Tomography - Thomas Brocher, Tom Parsons, Michael A. Fisher, Anne M. Tréhu, George D. Spence and the SHIPS Working Group*.


123°40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'


Arc Explorer
Themes visible in the above map: TOMO1KM(Image), LATITUDE/LONGITUDE and COASTLINE. When the tomography data is viewed in Arc Explorer, please note that the TOMO1KM(Image) theme is an image and therefore not able to be queried. The TOMOBAR (Image) provides a scale for TOMO1KM (Image).
48°30'
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10'
05'
48°00'
123°40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'



Arc Explorer
Themes visible in the above map: TOMO3KM(Image), LATITUDE/LONGITUDE and COASTLINE. When the tomography data is viewed in Arc Explorer, please note that the TOMO3KM(Image) theme is an image and therefore not able to be queried. The TOMOBAR (Image) provides a scale for TOMO3KM (Image).
48°30'
25'
20'
15'
10'
05'
48°00'
123°40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'

 

 

Links: Clicking on the kilometre labels to the left of and below the map will link to figures of the tomographic cross sections.

Arc Explorer
Themes visible in the above map: TOMO5KM(Image), LATITUDE/LONGITUDE and COASTLINE. When the tomography data is viewed in Arc Explorer, please note that the TOMO5KM(Image) theme is an image and therefore not able to be queried. The TOMOBAR (Image) provides a scale for TOMO5KM (Image).

Three-Dimensional Tomography in the Eastern Juan de Fuca Strait: Preliminary Results from SHIPS, the 1998 Seismic Hazards Investigation in Puget Sound.
Tom Brocher, Tom Parsons, Michael A. Fisher, Anne M. Tréhu, George D. Spence and the SHIPS Working Group

The SHIPS Working Group includes Uri S. ten Brink, Thomas L. Pratt, Robert S. Crosson, Kenneth C. Creager, Kate C. Miller, Victoria E. Langenheim, Roy A. Hyndman, David C. Mosher, Barry C. Zelt, Richard E. Blakely, Nicolas I. Christensen, Ray E. Wells, and Craig S. Weaver

In March 1998 SHIPS participants conducted onshore-offshore wide-angle and multichannel seismic (MCS) reflection profiling throughout the Puget Lowland using a 79 or 110 liter (4838 or 6730 cu.in.) air gun array (Fisher et al., 1999). MCS profiling was performed in Puget Sound, the Juan de Fuca Strait, and the Strait of Georgia. The air gun shots were recorded by a temporary array of 210 seismographs deployed around Puget Sound and in the Juan de Fuca Strait (Brocher et al., 1999). temporary array of 210 onshore seismographs deployed around Puget Sound and the Strait of Juan de Fuca (Brocher et al., 1999). All seismographs recorded the vertical geophone component and of these 75 seismographs recorded two horizontal geophone components in addition.

Travel Time Inversion for the Upper Crustal Velocity Structure of Puget Lowland

A total of 977,000 P wave first-arrival travel times from controlled sources were included in our velocity modeling. We estimate picking errors to be 100 ms (one cycle at 10 Hz). All arrivals used were refractions from the upper crust generally at source-receiver offsets less than 100 km. The SHIPS data were acquired with a shot spacing between 50 and 150 m and a receiver spacing of 5 to 15 km. Note that the absence of deeper source and long offsets limit the deeper ray coverage and resolution of the model.

To determine the crustal velocities, we applied the 3-D tomographic technique of Hole (1992), and followed the same procedures as detailed by Parsons et al. (1999). This technique applies a finite difference solution to the eikonal equation (Vidale [1990]; updated by Hole and Zelt [1995]) to calculate first arrival times through a gridded slowness model. An iterative nonlinear inversion is performed as a back projection along ray paths determined from the forward modeling step.

We used hit counts to determine ray coverage and checkerboard tests to estimate the spatial and velocity resolution of the solution. Because all of our sources are located at the surface, maximum ray densities decrease with depth as the ray coverage becomes more uniform. The checkerboard tests reveal that the spatial and relative velocity resolution varies with depth. The checkerboard tests indicate that anomalies >10 km across in the lateral dimensions are generally resolvable.

