SHIPS - Georgia Basin and Vancouver Island

Brocher, T.M., T.L. Pratt, G.D. Spence, M. Riedel, and R.D. Hyndman, 2003, Wide-angle seismic recordings from the 2002 Georgia Basin Geohazards Initiative, Northwestern Washington and British Columbia, U.S. Geological Survey Open-File Report 03-160, 34 p.

This report describes the acquisition and processing of shallow-crustal wide-angle seismic-reflection and refraction data obtained during a collaborative study in the Georgia Strait, western Washington and southwestern British Columbia. The study, the 2002 Georgia Strait Geohazards Initiative, was conducted in May 2002 by the Pacific Geoscience Centre, the U.S. Geological Survey, and the University of Victoria. The wide-angle recordings were designed to image shallow crustal faults and Cenozoic sedimentary basins crossing the International Border in southern Georgia basin and to add to existing wide-angle recordings there made during the 1998 SHIPS experiment. We recorded, at wide-angle, 800 km of shallow penetration multichannel seismic-reflection profiles acquired by the Canadian Coast Guard Ship (CCGS) Tully using an air gun with a volume of 1.967 liters (120 cu. in.). Prior to this reflection survey, we deployed 48 Refteks onshore to record the airgun signals at wide offsets. Three components of an oriented, 4.5 Hz seismometer were digitally recorded at all stations. Nearly 160,300 individual air gun shots were recorded along 180 short seismic reflection lines. In this report, we illustrate the wide-angle profiles acquired using the CCGSTully, describe the land recording of the air gun signals, and summarize the processing of the land recorder data into common-receiver gathers. We also describe the format and content of the archival tapes containing the SEGY-formated, common-receiver gathers for the Reftek data. Data quality is variable but the experiment provided useful data from 42 of the 48 stations deployed. Three-fourths of all stations yielded useful first-arrivals to source-receiver offsets beyond 10 km: the average maximum source-receiver offset for first arrivals was 17 km. Six stations yielded no useful data and useful first-arrivals were limited to offsets less than 10 km at five stations. We separately archived our recordings of 86 local and regional earthquakes ranging in magnitude from 0.2 to 4.3 and 16 teleseisms ranging in magnitude 5.5 to 6.5.

Dash, R.K., G.D. Spence, M. Riedel, R.D. Hyndman, T.M. Brocher, and T.L. Pratt, 2003, Crustal velocity structure from first arrival seismic tomography in the Strait of Georgia, British Columbia and Washington, Eos Trans. AGU, 84(46), Fall Mtg. Suppl., Abs. S42A-0150.

Seismic refraction/wide-angle reflection data were collected in May 2002 in the Strait of Georgia, southwestern British Columbia and northwestern Washington. The experiment, part of the Georgia Basin Geohazards Initiative, provides knowledge about potential geohazards associated with near-surface crustal faulting in the strait. The last significant earthquake (M = 4.6 in 1997) within the strait reflects the regional seismic hazard. The seismic survey was aimed at studying the three-dimensional (3D) velocity structure of the shallow upper crust particularly the Cenozoic sedimentary basins, the Outer Island fault and the Lummi Island fault. The data were acquired using 48 Reftek stations deployed onshore to record the signals from a 120 cu. in. airgun fired at a nominal interval of 5 sec or 12 m. Single channel reflection data were collected simultaneously. The experiment provided useful data from 42 of the 48 stations deployed. The data quality at many receivers was very good. The average maximum source-receiver offset for first arrivals was 17 km with a maximum offset of 35 km. Preliminary 2D velocity modeling indicates three layers in the shallow upper crust down to a maximum depth of 3 km. The uppermost layer in the strait represents recent unconsolidated sediments. The second layer consists of basin fill sediments (velocity 4.1- 4.4 km/s) associated with the Upper Cretaceous Nanaimo Group. These sediments occur at the surface within the Canadian Gulf Islands but at a depth of about 700 m within the strait. The underlying basement layer (velocity 5.3 - 5.7 km/s) at a depth of 1.7 km possibly represents the volcanic Wrangellia terrane, extending from Vancouver Island beneath the strait. The wide distribution of receivers in the islands and shots in the surrounding waters provides excellent constraints for 3D inversion of first arrival travel times and hence for a high-resolution 3D image of the shallow structure.

