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Gualandi, Adriano
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Gualandi, Adriano
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- PublicationOpen AccessInterplay Between Seismic and Aseismic Deformation on the Central Range Fault During the 2013 Mw 6.3 Ruisui Earthquake (Taiwan)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.
29 7 - PublicationOpen AccessSpace–time evolution of crustal deformation related to the Mw 6.3, 2009 L'Aquila earthquake (central Italy) from principal component analysis inversion of GPS position time-seriesIn this work, we present a study of the coseismic and post-seismic crustal deformation associ- ated to the Mw 6.3, 2009 April 6 L’Aquila earthquake from the analysis of GPS displacement time-series. We use a principal component decomposition-based inversion method to study the space- and time-dependent evolution of slip on faults without any a priori assumption on the model used to characterize the temporal evolution of crustal deformation. The method adopted allows us to account for the initial post-seismic deformation in estimating the coseismic dis- placements, in a consistent manner for the whole GPS network. We use elastic dislocation theory and a least-squares procedure to invert for the slip distribution on the mainshock fault (Paganica fault) and a second fault (Campotosto fault), where a Mw 5.2 aftershock occurred on April 9. The geometries for these faults are obtained from a singular value decomposition of precisely relocated aftershocks. We find that the use of complex fault geometries is not justified by the GPS observations available. An inversion that accounts for post-seismic slip to occur on both the Paganica and Campotosto faults provides a better fit to the GPS time-series observations, than using only the Paganica fault segment, at a 95 per cent confidence level. Within our resolution, afterslip regions do not migrate over time and are localized on fault patches that are approximately complementary to those of coseismic slip. We find that the position of some relevant afterslip patches is different if the inversion is performed assuming a fixed rake or not. We estimate the parameter a – b of rate- and state-dependent friction on those fault regions accommodating afterslip that are robustly characterized in our inversions. We find values of the order of 10−3, which is near the transition from potentially unstable to nominally stable friction. These results are in agreement with laboratory measurements performed on typical rocks of the L’Aquila region.
29 32 - PublicationOpen AccessWeak upper-mantle base revealed by postseismic deformation of a deep earthquakeMantle viscosity plays a key role in the Earth's internal dynamics and thermal history. Geophysical inferences of the viscosity structure, however, have shown large variability depending on the types of observables used or the assumptions imposed [1-3]. Here, we study the mantle viscosity structure by using the postseismic deformation following a deep (approximately 560 km) earthquake located near the bottom of the upper mantle. We apply independent component analysis [4] to geodetic time series to successfully detect and extract the postseismic deformation induced by the moment magnitude 8.2, 2018 Fiji earthquake. To search for the viscosity structure that can explain the detected signal, we perform forward viscoelastic relaxation modelling [5,6] with a range of viscosity structures. We find that our observation requires a relatively thin (approximately 100 km), low-viscosity (10^17 to 10^18 Pa s) layer at the bottom of the mantle transition zone. Such a weak zone could explain the slab flattening [7] and orphaning [8] observed in numerous subduction zones, which are otherwise challenging to explain in the whole mantle convection regime. The low-viscosity layer may result from superplasticity [9] induced by the postspinel transition, weak CaSiO3 perovskite [10], high water content [11] or dehydration melting [12].
25 6 - PublicationOpen AccessUnderstanding the Geodetic Signature of Large Aquifer Systems: Example of the Ozark Plateaus in Central United StatesThe 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 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.
24 27 - PublicationOpen AccessControls on Spatial and Temporal Patterns of Slope Deformation in an Alpine ValleyA comprehensive surface displacement monitoring system installed in the recently deglaciated bedrock slopes of the Aletsch Valley shows systematic reversible motions at the annual scale. We explore potential drivers for this deformation signal and demonstrate that the main driver is pore pressure changes of groundwater in fractured granitic mountain slopes. The spatial pattern of these reversible annual deformations shows similar magnitudes and orientations for adjacent monitoring points, leading to the hypothesis that the annually reversible deformation is caused by slope-scale groundwater elevation changes and rock mass properties. Conversely, we show that the ground reaction to infiltration from snowmelt and summer rainstorms can be highly heterogeneous at local scale, and that brittle-ductile fault zones are key features for the groundwater pressure-related rock mass deformations. We also observe irreversible long-term trends (over the 6.5 years data set) of deformation in the Aletsch valley composed of a larger uplift than observed at our reference GNSS station in the Rhone valley, and horizontal displacements of the slopes towards the valley. These observations can be attributed respectively to the elastic bedrock rebound in response to current glacier mass downwasting of the Great Aletsch Glacier and gravitational slope deformations enabled by cyclic groundwater pressure-related rock mass fatigue in the fractured rock slopes.
