In 2011, significant geological movement occurred deep beneath the Japan Trench, triggering earthquakes that reshaped the ocean floor and caused a catastrophic tsunami. This area, located off the east coast of Japan’s Tohoku region, is part of a massive section of the Earth’s crust that supports the Pacific Ocean. The Tektonic IC plate is subducting beneath Japan and delving deep into the Earth’s interior. Researchers theorize that rocks from this plate compressed against those below Japan, resulting in immense pressure buildup. Eventually, the lower plate slipped, releasing energy and causing an earthquake.
Plate slips can lead to two primary outcomes. Initially, smaller earthquakes occur several kilometers deep within the Earth’s crust, which are unlikely to generate tsunamis. Conversely, significant slip events, like the one in 2011 that reached the Japan Trench, distort the ocean floor, causing seawater displacement and, consequently, tsunamis. Such occurrences are classified as trench slip earthquakes.
Japan’s history of earthquakes associated with tsunamis suggests that this phenomenon is not solely limited to the events of 2011. Research led by Charlotte Peiser and her team focused on the sediments of the Japan Trench, aiming to reveal the geological history of these trench-slip earthquakes.
Over time, the Japanese archipelago deposits sediment that fills ocean trenches, creating horizontal layers. Trench-slip earthquakes can bend and fold these layers. The research team posited that identifying and dating these folds in the Japan Trench would allow them to catalog past trench-slip earthquakes in the region.
These earthquakes occur repetitively; thus, more recent ones can obscure the geological features created by older events. To analyze this, researchers selected a study site within the Japan Trench, approximately 100 kilometers (60 miles) north of the most intense seismic activity. This location, being distant from the epicenter, has less severe deformation, facilitating the identification of distinct earthquakes.
Peiser and her team compiled data from previous studies at this site to reconstruct the earthquake history of Japan. The data comprised three major elements, two of which were images showcasing sediment layers in the ocean trenches generated from vibrational reflections deep underground. Seismic profiles were used in this analysis.
The first seismic profile documented the entire ocean trench, extending over 1 kilometer (approximately 0.6 miles) deep, with a clarity of 5 meters (16 feet). Consequently, layers thinner than 5 meters would not be observable in the captured data. The second seismic profile focused solely on the upper 40 meters (130 feet) of sediment, successfully detecting layers as thin as 10 centimeters (4 inches).
The final piece of data utilized was a 36-meter (120-foot) long sediment core extracted from the trench’s floor. Sediment core samples have previously been correlated with historic seismic events. The analysis revealed that two significant earthquakes, the Kyotoku earthquake of 1454 AD and the Jogan earthquake of 869 AD, were likely associated with tsunamis.
Peiser’s team accurately identified the seismic layers associated with the Keitoku and Jogan earthquakes from high-resolution seismic profiles of the ocean trench. In these detailed datasets, they observed deformation in the sediment up to the layer linked to the Jogan earthquake but not above it, indicating trench deformation coincided with the 869 AD event.
Although high-resolution data only captured deposits from recent earthquakes since 869 AD, the low-resolution profiles depicting the entire trench revealed a deeper, older historical record. Upon further investigation of the trench’s sediments, they discovered that deformation from the 869 AD earthquake extended from where the plate collided with Japan. They recognized at least six similarly deformed layers, each suggesting separate trench-slip events, though the dates of these occurrences remain uncertain.
Peiser et al. concluded that trench-slip earthquakes are a prevalent phenomenon in the Japan Trench, with a history extending over many years. They anticipate that ongoing studies will enhance the understanding of Japan’s tsunami risk.
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Source: sciworthy.com