The science of aftershocks: What they mean for Japan, beyond

Apr 12, 2011

Is Japan in store for more seismic trouble? Join James Whitcomb, section head of the Division of Earth Science in the National Science Foundation?s Directorate of Geosciences, as he explains the significance of aftershocks and what sort of trouble they could bring to the country.

Have a question? Ask now.

Hi everyone.  Thanks for joining me.  Although I am not currently an active researcher, my background is earthquakes and my section at the National Science Foundation supports much of the earthquake research done worldwide.  Any questions about earthquakes and their aftershocks are welcome, and I will try to answer as many as I can.

Is it true this latest earthquake has probably increased the instability of the faults and that years of earthquakes are now more likely?

It is generally accepted that the earthquake increased the stress on the adjoining faults.  For an earthquake of this size, the aftershock series will continue for many years, but at a decreasing rate with time.

How are tremors classified as either an aftershock or a new earthquake? As I understood it, there was a time limit for aftershocks and after that they were considered earthquakes in their own right. But with the latest tremors in Japan still being called aftershocks, I'm confused.

There is nothing thus far discovered that distinguishes aftershocks from new earthquakes except for the relationship in time and space with the major event.  Thus, they will be called aftershocks if they are close to the main event until the seismicity rate drops to what is considered "normal".

There now have been significant quakes on 3 corners of the Pacific plate within the last few years. What does this do to the probability of a significant quake in the NE corner, off the coast of Oregon/Washington?

Scientists have not yet found any clear relationship between the timing of earthquakes on opposing sides of the Pacific plate.  However, this is a subject of ongoing research.

I've read that buildings damaged in a major earthquake can be given the coup de grace by aftershocks. Japan has for years required buildings to have suspension and shock absorbing components to prevent building from collapsing during an earthquake. Can these components become damaged in a major earthquake and cease to protect building afterwards?

Depending on the design of the absorbing components, it is possible that they have been damaged to the point that they have lost their ability to protect the structure.  Earthquake engineers will have to inspect the components to determine what actions need to be taken.

I understand that the quake in the Tohoku area may actually have increased the stresses in the fault-line further south near Tokyo, perhaps making the long predicted quake in the area more likely. If so, would this stress also make the Todai quake even stronger than if the Tohoku quake had not occurred? Could the strong aftershocks actually trigger a major quake in the Tokyo area? Finally, how do you determine that a quake is an aftershock rather than a new quake. Thanks David in Tokyo

Modeling of the stress changes due to the Tohoku earthquake show stress increases to the north and south of the main rupture surface, including the Tokyo area.  However, we are not able to translate this information into a better prediction of the timing of future events.  This stress increase does not necessarily translate into a stronger earthquake because that depends on an extensive history of stress build-up that extends for hundreds to thousands of years in the past.  While an aftershock could possibly trigger an earthquake nearby, it is difficult to predict this in advance.

I survived the El Salvador Earthquakes of 2001 (January 11 and February 11) which were 7.0 if I recall well. It's been ten years and there have been at leas 3 or 4 shakes magnitude 5.0 or bigger almost every year. In the case of Japan, do you believe aftershocks or related earthquakes will last for a decade?

For an earthquake of magnitude 9.0, the aftershock series could easily last for a decade with decreasing seismic frequency with time.  The aftershock series is judged over when it reaches the "normal" earthquake frequency level, which is quite high in Japan.

I live in the Pacific Northwest. With recent megaquakes in the other three corners of the Pacific Plate (Chile, New Zealand, Japan) are we next?

Right now, scientists can't find any physical or statistical reason to use the Chile, New Zealand, and Japan earthquakes to predict the timing of a future event in the Pacific Northwest.  However, we know that a great earthquake occured there in 1700, and that we must do everything we can to prepare for a recurrence of that event.

How do you differentiate between an aftershock and just another earthquake in the same area?

As I said before, an aftershock and a "normal" earthquake cannot be distinguished from one another.  An aftershock is within the same area, approximately within a fault length, and some time after the main event.  It is no longer an aftershock when the seismicity has dropped to a normal level for that region.

It appears many in the U.S. are being complacent about an earthquake similar to the one that occured in Japan happening near a U.S. nuclear plant. If the disaster in Japan won't convince them of the dangers, what will?

The Tohoku earthquake is a wakeup call for everyone in seismically active areas, or on vulnerable coastlines, to evaluate earthquake and tsunami hazards to nuclear plants, the built environment in general, and evacuation plans.

Do you expect other strong earthquakes to happen around the Pacific Rim soon? Since the one in Japan was so strong, is that an indication that other plates will need to move, too? A domino effect?

Earthquakes can happen around the Pacific rim, the so-called "ring of fire", at any time.  However, we haven't been able to make a clear connection between the timing of events that are distant from one another.  That is, we haven't been able to confirm a domino effect around the Pacific rim.

It seems that one reason the ground shook so long in the recent quake is that as the rupture on the fault spread from the epicenter, it got strong again further south on the fault. It was almost like a double earthquake. How often does that happen, and does it tell you anything about the possibility of a new quake further down the fault?

That is essentially correct.  The duration of the shaking is directly a function of how extensive is the fault rupture surface.  It can be considered to be multiple earthquakes on the fault surface and the further you are from each rupture, the longer it takes for seismic waves to arrive.  This is a general characteristic of great earthquakes which by definition have extensive fault surfaces.  It is hard to say what it tells you about the time of the next earthquake further down the fault.

Thanks for all the questions and thanks to the producer of the chat.

In This Chat
James Whitcomb
James H. Whitcomb is the Section Head for the Deep Earth Processes Section in the Earth Sciences Division of the National Science Foundation (NSF). From 2007-2008 he was Acting Division Director and from 1999 – 2001 he was Acting Deputy Division Director. From 1989 - 1999, he served as Program Director for the Geophysics Program. He was in charge of the development of the NSF EarthScope initiative and was the coordinator for the Southern California Earthquake Center, an NSF Science and Technology Center, from its inception in 1990.
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