Graindorge, D., G. Spence, P. Charvis, J.Y. Collot, R.D. Hyndman, and A.M. Tréhu, 2003, Crustal structure beneath Juan de Fuca Strait and southern Vancouver Island from seismic and gravity analyses, J. Geophys. Res., 108, B10, doi:10.1029/2002JB001823, EPM 5-1 to 5-23. PDF

Wide-angle and vertical incidence seismic data from Seismic Hazards Investigations in Puget Sound (SHIPS), gravity modeling, and seismicity are used to derive two dimensional crustal models beneath the Strait of Juan de Fuca. The Eocene volcanic Crescent-Siletz terrane is significantly thicker than previously recognized and extends from near the surface to depths of 22 km or greater. For the northern strait, a weak midcrustal reflector, dipping east from 12- to 22-km depth, is inferred from wide-angle reflections. A stronger deeper reflector, dipping eastward from 23- to 36-km depth, is associated with the top of ‘‘reflector band E,’’ a zone of high reflectivity on coincident Multichannel Seismic (MCS) data, interpreted as a shear zone. A high-velocity zone (7.60 ± 0.2 km s 1) between these reflectors is interpreted as a localized slice of mantle accreted with the overlying Crescent-Siletz terrane. For the southern strait, no deep high-velocity layer is observed and the E-band reflectivity is weaker than to the north. A strong deep reflector, interpreted as the oceanic Moho dips eastward from 35 to 42 km. Seismicity within the subducting slab occurs mainly above the inferred oceanic Moho. Gravity modeling, constrained by the wide-angle seismic models and seismicity, is consistent with the inferred large thickness of Crescent-Siletz and high-density rocks (3030 kg m 3) in the lower crust.

Hayward , N., Nedimovic, M. Cleary, and A.J. Calvert, Identifying faults and their recent motion in eastern Strait of Juan de Fuca, NEHRP Contract Report 02HQGR0055,35 pp., 2003. PDF

Multi-channel seismic reflection data, acquired in the SHIPS (Seismic Hazards Investigation in Puget Sound) survey are used to interpret the faulting and structure of the eastern Strait of Juan de Fuca. A number of major fault zones, including the Devils Mountain, a left-lateral transpressional fault zone, and the Southern Whidbey Island fault zone, underlie the region of large prehistoric earthquakes. Numerous Pleistocene glaciations and associated erosion and deposition have resulted in the surface masking of faults, which are now most easily investigated using seismic data. In this study, first arrival tomographic velocity models derived from the seismic data are used to accurately characterise the shallow P-wave velocity structure across fault zones and aid in the identification of potentially active faults, which may pose a significant seismic hazard to local communities.

Seismic reflection data from SHIPS lines JDF-1, JDF-2, JDF-3. JDF-4 JDF-5, JDF-6, PS-2 and SG-1 were reprocessed, using variable shot spacing geometry, to improve the quality of seismic images and allow for more detailed interpretation of the near surface. First arrival tomographic inversion velocities were calculated using an iterative two - dimensional inversion algorithm based on a finite-difference solution to the eikonal equation. With far offsets of approximately 2600 m, a high density of subsurface raypaths and a velocity grid spacing of 25 m, a high-resolution estimate of P-wave velocity structure is calculated for depths in the range of 500-1200 m. These P-wave velocity models provide information on the variation of physical properties with depth and across faults, which when overlain directly upon seismic profiles significantly aid in the interpretation.

Seismic reflection profiles, of the Devils Mountain fault, suggest that primary pre-Quaternary motion to the east of 122.95° W was transferred to a large fault scarp identified on profiles south of the mapped E-W trend on the DMF. This fault scarp appears to be related to the westward extension on the Utsalady Point fault, which shows a similar, although slightly smaller scarp west of Whidbey Island. More recent deformation has been experienced on the eastward continuance of the DMF, which east of 122.95° W affects a shallow pre-Tertiary basement and thin overburden far north of the primary fault scarp. However, Quaternary deformation associated with the primary fault scarp appears to be of as large a magnitude as faulting on the DMF to the northeast.

