We derive a co-seismic slip model of the 2015 Mw 8.3 Illapel, Chile earthquake constrained by line-of-sight displacements from Sentinel-1 interferograms. Greens functions are calculated with 3D finite element models (FEMs). The FEMs simulate a non-uniform distribution of elastic material properties and a precise geometric configuration of the irregular topographical surface. The rupturing fault follows the curvilinear Peru-Chile Trench and Slab1.0. The optimal model that inherits heterogeneous material properties, provides a significantly better solution than that in a homogenous domain at the 95% confidence interval. The best-fit solution for the domain having a non-uniform distribution of material properties reveals a triangular slip zone. Slip is concentrated near the trench with a dip-slip up to 7.75 m, giving rise to a moment magnitude of Mw8.22 in general agreement with the seismological estimate. This methodology allows us to integrate multiple datasets of geodetic observations with seismic tomography, to achieve a better understanding of seismic ruptures within crustal heterogeneity and fault curvature.

The eruption cycle of a volcano is controlled by the subsurface migration and storage of magma. The specific characteristics of the magma migration and spatial distribution of material properties produce a specific deformation signature on the Earth’s surface. Inverse analyses of geodetic data are used to optimize characteristic geometric and mechanical parameters of the volcanic system and hence provide information on the subsurface magmatic system. This study uses interferometric synthetic aperture radar data from a 1997 co- and post-eruptive interval for Okmok volcano to estimate the location of the magma reservoir and constrain finite element-based viscosity models of a thermally-weakened viscoelastic rind surrounding the reservoir. For the first time, approximately 10 years of pre-and post-eruption interferometric synthetic aperture radar data are analyzed to recover a magma reservoir pressurization history using both purely elastic and coupled elastic-viscoelastic models. The findings show that low viscosities surrounding the magma reservoir relax stresses rapidly enough to allow prediction of the more realistic viscoelastic pressurization histories to be calculated as a scaled version of the relatively simple but computationally efficient elastic models which allows for quick analysis of volcano hazards while maintaining fidelity to the actual physical system. This offers insights into how the shallow rheologic structure of magmatic systems can influence the predictions of transient deformation and estimates of the time-dependent magma budget.

Two major earthquakes of Mw7.8 and Mw7.5 ruptured the Southern East Anatolian Fault (SEAF) and the Savrun‐Çardak‐Sürgü fault (SCSF), devastating southeast Türkiye and northwest Syria on 6 February 2023. We adopt innovative nonlinear and linear approaches to analyze the coseismic ground displacements and estimate the complex slip geometry. Unlike conventional analytical solutions that simplify crust heterogeneity, finite‐element fault models invert the displacement data and simulate the dual‐fault geometry with non‐uniformly distributed shallow crustal materials. Our results suggest the west‐dipping SEAF and north‐dipping SCSF accommodate earthquake slips of >10 m. Their respective slip distributions and proximal aftershocks correlate spatially with local seismic velocity anomalies (i.e., ΔVp and ΔVs), which implies differences in structural control along these two faults and provides insights into assessing the seismic hazard of mixed incipient‐mature fault systems.

The Coso geothermal field is a major geothermal power production site in the western United States. It has been observed that low-magnitude seismic events (M < 3.71) are unevenly distributed in three distinct zones, namely, nearfield (<3 km), midfield (3-6 km), and farfield (> 6 km) from the Coso geothermal plant. These zones exhibit distinct changes in earthquake location before and during geothermal production episodes that began in 1986. After 1986, the midfield region of the main flank experiences a significantly lower seismicity rate than the surrounding areas before production episodes. During 2014-2019, the farfield earthquakes cluster in the eastern and western parts of the greater Coso area, which is discernably different from how those pre-production earthquake events were distributed along the conjugate NW-SE and SW-NW trending structures across the main flank. Here, we analyze the stage of stress with finite-element-based poroelastic simulations to illustrate how the spatiotemporal evolution of the seismicity is associated with the pattern of stress perturbations caused by fluid migration amid the operations of geothermal power plants. Generally, similar to 70% of co-production seismicity is found in zones of increased Coulomb stress between 2014 and 2019 at >99% confidence. Meanwhile, the midfield zone of seismic paucity overlaps with the zone of decreasing pore-fluid pressure. Overall, the results provide a physical explanation of how decadal geothermal operations at Coso have perturbed stress-field changes and contributed to the evolving characteristic seismic pattern, shedding insights into assessing the seismic hazard in other geothermal settings.

