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Over the last 3 decades, GPS measurements have been instrumental in quantifying tectonic plates current motion and deformation. Complex patterns of deformation along the plate boundaries revealed heterogeneous coupling on the plates interface and imaged seismic segments at different stages of their seismic cycle. Along the South-American trench in Chile, where large earthquakes occur frequently, continuous GPS observations (cGPS) captured both the long-term plate motion and the transient deformations associated with the seismic cycle. Over the years, a network of hundreds of cGPS stations has been deployed all across the South-American continent by many different institutions for many different sorts of purposes ranging from geographic reference to tsunami early warning. We report here on the processing of 20 years (2000–2020) worth of data over a selection of cGPS stations, devoted to the quantification and analysis of the deformation along the Chilean subduction zone between 18°S and 40°S. We use all available data near the trench in Chile and a less dense network inside the continent where the gradient of deformation is lesser. Our database, named SOAM_GNSS_solENS, provides time series of precise daily station position, obtained from double difference (DD) processing and expressed in the International Terrestrial Reference Frame 2014 (ITRF14). These time series allow to quantify, with sub-millimetric precision, any kind of ongoing deformation process, either from tectonic origin such as interseismic deformation, co- and post-seismic displacements associated with earthquakes, transient deformation associated to seismic swarms and/or a-seismic slow-slip events, or of other origin such as hydrological loading (for ex., the Amazonian basin load) or any other type of loading affecting the surface of the Earth (tides, atmosphere, etc.). We also provide a database of coseismic displacements associated with close to 60 earthquakes of Mw larger than 6.5 that occurred over the last 20 years within the observation area. All time series are directly accessible through a deposit and we plan to make them available through a web interface that will allow any user to perform elementary operations like estimating offsets, detecting outliers, detrending, filtering and stacking. That database will evolve with time, aggregating more data. In the future, we also plan to complement that database with a rapid solution in quasi real time processed in Precise Point Positioning (PPP), and with hourly atmospheric delays associated with water vapor content of the lower layer of the atmosphere.

In this paper, we propose an approach to determine seven parameters of the Helmert transformation by transforming the coordinates of a continuous GNSS network from the World Geodetic System 1984 (WGS84) to the International Terrestrial Reference Frame. This includes (1) converting the coordinates of common points from the global coordinate system to the local coordinate system, (2) identifying and eliminating outliers by the Dikin estimator, and (3) estimating seven parameters of the Helmert transformation by least squares (LS) estimation with the "clean" data (i.e. outliers removed). Herein, the local coordinate system provides a platform to separate points' horizontal and vertical components. Then, the Dikin estimator identifies and eliminates outliers in the horizontal or vertical component separately. It is significant because common points in a continuous GNSS network may contain outliers. The proposed approach is tested with the Géoazur GNSS network with the results showing that the Dikin estimator detects outliers at 6 out of 18 common points, among which three points are found with outliers in the vertical component only. Thus, instead of eliminating all coordinate components of these six common points, we only eliminate all coordinate components of three common points and only the vertical component of another three common points. Finally, the classical LS estimation is applied to "clean" data to estimate seven parameters of the Helmert transformation with a significant accuracy improvement. The Dikin estimator's results are compared to those of other robust estimators of Huber and Theil-Sen, which shows that the Dikin estimator performs better. Furthermore, the weighted total least-squares estimation is implemented to assess the accuracy of the LS estimation with the same data. The inter-comparison of the seven estimated parameters and their standard deviations shows a small difference at a few per million levels (E-6). [ABSTRACT FROM AUTHOR]

Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain (1-8) suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods (9-11). Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude [M.sub.w] = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50-70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with [M.sub.w] = 8.0 in the Pisco area is estimated to be 250 years., Imaging the patchwork made by areas of faults that mostly slip during earthquakes (governed by a rate-weakening friction law) and areas that mostly slip aseismically (governed by a rate-strengthening friction [...]

We combine Global Positioning System (GPS) velocity field with Interferometric Synthetic Aperture Radar (InSAR) results to study the interseismic deformation across the western Altyn Tagh fault (ATF) at longitude 86° E. GPS and InSAR data are consistent after correcting for the contribution from vertical deformation in the InSAR line-of-sight map. InSAR and GPS data identify an area of ∼2 mm a−1 sinistral shear and ∼6 mm a−1 of NS shortening located ∼150 km south of the ATF near the Manyi fault system. Excluding the data located in that area, Bayesian inversion of a 2-D profile across the ATF indicate a locking depth of 14.8 ± 3.5 km and a slip rate of 8.0 ± 0.4 mm a−1, lying at the lower range of previously published estimates. In addition, we find no significant offset between the fault at depth and the surface fault trace and no asymmetry of the interseismic profile that implicitly reveal lateral variations of the elastic strength across the ATF. Detailed analysis of InSAR profile across the fault show no distinguishable surface creep along the western ATF. Our study highlights how different data sets, data selection and model assumption might impact results on the ATF slip rate, locking depth and rheological contrast across the fault. [ABSTRACT FROM AUTHOR]