SHIPS - Seattle Fault and Basin

Brocher, T.M., T. Parsons, K. C. Creager, R. S. Crosson, N. P. Symons, G. Spence, B.C. Zelt, P.T.C. Hammer, R. D. Hyndman, D.C. Mosher, A. M. Tréhu, K. C. Miller, U.S. ten Brink, M.A. Fisher, T. L. Pratt, M.G. Alvarez, B.C. Beaudoin, and C.S. Weaver, 1999, Wide-angle seismic recordings from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS), western Washington and British Columbia, U.S. Geological Survey Open-File Report 99-314, 110 p. http://geopubs.wr.usgs.gov/open-file/of99-314/

This report describes the acquisition and processing of deep-crustal wide-angle seismic reflection and refraction data obtained in the vicinity of Puget Lowland, the Strait of Juan de Fuca, and Georgia Strait, western Washington and southwestern British Columbia, in March 1998 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As part of a larger initiative to better understand lateral variations in crustal structure along the Cascadia margin, SHIPS participants acquired 1000 km of deep-crustal multichannel seismic-reflection profiles and 1300 km of wide-angle airgun shot lines in this region using the R/V Thompson and R/V Tully. The Tully was used to record airgun shots fired by the Thompson in two different geometries: (1) expanding spread profiles (ESPs) and (2) constant offset profiles (COPs). Prior to this reflection survey, we deployed 257 Reftek and 15 ocean-bottom seismic recorders to record the airgun signals at far offsets. All data were recorded digitally on large-capacity hard disks. Although most of these stations only recorded the vertical component of motion, 95 of these seismographs recorded signals from an oriented 3-component seismometer. By recording signals generated by the Thompson's marine air gun array, operated in two differing geometries having a total volume of 110 and 79 liters (6730 and 4838 cu. in.), respectively, the arrays of wide-angle recorders were designed to (1) image the crustal structure, particularly in the vicinity of crustal faults and Cenozoic sedimentary basins, (2) determine the geometry of the Moho, and (3) image the subducting Gorda and Juan de Fuca plates. Nearly 33,300 air gun shots were recorded along several seismic lines. In this report, we illustrate the expanding spread profiles acquired using the Thompson and Tully, describe the land and ocean-bottom recording of the air gun signals, discuss the processing of the land recorder data into common receiver gathers, and illustrate the processed wide-angle seismic data collected using the Refteks and ocean-bottom seismometers. 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 SHIPS appears to have successfully obtained useful data from almost all the stations deployed to record the airgun shots. Several interesting arrivals were observed: including refractions from the sedimentary basin fill in several basins, refractions from basement rocks forming the upper crust, Pg, refractions from the upper mantle, Pn, as well as reflections from within the crust and from the top of the upper mantle, PmP. We separately archived more than 30 local earthquakes recorded by the Reftek array during our deployment.

Brocher, T.M., T. Parsons, M.A. Fisher, A.M. Tréhu, G.D. Spence, and the SHIPS Working Group, 2000, Three-dimensional tomography in the eastern Strait of Juan de Fuca: Preliminary results from SHIPS, the 1998 Seismic Hazards Investigation in Puget Sound, in Mosher, D.C., and S.Y. Johnson (eds.), Rathwell, G.J., R.B. King, and S.B. Rhea (compilers), Neotectonics of the eastern Strait of Juan de Fuca; a digital geologic and geophysical atlas, Geological Survey of Canada Open File Report3931.

Brocher, T.M., T. L. Pratt, K.C. Miller, A.M. Tréhu, C.M. Snelson, C.S. Weaver, K. C. Creager, R.S. Crosson, U.S. ten Brink, M.G. Alvarez, S.H. Harder, and I. Asudeh, 2000, Report for explosion and earthquake data acquired in the 1999 Seismic Hazards Investigation in Puget Sound (SHIPS), Washington, U.S. Geological Survey Open-File Report 00-318, 85 p. http://geopubs.wr.usgs.gov/open-file/of00-318/

