Massive slip trigger of the 2011 Tohoku Earthquake clarified by JAMSTEC and University of Tsukuba

A research group led by Researcher Hanaya Okuda from the Kochi Institute for Core Sample Research, Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star) and Executive Director Shuichi Kodaira of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Professor Kohtaro Ujiie from the Institute of Life and Environmental Sciences at the University of Tsukuba, in collaboration with the University of Nevada, USA and others, announced that they have revealed a significant contrast in the physical properties of rocks above and below the plate boundary fault in the Japan Trench subduction zone that caused the 2011 Tohoku Earthquake, which may have led to the massive slip.

This was discovered through analysis of drilling surveys (Expedition Tracking Tsunamigenic Slip Across the Japan Trench: JTRACK) conducted in 2024. It was also found that the subduction zone plate boundary fault has an elevation difference of up to 15 meters within a range of 100 meters. These findings are expected to contribute to elucidating the mechanism of massive earthquakes occurring in the Japan Trench. The results were published in Science on December 19.

The 2011 Tohoku Earthquake was a plate boundary earthquake that occurred in the Japan Trench where the Pacific Plate subducts beneath the North American Plate, and was the largest earthquake in Japan's recorded history. Emergency surveys conducted immediately after the earthquake by JAMSTEC and others revealed that a slip exceeding 50 meters occurred in the deepest trench area of the Japan Trench, generating a massive tsunami.

In addition to the Integrated Ocean Drilling Program (IODP) Expedition 343, the Japan Trench Fast Drilling Project (JFAST) was conducted in 2012. In JFAST, the scientific research vessel D/V Chikyu was used to directly drill the plate boundary fault that significantly slipped during the earthquake and analyzed fault samples of the fault. It was found that the plate boundary fault contained abundant slippery clay minerals, and all the stress stored in the fault was released during the earthquake. However, this survey was limited to one location at specific depths, and the spatial extent in the geological structure remained unclear.

This time, the research group analyzed geological samples obtained from IODP Expedition 405, JTRACK, which was conducted from September to December 2024 following JFAST. In this expedition, Chikyu was used to revisit the JFAST survey area (Site C0019), and drilling was conducted at two locations along the Japan Trench from the plate boundary fault zone. The research group collected core samples at each drilling location and succeeded in understanding the overall situation at the plate boundary. Pipes were extended from Chikyu to perform surveys, including drilling 950 meters from a seafloor depth of 6,930 meters to further obtain core samples. Long core samples, including those from above and below the plate fault, were successfully obtained. In addition, undeformed core samples before the plate subducted were obtained at a site (Site C0026) offshore from Site C0019. Specifically, Site C0026 is offshore from the Japan Trench, which is located between Sites C0019 and C0026. Combined with JFAST, drilling was conducted at five locations within a range of 100 meters in the fault that triggered the 2011 Tohoku Earthquake.

When observing these core samples, the plate boundary fault was formed between mudstone of the frontal prism, which was formed as a result of being pressed onto the North American Plate, and pelagic clay, which was accumulated on the Pacific Plate, and had a clear boundary with lithofacies above and below. In particular, it was found that the deformation at the plate boundary fault was concentrated in fault gouge approximately 1 centimeter thick.

It was also revealed for the first time that there is a contrast: density and P-wave velocity are high above the plate boundary fault and low below it. This physical property contrast existed before subduction, but it was found to be further amplified by the effects of compression associated with subduction.

Furthermore, obtaining multiple core samples at Site C0019 revealed that the position of the plate boundary fault has an elevation difference of up to 15 meters within a horizontal range of 100 meters, with spatial irregularities. Structures were also observed below the fault where the distribution of Pacific Plate pelagic clay from before subduction appears to have become heterogeneous due to subduction.

The results of these analyses make it conceivable that there is a physical property contrast between the rocks above and below the fault, where fault gouge rich in clay minerals was formed and deformation was concentrated, resulting in massive slip. Although the plate boundary fault has irregularities, the massive slip is believed to have overcome these irregularities.

Okuda commented: "These results are the first findings obtained from JTRACK. By integrating them with analyses from other drilling surveys in the future, we hope to elucidate the mechanism of massive earthquakes occurring in the Japan Trench. I recently participated in a drilling survey conducted at Site C0027 during International Ocean Drilling Programme (IODP3) Expedition 502. At this location, there are submarine volcanoes called petit-spots. In the future, I would like to also research how heterogeneous structures on the subducting plate like submarine volcanoes are involved in earthquakes in the Japan Trench."

Journal Information
Publication: Science
Title: Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench
DOI: 10.1126/science.ady0234