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Japan's earthquake warning system
From: David Farber <dave () farber net>
Date: Mon, 26 Mar 2007 19:06:39 -0400
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 ------------------------------------------- Archives: http://v2.listbox.com/member/archive/247/@now Powered by Listbox: http://www.listbox.com
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- Japan's earthquake warning system David Farber (Mar 26)
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