Investigating geologic and geodetic vertical motion discrepancies of the Southern San Andreas Fault System Garrett M. Thornton and Bridget R. Smith-Konter Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968 gmthornton@miners.utep.edu An accurate characterization of the 3D spatial and temporal distribution of earthquake cycle deformation is an essential component of seismic hazard mitigation along the San Andreas Fault System (SAFS). Arrays of seismometers along the SAFS provide tight constraints on the coseismic processes of the earthquake cycle, but geodetic and geologic measurements are needed for understanding the slower processes. Furthermore, while the horizontal velocity field of the SAFS has been thoroughly examined, vertical velocity measurements are often ignored. Here we investigate the relationship between available vertical geologic data from the SCEC Vertical Motion Database (Niemi et al., 2008) and geodetic data primarily from the EarthScope Plate Boundary Observatory. The geologic dataset comprises over 1800 observations located in southern California ranging from -7 to 14 mm/yr with uncertainties on the order of 0.5 mm/yr. These data reflect the motions of rocks ranging in age from 10 Ka to 1 Ma and primarily reflect long time-scale tectonic motions, however these data are spatially limited to a 24,000 km2 region west of the SAFS. The geodetic dataset reflects GPS vertical velocities from 1100 stations distributed throughout California, ranging from -50 to 18 mm/yr with an average uncertainty of 2.7 mm/yr. These data reflect average vertical motions over the past 20 years associated with both tectonic events and anthropogenic effects such as groundwater pumping and hydrocarbon extraction. Initial comparison of the geologic and geodetic vertical motion data in California reveals a poor agreement (Fig. 1). The fundamental issue is that these datasets are not spatially co-located, thus we explore several different surface and triangulation techniques for optimal analysis of the data. While the number of available data points for comparison largely depends on the chosen interpolation technique, all methods suggest that the relationship between geologic and geodetic vertical data is not 1:1. In regions of subsidence, for example, the geodetic rates are often twice as fast as the geologic rates. In several locations the observations are even anticorrelated, suggesting that the respective timescales of the data play an important role. Since anthropogenic effects may contaminate some of the geodetic data signal, we also use a vertical velocity crustal deformation model of the SAFS to help identify locations where tectonic motions should dominate the data signals. We adjust the elastic plate thickness of the model to explore the sensitivity of data to this parameter and find that both datasets correlate best with a thick elastic plate. As we continue to explore the vertical motion discrepancies of the SAFS using regional tide gauge records along the California coast, we emphasize the fundamental need for future deployment of geodetic arrays in locations complimentary to existing geologic observations.

Fig. 1. Vertical motion differences between geologic and geodetic datasets. This map reveals significant variations in the two datasets (geologic-geodetic), with colors saturated at +/- 4 mm/yr. A simple blockmedian surface interpolation was used to construct this map.