This report describes the acquisition, processing, and quality of seismic reflection and refraction data obtained in the Seattle basin, central Puget Lowland, western Washington, in September 1999 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As a sequel to the 1998 SHIPS air gun experiment (also known as “Wet SHIPS”), the 1999 experiment, nicknamed “Dry SHIPS”, acquired a 112-km-long east-west trending multichannel seismic-reflection and refraction line in the Seattle basin. One thousand and eight seismographs were deployed at a nominal spacing of 100 meters and 29 shot points were detonated at approximately 4 km intervals along the seismic line. The wide-angle seismic profile was designed to (1) determine the E-W geometry of Seattle basin, (2) measure the seismic velocities within the basin, and (3) define the basement structure underlying the Seattle basin. In this report, we describe the acquisition of these data, discuss the processing and merging of the data into common shot gathers, and illustrate the acquired profiles. We also describe the format and content of the archival tapes containing the SEGY-formatted, common-shot gathers. Data quality is variable but useful data were acquired from all 29 shot points fired along the Dry SHIPS seismic line. The data show pronounced travel time delays associated with the low velocity sedimentary rocks filling the Seattle basin.

Thirty-five REFTEK stations, deployed at 4 km intervals along the Dry SHIPS line, recorded 26 regional earthquakes and blasts and 17 teleseismic events, including the main shock and several aftershocks of the M w = 7.6 Chi-Chi (Taiwan) earthquake of 9/20/1999. The teleseismic recordings of the Chi-Chi ( Taiwan) mainshock provide useful signals down to 10 second periods. They document a significant (factor between 5 and 10) focusing of compressional- and shear-wave energy by the Seattle basin at periods between 1 and 2 seconds relative to “bedrock” sites east and west of the basin. Signal durations in the Seattle basin were also substantially increased relative to “bedrock” sites in the Olympic peninsula and Cascade foothills.

Brocher, T.M., T. L. Pratt, C.S. Weaver, A.D. Frankel, A.M. Tréhu, C.M. Snelson, K.C. Miller, S.H. Harder, U.S. ten Brink, K. C. Creager, R.S. Crosson, and W. P. Steele, 2000, Urban seismic experiments investigate the Seattle fault and basin, Eos, Trans. AGU, v. 81, no. 46, p. 545, 551-552. PDF

In the past decade earth scientists have come to recognize the seismic hazards posed by crustal faults and sedimentary basins to Seattle, Washington (Figure 1). In 1998, the U.S. Geological Survey and its collaborators initiated a series of urban seismic studies of the upper crust to better map seismogenic structures and sedimentary basins in the Puget Lowland. We call these studies the Seismic Hazard Investigations of Puget Sound (SHIPS).

In March 1998 we conducted our first SHIPS study, an investigation of the upper crustal structure of the Puget Lowland, nicknamed Wet SHIPS, using marine airgun sources and land recorders [Fisher et al., 1999]. In September 1999, we obtained a seismic refraction line to study the upper crustal structure in the Seattle area, in a landbased study nicknamed Dry SHIPS [Brocher et al., 2000]. In March 2000, we recorded the demolition of the Seattle Kingdome sports stadium using a dense array of seismic recorders for a detailed site response study, nicknamed Kingdome SHIPS.

Brocher, T.M., T. Parsons, R.A. Blakely, N.I. Christensen, M.A. Fisher, R.E. Wells, and the SHIPS Working Group, 2001, Upper crustal structure in Puget Lowland, Washington: Results from the 1998 Seismic Hazards Investigation in Puget Sound, J. Geophys. Res., 106, 13,541-13,564. PDF

