A Geo-Referenced 3D Model of the Juan de Fuca Slab and Associated Seismicity J.L. Blair, P.A. McCrory, and D.H. Oppenheimer U.S. Geological Survey F. Waldhauser Lamont-Doherty Earth Observatory 2011, revised 2013 U.S. Geological Survey Data Series 633, Version 1.2 OVERVIEW OF THE SLAB GIS PROJECT We present a Geographic Information System (GIS) of a new 3-dimensional (3D) model of the subducted Juan de Fuca Plate beneath western North America and associated seismicity of the Cascadia subduction system. The GIS project accompanies a publication that provides first- order correlations between the occurrence of Wadati- Benioff zone earthquakes and slab geometry, temperature, and hydration state (McCrory and others, 2012). The geo- referenced 3D model was constructed from weighted control points that integrate depth information from hypocenter locations and regional seismic velocity studies. We used the 3D model to differentiate earthquakes that occur in the Cascadia forearc above the Juan de Fuca Plate surface from earthquakes that occur below the surface within the underlying Juan de Fuca Plate. Seismicity below the slab surface is most prevalent where the slab is warped beneath northwestern California and western Washington suggesting that slab flexure, in addition to expected metamorphic processes such as dehydration embrittlement, is required to trigger seismicity within the subducted oceanic plate. Beneath northern California and southern Vancouver Island, upper plate epicenters co-locate with slab epicenters where the slab ranges from 10 to 30 km in depth. However, beneath western Washington, slab epicenters are deeper, concentrated between slab depths of 25 to 45 km. And upper plate epicenters are concentrated east of the 40-km depth contour, thus somewhat offset from the slab epicenters. Double- differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern California that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the interface that includes the 1992 M7.1 Cape Mendocino earthquake. This GIS project of the Cascadia subduction system supersedes the one previously published by McCrory and others (2006). Our new slab model updates the prior model with new constraints. The most significant updates to the model include: (1) weighted control points to incorporate spatial uncertainty; (2) an additional gridded slab surface based on the Generic Mapping Tools (GMT) Surface program which constructs surfaces based on splines-in-tension (see expanded description below); (3) double-differenced hypocenter locations in northern California to better constrain slab location there; and (4) revised slab shape based on new hypocenter profiles that incorporate routine depth uncertainties as well as data from new seismic-reflection and seismic-refraction studies. We used two different methods to construct the slab geometry model. The first method employed hand contouring of geo-referenced weighted control points; we then converted the contours into a 5-minute grid using ArcGIS (Drafted_slab_XYZ_Grid_GCS_WGS84.txt). This version was primarily designed as a tool to separate earthquakes above the slab surface from earthquakes below the slab surface, and as such the model conforms closely to apparent warps and buckles in the slab geometry. The second method employed to construct slab geometry used polynomial splines of the same weighted control points to calculate the location of the slab surface. We constructed a 10-minute grid from the control points and then contoured it using the GMT Surface script (Wessel and Smith, 1998) that interpolates and grids clustered noisy 3D data using Green?s functions for splines-in- tension to reduce artifacts (Wessel and Bercovici, 1998). The computer-generated version was partly intended to serve as a check for bias in the hand-generated version. Although the computer-generated version of the slab geometry replicates the general shape of the hand- contoured version, it also generated obvious distortions owing primarily to the uneven distribution of data points. Therefore, we modified the computer-generated version by smoothing out these distortions to provide a simplified contour map of the slab and a 5-minute grid (Modeled_slab_XYZ_Grid_GCS_WGS84.txt); this smoother geometry is suitable for investigating strain, stress, episodic tremor and slip (ETS), and other phenomena associated with the Cascadia subduction boundary. We also provide XYZ files of earthquakes above and below the hand-contoured slab surface, and a 3D fly-through animation of the model for use as a visualization tool. In the 3D animation, the slab surface is denoted by a transparent green surface; slab contours (at 10-km contour intervals with supplemental 5-km intervals) are denoted by gray lines. Earthquakes below the slab surface are denoted by orange dots; earthquakes above the slab surface are denoted by white dots. Spreading ridge plate boundaries are denoted by red lines; transform fault plate boundaries are denoted by green lines; the trench axis is denoted by a light yellow line; seismically active arc volcanoes are denoted by red cones. The coastline and statelines are denoted by light yellow lines; major cities are denoted by yellow cubes. Vertical exaggeration of the shaded relief bathymetry and topography is 3X; vertical exaggeration of the slab surface and hypocenters is 2500X. McCrory, P.A., Blair, J.L., Oppenheimer, D.H., and Walter, S.R., 2006, Depth to the Juan de Fuca slab beneath the Cascadia subduction margin: