Comparing GPS and Geologic Vertical Velocities in Southern California

Abstract:

Horizontal geologic and geodetic slip rate discrepancies for active faults in Southern California are well documented, however this discrepancy is significantly larger in the vertical direction. In turn, rheological models used to constrain fault slip rates generally exclude vertical geodetic motions. Here we compare geologic vertical rates from the Southern California Earthquake Center Vertical Motion Database and vertical GPS velocities from the EarthScope Plate Boundary Observatory (PBO). These two data sets are not necessarily co-located in space and the geologic data are observed using four different markers: thermo-chronologic (TH), river terraces (RT), stratigraphic horizons (SH) and marine terraces (MT). Therefore, we extract subsets of the geologic data and perform geostatistical analysis of these data to better understand their spatial dependency. Overall, the combined raw data correlation coefficient (R) is ~0.1, while for the subset data, R ranges from -0.3 to 0.3. The geologic data from SH primarily indicate subsidence, but the geologic data from other sources (TH, MT, and RT) primarily indicate uplift, though the SH and RT samples are mostly from the same geographic location. The GPS data are also likely to be contaminated by high-frequency non-tectonic noise (e.g., groundwater withdrawal). Therefore, we also compare the geologic data to a smoothed GPS velocity field derived from a statistically robust interpolation technique that reduces non-tectonic noise. Our comparison with the interpolated GPS data shows an increased combined correlation (R = 0.29), but overall the agreement is not strong. Moreover, average geologic vertical velocities appear to be one order smaller in magnitude than geodetic rates. While the agreement between these two vertical motion rates is not necessarily warranted, the discrepancy is likely related to measurement bias caused by the use of different geologic markers, as well as non-tectonic motion in the geodetic data.