Project Overview and Motivation Tectonic Setting

Transcription

1 Project Overview and Motivation We propose an open-access 3D seismic reflection program across the Cascadia subduction system, offshore Grays Harbor, Washington (Fig. 1). The principal goals of this project are (1) to acquire a 3D seismic data set that will address critical science questions and leverage other infrastructure and scientific investments in the Cascadia subduction margin; (2) to expand the user base of the R/V Langseth by providing targeted training and at-sea experience for students, postdocs and early-career scientists; and (3) to provide a test case of a new open-access model that has been espoused by the scientific community for Langseth seismic data. Figure 1. Location of proposed 3D survey (red outline; solid red box on inset). The survey is 115 km long so as to extend from the trench to the upper continental shelf; the 15.5-km width is governed by ship endurance and survey configuration constraints. This proposal follows recommendations adopted at a March 2010 community workshop on the future of marine seismology ( This workshop was convened to develop community responses to several critical challenges facing the Langseth community, including high operational costs, few conducted cruises (~1-4/year), and long timelines to public data release (in practice, often >5 years). Because of these challenges, coupled with the increased specialization required to acquire, process and interpret 3D data, Langseth data may not be achieving their full scientific and educational impact. This proposal, which may be a first step in tackling these issues, represents the culmination of encouragement by, and contributions from, a Cascadia 3D Working Group that numbers over 50 scientists and students. We view this proposal as an umbrella proposal that will enable many future data analysis proposals; the role of the five project PI s is simply to shepherd this project through the planning and data acquisition phases. Because of this open-access nature, this proposal will depart from several of the usual tenets of NSF proposal writing. The data acquired here will be available to, and ultimately analyzed by, many scientists who are not PI s of this proposal; therefore, we do not request funding beyond data acquisition and industry processing. The Cascadia margin hosts a wide variety of important geological processes; therefore, we will not restrict our focus to only one or two key problems. Although this survey is designed to enable testing of critical hypotheses, others will likely conduct those tests; therefore, we make no claims to deliver conclusive hypothesis tests. Finally, although our project will impact students and early-career scientists through at-sea experience and short-course training in 3D seismology, we do not request science salary support for our own students and postdocs. We hope that reviewers of this proposal will be open-minded about its unorthodox nature. Tectonic Setting

2 In the past several decades, it has become widely recognized that despite very limited and localized seismicity on the plate interface (Trehu et al., 2008; Williams et al., in press), the Cascadia subduction zone is subject to megathrust earthquakes. Extensive paleoseismic work onshore and offshore has resulted in a 10,000 yr history of past quakes that suggests an average recurrence rate of ~500 years for magnitude ~9 events that rupture the entire subduction zone from northern California to the Nootka fault offshore Vancouver Island (e.g. Atwater, 1987; Adams, 1990; Nelson et al., 1995; Atwater and Hemphill-Haley, 1997; Goldfinger et al., 2003, in press). The last such event occurred on January 26, 1700 based on modeling of an orphan tsunami recorded in Japan (Satake et al., 1996; Atwater et al., 2005). The paleoseismic record, however, varies along strike, with smaller, more frequent events south of 45 o N and slip in the north only during the largest events (Goldfinger et al., in press). This apparent temporal variability in the spatial extent of slip, and consequently the magnitude of the resultant earthquake, is shown in Figure 2. Similar segmentation of the margin is reflected in several other parameters of the forearc, such as the along-strike length of forearc basins (Wells et al., 2003). Elsewhere, large forearc basins, as reflected by gravity anomalies, are frequently associated with large earthquakes, with event initiation occurring near the edge of the basin and maximum slip occurring in the center of the basin (Wells et al, 2003; Song and Simon, 2003; McGuire and Llenos, 2007). The growing recognition that Cascadia was a stealth subduction zone led in the 1980s and 90s to a series of onshore and offshore active-source, and broadband seismic experiments to image crustal structure along the arc, some results of which are summarized in Figure 3. Figure 2. Schematic illustration of the heterogeneity of slip regimes on a subduction plate boundary. The project will image the characteristics of slip within what geodetic, paleoseismic and geologic data suggest is a long locked patch of the megathrust seismogenic zone. It will also provide for the first time, a 3D seismic image of how the deep megathrust develops from a frontal thrust that is located just above basement in a region of landward verging folds. Compared to the region imaged in 3D in Nankai as part of SEIZE (e.g. Moore et al. (2009); Bangs et al., 2009), this segment of Cascadia represents the opposite end member for frontal thrust tectonics. Geodetic (GPS, tide gauge and leveling) data also suggest that the shallow, predominantly offshore, part of the plate boundary is currently locked and storing stress that will be released in the next megathrust earthquake (e.g. McCaffrey et al., 2007; Burgette et al., 2009; Weldon et al., in press), although there is considerable controversy about the significance of along-strike variations in apparent locking pattern and in the up-dip and down-dip extent of the locked zone. An apparent change in coupling behavior near 45 N is correlated with the apparent along-strike change in recurrence interval determined by paleoseismic methods. The northern part of the forearc off Washington is also characterized by a long, continuous basin that is not interrupted by along-strike basement highs (Wells et al., 2003), a feature that has been correlated with large earthquakes on a global basis (Wells et al., 2003; Song and Simon, 2003; Llenos and McGuire, 2007), although the correlation is imperfect (Teel et al., 2009). Nonetheless, when combined

