Surface Gravity Changes Research Articles

Surface deformations and gravity changes caused by pressurized finite ellipsoidal cavities

SUMMARY We develop quasi-analytical solutions for the surface deformation field and gravity changes due to the pressurization of a finite (triaxial) ellipsoidal cavity in a half-space. The solution is in the form of a non-uniform distribution of triaxial point sources within the cavity. The point sources have the same aspect ratio, determined by the cavity shape, while their strengths and spacing are determined in an adaptive manner, such that the net point-source potency per unit volume is uniform. We validate and compare our solution with analytical and numerical solutions. We provide computationally efficient MATLAB codes tailored for source inversions. This solution opens the possibility of exploring the geometry of shallow magma chambers for potential deviations from axial symmetry.

Analytical Solutions for Gravity Changes Caused by Triaxial Volumetric Sources

AbstractVolcanic crises are often associated with magmatic intrusions or the pressurization of magma chambers of various shapes. These volumetric sources deform the country rocks, changing their density, and cause surface uplift. Both the net mass of intruding magmatic fluids and these deformation effects contribute to surface gravity changes. Thus, to estimate the intrusion mass from gravity changes, the deformation effects must be accounted for. We develop analytical solutions and computer codes for the gravity changes caused by triaxial sources of expansion. This establishes coupled solutions for joint inversions of deformation and gravity changes. Such inversions can constrain both the intrusion mass and the deformation source parameters more accurately.

A free-geometry geodynamic modelling of surface gravity changes using Growth-dg software

Globally there is abundant terrestrial surface gravity data used to study the time variation of gravity related to subsurface mass and density changes in different geological, geodynamical and geotechnical environments. We present here a tool for analysing existing and newly acquired, 4D gravity data, which creates new findings from its reuse. Our method calculates in an almost automatic way the possible sources of density change responsible for the observed gravity variations. The specifics of the new methodology are: use of a low number of observation points, relatively small source structures, low signal/noise ratio in the data, and a free 3D source geometry without initial hypothesis. The process is based on the non-linear adjustment of structures defined by aggregation of small cells corresponding to a 3D section of the sub-floor volume. This methodology is implemented in a software tool, named GROWTH-dg, which can be freely downloaded for immediate use, together with a user manual and application examples.

Minimum Detectable Mass and Volume Fluxes During Magmatic Recharge at High Prominence Volcanoes: An Application to Erciyes Dağ Volcano (Turkey)

Magma reservoir recharge is widely recognised as a precursor of eruptive activity. However, the causative relationships between reservoir rejuvenation and surface observables such as gravitational potential field changes and ground deformation are still poorly understood. At intermediate and silicic intra-plate volcanoes where crustal mechanical heterogeneity combined with high-prominence are expected to fundamentally affect the crustal stress and strain relationship, protracted period of repose and absence of monitoring data raise questions about the detectability of magma recharge. Here we report results from integrated geodetic forward modelling of ground displacements and gravity changes from reservoir recharge at Erciyes Dağ, a large prominence (∼2,800 m), yet poorly studied, stratovolcano of the Central Anatolian Volcanic Province in Turkey. The most recent eruption at ∼7000 BC, close proximity to the Kayseri Metropolitan Area and absence of dedicated volcano monitoring set a precedent to explore stealth magmatic processes at the volcano. Using finite element analysis we systematically explore the influence of subsurface mechanical heterogeneities and topography on surface deformation and gravity changes from magmatic recharge of Erciyes Dağ’s reservoir. We show that whilst crustal heterogeneity amplifies ground displacements and gravity variations, the volcano’s substantial prominence has the opposite effect. For generic reservoir pressure and density changes of 10 MPa and 10 kg m−3 predicted vertical displacements vary by a factor of 5 while residual gravity changes vary by a factor of 12 between models ignoring topography or mechanical heterogeneity and those that do not. We deduce reservoir volume and mass changes of order 10–3 km3 and 1010 kg, respectively, at the detectability limit of conventional surveying techniques at the volcano. Though dependent on model assumptions, all results indicate that magma recharge at Erciyes Dağ may go undetected at fluxes 1) sufficient to maintain an active reservoir containing eruptable magma and 2) similar to those reported for intermediate/silicic volcanoes with repose times of 100–1,000s of years (e.g., Parinacota) and persistently active mafic volcanoes such as Mt. Etna and Stromboli. Our findings may be utilised to inform integrated geodetic and gravimetric monitoring at Erciyes Dağ and other large prominence silicic volcanoes and could provide early insights into reservoir rejuvenation with implications for the development of disaster risk reduction initiatives.

