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This report highlights the usefulness and applicability of various gravimetric methods for studying earthquakes and volcanic activities. A high-resolution gravity anomaly map of Japan reveals areas with very steep horizontal gradients, where potential seismic faults are likely to be buried. Such traditional geoprospecting is coupled with novel cosmic-ray radiography to produce a fine-resolution (

The gravity signal originating from magma mass movement in a volcanic conduit is retrieved from the hydrologically disturbed absolute gravity data obtained at Asama Volcano (Central Japan) in 2004, using a three-dimensional hydrological model. We improve the hydrological model of the previous study using realistic soil parameters and boundary conditions, to better estimate the spatiotemporal land-water distributions and the consequent hydrological gravity disturbances. The newly estimated gravity disturbances agree with the absolute gravity values observed by FG5 gravimeters in 2004–2009 within about 2.6 μGal, by additionally accounting for the excess discharge of groundwater mass associated with a sloping impermeable surface below the discharge area. After the gravity disturbance of 20 μGal amplitude is subtracted from the absolute gravity data observed during the 2004 eruptive event, the gravity residual of 5 μGal amplitude shows a significant decrease in synchronization with eruptions, because the ascending magma mass in the conduit affects the upward attraction force to the gravimeters installed on the flank of Asama Volcano. The magma head altitude, to which the residual gravity is converted assuming a homogeneous linear density in the conduit, shows a comprehensive agreement of the time variation in the magma head with those in other volcanic observations, such as gas emission rate and earthquake frequency. By correcting the hydrological gravity disturbances using this hydrological model and simultaneously obtained meteorological data in real time, spatiotemporal variations in the magma mass can be instantaneously monitored at Asama Volcano, even before eruptions during future volcanic events.

We have developed an integrated processing method for muon radiography and gravity anomaly data for determining the 3-D density structures of volcanoes with a higher spatial resolution than is possible by conventional gravity inversion. In the present paper, we demonstrate the performance of the proposed method by performing numerical tests using synthesized data, and we present the results obtained by applying the proposed method to a volcano, Showa-Shinzan lava dome, Hokkaido, Japan. We obtained the detailed shape of a vent beneath the dome and detected the presence of solidified dense lava near the top of the dome. The results demonstrate the advantage of a hybrid measurement based on both gravity and muon radiography for imaging small structures with sizes of a few hundreds of meters near the surface of a volcano.

This study investigates data-processing methods and examines the precipitation effect on gravity measurements at the Dali gravity network, established in 2005. High-quality gravity data were collected during four measurement campaigns. To use the gravity data validly, some geophysical corrections must be considered carefully. We first discuss data-processing methods using weighted least-squares adjustment with the constraint of the absolute gravity datum. Results indicate that the gravity precision can be improved if all absolute gravity data are used as constraints and if calibration functions of relative gravimeters are modeled within the observation function. Using this data-processing scheme, the mean point gravity precision is better than 12 μgal. After determining the best data-processing scheme, we then process the gravity data obtained in the four measurement campaigns, and obtain gravity changes in three time periods. Results show that the gravity has a remarkable change of more than 50 μgal in the first time period from Apr–May of 2005 to Aug–Sept of 2007. To interpret the large gravity change, a mean water mass change (0.6 m in height) is assumed in the ETOPO1 topographic model. Calculations of the precipitation effect on gravity show that it can reach the same order of the observed gravity change. It is regarded as a main source of the remarkable gravity change in the Dali gravity network, suggesting that the precipitation effect on gravity measurements must be considered carefully.

We report here on continuous absolute gravity measurements made between February 2011 and March 2012 and repeated relative gravity measurements in the vicinity of Shinmoe-dake volcano, which commenced erupting in late January 2011. We find that 20 of 24 eruptive events are associated with precursory short-term gravity decreases occurring over 5–6 hours followed by quick recoveries lasting 1–2 hours. Also evident are significant long-term gravity changes arising principally from hydrological processes around the volcano, where annual precipitation exceeds 5,000 mm. To isolate the gravity signal associated with volcanic processes, we compared gravity measurements made at 15 sites in March 2011 and again in March 2012. The gravity changes and crustal deformation observed during the one year period are well explained by 6×106 m3 inflation of a magma reservoir at a depth of 9 km and intrusion at shallower depths of a dike with dimensions of 10 km × 0.5 km × 0.5 m.

