Coseismic Gravity Changes Research Articles

Results from parallel observations of superconducting and absolute gravimeters and GPS at the Hsinchu station of Global Geodynamics Project, Taiwan

The Hsinchu (HS) superconducting gravimeter (SG, serial T48) station is a newly established site in the Global Geodynamics Project (GGP). Simultaneous observations of T48, three FG5 absolute gravimeters, and GPS at four stations are studied. GPS shows few mm a−1 of horizontal and vertical motions around HS. The calibration factor and drifting rate of T48 are −75.96 ± 0.07 μGal V−1 and 0.2 ± 0.7 μGal a−1 (1 μGal = 10−8 m s−2). Both the SG and absolute gravity records contain trends of about 2–3 μGal a−1. The ocean tide gravity effects (OTGEs) were estimated from NAO.99b, FES2004, and CSR4.0, and their amplitudes agree with the SG observations at the submicroGal level, but their phases differ from the observations up to 10°. The Newtonian effect of ocean tide contributes 20% to the total OTGE at HS, and it is larger at islands in the Taiwan Strait. The inelastic body tide model of Dehant et al. (1999) is more consistent with the SG observations than the elastic model. Modeled gravity‐atmosphere admittances based on an exponential distribution of air mass explain well the observed admittances. The average gravity‐atmosphere admittance during typhoons is 30% larger than that in a nontyphoon time. A list of coseismic gravity changes from T48 caused by earthquakes over 2006–2007 is given for potential studies of fault parameters. The modeled effects of atmospheric pressure, groundwater, soil moisture, and polar motion explain the FG5 observed gravity trend to 1.1 μGal a−1. Seasonally, the groundwater‐induced gravity change contributes the most to the SG residual gravity, but its phase leads the latter by 63 days.

Effects of Earth’s Lateral Inhomogeneous Structures on Coseismic Gravity Changes

Based on the perturbation method, we present a new method to study the effects of Earth’s laterally inhomogeneous structures on coseismic gravity changes caused by dislocations within a 3-D heterogeneous spherical earth model. We describe this method by six independent dislocations: A vertical strike-slip, two vertical dip-slips perpendicular to each other, and three tensile openings on three perpendicular planes. We derived the calculation formulae for the six independent dislocations. A combination of the six independent dislocations is useful to compute the effects on coseismic gravity changes resulting from an arbitrary seismic source at an arbitrary position.

General formulations of global co-seismic deformations caused by an arbitrary dislocation in a spherically symmetric earth model-applicable to deformed earth surface and space-fixed point

Based on the authors' previous work, co-seismic deformations for a spherical symmetric earth model are summarized and reformulated. Unified expressions presented herein accommodate physical deformations: displacement, potential, gravity, geoid and strain changes. The corresponding Green's functions are derived by combining spheroidal and toroidal deformations. Sign errors in previous publications are corrected in these new formulas. These expressions are developed basically for a deformed earth surface because most traditional geodetic measurements are performed on the terrain surface. However, through development of space geodetic techniques, such as the satellite gravity missions, co-seismic gravity changes can be detected from space. In this case, the above dislocation theory (e.g., the co-seismic gravity change) cannot be applied directly to the observed data because the data do not include surface crustal deformation (the free air gravity change). Correspondingly, the contribution by the vertical displacement part must be removed from the traditional expressions. For this purpose, we present the corresponding expressions applicable to space observations. Some numerical technical problems are discussed. In addition, a smoothing technique is necessary to damp the high-frequency contribution so that the theory can be applied reasonably. Global co-seismic deformations caused by the 2004 Sumatra—Andaman earthquake (M9.3) are studied as an application of the new Green's function. That earthquake caused a global deformation detected by GPS, strain metres and even a satellite gravity mission. These global deformations are calculated based on the derived Green's functions and the seismic-wave derived earth model. A segment-summation scheme is used considering the slip distribution on a limited fault plane. The results are useful for interpreting observed deformations, especially those in the far field. The earthquake reveals global co-seismic deformations and effects of spherical curvature and the earth's layered structure. Comparisons between results for a spherical earth mode and a half-space model show a large discrepancy at an epicentral distance of about 1000 km, implying that effects of spherical curvature and layer structure are considerably large. In addition, the theoretical results are compared with the real observed strain steps, horizontal displacements and gravity changes caused by that earthquake. Good agreement validates the results of the current theoretical work. Finally, we discuss the application the above theory to the GRACE data through several case studies.

Coseismic change and precipitation effect in temporal gravity variation at Inuyama, Japan: A case of the 2004 off the Kii peninsula earthquakes observed with a superconducting gravimeter

Abstract Coseismic gravity change, correlated with the 2004 off the Kii peninsula earthquakes (∼M7), was detected with a superconducting gravimeter installed at the Inuyama station of Nagoya University, central Japan, where distances from epicenters were ∼250 km. The influence of precipitation in the vicinity of the site just before the earthquakes was included in the observed temporal gravity variation. Therefore the precipitation effect was removed using a simple tank model with the Radar-AMeDAS precipitation data. As a result, the magnitude of the coseismic gravity decrease was ∼10 nm s −2 (1 μGal). The gravity changes were calculated using a dislocation model with two fault models those defined by GPS estimates. These theoretical values corresponded to the observed value in the order of 1 μGal.