Magmatism is a known driver of flank instability at volcanoes where flank slip has been observed. Studies of instability at Kīlauea, Piton de la Fournaise, and Etna imply that long-term flank motion likely requires the presence of a layer accommodating the sliding, and a force, such as magma intrusion, that promotes slip. We present a parametric study using 2D Finite Element Models (FEMs), to assess how edifice aspect ratio, detachment fault geometry, asymmetric buttressing, and intrusion depth affect the potential for development of magma-driven flank instability at volcanoes. We quantify whether the tested conditions would favor flank slip based on the Coulomb Stress Changes (CSCs) associated with endmember scenarios and showcase the expected surface displacements for each scenario, to highlight their deviations from half-space models. Development of instability is more likely when flank slip is along a shallower-dipping receiver fault and the dike intrusion spans the edifice, regardless of edifice steepness. Another favorable scenario occurs in steep edifices with a steeply-dipping receiver fault when the dike is beneath the edifice. Buttressing slightly enlarges the region with conducive CSCs on shallow-dipping faults when the dike is within the edifice but shrinks the region with conducive CSCs in the steep edifice case with steeply-dipping faults. We also find that neglecting topography yields different magnitudes and extents of surface deformation, especially for steeper volcanic edifices. This topographical effect is more important when modeling horizontal displacements and stress fields induced by shallower intrusions.

The Longmen Shan Thrust Belt, which lies at the convergence of the eastern Tibetan Plateau and Sichuan Basin, is characterized by a topographic relief of 4000 m over a distance of 100 km. While the rock underlying the Sichuan Basin is strong, the crust underlying the Tibetan Plateau is weak. Four models that simulate deformation caused by distributions of dislocations along a geometrically complex fault system are developed to investigate how the contrast in elevational and crustal heterogeneity influences coseismic deformation of the 2008 M(w)7.9 Wenchuan earthquake. The models are configured with an assembly precise ramp-acollement structures and planar fault geometries. The coseismic slip distribution is estimated by inverting three component GPS observations with Green's functions calculated for each model configuration. By comparing the slip distribution and residuals among the displacement predictions from these four models and measurements of GPS, the crustal heterogeneity model robustly explained most of the observed GPS horizontal and vertical coseismic displacements. Results revealed four composite dislocation asperities distributed in the shallow crust and a slip of 4-8 m on the sub-surface atonement fault with a depth of similar to 20 km. The resulting geodetic moment is 8.10 x 10(20) Nm, which is in agreement with the seismological estimates. The deep slip predictions obtained with the rampatonement fault geometry indicate that the eastern Tibetan Plateau was thrust over the Sichuan Basin rather than the plateau being uplifted owing to the underlying weak crust.

The 2018 M7.2 Pinotepa earthquake ruptured a shallow slab section along the Middle America subduction zone. We demonstrate how a geodetic Green's function (GF) library and efficient modeling algorithm can rapidly resolve earthquake slip and contribute to early warning systems. The source is characterized with InSAR data and a finite-element model mimicking realistic slab geometry (Slab1.0) and velocity structures (CRUST2.0) that are not considered in the conventional homogeneous (HOM)- or layered(1-D)-crust solutions. The rupture is imaged 18 min after geodetic data are downloaded to a 16-CPU-thread workstation. Nearby M(w)8.6-megathrust scenarios are also studied with synthetic GPS data in the Guerrero/Oaxaca area. Our results show that earthquake slip solutions are sensitive to the materials of elastic-modeling domains. Attaining smaller misfits or sums of squared errors, models by 3-D GFs significantly better recover coseismic displacements than those estimated with 1-D GFs or HOM GFs at 95% confidence. Plain Language Summary A M(w)7.2 earthquake took place near the city of Pinotepa Nacinoal in Oaxaca, Mexico, on 16 February 2018 and became the third most significant event striking southern Mexico since mid-2017, after the M(w)8.2 Tehuantepec and M(w)7.1 Puebla-Morelos earthquakes. A new algorithm of deformation modeling is designed to use a fault library to efficiently characterize the earthquake source. This library accounts for fault slip within both rock and structural complexities documented within the Middle America subduction zone. We further study 100 M(w)8.6 megathrust scenarios and discover that an elastic model of these complexities is necessary to better recover the slip and surface displacements than the customary layered or uniform-crust solutions. These M(w)8.6 scenarios are the largest expected events based on historical records. Our algorithm images the Pinotepa earthquake rupture 18 min after the ground displacement data are downlinked from the satellite and processed in local machines, providing an alternative of evaluating the earthquake mechanism in addition to other seismological methods. The majority of fault slip (up to 1.2 m) is located over a gently dipping slab segment at a depth of similar to 21 km. These results illuminate the effectiveness and applicability of the proposed algorithm for real-time analyses, without compromising model accuracy to analytical solutions.