A new three-dimensional (3-D) model shows seismic velocities beneath the Puget Lowland to a depth of 11 km. The model is based on a tomographic inversion of nearly one million first-arrival travel times recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS), allowing higher-resolution mapping of subsurface structures than previously possible. The model allows us to refine the subsurface geometry of previously proposed faults (e.g., Seattle, Hood Canal, southern Whidbey Island, and Devils Mountain fault zones) as well as to identify structures ( Tacoma, Lofall, and Sequim fault zones) that warrant additional study. The largest and most important of these newly identified structures lies along the northern boundary of the Tacoma basin; we informally refer to this structure here as the Tacoma fault zone. Although tomography cannot provide information on the recency of motion on any structure, Holocene earthquake activity on the Tacoma fault zone is suggested by seismicity along it and paleoseismic evidence for abrupt uplift of tidal marsh deposits to its north. The tomography reveals four large, west to northwest trending low-velocity basins ( Tacoma, Seattle, Everett, and Port Townsend) separated by regions of higher velocity ridges that are coincident with fault-bounded uplifts of Eocene Crescent Formation basalt and pre-Tertiary basement. The shapes of the basins and uplifts are similar to those observed in gravity data; gravity anomalies calculated from the 3-D tomography model are in close agreement with the observed anomalies. In velocity cross sections the Tacoma and Seattle basins are asymmetric: the basin floor dips gently toward a steep boundary with the adjacent high-velocity uplift, locally with a velocity “overhang” that suggests a basin vergent thrust fault boundary. Crustal fault zones grow from minor folds into much larger structures along strike. Inferred structural relief across the Tacoma fault zone increases by several kilometers westward along the fault zone to Lynch Cove, where we interpret it as a zone of south vergent faulting overthrusting Tacoma basin. In contrast, structural relief along the Seattle fault zone decreases west of Seattle, which we interpret as evidence that the N-S directed compression is being accommodated by slip transfer between the Seattle and Tacoma fault zones. Together, the Tacoma and Seattle fault zones raise the Seattle uplift, one of a series of east-west trending, pop-up structures underlying Puget Lowland from the Black Hills to the San Juan Islands.

Brocher, T.M., C.S. Weaver, and R.S. Ludwin, 2003, Assessing hypocentral accuracy and lower magnitude completeness in the Pacific Northwest using seismic refraction detonations and cumulative frequency-magnitude relationships, Seism. Res. Lett., 74, 773-790. PDF

Absolute hypocentral location errors are traditionally estimated from the errors in the location of quarry blasts. Catalog completeness has been evaluated by examining cumulative frequency-magnitude relationships (e.g., Rydelek and Sacks, 1989). In the Pacific Northwest, however, the number of quarries with well-defined blasting schedules is relatively sparse (e.g., Benson et al., 1992). Seismic refraction detonations provide an independent assessment of actual location errors of surface events in the Pacific Northwest and elsewhere. Because the detonation yields are also known, their reported magnitudes can be used to investigate the detection and location thresholds for low magnitude earthquakes. As the number and distribution of seismic stations in the Pacific Northwest expanded (Figure 1), the location accuracy of the networks and the completeness of their catalogs has improved with time. Because the station coverage is not uniform geographically, these network properties vary with location.

In this note we use 72 refraction detonations listed in the Advanced National Seismic System (ANSS) and Pacific Northwest Seismic Network (PNSN) catalogs for the Pacific Northwest to investigate the hypocentral accuracy and completeness of the earthquake catalog as a function of time and location since 1984. These reported detonations had an average charge size of 939 kg and yielded an average coda magnitude of 1.65. Applying magnitude-versus-charge-size relationships to 64 detonations not listed in the ANSS and PNSN catalogs permits us to extend our analysis back to 1965. Brocher (2003a) noted several reasons why the location errors for surface detonations may not be fully representative of location errors for tectonic earthquakes. Nonetheless, quarry blasts and refraction detonations represent seismic sources whose origin times and locations are precisely known, and they provide an independent measure of the quality of location solutions by the network. Furthermore, the regional variation in the ability of a seismic network to locate surficial detonations, which can be measured, presumably reflects the regional variation in the ability of the network to locate shallow (near surface to a few km depth) earthquakes, which cannot be measured. We sought to determine whether regional variations exist in the capability of the Pacific Northwest Seismic Network (PNSN) to locate earthquakes in Oregon and Washington.

Brocher, T.M., R.J. Blakely, and R.E. Wells, 2004, Interpretation of the Seattle Uplift, Washington, as a passive roof duplex, Bull. Seism. Soc. Am., 94, 1379-1401. PDF

We interpret seismic lines and a wide-variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short-wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults are inferred to core these shallow folds and to root into a shallow roof thrust. Although currently aseismic, some of the shallow thrusts above the triangle zone have ruptured to the surface in the late Holocene. Ages from offset soils in trenches across the fault scarps and from abruptly raised shorelines indicate that both the floor and roof thrusts of the Seattle and Tacoma faults ruptured about 1100 years ago.

Brocher, T.M., R. J. Blakely, R. E. Wells, B. L. Sherrod, and K. Ramachandran , 2005, The Transition Between N-S and NE-SW Directed Crustal Shortening in the Central and Northern Puget Lowland: New Thoughts on the Southern Whidbey Island Fault, Eos, Trans. Amer. Geophys. Union, in press.

