Title:Remote triggering of earthquakes as a potential stress-meter: the case of the 2016 M7.3 Kumamoto (Kyushu) mainshock
Presenter: Associate Professor Bogdan Enescu
Institution: Department of Geophysics, Kyoto University
Time: 20180517-16:00:00
Location: Room 2829, No. 2 Science Building.

Introduction. Distant earthquake triggering is relatively scarce in Japan (Harrington and Brodsky, 2006), except for the remote seismicity activated following the 2011 M9.0 Tohoku-oki earthquake (e.g., Opris et al., 2018). Here we report on a widespread remote triggering of small events after the 2016 M7.3 Kumamoto earthquake and discuss implications on the stress-state at inland active faults and volcanoes throughout Japan
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Data and methods. We have processed waveform data recorded at high-sensitivity Hi-net and broadband F-net stations (NIED) allover Japan. The waveforms were scrutinized in high and low frequency ranges to detect remote events occurred during the passage of mainshock surface waves. Dynamic stresses were estimated using the approach of Peng et al. (2009).

Results. The remotely activated seismicity correlates well with the passage of the surface waves from the Kumamoto earthquake. Triggering was observed at volcanoes in Hokkaido (at an epicentral distance of ~1630 km), Tohoku, Chubu, Izu Peninsula, and southern part of Kyushu, as well as active fault areas in Wakayama, Tottori and Noto Peninsula. The remote dynamic stresses are significant, ranging from several to tens of kPa. Since most of the remotely triggered events occurred at volcanoes, we suggest that excitation of fluids is the main activation mechanism. The strong shaking, due to a clear directivity effect, may explain the observed spatial distribution of triggered earthquakes as well as their long-range extent.

Conclusions. It is very important to understand why such widespread remote activation has not been observed before in Japan, at similar dynamic stress levels. We found that many of the regions activated this time have been also activated after the 2011 Tohoku-oki earthquake. We hypothesize that mechanical weakening of a pressurized crust, due to the 2011 megathrust, might be responsible for an increased trigger-ability. The earthquake triggering at some active crustal faults might be related to the regional levels of tectonic stress. All these observations suggest that remote triggering could be used as a stress-meter at volcanoes and active faults and thus might have applications in earthquake hazard assessment and forecasting.

References
Opris, A., Enescu, B., Yagi, Y., and J. Zhuang, Triggering and decay characteristics of dynamically activated seismicity in Southwest Japan, Geophys. J. Int., 212, 2, 1010-1021, doi: 10.1093/gji/ggx456, 2018.
Enescu, B., Shimojo, K., Opris, A., and Y. Yagi, Remote triggering of seismicity at Japanese volcanoes following the 2016 M7.3 Kumamoto earthquake, Earth, Planets and Space, 68:165, doi: 10.1186/s40623-016-0539-5, 2016.

Homepage:www-seis.kugi.kyoto-u.ac.jp/~benescu/e-index.html