Coseismic gravity changes are interesting signals to be studied with precise gravity observations by means of the superconducting gravimeter (SG). Using the data from two SG stations in Japan, Matsushiro and Kamioka, an attempt is made to detect permanent gravity changes caused by the two recent earthquakes, the 2007 Noto Hantou earthquake ( M w = 6.9) and the 2007 Niigataken Chuetsu-oki earthquake ( M w = 6.8). Although these earthquakes are much smaller in magnitude than the 2003 Tokachi-oki earthquake studied by Imanishi et al. (2004) , the expected magnitude of the coseismic signals reaches the observable level of the order of 0.1 μGal (=1 × 10 −9 m s −2 ), owing to the fact that the epicentral distances are relatively short (120–170 km). A total of four possible combinations of source and receiver are investigated. In two cases in which the epicentral distances are shorter, there are instrumental offsets in the gravimeter records inhibiting estimation of gravity changes of natural origin. In the remaining two cases, the observed gravity changes are in agreement with theoretical predictions at least in their sign, but the differences between them are unexplained. Our result indicates the difficulty in identification of the coseismic signals caused by typical inland earthquakes.

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.

SUMMARY We present a numerical formulation for computing elastic deformations caused by a dislocation on a ¢nite plane in a spherically symmetric earth. It is based on our previous work for a point dislocation (Sun & Okubo 1993). The formulation enables us to compute the displacement, potential and gravity changes due to an earthquake modelled as spatially distributed dislocations. As an application of the ¢nite-fault dislocation theory, we make a case study of the theoretical and observed gravity changes. The computed results are in excellent agreement with the observed gravity changes during the earthquake. The gravity changes in the near ¢eld can reach some 100 ]gal, which can be easily detected by any modern gravimeter. In the far ¢eld they are still signi¢cantly large:jdgj > 10 ]gal within the epicentral distance h 1 ]gal within h 0:1 ]gal within h 0:01 ]gal globally. We also calculate the geoid height changes caused by the 1964 Alaska earthquake and by the same earthquake with revised parameters and an assumed barrier. We ¢nd that the earthquake should have caused geoid height changes as large as 1.5 cm.

Employing both absolute and relative gravimeters, we carried out a hybrid microgravity survey at Miyakejima volcano, Japan. We detected significant gravity changes, e.g., exceeding 1000 At gal, during the course of our measurements since June 1998. The spatio-temporal gravity changes provide us with strong evidence of preexisting vacant space beneath the island before the summit collapse of the volcano. Also, our data are totally in conflict with the so-called magma drainback model, suggesting that the magma flowed out laterally.

With high-resolution continuous gravity recordings from a regional network of superconducting gravimeters, we have detected permanent changes in gravity acceleration associated with a recent large earthquake. Detected changes in gravity acceleration are smaller than 10 –8 meters seconds –2 (1 micro-Galileo, about 10 –9 times the surface gravity acceleration) and agree with theoretical values calculated from a dislocation model. Superconducting gravimetry can contribute to the studies of secular gravity changes associated with tectonic processes.

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.

[1] Today, some tens of million years after its creation by the collision between India and Asia, the Tibetan Plateau is the highest and largest plateau on Earth. Results of geological and tectonic studies indicate that the plateau is extending. However, almost no quantitative evidence shows whether the plateau is still uplifting or thickening nowadays. Herein, we present geodetic evidence of mass loss beneath the Tibetan Plateau and increasing crust thickness. Combined absolute gravity and Global Positional System (GPS) measurements at three stations in southern and southeastern Tibet during two decades reveal uplifting of the Tibetan Plateau at a millimeter-per-year level, but its underlying mass is diminishing, indicating that the crustal thickness is increasing at an annual millimeter to decimeter level.

