Despite remarkable successes achieved by Differential InSAR, estimations of low tectonic strain rates remain challenging in areas where deformation and topography are correlated, mainly because of ...the topography‐related atmospheric phase screen (APS). In areas of high relief, empirical removal of the stratified component of the APS may lead to biased estimations of tectonic deformation rates. Here we describe a method to correct interferograms from the effects of the spatial and temporal variations in tropospheric stratification by computing tropospheric delay maps coincident with SAR acquisitions using the ERA‐Interim global meteorological model. The modeled phase delay is integrated along vertical profiles at the ERA‐I grid nodes and interpolated at the spatial sampling of the interferograms above the elevation of each image pixel. This approach is validated on unwrapped interferograms. We show that the removal of the atmospheric signal before phase unwrapping reduces the risk of unwrapping errors in areas of rough topography.
Key Points
A reliable estimate of tropostatic phase delay in InSAR data
Systematic correction of InSAR data enhancing measurement precision
Allows for a more reliable unwrrapping over rough topography
Interferometric synthetic aperture radar data are used to map the interseismic velocity field along the Haiyuan fault system (HFS), at the north‐eastern boundary of the Tibetan plateau. Two M ∼ 8 ...earthquakes ruptured the HFS in 1920 and 1927, but its 260 km‐long central section, known as the Tianzhu seismic gap, remains unbroken since ∼1000 years. The Envisat SAR data, spanning the 2003–2009 period, cover about 200 × 300 km2 along three descending and two ascending tracks. Interferograms are processed using an adapted version of ROI_PAC. The signal due to stratified atmospheric phase delay is empirically corrected together with orbital residuals. Mean line‐of‐sight velocity maps are computed using a constrained time series analysis after selection of interferograms with low atmospheric noise. These maps show a dominant left‐lateral motion across the HFS, and reveal a narrow, 35 km‐long zone of high velocity gradient across the fault in between the Tianzhu gap and the 1920 rupture. We model the observed velocity field using a discretized fault creeping at shallow depth and a least squares inversion. The inferred shallow slip rate distribution reveals aseismic slip in between two fully locked segments. The average creep rate is ∼5 mm yr−1, comparable in magnitude with the estimated loading rate at depth, suggesting no strain accumulation on this segment. The modeled creep rate locally exceeds the long term rate, reaching 8 mm yr−1, suggesting transient creep episodes. The present study emphasizes the need for continuous monitoring of the surface velocity in the vicinity of major seismic gaps in terms of seismic hazard assessment.
Key Points
Imaging spatial variations of interseismic strain along a strike‐slip fault
InSAR data resolves shallow slip anomaly between locked sections of a fault
Method developments improve InSAR data resolution to a subcentimeter level
We measure ground motion around the Lake Mead, Nevada, using synthetic aperture radar interferometry. The lake water level has fluctuated through time since impoundment in 1935. To quantify the ...deformation due to water level variations over the past decade, and to constrain the crust and mantle rheological parameters in the lake area, we analyze 241 interferograms based on 43 ERS images acquired between 1992 and 2002. All interferograms have a high coherence due to arid conditions. Most of them show strong atmospheric artefacts. Tropospheric phase delays are estimated and corrected for each interferogram by analyzing the phase/elevation correlation. Corrections are validated using data from the ERA40 global atmospheric reanalysis. Corrected interferograms are inverted pixel by pixel to solve for the time series of ground motion in the lake area. Temporal smoothing is added to reduce random atmospheric artefacts. The observed deformation is nonlinear in time and spreads over a 50 × 50 km2 area. We observe a 16 mm subsidence between 1995 and 1998 due to an 11 m water level increase, followed by an uplift due to the water level drop after 2000. We model the deformation, taking into account the loading history of the lake since 1935. A simple elastic model with parameters constrained by seismic wave velocities does not explain the amplitude of the observed motion. The two‐layer viscoelastic model proposed by Kaufmann and Amelung (2000), with a mantle viscosity of 1018 Pa s, adjusts well the data amplitude and its spatiotemporal shape.