Hayward , N., M. Nedimovic, M. Cleary, and A.J. Calvert, Structural variation along the Devils Mountain fault zone, northwestern Washington, Canadian Earth Sciences J., in press. PDF

The eastern Juan de Fuca Strait is subject to long-term, north-south oriented, shortening. The observed deformation is interpreted to result from the northward motion of the Oregon block, which is being driven north by oblique subduction of the oceanic Juan de Fuca plate. Seismic data, acquired during the SHIPS (Seismic Hazards Investigation in Puget Sound) survey are used, with coincident first arrival tomographic velocities, to interpret structural variation along the Devil's Mountain fault zone in the eastern Juan de Fuca Strait. The Primary fault of the Devil's Mountain fault zone developed at the northern boundary of the Everett basin, during north-south oriented Tertiary compression. Interpretation of seismic reflection data suggests that, based on their similar geometry, including the large magnitude of pre-Tertiary basement offset, the Primary fault of the Devil's Mountain fault west of ~122.95° W and the Utsalady Point fault represent the main fault of the Tertiary Devil's Mountain fault zone. The Tertiary Primary fault west of ~ 122.95° W was probably kinematically linked to faults to the east (Utsalady Point, Devil's Mountain, and another to the south), by an oblique NNE trending transfer zone or ramp. Left-lateral transpression controlled the Quaternary evolution of the Devil's Mountain fault zone. Quaternary Primary fault offsets are smaller to the east of ~122.95° W, suggesting that stress here was in part accommodated by the prevalent oblique compressional structures to the north. Holocene deformation has focussed on the Devil's Mountain, Utsalady Point and Strawberry Point faults to the east of ~ 122.8° but has not affected the Utsalady Point fault to the west of ~ 122.8° W.

Johnson, S.Y., Dadsman, S.V., Mosher, D.C., Blakely, R.J., and Childs, J.R., 2001, Active tectonics of the Devils Mountain fault and related structures, northern Puget Lowland and eastern Strait of Juan de Fuca region, Pacific Northwest, U.S. Geological Survey Professional Paper 1643 (65 p. text, 2 tables, 38 figs., 2 plates). or

Information from marine high-resolution and conventional seismic-reflection surveys, aeromagnetic mapping, coastal exposures of Pleistocene strata, and lithologic logs of water wells is used to assess the active tectonics of the northern Puget Lowland and eastern Strait of Juan de Fuca region of the Pacific Northwest. These data indicate that the Devils Mountain fault and the newly recognized Strawberry Point and Utsalady Point faults are active structures and represent potential earthquake sources.

Lowe, C., S. A. Dehler, and B. C. Zelt (2003). Basin architecture and density structure beneath the Strait of Georgia, British Columbia, Can. J. Earth Sci., 40 (7), 965-981. PDF

Georgia Basin is located within one of the most seismically active and populated areas on Canada’s west coast. Over the last decade, geological investigations have resolved important details concerning the basin’s shallow structure and composition. Yet, until recently, relatively little was known about deeper portions of the basin. In this study, new seismic velocity information is employed to develop a 3-dimensional density model of the basin. Comparison of the calculated gravity response of this model with the observed gravity field validates the velocity model at large scales. At smaller scales, several differences between model and observed gravity fields are recognized. Analysis of these differences and correlation with independent geoscience data provide new insights into the structure and composition of the basin-fill and underlying basement. Specifically, four regions with thick accumulations of unconsolidated Pleistocene and younger sediments, which were not resolved in the velocity model, are identified. Their delineation is particularly important for studies of seismic ground-motion amplification and offshore aggregate assessment. An inconsistency between the published geology and the seismic structure beneath Texada and Lasqueti Islands in the central Strait of Georgia is investigated; however, the available gravity data cannot preferentially validate either the geologic interpretation or the seismic model in this region. We interpret a northwest-trending and relatively linear gradient extending from Savory Island in the north to Boundary Bay in the south as the eastern margin of Wrangellia beneath the basin. Finally, we compare Georgia Basin with the Everett and Seattle basins in the southern Cascadia fore arc. This comparison indicates that while a single mechanism may be controlling present-day basin tectonics and deformation within the fore arc this was not the case for most of the Mesozoic and Tertiary time periods.