Separation of coseismic and postseismic gravity changes for the 2004 Sumatra-Andaman earthquake from 4.6 yr of GRACE observations and modelling of the coseismic change by normal-modes summation

This paper is devoted to the simultaneous determination of the coseismic and postseismic gravitational changes caused by the great 2004 December 26 Sumatra–Andaman earthquake from the time-variable global gravity fields recovered by the Gravity Recovery And Climate Experiment (GRACE) mission. Furthermore, a complete modelling of the elasto-gravitational response of a self-gravitating, spherically layered, elastic earth model is carried out using a normal-modes summation for comparison with the observed coseismic gravitational change. Special attention is paid to the ocean mass redistribution. Special care is paid during the inversion of the data to avoid contamination of tectonic gravity changes by ocean tidal model errors, seasonal and interannual signals originating from continental hydrology and oceanic circulation as well as contamination of the coseismic gravity change by the postseismic relaxation. We use a 4.6-yr-long time-series of global gravity solutions including 26 months of postseismic data, provided by the Groupe de Recherche en Géodésie Spatiale (GRGS). For comparison, the Release-04 solutions of the Center for Space Research (CSR) are also investigated after a spectral windowing or a Gaussian spatial smoothing. Results are shown both in terms of geoid height changes and gravity variations. Coseismic and postseismic gravitational changes estimated from the different gravity solutions are globally similar, although their spatial extent and amplitude depend on the type of filter used in the processing of GRACE fields. The highest signal-to-noise ratio is found with the GRGS solutions. The postseismic signature has a spectral content closer to the GRACE bandwidth than the coseismic signature and is therefore better detected by GRACE. The coseismic signature consists mainly of a strong gravity decrease east of the Sunda trench, in the Andaman Sea. A gravity increase is also detected at a smaller scale, west of the trench. The model for the coseismic gravity changes agrees well with the coseismic signature estimated from GRACE, regarding the overall shape and orientation, location with respect to the trench and order of magnitude. Coseismic gravity changes are followed by a postseismic relaxation that are well fitted by an increasing exponential function with a mean relaxation time of 0.7 yr. The total postseismic gravity change consists of a large-scale positive anomaly centred above the trench and extending over 15° of latitude along the subduction. After 26 months, the coseismic gravity decrease has been partly compensated by the postseismic relaxation, but a negative anomaly still remains south of Phuket. A dominant gravity increase extends over 15° of latitude west of the trench, being maximal south of the epicentre area. By investigating analyses of two global hydrology models and one ocean general circulation model, we show that our GRACE estimates of the coseismic and postseismic gravitational changes are almost not biased by interannual variations originating from continental hydrology and ocean circulation in the subduction area and in the central part of the Andaman Sea, while they are biased by several μGal in the Malay Peninsula.

Implications of postseismic gravity change following the great 2004 Sumatra‐Andaman earthquake from the regional harmonic analysis of GRACE intersatellite tracking data

We report Gravity Recovery and Climate Experiment (GRACE) satellite observations of coseismic displacements and postseismic transients from the great Sumatra‐Andaman Islands (thrust event; Mw ∼9.2) earthquake in December 2004. Instead of using global spherical harmonic solutions of monthly gravity fields, we estimated the gravity changes directly using intersatellite range‐rate data with regionally concentrated spherical Slepian basis functions every 15‐day interval. We found significant step‐like (coseismic) and exponential‐like (postseismic) behavior in the time series of estimated coefficients (from May 2003 to April 2007) for the spherical Slepian functions. After deriving coseismic slip estimates from seismic and geodetic data that spanned different time intervals, we estimated and evaluated postseismic relaxation mechanisms with alternate asthenosphere viscosity models. The large spatial coverage and uniform accuracy of our GRACE solution enabled us to clearly delineate a postseismic transient signal in the first 2 years of postearthquake GRACE data. Our preferred interpretation of the long‐wavelength components of the postseismic gravity change is biviscous viscoelastic flow. We estimated a transient viscosity of 5 × 1017 Pa s and a steady state viscosity of 5 × 1018–1019 Pa s. Additional years of the GRACE observations should provide improved steady state viscosity estimates. In contrast to our interpretation of coseismic gravity change, the prominent postearthquake positive gravity change around the Nicobar Islands is accounted for by seafloor uplift with less postseismic perturbation in intrinsic density in the region surrounding the earthquake.

