- Open Access
Tomographic image of low P velocity anomalies above slab in northern Cascadia subduction zone
Earth, Planets and Space volume 53, pages285–293(2001)
At the Cascadia margin the Juan de Fuca plate is subducting beneath the North America plate, causing active seismicity within both plates. Earthquakes occur down to a maximum depth of 80 km within the descending oceanic plate and to about 30 km in the overriding continental plate. We use a method of seismic tomography to invert 28,230 P wave arrival times from 2666 local earthquakes that occurred in and around Vancouver Island from 1970 to 1990. The tomography model uses about 30 km horizontal and 12–19 km vertical grid spacing and assumes that the seismic velocity perturbations vary continuously between grid points. Velocity structures can be obtained to a depth of 65 km. The obtained tomographic image shows an extensive low velocity zone above the subducted slab at about 45 km depth and patches of low velocities at shallower depths just seaward of the volcanic front. The deeper extensive low velocity zone may indicate the presence of partially hydrated mantle, most likely serpentinite, as a result of slab dehydration associated with the transformation of metabasalt to eclogite. One of the shallow low velocity patches coincides with an abrupt increase in surface heat flow and may reflect the presence of partial melts or water in the crust.
Auld, D. R., S. E. Dosso, D. W. Oldenburg, and L. K. Law, Monitoring temporal change in conductivity in the central Vancouver Island region, an area with past large earthquakes, Can. J. Earth Sci., 29, 601–608, 1992.
Benz, H. M., G. Zandt, and D. H. Oppenheimer, Lithospheric structure of northern California from teleseismic images of the upper mantle, J. Geophys. Res., 97, 4791–4807, 1992.
Bostock, M. G. and J. C. VanDecar, Upper mantle structure of the northern Cascadia subduction zone, Can. J. Earth Sci., 32, 1–12, 1995.
Cassidy, J. F. and R. M. Ellis, Shear wave constraints on a deep crustal reflective zone beneath Vancouver Island, J. Geophys. Res., 96, 19,843–19,851, 1991.
Cassidy, J. F. and R. M. Ellis, S-wave velocity structure of the northern Cascadia subduction zone, J. Geophys. Res., 98, 4407–4421, 1993.
Christensen, N., Elasticity of ultrabasic rocks, J. Geophys. Res., 71, 5921–5931, 1966.
Clowes, R. M., C. A. Zelt, J. R. Amor, and R. M. Ellis, Lithospheric structure in the souther Canadian Cordillera from a network of seismic refraction lines, Can. J. Earth Sci., 32, 1485–1513, 1996.
Clowes, R. M., D. J. Baird, and S. A. Dehler, Crustal structure of the Cascadia subduction zone, southwestern British Columbia from potential field and seismic studies, Can. J. Earth Sci., 34, 317–335, 1997.
Cook, F. A., A. G. Green, P. S. Simony, R. A. Price, R. Parrish, B. Milkereit, P. L. Gordy, R. L. Brown, K. C. Coflin, and C. Patenaude, Lithoprobe seismic reflection structure of the southern Canadian Cordillera: Initial results, Tectonics, 7, 157–180, 1988.
Crosson, R. S. and T. J. Owens, Slab geometry of the Cascadia subduction zone beneath Washington from earthquake hypocenters and teleseismic converted waves, Geophys. Res. Lett., 14, 824–827, 1987.
Davis, E. E. and R. D. Hyndman, Accretion and recent deformation of sediments along the northern Cascadia subduction zone, Geol. Soc Am. Bull., 101, 1465–1480, 1989.
Flück, P., R. D. Hyndman, and K. Wang, Three-dimensional dislocation model for great earthquakes of the Cascadia subduction zone, J. Geophys. Res., 102, 20,539–20,550, 1997.
Harris, R. A., H. M. Iyer, and P. B. Dawson, Imaging the Juan de Fuca plate beneath southern Oregon using teleseismic P wave residuals, J. Geophys. Res., 96, 19,879–19,889, 1991.
Hasegawa, A., D. Zhao, S. Hori, A. Yamamoto, and S. Horiuchi, Deep structure of the northeastern Japan arc and its relationship to seismic and volcanic activity, Nature, 352, 683–689, 1991.