A 3-D model for sorting earthquakes: U.S. Geological Survey Data Series DS-91, 1 CD-ROM, v. 1.2, available at http://pubs.usgs.gov/ds/91/; last accessed 24 Oct 2011) McCrory, P.A., Blair, J.L., Waldhauser, F., and Oppenheimer, D.H., 2012, Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity: Journal of Geophysical Research, v. 117, n. B09306, 23 pp., doi:10.1029/2012JB009407. Wessel, Paul, and Bercovici, David, 1998, Interpolation with splines in tension: A Green's Function Approach: Mathematical Geology, v. 30, no. 1, p. 77-93. Wessel, Paul, and Smith, W.H.F, 1998, New, improved version of the Generic Mapping Tools Released: Eos Transactions, American Geophysical Union, v. 79, p. 579. FILES 1_README.txt Overview of the slab-grid project - this file DS633_Animation A folder containing the following PNW_animation_Readme.txt Text explanation of content portrayed in the PNW_animation. animation files: PNW_animation.mp4 3D animation in MPEG-4 (Moving Picture Experts Group) format PNW_animation.mov 3D animation in MOV (QuickTime Movie) format PNW_animation_cellular.m4v 3D animation in M4V format; this file was downsampled for viewing on cellular telephones DS633_Data.zip A compressed folder containing GIS ASCII data, GIS binary data, and metadata files: DS633_ASCII_GIS_Data A folder containing the following GIS files in (either comma- or tab-separated) ASCII format: ANSS_hypos_above_slab_GCS_WGS84.txt Sorted earthquakes above slab surface in Geographic Coordinate System WGS 1984. List of North American seismic events selected from ANSS catalog earthquakes See McCrory and others (2012) for explanation of parameters used to select earthquakes ANSS_hypos_below_slab_WGCS_GS84.txt Sorted earthquakes below slab surface in Geographic Coordinate System WGS 1984. List of Juan de Fuca seismic events selected from ANSS catalog earthquakes See McCrory and others (2012) for explanation of parameters used to select earthquakes. CNSN_hypos_above_slab_GCS_WGS84.txt Sorted earthquakes above slab surface in Geographic Coordinate System WGS 1984. List of North American seismic events selected from CNSN catalog earthquakes See McCrory and others (2012) for explanation of parameters used to select earthquakes CNSN_hypos_below_slab_WGCS_GS84.txt Sorted earthquakes below slab surface in Geographic Coordinate System WGS 1984. List of Juan de Fuca seismic events selected from CNSN catalog earthquakes See McCrory and others (2012) for explanation of parameters used to select earthquakes. Drafted_slab_XYZ_Grid_GCS_WGS84.txt Geo-referenced grid of depth to Juan de Fuca slab surface in Geographic Coordinate System WGS 1984. Grid based on hand- contoured data points. File in XYZ format using decimal degrees and kilometers; cell size is equivalent to 5 km. See McCrory and others (2012) for explanation of method used to create grid Modeled_slab_XYZ_Grid_GCS_WGS84.txt Geo-referenced grid of depth to Juan de Fuca slab surface in Geographic Coordinate System WGS 1984. Grid based on contouring data points with GMT Surface program. File in XYZ format using decimal degrees and kilometers; cell size is equivalent to 5 km. See McCrory and others (2012) for explanation of method used to create grid Drafted_slab_contours_GCS_WGS84.txt Geo-referenced iso-contours of depth to Juan de Fuca slab surface in Geographic Coordinate System WGS 1984. Contours based on hand-contoured data points. File in XYZ format using decimal degrees and kilometers. See McCrory and others (2012) for explanation of method used to create contours Modeled_slab_contours_GCS_WGS84.txt Geo-referenced iso-contours of depth to Juan de Fuca slab surface in Geographic Coordinate System WGS 1984. Contours based on contouring data points with GMT Surface program. File in XYZ format using decimal degrees and kilometers. See McCrory and others (2012) for explanation of method used to create contours DS633_Binary_GIS_Data A folder containing the GIS files (shapefiles, and ESRI Grid files) utilized in the .mov and .mp4 files, along with ESRI ArcScene (9x and 10) .sxd files to be opened in ArcScene. Requires ArcGIS software with 3D Analyst extension. DS633_Metadata (also provided outside of the data package) A folder containing metadata files for the ASCII GIS data and binary GIS data DS633_Location_Map.pdf A location map for reference to locations and features discussed in the 1_README.txt and PNW_animation_Readme.txt DS633_Metadata A folder containing metadata files for the ASCII GIS data and binary GIS data The suggested citation for v. 1.0 of this publication was: Blair, J.L., McCrory, P.A., Oppenheimer, D.H., and Waldhauser, F., 2011, A Geo-referenced 3D model of the Juan de Fuca Slab and associated seismicity: U.S. Geological Survey Data Series 633, v.1.0, http://pubs.usgs.gov/ds/633/. ----------------------------------------------------- version 1.1. 1_README.txt file updated to change McCrory and others (in press) citation to McCrory and others (2012). DS633_Data.zip updated with revised Drafted_slab_XYZ_Grid_GCS_WGS84.txt and Drafted_slab_contours_GCS_WGS84.txt files exchanged for previous versions in order to be consistent with McCrory and others (2012). version 1.2. Corrected minor issues with data package. The suggested citation for this publication is: Blair, J.L., McCrory, P.A., Oppenheimer, D.H., and Waldhauser, F., 2011, revised 2013, A Geo-referenced 3D model of the Juan de Fuca Slab and associated seismicity: U.S. Geological Survey Data Series 633, v.1.2, http://pubs.usgs.gov/ds/633/.