Numerical study of reservoir permeability effects on gravity changes associated with CO2 geological storage: implications for gravimetric monitoring feasibility

AbstractGravimetric methods are used for monitoring the migration of CO2 stored in the reservoir. Reservoir permeability is crucial for injectivity and potentially important for gravity changes associated with CO2 storage. Nevertheless, its effects on gravity changes have not been investigated systematically. After conducting numerical simulations of CO2 injection and storage using models with varying permeability conditions (50–1000 mD), we calculated surface gravity changes. Results show that two mechanisms affect gravity decrease, related to differences in CO2 density and in CO2 spatial patterns. Under low‐permeability conditions, gravity decrease is minimal during injection because of increases in pressure and the resulting relative high density of CO2. Gravity decrease is enhanced after injection ceases because of the pressure release and resulting CO2 expansion. These effects are strong at shallow depths, at which CO2 is near the critical point and highly compressible. Under higher permeability conditions, rapid spreading of the CO2 plume causes smaller gravity reduction rate after some decades of injection. After injection ceases, upward CO2 migration because of buoyancy and plume spreading increase gravity above the injection zone. The time required for gravity change detection depends on permeability. Detection during injection can be made earlier under higher permeability conditions and under lower permeability conditions after injection ceases. These points suggest that permeability affects the feasibility of gravimetric methods. Moreover, designing monitoring programs based on permeability and depth conditions is necessary for successful monitoring. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Upper mantle density and surface gravity change in Fennoscandia, determined from GRACE monthly data

Precise gravity measurements have been repeatedly observed in Fennoscandia since the 1960-ties, first by relative gravimeters, then by absolute gravimeters, for studying the temporal change of gravity related with the on-going glacial isostatic adjustment (GIA). The results of such studies are vital for understanding GIA processes and tuning GIA and Earth structure models.This study uses monthly repeated data from the twin satellite mission GRACE for 164 months between 2003 and 2016 for estimating the temporal change of surface gravity, its ratio (f) to the land uplift rate as well as the upper mantle density related with the viscous mass flow in the mantle. The maximum negative change of gravity is estimated to −1.8 μGal/yr, and f is estimated to −0.172 ± 0.018 μGal/mm.The solutions for f from the three independent techniques (relative, absolute and satellite gravity observations) were found to agree statistically without significant biases, and they were merged in a joint solution to −0.166 ± 0.004 μGal/mm, corresponding to a mean upper mantle mass flow density of 3402.5 ± 95 kg/m3, which decreases towards the uplift center.

A study of the monsoonal hydrology contribution using a 8-yr record (2010–2018) from superconducting gravimeter OSG-060 at Djougou (Benin, West Africa)

SUMMARY We analyse a nearly 8-yr record (2010–2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). After tidal analysis removing all solid Earth and ocean loading tidal contributions and correcting for the long-term instrumental drift and atmospheric loading, we obtain a gravity residual signal which is essentially a hydrological signal due to the monsoon. This signal is first compared to several global hydrology models (ERA, GLDAS and MERRA). Our superconducting gravimeter residual signal is also superimposed onto episodic absolute gravity measurements and to space gravimetry GRACE data. A further comparison is done using local hydrological data like soil moisture in the very superficial layer (0–1.2 m), water table depth and rainfall. The temporal evolution of the correlation coefficient between the gravity observation and both the soil moisture and the water table is well explained by the direct infiltration process of rain water together with the lateral transfer discharging the water table. Finally, we compute the water storage changes (WSC) using a simulation based on the physically based Parflow-CLM numerical model of the catchment, which solves the water and energy budget from the impermeable bedrock to the top of the canopy layer using the 3-D Richards equation for the water transfers in the ground, the kinematic wave equation for the surface runoff and a land surface model (CLM) for the energy budget and evapotranspiration calculation. This model forced by rain is in agreement with evapotranspiration and stream flow data and leads to simulated water storage changes that nicely fit to the observed gravity signal. This study points out the important role played by surface gravity changes in terms of a reliable proxy for water storage changes occurring in small catchments.