Mosher, D.C., Cassidy, J.F., Lowe, C., Mi, Y., Hyndman, R.D., Rogers, G.C., and M.A. Fisher (2000). Neotectonics in the Strait of Georgia: Tentative correlation of seismicity with shallow geologic structure in southwestern British Columbia, Geological Survey of Canada, Current Research 2000-A22, 1-9. PDF

Multichannel seismic reflection data within the Strait of Georgia provide the first tentative correlation of geological structure with recent seismicity. Reflection data show broad folding of sedimentary rocks in the southernmost part of the strait. To the north are two broad (5-10 km wide) deformation zones with interpreted normal faults. Both zones have an associated magnetic anomaly. The northern deformation zone, 30 km west of Vancouver, correlates with the location of a number of recent shallow earthquakes. Detailed analyses of an M=4.6 event at this site (June 1997) describe a shallow (2-4 km depth) reverse thrust on a northern dipping (~50°), east-trending fault plane. The fault dip direction and strike agree with seismic reflection interpretations and the magnetic anomaly trend. These data are required to provide fundamental geological information for addressing issues of strain partitioning in the Cascadia forearc.

Mosher, D.C., and S.Y. Johnson (eds.), Rathwell, G.J., R.B. King, and S.B. Rhea (compilers), 2000, Neotectonics of the eastern Strait of Juan de Fuca; a digital geologic and geophysical atlas, Geological Survey of Canada Open File Report 3931.

Ramachandran, K., 2001, Velocity structure of S.W. British Columbia, and N.W. Washington, from 3-D non-linear seismic tomography, Ph.D. thesis, Univ. of Victoria, B.C., 198 pp. PDF

This thesis applies three-dimensional (3-D) non-linear seismic tomography to image crustal/upper mantle structure of S.W. British Columbia and N.W. Washington. Two tomographic inversions are carried out including high-resolution imaging of upper crustal structure using controlled source data, and deeper imaging by simultaneous inversion of controlled source and earthquake data.

Non-linear first arrival travel-time tomography is applied to controlled source data from the Seismic Hazards Investigation of Puget Sound (SHIPS) experiment conducted in 1998. Nearly 175,000 first arrival travel-times are inverted to obtain a minimum structure upper crustal velocity model to a depth of 12 km with a cubical cell size of 1 km. Results from checkerboard tests for this velocity model indicate a lateral resolution of 20 km and above. The main geological and structural features in the study area are well defined by this velocity model. The structural outline of the sedimentary basins in the Straits of Georgia and Juan de Fuca are distinctly mapped. The Crescent Terrane is mapped beneath southern Vancouver Island with velocities up to 7 km/s that correlate well with the presence of gabbro in the subsurface. The northwest-southeast structural trend observed in the Strait of Georgia correlates with the observed seismicity. Shallow seismicity observed at the southern tip of Vancouver Island correlates with the location of the Leech River Fault.

An earthquake tomography algorithm was developed for joint estimation of hypocentral and velocity parameters, and tested on a synthetic data set. Using this algorithm, tomographic inversion was performed simultaneously on earthquake and controlled source data from southwestern British Columbia and northwestern Washington. Approximately 15,000 first arrivals from 1,400 earthquakes and 40,000 first arrivals from the SHIPS experiment were simultaneously inverted for hypocentral parameters and velocity structure. Model resolution studies indicate a lateral resolution of 30 km and above. Upper-crustal earthquakes close to southern Vancouver Island correlate with the velocity contrasts associated with the Leech River, Southern Whidbey Island, and Darrington-Devils Mountain faults. Three mafic to ultramafic high velocity units are identified at approximately 25 km depth beneath the Crescent Terrane and above the subducting Juan de Fuca crust. The continental crust and subducting Juan de Fuca crust and mantle are well mapped. The transition zone to continental mantle occurs at 35 km depth beneath the eastern Strait of Georgia. The slab seismicity beneath the Strait of Georgia at depths > 65 km lies below a low velocity zone mapped in the mantle wedge at depths of about 45–55 km. This low velocity zone may be indicative of the presence of fluids released during the phase change from basalt/gabbro to eclogite in the subducting slab.

