Investigating vertical motion discrepancies in Southern California using geologic, geodetic, and well log data

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Geodetic and geologic vertical velocity measurements record uplift and subsidence throughout Southern California that provide, in some regions, very different perspectives of vertical deformation. In this study we compare vertical geologic velocities from the SCEC Vertical Motion Database and GPS velocities from the EarthScope Plate Boundary Observatory. Analyzing the relationship between geologic and GPS vertical data is nontrivial, as these data sets differ in geographic coverage area, sampling time scales, uncertainties, spatial resolution, and signal source. As these data sets are not spatially co-located, several different interpolation techniques were utilized for optimal analysis of the data. Our major finding is that the geologic and GPS vertical motions in Southern California are not well correlated, regardless of the technique used to compare the data sets. In particular, we identify significant discrepancies north of San Diego and north and west of Los Angeles. Using vertical velocities estimated by a 3-D earthquake cycle deformation model, geologic and GPS data show modest correlation when limiting the data sets to vertical motions that are dominated by large-scale tectonic motions, as opposed to those possibly due to local deformation sources. Since anthropogenic effects may contaminate some of the GPS data signal, a first-order groundwater correction was developed to isolate groundwater deformation recorded in the GPS data. Our results suggest a slight improvement in the correlation between geologic and GPS data corrected for groundwater deformation, although future work is needed to develop a heterogeneous ground water to vertical motion ratio model.

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