Monday, June 16, 2008

Earthquakes, volcanoes, and landslides in Japan

The rural setting of Saturday's M 6.8 earthquake in northeastern Honshu, Japan* has fortunately meant that many fewer deaths occurred than during the 1995 Kobe earthquake. (From news reports and photos, it looks like landslides were responsible for a lot of the destruction. Dave's landslide blog has photos.)

So the surface is where the destruction is, but I'm also interested in what's going on deeper down. If you use the Google Earth plug-ins for the USGS real-time earthquakes and the Smithsonian Institution's volcanoes, you can see where the fault is compared to the volcanic arc:

It's in the arc. Right in the middle of the arc. One volcano, Kurikoma, lies above the aftershock zone. And it's acting up. (News report here.)

So here are my somewhat random thoughts:

- The earthquake was along a thrust fault. I know of arcs that are extensionally collapsing, which involves normal faults. (The Marianas arc is a good example.) I know of arcs with strike-slip faults running through the line of volcanoes. (The Sumatra fault, for instance, is like this - there's a strike-slip fault that runs along the arc, so the plate movement is partitioned into two zones - subduction at the trench that caused the 2004 tsunami, and strike-slip movement within the arc.) And I know of studies (including one of mine) arguing that thrust faults moved during the growth of ancient plutons. But modern thrust faults in the middle of arcs - not thrust faulting partitioned into the wedge above the subduction zone, or in the back-arc (like in parts of the Andes) - I'm not sure if I've seen a thrust earthquake in an arc before. I know there are thrust earthquakes in the Sea of Japan, and that there is compression between Japan and Asia, so the fault mechanism isn't surprising. But the location is. Is this a case where the magmatism created a weaker zone in the crust and allowed thrust faulting to take place?

- Did the earthquake (either due to stress changes, or due to the passage of seismic waves) cause Kurikoma to start acting up?

- What's going to happen next?

* The Japan Meteorological Agency measured the earthquake as magnitude 7.2, but I don't know which magnitude scale they're using for it. I would rather use moment magnitude consistently to think about earthquake size, and I know that's what the USGS is reporting, so I'll stick with that.


Erik said...

I believe the JMA has their own magnitude scale. It should be pretty close to the USGS number up to around 7.5 though, so the difference here is probably somewhere in the interpretation.

Alessia Maggi said...

I've just taken a look at the JMA site, and they now have this earthquake at M7.0