On the basis of the distribution of seismic sources we sought to resolve velocity anomalies >10 km across the lateral dimensions and >2 km thick in the vertical dimension. We thus applied a 5-km-wide by 1-km-high smoothing filter during the final iteration, which yielded a RMS travel time misfit of 0.09 s.

Port Townsend Basin

We informally use the name, Port Townsend basin, for the 40-km-long by 15-km-wide velocity low that is bounded by the confluence of Southern Whidbey Island, Leech River and Devils Mountain-Darrington faults in the eastern Juan de Fuca Strait. Most of the basin lies offshore (see Structure Map). N-S cross sections (Figure 2) show that the basin is asymmetric, thickening southward, consistent with industry and SHIPS seismic reflection data. In E-W cross section (Figure 1) the basin appears more symmetric than in NS cross section. The basin lacks a prominent gravity anomaly, and as is evident on most depth slices the basin has relatively high velocities, suggesting that the basin is thinner than the Seattle, Tacoma or Everett basins. The lower boundary of the basin is approximately identified as the 5.5 km/s contour (i.e., base of yellow on velocity color scale).

Sequim Fault

The southern boundary of the low velocity anomaly defining the Port Townsend basin (see Structure Map) coincides with an E-W trending aeromagnetic anomaly bounding Eocene volcanic rocks to the south and a thick section of Tertiary sedimentary rocks to the north. This relationship has lead to its interpretation as a north side down structure (fault F of Gower et al., 1985). We refer to this fault informally as the Sequim fault, for the town of Sequim lying just north of the fault. Based on Mobil seismic reflection line W70-13, Johnson et al. (1996) interpreted the Sequim fault as a south-dipping, north-vergent thrust fault. The tomography model is consistent with the continuation of the Sequim fault as far east as the Southern Whidbey Island fault and show that the south side of the fault is structurally higher than the north side. Structural relief on the Sequim fault is largest in the vicinity of the Port Townsend basin.

Devils Mountain fault

The San Juan Islands, the southeastern tip of Vancouver Island, and the eastern Juan de Fuca Strait west of the Leech River fault, all located north of the Devils Mountain fault, are associated with relatively high velocities at all depths in the tomography model (see Structure Map). SHIPS seismic reflection lines JDF 1 through 4 show only thin sediments covering the basement rocks north of the Devils Mountain fault. The tomography model indicates that the north side of the Devils Mountain fault is structurally high, consistent with its interpretation as a north-dipping, south-vergent thrust fault by Johnson et al. (in prep.). In NS cross section (Figure 2) structural relief on the Devils Mountain fault inferred from the tomography model increases to the west, reaching its maximum within the Port Townsend basin. Industry seismic reflection data presented by Johnson et al. (in prep.) also show this E-W variation in structural relief.

ACKNOWLEDGMENTS

*The SHIPS Working Group includes Uri S. ten Brink, Thomas L. Pratt, Robert S. Crosson, Kenneth C. Creager, Kate C. Miller, Victoria E. Langenheim, Roy A. Hyndman, David C. Mosher, Barry C. Zelt, Richard J. Blakely, Nicolas I. Christensen, Ray E. Wells, and Craig S. Weaver.

This work was supported by the USGS Marine and Coastal Studies Programs, USGS Seattle Area Natural Hazards Initiative, USGS Venture Capital Fund, and external grants from the USGS National Earthquake Hazards Reduction Program to Oregon State University, the University of Texas El Paso, and the University of Washington. Additional support was given by the Geological Survey of Canada and the U.S. Minerals Management Service.

