Geologists look to past for answers on future tsunami threats
IMAGE: VIRGINIA TECH AND JAPANESE RESEARCHERS EXTRACT SEDIMENT CORE FROM THE KUJUKURI COASTAL PLAIN IN JAPAN. PHOTO COURTESY OF TINA DURA view more
CREDIT: TINA DURA
Multiple large and destructive tsunamis in the past few decades in the Indian Ocean (2004), Chile (2010), and Japan (2011) have underscored the threat that tsunamis pose to coastal regions, ushering in a new era of research aimed at better predicting areas threatened by the fast-developing natural disasters.
But documenting and examining recent events does not provide enough information to fully characterize coastal hazards, said Tina Dura, an assistant professor of coastal hazards in Virginia Tech’s Department of Geosciences. Dura is taking a new approach to help overcome that information deficit. She’s looking at the past, with an aim to understand what may lie ahead.
“It is still too difficult to predict when a tsunami may strike if we only consider the recent past,” Dura said. “Some coastlines have not experienced a recent large tsunami, so we may underestimate the potential for infrequent, but large and destructive tsunami events,” Dura said. “We hope to change that by using geological history as our guide.”
In another study published earlier this year, Dura, along with Jessica DePaolis, a Ph.D. student in the Coastal Hazards Lab, discovered important geologic evidence along the coast of north-central Chile that should help improve earthquake and tsunami hazards assessments along this coastline. Robert Weiss and Ben Gill, also members of the Department of Geosciences, contributed to the study.
“This study describes the first geologic evidence of past tsunami inundation along the north-central Chile coast,” said Dura, who is an affiliated member of the Center for Coastal Studies at Virginia Tech. “Every new tsunami deposit we describe helps paint a more complete picture of how tsunamis have behaved in the past and what we can expect in the future.”
Their findings were published in Quaternary Science Reviews, a peer-reviewed journal covering earth, climate, and life interactions.
“Visiting north-central Chile and seeing the tsunami evidence firsthand really drove home the power of this hands-on geologic effort to better understand tsunami hazards,” said DePaolis. “Leading the data collection and writing of the study was a big challenge but it was made possible by a great group of collaborators from Chile and the U.S.”
The study focuses on a region of north-central Chile that lies along an active portion of a subduction zone that has produced multiple earthquakes larger than magnitude 8 over the past century, often accompanied by tsunamis. However, a combination of several factors, including a semi-arid climate, sand-dominated coastline, and historically sparse population, has limited the historical (written and instrumental) and geologic records of tsunamis in this region. A recent magnitude 8.3 earthquake and accompanying tsunami in north-central Chile on Sept. 16, 2015, provided a unique opportunity to examine the nature of tsunami deposits in the region.
Dura’s Coastal Hazards Lab uses sand beds deposited by tsunamis and preserved in coastal environments to reconstruct past tsunami inundation over hundreds to thousands of years, helping better define coastal hazards in a region.
Dura was part of research that appeared in Nature Geoscience today examining seismic risk of earthquakes and associated tsunamis in the Tokyo region. They used a combination of geological evidence of tsunami inundation along with tsunami modeling to determine what plate boundaries past regional tsunamis have originated from. The work revealed that earthquakes along a previously unconsidered plate boundary have produced significant tsunami inundation in the past, adding another possible source for earthquakes in the Tokyo region and tsunamis in the Pacific Ocean.
The Coastal Hazards Lab group employed both field-based (stratigraphic mapping, sediment descriptions, surveying) and laboratory-based methods (particle size, geochemical, modeling, and dating analyses) to identify two anomalous sand beds in a coastal wetland in Tongoy Bay, north-central Chile. Satellite imagery taken before and after the 2015 earthquake, and post-earthquake surveys show the younger sand bed was deposited by the 2015 tsunami.
An older sand bed at the site was previously undocumented and was remarkably similar in composition to the 2015 tsunami sand bed. However, the older sand bed was slightly thicker and extended further inland than the 2015 tsunami sand, suggesting it was deposited by a larger event.
Radionuclide dating, which using cesium and lead to calculate the recent rate of sedimentation in the wetland, constrained the age of the older sand bed to 1922, when a large earthquake and tsunami occurred to the north of the study site. Analysis of historical maps showing the land use history of the wetland further supported the timing of sand bed deposition in the wetland to the early 20th century.
