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The Effects of the Tohoku Earthquake on Regional Seismicity in Japan

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The Effects of the Tohoku Earthquake on Regional Seismicity in Japan Understanding Earthquake Risk in Japan Following the Tohoku-Oki Earthquake of March 11, 2011 Copyright 2012 AIR Worldwide. All rights reserved. Information in this document is subject to change without notice. No part of this document may be reproduced or transmitted in any form, for any purpose, without the express written permission of AIR Worldwide (AIR). Trademarks AIR Worldwide is a registered trademark. CLASIC/2 and ALERT are trademarks of AIR Worldwide. Confidentiality AIR invests substantial resources in the development of its models, modeling methodologies and databases. This document contains proprietary and confidential information and is intended for the exclusive use of AIR clients who are subject to the restrictions of the confidentiality provisions set forth in license and other nondisclosure agreements. Contact Information If you have any questions regarding this document, contact: AIR Worldwide 131 Dartmouth Street Boston, MA 02116-5134 USA Tel: (617) 267-6645 Fax: (617) 267-8284 2 CONFIDENTIAL Table of Contents Table of Contents 1 Executive Summary ..................................................................................................................................... 5 2 Introduction ................................................................................................................................................ 11 2.1 The AIR Earthquake Model for Japan ................................................................................................ 11 3 The Effects of the Tohoku Event on Earthquake Risk in Japan ........................................................... 15 3.1 Japan’s Tectonic Setting—An Overview ............................................................................................ 16 3.2 Implications of the Tohoku Earthquake for the State of Stress along the Japan Trench ............. 17 3.3 Implications of the Tohoku Earthquake for the State of Stress in the Kanto Plain, Sagami Trough, and Regional Faults .......................................................................................................................... 25 4 Conclusions ................................................................................................................................................ 29 4.1 Summary of Apparent Changes in Regional Risk ............................................................................ 29 5 Research Notes – The Effects of the Tohoku Earthquake on Regional Seismicity in Japan ............. 31 5.1 Changes in Coulomb Stress Failure in the Kanto Plain and for 98 Major Faults in Japan .......... 36 6 References ................................................................................................................................................... 46 7 About AIR Worldwide .............................................................................................................................. 49 List of Figures Figure 1. Source Zones within the Japan Trench ............................................................................................... 6 Figure 2. Faults Potentially Affected by the Tohoku-oki Earthquake (faults delineated in red are explicitly referenced in this document) ........................................................................................................ 8 Figure 3. AIR’s Modeled Tsunami Footprint Following the Tohoku Earthquake and Close-up of the Inundated Region near Sendai ..................................................................................................................... 13 Figure 4. Tectonic Setting of Japan and Major Historical Earthquakes ........................................................ 16 Figure 5. M9.0 Tohoku Earthquake and M7.9 Aftershock (contours) with Footprints of Historical Earthquakes (left panel) and Subduction Zones of the Japan Trench (right panel) .............................. 18 Figure 6. Tectonics of the “Boso-oki Segment” Offshore of the Kanto Plain ............................................... 23 Figure 7. Surface Projection of three Rupture Scenarios under the Kanto Plain: Pacific Interface Subduction (left panel), Philippine Sea Interface Subduction (middle panel) and Philippine Sea Intraplate Subduction (right panel) ............................................................................................................. 26 Figure 8. Faults Potentially Affected by the Tohoku-oki Earthquake........................................................... 30 3 CONFIDENTIAL List of Tables Figure 9. Tectonic Forces create Secular Stress on a Fault .............................................................................. 32 Figure 10. Schematic Diagram of a Stochastic Renewal Model used to Estimate Time-Dependent Rupture Probabilities..................................................................................................................................... 33 Figure 11. Decay of Friction Coefficient with Change in Slip Velocity......................................................... 34 Figure 12. Seismicity Ratios for Four Different Cases ..................................................................................... 35 Figure 13. Aftershock Duration and Fault Loading Rates for Selected Large Earthquakes in Different Tectonic Settings, 1 kyr = 1,000 years (Stein et al. 2009) ............................................................................ 36 Figure 14. Subduction Segments and 98 Major Faults used to Determine Changes in Coulomb Stress Failure .............................................................................................................................................................. 37 Figure 15. Changes in the CFS on the Pacific Plate Interface beneath the Boso Peninsula due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4 ................................................ 38 Figure 16. Comparison of CFSC values based on coseismic displacements, coseismic and postseismic displacements, (both from Ozawa et al., 2011,) and coseismic displacement (California Institute of Technology), Friction Coefficient = 0.4. ....................................................................................................... 39 Figure 17. The rate of M ≥ 4 Earthquakes Increased Significantly after Tohoku ......................................... 40 Figure 18. CFS Changes at the Philippine Sea Interface for Kanto and Genroku using Right-Lateral Strike-Slip in the Sagami Trough due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4 .............................................................................................................................................. 43 Figure 19. CFS Changes at the Philippine Sea Interface for Kanto and Genroku using Oblique Thrust and Right-Lateral Strike-Slip in the Sagami Trough due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4 .................................................................................................................... 43 Figure 20. CFS Changes for Crustal Faults with CFSC ≥ 0.1 Bars due to Coseismic Displacement (Ozawa et al., 2011) ....................................................................................................................................................... 44 Figure 21. CFS Changes for Crustal Faults with CFSC ≥ 0.1 Bar due to Coseismic and Postseismic Displacement (Ozawa et al., 2011) ............................................................................................................... 44 Figure 22. CFS Changes for Crustal Faults with CFSC ≤ -1.0 Bar due to Coseismic Displacement, (Ozawa et al., 2011) ........................................................................................................................................ 45 List of Tables Table 1. Summary of Risk Changes for Subduction Zones and Faults Potentially Affected by the Tohoku-oki Earthquake .................................................................................................................................. 9 Table 2. Number of Large Earthquakes in the Japan Trench since 1600 ...................................................... 17 Table 3. Seismic Source Zones of the Japan Trench as Identified by HERP in the 2007 National Seismic Hazard Maps .................................................................................................................................................. 18 Table 4. Summary of Risk Changes for Subduction Zones and Faults Potentially Affected by the Tohoku-oki Earthquake ................................................................................................................................ 30 4 CONFIDENTIAL Executive Summary 1 Executive Summary Japan has a long and well-documented history of large earthquakes. This has fostered a deep awareness of earthquake risk throughout the nation, which continues to develop some of the highest standards in the world for building codes and risk mitigation practices. The country’s complex and active tectonics has been the subject of extensive research leading to extremely strict and heavily enforced practices aimed at minimizing the damaging effects of earthquakes. Yet not even Japan was prepared for the Tohoku-oki earthquake and tsunami, which devastated the country on March 11, 2011. Conventional wisdom, upheld by the scientific community and Japan’s own Headquarters for Earthquake Research Promotion (HERP), maintained that it was not possible for such an
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