Ramachandran, K., S.E. Dosso, C.A. Zelt, G.D. Spence, R.D. Hyndman, and T.M. Brocher, 2004, Upper crustal structure of southwestern British Columbia from the 1998 Seismic Hazards Investigation in Puget Sound, J. Geophy. Res., 109, B9, B09303, doi: 10.1029/2004JB003092. PDF

This paper applies nonlinear three-dimensional travel time tomography to refraction data recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) to derive the first large-scale, high-resolution upper crustal velocity model for southwestern British Columbia. A minimum structure P wave velocity model is constructed using 175,000 first arrival travel times picked from data recorded by 58 temporary onshore stations. The model details forearc crustal structures related to terrane accretion and subsequent basin formation to a depth of about 10 km. The Metchosin igneous complex (correlative with the Eocene Crescent-Siletz Terrane in Washington) is imaged as a laterally extensive WNW trending high-velocity anomaly underlying southernmost Vancouver Island and much of the Strait of Juan du Fuca. Northeast of the Strait of Georgia, the southwesterly dip of the contact between the Wrangellia terrane rocks of Vancouver Island and the Coast Plutonic Complex suggests Wrangellia rocks are downfaulted against the plutonic complex. At the southwestern end of the Strait of Juan de Fuca, the 50 km long WNW trending Clallam basin has a maximum thickness of 5–6 km. Near the eastern end of the Strait of Juan de Fuca, Port Townsend basin has an inferred thickness of approximately 4–5 km. The southern end of the 9 km thick Georgia basin is bounded by a high-velocity basement ridge. Beneath the Strait of Georgia, clusters of well-located earthquakes have a prominent NW trend and coincide spatially with rapid lateral velocity changes. Clusters of microearthquakes there are associated with the intersection of several east trending structural highs within this NW trend.

Ramachandran, K., S.E. Dosso, C.A. Zelt, G.D. Spence, R.D. Hyndman, and T.M. Brocher, 2005, Forearc structure beneath southwestern British Columbia: A 3-D tomography velocity model, J. Geophy. Res., 110, B02303, doi:10.1029/2004JB003258. PDF

This paper presents a three-dimensional compressional wave velocity model of the forearc crust and upper mantle and the subducting Juan de Fuca plate beneath southwestern British Columbia and the adjoining straits of Georgia and Juan de Fuca. The velocity model was constructed through joint tomographic inversion of 50,000 first-arrival times from earthquakes and active seismic sources. Wrangellia rocks of the accreted Paleozoic and Mesozoic island arc assemblage underlying southern Vancouver Island in the Cascadia forearc are imaged at some locations with higher than average lower crustal velocities of 6.5–7.2 km/s, similar to observations at other island arc terranes. The mafic Eocene Crescent terrane, thrust landward beneath southern Vancouver Island, exhibits crustal velocities in the range of 6.0–6.7 km/s and is inferred to extend to a depth of more than 20 km. The Cenozoic Olympic Subduction Complex, an accretionary prism thrust beneath the Crescent terrane in the Olympic Peninsula, is imaged as a low-velocity wedge to depths of at least 20 km. Three zones with velocities of 7.0–7.5 km/s, inferred to be mafic and/or ultramafic units, lie above the subducting Juan de Fuca plate at depths of 25–35 km. The forearc upper mantle wedge beneath southeastern Vancouver Island and the Strait of Georgia exhibits low velocities of 7.2–7.5 km/s, inferred to correspond to 20% serpentinization of mantle peridotites, and consistent with similar observations in other warm subduction zones. Estimated dip of the Juan de Fuca plate beneath southern Vancouver Island is 11 , 16 , and 27 at depths of 30, 40, and 50 km, respectively.

Riedel, M., V. Barrie, P. Hill, T.M. Brocher, T.L. Pratt, and R.D. Hyndman, 2002, Mapping of near Surface Active Faults in Georgia Strait, Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract S22B-1031, 2002.