3 Cascadia 3D Proposal, Draft 4c with paleoseismic and geodetic results, the data suggest that the northern Cascadia forearc is similar to other forearcs that generate earthquakes with rupture planes that extend for hundreds of kilometers and generate large tsunamis. The factors that control frictional heterogeneity within the nominal locked zone and the up-dip and down-dip boundaries of this zone remain poorly understood (Fig. 3). Figure 3. (A) Track lines of MCS and OBS seismic lines acquired in the 1980s and 90s. The red dash near 44oN represents the only existing 3D data from Cascadia [a high-resolution site survey for ODP Leg 204 (Trehu et al., 2003), which was repeated in 2008 (Bangs et al., 2009)]. Two lines from the 1996 ORWELL survey (Flueh et al., 1998; Fisher et al., 2008) are close to the region selected for this proposed crustalscale 3D survey and have been used to help identify project objectives and plan survey parameters. It is important to note that the Langseths s airgun array is stronger and better tuned and that the data quality will be far superior to that acquired by ORWELL. (B) Morphology of the Cascadia accretionary complex. Numbers indicate the sediment thickness at the deformation front. Note the marked change in character from seaward vergent to landward vergent folding near 45oN. (C) Schematic crustal sections across the forearc (Trehu, 2010; see references for a listing of studies used to construct this diagram). The discovery in Cascadia and Japan of episodic tremor and slip (ETS), during which large patches of the plate boundary downdip of the megathrust seismogenic zone slip slowly over periods of several days or weeks (Dragert et al., 2001), accompanied by seismic tremor (Obara, 2002; Rogers and Dragert, 2003), is perhaps one of the more exciting discoveries in Earth science of the past decade. Total moment released in a particular ETS event is comparable to a mag earthquake (e.g. Szelinga et al., 2007; Chapman and Melbourne, 2009). Much ongoing research is directed towards determining the temporal and spatial segmentation of ETS activity both on the scale of a single patch (e.g. Wech and Creager, 2008; Ghosh et al., 2009) and along the entire subduction zone (e.g. Brudzinski and Allen, 2007; Boyarski et al., 2010, in preparation). In the past decade, ETS has been found to be a widespread phenomenon that has been observed along plate boundaries of all types, although mechanisms for its generation are debated. Why 3D, why Cascadia, and why Grays Harbor? 3

4 Three-dimensional seismic reflection data provide a unique view of geological structures and processes, at horizontal dimensions from 10 s of km down to 10 s of meters, and at vertical resolution down to ~10 m. In geological environments where the key processes are expressed at those scales, and where they respond to or create patterns that cannot be adequately captured in two-dimensional transects, three-dimensional imaging is required. In many cases, the geological structures associated with key processes are either completely missed, or produce misleading interpretations, when sampled only in 2D transects. As inherently complex, three-dimensional systems, subduction zones are natural targets for 3D imaging (e.g., Moore et al., 2009). The Cascadia subduction zone hosts diverse geological processes that are poised to produce breakthrough science if they can be imaged in 3D. As will be described in more detail later, the dataset we propose will provide fundamental knowledge on the location of the (poorly known) plate boundary offshore Washington, the role of sediments and plate boundary roughness in controlling large earthquakes, strain partitioning in accretionary prisms, fluid budgets and dewatering of forearcs, geohazards (earthquakes and tsunami) from splay faults, plate boundary conditions updip of a zone of ETS, and the geological controls on methane hydrates and an extensive methane venting system that hosts unusual seafloor and water-column ecosystems. In addition, the Cascadia margin is becoming the focus of an impressive array of scientific initiatives and infrastructure investment. These include Earthscope, the Plate Boundary Observatory (PBO), the Ocean Observatories Initiative (OOI), GeoPRISMS (the MARGINS successor program), and the ARRA Cascadia Initiative, which is investing in new ocean-bottom seismometers (OBS), and onshore seismometers and geodetic stations. Indeed, GeoPRISMs (the MARGINS successor program) has recently selected Cascadia as a focus site, and the ARRA OBS deployment plan calls for a concentration of instruments to coincide with our proposed 3D corridor (Fig. 4). The CAFÉ transect of portable seismometer deployments onshore (Abers et al., 2009) lies directly onshore from our proposed corridor, and the Ocean Observatories Initiative Grays Harbor array will be deployed immediately adjacent to our 3D box. Open-access 3D images of the subduction zone offshore will complement and leverage those investments, and the confluence of assets in the area guarantees high interest and a long shelf life for the 3D data. Figure 4. Detail of proposed ARRA oceanbottom seismometer positions, showing a focused deployment designed at the October 2010, Portland, workshop to coincide with our proposed 3D reflection survey. Given the limited area that can be imaged in a single 3D campaign, we have selected the area that we believe offers the highest bang for the buck in the first 3D survey of the Cascadia subduction margin: offshore Grays Harbor, Washington. We chose this region for several reasons: (1) This area marks the central part of a large segment of the Cascadia subduction zone that only ruptures in large earthquakes. (2) This segment is close to Seattle, the largest population center on the Cascadia subduction zone. (The Grays Harbor region is as close to Seattle as offshore seismics can be conducted, given the Olympic Marine Sanctuary.) (3) The large forearc basin centered on this study area will enable hypothesis testing about the characteristics that allow 4

