Depth to the Juan de Fuca Slab Beneath the Cascadia Subduction Margin— A 3-D Model for Sorting Earthquakes By Patricia A. McCrory, J. Luke Blair, David H. Oppenheimer, and Stephen R. Walter U.S. Geological Survey, Menlo Park, California Abstract above and below the slab surface and a 3-D animation or fly-through showing a shaded- We present an updated model of the relief map with plate boundaries, the slab Juan de Fuca slab beneath southern British surface, and hypocenters for use as a Columbia, Washington, Oregon, and visualization tool. northern California, and use this model to separate earthquakes occurring above and below the slab surface. The model is based Introduction on depth contours previously published by The Juan de Fuca oceanic plate descends Flück and others (1997). Our model beneath the Pacific Northwest continental attempts to rectify a number of margin along the Cascadia subduction fault shortcomings in the original model and (fig. 1). The location and shape of the update it with new work. The most subducted oceanic plate or slab beneath the significant improvements include (1) a margin is a key element for understanding gridded slab surface in geo-referenced the seismotectonics of this complex plate (ArcGIS) format, (2) continuation of the slab boundary. Of particular interest are surface to its full northern and southern seismicity patterns in North American edges, (3) extension of the slab surface from continental crust above the Juan de Fuca 50-km depth down to 110-km beneath the slab that may correlate with mapped, Cascade arc volcanoes, and (4) revision of shallow crustal faults (see for example, the slab shape based on new seismic- McCrory, 2002). To isolate seismic events in reflection and seismic-refraction studies. the upper plate relevant to shallow crustal We have used this surface to sort faults requires identifying the subduction earthquakes and present some general fault interface. To this end we created a observations and interpretations of geo-referenced, three-dimensional (3-D) seismicity patterns revealed by our analysis. model of the interplate boundary that For example, deep earthquakes within the allowed us to separate earthquakes Juan de Fuca Plate beneath western occurring above and below the Juan de Fuca Washington define a linear trend that may slab surface. This model surface also has mark a tear within the subducting plate broad application to studies of interplate Also earthquakes associated with the strain accumulation, slab seismicity, and slab northern stands of the San Andreas Fault metamorphism. This contribution provides abruptly terminate at the inferred southern three data sets—(1) a 3-D grid of depth to boundary of the Juan de Fuca slab. In the slab surface in kilometers below sea addition, we provide files of earthquakes level, (2) a catalog of earthquakes occurring U.S. Geological Survey Data Series 91 1 above the surface, and (3) a catalog of typically does not produce a strong earthquakes occurring below the surface. In reflector or sharp P-wave velocity gradient. addition, we provide a 3-D animation of the We infer this surface to be located about 7 combined data sets as a visualization tool. km above the Moho on the basis of the young age of the slab (Gutscher and others, 2000; Tréhu and others, 2002). Approach For the deeper portion of the We base our new model of the Juan de subducted slab, we primarily rely on studies Fuca slab surface on depth contours of teleseismic travel-time data and previously published by Flück and others hypocenter profiles in areas where (1997) (fig. 2A), adjusted in places by results earthquakes occur down to depths of 70 to of recent seismic surveys. The Flück and 100 km. In general, our slab model is drawn others (1997) depth contours were based as a smooth surface with constant dip on three main data types—(1) regional between wide-angle transects, teleseismic earthquake hypocenters, (2) teleseismic transects, and hypocenter profiles (fig. 2B). travel-time data, and (3) seismic reflection In areas where overlapping data sets yield and refraction transects. We updated the conflicting depth assignments, the data shape of their surface using recent wide- providing the smoothest contours are angle seismic results described in the next preferentially chosen. This choice may section (see table 1 for references) (fig. 2B). sometimes result in over-smoothing the We extended the surface east to beneath slab’s actual geometry, but resolving local the Cascade volcanic arc using hypocenter complexities is not possible with currently and teleseismic data. We modeled the slab available data. surface to its full northern and southern We use the resultant surface (fig. 3) to edges using both a regional study of first- separate those earthquakes that occur in arrival data from a 1994 Mendocino North American continental lithosphere earthquake (fig. 2C) and a reconstruction of from those that occur in Juan de Fuca Juan de Fuca Plate magnetic isochrons oceanic lithosphere. Uncertainties in the (Wilson, 2002). These new constraints and location