Positive Gravity Change Research Articles

Hydrological and volcano-related gravity signals at Mt. Somma–Vesuvius from ∼20 yr of time-lapse gravity monitoring: implications for volcano quiescence

SUMMARY We report on about 20 yr of relative gravity measurements, acquired on Mt. Somma–Vesuvius volcano in order to investigate the hydrological and volcano-tectonic processes controlling the present-day activity of the volcano. The retrieved long-term field of time gravity change (2003–2022) shows a pattern essentially related to the subsidence, which have affected the central part of the volcano, as detected by the permanent GNSS network and InSAR data. After reducing the observations for the effect of vertical deformation, no significant residuals are found, indicating no significant mass accumulation or loss within the volcanic system. In the north-western sector of the study area, at the border of the volcano edifice, however, significant residual positive gravity changes are detected which are associated to ground-water rebound after years of intense exploitation of the aquifers. On the seasonal timescale, we find that stations within the caldera rim are affected by the seasonal hydrological effects, while the gravity stations at the base of the Vesuvius show a less clear correlation. Furthermore, within the caldera rim a multiyear gravity transient is detected with an increase phase lasting about 4 yr followed by a slower decrease phase. Analysis of rain data seem to exclude a hydrological origin, hence, we hypothesize a deeper source related to the geothermal activity, which can be present even if the volcano is in a quiescent state. We infer the depth and volume of the source by inverting the spatial pattern of the gravity field at the peak of the transient. A volume of fluids of 9.5 × 107 m3 with density of 1000 kg m−3 at 2.3 km depth is capable to fit reasonably well the observations. To explain the gravity transient, simple synthetic models are produced, that simulate the ascent of fluids from a deep reservoir up to the depth of 2.3 km and a successive diffusion within the carbonate aquifer hosting the geothermal system. The whole process appears to not significantly affect the seismicity rate and the deformation of the volcano. This study demonstrates the importance of a 4-D gravity monitoring of a volcano to understand its complex gravity signals that cover different spatial and temporal scales. Discriminating the different contributions that mix up in the observed gravity changes, in particular those due to hydrologic/anthropogenic activities form those due to the geothermal dynamics, is fundamental for a complete and reliable evaluation of the volcano state.

Gravity changes caused by crustal fluids invasion: A perspective from finite element modeling

Gravity changes related to large earthquake preparation are reported in broad seismogenic source regions, and crustal fluids may change the gravity potential and trigger earthquakes. However, what is the major factors affecting crustal fluids movement in the upper 10 km of the continental crust and altering the gravity potential is still poorly understood. In this study, we evaluated the gravity changes caused by invasion of crustal fluids into the depth of 6–10 km using finite element based numerical modeling. We conducted a series of numerical experiments to investigate the influences of permeability distribution, the fluid source, and the invasion depth on gravity changes. The sensitivity analysis and comparison results suggest that the invasion of high-pressure fluids increases fluid density and produces positive gravity changes within a decade. Fluid pressure is the key factor for the variations of fluid density, while temperature exerts a minor influence. Consistent with the dissipation of fluid pressure, the corresponding gravity changes are fast (>3 μGal/year) in the first 2–3 years of fluid invasion and become increasingly slow later. Therefore, the first few years are the more suitable time for monitoring the gravity changes caused by crustal fluids invasion. The fluid-related gravity changes are positively correlated to the porosity and permeability of rocks adjacent to the fluid source, the salinity and the scale of crustal fluids, and the initial temperatures, and are negatively correlated to the invasion depth. Transient invasion of crustal fluids can be detected by means of the high precision gravity observation under certain conditions subjected to the initial geological characteristics of the continental crust and the geochemical evolution of the fluid sources.