38 34 - PublicationOpen AccessHydrologically Induced Karst Deformation: Insights From GPS Measurements in the Adria-Eurasia Plate Boundary Zone(2018)
; ; ; ; ; ; ; ; ; ; ; ;; ; ;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 systems782 60 - PublicationRestrictedModeling earthquake effects on groundwater levels: evidences from the 2012 Emilia earthquake (Italy)(2016-08)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; Changes in water level are commonly reported in regions struck by a seismic event. The sign and amplitude of such changes depend on the relative position of measuring points with respect to the hypocenter, and on the poroelastic properties of the rock. We apply a porous media flow model (TOUGH2) to describe groundwater flow and water‐level changes associated with the first ML5.9 mainshock of the 2012 seismic sequence in Emilia (Italy). We represent the earthquake as an instantaneous pressure step, whose amplitude was inferred from the properties of the seismic source inverted from geodetic data. The results are consistent with the evolution recorded in both deep and shallow water wells in the area and suggest that our description of the seismic event is suitable to capture both timing and magnitude of water‐level changes. We draw some conclusions about the influence of material heterogeneity on the pore pressure evolution, and we show that to reproduce the observed maximum amplitude it is necessary to take into account compaction in the shallow layer.632 2 - PublicationOpen AccessMechanical Response of Shallow Crust to Groundwater Storage Variations: Inferences From Deformation and Seismic Observations in the Eastern Southern Alps, Italy(2021)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; 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 km 2 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.336 29 - PublicationOpen AccessNew insights into active tectonics and seismogenic potential of the Italian Southern Alps from vertical geodetic velocities(2020-09-04)
; ; ; ; ; ; ; ; ; ; ; ; ; This study presents and discusses horizontal and vertical geodetic velocities for a low strain rate region of the south Alpine thrust front in northeastern Italy obtained by integrating GPS, interferometric synthetic aperture radar (InSAR) and leveling data. The area is characterized by the presence of subparallel, south-verging thrusts whose seismogenic potential is still poorly known. Horizontal GPS velocities show that this sector of the eastern Southern Alps is undergoing ∼1 mm a−1 of NW–SE shortening associated with the Adria–Eurasia plate convergence, but the horizontal GPS velocity gradient across the mountain front provides limited constraints on the geometry and slip rate of the several subparallel thrusts. In terms of vertical velocities, the three geodetic methods provide consistent results showing a positive velocity gradient, of ∼ 1.5 mm a−1, across the mountain front, which can hardly be explained solely by isostatic processes. We developed an interseismic dislocation model whose geometry is constrained by available subsurface geological reconstructions and instrumental seismicity. While a fraction of the measured uplift can be attributed to glacial and erosional isostatic processes, our results suggest that interseismic strain accumulation at the Montello and the Bassano–Valdobbiadene thrusts it significantly contributing to the measured uplift. The seismogenic potential of the Montello thrust turns out to be smaller than that of the Bassano–Valdobbiadene fault, whose estimated parameters (locking depth equals 9.1 km and slip rate equals 2.1 mm a−1) indicate a structure capable of potentially generating a Mw>6.5 earthquake. These results demonstrate the importance of precise vertical ground velocity data for modeling interseismic strain accumulation in slowly deforming regions where seismological and geomorphological evidence of active tectonics is often scarce or not conclusive.626 31 - PublicationOpen AccessPoroelasticity and Fluid Flow Modeling for the 2012 Emilia-Romagna Earthquakes: Hints from GPS and InSAR DataThe Emilia-Romagna seismic sequence in May 2012 was characterized by two mainshocks which were close in time and space. Several authors already modeled the geodetic data in terms of the mechanical interaction of the events in the seismic sequence. Liquefaction has been extensively observed, suggesting an important role of fluids in the sequence. In this work, we focus on the poroelastic effects induced by the two mainshocks. In particular, the target of this work is to model the influence of fluids and pore-pressure changes on surface displacements and on the Coulomb failure function (CFF). The fluid flow and poroelastic modeling was performed in a 3D half-space whose elastic and hydraulic parameters are depth dependent, in accordance with the geology of the Emilia-Romagna subsoil. The model provides both the poroelastic displacements and the pore-pressure changes induced coseismically by the two mainshocks at subsequent periods and their evolution over time. Modeling results are then compared with postseismic InSAR and GPS displacement time series: the InSAR data consist of two SBAS series presented in previous works, while the GPS signal was detected adopting a variational Bayesian independent component analysis (vbICA) method. Thanks to the vbICA, we are able to separate the contribution of afterslip and poroelasticity on the horizontal surface displacements recorded by the GPS stations. The poroelastic GPS component is then compared to the modeled displacements and shown to be mainly due to drainage of the shallowest layers. Our results offer an estimation of the poroelastic effect magnitude that is small but not negligible and mostly confined in the near field of the two mainshocks. We also show that accounting for a 3D fault representation with a nonuniform slip distribution and the elastic-hydraulic layering of the half-space has an important role in the simulation results.
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