EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

An unconsidered source of earthquakes and tsunamis from the Kanto region of Japan

Jessica Pilarczyk1, Yuki Sawai2, Yuichi Namegaya2, Toru Tamura2, Koichiro Tanigawa2, Dan Matsumoto2, Tetsuya Shinozaki2, Osamu Fujiwara2, Masanobu Shishikura2, Yumi Shimada2, Tina Dura3, Ben Horton4, Andrew Parnell5, and Christopher Vane6
Jessica Pilarczyk et al.
  • 1Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, Burnaby, B.C., Canada
  • 2Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan
  • 3Department of Geosciences, Virginia Tech, Blacksburg, VA, USA
  • 4Earth Observatory of Singapore, Nanyang Technological University, Singapore
  • 5Hamilton Institute and Insight Centre for Data Analysis, Maynooth University, Kildare, Ireland
  • 6British Geological Survey, Keyworth, Nottingham, UK

The assessment of seismic hazards along subduction zone coastlines provides important information regarding the frequency and magnitude of earthquakes and tsunamis that can be expected in the future.  Unlike many subduction zone coastlines that involve one tectonic plate subducting under another, seismic hazard assessments for the Kanto region of Japan are complicated by the presence of a nearby triple junction; where one continental (CON) and two oceanic plates (PHS, PAC) collide.   The CON/PHS (Sagami Trough) and CON/PAC (Japan Trench) boundaries are recognized earthquake sources.  However, historical and geological evidence of a large PHS/PAC (Izu-Bonin Trench) earthquake has been lacking and decades worth of instrumental data point to low seismicity along this boundary.  Here we show that two unusually large tsunamis are evidenced by sandy deposits preserved along 50 km of coastline in the Kanto region.  The oldest of them, deposited about 1,000 years ago, contains evidence consistent with tsunami deposits reported elsewhere (e.g., marine foraminifera, rip-up clasts, pebbles, erosional base) and represents a previously unknown prehistoric earthquake.  In computer simulations, this earthquake deposited sand that extended too far inland to represent any known historical earthquake originating from the CON/PHS and CON/PAC boundaries alone.  Rather, the greater inland inundation points to significantly greater displacement on the CON/PHS and CON/PAC boundaries, which may be unrealistic, or much smaller displacement along the previously unconsidered PHS/PAC megathrust.  This plate-boundary fault adds another source for earthquakes in Tokyo and tsunamis in the Pacific Ocean.

How to cite: Pilarczyk, J., Sawai, Y., Namegaya, Y., Tamura, T., Tanigawa, K., Matsumoto, D., Shinozaki, T., Fujiwara, O., Shishikura, M., Shimada, Y., Dura, T., Horton, B., Parnell, A., and Vane, C.: An unconsidered source of earthquakes and tsunamis from the Kanto region of Japan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13502,, 2022.