We use 20 years of Synthetic Aperture Radar acquisitions by the ERS and Envisat satellites to investigate the spatial and temporal variations of strain rates along the 35-km long creeping section of ...the Haiyuan fault, at the north eastern boundary of the Tibetan plateau. We then use the derived displacements to infer the faultʼs frictional properties and discuss the relationship between creep and the seismic behavior of the fault. Located in between a millennial seismic gap and the 1920 M8 surface rupture trace, this section has an average creep rate of 5±1 mm/yr, about the interseismic loading rate. The comparison of average surface velocity profiles derived from SAR interferometry across the creeping section reveals a creep rate increase and/or a creep migration to shallower depth between the 1990s and the 2000s. We apply a smoothed time series analysis scheme on Envisat InSAR data to investigate the creep rate variations during the 2004–2009 time period. Our analysis reveals that the creep rate accelerated in 2007, although data resolution does not allow to better constrain the onset of creep acceleration and its amplitude. Both decadal and short term transient behaviors are coeval with the largest earthquakes (M∼4–5) along the fault segment in recent years. From the precise mapping of the surface fault trace, we use the fault strike variations and the Mohr circle construction to compute the along-strike distribution of the friction coefficient along the creeping segment and compare it with the observed distribution of the creep rate. We find that the creep rate scales logarithmically with the friction coefficient, in agreement with the rate-and-state friction law in a rate strengthening regime. The estimated value of δμ/δlogV∼2×10−3 indicates that the earthquakes occurring along the creeping section cannot be the cause for a significant change in the overall segmentʼs creep rate and that the recorded micro seismicity is most likely creep-driven. Finally, given the size and frictional properties of the creeping section, we estimate, based on previous models of dynamic rupture simulations, a 0–20% probability for a rupture to break through this section. Together with the geometrical configuration of the Haiyuan fault, these results suggest that the creeping segment may act as a persistent barrier to earthquake propagation.
The Western European Alps display measurable surface deformation rates from leveling and GNSS data. Based on the time‐series analysis of 4 years of Sentinel‐1 data, we propose for the first time an ...InSAR‐based mapping of the uplift pattern affecting the Western Alps on a ∼350 × 175‐km‐wide area. This approach provides a denser spatial distribution of vertical motion despite the high noise level inherent to mountainous areas and the low expected deformation signal. Our results show consistency with other geodetic measurements at the regional scale, and reveal smaller‐scale spatial variations in the uplift pattern. Higher uplift rates are found within the external crystalline massifs compared to surrounding areas, in agreement with the variations expected from recent deglaciation and long‐term exhumation data. This work brings the first InSAR‐based geodetic clue of differential uplift within the Alpine belt in response to the surface and deep processes affecting the belt.
Plain Language Summary
The surface of the Earth is constantly moving from about a few millimeters to a few centimeters per year in response to geological processes such as tectonic plate motions and gravitational re‐equilibrium. In the Western European Alps, where the mountain‐building phase of the belt is over, the processes at the origin of surface motions are under debate. Mapping surface displacements at the highest possible resolution is mandatory to better understand the evolution of the Alpine belt. Among other techniques, radar satellites can provide measurements of such displacements. Here, radar images from the recent Sentinel‐1 satellite allow us to improve the spatial resolution of vertical surface displacements in the Western Alps compared to previous techniques. Thanks to the unmatched frequency of acquisitions from this satellite, we are able for the first time in our study area to measure small‐scale spatial variations within the vertical motions. These variations appear correlated to distinct geological units. This brings new insights into the geological processes acting nowadays on the Western Alps.
Key Points
We use four years of Sentinel‐1 radar acquisitions to derive interseismic line‐of‐sight velocities over the Western European Alps
InSAR‐derived velocities are consistent with other geodetic studies and provide increased spatial resolution of surface velocity patterns
Short‐wavelength spatial variations show locally higher velocities localized on crystalline massifs, interpreted as higher uplift rates
Oblique convergence across Tibet leads to slip partitioning with the coexistence of strike-slip, normal and thrust motion on major fault systems. A key point is to understand and model how faults ...interact and accumulate strain at depth. Here, we extract ground deformation across the Haiyuan Fault restraining bend, at the northeastern boundary of the Tibetan plateau, from Envisat radar data spanning the 2001–2011 period. We show that the complexity of the surface displacement field can be explained by the partitioning of a uniform deep-seated convergence. Mountains and sand dunes in the study area make the radar data processing challenging and require the latest developments in processing procedures for Synthetic Aperture Radar interferometry. The processing strategy is based on a small baseline approach. Before unwrapping, we correct for atmospheric phase delays from global atmospheric models and digital elevation model errors. A series of filtering steps is applied to improve the signal-to-noise ratio across high ranges of the Tibetan plateau and the phase unwrapping capability across the fault, required for reliable estimate of fault movement. We then jointly invert our InSAR time-series together with published GPS displacements to test a proposed long-term slip-partitioning model between the Haiyuan and Gulang left-lateral Faults and the Qilian Shan thrusts. We explore the geometry of the fault system at depth and associated slip rates using a Bayesian approach and test the consistency of present-day geodetic surface displacements with a long-term tectonic model. We determine a uniform convergence rate of 10 8.6–11.5 mm yr−1 with an N89 81–97°E across the whole fault system, with a variable partitioning west and east of a major extensional fault-jog (the Tianzhu pull-apart basin). Our 2-D model of two profiles perpendicular to the fault system gives a quantitative understanding of how crustal deformation is accommodated by the various branches of this thrust/strike-slip fault system and demonstrates how the geometry of the Haiyuan fault system controls the partitioning of the deep secular motion.