Surface volume and gravity changes due to significant earthquakes occurred in central Italy from 2009 to 2016

We have modelled the surface volume and gravity changes caused by four seismic events: three mainshocks (moment magnitude Mw 6.0, 5.9, 6.5) occurred during the last seismic period started on 2016, August 24 in central Italy, and the 2009, April 6 L’Aquila Earthquake (Mw 6.3). Our calculations start from the source parameters estimated by the inversion of the largest dataset of Interferometric Synthetic Aperture Radar (InSAR) and global positioning system observations ever managed in Italy after earthquake occurrences, based on the half-space elastic dislocation theory. The vertical displacements modelled after the 2016 events allow to infer a substantial unbalance between the subsided and uplifted volumes. In particular, we detected ~ 106 × 106 m3 of hangingwall subsidence against ~ 37 × 106 m3 of footwall uplift, that accounts for ~ 74% of the total volume mobilization. From the ratio between the footwall and total deformed volumes, we have computed an average fault dip of ~ 47°, in line with the values retrieved by seismological methods. The total gravity variations which affected the study area are of the order of ~ 1 μGal (1 μGal = 10−8 ms−2) in the far field, and ~ 170 μGal in the near field. The area affected within a gravity change of 1 μGal is ~ 140 km long and ~ 57 km wide, parallel to the Apennines mountain chain. The larger contribution is given by positive variations which account for the tensional style of deformation and larger subsided area. The significant gravity variations modelled from the coseismic deformations point out the need to update our knowledge about the absolute gravity field in Italy carrying out extensive measurements, and to align Italy to the recent international standards about national gravity and height networks (International Association of Geodesy, IAG Report, Commission 2—gravity field, http://www.iag-commission2.ch , 2015).

Benchmarked computation of time-domain viscoelastic Love numbers for adiabatic mantles

SUMMARYThe viscoelastic load Love numbers encapsulate the Earth’s rheology in a remarkably efficient fashion. When multiplied by a sudden increment of spherical harmonic load change, they give the horizontal and vertical surface displacements and gravity change at all later times. Incremental glacial load changes thus need only be harmonically decomposed, multiplied by the Love numbers and summed to predict the Earth’s response to glacial load redistributions. The computation of viscoelastic Love numbers from the elastic, viscous and adiabatic profiles of the Earth is thus the foundation upon which many glacial isostatic adjustment models are based. Usually, viscoelastic Love numbers are computed using the Laplace transform method, employing the correspondence principle to convert the viscoelastic equations of motion into the elastic equations with complex material parameters. This method works well for a fully non-adiabatic Earth, but can accommodate realistic partially adiabatic and fully adiabatic conditions only by changing the Earth’s density profile. An alternative method of Love number computation developed by Cathles (1975) avoids this dilemma by separating the elastic and viscous equations of motion. The separation neglects a small solid-elastic/fluid-elastic transition for compressible deformation, but allows freely defining adiabatic, partially adiabatic or fully non-adiabatic profiles in the mantle without changing the Earth’s density profile. Here, we update and fully describe this method and show that it produces Love numbers closely similar to those computed for fully non-adiabatic earth models computed by the correspondence principle, finite element and other methods. The time-domain method produces Love numbers as good as those produced by other methods and can also realistically accommodate any degree of mantle adiabaticity. All method implementations are available open source.