Co-Seismic and Post-Seismic Temporal and Spatial Gravity Changes of the 2010 Mw 8.8 Maule Chile Earthquake Observed by GRACE and GRACE Follow-on

The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-on (GRACE-FO) satellites are important for studying regional gravitational field changes caused by strong earthquakes. In this study, we chose Chile, one of Earth’s most active seismic zones to explore the co-seismic and post-seismic gravitational field changes of the 2010 Mw 8.8 Maule earthquake based on longer-term GRACE and the newest GRACE-FO data. We calculated the first-order co-seismic gravity gradient changes (GGCs) and probed the geodynamic characteristics of the earthquake. The earthquake caused significant positive gravity change on the footwall and negative gravity changes on the hanging wall of the seismogenic fault. The time series of gravity changes at typical points all clearly revealed an abrupt change caused by the earthquake. The first-order northern co-seismic GGCs had a strong suppressive effect on the north-south strip error. GRACE-FO results showed that the latest post-seismic gravity changes had obvious inherited development characteristics, and that the west coast of Chile maybe still affected by the post-seismic effect. The cumulative gravity changes simulated based on viscoelastic dislocation model is approximately consistent with the longer-term GRACE and the newest GRACE-FO observations. Our results provide important reference for understanding temporal and spatial gravity variations associated with the co-seismic and post-seismic processes of the 2010 Maule earthquake.

Integrating Reservoir Dynamics, Crustal Stress, and Geophysical Observations of the Laguna del Maule Magmatic System by FEM Models and Data Assimilation

AbstractThe Laguna del Maule volcanic field is a large rhyolitic magmatic system in the Chilean Andes, which has exhibited frequent eruptions during the past 20 ka. Rapid surface uplift (>20 cm/year) has been observed since 2007 accompanied by localized earthquake swarms and microgravity changes, indicating the inflating magma reservoir may interact with a preexisting weak zone (i.e., Troncoso fault). In this investigation, we model the magma reservoir by data assimilation with Interferometric Synthetic Aperture Radar data. The reservoir geometry is comparable to the magma body inferred by seismic tomography, magnetotelluric, and gravity studies. The models also suggest that a weak zone, which has little effect on surface displacement, is important as a fluid transport channel to promote earthquakes and microgravity changes. In particular, concentrated dilatancy within the weak zone facilitates the microfracture formation during reservoir inflation. High‐pressure fluid can inject into the weak zone from the magma reservoir to trigger earthquakes and further migrate upward to create positive gravity changes by occupying unsaturated storages. The pore pressure will then decrease, halting the seismicity swarm until the next cycle. This “hydrofracturing” process may release some accumulated stress along the magma reservoir delaying an eventual eruption in turn. Besides, the resultant models are propagated forward in time to evaluate potential stress trajectories for future unrest.

Seismic b-value anomalies in the Sumatran region: Seismotectonic implications

The spatial variations in the seismic b-value, seismic moment, Bouguer gravity change and fault-plane solution are estimated in the Sumatran region based on homogeneous seismicity data from June 2005 to March 2012 to investigate the seismotectonic characteristics and tectonic activity between the 2004 Sumatran earthquake and the 2012 East Indian Ocean earthquakes. Based on changes in the seismicity rate, this time period is divided into five intervals with different earthquake distributions and faulting styles. The seismic b-values, faulting mechanisms and seismic moment release values are mapped throughout the Sumatran region, and the gravity change every two years between the 2004 event and the 2012 events are determined from satellite (GRACE) gravity data. The b-values vary from 0.45 to 1.59, and an inverse correlation is observed between the b-values and convergence rates. Low b-values are located at depths between 20 km and 40 km in the subduction zone and between 20 km and 35 km in the oceanic plate; the lowest b-values are located at a depth of approximately 28 km in both regions, which indicates a homogeneous fault system between them. The areas with low b-values are associated with megathrust and strike-slip mechanisms, while the area with the lowest b-values is dominated by a megathrust mechanism. Seismically active regions have low b-values associated with a high stress accumulation and high seismic moment release. Low b-value areas are located mainly along the Andaman-Sunda Trench and Andaman-Nicobar-Sumatra forearc. Temporal variations in the seismicity distribution and faulting style are recognized with temporal variations in the b-values in each period. Relationships exist between the epicenter of the 2004 event and the location of the maximum Bouguer gravity change in 2012 and between the epicenters of the 2012 events and the location of the maximum Bouguer gravity change in 2004. Furthermore, a zone of continuous 2-year positive gravity change is located near the Nicobar Islands. The 2004 event boost the 2012 events and intraplate seismicity via stress transfer. The seismicity distributions in individual periods within the studied timeframe highlight the active and locked portions of the fault systems.

