A new source of earthquakes and tsunamis has been identified in the greater Tokyo area

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Jessica Bellarczyk and colleagues from the Geological Survey of Japan’s rice paddy core on the Poso Peninsula reveal geological evidence of a 1,000-year-old tsunami. Credit: SFU

Researchers have discovered geological evidence that unusually large earthquakes and tsunamis from the Tokyo region – located near tectonic plate boundaries that are known to be a source of seismic hazard – can be traced back to a previously unconsidered plate boundary. The team, led by Jesica Bilarczyk, an Earth scientist at Simon Fraser University, published its research today in Nature Geoscience.

The team’s groundbreaking discovery presents a new and understudied seismic hazard to Japan with implications for countries on the Pacific Rim, including Canada.

Pilarczyk points to low-lying areas such as the Delta, Richmond and Port Alberni as potentially vulnerable to tidal waves originating from this area.

In 2011, eastern Japan was hit by a massive 9-magnitude earthquake – creating the largest rupture zone of any earthquake originating from the Japan Trench. It caused the Fukushima Daiichi nuclear disaster and a tsunami that traveled thousands of miles – affecting the shores of British Columbia, California, Oregon, Hawaii and Chile.

Over the past decade, Bellarczyk and an international team of collaborators have worked with the Japan Geological Survey to study Japan’s unique geological history. Together, they discovered and analyzed sandy sediments from the Poso Peninsula area (50 kilometers east of Tokyo) that they attribute to an unusually large tsunami that occurred about 1,000 years ago.

Until now, scientists did not have historical records to ascertain whether a portion of the Philippine Sea/Pacific Ocean plate boundary near the Poso Peninsula was capable of generating large tsunamis similar in size as the Tohoku event in 2011.

Jessica Bellarczyk (SFU) and collaborator Tina Dora (Virginia Tech) sample sediment samples from rice fields in the Greater Tokyo area that contain evidence of a 1,000-year-old earthquake that likely originated from an earthquake source not considered historical. Credit: SFU

Using a combination of radiocarbon dating, geological and historical records, and paleoecology, the team used 13 hypothetical and historical models to assess each of the three plate boundaries, including the Continental Sea/Philippine Plate boundary (Sagami Basin), and the Continental/Pacific Plate boundary (Trench). Japan) and the Philippine Sea/Pacific Plate boundary (Izu Bonin Trench) as sources of a 1000-year-old earthquake.

Bellarczyk reported that modeled scenarios suggest that the source of the 1,000-year-old tsunami originated from the offshore region off the Poso Peninsula—and smaller ones (eg, potential earthquakes of minimal strength), are related to what has not been considered before. Izu-Bonin Trench on the border of the Philippine Sea and the Pacific Plates.

“Seismic risk assessments in the Tokyo region are complicated by the ‘triple trench junction’, whereby not only is the Philippine Sea-oceanic plate bound on a continental plate but is also being driven to extinction by the Pacific plate.” Earth sciences at SFU who holds the Canada Research Chair in Natural Hazards. “Strong earthquakes and accompanying tsunamis are historically recognized hazards from the continental/Philippine (Sagami Basin) and continental/Pacific (Japan Trench) plate boundaries but not from the Philippine Sea/Pacific boundary alone.”

Pilarczyk hopes that these findings will be used to produce more informed seismic hazard maps for Japan. It also says that this information could be used by remote field sites, including Canada, to inform building practices and emergency management strategies that would help mitigate the devastating consequences of an earthquake similar to that of 1,000 years ago.

Was the 1700s Cascadia earthquake part of a chain of earthquakes? More information: Jessica E. Pilarczyk et al, Another source for the Tokyo earthquake and Pacific tsunami, Nature Geoscience (2021). DOI: 10.1038/s41561-021-00812-2, www.nature.com/articles/s41561-021-00812-2 Submitted by Simon Fraser University

Quote: New source of earthquakes and tsunamis identified in greater Tokyo area (2021, September 2), retrieved September 2, 2021 from https://phys.org/news/2021-09-source-earthquakes-tsunamis-graduate-tokyo. html

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