Ground Deformation Field Research Articles

Surface displacements on faults triggered by slow magma transfers between dyke injections in the 2005–2010 rifting episode at Dabbahu–Manda–Hararo rift (Afar, Ethiopia)

The rifting episode that occurred in Dabbahu–Manda–Hararo (Ethiopia) between 2005 and 2010 during which 14 dyke intrusions were emitted, was a unique opportunity to study interactions between tectonic deformation and magmatic processes. While magmatism has been shown to control primarily the spatial and temporal distribution of dyke intrusions during this accretion sequence, the role of faults in accommodating plate spreading in rift segments is poorly understood. During interdyking periods, transient ground deformation due to magma movement is generally observed. Investigating such a small-scale deformation and in particular the movement along faults during these periods will help understanding the factors that trigger fault movement in magmatic rifts. We analyse fault activity during three interdyking periods: 2006 December–June (d0–d1), 2007 January–July (d5–d6) and 2009 November–January (d10– d11). The time–space evolution of surface displacements along ∼700 faults is derived from pairs of ascending and descending SAR interferograms. Surface slip distributions are then compared with codyking ground deformation fields. The results show that faults are mainly activated above the areas affected by magma emplacement during interdyking periods. A detailed analysis of brittle deformation during the six months following the 2005 September intrusion shows asymmetric deformation on the rift shoulders, with significant opening on faults located to the west of the dyke. We explain this feature by the activation of westward dipping pre-existing faults, with block rotations in between. In addition, we observe that the strip encompassing the activated faults narrows by 30 per cent from co-to interdyking period. This suggests that magma keeps migrating to shallower depths after the dyke intrusion. During a rifting episode, activation of faults in a pre-existing fracture network therefore seems to be mainly controlled by deep magma processes.

The ground deformation field induced by a listric thrust fault with an overburden soil layer

The surface deformation field induced by a listric thrust fault with a thick, overburden soil layer is studied in this paper by the finite element method (FEM). The results show: (a) The maximum slip induced by the buried fault is not located at upper tip of the fault, but below it. (b) The vertical displacement changes remarkably near the fault, forming a fault scarp. With the increase of the soil layer thickness, the height of the scarp is decreased for the same earthquake magnitude. (c) The strong strain zone on the surface is localized near the projection of the fault tip on the ground surface. The horizontal strains in the zone are in tension above the hanging wall and in compression above the foot wall, and the vertical strains in the zone are vice versa, which is favorable for tensileshear, compression-shear fissures above hanging wall and foot wall, respectively.

New insights into the 2012 Emilia (Italy) seismic sequence through advanced numerical modeling of ground deformation InSAR measurements

AbstractWe provide new insights into the two main seismic events that occurred in 2012 in the Emilia region, Italy. We extend the results from previous studies based on analytical inversion modeling of GPS and RADARSAT‐1 InSAR measurements by exploiting RADARSAT‐2 data. Moreover, we benefit from the available large amount of geological and geophysical information through finite element method (FEM) modeling implemented in a structural‐mechanical context to investigate the impact of known buried structures on the modulation of the ground deformation field. We find that the displacement pattern associated with the 20 May event is consistent with the activation of a single fault segment of the inner Ferrara thrust, in good agreement with the analytical solution. In contrast, the interpretation of the 29 May episode requires the activation of three different fault segments and a block roto‐translation of the Mirandola anticline. The proposed FEM‐based methodology is applicable to other seismic areas where the complexity of buried structures is known and plays a fundamental role in the modulation of the associated surface deformation pattern.

