurred?
Harley Benz: They can go to earthquake.usgs.gov and on our homepage, they will see over in earthquakes in the news, they’ll see a link there that will take them to the first event. And we’ll be posting more information on the second event. So all the kinds of detailed modeling that USGS does with respect to these earthquakes can be found there at earthquake.usgs.gov.
Scott Horvath: Great. Is there anything else before we go ahead in this interview that you might want to add?
Harley Benz: Yes. We’ve been looking at satellite photos of this area, and there are a number of very small villages in the epicenter region. If they weren’t significantly damaged by the first earthquake, they’re likely to be weakened or damaged by the second earthquake. And we probably expect to see aftershocks in the magnitude 4 and 5 range. And unfortunately, given that these two earthquakes back to back had weakened much of the building stock in the area, we’re likely to see some of these smaller events causing further damage. So it’s very problematic here, the Pakistan government’s going to have a lot of cleanup to do on this area.
Scott Horvath: Right. I’m sure you’re pretty busy right now dealing with all these data coming in as well.
Harley Benz: Right. We’re modeling each of these events to make sure we have everything right with them and then following up on the aftershocks so that if there are significant aftershocks, we can put out the information very quickly for the emergency response agencies.
Scott Horvath: Excellent. Well, Harley, thank you very much for providing us with that great information and thanks for joining me on the phone this morning.
Harley Benz: You’re welcome.
Scott Horvath: That does it for this episode of the USGS CoreCast. Thanks for listening and don’t forget to visit our website at usgs.gov/corecast where you can find more information, links and transcriptions for this episode and other episodes in the past. Also, don’t forget that you can go to earthquake.usgs.gov to get the latest information on these two earthquakes as well as the main USGS homepage at www.usgs.gov.
As a reminder, the USGS CoreCast is a product of the U.S. Geological Survey, Department of the Interior. Until next time, I’m Scott Horvath saying keep your cool.
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