5 a plate boundary to slip in large events. (4) This region is at risk from tsunamis from megathrust events and from splay faults on the shelf and slope that are poorly mapped. (5) 3D imaging will identify the location of the plate boundary, which is poorly constrained offshore, because of the low levels of plate boundary seismicity. (6) This survey will link to recent onshore broadband experiments (e.g., CAFE, Abers et al., 2009); together, these projects will provide unprecedented views of the subduction system from trench to volcanic arc. Scientific Objectives The survey we propose will provide unique insights into multiple important geological processes that occur in subduction zones; here we summarize several outstanding scientific objectives. This is not an exhaustive list; the panoply of processes expressed on subduction margins extends beyond those listed here. Space limitations prevent us from describing each of these scientific goals in the full detail they deserve; entire proposals can (and, we hope, will!) be written on each of these topics. Moreover, because of their unique resolution and synoptic view into geological structures, 3D data always provide surprises. This discovery aspect of 3D imaging is difficult to predict but worth remembering. Where is the offshore plate boundary in the Cascadia subduction zone? The location of the plate boundary is difficult to pinpoint in much of the Cascadia subduction zone, due to the relatively low levels of Wadati-Benioff zone (McCrory et al., 2004) and plate boundary (Trehu et al., 2008) seismicity. Recent onshore teleseismic work has detected a prominent low-velocity zone beneath the forearc (e.g. Xi and Nabelek, 1999; Bostock et al., 2002; Audet et al., 2010). Several studies have concluded that this LVZ must represent descending oceanic crust (Audet et al., 2010); however, this interpretation is puzzling because the layer appears to change thickness significantly along strike. By illuminating the tectonic development of the plate boundary offshore, the proposed survey may help resolve whether the teleseismic LVZ represents (1) variably imbricated subducting oceanic crust, (2) oceanic crust that has undergone metamorphism, or (3) a combination of oceanic crust and overlying sediments, perhaps in a subduction channel. Fig. 5. Migrated S-wave (top) and P-wave (bottom) teleseismic images from Abers et al. (2009), showing earthquake locations (cirles and stars). Coastline is at 0 km. Proposed 3D seismic survey would be just offshore of this teleseismic image and would provide critical high-resolution images of the downgoing plate structure that would inform the interpretation of this image and others like it (e.g., Rondenay et al., 2008; Audet et al. 2010). Controls on megathrust earthquakes: sediments and plate boundary roughness. Wells et al. (2003) and Song and Simon (2003) showed a strong correlation between great earthquake rupture areas and forearc basins mapped by global gravity data, suggesting that forearc structure is linked to great earthquakes (Mw 8.5+) and deeper subduction processes by basal erosion. Along Cascadia, five such large basins may indicate potential asperities at depth. 5

6 Our proposed cross-section crosses the longest of these, the Willapa Basin. We will image lateral variations in plate roughness and potential asperities in the plate interface beneath this basin. One of the fundamental unanswered questions of earthquake seismology is what controls whether an earthquake rupture continues to propagate (e.g Brodsky et al. 2009). Because the historic record of megathrust events is sparse, it is difficult to find common characteristics (Scholl et al., 2010). Most megathrust earthquakes have occurred where the downgoing plate is relatively young (and therefore buoyant) and covered by a thick layer of sediment. (The 1964 Gulf of Alaska is an exception, but the presence of overthickened Yakutat Block crust may increase the buoyancy of the subducted plate there.) Moreover, topographic features on the subducting plate have been correlated with segment boundaries during large earthquakes (e.g. Sparkes et al., 2009). These observations lead to the hypothesis that fault planes that are capable of very large ruptures are smooth and form high enough in the accretionary prism to bypass basement roughness on the subducting plate but deep enough to be within sedimentary rocks characterized by frictional parameters resulting in velocity weakening. Such fault planes should be imageable with seismic reflection data. However, scattering from complicated seafloor and shallow sedimentary topography, and high attenuation in gas-rich accreted sediments, may explain the fact that the decollement cannot be followed for more than ~10-40 km landward of the deformation front in most seismic reflection data from Cascadia (e.g. MacKay, 1995; Trehu et al., 1995; Gulick et al. 1998; Fisher et al., 1999, 2008; Flueh et al. 1998; Booth-Rea et al., 2008), with only occasional observations of plate boundary reflectivity further landward. A 3D swath across the accretionary complex off Washington acquired with a broadband source will allow us to separate effects of shallow scattering from true variation in plate boundary reflectivity. Quantification of pore fluid pressure, fluid budgets, and upstream inputs to the ETS zone. Subducted sediment carries a large volume of water down-dip along the subduction interface. In low-permeability marine sediments, the rates of tectonically driven burial and mineral dehydration can outpace fluid escape, leading to poorly drained conditions and significant pore fluid overpressures both within the subducting sediment and along the plate interface [e.g., Neuzil, 1995; Tobin & Saffer, 2009]. Such fluid-rich and overpressured zones are interpreted to be present in many subduction zones (e.g., Nankai [Tobin & Saffer, 2009; Skarbek & Saffer, 2009; Tsuji et al., 2008; Screaton et al., 2002], Costa Rica [Ranero et al., 2008], Cascadia [Yuan et al., 1994; Audet et al., 2009]), and are hypothesized as a potential cause of overall mechanical weakness, as well as a prerequisite for episodic tremor and slip (ETS) [e.g., Ito et al., 2006?]. Fluids are also thought to impact the structure of the accretionary prism, as variations in pore pressure may explain along-margin changes from landward- to seawarddipping frontal thrusts (e.g. MacKay et al., 1995; Underwood et al., 2002), and may be the cause of the anomalous pattern of apparent heat flow offshore Cascadia determined from BSR observations (e.g. Hyndman et al., 1993; Trehu, 2006). Faults (e.g. Sample et al., 1993) and permeable stratigraphic horizons (Trehu et al., 2004) in the prism can act as conduits for the fluids to the seafloor. Fluid-rich sediment can be subducted either via trench sediment entering the subduction channel or by basal erosion of the upper plate. Although no sediment appears to be subducted at the deformation front at present along the segment of accretionary complex chosen for this survey (Flueh et al., 1997), we speculate that the plate boundary moves up through the sediment column. Fluids have a strong impact on seismic velocities (Yuan et al., 1994) and the reflection character of the plate interface and can be identified/interpreted from variations in those properties [e.g., recent Phil Barnes New Zealand paper; Ranero et al., 2008]. Flueh et al. (1997) note variations in the strength of the interplate reflection along the Cascadia margin on 2D lines but are unable to distinguish between processing artifacts and true variations, and thus cannot resolve in situ fluid conditions. The high reflectivity of landward-verging faults in the 6