The discovery of a 1922 tsunami sand in north-central Chile shows that the 1922 tsunami produced significant inundation beyond the southern end of the earthquake rupture area. This provides additional insight into the type of ruptures that create high tsunamis along the north-central Chile coast. Dura said the results demonstrate that careful site selection and a multidisciplinary (e.g., stratigraphic, historical, and modeling) approach may help improve earthquake and tsunami histories along similar arid subduction zone coastlines.
“We need more studies like this; it’s a crucial step,” said Robert Weiss, director of Virginia Tech’s Center for Coastal Studies. “This is essential information that the Chilean government or the towns within the potentially threatened region could take action with to help mitigate the human toll of a tsunami.”
JOURNAL
Nature Geoscience
ARTICLE PUBLICATION DATE
2-Sep-2021
Ground-breaking work from SFU identifies new source for earthquakes and tsunamis in the Greater Tokyo Region
IMAGE: JESSICA PILARCZYK AND COLLEAGUES FROM THE GEOLOGICAL SURVEY OF JAPAN CORE RICE PADDIES ON THE BOSO PENINSULA TO UNCOVER GEOLOGICAL EVIDENCE FOR A TSUNAMI FROM 1,000 YEARS AGO. view more
CREDIT: SFU
Researchers have discovered geologic evidence that unusually large earthquakes and tsunamis from the Tokyo region—located near tectonic plate boundaries that are recognized as a seismic hazard source—may be traceable to a previously unconsidered plate boundary. The team, headed by Simon Fraser University Earth scientist Jessica Pilarczyk, has published its research today in Nature Geoscience.
The team’s ground-breaking discovery represents a new and unconsidered seismic risk for Japan with implications for countries lining the Pacific Rim, including Canada.
Pilarczyk points to low-lying areas like Delta, Richmond and Port Alberni as potentially vulnerable to tsunamis originating from this region.
In 2011, eastern Japan was hit with a massive magnitude 9 quake – creating the largest rupture area of any earthquake originating from the Japan Trench. It triggered the Fukushima Daiichi nuclear disaster and a tsunami that travelled thousands of miles away—impacting the shores of British Columbia, California, Oregon, Hawaii and Chile.
For the past decade, Pilarczyk and an international team of collaborators have been working with the Geological Survey of Japan to study Japan’s unique geologic history. Together, they uncovered and analyzed sandy deposits from the Boso Peninsula region (50 km 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 if a portion of the Philippine Sea/Pacific plate boundary near the Boso Peninsula was capable of generating large tsunamis similar in size as the Tohoku event in 2011.
Using a combination of radiocarbon dating, geologic and historical records, and paleoecology, the team used 13 hypothetical and historical models to assess each of the three plate boundaries, including the Continental/Philippine Sea plate boundary (Sagami Trough), the Continental/Pacific plate boundary (Japan Trench) and the Philippine Sea/Pacific plate boundary (Izu-Bonin Trench) as sources of the 1,000-year-old earthquake.
CAPTION
Jessica Pilarczyk (SFU) and collaborator Tina Dura (Virginia Tech) sample sediment cores from rice paddies of the Greater Tokyo Region that contain evidence for an earthquake from 1,000 years ago that potentially originated from a historically unconsidered earthquake source.
CREDIT
SFU
Pilarczyk reports that the modeled scenarios suggest that the source of the tsunami from 1,000 years ago originated from the offshore area off the Boso Peninsula — the smallest of which (for example, possible earthquakes with the lowest minimum magnitude), are linked to the previously unconsidered Izu-Bonin Trench at the boundary of the Philippine Sea and Pacific plates.
“Earthquake hazard assessments for the Tokyo region are complicated by the’ trench-trench triple junction’, where the oceanic Philippine Sea Plate not only underthrusts a continental plate but is also being subducted by the Pacific Plate.”says Pilarczyk, an assistant professor of Earth sciences at SFU who holds a Canada Research Chair in Natural Hazards. ”Great thrust earthquakes and associated tsunamis are historically recognized hazards from the Continental/Philippine Sea (Sagami Trough) 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 better informed seismic hazard maps for Japan. She also says that this information could be used by far-field locations, including Canada, to inform building practices and emergency management strategies that would help mitigate the destructive consequences of an earthquake similar to the one of 1,000 years ago.
JOURNAL
Nature Geoscience
METHOD OF RESEARCH
Data/statistical analysis
SUBJECT OF RESEARCH
Not applicable
ARTICLE PUBLICATION DATE
2-Sep-202
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