On Mon, 3 Jul 2000, meredith lamb wrote: > Any thoughts on all the continuing quakes in this region from > anyone? > They seem to be running across a plate tip area, almost like > it could be re-defining the plate boundary edge, and/or "breaking" > the tip away from the rest of the plate mass? > At the continuing rate of quakes over time....something must > be going on there....but who knows, they could cease soon too. > Anyone know of a web URL, with any english translations to > this activity? > Thanks, Meredith Lamb Meredith, Japan is probably the best studied seismic region in the world. They have literally carpeted the islands with seismometers. The quakes at shallow parts of the subduction zone show a group of linear trends parallel to the plate motion. Some have interpreted this to be the subduction of sea mounts which scrape along the bottom of Japan as they go down. I saw the plots once before and it looked fairly convincing because the arrangement of these seismicity lineations seems to be distributed geometrically much like the sea mounts to the east. I have also seen some of the most fantastic tomographic plots from Japan. If I come across any good web links I'll let you know where to find them. Most of the east coast of Asia exhibits a phenomenon known as "back-arc spreading." One explanation for this is that as the lithosphere sinks into the upper mantle it pulls some of the mantle above it down, which creates a sort of eddy current above the slab. The eddy then pulls the lithosphere above it along the surface towards the trench, thus causing spreading behind the subduction zone. Japan has a smaller back-arc region behind it than the areas to the south. But it does seem like it will be pulled further from Asia over time along with Kamchatka to the north. The rate of subduction under Japan is fairly high, and will probably persist for a long while to come. The lithosphere sinking to the north is older and cooler than that to the south, which might have helped to create the type of corner you are thinking about. It is interesting to think about back-arc spreading in terms of the western US when the Farallon plate used to go down on the west coast along the entire front. There are a lot of lower basins to the east (i.e. Basin and Range), which might have something to do with this type of activity. The corner of slab extending beneath Japan and under North Korea/far east China might be one of these areas where slabs sink into the upper mantle but begin to pile up at the 660 km discontinuity, awaiting the time when enough weight will be added for a "flushing" event to occur where it falls into the lower mantle. This is very fun stuff to think about... John Hernlund E-mail: hernlund@....... WWW: http://www.public.asu.edu/~hernlund/ ****************************************************************************** __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>