Hussong, D. M. and S. Uyeda, Tectonic processes and the history of the Mariana arc: A synthesis from results of DSDP Leg 60, Init. Rep. Deep Sea Drilling Proj., 60, 909–929, 1982.
Hyndman, R. D., Dipping seismic reflectors, electrically conductive zones, and trapped water in the crust over a subduction plate, J. Geophys. Res., 93, 13,391–13,405, 1988.
Hyndman, R. D., The Lithoprobe corridor across the Vancouver Island continental margin: The structural and tectonic consequences of subduction, Can. J. Earth Sci., 32, 1777–1802, 1995.
Hyndman, R. D. and T. J. Lewis, Review: The thermal regime along the southern Canadian Cordillera Lithoprobe corridor, Can. J. Earth Sci., 32, 1611–1617, 1995.
Hyndman, R. D., C. J. Yorath, R. M. Clowes, and E. E. Davis, The northern Cascadia subduction zone at Vancouver Island: Seismic structure and tectonic history, Can. J. Earth Sci., 27, 313–329, 1990.
Kisslinger, E. and J. C. Lahr, Tomographic image of the Pacific slab under southern Alaska, Eclogae Geol. Helv., 84, 297–315, 1991.
Kurtz, R. D., J. M. Delaurier, and J. C. Gupta, A magnetotelluric sounding across Vancouver Island detects the subducting Juan de Fuca plate, Nature, 321, 596–599, 1986.
Kurtz, R. D., J. M. Delaurier, and J. C. Gupta, The electrical conductivity distribution beneath Vancouver Island: A region of active plate subduction, J. Geophys. Res., 95, 10,929–10,946, 1990.
Lees, J. M. and R. S. Crosson, Tomographic inversion for three-dimensional velocity structure at Mount St. Helens using earthquake data, J. Geophys. Res., 94, 5716–5728, 1989.
Lees, J. M. and R. S. Crosson, Tomographic imaging of local earthquake delay times for three-dimensional velocity variation in western Washington, J. Geophys. Res., 95, 4763–4776, 1990.
Lewis, T. J., W. H. Bentkowski, E. E. Davis, R. D. Hyndman, J. G. Souther, and J. A. Wright, Subduction of the Juan de Fuca plate: Thermal consequences, J. Geophys. Res., 93, 15,207–15,225, 1988.
Lewis, T. J., W. H. Bentkowski, and R. D. Hyndman, Crustal temperatures near the Lithoprobe Southern Canadian Cordillera Transect, Can. J. Earth Sci., 29, 1197–1214, 1992.
Michaelson, C. A. and C.S. Weaver, Upper mantle structure from teleseismic P arrivals in Washington and northern Oregon, J. Geophys. Res., 91, 2077–2094, 1986.
Moran, S. C., J. M. Lees, and S. D. Malone, P wave crustal velocity structure in the greater Mount Rainier area from local earthquake tomography, J. Geophys. Res., 104, 10,775–10,786, 1999.
Peacock, S. M., Thermal and petrological structure of subduction zones, in Subduction: Top to Bottom, edited by G. Bebout, D. W. Scholl, S.H. Kirby, and J. P. Platt, pp. 119–133, American Geophysical Union, Washington, D. C., 1996.
Peacock, S. M. and K. Wang, Seismic consequences of warm versus cool subduction metamorphism: Examples from Southwest and Northeast Japan, Science, 286, 937–939, 1999.
Rasmussen, J. and E. Humphreys, Tomographic image of the Juan de Fuca plate beneath Washington and western Oregon using teleseismic P-wave travel times, Geophys. Res. Lett., 15, 1417–1420, 1988.
Riddihough, R. P., Gravity and structure of an active margin—British Columbia and Washington, Can. J. Earth Sci., 16, 350–363, 1979.
Rogers, G. C., Seismotectonics of British Columbia, PhD thesis, University of British Columbia, Vancouver, British Columbia, 247 pp., 1983.
