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Woodward Clyde Consultants 3 Juan de Fuca Plate Comparison Task JFP-2 Prepared for Washington Public Power Supply System Richland, Washington By Woodward-Clyde Consultants 100 Pringle Avenue Walnut Creek, CA 94596 February 1984 [hW \ .. - _ _ _ _ _ _ _ _ _ ' . | N% Consultants TABLE OF CONTENTS | Page 1.0 INTRODUCTION 1 2.0 COMPARATIVE DATA BASE 2 w 3.0 REVIEW OF PREVIOUS WORK 3 3.1 Qualitative Comparisons 3 3.2 Quantitative Comparisons 5 4.0 COMPARIS0N BETWEEN THE JUAN DE FUCA AND OTHER SUBDUCTION ZONES 5 4.1 - t egence Rate and Age of Subducting 6 U.shosphere at the Trench 4.2 Back-Arc Spreading 7 4.3 Depth of Seismicity 8 4.4 Trench Bathymetry 8 4.5 Dip of Benioff Zone 10 4.6 Seafloor Topography 10 4.7 Preseismic Quiescence 11 4.8 Focal Mechanisms in the Subducting Plate 13 d in the Vicinity of the Trench - 4.9 Focal Mechanisms in the Overriding Plate 14 _ 4.10 Focal Mechanisms in the Back-Arc Region 15 4.11 Transverse Structures / Segmentation 15 4.12 Heat Flow 17 5.0 SUMMARY 18 6.0 REFERENCES 20 | APPENDIX A - DESCRIPTION OF SUBDUCTION ZONE PARAMETERS - TABLES 1A AND 18 i -1- | l ) ! ._. .. -- I Woodward Clyde Consultants i ! ; LIST OF FIGURES 1 Subduction Zone Locations 2 Subduction. Zone Magnitude - Convergence Rate-Age Relationship LIST OF TABLES , 1A Seismicity Characteristics 1B Geologic / Geometric Characteristics 2A Seismicity Characteristics: References 28 Geologic / Geometric Characteristics: References . G G -11 - _ _ _ _ . _ _ _ _ _ _ . _ _ _ _ _ . _ . _ . _ _ __ .. _ _ _ _ . _ _ _ _ . _ , _ _ . _ . _ _ Woodward Clyde Consultants i JUAN DE FUCA PLATE COMPARISON TASK JFP-2 1.0 INTRODUCTION Presently available geologic, seismologic, and geodetic data permit a variety of interpretations regarding the nature of interaction along the shallow interface between the Juan de Fuca and North American plates. This results in a wide range of uncertainty in assessing the seismic potential of the int erface. As used in this report, seismic potential refers both to the ability of the interface to rupture and produce. large earthquakes and to the magnitude of these events. Interpretations of plate interaction and seismic potential include: present-day accumulation of strain on the plate interface using trilateration data (Savage and others,1981); a strongly coupled plate boundary with the potential for large thrust earthquakes based on comparison with other subduction zones (Heaton and Kanamori, in press); a locked plate interface interpreted from the orientation of P-axes along the Mt. St. Helens seismic zone (Weaver and Smith,1983); aseismic subduction based on leveling surveys and tide gauge measurements (Ando and Balazs,1979); and an unlacked subduction zone with aseismic subduction based on the lack of interplate earthquakes, the north-south orientation of P-axes of earthquake fault plane solutions, and amount of seismicity observed since 1900 (Rogers,1983). For most subduction zones the seismicity that has occurred during this century forms an adequate basis for evaluating the seismic potential of the zone (Ruff and Kanamori,1980).