Mass transfer processes in a post eruption hydrothermal system: Parameterisation of microgravity changes at Te Maari craters, New Zealand

Fluid transfer and ground deformation at hydrothermal systems occur both as a precursor to, or as a result of, an eruption. Typically studies focus on pre-eruption changes to understand the likelihood of unrest leading to eruption; however, monitoring post-eruption changes is important for tracking the return of the system towards background activity. Here we describe processes occurring in a hydrothermal system following the 2012 eruption of Upper Te Maari crater on Mt Tongariro, New Zealand, from observations of microgravity change and deformation. Our aim is to assess the post-eruption recovery of the system, to provide a baseline for long-term monitoring. Residual microgravity anomalies of up to 92 ± 11 μGal per year are accompanied by up to 0.037 ± 0.01 m subsidence. We model microgravity changes using analytic solutions to determine the most likely geometry and source location. A multiobjective inversion tests whether the gravity change models are consistent with the observed deformation. We conclude that the source of subsidence is separate from the location of mass addition. From this unusual combination of observations, we develop a conceptual model of fluid transfer within a condensate layer, occurring in response to eruption-driven pressure changes. We find that depressurisation drives the evacuation of pore fluid, either exiting the system completely as vapour through newly created vents and fumaroles, or migrating to shallower levels where it accumulates in empty pore space, resulting in positive gravity changes. Evacuated pores then collapse, causing subsidence. In addition we find that significant mass addition occurs from influx of meteoric fluids through the fractured hydrothermal seal. Long-term combined microgravity and deformation monitoring will allow us to track the resealing and re-pressurisation of the hydrothermal system and assess what hazard it presents to thousands of hikers who annually traverse the volcano, within 2 km of the eruption site.

Dynamic changes of gravity fields before and after the 2008 Wenchuan earthquake (Ms8.0)

Abstract: The pattern evolution and dynamic mechanism of the dynamic changes of regional gravity fields occurring before and after the Wenchuan M s8.0 earthquake are analyzed, based on five epochs of 1998–2007 mobile gravity data from the middle-south section of the north–south seismic belt, and two epochs of field research data collected after the 2008 Wenchuan earthquake in combination with GPS data, leveling observations, and geotectonic environment data. The regional dynamic gravity changes demonstrate the effects of the eastward flow of solid matter in the Qinghai-Tibetan plateau and the preparation of the 2008 Wenchuan earthquake (2–10 yr). The two most meaningful gravity indicators of the Wenchuan earthquake preparation are the positive (increasing) gravity changes occurring over many years in the southwest epicenter and the large-scale gradient zone of gravity variation, with the cumulative difference between the two sides of the gradient zone of gravity exceeding 200 μGal. The positive gravity changes may facilitate a constant energy accumulation and the gradient belt may support seismic shear breakage. Overall, the gravity changes associated with the earthquake preparation indicate a pattern of accelerating increase-decelerating increase-earthquake occurrence. The Songpan-Ganzi block generally displays a negative gravity change, providing evidence for a local upwarping of the deep crust-mantle and an interior expansion of the deep crust attributable to high temperatures. The viewpoint is consistent with the dilatant mechanism for earthquake preparation.

