Researchers have developed a new technique for quickly assessing the magnitude of large earthquakes, cutting down the time required in the case of the recent quake in Japan, for example, from about 20 minutes to just 2-3 minutes. Those crucial minutes would have helped with tsunami warnings and in making sure that even far-away communities like Tokyo had proper alerts as soon as possible, says Yehuda Bock of the University of California, San Diego, who developed the technique.
The strategy involves tying together real-time data coming from seismic instruments, which detect shaking, as well as Global Positioning System (GPS) instruments, which detect the absolute movements of the ground. Both devices are already installed in places such as Japan and California — the key is to ensure that they are delivering the right sort of data simultaneously, says Bock, who reported on his progress at the American Geophysical Union (AGU) meeting in San Francisco, California, on 8 December. Bock and his colleagues this month received funding to build and test a prototype upgrade device, and hope to have an initial system in place in California within six months.
Seismic instruments are very sensitive, but have a hard time discriminating between large quakes of magnitude 7 or higher in the first seconds or minutes of an earthquake, because the shaking simply goes off the scale. In the case of the March 2011 Tohoku earthquake in Japan, for example, the Japan Meteorological Agency estimated the quake’s magnitude as just 6.8 after 38 seconds, and 8 after a few minutes, says Bock. It was not until weaker seismic readings from much further away were added to the analysis that they could say, 20 minutes after the quake began, that it was a devastating magnitude 9 — 30 times stronger than a magnitude-8 quake.
Accelerometers add another layer of information, but their data take too long to process to be of use. GPS instruments are more useful. The station closest to the epicentre, for example, showed a 1.5-metre drop of the ground in the first 100 seconds of the quake. “That’s huge,” says Bock. This provides a quick and obvious indication of large vertical ground displacement — the thing that causes tsunamis — and can be combined with seismic data to quickly assess quake size. But most GPS networks were designed to provide long-term data about ground movement, not short-term information during earthquakes; they may be designed to take readings once every 30 minutes and deliver data once a day, for example. And they aren’t necessarily installed next to seismometers. Of Japan’s 1,200 GPS stations — all of which are real time — only 180 are close enough to seismic stations to be of use in this sort of system, and so far they haven’t been utilized this way.
Via Dr. Stefan Gruenwald