7 accretionary prism may result from elevated fluid content and/or pressure within or below the faults [e.g., Bangs et al., 2009], and detailed analysis of high-resolution seismic data will provide an avenue to evaluate these hypotheses. Sediments along the Cascadia margin would likely contain high volumes of fluid, as rapid deposition from Pleistocene glaciation would have trapped fluid near and outboard of the trench before it could escape. As discussed earlier, Cascadia is one of the primary study sites for the phenomenon of ETS. By constraining the amount of sediment being subducted, it should be possible to estimate the amount and distribution of water influx into and escaping from the "subduction conveyor" along this segment of the margin by estimating porosity of the subducted sediments and the evolution of porosity downdip from the trench, approaching the zone of ETS [e.g., Bray & Karig, 1985; Cochrane et al., 1996; Skarbek & Saffer, 2009]. An analysis of heat flow as indicated by seismic observations of the BSR can also be used to constrain sediment compaction, pore pressure and fluid flow. Throughout Cascadia, heat flow is lower than predicted by a steady-state model several tens of km landward of the deformation front; it then increases to approach the steady-state prediction (e.g. Hyndman et al., 1993; Wang and Davis, 1993; Trehu, 2006a,b). The data will image along-strike and along-dip variations in the depth to the BSR and allow correlation of apparent heat flow to sediment compaction and fluid conduits. By imaging variations in the reflectivity of the plate boundary and forearc faults, and variations in depth to the BSR, the community can probe the following questions: How is fluid distributed at the plate boundary interface? Is the plate boundary over-pressured, and does this vary spatially? Do faults in the forearc serve as fluid conduits to allow fluid escape? What volume of fluid is subducted down-dip into the zone of ETS? Geological controls on methane distribution in an accretionary margin. Current environmental interest in methane, CH4, is due to its greenhouse warming potential, which is >20 times that of carbon dioxide on a per molecule basis [Khalil and Rasmussen, 1995], and recent studies suggest that methane s radiative impact over its entire atmospheric residence time could be ~30% of the impact of CO 2 [Shindell et al., 2005]. Of this natural flux, marine seeps contribute an estimated ~15 23% [Kvenvolden and Rogers, 2005], although the sources, emission rates, and variability are very poorly known. Although CH 4 seepage exists on all continental margins, very few studies connect the underlying geological processes in the shelf sediments to the locations of seafloor methane seeps [Hovland et al., 2002; Judd et al., 2002]. Although the Washington margin has been recognized as a potential source of thermogenic hydrocarbons for some time [Palmer and Lingley 1989; Martin et al, 2007], with abundant images recording BSRs within the upper margin sediments [Booth-Rea et al, 2008], recent work in our proposed study area has discovered a system of methane vents that appear to support complex ecosystems. Recent cruises to the continental shelf off Grays Harbor, Washington, have discovered numerous active seafloor methane seeps at depths of m. These vents support rich ecosystems with organisms at all trophic levels, from bacterial mats to massive krill swarms to high-level predators, and methane gas bubbles from one well-defined pockmark were observed to reach the sea surface. 3.5 khz surveys around these vents show clouds of acoustic scatterers, which are caused by methane bubbles and zooplankton swarms, both of which have been observed by towed cameras and ROV surveys. These observations suggest that geologically controlled methane venting provides an important non-seasonal energy source to a shelf ecosystem that is one of the most productive commercial fisheries in the entire NE Pacific. The recently discovered methane vents on the WA margin, along with similar vents off Oregon [Collier and Lilley, 2005; Torres et al., 2010], point to a potentially widespread regional system of marine methane leakage from deep geological reservoirs into the ocean and atmosphere, with potential implications for climate and biological productivity. The sources of this gas, as well as the mechanisms that control the location of seafloor vents, are unknown. Our 7

