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The 2013 Ruisui earthquake represents the first unequivocal evidence of the activity of the Central Range fault (CRF) in central Longitudinal Valley, Taiwan. Using a joint Bayesian finite‐fault source inversion of Global Navigation Satellite System and strain time series, we infer that coseismic rupture occurred between 4 and 19 km depth with maximum slip of 0.5 m located near the hypocenter. We then apply a variational Bayesian independent component analysis approach to displacement signals to infer a 3‐months long afterslip located in the near‐source region. This observation represents the first evidence of aseismic slip on the CRF. Combining geodetic and seismological analysis with simulations based on rate‐and‐state friction mechanics, we analyze the interplay between seismic and aseismic deformation during the earthquake sequence. We observe that afterslip is the dominant postseismic deformation mechanism, with >95% of the moment being released aseismically in the postseismic phase and also likely represents the driving force controlling aftershock productivity. Finally, we infer the presence of a shallow velocity strengthening zone (∼0–4 km depth) associated with spatially heterogeneous slip during the postseismic phase with maximum slip of 0.18 m located above the zone of maximum coseismic deformation. Plain Language Summary: Tectonic faults display a broad range of slip patterns, ranging from fast slip (earthquakes) to episodic or continuous aseismic slip. Aseismic transient slip events are now widely observed in active regions and play an important role in stress redistribution in the Earth's crust. The Central Range fault (CRF) is the second most active fault in the Longitudinal Valley, in eastern Taiwan. During the past 15 years, the fault hosted large to destructive earthquakes, but little is known about the presence and the role of aseismic events on the fault deformation. The 2013 Ruisui earthquake reveals for the first time the presence of transient slip regions on the CRF, capable of sustaining aseismic deformation over months. Besides, slow stress relaxation on the fault plane may have also influenced the behavior of seismicity following the mainshock. Monitoring and characterizing the sources of aseismic slip is fundamental to identify areas with high seismic hazard on the fault and to gain more knowledge about the interactions between seismic and aseismic processes. Key Points: We present the first evidence for the detection of postseismic deformation on the Central Range fault, in the Longitudinal Valley (Taiwan)Afterslip represents the dominant deformation mechanism during the postseismic phase and controlled aftershock productionThe velocity strengthening region may have acted like a barrier, resisting the rupture propagation toward the surface during the earthquake [ABSTRACT FROM AUTHOR]

The Parkfield segment of the San Andreas Fault (SAF) sits at the transition between the locked Cholame segment to the South and the SAF creeping segment to the North. The Parkfield segment hosts regular ∼Mw $\sim {\boldsymbol{M}}_{\boldsymbol{w}}$6 earthquakes followed by postseismic deformation. Recent studies based on geodetic data have highlighted spatial and temporal variations of aseismic slip rate in addition to postseismic slip along this section of the fault. We combine Global Navigation Satellite Systems (GNSS) and seismicity data over the 2006–2018 period to detail a comprehensive picture of transient slip events. We produce a catalog of relocated seismicity and repeating earthquakes. We use a variational Bayesian independent component analysis decomposition on GNSS data to separate geodetic deformation due to non‐tectonic sources from signals of potential tectonic origin. We then reconstruct the temporal evolution of fault slip and detect potential slip transients. Those events, determined as mostly aseismic with the exception of one related to a Mw ${\boldsymbol{M}}_{\boldsymbol{w}}$4.8 earthquake, occur more frequently during the 2004 Mw ${\boldsymbol{M}}_{\boldsymbol{w}}$6 post‐seismic period than during the subsequent inter‐seismic phase. Our study illustrates the rich dynamics of seismic and aseismic slip during both post‐ and inter‐seismic periods along active faults. Plain Language Summary: Faults can slip abruptly, generating earthquakes and seismic waves, or slowly and aseismically. Such a slow motion has now been observed along multiple active faults and is recognized as one of the important processes influencing the Earth's crust stress field. Aseismic slip interacts with earthquakes, and monitoring its time evolution on faults help us in better modeling the seismic cycle. Here we focus our attention on the Parkfield fault segment, sitting along the San Andreas Fault in California in between a section that generated a large earthquake (M7.9) in 1857 and a 150‐km‐long central creeping section that slips aseismically. We dig in noisy time series of surface displacements measured by GNSS (Global Navigation Satellite System) to detect and model small variations of aseismic slip rate (10–100 mm/year) along the fault. We find multiple candidate slow slip events during the period following the 2004 Parkfield earthquake, confirming and expanding previous independent observations. While aseismic slip following earthquakes was first thought to be a steady release of tectonic stress, we show that, in Parkfield, it is evolving with sudden jumps and slowdown. Key Points: Independent Component Analysis of GPS time series highlights slip transients along the San Andreas Fault after the 2004 M6 Parkfield eventWe document the spatio‐temporal distribution of seismic and aseismic slip historySlow Slip Events are more frequent during the 2004 M6 post‐seismic period than the subsequent interseismic period [ABSTRACT FROM AUTHOR]

