David Hebert: Hello and welcome to the USGS CoreCast. This is Dave Hebert. A magnitude 5.8 earthquake occurred just south of the California-Mexico border today, December 30, 2009, just before 11 a.m. local time. This earthquake was pretty widely felt so I'm joined in the phone here by Southern California Regional Coordinator for the USGS Earthquake Program, Ken Hudnut. Ken, thank you very much for joining us.
Ken Hudnut: My pleasure.
David Hebert: Ken, where exactly did this earthquake take place?
Ken Hudnut: Well, it took place within the Colorado River Delta. So, it's about 50 kilometers southeast of El Centro and it's just to the northeast of Sierra Prieto, which is a volcano. There's a lot of associated earthquake activity near Sierra Prieto historically. So, this is just slightly to the northeast of where most of that prior activity has been going on.
David Hebert: OK. And what fault was this earthquake on?
Ken Hudnut: Well, that's a tough question. Because the Colorado River Delta is there, it has been burying the active faults with a lot of sediment. It looks like this is along, if you were to extrapolate the Imperial fault to the Southeast, it looks like it's along that trend.
David Hebert: OK. And there have been several significant aftershocks, is that correct?
Ken Hudnut: Yeah. The aftershocks are perking away. We're expecting more aftershocks. So, roughly over the next seven days we have about a 40% level hazard of a magnitude 5 or larger aftershock, what we call a strong aftershock. And through time, the odds go down but we're expecting basically a sort of a normal aftershock sequence. It has been what we've been seeing so far.
David Hebert: OK. And so, the USGS doesn't think this is a precursor to a larger earthquake event?
Ken Hudnut: Gosh, it's always risky to say that. There's always about a 5% chance that this is a foreshock to something bigger. We have been, of course, watching to see whether the activity is migrating off in any direction from where the largest event occurred. We're not really seeing any indication of that. So, we have not seen any activity associated with this north of the US-Mexico border. That's another thing we're looking at. So far, it has been quiet to the north of the US-Mexico border. There doesn't seem to be anything triggered up north.
David Hebert: OK. Now, how widespread was this earthquake felt as far as we can tell?
Ken Hudnut: Well, we had a lot of felt reports through our Did You Feel It? website, the USGS website where we pick up felt reports off the Internet. We had a lot of reports coming from the San Diego area and actually reports sent from over 400 different US zip codes and in 11 cities in Mexico people have been reporting the shaking that they felt.
So, it was widely felt. We have had some initial reports of light damage, I would say. We're not yet hearing any reports of strong damage. So, that's about consistent with our estimates at the USGS pager product. We have an estimate of about 1000 people exposed to severe shaking that's intensity 7 or 8. Larger numbers of people were likely exposed to strong to very strong shaking, closer to Calexico, Mexicali. But we're not yet getting in reports of major damage.
Ken Hudnut: OK. Now, that Did You Feel It? tool that you mentioned, that's something that any citizen can come to our earthquake website and report having felt an earthquake, is that correct?
David Hebert: Right. Anywhere around the world, people can come in on the USGS NEIC Did You Feel It? website and report their experience with the shaking. We really strongly encourage people to go in onto that website and report for us. We use that information in a variety of ways to try to help improve our real-time and real-time earthquake shaking data product, what we call Shake Map and also Did You Feel It? So, these are our ways of communicating back to everybody out there what our instruments got but also what people have felt and also our estimates based on our instrumental records of what people may have experienced.
So, the observations we get from people through the Internet help us to basically tune and calibrate the maps that we put out soon after an earthquake.
David Hebert: Great. And how many people roughly have weighed in on this earthquake so far via Did You Feel It?
Ken Hudnut: Last time I checked was a while ago, about an hour ago. And at that point, we had over 7000 reports already.
David Hebert: OK. Excellent.
Ken Hudnut: I'm sure many more have come in since then.
David Hebert: Certainly. Ken, did you have anything else you wanted to add about this earthquake?
Ken Hudnut: Well, so far we're not sure whether to expect more damage reports coming in. It can be typical that in a damaging earthquake it takes a while for the reports to roll in. We are working with our partners at CICESE in Ensenada. That's the academic institution in Ensenada that handles seismic monitoring in Baja Norte. Also, UNAM in Mexico City runs the national seismic network for Mexico.
So, we're working with our collaborators at those institutions. And people who go on the website can look up websites for those institutions. And on the USGS/NEIC website, we have links to their sites as well. So, they're doing a lot of the earthquake monitoring and putting up reports as well. And a lot of the information we're trying to get it translated into Spanish but on the CICESE and UNAM websites, that would be a good resource for people whose primary language is Spanish.
David Hebert: Excellent. Well, thank you very much for your time, Ken. I appreciate it.
Ken Hudnut: My pleasure.
David Hebert: And thank you to all of you out there for listening. If you'd like information about this or any other earthquake happening around the world, go to earthquakes.usgs.gov. Also, if you felt this earthquake and you haven't had a chance to weigh in yet, you can do it from that site. That's earthquakes.usgs.gov. And of course, you can get this and any other USGS CoreCast at usgs.gov/corecast.
CoreCast is a product of the US Geological Survey, Department of the Interior. Until next time. This is Dave Hebert. Thank you for listening and have a great day.