ABSTRACTThe 2017 Mw 6.5 Jiuzhaigou earthquake (JE) struck a rugged area of the Jiuzhaigou Valley in eastern Tibet that has experienced frequent seismic activity over the last few decades. We use finite‐element models (FEMs) and Sentinel‐1 Interferometric Synthetic Aperture Radar observations to characterize the earthquake source. The FEM domain accommodates a heterogeneous (HET) distribution of realistic crustal materials inferred by regional seismic tomography data. The HET‐derived source configurations yield a significantly smaller misfit, at the 95% confidence level, than that estimated for a homogeneous (HOM) half‐space. The former generally requires a lower degree of smoothing constraint, highlighting that the HET solutions are systematically more compatible with the surface observations than the HOM solutions. The magnitudes of induced Coulomb failure stress change (ΔCFS) estimated by the HET solution drastically differ (by >0.1 MPa) from those calculated by the HOM solution. The postearthquake stability of near‐field faults is generally overestimated by the HOM estimations, whereas some localities of negative ΔCFSHOM are predicted with positive ΔCFSHET. These results highlight the sensitivities of both slip and stress estimations to the complexity of the adopted elastic modeling domain, leading to more accurate aftershock hazard assessments. The HET‐resolved seismic rupture reveals two major slip asperities of magnitude up to 0.83 m distributed along the fault strike, which is coherent with the aftershock distribution. Two aftershock clusters are consistently found near or below these two peak‐slip zones, which are imaged by the HET model but absent in the HOM solution. The JE hypocenter and aftershocks are bounded below by a negative velocity anomaly (ΔVP, ΔVS down to −4%) at ∼18 km depth. Such low‐velocity layers of reduced strength may be relevant to the vertical distribution of seismicity and earthquake slip, which provide insights into assessing the seismic hazards and aftershock‐prone areas of the eastern Tibetan margin.

The large M7.3 aftershock occurred 17 days after the 2015 M7.8 Gorkha earthquake. We Investigate if this sequence is mechanically favored by the mainshock via time-dependent fluid migration and pore pressure recovery. This study uses finite element models of fully-coupled poroelastic coseismic and postseismic behavior to simulate the evolving stress and pore-pressure fields. Using simulations of a reasonable permeability, the hypocenter was destabilized by an additional 0.15 MPa of Coulomb failure stress change (Delta CFS) and 0.17 MPa of pore pressure (Delta p), the latter of which induced lateral and upward diffusive fluid flow (up to 2.76 mm/day) in the aftershock region. The M7.3 location is predicted next to a local maximum of Delta p and a zone of positive Delta CFS northeast of Kathmandu. About 60% of the aftershocks occurred within zones having either Delta p > 0 or Delta CFS > 0. Particularly in the eastern flank of the epicentral area, similar to 83% of the aftershocks experienced postseismic fluid pressurization and similar to 88% of them broke out with positive pore pressure, which are discernibly more than those with positive Delta CFS (71%). The transient scalar field of fluid pressurization provides a good proxy to predict aftershock-prone areas in space and time, because it does not require extraction of an assumed vector field from transient stress tensor fields as is the case for Delta CFS calculations. A bulk permeability of 8.32 x 10(-18) m(2) is resolved to match the transient response and the timing of the M7.3 rupture which occurred at the peak of the Delta CFS time-series. This estimate is consistent with the existing power-law permeability-versusdepth models, suggesting an intermediately-fractured upper crust coherent with the local geology of the central Himalayas. The contribution of poroelastic triggering is verified against different poroelastic moduli and surface flow-pressure boundaries, suggesting that a poroelastic component is essential to account for the time interval separating the mainshock and the M7.3 aftershock.