Gravity changes and crustal deformations before the Menyuan, Qinghai Ms6.4 earthquake of 2016

In this study, the relative gravity data (2012–2015), GPS data-derived horizontal deformation (2011–2014) and the background vertical deformation from the leveling measurements (1970–2011) in the northeastern margin of Tibetan Plateau were processed to systematically analysis the mechanism of temporal–spatial patterns and the relationship with Menyuan Ms6.4 earthquake. It can be summarized in the following: 1) The regional gravity changes, the GPS and the vertical deformational showed an intense spatial relationship: the gravity increased along with the direction of horizontal movement, and decreased with the crustal uplift and vice versa, which reflected the inherited characteristics of geotectonic activities. 2) The crustal deformations were closely related to the active faults. The contour lines of gravity changes and vertical deformation were generally along with the Qilian-Haiyuan fault (strike is NWW), and the crustal horizontal deformation showed left-lateral strike slip motion near the Qilian-Haiyuan fault. 3) Menyuan Ms6.4 earthquake occurred in the high negative gravity variation area and a high gradient formed in regions, positive and negative variation of gravity amount to 110 μGal. Specifically, a borderline of positive and negative gravity located in the south of epicenter along the north edge of Qilianshan fault and Lenglongling fault, as well as the vertical and/or horizontal deformation is intensely. The extrusion deformation, surface compression rate and gravity changes were obvious near the epicenter of 2016 Menyuan Earthquake.

Surface displacements, deformations and gravity changes due to underground heat source

Abstract Thermo-elastic strains and stresses play a considerable role in the stress state of the lithosphere and its dynamics, especially at pronounced positive geothermal anomalies. Topography has a significant effect on ground deformation. Two methods for including the topographic effects in the thermo-viscoelastic model are described. First we use an approximate methodology which assumes that the main effect of topography is due to the distance from the source to the free surface and permits to have an analytical solution very attractive for solving the inverse problem. A numerical solution (for 2D plain strain case) is also computed using finite element method (FEM). The numerical method allows to include the local shape of the topography in the modeling. In the numerical model the buried magmatic body is represented by a finite volume thermal source. The temperature distribution is computed by the higher-degree FEM. For analytical as well as numerical model solution only the forces of thermal origin are considered. The results show that for the volcanic areas with prominent topography, its effect on the perturbation of the thermo-viscoelastic solution (deformation and total gravity anomaly) can be quite significant. In consequence, neglecting the topography could give erroneous results in the estimated source parameters.

Hybrid gravimetry monitoring of Soultz-sous-Forêts and Rittershoffen geothermal sites (Alsace, France)

We investigate the feasibility of the hybrid gravity technique applied to two geothermal reservoirs in northern Alsace, France, namely the Soultz-sous-Forêts and Rittershoffen sites. Soultz-sous-Forêts site is the first enhanced geothermal system demonstration site producing electricity in France. Here a geothermal fluid at 165 °C allows to produce around 1.5 MW thanks to one injection well and one production well 5 km deep. Rittershoffen site is dedicated to an industrial use and it is designed to produce 24 MWth heat power with 2 wells around 2.5 km deep. The most recent production episodes of Rittershoffen and Soultz-sous-Forêts geothermal plants have started respectively on May and June 2016. To study underground mass redistribution, time-lapse relative microgravity measurements have been done since 2014 on a network designed ad hoc for Soultz-sous-Forêts site and since 2015 for Rittershoffen site. After tide and drift correction, double differences are calculated to retrieve the gravity variation at each measuring point compared to a reference time and station. Absolute gravity measurements have been also collected at one of the reference stations. Before the beginning of the production in 2016, the stability of the Soultz-sous-Forêts reference station was monitored through the repetition of absolute measurements and continuous gravity records. In 2016, regular ties between the reference stations and the Strasbourg gravity observatory STJ9 have been done. Several superconducting gravimeters operate continuously in STJ9. Thus, we approach the concept of hybrid gravity. Vertical deformation is also controlled thanks to six continuous GPS measurements: the height changes are less than 1 cm. So we consider that our gravity variations are only due to subsurface mass changes. For the Soultz-sous-Forêts network, we notice significant changes in agreement with the position of the injection and the production wells. The maximum gravity change is 31 ± 8 μGal. On the contrary, we do not detect any similar signal for the Rittershoffen network. A simplistic model using two spherical sources located at 5 and 2 km for Soultz-sous-Forêts and Rittershoffen sites respectively shows negligeable surface gravity changes when taking into account the known injection and production flow rates.