The Georgia Basin is an area of strong crustal seismicity but few fault traces have been identified. The last significant crustal earthquake, M = 4.6, occurred in 1997 at a depth of 3- 4 km, about 30 km west of Vancouver. Focal mechanism solutions and joint hypocenter relocation of the earthquake and aftershocks delineated an east-west striking fault that dips north at about 53 degrees. High-resolution seismic surveys and multibeam seafloor swath mapping have delineated an active fault zone and associated seafloor pockmarks in an adjacent area west of the epicenter. A high-resolution 3D seismic survey conducted in May 2002 covered the area of the pockmark field to further study the occurrence and significance of near surface faults close to urban centers such as Vancouver, Victoria and Nanaimo. One hundred inlines spaced at 50 m was shot perpendicular to the main chain of pockmarks (oriented NW-SE), with multichannel and single channel seismic, and high-resolution Huntec data. The main chain of pockmarks is associated with a Holocene fault, which pinches out at the seafloor. It appears to have Holocene movement of up to 40 m based on the displacement of major seismic horizons mapped across the fault. The pockmarks are underlain by gas-rich sediments that mask deeper reflectivity especially in the high-frequency Huntec data. The area of the epicenter itself was covered with several seismic lines crossing the epicenter location in multiple directions. No clear evidence for near surface tectonic deformation was found in the immediate area. Deeper sub-basement structures, imaged by the earlier conventional multichannel SHIPS project data, were tentatively correlated to the site of the 1997 earthquake, but these data do not provide a detailed image of the faults and deformation zones. The marine seismic data acquisition was complemented by a detailed land-based seismic study to further investigate the structure of the Georgia Basin. Fifty Reftek (IRIS-PASSCAL) stations were deployed along the Southern and Northern Gulf Islands and on the Vancouver mainland side. Several regional seismic lines shot in the Georgia Strait will be used for a high-resolution first-arrival time tomography study to delineate the P-wave velocity structure in the upper few kilometers of sediments of the Georgia Basin.

Zelt, B.C., R.M. Ellis, C.A. Zelt, R.D. Hyndman, C. Lowe, G.D. Spence, and M.A. Fisher, 2001, Three-dimensional crustal velocity structure beneath the Strait of Georgia, British Columbia, Geophys. J. Int., 144, 695-712. PDF

The Strait of Georgia is a topographic depression straddling the boundary between the Insular and Coast belts in southwestern British Columbia. Two shallow earthquakes located within the strait (M=4.6 in 1997 and M=5.0 in 1975) and felt throughout the Vancouver area illustrate the seismic potential of this region. As part of the 1998 Seismic Hazards Investigation of Puget Sound (SHIPS) experiment, seismic instruments were placed in and around the Strait of Georgia to record shots from a marine source within the strait. We apply a tomographic inversion procedure to first-arrival travel-time data to derive a minimum-structure 3-D P-wave velocity model for the upper crust to about 13 km depth. We also present a 2-D velocity model for a profile orientated across the Strait of Georgia derived using a minimum-parameter traveltime inversion approach.

This paper presents the first detailed look at crustal velocity variations with the major Cretaceous to Cenozoic Georgia Basin, which underlies the Strait of Georgia. The 3-D velocity model clearly delineates the structure of the Georgia Bain. Taking the 6 km/s isovelocity contour to represent the top of the underlying basement, the basin thickens from between 2 and 4 km in the northwestern half of the strait to between 8 and 9 km at the southeastern end of the study region. Basin velocities in the northeastern half are 4.5 to 6 km/s and primarily represent the Upper Cretaceous Nanaimo Group. Velocities to the south are lower (3-6 km/s) because of the additional presence of the overlying Tertiary Huntingdon Formation and more recent sediments, including glacial and modern Fraser River deposits. In contrast to he relatively smoothly varying velocity structure of the basin, velocities of the basement rocks, which comprise primarily Paleozoic to Jurassic rocks of the Wrangellia Terrane and possibly Jurassic to mid-Cretaceous granitic rocks of the Coast Belt, show significantly more structure, probably an indication of the varying basement rock lithologies. The 2-D velocity model more clearly reveals the velocity layering associated with the recent sediments, Huntingdon Formation and Nanaimo Group of the southern Georgia Basin, as well as the underlying basement. We interpret lateral variations in sub-basin velocities of the 2-D model as a transition from Wrangellian to Coast Belt basement rocks. The effect of the narrow, onshore-offshore recording geometry of the seismic experiment on model resolution was tested to allow a critical assessment of the validity of the 3-D velocity model. Lateral resolution throughout the model to a depth of 3-5 km below the top of basement is generally 10-20 km.

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