of the Moho reflector the changes they produced in the slab (approximately ±5 percent of its modeled surface model are described in the next depth) and the uncertainties in the location section. of the earthquakes (approximately ±3 km) A primary constraint on the location preclude identifying interplate earthquakes and shape of the slab surface comes from —those that occur on the Cascadia profiles of P-wave velocities obtained by subduction fault. However, only one modeling the travel times of active source earthquake to date has been identified as seismic experiments. The oceanic possibly occurring at the plate interface— Mohorovic discontinuity (Moho), which the 1992 (moment magnitude, Mw 7.1) Cape marks the boundary between the crust and Mendocino earthquake (Oppenheimer and upper mantle is typically observed as a others, 1993; National Earthquake strong reflector with a sharp velocity Information Center (NEIC) at http:// gradient on these profiles. Available wide- neic.usgs.gov/neis/eqlists/large_usa.html). angle experiments resolve this boundary to about 40- to 50-km deep, depending on the length of the transect. The top of the slab— the boundary of interest in this study— U.S. Geological Survey Data Series 91 2 TABLE 1. List of published references and unpublished data used to construct the grid of the Juan de Fuca slab surface (see fig. 2B for the location of transects and profiles). ORIGINAL SLAB DEPTHS FROM: Flück, Hyndman, and Wang, 1997 REVISED SLAB DEPTHS FROM: WIDE-ANGLE (ACTIVE SOURCE) TRANSECTS Transects H1H1’, H2’H2”, and H3H3’ Beaudoin and others, 1998 Transect JJ’ Brocher, Thomas M., and others, 1994, unpubl. data Transect NN’ Gerdom and others, 2000 Transect OO’ * Tréhu and others, 1994 Transect QQ’ * Tréhu and others, 1994 Transect SS’S” Parsons and others, 1999 Transects SS’ and UU’ Flueh and others, 1998 Transect YY’ and Region W Creager and others, 2002 Tréhu and others, 2002 Transect Z1Z1’ and Z2Z2’ * Hyndman and others, 1990 TELESEISMIC (PASSIVE SOURCE) TRANSECTS Transect LL’ Harris and others, 1991 Transect MM’ Bostock and others, 2002 Transect R1R1’, R2R2’, and R3R3’ Michaelson and others, 1986 SEISMICITY PROFILES Profile a1a1’ this study Profile a2a2’ McCrory, 2000 Profile bb’ Walter, 1986 Profile c1c1’ this study Profile c2c2’ this study Profile dd’ Chaio, 1991 Profile ee’ Zhao and others, 2001 RESISTIVITY TRANSECT Transect PP’ Wannamaker and others, 1989 WORK IN PROGRESS (TO BE INCLUDED IN FUTURE REVISIONS): Transect H2H2’ Tréhu, Anne M., and others, 1993; 1994, unpubl. data Transect KK’ Brocher, Thomas M., and others, 1994, unpubl. data Transect MM’ Nabelek, John L., 1993, unpublished data Transects TT,’ U’U”, VV’, and XX’ Pecher, Ingo, and others, 1996, unpublished data *References incorporated into Flück and others (1997) model U.S. Geological Survey Data Series 91 3 Main Improvements to Mendocino earthquake to map the slab edge. This earthquake occurred in the the Slab Model vicinity of the Mendocino fracture zone and Few wide-angle seismic reflection and its moment tensor solution (NEIC; http:// refraction transects had been conducted neic.usgs.gov/neis/bulletin/ along the Cascadia subduction margin when neic_vmaj_m.html) indicates right-lateral Flück and others (1997) constructed their slip occurred on a near vertical plane with a model (fig. 2A). Several new transects have strike of 095°, consistent with failure of the been completed along the continental fracture zone. Projecting this orientation margin of Washington, Oregon, and landward, the pattern of first-motion northern California (table 1) since that readings recorded by short period time. These new results prompted our seismometers operated by the Northern updating the 3-D slab geometry before California Seismic Network (NCSN) should sorting earthquakes into North American be consistent with this azimuth. That is, or Juan de Fuca Plate events. The main compressional or "up" first motions should change in slab shape occurs beneath the be recorded north of this projected Puget Sound region, where seismic azimuth. Instead, we observe a pattern that reflection and refraction experiments (for describes an azimuth of about 120° and example, the Seismic Hazard Investigation in intersects the coast near the Mendocino Puget Sound (SHIPS) Experiment; Brocher triple junction (fig. 2C). We interpret this and others, 1999; Brocher and others, 2002) pattern as imaging the southern edge of the have resulted in improved P-wave velocity subducted Juan de Fuca Plate. models that image the oceanic Moho. These Although we have not modeled the models depict a less pronounced curvature travel-time data, we interpret the unusual to the arched slab geometry (fig. 2B) first-motion pattern to be the result of a beneath western Washington than the seismic wavefront that diffracted into the earlier model. subducted slab near the Mendocino triple Geophysical transects already junction. The slab presumably has a higher completed in northern California (for velocity than the overlying continental example, Tréhu, 1994, written commun.; lithosphere.