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We want to hear your story! http://escholarship.org/reader_feedback.html Peer Reviewed Title: Investigation of the huge tsunami from the 2011 Tōhoku-Oki, Japan, earthquake using ocean floor boreholes to the fault zone Journal Issue: Oceanography, 27(2) Author: Mori, J Chester, F Brodsky, EE Kodaira, S Publication Date: 06-01-2014 Series: UC Santa Cruz Previously Published Works Permalink: http://escholarship.org/uc/item/91b18878 DOI: http://dx.doi.org/10.5670/oceanog.2014.48 Local Identifier: 999239 Abstract: Integrated Ocean Drilling Program (IODP) Expedition 343, named the Japan Trench Fast Drilling Project (JFAST), drilled ocean floor boreholes through the fault zone of the 2011 Tōhoku-Oki earthquake (M9.0) to enhance understanding of the rupture process and tsunami generation. This project investigated the very large fault slip that caused the devastating tsunami by making borehole stress measurements, sampling the plate boundary fault zone, and taking temperature measurements across the fault zone. The results show that the earthquake rupture occurred in a narrow fault zone (< 5 m). Based on both laboratory experiments on fault zone material and eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. temperature monitoring of the fault zone, we show that the fault had very low friction during the earthquake. The low friction properties of the fault zone might be attributed to high smectite clay content of the sediment. These physical and frictional characteristics contributed to the unusually large fault slip that occurred during the earthquake and generated the tsunami. © 2014 by The Oceanography Society. All rights reserved. Copyright Information: All rights reserved unless otherwise indicated. Contact the author or original publisher for any necessary permissions. eScholarship is not the copyright owner for deposited works. Learn more at http://www.escholarship.org/help_copyright.html#reuse eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. OceTHE OFFICIALa MAGAZINEnog OF THE OCEANOGRAPHYra SOCIETYphy CITATION Mori, J., F. Chester, E.E. Brodsky, and S. Kodaira. 2014. Investigation of the huge tsunami from the 2011 Tōhoku-Oki, Japan, earthquake using ocean floor boreholes to the fault zone. Oceanography 27(2):132–137, http://dx.doi.org/10.5670/oceanog.2014.48. DOI http://dx.doi.org/10.5670/oceanog.2014.48 COPYRIGHT This article has been published inOceanography , Volume 27, Number 2, a quarterly journal of The Oceanography Society. Copyright 2014 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. DOWNLOADED FROM HTTP://WWW.TOS.ORG/OCEANOGRAPHY SPECIAL ISSUE ON UNDERSEA NATURAL HAZARDS Investigation of the Huge Tsunami from the 2011 Tōhoku-Oki, Japan, Earthquake Using Ocean Floor Boreholes to the Fault Zone BY JIM MORI, FREDERICK CHESTER, EMILY E. BRODSKY, AND SHUICHI KODAIRA (left) Drilling vessel Chikyu used for Japan Trench Fast Drilling Project (JFAST) drilling operations. Photo courtesy of JAMSTEC (right) Remotely operated vehicle Kaiko used for tem- perature instrument retrieval from the borehole. ABSTRACT. Integrated Ocean Drilling Program (IODP) earthquake rupture occurred in a narrow fault zone (< 5 m). Expedition 343, named the Japan Trench Fast Drilling Project Based on both laboratory experiments on fault zone material (JFAST), drilled ocean floor boreholes through the fault and temperature monitoring of the fault zone, we show that zone of the 2011 Tōhoku-Oki earthquake (M9.0) to enhance the fault had very low friction during the earthquake. The understanding of the rupture process and tsunami generation. low friction properties of the fault zone might be attributed This project investigated the very large fault slip that caused the to high smectite clay content of the sediment. These physical devastating tsunami by making borehole stress measurements, and frictional characteristics contributed to the unusually sampling the plate boundary fault zone, and taking temperature large fault slip that occurred during the earthquake and measurements across the fault zone. The results show that the generated the tsunami. 