The continuous redistribution of water involved in the hydrologic cycle leads to deformation of the solid Earth. On a global scale, this deformation is well explained by the loading imposed by hydrological mass variations and can be quantified to first order with space‐based gravimetric and geodetic measurements. At the regional scale, however, aquifer systems also undergo poroelastic deformation in response to groundwater fluctuations. Disentangling these related but distinct 3D deformation fields from geodetic time series is essential to accurately invert for changes in continental water mass, to understand the mechanical response of aquifers to internal pressure changes as well as to correct time series for these known effects. Here, we demonstrate a methodology to accomplish this task by considering the example of the well‐instrumented Ozark Plateaus Aquifer System (OPAS) in the central United States. We begin by characterizing the most important sources of groundwater level variations in the spatially heterogeneous piezometer dataset using an Independent Component Analysis. Then, to estimate the associated poroelastic displacements, we project geodetic time series corrected for hydrological loading effects onto the dominant groundwater temporal functions. We interpret the extracted displacements in light of analytical solutions and a 2D model relating groundwater level variations to surface displacements. In particular, the relatively low estimates of elastic moduli inferred from the poroelastic displacements and groundwater fluctuations may be indicative of aquifer layers with a high fracture density. Our findings suggest that OPAS undergoes significant poroelastic deformation, including highly heterogeneous horizontal poroelastic displacements. Plain Language Summary: A number of hydrological processes can deform the solid Earth. Measuring this deformation through space‐based geodesy offers an opportunity to study these hydrologic processes and infer properties of the sub‐surface. In the case of an aquifer, surface displacements can arise from changes in total water mass, which load the Earth, as well as from changes in groundwater pressure which alter stresses in the aquifer and in the surrounding medium. In this study, we describe a methodology to extract and separate these distinct but related deformation signals from Global Navigation Satellite System time series and hence infer mechanical properties of the aquifer system by using satellite gravimetry data, local groundwater level measurements as well as a blind source separation technique. We also present a mathematical framework to study surface displacements resulting from variations in groundwater pressure in a medium with heterogeneous elastic properties. We demonstrate the methodology in the Ozark Plateaus Aquifer System in the central United States. Key Points: We characterize seasonal and multiannual fluctuations in groundwater levels with an Independent Component AnalysisWe separate and model the hydrological loading and poroelastic deformation fields captured by global navigation satellite systemWe infer relatively low elastic moduli from the extracted poroelastic displacements and groundwater fluctuations [ABSTRACT FROM AUTHOR]

The 2016–2017 Central Italy earthquake sequence struck the central Apennines between August 2016 and October 2016 with Mw ∈ [5.9; 6.5], plus four earthquakes occurring in January 2017 with Mw ∈ [5.0; 5.5]. We study Global Positioning System time series including near‐ and far‐field domains. We use a variational Bayesian independent component analysis technique to separate the post‐seismic deformation from signals caused by variation of the water content in aquifers at hundreds of meters of depth and of the soil moisture. For each independent component, realistic uncertainties and a plausible physical explanation are provided. We focus on the study of afterslip on the main structures surrounding the mainshock, highlighting the role played by faults that were not activated during the co‐seismic phase in accommodating the post‐seismic deformation. We report aseismic deformation occurring on the Paganica fault, which hosted the Mw 6.1 2009 L'Aquila earthquake, suggesting that static stress transfer and aseismic slip influence the recurrence time of nearby (∼50 km further south of the mainshocks) segments. A ∼2–3 km thick subhorizontal shear‐zone, clearly illuminated by seismicity, which bounds at depth the west‐dipping normal faults where the mainshocks nucleated, also shows aseismic slip. Since afterslip alone underestimates the displacement in the far‐field domain, we consider the possibility that the shear zone marks the brittle‐ductile transition, assuming the viscoelastic relaxation of the lower crust as a mechanism contributing to the post‐seismic displacement. Our results suggest that multiple deformation processes are active in the first 2 years after the mainshocks. Key Points: We study the post‐seismic deformation following the 2016–2017 Central Italy seismic sequence through Global Positioning System displacement time seriesWe map the afterslip distribution on faults co‐seismically activated and we investigate interaction among faultsWe infer the occurrence of multiple post‐seismic deformation mechanisms including afterslip and viscoelastic relaxation [ABSTRACT FROM AUTHOR]