Low angular momentum accretion of fractal medium on to Kerr black holes and the emergence of the acoustic manifold

For three different geometric configurations of matter and two different thermodynamic equations of state, low angular momentum, multi-transonic, axially symmetric accretion flow of matter having fractional dimension of mass distribution onto a rotating black hole has been studied by employing certain post-Newtonian pseudo-Kerr black hole potential. Such task has been accomplished mathematically by mapping the fractal nature of accreted medium onto its continuum counterpart. The difference between spin dependence of accretion dynamics of the fractal medium and the continuous medium has been highlighted. By employing a time dependent linear perturbation scheme, it has been demonstrated that accretion of matter with fractional dimension of density distribution can be considered as a natural example of classical analogue model. The corresponding acoustic surface gravity has been estimated in terms of accretion variables. The value of the surface gravity changes as the accreted matter makes a transition from its fractal nature to the corresponding continuum distribution.

Advance in prediction of body tide and ocean tidal loading

Earth’s body tide is the largest periodic perturbation in the solid Earth. We developed a new computer program ‘K2-Tide’, which predicts body tide accurately and efficiently for any place on the globe. K2-Tide has outgrown its former version K-Tide and JTIDAL, and its algorithm includes corrections for the permanent tide and the coupling between Earth’s spin and tidal deformation–those neglected in the two former ones. Other enhancements are the updated physical constants and compensation of mantle inelasticity. For any specified location and time span in the 21st century, K2-Tide computes tidally induced perturbations; surface displacement, gravity change, tidal potential and deviation of the vertical. Periodic changes of ocean water height associated with the ocean tides induce secondary deformation in the crust–ocean tidal loading (OTL). By using NAO.99 ocean tide model and the IASPEI Earth model, we constructed ‘K-OTL’, which similarly predicts OTL displacement and gravity changes (originally designed to be used in Korea). Twelve tidal constituents considered in K-OTL are M2, S2, K2, N2, O1, K1, P1, Q1, Mf, Mm, Ssa, and Sa. Amplitudes of 1 cm and 3 μGal are typical values of OTL deformation in major urban area of Korea. These OTL displacement and gravity show strong semidiurnal variation and become smaller at the eastern parts of Korea. As the amplitude and phase of ocean tide largely vary at different regions on the globe, so do the OTL effects.

Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben

The study is devoted to the monitoring of a geothermal reservoir by hybrid gravimetry combining different types of instruments (permanent superconducting gravimeter, absolute ballistic gravimeter, and micro-gravimeters) and different techniques of measurements (both time-discrete and recording data collection). Using a micro-gravimetric repetition network around a reference station, which is regularly measured, leads to the knowledge of the time and space changes in surface gravity. Such changes can be linked to the natural or anthropic activities of the reservoir. A feasibility study using this methodology is applied to two geothermal sites in the Alsace region (France) of the Rhine graben. We show the results in terms of gravity double differences from weekly repetitions of a network of 11 stations around the geothermal reservoir of Soultz-sous-Forets, separated into 5 loops during July–August 2013 and 2014 as well as preliminary results from 2 stations near Rittershoffen (ECOGI). We point out the importance of a precise leveling of the gravity points for the control of the vertical deformation. A first modeling of surface gravity changes induced by realistic geothermal density perturbations (Newtonian attraction) is computed in the frame of the existing geological model and leads to gravity changes below the μGal level being hence undetectable. However, and for the same case, borehole gravity modeling showed a significant anomaly with depth that can be used as a complementary monitoring method. We show that in the limit of our uncertainties (SD ~ 5 μGal), we do not detect any significant gravity change on the geothermal site of Soultz in agreement with the fact that there was indeed no geothermal activity during our analysis period. On the contrary, the measurements near Rittershoffen show a signal above the noise level which correlates in time with a production test but cannot be explained in terms of Newtonian attraction effects according to our basic numerical simulation.

