Toward a Continuous Monitoring of the Horizontal Displacement Gradient Tensor Field in Southern California Using cGPS Observations from Plate Boundary Observatory (PBO)

Abstract

Detection and characterization of strain transients provides opportunities for better understanding of crust and mantle rheology (Freed and Burgman, 2004; Freed et al. , 2007; Pollitz et al. , 2012), fault zone behavior (McGuire and Segall, 2003; Miyazaki et al. , 2011), and the mechanics of parts of the earthquake cycle (Dragert and Wang, 2011; Bartlow et al. , 2011). The relatively dense coverage of continuous GPS (cGPS) within western North America, through EarthScope Plate Boundary Observatory (PBO), enables the possibility of detecting crustal strain transient phenomena on a variety of spatial and temporal scales (Ohtani et al. , 2010; Ji and Herring, 2011). In this paper we present a geodetic network‐processing tool for detection of anomalous strain transients in Southern California, or within any region with reasonably dense cGPS coverage. The modeling procedure determines time‐dependent displacement gradient fields from cGPS time series (Hernandez et al. , 2005; Hernandez, 2007) and determines the significance of any departures of this field relative to a geodetic reference model. The derived field satisfies strain‐rate compatibility relations everywhere within the plate boundary zone on a sphere. Given sufficient spatial coverage of cGPS, the method can detect anomalous strain, either from shear or volume sources, and either on or off the known faults. In Determination of a Geodetic Reference Model, we present a brief description of the derivation of the geodetic reference model. We show from a benchmarking exercise that GPS observations in southern California (Shen et al. , 2011) are sufficiently dense to provide a reliable estimate of a reference model. We also show that a joint fitting of reference model strains, together with the sparser distribution of GPS from PBO stations, yields minimal strain‐rate artifacts. In Method for Obtaining Time‐Dependent Displacement Estimates from cGPS Observations, we introduce the method for interpolating the …