We hypothesize that the southern Whidbey Island fault (SWIF) is a NW-SE oriented fold and thrust belt accommodating NE-directed crustal shortening. The SWIF has been considered a dextral strike-slip fault based largely on two interpretations: (1) its northwest orientation in a region believed to be undergoing dominantly N-S compression, and (2) interpretation of industry seismic-reflection data across the SWIF as a flower structure, suggestive of transpressional faulting. Both interpretations require reconsideration based on evidence outlined below.

Recent GPS studies (e.g., Miller et al., 2001) have shown that the Puget Lowland is a zone of transition between N-directed compression to the south and NE-SW directed compression (parallel to the plate-convergence vector) to the north. While N-S compression provides an adequate explanation for the E-trending Seattle and Tacoma thrust faults to the south, recent paleoseismic and geophysical studies suggest that NE-SW compression producing NE-directed tectonic wedging (passive roof duplexing) dominates at the SWIF. Evidence for a SW-dipping floor thrust forming the base of the tectonic wedge is provided by gravity and seismic tomography models demonstrating higher structural relief of basement rocks to the south of the SWIF than to its north. Aeromagnetic anomalies, lidar studies, and paleoseismic evidence indicate a broader (~25 km wide) zone of abundant NE-side-up shallow reverse faults parallel to the SWIF than previously recognized. We interpret these faults as evidence for a zone of NW-oriented, NE-dipping splay faults soling into a shallow (3 to 4 km deep), NE-dipping detachment surface forming the top of the tectonic wedge. We re-examined oil industry seismic-reflection profiles across the SWIF, previously seen as evidence for transpressional faults, and find them more compatible with shallow thrust folds associated with shallow (upper 3 to 4 km) splay faults.

In sum, these observations are consistent with a blind NE-vergent wedge tip, accompanied by shallow, NE-dipping roof and splay thrusts of late Holocene age. Available data do not rule out a small component of transpressional faulting along the SWIF. Indeed, paleoseismic excavations at the Utsulady fault, which lies subparallel to and north of the SWIF, reveal late Holocene sinistral slip. Thus, the SWIF may represent the southernmost crustal fault in the central Puget Lowland that responds to plate convergence rather than northward migration of the forearc. To the north, along southern Vancouver Island, other fold and thrust belts (e.g., the Cowichan Fold and Thrust Belt) are clearly oriented perpendicular to NE-directed plate convergence as measured by GPS studies.

Calvert, A.J., M.A. Fisher, and SHIPS Working Group, 2001, Imaging the Seattle fault zone with high-resolution seismic tomography, Geophys. Res. Lett., 28, 2337-2340. PDF

The Seattle fault, which trends east-west through the greater Seattle metropolitan area, is a thrust fault that, around 1100 years ago, produced a major earthquake believed to have had a magnitude greater than 7. We present the first high resolution image of the shallow P-wave velocity variation across the fault fault obtained by tomographic inversion of first arrivals recorded on a seismic reflection profile shot through Puget Sound adjacent to Seattle. The velocity image shows that above 500 m depth the fault zone extending beneath Seattle comprises three distinct fault splays, the northernmost of which dips to the south at around 60 degrees. The degree of uplift of Tertiary rocks within the fault zone suggests that the slip-rate along the northernmost splay during the Quaternary is 0.5 mm/yr, which is twice the average slip-rate of the Seattle fault over the last 40 Ma.

Calvert, A.J., M.A. Fisher, S.Y. Johnson, and the SHIPS Working Group, 2003, Along-strike variations in the seismic velocity structure of the Seattle fault zone: Evidence for fault segmentation beneath Puget Sound, J. Geophys. Res. , 108(B1), doi:10.1029/2001JB001703. PDF

