PSN-L Email List Message

Subject: Re: Near S. Coast of Honshu, Japan
From: John Hernlund hernlund@.......
Date: Mon, 03 Jul 2000 13:15:22 -0700 (MST)

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

   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

   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@.......



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Larry Cochrane <cochrane@..............>