Rogers, G. C. and H. S. Hasegawa, a second look at the British Columbia earthquake of 23 June, 1946, Bull. Seism. Soc. Am., 68, 653–676, 1978.
Rogers, G. C., C. Spindler, and R. D. Hyndman, Seismicity along the Vancouver Island Lithoprobe Corridor, in Proceedings of the Project Lithoprobe: Southern Canadian Cordillera Transect Workshop, pp. 166–169, University of Calgary, Calgary, Alberta, 1990.
Spence, G. D. and N. A. Mclean, Seismic structure across the active subduction zone of western Canada, Can. J. Earth Sci, 36, 1999 (in press).
Spence, G. D., R. D. Hyndman, E. E. Davis, and C. J. Yorath, Seismic structure of the northern Cascadia accretionary prism: Evidence from new multichannel seismic reflection data, in Continental Lithosphere: Deep Reflections, Geodynamics 22, pp. 257–263, American Geophysical Union, Washington, D.C., 1991.
Stanley, D., A. Villasenor, and H. Benz, Subduction zone and crustal dynamics of western Washington: A tectonic model for earthquake hazards evaluation, U.S. Geological Survey Open-File Report 99–311, 1999.
Suyehiro, K., N. Takahashi, Y. Ariie, Y. Yokoi, R. Hino, M. Shinohara, T. Kanazawa, N, Hirata, H. Tokuyama, and A. Taira, Continental crust, crustal underplating, and low-Q upper mantle beneath and oceanic island arc, Science, 272, 390–392, 1996.
Taber, J. J. and S. W. Smith, Seismicity and focal mechanisms associated with the subduction of the Juan de Fuca plate beneath the Olympic Peninsula, Washington, Bull. Seism. Soc. Am., 75, 237–249, 1985.
VanDecar, J. C., Upper mantle structure of the Cascadia subduction zone from non-linear teleseismictravel-time inversion, Ph.D. thesis, University of Washington, Seattle, Washington, 1991.
Verdonck, D. and G. Zandt, Three-dimensional crustal structure of the Mendocino triple junction region from local earthquake travel times, J. Geophys. Res., 99, 23,843–23,858, 1994.
Wahlstrom, R. and G. C. Rogers, Relocation of earthquakes west of Vancouver Island, British Columbia, 1965–1983, Can. J. Earth Sci., 29, 953–961, 1992.
Wang, K., T. Mulder, G. C. Rogers, and R. D. Hyndman, Case for very low coupling stress on the Cascadia subduction fault, J. Geophys. Res., 100, 12,907–12,918, 1995.
Zelt, B. C., R. M. Ellis, and R. M. Clowes, Crustal velocity structure in the eastern Insular and southernmost Coast Belts, Can. J. Earth Sci., 30, 1014–1027, 1993.
Zelt, B. C., R. M. Ellis, and R. M. Clowes, Inversion of three-dimensional wide-angle seismic data from the southwestern Canadian Cordillera, J. Geophys. Res., 101, 8503–8529, 1996.
Zhao, D., A. Hasegawa, and S. Horiuchi, Tomographic imaging of P and S wave velocity structure beneath northeastern Japan, J. Geophys. Res., 97, 19,909–19,928, 1992.
Zhao, D., A. Hasegawa, and H. Kanamori, Deep structure of Japan subduction zone as derived from local, regional, and teleseismic events, J. Geophys. Res., 99, 22,313–22,329, 1994.
Zhao, D., D. Christensen, and H. Pulpan, Tomographic imaging of the Alaska subduction zone, J. Geophys. Res., 100, 6487–6504, 1995.
Zhao, D., H. Kanamori, H. Negishi, and D. Wiens, Tomography of the source area of the 1995 Kobe earthquake: evidence for fluids at the Hypocenter?, Science, 274, 1891–1893, 1996.
About this article
Cite this article
Zhao, D., Wang, K., Rogers, G.C. et al. Tomographic image of low P velocity anomalies above slab in northern Cascadia subduction zone. Earth Planet Sp 53, 285–293 (2001). https://doi.org/10.1186/BF03352385
- Subduction Zone
- Velocity Structure
- Velocity Perturbation
- Volcanic Front
- Tomographic Inversion