8 proposed survey will image the geological controls on this actively venting methane system, including the source, migration pathways, and controls on venting. Archive industry seismic profiles show numerous direct indicators of gas in the area (bright spots, flat spots, BSRs), as well as features that may contribute to gas migration and concentration, including listric normal faults, mud diapirs (Fig. 6), and turbiditic channels. Because of the nature of these structures, only 3D seismic data can illuminate the complex interactions that control methane venting. Our data will enable hypothesis-testing about the source and plumbing of both the shallow methane vent system and the methane hydrate system farther offshore, centered on two overarching questions that are proxies for the entire NE Pacific margin: (1) What are the sources of biogenic and thermogenic methane gas on the Washington shelf and slope, and are they linked or distinct? (2) What are the geological controls on methane venting on the Washington shelf? More specifically, these data will address the following issues: Are the sources of methane gas on the shelf and slope linked or distinct? Is the methane flux on the Washington shelf deeply sourced (thermogenic)? Are methane vents on the Washington margin controlled by structural elements, such as faulting, stratigraphic architecture, such as mud diapirs and turbiditic channels, or both? Are listric faults on the margin (Fig. 6) lubricated by fluid-charged zones? Subduction channel The Cascadia margin is a sediment-rich system, and the accretionary prism along the margin is actively growing. Along other sediment-rich margins, some thickness of sediment is commonly not accreted, but subducted down with the oceanic plate. Examples of sediment subduction include Ecuador [Sage et al., 2006; Calahorrano et al., 2008], southern Chile [von Huene et al., 1997; Diaz-Naveas, 1999; Ranero et al., 2006], Costa Rica [Kimura et al., 1997; Saffer, 2003]; and Nankai [Park et al., 2010]. This subduction of sediment carries water down between the plates, may lubricate the plate interface by smoothing local roughness, and has profound implications for the properties of the plate interface. Sediments in the subduction channel may also come from basal erosion of the overriding plate. In the region of our study, Flueh et al. [1997] interpret from 2D MCS data that the entire thickness of sediment on the subducting plate is accreted and none is carried down into a sediment channel. However, to the north near Vancouver Island, a series of reflectors >4 km thick known as the E-layer [Nedimovic et al., 2003; Calvert, 2004] is often interpreted as subducted sediments [Nedimovic et al,. 2003]. The discrepancy between these two regions could represent a large variation in the amount of sediment laterally along the margin (although this is not apparent at the trench), variations in the thickness of the subducted sediment layer with depth (representing a time dependence to subduction vs. accretion of sediments), or a processing/interpretation artifact between the two areas. Deep imaging in the Sonne MCS data [Flueh et al., 1997], unlike the SHIPS data [Nedimovic et al., 2003], was hampered by strong water-bottom multiples. The 3D nature of our survey and longer streamer cables will enable superior multiple attenuation, improving deep imaging. Questions that can be addressed regarding the subduction channel include: Is sediment subducted between the upper and lower plates at all sediment-rich margins? What effect do these sediments, if subducted, have on the properties of the plate interface? At what depth does dewatering of the sediment create a zone of fluid pressure at the plate boundary? What variability exists along the margin in the thickness of the subducted sediment channel, and what effect does this have on seismogenic margin segmentation? Accretionary prism structure and geohazards The accretionary prism off Washington is composed of thin mudstone deposits alternating with layers of fine sand/clay, indicative of the accretion of abyssal turbidite deposits (McNeill et al. 1997). Beneath the Pliocene strata lies the Eocene to middle Miocene mélange and broken formation (MBF) which appears to be underconsolidated and/or overpressured and mobilizes in 8

9 mud diapirs (McNeill et al., 1997). The folded upper Pliocene strata have been horizontally truncated during past low sea level stands and are partially overlain by modern sediments (Sternberg, 1986; McNeill et al., 1997). Structures in the accretionary prism are a complex mix of compressional, extensional, and strike-slip structures to accommodate the oblique convergence of the subducting Juan de Fuca plate, including thrust faults, fault-bend folds, and extensional faults that deform the accretionary wedge and overlying basin fill. Compressional faults in the accretionary prism dip landward in southern Oregon and off Vancouver Island, as commonly seen in global subduction settings, but seaward off northern Oregon and Washington. Extension in our area often takes the form of spectacular listric faults that contain tilted growth strata and turn sub-horizontal as they sole into the MBF at depths of ~2 km (Fig. 6). The MBF, which has velocities of km/sec, likely plays an important role in decoupling the deeper subduction-associated compression from regional normal faulting characteristic of the northern Cascadia continental shelf (McNeill et al., 1997). Some of the listric faults cut the seafloor and appear quite young in seafloor photos, suggesting a possible earthquake hazard. In places, amplitude anomalies suggestive of gas are associated with these faults, suggesting a possible role in gas migration. Features interpreted as mud diapirs are thought to result from the mobility of the MBF, but are only imaged on 2D seismic lines. The nature and structure of the transition between the active compressional faulting on the lower slope and the active extensional listric faults on the shelf and upper slope is uncertain (McNeill et al., 1997; Fisher et al., 1999). McNeill et al. (1997) suggest that the coeval E-W compression and E-W extension result from isolation of tectonic regimes between the slope and the lower shelf. While many of the normal faults sole into the MBF, other normal faults are planar and cut the MBF (McNeill et al., 1997). Some of these faults may potentially have strikeslip offset. The complex deformation pattern observed is indicative of strain partitioning along the margin, which has implications for seismic hazards. Along this part of the Washington margin, splay faults arising from the plate interface and cutting through the accretionary prism have not been identified. Our data will allow imaging of the interaction of these complicated fault systems and address the following questions: Do the high-angle faults imaged have both dip-slip and strikeslip offset? Do the high-angle faults or listric normal faults pose individual earthquake hazards? Are there unidentified splay faults from the plate boundary through the accretionary prism? Are these faults conduits for gas migration and are important in the methane system? Fig. 6. Archive industry seismic data on Line WO-48 (used in Fig. 5 of McNeill et al., 1997), showing clear listric normal faults. The fault at left becomes nearly horizontal and shows amplitude anomalies at its base. D = mud diapir; P = seafloor pockmark. Proposed Work R/V Langseth Cruise. We propose a cruise of R/V Langseth to acquire new seismic data on the WA continental margin. The 3D survey will comprise a 15.5 by 115 km area, from just seaward of the trench to the upper continental shelf, in a location selected to contain examples of all the key targets. The 3D survey is longer than it is wide, as that is the most efficient design to maximize survey area per cruise day (fewer turns). 9

