Improving model constraints for vertical deformation across the northern Cascadia margin




Wolynec, Lisa.

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Over the past decade, patterns of horizontal crustal motion observed along the Cascadia subduction zone (CSZ) from Global Positioning System (GPS) measurements have been used to derive locked subduction zone models with varying geometry and coupling factors. Although vertical crustal deformation estimates have been less abundant and less accurate than horizontal component observations, they provide key constraints to the models for estimating the extent of rupture for the next subduction thrust earthquake. In order to provide updated model constraint estimates, the contemporary vertical deformation pattern across the northern Cascadia margin was investigated through the combined application of GPS, repeated leveling, precise gravity, and monthly mean sea level measurements across southern Vancouver Island and repeated leveling on the mainland. To the first order, these estimates are consistent with across-margin tilt predictions from current dislocation models for the region. In their details, however, they reflect a more complex system than suggested by the simple models. Minor landward tilt across the margin at Tofino determined from the re-analyses of -8 years of continuous vertical GPS positions, -40 years of monthly mean sea levels and long-term time (decadal) intervals of repeat leveling surveys is distinctly different than the -3 mm yr'l of landward tilt observed at Neah Bay. While this difference may be minimized by allowing for a small amount of tilt induced at the southern stations from northward migration of the Cascadia forearc, differences in tilting of 3-4 mm yr'l between short- and long-term estimates of repeat leveling at Bamfield are attributed to transients. To a lesser degree, elevation changes across the margin at Tofino may also illustrate transients. As well, distinct differences in the magnitude of vertical deformation for stations to the north and south of Barkley Sound suggest that differential deformation may be occurring along the margin. Similarly, while repeat relative gravity measurements across the margin at Tofino indicate 3-7 mm yr-' of seaward tilt (at odds with results from all other methods), a temporal dependence of vertical deformation might also be evident from the long-term versus short-term tilt rates. However, although repeat absolute gravity estimates between 1995 and 2002 indicate little across-margin tilt, consistent with continuous GPS results, differences between the time series at the Ucluelet absolute gravity and GPS stations indicate that gravity observations could be influenced by episodic mass redistribution beneath western Vancouver Island. This suggests that gravity results might not be directly comparable to estimates from other geodetic methods in determining uplift rates. Extension of the vertical deformation profile eastward into the backarc using repeat leveling surveys indicates a broad region of uplift in the Pemberton area with respect to the coast, which is consistent with the vertical component at the continuous GPS station WSLR. Current dislocation models cannot account for the observed deformation. Therefore, modification of one model was attempted in which a weaker crustal zone, coincident with high heat flow near the Garibaldi Volcanic Arc, was included. A poor fit to the observed deformation rates indicates that further refinements must be made to such a model. Nonetheless, these results suggest a complex system of strain accumulation in the northern CSZ, which may result from a greater 3- dimensionality of the tectonic controls than current dislocation models of the region employ.