The Japan Meteorological Agency will introduce Thursday a new method of predicting the seismic intensity of a large-scale earthquake in an effort to accurately issue Earthquake Early Warnings (see below), even to areas distant from the quake's epicenter.
The agency's new warning system will be based on real data of ground motion detected by such equipment as seismometers across the country, regardless of the distance from the quake's focus, taking into account the lessons learned from the 2011 Great East Japan Earthquake when it could not provide warnings to affected areas including the Kanto region.
The introduction of the new system will help issue warnings to wider areas than now in the event of such huge earthquakes as one along the Nankai Trough, which runs off the Pacific coast of central to southwestern Japan, the agency said.
The agency currently estimates the focus and scale of an earthquake based on observed data of an initial tremor called a P-wave, that pushes and pulls the rock it moves at a high speed. It then predicts the seismic intensity of the principal strong motion called an S-wave -- which arrives after the P-wave -- and issues warnings to areas where a quake registering 4 or more on the Japanese seismic intensity scale of 7 is expected to occur.
In the event of a massive earthquake, however, cracks widen in the underground bedrock near the quake's focus, which has the effect of widening its epicentral region.
The current prediction method is based on the initially estimated location of the quake's focus, but has the flaw of underestimating its seismic intensity as the distance from the epicenter increases.
In the Great East Japan Earthquake, the agency issued warnings to only five prefectures in the Tohoku region based on its assessment that the quake's focus was off the Sanriku area, from which the jolt started.
However, the focal region actually had widened north and south for a total of about 500 kilometers, reaching the sea off Ibaraki Prefecture in the Kanto region. An intensity of 4 or more -- with a maximum seismic intensity of upper 6 -- was observed in 15 prefectures including the Kanto and Koshin regions as well as in Hokkaido, although warnings were not issued for these areas.
In light of the lessons learned from this case, the agency has developed an approach called the propagation of local undamped motion (PLUM) method. With this new method, the agency will predict the seismic intensity in areas distant from the epicenter, based on data observed with seismometers and seismic intensity meters that are set at about 1,500 locations across Japan -- not based on the location of the focus and the scale of the quake, which are estimated from the P-wave in the current method.
The new method will enable the agency to predict the seismic intensity regardless of the distance from the initially estimated location of the focus, which will likely improve the accuracy in distant areas at the time of a massive quake in which its focal region expands. A simulation by the agency of the Great East Japan Earthquake was able to predict strong tremors in the areas including the Kanto region, according to the agency.
The agency is scheduled to start using the new PLUM method at noon Thursday along with the current method. There may be a case in which the agency announces the first warning to the area near the epicenter based on the P-wave, and then issues the second and third warnings to more distant areas based on the seismic intensity estimated with the PLUM method, the agency said.
Shinya Tsukada, head of the agency's office of earthquake and tsunami disaster mitigation, said, "The accuracy of earthquake predictions is expected to improve especially for a massive earthquake with a magnitude of 8 or stronger" with the new method.
-- Earthquake Early Warnings
A system to predict seismic intensity in locations across Japan by estimating the focus and scale of an earthquake. If a quake is predicted to have a maximum seismic intensity of lower 5 or more on the Japanese seismic intensity scale of 7, a warning is issued automatically via mobile phones and other devices to areas that are expected to be hit by tremors with an intensity of 4 or more. The Japan Meteorological Agency started operating the system on Oct. 1, 2007. As of the end of December 2017, a total of 101 such warnings have been issued for 177 earthquakes with an intensity of lower 5 or more.
Read more from The Japan News at https://japannews.yomiuri.co.jp/