Multiscale InSAR Time Series (MInTS) analysis of surface deformation

We present a new approach to extracting spatially and temporally continuous ground deformation fields from interferometric synthetic aperture radar (InSAR) data. We focus on unwrapped interferograms from a single viewing geometry, estimating ground deformation along the line‐of‐sight. Our approach is based on a wavelet decomposition in space and a general parametrization in time. We refer to this approach as MInTS (Multiscale InSAR Time Series). The wavelet decomposition efficiently deals with commonly seen spatial covariances in repeat‐pass InSAR measurements, since the coefficients of the wavelets are essentially spatially uncorrelated. Our time‐dependent parametrization is capable of capturing both recognized and unrecognized processes, and is not arbitrarily tied to the times of the SAR acquisitions. We estimate deformation in the wavelet‐domain, using a cross‐validated, regularized least squares inversion. We include a model‐resolution‐based regularization, in order to more heavily damp the model during periods of sparse SAR acquisitions, compared to during times of dense acquisitions. To illustrate the application of MInTS, we consider a catalog of 92 ERS and Envisat interferograms, spanning 16 years, in the Long Valley caldera, CA, region. MInTS analysis captures the ground deformation with high spatial density over the Long Valley region.

Analysis of satellite and in situ ground deformation data integrated by the SISTEM approach: The April 3, 2010 earthquake along the Pernicana fault (Mt. Etna - Italy) case study

Etna is known worldwide as one of the most studied and monitored active volcanoes. Flank instability along the eastern and southern portions of Mt. Etna has been observed and measured thanks to geodetic networks and InSAR data analysis. The spreading area is bordered to the north by the east–west Pernicana Fault System (PFS) whose dynamics is often linked with the eruptive activity, as recently observed during the 2002–2003 eruption. A seismic sequence occurred from April 2–3, 2010, along the PFS with very shallow (a few hundred meters) mainshocks of magnitude 4.3 and 3.6. Explosions and ash emissions at the summit craters followed this swarm culminating a few days later (April 7–8). Despite their small magnitude, the earthquakes caused damage and significant surface fracturing along the PFS. In order to investigate and measure the deformations in the near field of the earthquakes, the SISTEM integration approach has been exploited. The SISTEM enabled integrating geodetic in situ ground deformation measurements (GPS and leveling) with satellite interferometric measurements (ENVISAT and ALOS), in order to obtain high resolution 3D displacement maps, allowing to overcome the limitations of each technique and take advantage of the particular features of each of them. The integrated ground deformation field evidenced that the medium behave elastically. We inverted the SISTEM results using an optimization algorithm based on the Genetic Algorithm (GA) in order to model the kinematics of the PFS associated to seismic swarm; the results are in good agreement with the field evidence and improved the knowledge on the kinematics of the PFS and Mt. Etna's unstable flank.

Magma, faults, and gravitational loading at Mount Etna: The 2002–2003 eruptive period

[1] Mount Etna is characterized by a complex structural setting that influences its evolution. In order to understand how the interaction between dike-forming intrusions and faulting influences the kinematics of the volcanic edifice, we developed a numerical model. It takes account of the topography, the medium heterogeneities, the gravitational loading, and the most active crustal discontinuities. A parameterization of the apparent coefficient of friction, as a function of the depth, has been considered in the range from 0.01 to 0.5. The density values used, Young's modulus, and Poisson's ratio range from about 1700 to 3200 kg/m3, 8.5 to 140 GPa, and 0.15 to 0.35, respectively. The model was applied to the 2002–2003 Etna eruption. The resulting deformation pattern was in agreement with the data provided by the continuous GPS stations and the seismological knowledge, obtaining displacements up to 1 m. Some discrepancies between the recorded ground deformation field and the modeled displacements allowed us to make some hypotheses on the volcano-tectonic (e.g., for the Pernicana fault) or regional-tectonic origin (Acireale–S. Alfio fault system) of the crustal discontinuities. Moreover, our results suggested that the Pernicana fault is characterized by a very low apparent coefficient of friction, less than 0.1. Furthermore, we evaluated the contribution of the gravitational loading and found that it implies a variation of about 10% of the overall deformation pattern. Our results clearly showed that the weight of the volcanic edifice acts in opposition to the magmatic intrusions. Conversely, we found that the presence of medium heterogeneities may favor the eastern flank movements toward the SE and the spreading of the summit area, playing a fundamental role in ascending magma. Finally, we investigated the shallow sliding model and found that dike-forming intrusions produce negligible displacements, less than 1 mm, along the subhorizontal detachment surface. Therefore, the gravitational loading during dike-forming intrusions is not able to trigger sliding processes along this plane.

