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Seismic Structure, Intraslab Earthquakes and Processes, and Earthquake Hazards THE CASCADIA SUBDUCTION ZONE AND RELATED SUBDUCTION SYSTEMS Seismic Structure, Intraslab Earthquakes and Processes, and Earthquake Hazards Editors: Stephen Kirby, Kelin Wang and Susan Dunlop U.S. Geological Survey Open-File Report 02–328 Geological Survey of Canada Open File 4350 August 2002 The Cascadia Subduction Zone and Related Subduction Systems— Seismic Structure, Intraslab Earthquakes and Processes, and Earthquake Hazards Edited by Stephen Kirby1, Kelin Wang2, and Susan Dunlop3 Released simultaneously as U.S. GEOLOGICAL SURVEY OPEN-FILE REPORT 02–328 and GEOLOGICAL SURVEY OF CANADA OPEN FILE 4350 Version 1.0 2002 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although all data and software released on this CD-ROM have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related material and (or) the functioning of the software. U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 1 U.S. Geological Survey MS 977, 345 Middlefield Road, Menlo Park, California, 94025, USA 2 Pacific Geoscience Centre, Geological Survey of Canada, PO Box 6000, Sidney, British Columbia, V8L 4B2, Canada 3 Centre for Earth and Ocean Research, University of Victoria, P.O. Box 3055, STN CSC, Victoria, BC, V8W 3P6, Canada ISBN: 0-607-99675-7 This publication and any updates to it can be found online at: http://geopubs.wr.usgs.gov/open-file/of02-328/ The Cascadia Subduction Zone and Related Subduction Systems | iii Preface The following report is the principal product of an international workshop titled “Intraslab Earthquakes in the Cascadia Subduction System: Science and Hazards” and was sponsored by the U.S. Geological Survey, the Geological Survey of Canada and the University of Victoria. This meeting was held at the University of Victoria’s Dunsmuir Lodge, Vancouver Island, British Columbia, Canada on September 18–21, 2000 and brought 46 participants from the U.S., Canada, Latin America and Japan. This gathering was organized to bring together active research investigators in the science of subduction and intraslab earthquake hazards. Special emphasis was given to “warm-slab” subduction systems, i.e., those systems involving young oceanic lithosphere subducting at moderate to slow rates, such as the Cascadia system in the U.S. and Canada, and the Nankai system in Japan. All the speakers and poster presenters provided abstracts of their presentations that were a made available in an abstract volume at the workshop. Most of the authors subsequently provided full articles or extended abstracts for this volume on the topics that they discussed at the workshop. Where updated versions were not provided, the original workshop abstracts have been included. By organizing this workshop and assembling this volume, our aim is to provide a global perspective on the science of warm-slab subduction, to thereby advance our understanding of internal slab processes and to use this understand­ ing to improve appraisals of the hazards associated with large intraslab earthquakes in the Cascadia system. These events have been the most frequent and damaging earthquakes in western Washington State over the last century. As if to underscore this fact, just six months after this workshop was held, the magnitude 6.8 Nisqually earthquake occurred on February 28th, 2001 at a depth of about 55 km in the Juan de Fuca slab beneath the southern Puget Sound region of western Washington. The Governor’s Office of the State of Washington estimated damage at more than US$2 billion, making it among the costliest earthquakes in U.S. history. iv | The Cascadia Subduction Zone and Related Subduction Systems | v Table of Contents SUMMARY AND INTRODUCTION Workshop consensus findings and recommendations Stephen H. Kirby, Kelin Wang, Workshop Steering Committee (Garry C. Rogers, Robert S. Crosson, Anne M. Tréhu, Chris Goldfinger and Tom M. Brocher) and the workshop participants ...................................................................................... 1 Introduction to a global systems approach to Cascadia slab processes and associated earthquake hazards Stephen H. Kirby and Kelin Wang .................................................................................................................................................................................................. 5 CASCADIA SUBDUCTION SYSTEM: SETTING AND STRUCTURE The Juan de Fuca plate and slab: Isochron structure and Cenozoic plate motions Douglas S. Wilson ........................................................................................................................................................................................................................................ 9 Offshore structure of the Juan de Fuca plate from marine seismic and sonar studies Chris Goldfinger, Robert P. Dziak and Chris Fox ............................................................................................................................................................. 13 Cascadia microplate models and within-slab earthquakes Ray E. Wells, Richard J. Blakely and Craig S. Weaver ................................................................................................................................................. 17 Geometry of the subducting Juan de Fuca plate: New constraints from SHIPS98 Anne M. Tréhu, Tom M. Brocher, Kenneth C. Creager, Mike A. Fisher, Leiph A. Preston, George Spence and the SHIPS98 Working Group ............................................................................................................................................................................................... 25 Three-dimensional structure of the Cascadia forearc region from SHIPS active experiment and earthquake observations: Tomographic inversion provides a high-resolution view into the core of the Cascadia forearc complex Robert S. Crosson, Neil P. Symons, Kenneth C. Creager, Leiph A. Preston, T. Van Wagoner, Tom M. Brocher, Mike A. Fisher and the SHIPS98 Working Group .......................................................................................................................................................... 33 Velocity structure of western Washington from local earthquake tomography Antonio Villaseñor, D. Stanley and Harley Benz ............................................................................................................................................................ 35 Three-dimensional reflection image of the subducting Juan de Fuca plate Kenneth C. Creager, Leiph A. Preston, Robert L. Crosson, T. van Wagoner, Anne Tréhu and the SHIPS98 Working Group ............................................................................................................................................................................................... 37 Modelling post-critical Moho reflections from intraslab earthquakes in the Cascadia subduction zone Jeff Shragge, Michael Bostock and Garry C. Rogers .................................................................................................................................................... 43 Geometry and membrane deformation rate of the subducting Cascadia slab Ling-Yun Chiao and Kenneth C. Creager ............................................................................................................................................................................... 47 Crustal structure of northern Juan de Fuca plate and Cascadia subduction zone—new results, old data Ron M. Clowes ........................................................................................................................................................................................................................................... 55 Seismic reflection imaging of the Cascadia plate boundary offshore Vancouver Island Andrew Calvert ........................................................................................................................................................................................................................................... 59 An image of the Cascadia subduction zone across central Oregon Stephane Rondenay and Michael G. Bostock .................................................................................................................................................................. 63 CASCADIA SLAB EARTHQUAKES Intraslab earthquakes beneath Georgia Strait/Puget Sound Garry C. Rogers and Robert S. Crosson ................................................................................................................................................................................ 65 vi | Precise relocations of slab seismicity in the northern Cascadia subduction zone John F. Cassidy and Felix Waldhauser ...................................................................................................................................................................................... 69 Large earthquakes under our feet and deep beneath the Puget Lowland and Georgia Strait: How often do they occur, how big can they get and what are their likely effects? Stephen H. Kirby, Kenneth C. Creager and Leif A. Preston ..................................................................................................................................
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