[1] We quantify the spatial pattern of uplift rate in western Oregon and northernmost California using tidal and leveling records to better understand the pattern of interseismic locking on the Cascadia subduction zone. We extend relative sea level time series of the six primary NOAA tide gauges to include all observations from 1925 to 2006. Previously unidentified bench mark instability biases portions of tidal records by 1.6 mm a À1 before correction. We determine precise relative uplift rates at the six tidal sites with an adjustment that includes rates of differenced time series as additional constraints. Our analysis of National Geodetic Survey leveling data between tide gauges corrects errors in 1941 leveling, and 184 secondary ties double the number of highest quality uplift rate estimates. Relative uplift rates from leveling are adjusted to the tidal rates, accounting for uncertainties in both data types. Tidal and leveling uplift rates agree within error for all but one of the coastal segments, where we infer systematic leveling error affects the 1988 line. Uplift rates are made absolute using an interval and location-specific geocentric sea level rise rate of 2.3 ± 0.2 mm a À1 . Total propagated one sigma errors for the absolute uplift rates of bench marks are $0.4 mm a À1 . Along-strike changes in uplift rate near 45°N and 42.8°N require two distinct changes in locking depth, as inferred from elastic dislocation modeling. The along-strike changes in locking on this portion of the Cascadia subduction zone interface correlate to the western and southern extent of the mafic Siletzia block in the fore arc.

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Figure 1. A: Major plate tectonic features of Central America. Names of principle plates are in bold and underlined. SOAM-South America, CARIB-Caribbean, SPDB-South Panama deformed belt, NPDB-North Panama deformed belt, CCRDB-Central Costa Rica deformed belt, PPB-paleoplate boundary, RJ-ridge jump. Inset shows location of B. Major tectonic features are compiled from Mackay and Moore (1990), Silver et al. (1990), Meschede et al. (1998), Barckhausen et al. (2000), and Marshall et al. (2000). Modern plate motion relative to Caribbean is from DeMets et al. (1990) and Protti et al. (1995). Small numbers give age of Cocos seafloor. B: Generalized geologic map of Costa Rica, bathymetry of Cocos plate and location of study area on Península de Nicoya (inset). FSG, Fisher seamount group; QP, Quepos Plateau; CR, Cocos Ridge. Geologic units: Nicoya Complex, random dash pattern; active arc volcanic rocks, random double-dash pattern; extinct-arc volcanic rocks, stipple pattern; Cretaceous and Cenozoic fluvial to marine clastic and volcaniclastic rocks, horizontal line pattern. Bathymetric data are from von Huene and Fluh (1994). Magnetic anomolies are from Barckhausen et al. (2000). Contour interval is 100 m, bold, and labeled every 1 km. Black dot with surrounding hachured area shows epicenter and aftershock extent of March 25, 1990, Golfo de Nicoya subduction earthquake (Mw ‫؍‬ 7.0, depth ‫؍‬ 20 km) and associated focal mechanism (Protti et al., 1995).