Finite element modeling of ground deformation and gravity field at Mt. Etna

An elastic 3-D axi-symmetric model based on Finite Element Method (FEM) is proposed to compute ground deformation and gravity changes caused by overpressure sources in volcanic areas. The numerical computations are focused on the modeling of a complex description of Mt Etna in order to evaluate the effect of topography, medium heterogeneities and source geometries. Both ground deformation and gravity changes are investigated by solving a coupled numerical problem considering a simplified ground surface profile and a multi-layered crustal structure inferred from seismic tomography. The role of the source geometry is also explored taking into account spherical and ellipsoidal volumetric sources. The comparison between numerical results and those predicted by analytical solutions disclosed significant discrepancies. These differences constrain the applicability of simple spherical source and homogeneous half-space hypotheses, which are usually implicitly assumed when analytical solutions are applied.

Coseismic deformation due to the 2007 Chuetsu-oki earthquake (Mw= 6.8)

AbstractGlobal Positioning System (GPS) and Synthetic Aperture Radar Interferometry (InSAR) detected substantial ground deformation due to the 2007 Chuetsu-oki earthquake (Mw= 6.8); GPS observation detected a horizontal deformation of up to about 170 mm and subsidence of up to 30 mm, and InSAR detected up to 290 mm of line-of-sight changes. A fault model is proposed to fit the ground deformation field as well as the aftershock distribution. Our model shows that a northwest-dipping fault to the north and a southeast-dipping fault to the south, consistent with the aftershock distribution, fits well with the observed data. However, our model also suggests that the modeled faults are likely to extend to shallower depths, where aftershocks are terminated. Combining our model with the subsurface structure suggests that the earthquake rupture initiated at depth and propagated into shallower depths that are not capable of nucleating an earthquake because of the presence of unconsolidated sediments.

Modelling of ground deformation and gravity fields using finite element method: an application to Etna volcano

SUMMARY Elastic finite element models are applied to investigate the effects of topography and medium heterogeneities on the surface deformation and the gravity field produced by volcanic pressure sources. Changes in the gravity field cannot be interpreted only in terms of gain of mass disregarding the ground deformation of the rocks surrounding the source. Contributions to gravity changes depend also on surface and subsurface mass redistribution driven by dilation of the volcanic source. Both ground deformation and gravity changes were firstly evaluated by solving a coupled axisymmetric problem to estimate the effects of topography and medium heterogeneities. Numerical results show significant discrepancies in the ground deformation and gravity field compared to those predicted by analytical solutions, which disregard topography, elastic heterogeneities and density subsurface structures. With this in mind, we reviewed the expected gravity changes accompanying the 1993‐1997 inflation phase on Mt Etna by setting up a fully 3-D finite element model in which we used the real topography, to include the geometry, and seismic tomography, to infer the crustal heterogeneities. The inflation phase was clearly detected by different geodetic techniques (EDM, GPS, SAR and levelling data) that showed a uniform expansion of the overall volcano edifice. When the gravity data are integrated with ground deformation data and a coupled FEM modelling was solved, a mass intrusion could have occurred at depth to justify both ground deformation and gravity observations.

Estimation of Ground Strain Using Accelerograms Recorded by Two Dense Seismic Arrays at Lotung, Taiwan