Around 1100 years ago, the Seattle fault, which trends east-west beneath Puget Sound and the greater Seattle metropolitan area, experienced a M > 7 earthquake. We present high-resolution images of the shallow P wave velocity variation across the fault zone. These images were obtained by tomographic inversion of the first arrivals recorded along two north-south oriented seismic reflection lines shot within Puget Sound near Seattle. Just beneath the seafloor, the fault zone includes uplifted Tertiary rocks with seismic velocities in the range of 2300 to 2600 m s 1. These velocities contrast markedly with values of 1600 m s 1 in shallow Holocene sediments. South of the Seattle fault zone volcanic rocks of the Crescent Formation, which exhibit velocities >3700 m s 1, are identified at depths of only 900 m. Seismic velocities of around 2600 m s 1, which represent Oligocene rocks, are found in the hanging wall of the Seattle fault beneath eastern Puget Sound. In the west, lower, 2300 m s 1 seismic velocities occur, probably due to the presence of Miocene rocks, which are not found in the east. Along-strike velocity variations arise from the folding of Tertiary rocks and the presence of distinct fault splays, including a north striking tear fault characterized by depressed seismic velocities that was intersected by the eastern seismic line. Along-strike differences in the uplift of Tertiary rocks beneath Puget Sound are likely associated with the existence of a segment boundary of the Seattle fault system.

Crosson, R.S., N.P. Symons, K.C. Creager, L.A. Preston, T. Van Wagoner, T.M. Brocher, M. Fisher, and the SHIPS Working Group, 2002, Three-dimensional structure of the Cascadia forearc region from SHIPS active experiment and earthquake observations: tomographic inversion provides a high-resolution view into the core of the Cascadia forearc complex, U.S. Geological Survey Open-File Report 02-328 and Geological Survey of Canada Open File 4350, p. 33-34. http://geopubs.wr.usgs.gov/open-file/of02-328/

Phase I of SHIPS (Seismic Hazards in Puget Sound) was a large-scale airgun deployment in the inland waterways of western Washington State and western British Columbia. The coincidence of SHIPS travel time data in the Puget Sound region with earthquake arrival time data from the Pacific Northwest Seismograph Network has allowed us to carry out high-resolution joint active source and earthquake travel time inversion (tomography) for P wave velocity structure to depths of approximately 60 km over a portion of the Cascadia forearc margin. A total of about 70,000 P wave arrival observations were used in our initial effort; approximately 1000 earthquakes comprise 45% of the total number of P wave observations. Excellent constraint of the shallow structure by SHIPS data allows greatly improved resolution of the deep structure through earthquake observations, compared to inversion using earthquake data alone. The three-dimensional velocity model fits both airgun and earthquake picks to approximately 0.1 second root-mean-squared (rms) overall. Simple variance reduction from a best fitting onedimensional model to our preferred three-dimensional model exceeds 90%.

After inversion, earthquake hypocenters migrate in general to slightly shallower depths relative to depths computed with a best fitting one-dimensional starting model. Increasing the average velocity of the one-dimensional model could bring the one-dimensional and three-dimensional depths into closer agreement; however, because of very significant three-dimensional effects, such a model may still not provide a satisfactory one-dimensional average earth model. Earthquakes in the Wadati– Benioff zone are moved an average of about three kilometers shallower but generally maintain the average dip of the zone (about 14°). The scatter of Wadati–Benioff zone earthquakes about a plane is also reduced by a factor of about two in standard deviation in going from one-dimensional to three-dimensional models. Mafic Crescent/Siletz volcanic rocks form the basement of the Puget basin region with P wave velocity exceeding 7 km/s at 20 km depth. In the central Puget basin, Crescent/Siletz is in turn underlain at a depth of about 30 km by lenses of lower-velocity, subducted accretionary prism rocks which are gradational with rocks of the Olympic core further to the west. This suggests that the Olympic core rocks are mechanically emplaced by subduction beneath the Crescent/Siletz eastward of the Olympic Mountains, producing a local low velocity zone in the deep crust beneath the Puget basin (although in the conventional sense “crust” may be ill-defined in this region). The P wave velocity reversal produced by this emplacement process is typically about 0.5 km/s. The relationship between Olympic core rocks and the Crescent/Siletz terrane is consistent with models of “unroofing” of the Olympic core [e.g., Brandon et al., 1998].

Fisher, M.A., T. M. Brocher, R. D. Hyndman, A. M. Tréhu, C. S. Weaver, K. C. Creager, R. S. Crosson, T. Parsons, A. K. Cooper, D. Mosher, G. Spence, B. C. Zelt, P. T. Hammer, U. ten Brink, T. L. Pratt, K. C. Miller, J. R. Childs, G. R. Cochrane, S. Chopra, and R.Walia, 1999, Seismic survey probes urban earthquake hazards in Pacific Northwest, EOS, Trans. Amer. Geophys. Un., v. 80, no. 2, p. 13-17.

