Resolution of GPS data from the 2004 Mw6.0 Parkfield Earthquake
PAGE, M.T., Department of Physics, University of California, Santa Barbara, CA 93106, firstname.lastname@example.org; CUSTODIO, S., Department of Earth Science, University of California, Santa Barbara, CA 93106, email@example.com; ARCHULETA, R.J., Department of Earth Science, University of California, Santa Barbara, CA 93106, firstname.lastname@example.org; CARLSON, J.M., Department of Physics, University of California, Santa Barbara, CA 93106, email@example.com.
The long-awaited 2004 Mw6.0 Parkfield Earthquake provides a unique opportunity to probe the resolution limits of source inversions due to the large amount of near-field seismic stations. This earthquake was well recorded by a dense network of strong-motion seismographs and GPS 1-Hz receivers. We investigate the spatial resolution of the GPS data, which provides a constraint on the static field. Even for a well-recorded earthquake such as Parkfield, static GPS inversions are poorly resolved at depth and near the edges of the fault. We demonstrate how in underdetermined inversions such as this, it is possible to obtain structure in poorly resolved areas that is not real. Furthermore, bootstrapping is unlikely to give correct error bounds in these regions. As such, much of the structure shown in GPS inversions of Parkfield is highly uncertain. We demonstrate that a nonuniform grid whose grid spacing matches the local resolution length on the fault outperforms small uniform grids, which generate spurious structure in poorly resolved regions, and large uniform grids, which lose recoverable information in well-resolved areas of the fault. With a synthetic test, we show that our nonuniform grid correctly averages out small-scale structure in poorly resolved areas while recovering small-scale structure near the surface.