To test Eastern Tibet crustal thickening modes, we compare 2‐D numerical models of two emblematic end‐member models, with either an obstacle in the low viscosity lower crust or a thrust embedded in ...the high viscosity one. We show that the obstacle halts the viscous lower crustal flow potentially initiated by the weight of the high Central Tibet, generating a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. On the contrary, including a low viscosity discontinuity in the upper crust, mimicking a shallow steep listric fault as inferred in the region, reproduces a sharper exhumation profile, as constrained from thermo‐kinematic inversions of thermochronological data, and the lack of foreland basin, as observed in the field. Moreover, such fault drives deformation throughout the entire crust, suggesting a deep crustal ductile shear zone limiting the more ductile deformation in the lower crust although no discontinuity is imposed.
Plain Language Summary
The role of thrusting in crustal thickening during the formation of Tibet, the world's largest and highest orogenic plateau, constitutes one of the main controversies in earth sciences. In Eastern Tibet in particular, two end‐members based on two contrasting controversial hypotheses can be tested: the thickening is dominated either by the flow of the lower Tibetan crust halted by the hard Sichuan craton, or by thrusting of the Tibetan upper crust. Here, we present 2‐D crustal numerical models of a shallow steep listric thrust (as inferred in the region) embedded in the high viscosity upper crust, and we show that such model reproduces the exhumation profile constrained from thermochronological data and the lack of foreland basin observed in the field. Interestingly, we also show that such upper crustal thrust drives upward the more ductile lower crust albeit no discontinuity is imposed. On the contrary, by using a model driven by an overpressure in the lower crust, we show that the obstacle halts the viscous lower crustal flow and generates a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet.
Key Points
2‐D numerical models of thrusts embedded in the high viscosity upper crust, to test thermo‐kinematic models based on thermochronology data
accommodation in the lower crust by ductile flow of the deformation induced by the high angle thrust in the upper crust
predicting exhumation rates and subsidence patterns that are compatible with the measured ones in Eastern Tibet
Due to the limited resolution at depth of geodetic and other geophysical data, the geometry and the loading rate of the ramp‐décollement faults below the metropolitan Los Angeles are poorly ...understood. Here we complement these data by assuming conservation of motion across the Big Bend of the San Andreas Fault. Using a Bayesian approach, we constrain the geometry of the ramp‐décollement system from the Mojave block to Los Angeles and propose a partitioning of the convergence with 25.5 ± 0.5 mm/yr and 3.1 ± 0.6 mm/yr of strike‐slip motion along the San Andreas Fault and the Whittier Fault, with 2.7 ± 0.9 mm/yr and 2.5 ± 1.0 mm/yr of updip movement along the Sierra Madre and the Puente Hills thrusts. Incorporating conservation of motion in geodetic models of strain accumulation reduces the number of free parameters and constitutes a useful methodology to estimate the tectonic loading and seismic potential of buried fault networks.
Key Points
We use a Bayesian approach to identify fault geometry and slip rates compatible with geodesy and conservation of motion
The surface velocity field in Southern California is in agreement with a large‐scale decollement that partitions deep‐seated motion
Structural geology and long‐term slip compatibility laws help constraining the kinematics of buried faults below Los Angeles
The main limiting factor on the accuracy of Interferometric SAR measurements (InSAR) comes from phase propagation delays through the troposphere. The delay can be divided into a stratified component, ...which correlates with the topography and often dominates the tropospheric signal, and a turbulent component. We use Global Atmospheric Models (GAM) to estimate the stratified phase delay and delay-elevation ratio at epochs of SAR acquisitions, and compare them to observed phase delay derived from SAR interferograms. Three test areas are selected with different geographic and climatic environments and with large SAR archive available. The Lake Mead, Nevada, USA is covered by 79 ERS1/2 and ENVISAT acquisitions, the Haiyuan Fault area, Gansu, China, by 24 ERS1/2 acquisitions, and the Afar region, Republic of Djibouti, by 91 Radarsat acquisitions. The hydrostatic and wet stratified delays are computed from GAM as a function of atmospheric pressure
P, temperature
T, and water vapor partial pressure
e vertical profiles. The hydrostatic delay, which depends on ratio
P/
T, varies significantly at low elevation and cannot be neglected. The wet component of the delay depends mostly on the near surface specific humidity. GAM predicted delay-elevation ratios are in good agreement with the ratios derived from InSAR data away from deforming zones. Both estimations of the delay-elevation ratio can thus be used to perform a first order correction of the observed interferometric phase to retrieve a ground motion signal of low amplitude. We also demonstrate that aliasing of daily and seasonal variations in the stratified delay due to uneven sampling of SAR data significantly bias InSAR data stacks or time series produced after temporal smoothing. In all three test cases, the InSAR data stacks or smoothed time series present a residual stratified delay of the order of the expected deformation signal. In all cases, correcting interferograms from the stratified delay removes all these biases. We quantify the standard error associated with the correction of the stratified atmospheric delay. It varies from one site to another depending on the prevailing atmospheric conditions, but remains bounded by the standard deviation of the daily fluctuations of the stratified delay around the seasonal average. Finally we suggest that the phase delay correction can potentially be improved by introducing a non-linear dependence to the elevation derived from GAM.
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