Dynamics of a large, restless, rhyolitic magma system at Laguna del Maule, southern Andes, Chile

Explosive eruptions of large-volume rhyolitic magma systems are common in the geologic record and pose a major potential threat to society. Unlike other natural hazards, such as earthquakes and tsunamis, a large rhyolitic volcano may provide warning signs long before a caldera-forming eruption occurs. Yet, these signs—and what they imply about magma-crust dynamics—are not well known. This is because we have learned how these systems form, grow, and erupt mainly from the study of ash flow tuffs deposited tens to hundreds of thousands of years ago or more, or from the geophysical imaging of the unerupted portions of the reservoirs beneath the associated calderas. The Laguna del Maule Volcanic Field, Chile, includes an unusually large and recent concentration of silicic eruptions. Since 2007, the crust there has been inflating at an astonishing rate of at least 25 cm/yr. This unique opportunity to investigate the dynamics of a large rhyolitic system while magma migration, reservoir growth, and crustal deformation are actively under way is stimulating a new international collaboration. Findings thus far lead to the hypothesis that the silicic vents have tapped an extensive layer of crystal-poor, rhyolitic melt that began to form atop a magmatic mush zone that was established by ca. 20 ka with a renewed phase of rhyolite eruptions during the Holocene. Modeling of surface deformation, magnetotelluric data, and gravity changes suggest that magma is currently intruding at a depth of ~5 km. The next phase of this investigation seeks to enlarge the sets of geophysical and geochemical data and to use these observations in numerical models of system dynamics. INTRODUCTION Caldera-scale rhyolitic volcanoes can rapidly deposit hundreds of cubic kilometers of ash over several million square kilometers, threatening people and agriculture at the scale of an entire continent (Sparks et al., 2005; Lowenstern et al., 2006; Self, 2006). Sooner or later, Earth will experience another eruption of this magnitude (Lowenstern et al., 2006; Self and Blake, 2008); consequently, there is a need to gather comprehensive information and create multi-scale models that realistically capture the dynamics leading to these destructive events. Most of our current understanding of this type of volcanic system has been gleaned from the study of eruptive products long after the catastrophic eruption, including voluminous ash flow deposits, such as the Bishop, Bandelier, Huckleberry Ridge, and Oruanui Tuffs (Lowenstern et al., 2006; Hildreth and Wilson, 2007; Bachmann and Bergantz, 2008; Wilson, 2008). The most recent rhyolitic “super-eruption” produced the Oruanui Tuff 26,500 years ago in New Zealand. Even in this relatively recent case, the geologic evidence has been partly obliterated by caldera-collapse, erosion, and burial (Wilson et al., 2005). Moreover, probing the present-day structures beneath a number of calderas using seismic tomography (e.g., Romero et al., 1993; Steck et al., 1998; Farrell et al., 2014) or other geophysical measures (e.g., Lowenstern et al., 2006; Battaglia et al., 2003; Tizzani et al., 2009) has not detected eruptible domains of crystal-poor melt in the shallow crust, nor has it captured the dynamics that preceded these large eruptions. This paper focuses on the Laguna del Maule Volcanic Field, Chile, a large, potentially hazardous, rhyolitic magmatic system, where an alarming rate of surface uplift for the past seven years and concentrated swarms of shallow earthquakes prompted Observatorio Volcanologico de los Andes del Sur (OVDAS) to declare in March 2013 a yellow alert, signaling a potential eruption within months or years. Straddling the Andean range crest at 36° S (Fig. 1A), this volcanic field features: (1) 13 km of rhyolite that erupted both explosively and effusively during the past 20 k.y.; (2) a zone of low electrical resistivity in the shallow crust below the deforming area; (3) widespread elevated CO 2 concentrations; and (4) a negative (~10 mGal) Bouguer anomaly and preliminary evidence for a positive dynamic gravity signal indicating mass addition. The underlying magma system has been sampled by eruptions numerous times since its apparent inception in the late Pleistocene, including a dozen crystal-poor, glassy rhyolitic lavas during the Holocene. Linking the assembly and evolution of this