This study reveals how modeling configurations of forward and inverse analyses of coseismic deformation data influence the estimations of seismic and tsunami sources. We illuminate how the predictions of near‐field tsunami change when (1) a heterogeneous (HET) distribution of crustal material is introduced to the elastic dislocation model, and (2) the near‐trench rupture is either encouraged or suppressed to invert spontaneous coseismic displacements. Hypothetical scenarios of megathrust earthquakes are studied with synthetic Global Positioning System displacements in Cascadia. Finite‐element models are designed to mimic the subsurface heterogeneity across the curved subduction margin. The HET lithospheric domain modifies the seafloor displacement field and alters tsunami predictions from those of a homogeneous (HOM) crust. Uncertainties persist as the inverse analyses of geodetic data produce nonrealistic slip artifacts over the HOM domain, which propagates into the prediction errors of subsequent tsunami arrival and amplitudes. A stochastic analysis further shows that the uncertainties of seismic tomography models do not degrade the solution accuracy of HET over HOM. Whether the source ruptures near the trench also controls the details of the seafloor disturbance. Deeper subsurface slips induce more seafloor uplift near the coast and cause an earlier arrival of tsunami waves than surface‐slipping events. We suggest using the solutions of zero‐updip‐slip and zero‐updip‐slip‐gradient rupture boundary conditions as end‐members to constrain the tsunami behavior for forecasting purposes. The findings are important for the near‐field tsunami warning that primarily relies on the near‐real‐time geodetic or seismic data for source calibration before megawaves hit the nearest shore upon tsunamigenic events.

The migration of magma within a volcano produces a deformation signature at the Earth’s surface. Inverse models of geodetic data estimate parameters that characterize the magma migration. These characterizations are tied to the specific model that relates migration to the observed deformation. A model is a simplified representation of a natural system. A modeler is tasked with the challenge of designing a model that represents the system, in the context of the available data and purpose of the model. This chapter presents a systematic approach to quantitatively simulate geodetic data with finite element models (FEMs) in the framework of a deformation modeling protocol. This chapter will (1) address the design and execution of FEMs that can account for the geophysical complexity of a volcano deformational system and (2) define techniques for including FEMs in both linear and nonlinear inverse methods to characterize a magmatic system based on observed geodetic data. With these techniques, researchers can estimate magmatic migration within active volcanoes and understand how uncertainties in the data propagate into predictions. These estimates comprise some measure of central tendency, a sense of uncertainty, and a quantification of biases.

Two months after the 2016 Amatrice earthquake (AE), a strong (~M6) earthquake (Visso earthquake, VE) struck the town Visso, Italy, 20 km north of the AE epicenter. Between these two events, the aftershocks migrated gradually toward to the VE epicenter at a rate of ~0.4 km/d, indicating propagation of pore pressure front. We use finite element models to simulate the postseismic fully coupled poroelastic response. The results show that the pore fluid flows (up to 50 nm/s) both horizontally and vertically into the VE hypocenter since the AE and destabilized the area with extra ~70% of Coulomb failure stress. Majority of nearby aftershocks (>80%) tend to cluster within the zones of coseismic depressurization where fluid flow converges. A maximum ΔCFS of ~35 kPa is calculated at the VE hypocenter during its rupture by a crustal permeability, 10–16 ± 0.7 m2, suggesting that an intermediately fractured crust allows maximum rupture tendency for the VE during poroelastic fluid recovery.