In this study, ground strains over a soft sediment-filled plain were estimated for nineteen moderate earthquakes in northeastern Taiwan. To this end, abundant accelerograms recorded by two independent seismic arrays, LLSST and SMART1, were analyzed. These two dense seismic arrays are both highly overlapping in terms of space and operation duration. Spatial analyses herein for the LLSST array suggest that good estimates of ground strains can be obtained using only three 3-component seismometers recording at stations configured in a simple triangle with maximum separation of about 100 m. In the case of the Hualien earthquake (ML = 6.5) of 20 May 1986, a peak shear strain of 210 µ occurs with direct S-waves whilst strain of 186 µ occurs with surface waves dominated by a period of about 2.5 sec. However, the peak vertical gradient accompanied by shear waves is in the range 1500 ~ 2000 µ , which is larger than that of horizontal differential motion by a factor of about 10. For further applications in mapping ground deformation fields at a given region, the single-station method is examined by comparing its estimates with those of a displacement gradient algorithm. A moving crosscorrelation method is proposed to obtain phase velocities for a sequence of waves, particularly when ground motion contains obvious surface-wave energy. Consequently, estimates of peak horizontal strain at the LLSST site are in the range 1 ~ 150 µ for the nineteen moderate earthquakes and the logarithm of these appears to be proportional to earthquake magnitudes.

Co-seismic ground deformation and source parameters of Mani M7.9 earthquake inferred from spaceborne D-InSAR observation data

We obtain the co-seismic ground deformation field of the Mani M7.9 earthquake on November 8, 1997 through three-pass differential interferometric processing. Then we get the geometric and kinematics parameters of this event by the elastic half-space model and forward modeling with the InSAR data. The following results have been obtained: (i) The deformation fields on both sides of the seismogenic fault are affected by the co-seismic deformation field even in 110 km away from the fault. The loss of phase coherence belt caused by the zone of ground rupture crosses the image from east to west with a length of an area 110 km. (ii) The maximum horizontal displacement caused by the earthquake reaches 7.96 m. (iii) The earthquake fault can be divided into four parts, and the deformation field of the middle two parts is bigger than that of the other two, with lengths of 27 km and 37 km, respectively. And their average sliding values on the rupture surface are 6500 mm and 6000 mm, respectively, and their depths both are 35 km. The segment of 27 km length is the major rupture surface of this earthquake. The west part and the east part of the fault have lengths of 23 km and 26 km, respectively. The sliding value of the west one is 4000 mm and that of the east one is 5800 mm. They extend to depths of 20 km and 18 km, respectively.

The 1995 Kozani-Grevena (northern Greece) earthquake revisited: an improved faulting model from synthetic aperture radar interferometry

Previously, geodetic data associated with earthquakes have been widely modelled using co-planar rectangular dislocations in an elastic half-space. However, such models appear inadequate when complex geometries such as variations in strike and dip or multiple fault segments are involved. Here we revisit the 1995 Ms = 6.6 Kozani–Grevena earthquake, and use synthetic aperture radar (SAR) interferometric measurements, tectonic observations and seismological data to constrain a fault model with a realistic geometry. We undertake a critical analysis of all available SAR data, including characterization of atmospheric artefacts. These are partially removed and the possibility that such effects are misidentified as secondary faulting is examined. Three well-correlated interferograms provide an accurate and complete description of the ground deformation field associated with the event. To take into account the complexity of the fault system activated during the earthquake, we construct a 3-D fault model, composed of triangular elements, that is geometrically more consistent with surface ruptures than those of previous studies. Using first trial-and-error and then iterative inversion, we explore the ranges of geometric parameters that can explain the data. We obtain an average final model and its standard deviation, with small slip amplitude at the surface, consistent with the field observations, and with slip as large as 2.5 m at depth. This model is compared with those previously published. We conclude that an antithetic fault is not required to explain the SAR data.

Displacement Field of Dislocation in a Twophase Saturated Medium in Semi‐Infinite Space

AbstractThe elastic dislocation theory is an important tool to study sources of earthquakes. Using the dislocation theory of the single‐phase medium, the obtained ground deformation field is stable and constant. However, the observed ground deformation field after earthquakes is varied which may be associated with fluids in rocks. The purpose of this study is to acquire the displacement field in a two‐phase saturated medium in semi‐infinite space. Our method is utilizing the cylinder function expression of the Green function for a two‐phase saturated medium in semi‐infinite space under a concentrated force, via vector transformation of the Helmholtz equation, and then superimposing the effect field of a free surface. Combining the DD model of Nur and assumption on water in and water out, our result can explain the variations of the ground deformation field before and after earthquakes.