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Tuesday, July 29 at 4 p.m.

Earthquake Shakes Los Angeles

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Dr. Morgan Page
Mendenhall Postdoctoral Fellow, U.S. Geological Survey
Tuesday, July 29, 2008; 4:00 PM

An earthquake shook Los Angeles late this morning, measuring 5.4 in magnitude. There have been no reported injuries and media reports said the quake was felt as far south as San Diego and as far east as Las Vegas, Nevada.

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Dr. Morgan Page, Mendenhall Postdoctoral Fellow at the U.S. Geological Survey in Pasedena, was online Tuesday, July 29 at 4 p.m. ET to discuss this afternoon's earthquake.

The transcript follows.

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washingtonpost.com: I apologize for the delay, but we will be joined by Dr. Karen Felzer at approximately 4:30 ET. Please standby...

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Dr. Morgan Page: Hello everyone! My name is Morgan Page. I am a researcher at the U.S. Geological Survey in Pasadena, CA. It's been quite an eventful day here in Southern CA. The earthquake which occurred this morning near Chino Hills, is now estimated to be approximately magnitude 5.4.

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Anonymous: How 'substantial' is 5.8 on the Richter scale?

Dr. Morgan Page: We don't actually use the Richter scale anymore. It's somewhat antiquated. A 5.8 is a moderate earthquake. However, as more data has been processed, we now believe the earthquake is closer to a 5.4. This is not a Richter scale magnitude, but a moment magnitude, which is a more precise measure of the energy in an earthquake. Each additional unit on the scale has 30 times the energy, so a 5.4 earthquake has 30 times the energy of a 4.4, and so on.

This earthquake is moderate in size. There may slight damage to some buildings.

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San Diego, Calif.: Did the quake occur on the San Andreas fault?

Dr. Morgan Page: No, the San Andreas fault is further east. This earthquake occurred near two smaller faults, the Whittier Fault and the Chino Fault. We aren't yet sure which fault the earthquake was on.

Actually, with smaller earthquakes, it's not always easy to tell if the earthquake happened on a particular fault or not! There are many smaller faults we don't know about. The fault system in California is like a shattered piece of glass. There aren't just a few cracks -- there are many little faults all over that, together, as a fault network, accommodate the plate motion.

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Virginia: Was there any indication that this would happen?

Dr. Morgan Page: Not really. Most earthquakes don't have foreshocks. In Southern California earthquakes can happen at any time, so we must always be prepared! So while this earthquake occurred without any warning, earthquakes are certainly to be expected in this part of the world.

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Anonymous: What kind of structural damage did the quake cause, if any?

Dr. Morgan Page: An earthquake of this size might cause slight structural damage, but as of now I haven't heard of any reports of damage.

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West L.A.: What are the likely range and time frame for aftershocks?

Dr. Morgan Page: Aftershocks become less likely as more time passes from the mainshock, and less likely farther from the epicenter.

In the next week, we can expect about 12-40 small aftershocks (magnitudes 3-5). There is about a 5-10 percent probability that one of the aftershocks of this earthquake will be larger than this earthquake (a magnitude 5.4), in which case one could call this earthquake a foreshock.

The largest aftershock is usually around 1.2 magnitude units smaller than the mainshock, so in this case we can expect to have a magnitude 4.2. But there is a lot of variation in this number, so 4.2 is just an estimate.

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San Diego, Calif -- Former East Coaster: Can you please discuss the idea that a major earthquake is predicted to hit Southern California in the next 30 years? Thank you!

Dr. Morgan Page: The probability of a magnitude 6.7 or greater earthquake occurring in the Los Angeles in the next 30 years is 97 percent. So it's almost certain.

The probabilities get smaller for even larger earthquakes.

For a magnitude 7 or greater, the probability over 30 years is 82 percent. For magnitude 7.5 or greater, it's 37 percent. For 8 or greater, it's 3 percent.

Source: Forecasting California's Earthquakes-What Can We Expect in the Next 30 Years?

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Philadelphia: This earthquake was not along the San Andreas fault line. Do these fault lines affect each other? Could this earthquake have caused any unsettling along the San Andreas fault line?

Dr. Morgan Page: This is a good question! Scientists are still in disagreement as to just how much earthquakes on one fault can affect other faults nearby. There could be effects such as a stress shadow, where an earthquake on one fault lowers the stress on a nearby fault, thus lowering the probability of earthquakes. Or the opposite could occur, where a stress increase could trigger events on nearby faults.

One thing that is certain is that aftershocks will occur. Aftershocks are most likely in the days immediately following the earthquake, and at locations nearby. It is more likely that aftershocks will occur closer to Chino Valley that far away to the east on the San Andreas Fault.

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Washington, D.C.: Are the people living in California on borrowed time? I keep hearing Arizona/Nevada will become beach front property.

Dr. Morgan Page: Earthquakes can happen at any time in California. But we're not going to "fall into the ocean" or anything like that. The Pacific plate is moving to the north, while the North American Plate is moving to the south, parallel to each other. They aren't separating.

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Los Angeles: Well that was exciting. I was wondering how much science has advanced in terms of predicting earthquakes. Are we getting close to a time when we will be able to get warnings, even if only by a few minutes?

Dr. Morgan Page: Possibly! Although the warning time is more likely to be seconds, not minutes. Japan has implemented an early warning system, but it only works for people farther away from the epicenter. It works because telecommunications can travel faster than waves in the Earth.

Sometimes you do get a little bit of warning, but in a different way. Earthquake waves contain both P waves, which travel faster and thus arrive first, and S waves, which arrive later but carry 10 times the energy of P waves. Here in Pasadena I felt the P wave a few seconds before the S wave arrived, so I was able to get under my desk before the stronger shaking arrived.

If someday we do have an early warning system, it might give enough warning for people to jump under their desks, or for trains to begin braking, or for surgeons to stop cutting and start clamping.

However, for the people closest to the epicenter, there will be no warning, because they will be the first to know!

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Be prepared: Hi Dr. Page. You say that we need to "be prepared" -- what does that constitute? What can the average person do to prepare for a potential earthquake?

Dr. Morgan Page: Excellent question!

You should be prepared to get by for 72 hours on our own, without relying on city services that may be disrupted in a large earthquake. That means having enough water and food on hand for three days, flashlights and batteries, and a first aid kit.

Large items of furniture that can topple, such as bookcases, should be secured to the wall. (In the Northridge earthquake several people died when their bookcases fell on them.) Also, your hot water heater should be bolted to the wall, and after an earthquake the gas valve should be shut off if you suspect a leak, in order to prevent fires.

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Seattle: Were there areas that were stronger hit than others?

Dr. Morgan Page: The worst shaking is close to the epicenter, but there are a number of other factors that can influence the ground motion. Two of the biggest factors are:

Rupture directivity - since the earthquake ruptures not just at a single point, but on a fault, the direction that it ruptures is important. Stronger ground motion is felt in the direction that the rupture occurs (for example, if an earthquake ruptures on a plane from the south to the north, stronger shaking would be felt to the north).

Type of sediment - This is a big factor in Los Angeles. LA is located on a basin, which tends to make the ground motion stronger and last longer. For this reason, if you could choose between building your house on filled land or an a rock, from an earthquake standpoint you should choose the rock.

You can see a map of the shaking felt in this earthquake here.

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Dr. Morgan Page: Thank you for all of your questions! For more information about preparing for earthquakes, you can visit this website. Thanks.

Morgan Page, USGS

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