Japan's earthquake warning system

[David Farber <dave () farber net>]

Begin forwarded message:

From: Rod Van Meter <rdv () tera ics keio ac jp>
Date: March 26, 2007 6:32:40 PM EDT
To: David Farber <dave () farber net>
Subject: Japan's earthquake warning system
Reply-To: rdv () tera ics keio ac jp

Dave, for IP if you wish...

By now everyone has heard about the M6.9 earthquake yesterday in
northwestern Honshu (Japan's main island).  Its magnitude is, roughly,
representative of the energy released but doesn't give an indication of
the local shaking.  Japan also uses a subjective system of degrees of
local shaking, where 1 is detectable only by sensitive people and 6
brings down buildings.  In some places, this one reached a strong 6.
Fortunately, the loss of life and injuries were small relative to the
size of the quake; it could have been much, much worse.

There have been, for months, brief tidbits in the press about Japan's
new earthquake warning system, which was put into service shortly before
this earthquake.  Unfortunately everything I have read has been quite
short on details (there are probably more thorough descriptions in the
Japanese geek press, but I don't keep up with that).

The system detects a quake's initial P wave, which is the first jolt you
feel, before the more severe S wave reaches you.  (Both start out at the
epicenter essentially simultaneously, but P waves travel faster; about
5km/s in rock, while S waves are about 60% of that.)

Reportedly, in this case, with the epicenter nearby, the warning period
was about 5 seconds.  The warnings were distributed over a dedicated
fiber network, as I understand it, to about 50 locations near Ishikawa.
I'm not sure what those locations *are*, though; probably government
agencies and maybe the broadcasters.  Five seconds isn't long enough to
distributed the information to the general population except possibly by
TV and radio automated broadcast.

Five seconds is long enough to turn off the stove and step away, or dive
under a table, but not do much else.  You certainly can't stop a
shinkansen (bullet train) in that amount of time.  For quakes farther
away, the warning could reach as much as a minute.

Officials praised the tsunami warning system, saying it helped them get
the warning out in about one minute, rather than the two to three it
used to take them.  I seem to remember from something I saw last fall
that the biggest difference here was a large compute farm that allowed
them to decide what kind of tsunami might be coming.

In this case, they predicted a tsunami of up to 50cm, which is up into
the mildly dangerous range, though what actually arrived was 10-20cm.

A few months ago some MAJOR quakes north of Hokkaido resulted in tsunami
warnings for much of the Hokkaido coast, but the number of people who
actually fled was only a small percentage of the population.
Fortunately in that case, the waves were also small.

Sorry I don't have more detail, but if I run across more will forward.
Likewise, I'm happy to hear from anyone who does know more...

Regards,

                --Rod




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