Viscous lithospheric structure beneath Sumatra inferred from post-seismic gravity changes detected by GRACE

Satellite gravity data of the regional rheological structure of the lithosphere in the vicinity of Sumatra is used as evidence to enable a better understanding of the regional geodynamic environment. The data is interpreted using the theory of post-seismic viscoelastic relaxation. Co- and post-seismic changes in the gravity field resulting from the 2004 Mw9.3 Sumatra earthquake were calculated from Gravity Recovery and Climate Experiment (GRACE) satellite data. A spatial Gaussian filter, 500 km wide, was used in the calculation. The results indicate that there were significant co-seismic jumps in both uplifted and subducted regions. The magnitude of the jump in the subducted zone was ∼9×10−8 m/s2, more significant than the ∼2×10−8 m/s2 jump observed in the uplifted zone. However, a positive gravity change occurred in the uplifted zone very soon after the earthquake. The rheological structure of the lithosphere has a great effect on deformation and its determination is a fundamental part of developing reliable numerical simulations in geodynamics. Based on the temporally-variable gravity field observed by GRACE, the viscous lithospheric structure of the Sumatra area is investigated with a self-gravitating, half space, viscoelastic earth model. The estimated viscosity is of the order of 1.0×1018 Pa·s and there are differences in the rheological parameters on the two sides of the fault. The factors that affect the viscosity are discussed in connection with the tectonic structure of the Sumatra area.

Magma storage, eruptive activity and flank instability: Inferences from ground deformation and gravity changes during the 1993–2000 recharging of Mt. Etna volcano

A long recharging period characterized Mount Etna volcano during 1993–2000 before the main explosive–effusive 2001 and 2002–2003 flank eruptions. The joint analysis of ground deformation and gravity data over the recharge period reveals that different phenomena occurred within Etna's plumbing system and clearly indicates two phases spanning 1993–1997 and 1997–2000, respectively. The first phase was characterized by magma storage and accumulation at an intermediate depth (2–6 km below sea level), which provoked overall inflation and positive gravity changes. During the second phase, the magma started to rise and intrude at shallower levels favoring the movement of the unstable eastern flank, which accelerated its sliding toward the East. The shallower magma accumulation also caused the gas exsolution, associated with increasing explosive activity at the summit craters. Negative gravity changes were detected during this phase. The gravity measurements, independent of the same result obtained by geochemical studies, suggest that only 20–30% of the magma volumes supplied in the plumbing system were then erupted. The complex dynamics of rising magma beneath Mount Etna makes ground deformation and gravity measurements complementary, being able to detect different effects of magma emplacements beneath the surface. Our results also highlight how the joint use of ground deformation and gravity observations may be crucial in identifying the nature and rate of an impending period of volcanic eruptions.

Gravity Changes Associated with the Ms8.0 Wenchuan Earthquake Detected by GRACE

AbstractTime‐varying gravity field and surface density distribution on and around Chinese Mainland are calculated by the GRACE satellite gravity data. Time series of gravity changes at typical locations are also obtained. At the same time, time series of displacements at GPS fiducial stations of WUSH, LHAS, KUNM and LUZH are obtained in the regional reference frame. Results from GRACE gravity observations indicate that there was significant negative gravity change along the Himalaya arc after the Sumatra earthquake, particularly during the period of 2006 to 2008. Meanwhile there was also obvious negative gravity change along the northwestern boundary of the Xiyu block. While there appeared an arc with positive gravity change along the northern and eastern boundaries of the Tibetan plateau in 2007. And positive gravity change was remarkable in the southern and middle segments of the North‐South seismic belt in 2008. This trend of gravity variation after the Sumatra earthquake began to change only after the Wenchuan earthquake. Displacements at four GPS stations recorded co‐seismic signals of the great Sumatra earthquake, and then the horizontal displacement vectors at WUSH, LHAS and KUNM showed significant changes in their general trend until the occurrence of the Ms8.0 Wenchuan earthquake. The changes in the ground mass in the Tibetan plateau and its surrounding areas as revealed by the GRACE satellites have provided new observation approach and data for the explanation of the dynamic mechanism of the Wenchuan earthquake. A preliminary explanation and discussion on the characteristics of the time variable changes in the gravity field observed by GRACE and the dynamic mechanism of the Wenchuan earthquake are presented in this paper by combining the features of regional tectonic movements and displacements of GPS observations.

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.