Magnitude 4.7 in Greater Los Angeles Area
Late on May 17, 2009, a magnitude 4.7 earthquake struck in the Greater Los Angeles area.
We spoke with Ken Hudnut, a geophysicist with the U.S. Geological Survey to fill us in on the details.
Scott Horvath: Welcome, and thanks for listening to the USGS CoreCast, I'm Scott Horvath. This morning we're doing an interview with Ken Hudnut, a geophysicist at the US Geological Survey, and we're going to be talking about the 4.7 that struck late last night in Southern California.
So, Ken, thank you for joining us today.
Ken Hudnut: My pleasure.
Scott Horvath: Can you give us a summary about the earthquake event that occurred in Southern California late last night?
Ken Hudnut: Sure. We had a magnitude 4.7 earthquake last night, 8:39 pm local time. It was in a highly populated area just a few miles east of Los Angeles International Airport, so a lot of people felt this one. It was at a depth of about 9 miles. So, given that location and magnitude, it's not surprising that it was felt widely.
We did expect from our USGS ShakeMap and pager initial results that the damage would be light and from what I heard on the news out here, so far that's been the case. There was some light damage but also very widely felt. We've had over 40,000 responses on the "Did You Feel It?" website.
It looks like this earthquake was almost in the same identical location as the 1920 event that was called the "Inglewood earthquake" at that time and Richter's book has summarized that. That is the basis for calling it the Inglewood fault and then it was also the basis for thinking that the fault was active. And so, it was later recognized that this is connected down through the oil fields that were well known by that time. But the 1920 earthquake indicated that it could be also seismically active and maybe even a source of future damaging earthquakes.
So, when the 1933 Long Beach earthquake happened on that same fault zone, that was recognized to connect with where the 1920 event had been and so they renamed it at that point the Newport-Inglewood Fault Zone.
We now know that it's at least about a 75 kilometer long fault zone and it appears to connect through the offshore with the Rose Canyon Fault that goes all the way on down to San Diego. So, at this point, we would expect after aftershocks. Maybe the probability of a felt aftershock at this point is well below 50-50 odds. So, we wouldn't be surprised if we get one...
Scott Horvath: Right.
Ken Hudnut: ...but, you know, and that probability will go down with time.
Scott Horvath: Wow, so, all right so, as far as - I was listening to the radio this morning and I heard several reports from residents saying that they felt their houses or their location move up and down as opposed to side to side like they normally do. Is that something that's typical with feeling the movement in a certain direction based upon maybe the type of fault or intensity of a quake?
Ken Hudnut: What we know about this earthquake is that the initial fault plane indications have been confirmed at this point. It looks like it was consistent with slip in Newport-Inglewood Fault which is a right lateral fault. So, for people to feel up and down motion that must mean that they are very close in - to the earthquake and these waves would have come up and been vertically incident to their plates where they were.
So, basically if you have a lateral slip earthquake, you can certainly imagine that in some places the initial first motions would be upward directed or downward directed. It all depends on how the energy radiates out from the source of the earthquake. Most of the larger ground motions from a distant strike-slip earthquake, you'd expect to be horizontal motions but if you're in close even to a strike-slip earthquake you can imagine having some vertical movements.
So, I wouldn't say it's unexpected. It's just that this happens to have been in close to a lot of people and so also pretty deep. So, that might explain why some people felt it that way and that it was an unusual and different kind of sensation for them.
Scott Horvath: Right. And because you said that it was pretty deep that it's about a little over 9 miles, and you said based upon a pager - a map that it - you expected like damage from that, and that's a pretty deep quake as far as that area is concerned, right? As far as the -
Ken Hudnut: Yes, although not a typical. Some of the earthquakes that we've had on Santa Monica Bay, if I'm not mistaken, were also down around that depth range and the magnitude 4.4 that we recently had out in the Western Santa Monica mountains sort of Northwest of Point Fermin and Southwest of Westlake Village, that was at a similar depth.
So, we have earthquakes that are down around the base of what we call the seismogenic part of the crust and actually come to think of it, the Chino Hills event which happened last July, that was the last widely felt event here in LA area. That was also down in the same depth range so, it's not at all uncommon to have earthquake activity down in that depth range here in and around Los Angeles.
Scott Horvath: OK, great. Well, is there anything else that you'd like to add before we - before I let you go?
Ken Hudnut: I can't think of anything else right now. Thanks very much for your interest.
Scott Horvath: Great. I appreciate it. And that does it for this episode of the USGS CoreCast. Thanks for listening. And don't forget that you can visit earthquakes.usgs.gov for more information and maps on this earthquake and future earthquakes. USGS CoreCast is a product of the US Geological Survey, Department of the Interior.
Until next time, I'm Scott Horvath saying, keep it cool.
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