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Teachable Moment: Earthquakes in Japan

Shaking from a shallow earthquake Shaking from a deep earthquake

L and I returned to Japan yesterday, with flights timed so that our planes converged from opposite points on the globe on Tokyo in time to meet up at the airport. We've been home less than 24 hours and we've already felt three earthquakes. We are both doing fine and there was no damage anywhere near us. I know I said it would be hard to post to the blog, but there is nothing like a fascinating feat of nature to get me excited about blogging again. Continue reading this message (click on the link below) to learn more about earthquakes...

The first two earthquakes we felt were the Niigata event and its largest aftershock. You've probably heard about it (perhaps you read this awful article about it from AP/CNN whose headline claims that there was a radioactive leak in a nearby nuclear reactor but whose body contradicts that, or a slightly less sensationalist article from our local newspaper). The map on the left shows the shaking intensity recorded by instruments all over Japan during the first of the three events we felt -- pretty strong! For us in Tokyo, the shaking felt fairly gentle. The second earthquake of the day was an aftershock of this event, and it was even more gentle so that L didn't even feel it when sitting beside me.

The third earthquake, however, felt very different. The shaking, while also relatively mild, felt like it lasted for a much longer time. When we looked online, we saw the shaking intensity map shown on the right (click to enlarge it). Both earthquakes were about the same distance away from Tokyo and both were recorded as magnitude 6.6 on the JMA website, which means that the two events released the same amount of energy. Despite that fact, the Niigata earthquake (left image) has much stronger shaking intensity. A red 'X' on each image shows the earthquake epicenter (the point on the map above where the earthquake started rupturing the fault). The Niigata earthquake (left image) follows the 'usual' pattern where shaking is strongest close to the earthquake epicenter and dies out as you get further away. But look at the really strange pattern from the earthquake on the right. The strongest shaking is actually along the Pacific coast in Hokkaido (the northern island), despite the fact that the epicenter is way off to the west in the Sea of Japan. What's up?


The map above shows the location of earthquakes in Japan over a ten year period. There are a lot of them. The colors show the depth of different earthquakes. The bottom graph shows a slice through the earth (a cross section) with the same earthquakes. The progression of colors seem to line up as a straight line, which is the evidence geologists used to discover "subduction zones" where one plate gets pushed beneath the other (see the image below). The plate boundary in Japan is on the Pacific coast (eastern side), but both of the earthquakes we felt yesterday are on the western side of Japan (probably at around distance of "200 km" on the horizontal-axis of the bottom plot). The Niigata earthquake, that produced damage like a bullseye started around 17 km deep (orange color). The other earthquake that produced the weird shaking pattern started at a depth of over 370 km (purple color off the bottom of this cross section). Shaking energy from the deeper event had to travel all the way from 370 km down up to the surface. During that longer journey, more of the energy got absorbed so that the peak shaking was less than the peak shaking from the shallow Niigata earthquake. Even though the earthquakes were about the same 200-250 km away horizontally, the depth difference also explains the longer shaking. Because of this extra vertical distance, the faster P-waves got further and further ahead of the slower S-waves as they raced to the surface, so that meant that there was a longer time between the first P-wave arrival and the end of the last S-wave shaking. Hence the deep earthquake shook us for a lot longer duration.

There's more advanced stuff we can learn from the deep earthquake as well (this stuff is a bit outside my area of expertise, so it might be wrong. Seismologist friends: be sure to make corrections). The plate sudbucting down into the earth is cold and dense (which is one of the reasons it sinks downward), and that makes earthquake waves travel more easily through it than the surrounding hotter and squishier earth. My guess is that energy from the deep earthquake stayed trapped within the subducting plate all the way up to the surface, which is why shaking was most intense nearest to where that plate reaches the surface along the eastern coast of Japan instead of closer to the epicenter. Even though it would have been a shorter distance to travel vertically, it was easier for the waves to go the longer distance through the plate. Also, shaking is much more intense to the north of Tokyo than to the south. This could be because Tokyo is at the boundary between three (or perhaps four) tectonic plates. If my guess is correct, then the deep earthquake occurred within the Pacific Plate and propagated only within it -- stations on the adjacent Philippine Sea Plate to the south hardly felt a thing. Seismologists give this phenomenon a fancy name called a "wave guide."

We've been keeping our list of felt earthquakes up-to-date. Check it out here.

Comments

I am so glad you posted this, Matt. even though I know you are in tokyo, it is reassuring to read that the two of you are ok.

Thanks for all the info Matt. I was hoping you'd have some insight/information and you had even more than I had hoped for. Such interesting stuff. Glad you are both ok. Hope the everything remains exciting yet safe there in Tokyo!
Melissa:)

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