The source geometry of the 24 August 2016 Amatrice earthquake is studied with Sentinel 1-A/B and Advanced Land Observation Satellite (ALOS)-2 Interferometric Synthetic Aperture Radar (InSAR) coseismic observations. Without presetting the fault geometry and location, we allow these geodetic data to constrain a planar fault and a listric dislocation through an innovative nonlinear approach. Finite-element models (FEMs) are built to simulate the earthquake deformation over a domain with a distribution of realistic crustal materials. Optimal fault geometries are resolved with the observed coseismic displacements in both homogeneous (HOM) half-spaces and heterogeneous (HET) FEMs. The HET-computed planar slip is modeled next to the hypocenter and estimated with a very compatible dip (47 degrees) similar to the moment tensor solution, while that of the HOM solution is offset by similar to 14 degrees. Mean-while, the former recovers the coseismic displacements significantly better than the latter at 95% confidence. This indicates that the inverse solutions of source analysis are sensitive to the presence of nonuniform rock materials in the elastic domain. Though abundant fault detachments are documented within the epicentral area, the slip distributions derived along optimized listric faults do not improve the prediction of surface movements estimated by the rectangular sources. This implies that a planar fault geometry is sufficient to describe the rupture of the Amatrice earthquake. The corresponding slip distribution reveals a maximum slip of similar to 1 m at 7 km depth, beneath a lithological boundary between the shallow weaker units and the underlying stronger Dolomite. A significant correlation is found between the slip magnitude and subsurface rock stiffness where stress is being accumulated, implying a possible structural control on the earthquake nucleation and propagation.

The Earth's surface deforms in response to earthquake fault dislocations at depth. Deformation models are constructed to interpret the corresponding ground movements recorded by geodetic data such GPS and InSAR, and ultimately characterize the seismic ruptures. Conventional analytical and latest numerical solutions serve similar purpose but with different technical constraints. The former cannot simulate the heterogeneous rock properties and structural complexity, while the latter directly tackles these challenges but requires more computational resources. As demonstrated in the 2015 M7.8 Gorkha, Nepal earthquake and the 2016 M6.2 Amatrice, Italy earthquake, we develop state-of-art finite element models (FEMs) to efficiently accommodate both the material and tectonic complexity of a seismic deformational system in a seamless model environment. The FEM predictions are significantly more accurate than the analytical models embedded in a homogeneous half-space at the 95% confidence level. The primary goal of this chapter is describe a systematic approach to design, construct, execute and calibrate FEMs of elastic earthquake deformation. As constrained by coseismic displacements, FEM-based inverse analyses are employed to resolve linear and nonlinear fault-slip parameters. With such numerical techniques and modeling framework, researchers can explicitly investigate the spatial distribution of seismic fault slip and probe other in-depth rheological processes.

SummaryWe test an innovative inversion scheme using Green's functions from an array of pressure sources embedded in finite-element method (FEM) models to image, without assuming an a-priori geometry, the composite and complex shape of a volcano deformation source. We invert interferometric synthetic aperture radar (InSAR) data to estimate the pressurization and shape of the magma reservoir of Rabaul caldera, Papua New Guinea. The results image the extended shallow magmatic system responsible for a broad and long-term subsidence of the caldera between 2007 February and 2010 December. Elastic FEM solutions are integrated into the regularized linear inversion of InSAR data of volcano surface displacements in order to obtain a 3-D image of the source of deformation. The Green's function matrix is constructed from a library of forward line-of-sight displacement solutions for a grid of cubic elementary deformation sources. Each source is sequentially generated by removing the corresponding cubic elements from a common meshed domain and simulating the injection of a fluid mass flux into the cavity, which results in a pressurization and volumetric change of the fluid-filled cavity. The use of a single mesh for the generation of all FEM models avoids the computationally expensive process of non-linear inversion and remeshing a variable geometry domain. Without assuming an a-priori source geometry other than the configuration of the 3-D grid that generates the library of Green's functions, the geodetic data dictate the geometry of the magma reservoir as a 3-D distribution of pressure (or flux of magma) within the source array. The inversion of InSAR data of Rabaul caldera shows a distribution of interconnected sources forming an amorphous, shallow magmatic system elongated under two opposite sides of the caldera. The marginal areas at the sides of the imaged magmatic system are the possible feeding reservoirs of the ongoing Tavurvur volcano eruption of andesitic products on the east side and of the past Vulcan volcano eruptions of more evolved materials on the west side. The interconnection and spatial distributions of sources correspond to the petrography of the volcanic products described in the literature and to the dynamics of the single and twin eruptions that characterize the caldera. The ability to image the complex geometry of deformation sources in both space and time can improve our ability to monitor active volcanoes, widen our understanding of the dynamics of active volcanic systems and improve the predictions of eruptions.