Surface gravity and deformation effects of water storage changes in China's Three Gorges Reservoir constrained by modeled results and in situ measurements

Abstract The water impoundment of China's Three Gorges Reservoir (TGR), with the largest dam in the world, makes a large mass concentrated and thus influences the surface gravity field and crustal deformation field. In the TGR area, water impoundment began in 2003, and the Earth is responding to the ongoing changes of water storage. These responses can be investigated using the gravimeter and the GPS. In this paper, using a water load model derived from Digital Elevation Model (DEM) data and elastic loading Green's function, we modeled the surface gravity and displacement changes in the front area of TGR caused by water storage variations. Predictive results are compared with the measurements derived from absolute gravity by A10, continuous gravity by gPhone and GPS time series at the sites of front area in TGR. The observations agree well with the prediction spatially and temporally. The quantitative comparison and analysis indicate that the ground gravity and the vertical displacement are more sensitive to water storage changes than the horizontal displacement. The predictions from the water load model are consistent with the in situ measurements reported in this work and therefore can be utilized for water effect corrections for exacting the further signals related to lithospheric dynamics and geological hazards, such as abnormal deformation of active faults and landslides.

Co- and post-seismic surface deformation and gravity changes of MS7.0 Lushan, earthquake

On April 20, 2013, an earthquake with magnitude 7.0 occurred in the southwest of the Longmenshan fault system in and around Lushan County, Sichuan Province, China. This devastating earthquake killed hundreds of people, injured 10 thousand others, and collapsed countless buildings. In order to analyze the potential risk of this big earthquake, we calculate the co- and post-seismic surface deformation and gravity changes of this event. In this work, a multilayered crustal model is designed, and the elastic dislocation theory is utilized to calculate the co- and post-seismic deformations and gravity changes. During the process, a rupture model obtained by seismic waveform inversion (Liu et al. Sci China Earth Sci 56(7):1187–1192, 2013) is applied. The time-dependent relaxation results show that the influences on Lushan and its surrounding areas caused by the MS7.0 Lushan earthquake will last as long as 10 years. The maximum horizontal displacement, vertical uplift, and settlement are about 5 cm, 21.24 cm, and 0.16 m, respectively; the maximal positive and negative values of gravity changes are 45 and −0.47 μGal, respectively. These results may be applied to evaluate the long-term potential risk caused by this earthquake and to provide necessary information for post-earthquake reconstruction.

Simulation of coseismic effects of the Ms7.0 Lushan earthquake

Using plane dislocation theory and the seismic-wave inversion results from the Institute of Geophysics, China Earthquake Administration and the Institute of Geodesy and Geophysics, Chinese Academy of Sciences models, the surface coseismic deformation and gravity changes caused by the 2013 Ms 7.0 Lushan earthquake are simulated. The simulations of coseismic gravity change and deformation indicate that the dislocation has dip-slip characteristics. The results also show that the coseismic deformation exhibits a symmetrical, positive-and-negative distribution, with the deformation usually being less than 10 mm in the farfield but up to 140 mm in the near-field. The gravity changes are concentrated on the fault-projection area, which is greatly affected by the vertical surface deformation. The gravity change and vertical deformation in the far field are usually less than and 5 mm, respectively, but reach and 330 mm, respectively, in the near field. The simulated results agree well with the measured results, which suggests a theoretical basis for the observed change in gravity before and after this earthquake.

Sea surface gravity changes observed prior to March 11, 2011 Tohoku earthquake

The 2011 Tohoku earthquake occurred via subduction of an oceanic tectonic plate, where we had no historical record of this size of earthquake. We have examined shipboard geophysical observations conducted above the rupture area for any indications before the earthquake. The location of the largest slip along the fault surface coincides with gravity changes measured at the sea surface separated by three months all before the earthquake. This gravity change cannot be explained by the local gravity gradient due to bathymetry along the cruise tracks. The measured gravity changes can be interpreted either as an uplift of ocean bottom or a density increase along the fault surface of which the time scale of evolution is about 3months. This observation may constrain the physical mechanism by which this large and slow slip can be generated along this fault.