VERTICAL CRUSTAL DEFORMATION IN CASCADIA FROM HISTORICAL LEVELING

The Cascadia subduction zone has been hypothesized as a location for a large megathrust earthquake by many researchers. Geodetic, GPS, and strain data provide evidence for active crustal strain buildup throughout the region and based on models of crustal deformation, the potential for a M>9 event has been suggested. Large earthquakes have also ocurred on crustal faults and within the subducting Juan de Fuca plate.

A better understanding of the vertical deformation throughout Cascadia will help model coupling along the plate boundary and eventually help us better assess the earthquake potential in the region. In this study, I analyze historical leveling data from Cascadia to produce a more comprehensive map of uplift rates.

Crustal uplift rates are calculated from repeated differential leveling. I calculate rates by taking the difference between elevations at common benchmarks and dividing by the time between surveys. I identify and remove outliers using a least square B-spline and median absolute deviation technique. The uplift rates are then adjusted to a common datum by a weighted least squares procedure using a bivariate cosine series as the basis function. The data are referenced to sea level by including uplift rates calculated from tide gauges (Weldon, personal communication). These uplift rates are weighted more heavily than the leveling data and are held fixed during the adjustment. Once outliers are identified and the data adjusted to a common datum, I smooth the data using an iterative, robust, weighted, moving average technique.

The results indicate several regions of uplift along the coast with some regions extending inland. Although the coastal uplift continues inland, landward tilting appears to dominate the deformation. The rate of landward tilting varies significantly along the coast with the fastest deformation occurring to the north. At about 45°N , there is little vertical deformation either at the coast or inland. In an earlier study I suggest that the subduction zone deformation is segmented (Verdonck, 1995). The lack of vertical deformation across central Oregon may represent a transition in subduction behavior between northern and southern segments.