Fisher, M.A., R.D. Hyndman, S.Y. Johnson, R.S. Crosson, U.S. ten Brink, A.J. Calvert, T.M. Brocher, R.E. Wells, and the SHIPS Working Group, 2003, Crustal structure and earthquake hazards of the Subduction Zone in the Central Part of Cascadia, U.S.Geological Survey Prof. Paper 1661-X, in press.

Parsons, T., R.J. Blakely, and T.M. Brocher, 2001, A simple algorithm for sequentially incorporating gravity observations in seismic traveltime tomography, International Geology Review, 43, 1073-1086. PDF

The geologic structure of the Earth’s upper crust can be revealed by modeling variation in seismic arrival times and in potential field measurements. We demonstrate a simple method for sequentially satisfying seismic traveltime and observed gravity residuals in an iterative 3-D inversion. The algorithm is portable to any seismic analysis method that uses a gridded representation of velocity structure. Our technique calculates the gravity anomaly resulting from a velocity model by converting to density with Gardner’s rule. The residual between calculated and observed gravity is minimized by weighted adjustments to the model velocity-depth gradient where the gradient is steepest and where seismic coverage is least. The adjustments are scaled by the sign and magnitude of the gravity residuals, and a smoothing step is performed to minimize vertical streaking. The adjusted model is then used as a starting model in the next seismic traveltime iteration. The process is repeated until one velocity model can simultaneously satisfy both the gravity anomaly and seismic traveltime observations within acceptable misfits. We test our algorithm with data gathered in the Puget Lowland of Washington State, USA (Seismic Hazards Investigation in Puget Sound (SHIPS) experiment). We perform resolution tests with synthetic traveltime and gravity observations calculated with a checkerboard velocity model using the SHIPS experiment geometry and show that the addition of gravity significantly enhances resolution. We calculate a new velocity model for the region using SHIPS traveltimes and observed gravity, and show examples where correlation between surface geology and modeled subsurface velocity structure is enhanced.

Pratt, T.L., Weaver, C.S., Brocher, T.M., Parsons, T., Fisher, M.A., Creager, K.C., Crosson, R.S., Hyndman, R.D., Spence, G., Trehu, A.M., Miller, K.C., and ten Brink, U.S., 2002, Understanding the seismotectonics of the Cascadia subduction zone: Overview and recent seismic work, in Fujinawa, Y., and Toshida, A., editors, Seismotectonics in Convergent Plate Boundary, Terra Scientific Publishing, Tokyo, p. 25-36.

Snelson, C.M., 2001, Investigating crustal structure in western Washington and in the Rocky Mountains: Implications for seismic hazards and crustal growth, Ph.D. thesis, Univ. Texas El Paso, 234 p. PDF

This dissertation consists of two seismic studies, one in western Washington State and one in the Rocky Mountains. The study in western Washington State is one component of the SHIPS (Seismic Hazards Investigations of Puget Sound) experiments, a continuing effort to define Cenozoic basin and fault geometry beneath the densely populated Puget Lowland. In September 1999, the U. S. Geological Survey and a number of university collaborators collected the “Dry” SHIPS seismic profile across the Seattle basin of western Washington State. The objectives of the “Dry” SHIPS study were to define the geometry of the Seattle basin in an E-W direction and to determine the structure of the eastern and western boundaries of the basin. In addition, the experiment was designed to test the hypothesis that N-S trending faults lie beneath Puget Sound or the adjacent Lowland.