SUMMARY We derive expressions in closed form which give the gravity potential changes caused by point dislocations. They enable us to evaluate coseismic changes in surface gravity and geoid height. Numerical simulation shows that a great earthquake could cause a geoid height change of order 1 m.

This paper presents a new approach to calculate dislocation Love numbers using observations of a satellite gravity mission (e.g. GRACE). The necessary condition is that the co-seismic potential change be sufficiently large to be detected by the gravity mission. Co-seismic deformations for each spherical harmonic degree n are decoupled. Therefore, dislocation Love numbers of degree n can be determined independently. The determinable maximum harmonic degree n depends on the seismic size, source type, magnitude, and the accuracy of a satellite gravity mission. For an arbitrary seismic source, all four types of dislocation Love numbers can be determined using data from only one seismic event because all deformation components are involved together. Only the concerned dislocation Love numbers can be computed for any one of the four types of sources. To prove the validity of the method proposed in this study, a simulation test is carried out by considering a similar case to the 2004 Sumatra earthquake (Mw 9.1). Results show that the method works well and guarantee the accuracy.

We find regional gravity decrease during an interseismic period at a subduction plate boundary where the land is steadily subsiding. This paradoxical finding poses a challenging problem on the subduction dynamics at converging plate boundaries.

Dense gravity measurements were carried out together with the GPS positioning along five survey lines across the Rokko fault system. The subsurface structure relevant to the 1995 Hyogo-ken Nanbu earthquake was inferred from the Bouguer anomaly. A hidden fault was discovered on the southwestward extension of the Koyo fault underneath the sedimentary layer. The extension runs just on the edge of the earthquake disaster belt. The thickness of the sedimentary layer was estimated to decrease gradually toward the mountain side. The wedge-like structure of the soft layer and the hidden fault scarp under the Kobe plain may serve as a focusing lens of seismic rays during the earthquake.

Gravity changes associated with variations in local land-water distributions have been observed at Isawa Fan in northern Japan, and modeled by hydrological equations. We solve the Richards equation numerically for the time variation in the vertical soil moisture distribution, which is then spatially integrated to estimate gravity changes due to the soil moisture distribution. In modeling Isawa Fan, we assume a simple hydrological model: a horizontally homogeneous soil in an infinite half-space. The estimated gravity is consistent with the observed gravity during a 50-day period within about 0.4 µgal root mean square, owing to both observed soil parameter values and the observation building geometry being incorporated into the hydrological model. However, the estimated gravity cannot fully reproduce annual gravity changes observed during a 2-year time frame, because the boundary conditions in the modeling determine only local water distributions and the resultant short-period gravity changes. Instead, the observed gravity over these 2 years can be reproduced within about 1.0 µgal root mean square, if the additional parameters of the annual gravity change (Aac and Aas) and the snowfall effect (As) are calculated by the function regression to the observed gravity with the least-squares method. The hydrological modeling techniques presented here can be utilized at all gravity sites in flat areas similar to Isawa Fan, such that hydrological effects in gravity data can be corrected and mass transfers associated with earthquakes and volcanoes can be monitored.

We carried out both absolute and relative gravity measurements in the Izu Peninsula just before and after the March 1997 earthquake swarm occurred. The measurements revealed significant absolute gravity changes, which we find to be made of three spatial components. The first one is located near Cape Kawana, and would be associated with the volcanic activity that caused the earthquake swarm. The second one would be associated with shallow and localized magma intrusion just beneath Ito. The third one may be due to a change in the deep region beneath the Kita-Izu fault system, which is considered to be a major tectonic line of this region. The gravity changes can be used to detect underground mass movement. For this purpose, we first use crustal movement observations to construct an elastic dislocation model with two tensile faults and a left lateral fault. Then we use the gravity changes to constrain the density of the material which filled the tensile faults. We find that the density is likely to be small, and that the gravity changes of the first component are reproduced well by the fault model. The smallness of the density implies that highly vesiculated magma or water would have injected into the faults.