132 Oceanography | Vol. 27, No. 2 INTRODUCTION along the Tōhoku coast. For example, 2. Obtain core samples of the plate When the Tōhoku-Oki earthquake the 1896 Sanriku earthquake had peak boundary fault zone to determine (M9.0) occurred off the east coast of tsunami heights comparable to those geologic structures and measure Japan on March 11, 2011, much of the of the 2011 earthquake (Figure 2). It physical properties of the fault zone. world was watching as the devastating is likely that faulting on the shallow Before this project, no one had tsunami inundated the northeast portion of the subduction zone also directly investigated a fault zone that coast of Honshu. This was the largest caused the large 1896 tsunami (Tanioka had recently moved tens of meters in earthquake and largest tsunami in Japan’s and Satake, 1996). The magnitude an earthquake. thousand-year recorded history of (about M8.5), and thus the source area 3. Determine the level of friction by destructive events, with over 18,000 lives for the 1896 earthquake, is much smaller measuring the temperature anomaly lost and great economic costs totaling compared to the 2011 event. This shows across the fault zone to estimate the 20 to 30 trillion Japanese yen. The that it is not necessary to have an M9 level of frictional heat generated tsunami heights were more than 10 m earthquake to generate 30 to 40 m tsu- during the earthquake. To resolve over several hundred kilometers of the nami heights—smaller M8 earthquakes a clear temperature signal, these coasts of Fukushima, Miyagi, and Iwate can also produce very large tsunamis. observations needed to be done Prefectures, and a maximum over 40 m Thus, for evaluating tsunami hazards, quickly after the earthquake, and this in Iwate Prefecture (Mori et al., 2011). the faulting on the shallow portion of was the main motivation for the rapid Although the ground motions were very the subduction zone is more important response of JFAST. intense (peak accelerations exceeding 1 g than the overall size of the earthquake. With these goals, a proposal for JFAST over a wide region), the shaking damage In order to investigate the tsunami- was written and quickly evaluated by was relatively small for such a large generating fault slip during the 2011 Integrated Ocean Drilling Program earthquake, and the tsunami caused the earthquake, JFAST included three main (IODP) scientific and operational panels primary impacts. Only 4.4% of the deaths scientific objectives: during 2011. Final project approval were attributed to seismic and landslides 1. Estimate the stress state in the was given in February 2012, and IODP effects. In response to this earthquake, region of the shallow fault from Expedition 343 sailed on April 1, 2012, scientists quickly began planning the borehole breakouts. within 13 months of the earthquake. Japan Trench Fast Drilling Project (JFAST) to study the shallow portion of the subduction fault, which was the main source area of the tsunami (Figure 1). One of the important features of the earthquake was the very large fault slip that occurred on the shallow portion of the subduction zone near the Japan Trench. This fault displacement of 40–60 m (e.g., Fujiwara et al., 2011) was the largest ever observed for any earth- quake in the world and astounded many scientists. Because the size of a tsunami is directly related to the amount of shal- low slip and resultant seafloor deforma- tion, understanding this unprecedented large fault movement was the main scientific focus for the drilling project. Figure 1. (A) Large red circle shows location of Japan Trench Fast Drilling Project (JFAST) site C0019 within the large slip area of the 2011 Tōhoku-Oki earthquake. (B) Seismic data showing the plate Very large tsunamis with heights of boundary fault. (C) Interpreted geological structure. Red vertical lines in B and C show the borehole 30 to 40 m have occurred previously location. Figure from Chester et al. (2013) Oceanography | June 2014 133 SHIP OPERATIONS preparations enabled the very fast imple- sensors could not be installed during IODP Expedition 343 was carried out mentation of the expedition. This is the two months of the main expedition. on the drilling vessel Chikyu, operated the only academic research vessel with However, additional ship time was made by the Japan Agency for Marine-Earth the very deep water drilling capability available in technical Expedition 343T in Science and Technology (JAMSTEC; see needed for this project. Two months of July 2012 for a