One of these faults may form the eastern boundary of the Siletz terrane. The “Dry” SHIPS data are characterized by travel time advances associated with the Siletz terrane to the west and the Cascades to the east and by delays of as much as 2 s in the Seattle basin. P-wave 3-D tomographic results show that the basin is about 70 km wide and contains sedimentary strata with velocities increasing gradually from 1.8 - 4.5 km/s. The contact with underlying basement rocks is characterized by a rapid increase in velocity from 4.5 to 5.0 km/s. At its center, the basin is 6 - 7 km deep along this profile. This result is consistent with results from a N-S trending reflection line collected in 1998 during the “Wet” SHIPS phase of the project that is tied to well control. The symmetry of the Seattle basin is consistent with thrust loading as the major contributor to the formation of the basin. The lower velocities within the upper part of the basement found east of the Puget Sound may be indicative of pre-Tertiary basement rocks of the Cascades. This change is probably an expression of the Coast Range Boundary fault, which has previously been interpreted from gravity and magnetic data. Density modeling tied to the velocity model shows that the Olympic accretionary wedge is indistinguishable from surrounding rocks below a depth of about 20 km. The contact between the Siletz and Pre-Tertiary basement rocks is a subtle contact as inferred from the velocity and gravity models.

The study in the Rocky Mountains is one component of the Continental Dynamics - Rocky Mountains Project (CD-RoM '99), a collaborative interdisciplinary study involving 14 American universities and the University of Karlsruhe, Germany that focuses on Precambrian features and their effects on Phanerozoic deformation. One of the major field efforts in the CD-RoM project took place during August, 1999. The University of Texas at El Paso and the University of Karlsruhe, with the assistance of several other institutions, collected data along a ~ 950 km long seismic refraction/wide-angle reflection profile extending from Fort Sumner, New Mexico to the Gas Hills, Wyoming. Station spacing was nominally 800 m using ~ 600 instruments during two deployments. Eleven shots were fired ranging in size from 167.2 - 4540.9 kg and were nominally spaced at ~ 100 km intervals along the profile. The profile crosses major structural features of the continent including the Jemez lineament, the Colorado mineral belt, and the Cheyenne belt (a prominent Proterozoic suture).

Velocity modeling, employing several techniques, indicates that crustal thicknesses ranges from ~ 45 to 55 km in New Mexico and Colorado. In northern Colorado, the crust begins to thin from ~ 50 and reaches ~ 40 km in Wyoming, north of the Cheyenne belt. A mid-crustal interface is very prominent within the data and can be thought of as the Conrad discontinuity. This interface falls at depths of about 25 to 30 km and is a discontinuity below which velocities increase to about 6.8 km/s. A high-velocity lowermost crustal layer with a thickness ranging from 5 to 10 km is evident in the Southern Rocky Mountains - Great Plains (SRM-GP) portion of the model. The velocity of this layer ranges from 7.0 to 7.4 km/s, a value that is consistent with a composition of mafic garnet granulite. One interpretation of this high-velocity lower crustal layer is that it originally formed during assembly of the Proterozoic terranes. Magmatic underplating at 1.4 Ga may have increased the thickness of this layer beneath the SRM-GP. This is not to say that the depth to the Moho has not been locally modified during Phanerozoic events, but that major modification took place during the Precambrian.

Snelson, C. M., T.M. Brocher, K.C. Miller, T.L. Pratt, and A.M. Tréhu, 2005, Seismic amplification within the Seattle basin, Washington: Insights from SHIPS seismic tomography experiments, Bull. Seism. Soc. Am., in journal review. PDF

Recent observations indicate that the Seattle sedimentary basin, underlying Seattle and other urban centers in the Puget Lowland, Washington, amplifies long period (1 to 5 s) weak ground motions by factors of 10 or more. We computed east-trending P- and S-wave velocity models across the Seattle basin from Seismic Hazard Investigations of Puget Sound (SHIPS) experiments in order to better characterize the seismic hazard the basin poses. The 3-D tomographic models, which resolve features to a depth of 10 km, for the first time define the P- and S-wave velocity structure of the eastern end of the basin. The basin, which contains sedimentary rocks of Eocene to Holocene age, is broadly symmetric in east-west section and reaches a maximum thickness of 6 km along our profile beneath north Seattle. A comparison of our velocity model with coincident amplification curves for weak ground motions produced by the 1999 Chi-Chi earthquake suggests that the distribution of Quaternary deposits, and reduced velocity gradients in the upper part of the basement east of Seattle, have significance in forecasting variations in seismic wave amplification across the basin. Specifically, eastward increases in the amplification of 0.2 to 5 Hz energy correlate to locally thicker unconsolidated deposits and a change from Crescent Formation basement to pre-Tertiary Cascadia basement. Amplification curves at 7 to 9 s periods mirror the E-W symmetry of the basin. Seismicity within the Seattle basin along the profile lines up with this inferred basement contact and coincides with proposed strike-slip faults of Eocene age.