10 Cascadia 3D Proposal, Draft 4c Because our target depths for 3D imaging extend down to the subducting slab, we will use four 6-km-long streamers spaced 200 m apart. We will shoot alternate double-string (18-gun, 3300 cu.in.) sources every 25 m. To shoot this box will require 39 sail lines spaced 400 m apart in a single racetrack pattern. This geometry will provide 8 subsurface lines per sail line, with 50 m cross-line spacing. Acquisition of 3D MCS data requires data collection beyond the area in which final imaging is desired, due to the need to build complete CDP fold and to record energy from diffractions which will later be migrated into the inner volume. The 15.5 x 115 km area is the entire shooting area, including the migration aperture: flat reflectors will be fully imaged within this box. For dipping targets, the width of the fully migrated volume shrinks as a function of reflector dip (migrated widths are 11.5 km for 10 dips and 7.5 km for 20 dips). However, we note that, in general, the steepest dips will occur in the in-line direction of shooting, so these estimates of the migration aperture are conservative, and in many areas, more of the full width will contain fully migrated data. The cruise plan calls for a single 44-day leg of Langseth, with 42 science days at sea, plus transit, broken down as follows: 4 days to deploy the 4x6 km 3D streamers and guns; 29 days to shoot the primary 3D box; 7.5 days for infill 3D shooting and maintenance; and 1.5 days to recover the 3D seismic gear. Midway through the cruise, a contracted chase boat will conduct an at-sea crew swap of the student participants (see Broader Impacts). During the cruise, we will record important ancillary data, including EM120 multibeam bathymetry, 3.5 khz (Langseth Knudsen system), ADCP, and gravity data. This cruise meets the endurance of the Langseth for 3D shooting, given the fuel consumption rate. That constraint, together with (1) the scientific imperative to traverse the entire margin from the trench to the upper continental shelf, (2) NSF feedback on cruise cost, and (3) significant cost inefficiencies that result from breaking the program into two legs, dictate the 15.5-km width of the 3D box. Fig. 7. Example of Blake Ridge 3D seismic data set, as visualized in OpendTect software. This data set has been uploaded to the dgb Open Data Repository, where it is freely available in OpendTect format. We propose a similar release path (as well as standard UTIG and LDEO data bases) for our proposed 3D data. Data Processing and Release Plan. In order to ensure that the data acquired here are processed and released in a timely fashion (within ~6 months of the cruise), we will contract a commercial processing firm to do the initial data processing. Prestack data will be immediately available through the LDEO data center. A 3D prestack time migrated (PSTM) data volume will be publicly released, both in SEGY format through the UTIG data base and in OpendTect format on the OpendTect Open Seismic Repository ( dgb Earth Sciences have agreed to convert the 3D data into OpendTect format (see letter of support in Appendix). We have successfully tested this approach using the 2000 Blake Ridge 3D data set, which we provided to dgb for conversion to OpendTect format, and which is now downloadable on their site (Fig. 7). There are several key advantages of using the OpendTect software: it is free, multiplatform (Mac, Windows, Linux), and open-source; dgb s willingness to convert the data into OpendTect format substantially increases data access; the software is user-friendly, which 10

11 lowers the bar for users to access and work with the data (in our experience, geology undergraduate students can quickly learn this software); and the participants in our education program will be trained in the use of this software. Budget Our budget request to NSF includes only the funding necessary to accomplish the planning and acquisition of this 3D data set, to conduct training and outreach activities, and to have the data set commercially processed. Once processed, the data will be immediately made available to the scientific community for data analysis; we anticipate that numerous proposals will be submitted for data analysis. The community training component (short courses in Laramie) will be funded by a matching grant of $95,308 from the University of Wyoming s School of Energy Resources (SER). This grant will cover the costs of travel, housing, per diem, and local transportation of ~20 participants for the two short courses, as well as one month of salary for Holbrook to prepare and present the short courses. Project Deliverables We will produce one major, tangible product from this work: a public, commercially processed 3D PSTM data volume across the Cascadia subduction zone. A multitude of later, detailed analyses will be possible using this data set (e.g., prestack depth migration), which will be immediately available for the community to conduct. The availability of these data should stimulate data analysis and interpretation proposals to NSF; hopefully this will result in an array of analysis approaches, applied by a broad variety of researchers. We expect this to be an important test case for future open-access 3D cruises. In addition, we will advertise the data set by writing an EOS article announcing and describing the 3D data set, and via an oral and poster presentation at the first AGU meeting following production of the processed data set. Finally, we expect that our project will produce meaningful intangible products, in the form of an energized and enthusiastic community of students and early-career scientists who are conversant in the technical aspects of 3D seismology, experienced in at-sea 3D acquisition, and trained in the analysis and interpretation of 3D seismic data volumes. Broader Impacts: Education and Outreach In addition to science goals, our project places strong emphasis on an integrated and comprehensive education and outreach program. A keystone of our proposal and, indeed, one of its primary motivations is a comprehensive education program that will (1) provide training and at-sea experience in 3D seismic methods to early-career scientists and graduate students in seismology; and (2) involve high school students from a local coastal Native American Nation high school, who will learn about the continental margin environment around them, work with University students and faculty in the intensive environment of a sea-going scientific program, benefit long-term from programs incorporating these research experiences, and be exposed to marine sciences as possible career alternatives. We describe these efforts below. Langseth 3D Seismic Education Program. The primary educational goal of the Langseth program is to begin to train the next generation of scientists in 3D reflection seismology. Why is this needed? The Langseth has introduced an entirely new capability into the U.S. academic community: swath 3D seismic imaging. Designing, acquiring, processing, and interpreting 3D seismic surveys are substantially more complex than for 2D seismic data. While a relatively large community of trained 2D users exists, the user base of academic scientists skilled in 3D seismic work is small (and graying). Moreover, the academic active-source seismology community has not attracted many new young scientists, due to several factors, including the gap in service between Ewing and Langseth, perceived challenges in obtaining NSF funding, and increased 11