The exploitation of ever increasing Interferometric Synthetic Aperture Radar (InSAR) data sets to monitor the Earth's surface deformation is an important goal of today's geodesy. In this study our observations consist of deformations along the Line‐Of‐Sight direction of the satellite. Our observations are the result of the combination of a multitude of sources (either volcano‐tectonic or nontectonic deformation). In most cases, we are facing a Blind source separation (BSS) problem. Natural approaches to tackle BSS problems are multivariate statistical techniques that attempt to decompose the data set into a limited number of statistically independent sources, under the assumption that the different physical mechanisms contributing to the observations have independent footprints in space and/or time. We show the capabilities of a variational Bayesian independent component analysis (vbICA) algorithm in dealing with synthetic InSAR time series and compare it to the commonly used FastICA algorithm. We explore the effectiveness of the spatial and temporal mode decompositions. We apply vbICA on data relative to the San Joaquin Valley and the Central San Andreas fault (CSAF), California, spanning the time range 2015/03/01–2019/07/14. The proposed approach likely isolates the contribution of shallow and deep aquifers to the surface deformation as well as the elastic and inelastic deformation. We present a 1‐dimensional compaction estimation of the elastic and inelastic storage coefficients adopting a formalism that takes into account the last century water level history. Concerning the CSAF, the algorithm helps separating tectonic loading from seasonal behavior concentrated in the Quaternary sediments of the Salinas Valley. Key Points: Variational Bayesian independent component analysis on Interferometric Synthetic Aperture Radar displacement time series performs better than FastICAApplication to San Joaquin Valley allows to isolate elastic and inelastic compaction and potential hydrocompaction contributionsApplication to Central San Andreas fault zone allows to separate seasonal behavior of Quaternary sediments from creep on the fault [ABSTRACT FROM AUTHOR]

Changes in continental water storage generate vertical surface deformation, induce crustal stress perturbations, and modulate seismicity rates. However, the degree to which regional changes in terrestrial water content influence crustal stresses and the occurrence of earthquakes remains an open problem. We show how changes in groundwater storage, computed for a ∼1,000 km2 basin, focus deformation in a narrow zone, causing large horizontal, nonseasonal displacements. We present results from a karstic mountain range located at the edge of the Adria‐Eurasia plate boundary system in Northern Italy, where shortening is accommodated across an active fold‐and‐thrust belt. The presence of geological structures with high permeabilities and of deeply rooted hydrologically active fractures focus groundwater fluxes and pressure changes, generating transient surface horizontal displacements up to 5 mm and perturbations of crustal stress up to 25 kPa at seismogenic depths. The background seismicity rates appear correlated, without evident temporal delay, with groundwater storage changes in the hydrological basin. With no evidence of pore pressure propagation from the hydrologically active fractures, seismicity modulation is likely affected by direct stress changes on faults planes. Plain Language Summary: The natural water cycle, by changing how water is stored on the continents, can cause nonnegligible deformation at the Earth's surface. Redistribution of water masses has long been known to alter the state of stress in the crust and potentially modulate seismicity rates. However, the degree to which regional changes in groundwater storage influence crustal stresses and the occurrence of earthquakes at fault scales remains an open problem. We study a karst area located in a tectonically active region of Northern Italy, where plate convergence is accommodated across a complex system of faults and folds. We use GPS, hydrological, meteorological and seismological observations, integrated by hydrological and mechanical models, to show that there is a direct elastic connection between changes in groundwater storage, crustal deformation, and seismicity rates. We show that hydrologically active fractures and seismically active fractures might be disjoint, and that pore pressure propagation is not required to generate stress changes at seismogenic depths. Indeed, the convergence of water from upstream catchment toward permeable fractures connected to the surface can generate large pressure changes on the wall of these fractures, causing horizontal displacements and perturbations of the crustal stress that modulate background seismicity rates. Key Points: Regional groundwater storage changes in heterogeneous media modulate horizontal transient deformationWater pressure changes in shallow permeable fractures cause large elastic stress changes at seismogenic depthBackground seismicity rates are correlated with groundwater storage changes [ABSTRACT FROM AUTHOR]

