Wavelet-based InSAR (WabInSAR) time series algorithm uses an improved filtering scheme that combines and inverts a large set of unwrapped interferograms to generate an accurate time series of the surface motion. This approach applies a variety of sophisticated wavelet based filters to estimate the interferometric phase noise and to reduce the effects of systematic and random artefacts, such as spatially correlated and temporal uncorrelated components of the atmospheric delay, and the digital elevation model and orbital errors. [Shirzaei 2013  JGR]

Random search approaches, such as Simulated Annealing (SA) and Genetic Algorithm (GA) are investigated and utilized in an iterated manner. The iterated approach helps to prevent GA in general and SA in particular from getting trapped in local minima, and it also increases redundancy for exploring the search space. A statistical competency test is applied for estimating the confidence interval of the inversion source parameters, considering their internal interaction through model, the effect of the model deficiency, as well as the observational error. [Shirzaei & Walter 2009 JGR]

Geodetic observations of surface deformation associated with volcanic activities can be used to constrain volcanic source parameters and their kinematics. Simple analytical models, such as point and spherical sources, are widely used to model deformation data. The inherent nature of oversimplified model geometries makes them unable to explain fine details of surface deformation. Current non-parametric, geometry-free inversion approaches resolve the distributed volume change, assuming it varies smoothly in space, which may detect artificial volume change outside magmatic source regions. To obtain a physically meaningful representation of an irregular volcanic source, we devise a new sparsity-promoting modeling scheme assuming active magma bodies are well-localized melt accumulations, namely outliers in the background crust. First, surface deformation data are inverted using a hybrid L1- and L2-norm regularization scheme to solve for sparse volume change distributions. Next, a boundary element method is implemented to solve for the displacement discontinuity distribution of the reservoir, which satisfies a uniform pressure boundary condition. The inversion approach is thoroughly validated using benchmark and synthetic tests, of which the results show that source dimension, depth, and shape can be recovered appropriately. We apply this modeling scheme to deformation observed at Kilauea summit for periods of uplift and subsidence leading to and following the 2007 Father’s Day event. We find that the magmatic source geometries for these periods are statistically distinct, which may be an indicator that magma is released from isolated compartments due to large differential pressure leading to the rift intrusion. [Zhai & Shirzaei 2017 JGR]

Understanding the evolution of aseismic slip enables constraining the fault seismic budget and provides insight into dynamics of creep. Inverting the time series of surface deformation measured along the Central San Andreas Fault obtained from Interferometric Synthetic Aperture Radar in combination with measurements of repeating earthquakes, we constrain the spatiotemporal distribution of creep during 1992-2010. We identify a new class of intermediate-term creep rate variations that evolve over decadal-scale, releasing stress on the accelerating zone and loading adjacent decelerating patches. We further show that in short-term (< 2-year period), creep avalanches, i.e. isolated clusters of accelerated aseismic slip with velocities exceeding the long-term rate, govern the dynamics of creep. The statistical properties of these avalanches suggest existence of elevated pore pressure in the fault zone, consistent with laboratory experiments.

Models of Himalayan neotectonics generally attribute most active mountain building in the range to slip on the Himalayan Sole Thrust (HST), which accommodates underthrusting of the Indian Plate beneath Tibet. However, the geometry of the HST and thus how slip along it causes uplift of the High Himalaya are unclear. We show that the 2015 Gorkha earthquake sequence adds important clarity to the architecture of the HST, suggesting that the canonical view of how the Himalaya grow may require revision. Inversion of surface deformation patterns for the event, as revealed by InSAR and GPS data, implies that the HST extends as a planar gently-dipping fault surface from near the Himalayan thrust front northward for at least 100 km, well north of the main physiographic transition demarcating the southern flank of the High Himalaya, implying that building of the high range cannot be attributed solely to slip along HST over a steep ramp, as has commonly been inferred.  [Whipple et al. 2016 NGeo]

In recent decades, high groundwater extraction rates, often coincident with periods of severe drought, result in the widespread decline of water levels. Overexploitation of aquifers also causes land subsidence, which poses a severe threat to infrastructure. Tucson, Arizona experiences land subsidence coupled with the depletion of groundwater, a critical water resource for the desert city. To understand the spatiotemporal evolution of land subsidence and its implications for aquifer properties, we examine long time series of surface deformation and head levels. Measurements at extensometer stations indicate rapid compaction of fine-grained material up to 8.5 mm/yr from 1990 to 2005, which results in permanent storage volume losses up to 4.1%. The analysis of densely populated sets of interferograms generated from Envisat and RadarSAT C-band acquisitions yields multitemporal maps of surface deformation at unprecedented resolution. These maps reveal that subsidence significantly slows by the late 2000s, corresponding with the implementation of artificial recharge efforts. Subsequent to groundwater level recovery, we observe a brief 6.6-year interval of residual compaction, suggesting a high vertical hydraulic conductivity, which is then shown to be up to 9.8x10-4 m/day. We also estimate the average elastic and inelastic skeletal storage coefficients for the aquifer system to be 3.78x10-3 and 6.01x10-3, respectively. InSAR shows deformation nearly ceases by 2015, likely reducing hazards associated with earth fissuring and infrastructure damage. This study highlights successful outcomes of water management and conservation plans that preserve existing groundwater reserves and increase artificial recharge. [Miller et al. 2017 JGR]

The accelerated rate of decline in groundwater levels across California’s Central Valley results from over-drafting and low rates of natural recharge and is exacerbated by droughts. The lack of observations with adequate spatiotemporal resolution to constrain the evolution of groundwater resources poses serious challenges to water management efforts. Here, we present SAR interferometric measurements of high-resolution vertical land motion across the valley, revealing multiscale patterns of aquifer hydrogeological properties and groundwater storage change. Investigating the depletion and degradation of the aquifer-system during 2007 – 2010, when the entire valley experienced a severe drought, we find that ~2% of total aquifer-system storage was permanently lost, owing to irreversible compaction of the system. Over this period, the seasonal groundwater storage change amplitude of 10.11 ± 2.5 km3 modulates a long term groundwater storage decline of 21.32 ± 7.2 km3. Estimates for sub-basins show more complex patterns, most likely associated with local hydrogeology, recharge, demand and underground flow. Presented measurements of aquifer-system compaction provide a more complete understanding of groundwater dynamics and can potentially be used to improve water security. [Ojha et al. 2018 WRR]