Analysis of Superconducting Gravimeter measurements at MunGyung station, Korea

Abstract In 2005, Korea’s first Superconducting Gravimeter (SG) was installed at MunGyung (MG). For the installation of the SG, a new measuring hut with a grounded pier was built. The SG has been successfully recording since April 2005. To monitor the groundwater level changes around the SG, a borehole was drilled in September 2005. For analyzing the gravity data, the SG calibration factor of −64.548 μgal/V (μgal = 10 −8 m/s 2 ) was determined by a parallel registration with an absolute gravimeter. The noise magnitude (NM) at the MG site was determined and compared with other SG stations and the New Low Noise Model. The MG site can be classified within the NM of other SG stations. Additionally, noise caused by misaligned instrumental tilt has been investigated and spectrally analyzed. Covering the instrument with a polystyrene box has resulted in a decrease of this noise. After preprocessing, the tidal analysis has been performed and compared with the Wahr–Dehant tidal model parameters. Non-tidal gravity variations, caused by atmospheric pressure and ground water level variations, were calculated. According to the FES2002 ocean tide model, the ocean loading at MG site was calculated. All known gravity effects (i.e. solid Earth tides, polar motion, atmospheric pressure and ground water level induced gravity variations, and the ocean loading gravity effect) were then subtracted from the SG measured gravity data. The remaining gravity variations (gravity residuals) were spectrally analyzed for detection of unknown gravity effects. Co-seismic gravity changes were detected by the SG for earthquakes with epicenters within a distance of about 100 km from the MG station. The largest detected co-seismic gravity change was 0.06 μgal.

A general method for calculating co-seismic gravity changes in complex fault systems

A general method for calculating the total, dilatational, and free-air gravity for fault systems with arbitrary geometry, slip motion, and number of fault segments is presented. The technique uses a Green's function approach for a fault buried within an elastic half-space with an underlying driver plate forcing the system. The method is easily adapted to investigate a wide range of geophysical applications and is provided as a suite of Fortran90 routines. The Joshua Tree-Landers-Hector Mine sequence is examined and shows remarkable correlation of epicentres with dilatational gravity highs. We also demonstrate the use of the method for large thrusting events, using the Alaska Good Friday earthquake of 1964 as an example.

Spatiospectral localization of global geopotential fields from the Gravity Recovery and Climate Experiment (GRACE) reveals the coseismic gravity change owing to the 2004 Sumatra‐Andaman earthquake

Regional mass fluxes owing to transport and adjustment within the Earth system that are implicitly contained in the monthly Gravity Recovery and Climate Experiment (GRACE) global geopotential coefficients are revealed by localizing global spectra using spatiospectrally concentrated window functions. We have analyzed 45 monthly global GRACE harmonic coefficient series in order to find the coseismic signature associated with the 2004 great Sumatra‐Andaman earthquake. A significant gravity change after the earthquake is found in the time series of the GRACE coefficients after localization with a single band‐limited window centered near the north of the island of Sumatra. This change is undetectable from the original global coefficients or from coefficients localized elsewhere on the globe. A step function with its discontinuity at 26 December 2004 usefully models the coseismic gravity change. The localized GRACE coefficients contain the jumps (associated with the earthquake) up to degree and order 55, although not all of them within this band produce changes that are statistically significant. The gravity change calculated from the localized GRACE coefficients displays 30 μGal peak‐to‐peak variations that are very well correlated with an independently derived seismic model based on elastic dislocation theory.

Surface coseismic gravity changes caused by dislocations in a 3-D heterogeneous earth

SUMMARY This study describes an examination of surface gravity changes caused by dislocations within a 3-D heterogeneous earth. This new theory is described using six independent dislocations: a vertical strike-slip, two vertical dip-slips perpendicular to each other, and three tensile openings on three perpendicular planes. A combination of the six independent dislocations is useful to compute coseismic gravity changes resulting from an arbitrary seismic source at an arbitrary position. Based on the 3-D lateral inhomogeneous P-wave velocity model, we deduce the 3-D density and S-wave velocity models using the relation of Karato. Finally, numerical computations are performed for a location south of Japan (30 ◦ N, 135 ◦ E). We calculate the coseismic gravity changes resulting from the six independent dislocations for source depths of 100, 300 and 637 km, respectively. Numerical results show that the maximum 3-D effect varies concomitantly with the dislocation type and the source depth. For seismic problems, the effect of elastic parameter μ is dominant.

First detection of absolute gravity change caused by earthquake

Unequivocal coseismic gravity change was detected for the first time by absolute gravimetry. The measurements were done just one day before and only 7 days after a M6.1 earthquake (M6.1). The absolute gravity change was −6 microgal, significantly larger than the observational error of about 1 microgal. Of particular interest is that the observed spatial gravity variations agree remarkably well with those expected from an elastic dislocation model constructed on displacement data alone. The result strongly encourages us to perform a simultaneous inversion of both gravity and displacements to have better insight of earthquakes.