We report Global Positioning System (GPS) measurements of postseismic deformation following the 2015 Mw7.8 Gorkha (Nepal) earthquake, including previously unpublished data from 13 continuous GPS stations installed in southern Tibet shortly after the earthquake. We use variational Bayesian Independent Component Analysis (vbICA) to extract the signal of postseismic deformation from the GPS time series, revealing a broad displacement field extending >150 km northward from the rupture. Kinematic inversions and dynamic forward models show that these displacements could have been produced solely by afterslip on the Main Himalayan Thrust (MHT) but would require a broad distribution of afterslip extending similarly far north. This would require the constitutive parameter (a − b)σ to decrease northward on the MHT to ≤0.05 MPa (an extreme sensitivity of creep rate to stress change) and seems unlikely in light of the low interseismic coupling and high midcrustal temperatures beneath southern Tibet. We conclude that the northward reach of postseismic deformation more likely results from distributed viscoelastic relaxation, possibly in a midcrustal shear zone extending northward from the seismogenic MHT. Assuming a shear zone 5–20 km thick, we estimate an effective shear‐zone viscosity of ~3·1016–3·1017 Pa·s over the first 1.12 postseismic years. Near‐field deformation can be more plausibly explained by afterslip itself and implies (a − b)σ ~ 0.5–1 MPa, consistent with other afterslip studies. This near‐field afterslip by itself would have re‐increased the Coulomb stress by ≥0.05 MPa over >30% of the Gorkha rupture zone in the first postseismic year, and deformation further north would have compounded this reloading. Key Points: New GPS data show that postseismic deformation of the 2015 Mw7.8 Gorkha (Nepal) earthquake extended far northward into southern TibetKinematic and dynamic models suggest that the northward reach of deformation likely reflects the influence of viscoelastic relaxationAfterslip likely dominated postseismic deformation in the near field, transferring stress back to the (relocked) Gorkha rupture zone [ABSTRACT FROM AUTHOR]

The Groningen gas reservoir, situated in the northeast of the Netherlands is western Europe's largest gas reservoir. Due to gas production measurable subsidence and seismicity has been detected across this region, attributed to the deformations induced by reservoir pore pressure depletion. We investigate the surface displacement history using a principal component analysis‐based inversion method to combine a diverse set of optical leveling, interferometric synthetic aperture radar, and Global Positioning System data to better constrain reservoir compaction and subsidence history. The generated compaction model is then used in combination with prior pressure depletion models to determine a reservoir uniaxial compressibility. The best fitting model of uniaxial compressibility is time independent but spatially variable. The absence of evidence for any significant time delay between changes in depletion and compaction rates supports an instantaneous poroelastic reservoir response. The absence of nonlinear yielding at the largest compaction strains suggests that anelastic deformations are a minor part of reservoir compaction. Key Points: Seismicity is located within the reservoir levelsVariations and changes in gas extraction are not manifested in subsidence dataPressure depletion within reservoir levels is modeled using geodetic inversion [ABSTRACT FROM AUTHOR]

Abstract: We apply a blind source separation algorithm to the ground displacement time series recorded at continuous Global Positioning System (GPS) stations in the European Eastern Alps and Northern Dinarides. As a result, we characterize the temporal and spatial features of several deformation signals. Seasonal displacements are well described by loading effects caused by Earth surface mass redistributions. More interestingly, we highlight a horizontal, nonseasonal, transient deformation signal, with spatially variable amplitudes and directions. The stations affected by this signal reverse the sense of movement with time, implying a sequence of dilatational and compressional deformation that is oriented normal to rock fractures in karst areas. The temporal evolution of this deformation signal is correlated with the history of cumulated precipitations at monthly time scales. This transient horizontal deformation can be explained by pressure changes associated with variable water levels within vertical fractures in the vadose zones of karst systems. The water level changes required to open or close these fractures are consistent with the fluctuations of precipitation and with the dynamics of karst systems. [ABSTRACT FROM AUTHOR]

Deformation of the Earth's surface associated with redistributions of continental water mass explains, to first order, the seasonal signals observed in geodetic position time series. Discriminating these seasonal signals from other sources of deformation in geodetic measurements is essential to isolate tectonic signals and to monitor spatio‐temporal variations in continental water storage. We propose a new methodology to identify and extract these seasonal signals. The approach uses a variational Bayesian Independent Component Analysis (vbICA) to extract the seasonal signals and a gravity‐based deformation model to identify which of these signals are caused by surface loading. We test the procedure on two study areas, the Arabian Peninsula and the Nepal Himalaya, and find that the technique successfully extracts the seasonal signals with one or two independent components, depending on whether the load is stationary or moving. The approach is robust to spatial heterogeneities inherent to geodetic measurements and can help extract systematic errors in geodetic products (e.g., draconitic errors). We also discuss how to handle the degree‐1 deformation field present in the geodetic data set but not captured by the gravity‐based model. Key Points: We propose a procedure to isolate seasonal geodetic signals due to surface loads using a GRACE‐based deformation model and an independent component analysisGNSS and GRACE‐derived displacements yield consistent seasonal time evolutions in both the Arabian Peninsula and the Nepal HimalayaThe method is robust to topographic and elastic heterogeneities, simple moving loads and geodetic products errors [ABSTRACT FROM AUTHOR]