Reftek seismographs were provided by the IRIS/PASSCAL Instrument facility (at Stanford); we thank Marcos Alvarez, Bruce Beaudoin, Russ Sells and Allan Swisenbank for their support during SHIPS. Tom Burdette organized the fieldwork in the USA. The Reftek array was deployed by Tanni Abramovitz, Steve Azevedo, Becky Barnhart, Rick Benson, Jeff Brody, Tom Burdette, Dean Childs, Cormack Craven, Brian Creaser, Shari Curry, Katherine Favret, Nilanjan Ganguly, Alex Gerst, Pat Hart, Brett Hiett, Sonja Hofmann, Lisa Jackson, Charlotte Keller, Fiona Kilbride, Adrian Kropp, Holger Mandler, Karen Meagher, Yanpeng Mi, Uli Micksch, Diane Minasian, Anubrati Mukherjee, Janice Murphy, Bob Norris, Terry O'Donnell, Jr., Leiph Preston, Tony Qamar, Baishali Roy, Byron Ruppel, Kari Sauers, Gillian Shearer, Ray Sliter, Alex Smith, Cathy Snelson, Neill Symons, Lori Tapia-Piozet, George Thomas, Annette Veilleux, Andy Wuestefeld, Tom Yelin, Phil Young, and Tianson Yuan. Steve Azevedo, Jeff Brody, Brian Creaser, Katherine Favret, Brett Hiett, Leiph Preston, and Craig Tiballi processed the Reftek data.

We thank the Washington State Departments of Forestry and Parks and Recreation, Olympic National Forest and Park, the Bureau of Land Management, and several smaller parks and public entities for permission to access land under their jurisdiction. We thank the Weyerhaeuser Corp., International Paper Co., and numerous smaller property owners for permission to access their land.

References

Brocher, T.M., T. Parsons, K.C. Creager, R.S. Crosson, N.P. Symons, G.D. 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, K.E. Louden, and C.S. Weaver, Wide-angle seismic recordings from the 1998 Seismic Hazards Investigation of Puget Sound (SHIPS), western Washington and British Columbia, US Geological Survey Open-file Report 99-314, 110 pp, 1999.

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.C. Mosher, G. Spence, BC Zelt, P.T. Hammer, U.S. ten Brink, T.L. Pratt, K.C. Miller, J.R. Childs, G.R. Cochrane, S. Chopra, and R. Walia, Seismic survey probes urban earthquake hazards in Pacific Northwest, EOS, Trans. Am. Geophys. Un., 80, no. 2, 13-17, 1999.

Gower, H.D., J.C. Yount, and R.S. Crosson, Seismotectonic map of the Puget Sound region, Washington, US Geological Survey Misc. Invest. Series Map I-1613, scale 1:250,000, 1985.

Hole, J.A., Nonlinear high-resolution three-dimensional seismic travel time tomography, J. Geophys. Res., 97, 6553-6562, 1992.

Hole, J.A., and B.C. Zelt, 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 427-434, 1995.

Johnson, S.Y., C.J. Potter, J.M. Armentrout, J.J. Miller, C., Finn, and C.S. Weaver, The southern Whidbey Island fault, western Washington-An active structure in the Puget Lowland, Washington, Geol. Soc. Am. Bull., 108, 334-354, and oversized insert, 1996.

Johnson, S.Y., S.V. Dadisman, D.C. Mosher, R.J. Blakely, and J.R. Childs, Late Quaternary tectonics of the Devils Mountain fault and related structures, northern Puget Lowland, in prep., 2000.

Parsons, T., R.E. Wells, M.A. Fisher, E. Flueh, and U.S. ten Brink, Three-dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington, J. Geophys. Res., 104, 18,015-18,039, 1999.

Tabor, R. W., and W.M. Cady, Geologic map of the Olympic Peninsula, Washington, US Geological Survey Misc. Invest. Series Map I-994, 1978. Two sheets.

Vidale, J.E., Finite-difference calculation of traveltimes in three dimensions, Geophysics, 55, 521-526, 1990.

Wells, D.L., and K.J. Coppersmith, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bull. Seismol. Soc. Am., 84, 974-1002, 1994.

 

Reference citation:
Brocher, T., Parsons, T., Fisher, M.A., Tréhu, A.M., Spence, G.D., and the SHIPS Working Group, 2000. Three-dimensional tomography in the eastern Juan de Fuca Strait: preliminary results from SHIPS, the 1998 Seismic Hazards Investigation in Puget Sound,
in: Mosher, D.C. and Johnson, S.Y. (Eds.), Rathwell, G.J., Kung, R.B., and Rhea, S.B. (Compilers), Neotectonics of the eastern Juan de Fuca Strait; a digital geological and geophysical atlas. Geological Survey of Canada Open File Report 3931

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