12 hiring by industry. These facts raise significant concerns about the long-term viability of the research community best situated to use the remarkable, new national facility provided by Langseth. In addition, the substantial expense of 3D surveys places a heavy burden on proponents to conduct projects with broad community support and participation, in places where non-seismologists can extract value from the resulting images. These issues were thoroughly discussed at the March 2010 Incline Village workshop, and our proposal follows the consensus model for open-access projects. Our program will provide hands-on experience in 3D data acquisition aboard the Langseth and short courses in seismic processing and interpretation. Together these experiences should provide a cadre of ~20 young scientists trained in 3D reflection seismology. While this is only a start toward a robust Langseth community, we hope that, if successful, our project might serve as a template for broader community participation in future 3D seismic surveys. The 3D cruise (Fig. 1) will be 44 days in length; in order to maximize at-sea participation, we will cycle up to 30 participants through the cruise, split into three groups of 10 who will each spend ~14 days at sea and be transferred by a small boat from the nearby port of Westport, WA (40 miles from our work area). During the cruise, participants will conduct watchstanding duties, receive lectures on the project s science, and participate in initial data processing and interpretation. Because of the unusual staffing of our cruise, we will place particular emphasis on pre-cruise planning and will work very closely with the Operator (LDEO) to plan logistics, berthing, staffing, and processing. In addition to the cruise, Langseth participants will be offered two short courses (each of one week duration), which will be held at the University of Wyoming and funded by a matching grant from the UWyo School of Energy Resources: (1) a reflection seismology short course, and (2) a 3D seismic interpretation course using the industry-processed 3D volume from the Washington margin. Course 1 will be held prior to the cruise, and will cover basic reflection seismology principles, 3D planning and acquisition principles, and an introduction to 3D interpretation. Course 2 will be held ~6 months after the cruise and will focus on interpreting the 3D volume acquired here, using OpenDTect software in UWyo s two modern, 12-seat PC labs. Participants in cruises and short courses will not be principally from the PIs home institutions, but rather will be selected from nationwide applications (participating students from the PIs home institutions will be limited to at most two). The opportunity will be widely advertised, and the selection process will be open and fair; we will ask the Marcus Langseth Science Oversight Committee (MLSOC) to rank the applications for participation in the program. (Three PI s Holbrook, Kent, and Johnson are currently MLSOC members but will recuse themselves from this discussion.) Two key aspects of the post-cruise data handling will ensure the success of our community participation model: rapid commercial processing of the 3D data, and an open data policy. As discussed above, commercial 3D processing guarantees a fully interpretable product for use in the post-cruise 3D interpretation workshop, timely release of data, and rapid dissemination of research results. We will adhere to a fully open data policy, fully releasing all 3D seismic data, including the processed 3D volume, upon receipt. This will promote broad use of the data set, for both research and teaching, and help ensure maximum value of the results in the planning process of the GeoPRISMS Cascadia Focus Site. As a tangible wide-use education product, we will produce a mini-lesson incorporating 3D seismic data for the MARGINS Data in the Classroom project, thus guaranteeing an education product with a long shelf life and broad audience. Local Community Outreach. Several components of our proposed 3-D MCS survey have the potential to impact local communities; any possible negative impacts must be identified in the early planning stages of the experiment, and steps taken to mitigate this influence. To this end, the experiment should be (a) conducted with the full informed consent of the local communities and stakeholders, (b) the scientific rationale for the experiment should be explained in the clearest 12