Symons, N., 1998, Seismic velocity structure of the Puget Sound region from 3-D non-linear tomography, Ph. D. thesis, Univ. Washington, 168 p.

ten Brink, U.S., P.C. Molzer, M.A. Fisher, R. J. Blakely, R.C. Bucknam, T. Parsons, R.S. Crosson, and K.C. Creager, 2002, Subsurface geometry and evolution of the Seattle fault zone and the Seattle Basin, Washington, Bull. Seism. Soc. Am., 92, 1737-1753. PDF

The Seattle fault, a large, seismically active, east–west-striking fault zone under Seattle, is the best-studied fault within the tectonically active Puget Lowland in western Washington, yet its subsurface geometry and evolution are not well constrained. We combine several analysis and modeling approaches to study the fault geometry and evolution, including depth-converted, deep-seismic-reflection images, P-wave-velocity field, gravity data, elastic modeling of shoreline uplift from a late Holocene earthquake, and kinematic fault restoration. We propose that the Seattle thrust or reverse fault is accompanied by a shallow, antithetic reverse fault that emerges south of the main fault. The wedge enclosed by the two faults is subject to an enhanced uplift, as indicated by the boxcar shape of the shoreline uplift from the last major earthquake on the fault zone. The Seattle Basin is interpreted as a flexural basin at the footwall of the Seattle fault zone. Basin stratigraphy and the regional tectonic history lead us to suggest that the Seattle fault zone initiated as a reverse fault during the middle Miocene, concurrently with changes in the regional stress field, to absorb some of the north–south shortening of the Cascadia forearc. Kingston Arch, 30 km north of the Seattle fault zone, is interpreted as a more recent disruption arising within the basin, probably due to the development of a blind reverse fault.

Van Wagoner, T.M., R.S. Crosson, K.C. Creager, G.F. Medema, L.A. Preston, N.P. Symons, and T.M. Brocher, 2002, Crustal structure and relocated earthquakes in the Puget Lowland, Washington from high resolution seismic tomography, J. Geophys. Res., 107(B12), 2381, doi:10.10129/2001JB000710. PDF

The availability of regional earthquake data from the Pacific Northwest Seismograph Network (PNSN), together with active source data from the Seismic Hazards Investigation in Puget Sound (SHIPS) seismic experiments, has allowed us to construct a new highresolution 3-D, P wave velocity model of the crust to a depth of about 30 km in the central Puget Lowland. In our method, earthquake hypocenters and velocity model are jointly coupled in a fully nonlinear tomographic inversion. Active source data constrain the upper 10–15 km of the model, and earthquakes constrain the deepest portion of the model. A number of sedimentary basins are imaged, including the previously unrecognized Muckleshoot basin, and the previously incompletely defined Possession and Sequim basins. Various features of the shallow crust are imaged in detail and their structural transitions to the mid and lower crust are revealed. These include the Tacoma basin and fault zone, the Seattle basin and fault zone, the Seattle and Port Ludlow velocity highs, the Port Townsend basin, the Kingston Arch, and the Crescent basement, which is arched beneath the Lowland from its surface exposure in the eastern Olympics. Strong lateral velocity gradients, consistent with the existence of previously inferred faults, are observed, bounding the southern Port Townsend basin, the western edge of the Seattle basin beneath Dabob Bay, and portions of the Port Ludlow velocity high and the Tacoma basin. Significant velocity gradients are not observed across the southern Whidbey Island fault, the Lofall fault, or along most of the inferred location of the Hood Canal fault. Using improved earthquake locations resulting from our inversion, we determined focal mechanisms for a number of the best recorded earthquakes in the data set, revealing a complex pattern of deformation dominated by general arc-parallel regional tectonic compression. Most earthquakes occur in the basement rocks inferred to be the lower Tertiary Crescent formation. The sedimentary basins and the eastern part of the Olympic subduction complex are largely devoid of earthquakes. Clear association of hypocenters and focal mechanisms with previously mapped or proposed faults is difficult; however, seismicity, structure, and focal mechanisms associated with the Seattle fault zone suggest a possible high-angle mode of deformation with the north side up. We suggest that this deformation may be driven by isostatic readjustment of the Seattle basin.

Back to Top