11:15 AM - 11:30 AM
[MZZ41-03] Japanese New Guidelines for the Information of the Prospect of Seismic Activity after Big Earthquakes and their Applications
★Invited papers
Keywords:Kumamoto Earthquake, Prospect of Seismic Activity, Aftershock Probability, Seismic Information, Aftershock
A big earthquake of M6.5 with maximum seismic intensity 7 in Japan Meteorological Agency (JMA) seismic intensity scale occurred in Kumamoto Prefecture at 21:26 on 14 April, 2016 (Japan Standard Time). That was the beginning of the sequence of “The 2016 Kumamoto Earthquake”. After 18 hours of this earthquake, JMA issued prospect of aftershock activity that there was a possibility to suffer strong ground motion with JMA seismic intensity 5+ or 6- in some areas by aftershocks for about one week. The probability of aftershocks which cause JMA seismic intensity 6- in some areas was calculated to be 20% for the next 3 days from 16:00 on 15 April, and the number of the probability was announced by JMA. This prospect was based on guidelines determined by the Earthquake Research Committee (ERC) of the Headquarters of Earthquake Research Promotion (HERP) in 1998. However, after 10 hours of the issuance of the prospect, a bigger earthquake of M7.3 with JMA seismic intensity 7 occurred at 01:25 on 16 April in the same region as the first big earthquake of M6.5, and triggered distant earthquakes. The seismically active area was finally spread southwest to northeast up to about 150km long from Kumamoto Prefecture to Oita Prefecture. As this seismic activity was revealed that it was not a simple mainshock - aftershock patterns, JMA stopped issuance of the following information about prospect of aftershock activity. Instead of issuance of prospect, JMA called attention to people to high seismic activity and strong ground motion by big earthquakes on the basis of previous cases in that area.
With lessons learned from this, seismologists and JMA discussed under a framework of the HERP, and the ERC of HERP published new guidelines for the information of the prospect of aftershock activity after big earthquakes in August 2016. The points of the guidelines are followings.
(1) JMA calls attention to strong ground motion which is similar level to the first big earthquake for about one week after big earthquakes.
(2) If there were prior cases of foreshock - mainshock - aftershock series or earthquakes with similar magnitude which occurred in the short term near the big earthquakes, JMA calls attention to such cases.
(3) If active faults and assumed source regions of big thrust-type subduction-zone earthquakes existed near the big earthquakes, JMA explains the characteristics and calls keeping in mind them.
(4) After one week, if the active seismic activity continues, JMA issues aftershock probability. The probability is shown by magnification ratio which compares to the probability just after the biggest earthquake and before the big earthquakes.
(5) JMA uses a word “earthquake” instead of “aftershock” when JMA calls to attention to strong ground motion by aftershocks, because the word of “aftershock” gave some impression to people that bigger earthquakes would not occur.
We will introduce new Japanese guidelines and show some actual examples of its applications.
With lessons learned from this, seismologists and JMA discussed under a framework of the HERP, and the ERC of HERP published new guidelines for the information of the prospect of aftershock activity after big earthquakes in August 2016. The points of the guidelines are followings.
(1) JMA calls attention to strong ground motion which is similar level to the first big earthquake for about one week after big earthquakes.
(2) If there were prior cases of foreshock - mainshock - aftershock series or earthquakes with similar magnitude which occurred in the short term near the big earthquakes, JMA calls attention to such cases.
(3) If active faults and assumed source regions of big thrust-type subduction-zone earthquakes existed near the big earthquakes, JMA explains the characteristics and calls keeping in mind them.
(4) After one week, if the active seismic activity continues, JMA issues aftershock probability. The probability is shown by magnification ratio which compares to the probability just after the biggest earthquake and before the big earthquakes.
(5) JMA uses a word “earthquake” instead of “aftershock” when JMA calls to attention to strong ground motion by aftershocks, because the word of “aftershock” gave some impression to people that bigger earthquakes would not occur.
We will introduce new Japanese guidelines and show some actual examples of its applications.