Using GPS Imaging to Resolve Seasonal Strain in Central California

Meredith L. Kraner, William C. Hammond, Corné W. Kreemer, & Geoffrey Blewitt

Published August 15, 2016, SCEC Contribution #6797, 2016 SCEC Annual Meeting Poster #166

Recently, studies using data from modern GPS have revealed that crustal deformation can be influenced by seasonal and nontectonic factors such as temperature, hydrology, and atmospheric loads. Statistically significant variations in annual micro-seismicity have also been observed. Here we build on this new idea by developing a robust seasonal strain model of Central California and run a series of verification routines to validate our result. We study an 8-year period (2008-2016) such that we can additionally test whether drought had an effect on influencing the nontectonic seasonal signal.

We use monthly median positions of horizontal GPS measurements to build our strain model. These positions are detrended using robust MIDAS velocities (product of the Nevada Geodetic Laboratory), destepped using a Heavyside function, and demeaned to center the time series around zero. The stations we use are carefully selected using the vertical rate of the GPS time series during the drought period. This selection method allows us to exclude stations located on unstable, heavily subsiding ground and include stations on sturdy bedrock.

In building our time-dependent strain model, we first filter these horizontal monthly median displacements using a technique called “GPS Imaging.” This technique uses a median-spatial filter to remove outliers, enhance the signal common to multiple stations, and improve the overall robustness of the input displacements for the strain model. An important property of this technique is its ability to despeckle the displacement field while resolving sharp edges between contrasting spatial domains. We then use these filtered displacements to model our dilatational and shear strain field for each month of the time series.

We validate each step of our procedure by examining the potential pitfalls of our method. We test the resolution of the GPS Imaging output by performing checkerboard reconstruction tests to understand the geographic variation in spatial resolution of the GPS measurement network. We additionally explore how varying the a priori faulting constraints in the strain-rate tensor affects our strain model.

Key Words
time-dependent, seasonal, strain, transient

Citation
Kraner, M. L., Hammond, W. C., Kreemer, C. W., & Blewitt, G. (2016, 08). Using GPS Imaging to Resolve Seasonal Strain in Central California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy