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Andrew Alden

Does Sendai Stress Tokai?

By March 14, 2011

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After the great Sumatra subduction earthquake in 2004, crustal stress increased in the adjacent segment of the subduction zone, where a magnitude-8 event occurred four months later. So naturally I'm concerned about what the March 11 Sendai earthquake will do to the plate boundary on either side of the slip zone.

US Geological Survey scientist Ross Stein and coworkers in the US and Japan have been running the models and have initial results showing a rise in stress to the north, off northernmost Honshu. This is no surprise. That side of the subduction zone is a straightforward extension of the segment off Honshu, which is the boundary between the Pacific plate on the east and the North American plate—specifically, the Okhotsk microplate that has recently been delineated there—on the west.

The image below shows the southern end of the rupture zone.

sendai earthquake coulomb stress

I want to make it plain that this image is only an unpublished, first-take approximation. But what it appears to show is that the Sendai quake has slightly loaded the "Sagami trough megathrust" that marks the south end of the plate boundary. That boundary, unlike the one where the Sendai quake occurred, lies between the Pacific and Philippine Sea plates. (It's the same fault that accounted for the 1923 Kanto earthquake.) The plate dynamics are quite different there, and I can say that the stresses from last week's quake have probably not loaded the Tokai segment, which runs from west of the Boso Peninsula, left of the "S" in "Sagami", down to the lower left corner. It lies between the Philippine Sea plate and the Eurasian plate (Amur microplate).

So does the Sendai quake put Tokyo in greater danger? Not in an obvious way. Do not expect a definitive answer from the scientific community, and do not trust every TV commentator who may assert such an answer (unless it's Ross Stein).

Related:
Stress transfer from earthquakes
About subduction
Preliminary stress changes from the Sendai quake — USGS image

Comments

March 16, 2011 at 4:49 am
(1) Björn Lund says:

Hi,

One should point out that the stress increase/decrease in the image is calculated for receiver faults as those of the 1923 earthquake, strike 290, dip 25 and rake 150. The Tokai segment fault(s) is oriented quite differently, dipping to the west, so the stress pattern will be very different from the one in the image. I think the image should not be used to interpret how the Tokai segment was loaded or unloaded. I am hoping Stein et al will release more images soon!

Cheers,
Björn

March 16, 2011 at 10:16 pm
(2) Geology Guide says:

Thank you Björn, you are correct, as I said, that this figure shows only a preliminary analysis and should not be relied upon. It suggests that there is unlikely to be a notable effect on the Tokai segment. The crustal structure is very complex there and we should not expect a simple domino-effect link to the south although it is plausible to the north of the rupture.

March 18, 2011 at 11:59 am
(3) Shawn says:

Hi, I was wondering if you could comment on the Jim Berkland predictions. It seems like “junk science”… or whatever term one may choose to use, but it is starting to cause a bit of uneasiness here in Southern California among those that are less-informed. I’m having trouble finding any real response/analysis/debate to his methodology.

March 18, 2011 at 2:48 pm
(4) Geology Guide says:

Jim Berkland is completely sincere about his predictions and scrupulous in his calculations, but his results don’t stand up to scrutiny. I don’t know what he’s predicting these days, but it is surely no more useful and valid than flipping coins.

You’re having trouble finding discussions because we’re all sick of talking about his long-discredited methods.

March 18, 2011 at 4:12 pm
(5) Shawn says:

That is comforting, but was the response I was expecting. He is predicting a major seismic event on the west coast on or around 3/19, possibly in the LA region partially related to the recent fish kill along Redondo Beach, among other factors. Most of us are aware that earthquake prediction is still a long way off, but people still get really unsettled when they hear things like this and I’m seeing links to a recent interview getting passed around. Thanks for your response

March 22, 2011 at 2:05 pm
(6) Adrian jones says:

Andrew, I have hoped to see a clear diagram of the various plates involved in teh Sendai earthquake, but so far have not. I diagram in NYTimes showed Honshu sitting on two plates, North American and ??Eurasian or a subplate. If the Sendai area moved several metres, or 8 feet east, was this the whole island or just the North American plate, and if just the NA American plate then there should be a massive split half way down the island, but I have not seen any pictures or reports of that. In addition of course, if the Pacific plate slipped under the NA PLate, then why did the island on the NA plate move east, instead of relaxing westward. Many questions that I hope About Geology.com can clarify.
adrian j

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