The Meteorological Agency said it issued 73 quake warnings in the two months following the Great Eastern Japan Earthquake — four times the amount issued since the system debuted in October 2007 — but 64 percent of them were false alarms triggered by aftershocks.
The agency sends warnings when it detects P-waves traveling faster than the S-waves that humans can sense as tremors.
When a quake is expected to register lower 5 or higher on the Japanese seismic intensity scale, for example, warnings are sent to areas where the intensity is anticipated to be 4 or above.
But 47 of the 73 warnings were issued to areas where the seismic intensity registered 2 or lower, making them false alarms.
Before March 11, only 29 percent of all quake warnings were deemed false.
The reason for the false alarms is the agency’s seismograph, which interprets a situation in which several quakes are occurring almost simultaneously as one big temblor — a factor that wasn’t taken into consideration when the system was initially developed.
Before March 11, if the distance between two seismographs that detected P-waves at the same time was within 350 km, the quakes were considered to be one, but the agency shortened the distance to 150 km after the Tohoku catastrophe. Still, inappropriate warnings have been issued since mid-April when aftershocks were felt in Fukushima Prefecture.
In hopes of improving the system, the agency has earmarked ¥100 million for repairs and ¥240 million for 40 additional seismographs in the fiscal 2011 budget.
However, Osamu Kamigaichi of the agency’s Seismological and Volcanological Department said no drastic solution is in sight and the warnings should still be taken seriously.
“At the moment, there may be some false alarms, but if the warning is issued, it is almost certain an earthquake struck. So protect yourself for the first minute or two,” he said.