Deciphering tectonic, isostatic, oceanic and anthropogenic signals in combined GPS, absolute gravity, InSAR, and tide gauge data in the northern Cascadia subduction zone

Mazzotti, Stephane; Lambert, Anthony; Henton, Joseph; Dragert, Herb; Schmidt, Schmidt
1. Geological Survey of Canada, Sidney BC, Canada; smazzotti@nrcan.gc.ca

Geodetic measurements in southwestern Canada are affected by a variety of processes over different spatial and temporal scales. On a regional scale, the present-day interseismic locking of the Cascadia subduction thrust results in margin-normal shortening and tilting of the Cascadia forearc. Although clear ? and fairly simple ? in horizontal GPS data, this subduction-loading signal is not well resolved in geodetic data that sense vertical motion (e.g., leveling, absolute gravity, tide gauge, InSAR). This is due to the fact that those techniques are also sensitive to a variety of other processes on a regional scale (e.g., postglacial rebound, sea-level rise) and more local scales (e.g., anthropogenic subsidence.). Deciphering and quantifying these various processes and their respective effects can be attempted by combining the geodetic datasets and comparing them with numerical model predictions. This type of analysis also reveals limitations and discrepancies among the different techniques. For example, we estimate the 20th century regional sea-level rise to be either +1.8 ñ 0.2 mm/a or -0.5 ñ 0.3 mm/a, depending on whether the tide gauge data are aligned to the ITRF200 or to the regional absolute gravity reference frame. On a more local scale, the combination of geodetic data and numerical models can be used to discriminate between large-scale and local-source signals. We show the example of the Fraser River Delta (greater Vancouver), where local vertical motion measured by leveling, GPS, and InSAR can be partitioned between long-term tectonics (~0 mm/a) and natural sediment compaction (~2 mm/a), and transient anthropogenic effects (primarily induced load compaction, 5 ? 10 mm/a).