2:00 PM - 2:15 PM
[SSS10-11] Disappearance of the ETAS effect on the eastern margin of the Sea of Japan, ongoing since May 2011
Keywords:Eastern margin of the Sea of Japan, The M9 earthquake off the East coast of Tohoku district, The disappearance of the ETAS effect , Viscoelastic relaxation of the M9 earthquake, Role of pore fluid on ETAS effect
Seismicity in the Eastern margin of the Sea of Japan, including the aftershock area of the 1983 Central Japan Sea Earthquake, has been significantly quiescent since mid-May 2011, as evidenced by the raw cumulative earthquake graph. We showed the quiescence in the 2017 JPGU (SSS05-10) with a simple quantitative analysis as a change in the average rate of occurrence. Then in 2019, there was a M6.7 reverse fault earthquake in the southeast end of our studied area. Even aftershocks followed it, the number of earthquakes in that area is still lower than before 2011. This time, we succeeded in the analysis using the ETAS model for the quiescent area. We examine the long-term changes in deviatoric stress due to the 2011 M9-Tohoku earthquake by crustal deformation simulation that represents the lithosphere as an elastic layer of 60-km thickness above a semi-infinite viscoelastic body. We compare it with the seismicity change, and observed GNSS data such as Tobishima station.
Using the ETAS model, seismicity decreased 65 days after the Tohoku earthquake as in the previous report. Before 2011, nearly half (44%) of the earthquakes were caused by the ETAS effect, but two months after the M9, the ETAS effect disappeared in this region. Furthermore, the basic occurrence rate also dropped to 78%, thus reducing seismicity to56%×78%= 44%, less than half of the pre-2011 level. Even after the 2019 reverse fault earthquake, the ETAS effect has not recovered in this region. This is the first discovery of a case where the ETAS effect disappeared for more than eight years in a region where it had existed. The ETAS analysis of the Matsushiro earthquakes (Matsu'ura & Karakama, doi.org/10.1007/s00024-005-2672-0) suggests that both the pore fluid pressure and its volumetric occupied rate has depressed in the crust of the eastern margin of the Sea of Japan so far.
The M6.7 off the coast of Yamagata Prefecture in 2019 was a reverse fault earthquake with a compressive axis along the PAC subduction direction, which was common on the eastern margin of the Japan Sea before 2011 and was accompanied by aftershock activity. The level of aftershock activity was similar to that before 2011, but the value of p, which represents the rate of aftershock attenuation, was larger than before. This earthquake was of a type that was suppressed by the 2011 M9 event, but it is considered to have occurred even the effect of the suppression has not completely disappeared over the past eight years.
GNSS observations indicate that at the Tobishima station, shortening strain of 44 years in the PAC subduction direction before 2011 was released coseismically. Simulation of viscoelastic relaxation using a viscosity of 5×1018 Pa s predicts that more than half the level of coseismic change in the same direction will continue in the shallow crust over the next 300-400 years in this region. Supposing the recurrence interval of the 2011 earthquake is 600 years, in the latter 1/3-1/2 period, 100% of the strain energy from PAC subduction will accumulate to produce earthquakes similar to the 2019 event, but for the recent few hundred years, some will continue to be consumed in counteracting the effects of the 2011 earthquake. If we can increase the number of such cases, or if we will detect the recovery of the ETAS effect, it will help us to improve the estimation on the strain energy balance in the eastern margin of the Japan Sea and the accuracy of earthquake hazard estimates of shallow earthquakes in western coastal areas in Japan.
Using the ETAS model, seismicity decreased 65 days after the Tohoku earthquake as in the previous report. Before 2011, nearly half (44%) of the earthquakes were caused by the ETAS effect, but two months after the M9, the ETAS effect disappeared in this region. Furthermore, the basic occurrence rate also dropped to 78%, thus reducing seismicity to56%×78%= 44%, less than half of the pre-2011 level. Even after the 2019 reverse fault earthquake, the ETAS effect has not recovered in this region. This is the first discovery of a case where the ETAS effect disappeared for more than eight years in a region where it had existed. The ETAS analysis of the Matsushiro earthquakes (Matsu'ura & Karakama, doi.org/10.1007/s00024-005-2672-0) suggests that both the pore fluid pressure and its volumetric occupied rate has depressed in the crust of the eastern margin of the Sea of Japan so far.
The M6.7 off the coast of Yamagata Prefecture in 2019 was a reverse fault earthquake with a compressive axis along the PAC subduction direction, which was common on the eastern margin of the Japan Sea before 2011 and was accompanied by aftershock activity. The level of aftershock activity was similar to that before 2011, but the value of p, which represents the rate of aftershock attenuation, was larger than before. This earthquake was of a type that was suppressed by the 2011 M9 event, but it is considered to have occurred even the effect of the suppression has not completely disappeared over the past eight years.
GNSS observations indicate that at the Tobishima station, shortening strain of 44 years in the PAC subduction direction before 2011 was released coseismically. Simulation of viscoelastic relaxation using a viscosity of 5×1018 Pa s predicts that more than half the level of coseismic change in the same direction will continue in the shallow crust over the next 300-400 years in this region. Supposing the recurrence interval of the 2011 earthquake is 600 years, in the latter 1/3-1/2 period, 100% of the strain energy from PAC subduction will accumulate to produce earthquakes similar to the 2019 event, but for the recent few hundred years, some will continue to be consumed in counteracting the effects of the 2011 earthquake. If we can increase the number of such cases, or if we will detect the recovery of the ETAS effect, it will help us to improve the estimation on the strain energy balance in the eastern margin of the Japan Sea and the accuracy of earthquake hazard estimates of shallow earthquakes in western coastal areas in Japan.