A network of continuously recording GPS stations has operated in the Nicoya Peninsula of northern Costa Rica since 2002. We processed all available data from this network for the period of 2002–2011 to investigate the occurrence of Slow Slip Events (SSE) on the subduction interface between the Cocos and Caribbean plates. In order to overcome signal masking by high levels of tropospheric noise, we developed a new technique that facilitates detection of transient events in the presence of noise. We identified five significant SSEs during the 2002–2011 period, with event middle times in 2003, 2005, 2007, 2009 and 2011, with an average recurrence interval of 21 ± 6 months. Time series analysis shows that transient deformation imparts a signature similar to random walk. Removal of the SSEs and regional common mode errors from the time series reduced velocity uncertainty by nearly an order of magnitude. Limited available data for the 2003, 2005 and 2011 events preclude detailed characterization of these events. However, good spatiotemporal coverage of the 2007 and 2009 events suggest that both events had irregular duration and distribution. In the 2007 event, slow slip started in the northwest coastal area and migrated southeastward over a period of ∼1 month. The 2009 event had a significantly longer event duration and larger surface displacement. Stations in the northwest area observed two separate SSEs in 2008.6 and 2009.4, correlating well with the tremor episodes offshore, indicating a shallow SSE slip patch with shorter recurrence interval. Significant differences between the 2009 and 2007 events lead us to question the simple recurrence interval model for the SSE in Nicoya.

[1] We present a method to derive velocity uncertainties from GPS position time series that are affected by time-correlated noise. This method is based on the Allan variance, which is widely used in the estimation of oscillator stability and requires neither spectral analysis nor maximum likelihood estimation (MLE). The Allan variance of the rate (AVR) is calculated in the time domain and hence is not too sensitive to gaps in the time series. We derived analytical expressions of the AVR for different kinds of noises like power law noise, white noise, flicker noise, and random walk and found an expression for the variance produced by an annual signal. These functional relations form the basis of error models that have to be fitted to the AVR in order to estimate the velocity uncertainty. Finally, we applied the method to the South Africa GPS network TrigNet. Most time series show noise characteristics that can be modeled by a power law noise plus an annual signal. The method is computationally very cheap, and the results are in good agreement with the ones obtained by methods based on MLE.Citation: Hackl, M., R. Malservisi, U. Hugentobler, and R. Wonnacott (2011), Estimation of velocity uncertainties from GPS time series: Examples from the analysis of the South African TrigNet network,

Abstract. The knowledge of the crustal strain rate tensor provides a description of geodynamic processes such as fault strain accumulation, which is an important parameter for seismic hazard assessment, as well as anthropogenic deformation. In the past two decades, the number of observations and the accuracy of satellite based geodetic measurements like GPS greatly increased, providing measured values of displacements and velocities of points. Here we present a method to obtain the full continuous strain rate tensor from dense GPS networks. The tensorial analysis provides different aspects of deformation, such as the maximum shear strain rate, including its direction, and the dilatation strain rate. These parameters are suitable to characterize the mechanism of the current deformation. Using the velocity fields provided by SCEC and UNAVCO, we were able to localize major active faults in Southern California and to characterize them in terms of faulting mechanism. We also show that the large seismic events that occurred recently in the study region highly contaminate the measured velocity field that appears to be strongly affected by transient postseismic deformation. Finally, we applied this method to coseismic displacement data of two earthquakes in Iceland, showing that the strain fields derived by these data provide important information on the location and the focal mechanism of the ruptures.

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The Gulf of California, Mexico, accommodates ~90% of North America‐Pacific plate relative motion. While most of this motion occurs on marine transform faults and spreading centers, several fault segments in the central Gulf come close to peninsular Baja California. Here we present Global Positioning System and interferometric synthetic aperture radar data near the Ballenas transform fault, separating the peninsula from Angel de la Guarda Island. We observe interseismic motion between June 2004 and May 2009 and displacements associated with the 3 August 2009 Mw 6.9 earthquake. From the interseismic data we estimate a locking depth of 9–12.5 km and a slip rate of 44.9–48.1 mm/yr, indicating that faults east of Angel de la Guarda deform at negligible rates and that the Ballenas Transform accommodates virtually all of the relative motion between the North American plate and the Baja California microplate. Our preferred model for coseismic slip on a finite rectangular fault plane suggests 1.3 m of strike‐slip displacement along a vertical rupture plane that is 60 km long and extends from the surface to a depth of 13 km in the eastern Ballenas Channel, striking parallel to Baja California‐North America relative plate motion. These estimates agree with the seismic moment tensor and the location of the major foreshock and aftershocks and are compatible with the fault location identified from high‐resolution bathymetric mapping. The geodetic moment is 33% higher than the seismic moment in part because some afterslip and viscous flow in the first month after the earthquake are included in the geodetic estimate. Coulomb stress changes for adjacent faults in the Gulf are consistent with the location of smaller aftershocks following the 2009 main shock and suggest potential triggering of the 12 April 2012 Mw 6.9 Guaymas earthquake.