13 possible terms, (c) the results of the cruise and post-cruise analyses should be available to them in a readily-accessibile format, and (d) the experiment should be designed to have a maximum positive residual impact on these communities. The area of the proposed survey on the Washington margin off-shore Grays Harbor is the traditional and treaty fishing zone for the Quinault Indian Nation, and support from this community will be required at the earliest planning stages. The proposed survey area has very productive rockfish, tuna, sablefish and crab fisheries, is located on seasonal whale migration route, and is an active coastal transit zone for both commercial tug boats and U.S. Navy vessels. Although the Washington margin has been the site of many multichannel seismic surveys over the previous 30 years, our proposed extensive 3-D MCS survey with modern seismic instrumentation and conducted over a 40-day period will require frequent and early communications with the local communities, transparency regarding survey plans, and the maximum possible involvement by local high school students, both prior to and during the cruise. The intense scientific activity taking place on the WA margin over the next decade represents a unique opportunity for informing an under-served community in an economically depressed area of possibile career choices for local students that would not have occurred otherwise. We propose to take advantage of this opportunity by conducting several public and school information sessions in the communities involved in the survey area, and are proposing actual hands-on participation by the students by (a) pre-cruise involvement in the project planning by local teachers and tribal/community leaders, (b) conducting organized tours of the ship, where the context of the scientific experiment is fully demonstrated, and (c) participation of local students on the cruise, where they can act as liason with their shore-based communities. The Co-PIs of this proposal take the responsibility for positive outreach very seriously, and we are committed to having the local communities see some direct benefit from our survey. In addition to fulfilling the federal permitting requirements for the R/V Langseth (conducted exclusively through NSF and including potential impact on marine mammals, and operation permission from the U.S. Navy), we will contact the groups listed below to inform them of the survey plan, timing, and of the possible impact of the Langseth on their activities. Input from these groups will be solicited early in the cruise planning process: (a) Quinault Indian Nation and the non-tribal commercial fishing fleet; (b) Other coastal tribal groups who have fishing interests in the area; (c) Crab Fishermen Association; (d) Washington Marine Resources Committee; (e) Coastal Towboat Operators Association; (f) Grays Harbor community organizations; (g) Washington State Environmental Councils; and (h) NGOs with interests in the Grays Harbor area (i.e., Nature Conservancy, Oceana). Interactions with the local school boards, community colleges and science teachers in the area will also be done at this early phase. Workshops. We propose to hold two invitational workshops in the early planning stages of the cruise planning in order to allow implementation of suggestions from the participants, which would include community and state representatives, tribal leaders and professional association representatives, educators, NGOs, and the five proposal PIs. These would be funded by the present proposal and we have been strongly encouraged to proposeadditional organizational and financial support from Washington Sea Grant (Ms Penny Dalton, WASG Program Manager; pers. Comm., 2010). The workshops would be held on the WA coast, for easy local access. The agenda would include presentation of scientific goals, geological overview, draft cruise plan, outreach plans, educational components and question-and-answer periods. Input would be requested from the local groups regarding possible modification of the experimental plans and how best to communicate with local schools and community groups, and to identify possible unforeseen negative impacts. One goal would be opening of communication channels between communities and scientific personnel (PIs) regarding the MCS experiment. One workshop would be held early in the planning process (summer 2011) and one held prior to the cruise (spring 2012). 13

14 Public Lectures. In addition to the invitational workshops, we propose a series of public lectures in different locations on Washington coastal communities. These would be conducted by the five PIs. Given a total of 5 public lectures, two would be held prior to the cruise (early 2012), two during the cruise (Summer, 2012) and one post-cruise (Fall, 2012). If logistics allow, one of the public lectures would be coordinated with public visits to the Langseth, assuming one port is Astoria. Ship tours would emphasize educational aspects for local student groups rather than the traditional tourist visits, although non-educational groups would also be encouraged to see the ship. Funds for the lecture series and ship tours are included in this proposal. Chase Boat Chartering. The Langseth MCS 3-D survey will require the chartering of a lead boat to precede the ship during the cruise, for the duration of the survey. We have conducted discussions regarding this issue with the Langseth group at Lamont, and these boats would be chartered from the local community, with the Quinault fishing boats given priority. These boats have a 3 or 4 day endurance, requiring a rotational cycle of different boats during the full cruise. These boat-rotation cycles provide opportunities for (1) additional ship-of-opportunity science, (2) rotation of personnel, and (3) short rotations of students onto the Langseth, where 1 or 2 berths for this purpose will be reserved during the cruise. The charters would also provide income to the local fishing fleet, which is largely idle during the non-fishing season. Student Participation. Our planned study will contribute to the broader educational goals of NSF for education and human resource development. We plan to select four high school students from a local Quinault reservation to participate on the cruise so that it may advance their education and future career goals. These students will participate by assisting watch standers, completing daily educational exercises led by a graduate student, and recording their experiences in an online video blog. At conclusion of the cruise, the video blog and exercises will be published as an online resource for high school students and educators (similar to the virtual voyage college curriculum developed by Don Reed). Summary and Expected Results We expect to produce a PSTM 3D seismic data volume that will provide fundamental new knowledge on geological processes on the Cascadia subduction zone. The images provided will address key questions about the location, dip, and roughness of the slab beneath the Washington margin, fluid budgets on the margin, the presence or absence of a subduction channel of downgoing sediments, structural controls on earthquake and tsunami geohazards in the region, and the distribution and controls on methane in the margin sediments. Our project follows a new model for community participation and open data access that will maximize scientific productivity and provide an alternative template that may be useful for future 3D work on the Langseth. The educational component of our project will combine coursework and at-sea experience targeted at students and early-career scientists in seismology. We also plan a shipboard program targeting representative high school students from the Quinault community, who would act as liason with their local communities, via web sites with personal experiences, videos of shipboard operations, and cruise results. The tangible products of this education program will include (1) a cadre of early-career U.S. scientists and students with training and experience in the design, acquisition, processing, and interpretation of 3D seismic surveys; (2) a mini-lesson incorporating 3D seismic images of methane systems for the MARGINS Data in the Classroom project; (3) videos and blogs of the science experience produced by tribal students who participate in the cruise. Finally, our project will enable important synergies to ongoing and nascent science and infrastructure programs in the area, including GeoPRISMS, the Ocean Observatories Initiative, the ARRA seismometer deployment, and Earthscope. 14