Wenke Sun

Evaluation of glacier changes in high‐mountain Asia based on 10 year GRACE RL05 models

In this paper, 10 years of time-variable gravity data from the Gravity Recovery and Climate Experiment Release 05 have been used to evaluate the glacier melting rate in high-mountain Asia (HMA) using a new computing scheme, i.e., the Space Domain Inverse method. We find that in HMA area, there are three different kinds of signal sources that should be treated together. The two generally accepted sources, glacier melting and India underground water depletion, are estimated to change at the rate of −35.0 ± 5.8 Gt/yr (0.09 mm/yr sea level rising) and −30.6 ± 5.0 Gt/yr, respectively. The third source is the remarkable positive signal (+30 Gt/yr) in the inner Tibetan Plateau, which is challenging to explain. Further, we have found that there is a 5 year undulation in Pamir and Karakoram, which can explain the controversies of the previous studies on the glacier melting rate here. This 5 year signal can be explained by the influence of Arctic Oscillation and El Nino–Southern Oscillation.

Global displacements caused by point dislocations in a realistic Earth model

We define dislocation Love numbers [h nm ij , l nm ij , k nm ij , l nm t , ij ] and Greens functions to describe the elastic deformation of the Earth caused by a point dislocation and study the coseismic displacements caused in a radially heterogeneous spherical Earth model. We derive spherical harmonic expressions for the shear and tensile dislocations, which can be expressed by four independent solutions : a vertical strike slip, a vertical dip slip, a tensile opening in a horizontal plane, and a tensile opening in a vertical plane. We carry out calculations with a radially heterogeneous Earth model (1066A). The results indicate that the dominating deformations appear in the near field and attenuate rapidly as the epicentral distance increases. The shallower the point source, the larger the displacements. Both the Earths curvature and vertical layering have considerable effects on the deformation fields. Especially the vertical layering can cause a 10% difference at the epicentral distance of 0.1°. As an illustration, we calculate the theoretical displacements caused by the 1964 Alaska earthquake (m w = 9.2) and compare the results with the observed vertical displacements at 10 stations. The results of the near field show that the vertical displacement can reach some meters. The far-field displacements are also significant. For example, the horizontal displacements (u ψ ) can be as large as 1 cm at the epicentral distance of 30°, 0.5 cm at about 40°, magnitudes detectable by modern instrument, such as satellite laser ranging (SLR), very long baseline interferometry (VLBI) or Global Positioning System (GPS). Globally, the displacement (u r ) caused by the earthquake is larger than 0.25 mm.

An increase in the rate of global mean sea level rise since 2010

The global mean sea level (GMSL) was reported to have dropped 5 mm due to the 2010/2011 La Nina and have recovered in 1 year. With longer observations, it is shown that the GMSL went further up to a total amount of 11.6 mm by the end of 2012, excluding the 3.0 mm/yr background trend. A reconciled sea level budget, based on observations by Argo project, altimeter, and gravity satellites, reveals that the true GMSL rise has been masked by El Nino–Southern Oscillation-related fluctuations and its rate has increased since 2010. After extracting the influence of land water storage, it is shown that the GMSL has been rising at a rate of 4.4 ± 0.5 mm/yr for more than 3 years, due to an increase in the rate of both land ice loss and steric change.

Global co-seismic displacements caused by the 2004 Sumatra-Andaman earthquake (Mw 9.1)

This paper presents and discusses the global co-seismic displacements caused by the 2004 Sumatra-Andaman earthquake, using quasi-static dislocation theory for a spherically symmetric earth model (Sun et al., 1996). Theoretical calculations are performed with a heterogeneous slip distribution fault model based on Ammon et al. (2005). Results show that the co-seismic horizontal displacements are large to the north-east and southwest of the fault plane. Even as far as 6000 km from the epicenter, more than 1 millimeter co-seismic horizontal displacements raised from the earthquake. This paper has three contributions: to validate the fault model (Ammon et al., 2005) by geodetic data; to interpret the displacements observed by GPS; and to provide a reference for other researchers or for other geodetic applications. Overall, the modelled and observed displacements basically agree with each other in both the near field and far field. The calculated displacements are a generally smaller than the observed ones, since considerable moment is released by slow-slips and/or aftershocks which has not been included in the fault model.

Gravity and uplift rates observed in southeast Alaska and their comparison with GIA model predictions

gravity change rates (unit: mGal/yr, 1 mGal = 10 8 ms 2 ) over the 6 sites are estimated to be 4.50 0.76 and 4.30 0.92 by only using our data and also using the 1987 data, respectively. We computed the uplift and gravity rates predicted by ice load models for three different time intervals: Last Glacial Maximum (LGM), Little Ice Age (LIA) and Present-Day (PD). Except for 1–2 examples, the predictions recover the observed rates within the observation errors. We also estimated the viscous portion of the ratio (unit: mGal/mm) of the observed gravity rate to the uplift rate by correcting for the effects of the Present-Day Ice Mass Change (PDIMC). Two PDIMC models are compared, which are called here as UAF05 and UAF07. Mean ratios are estimated to be 0.205 0.089 and 0.183 0.052 for the cases using UAF05 and UAF07, respectively. The predicted mean ratios are 0.166 0.001 and 0.171 0.002 for the cases using both the LGA and LIA ice models and only using the LIA ice model, respectively. We have confirmed that our AG and GPS observations detect the rates and ratios reflecting an early stage of viscoelastic relaxation mainly due to the unloading effects after the LIA.

Formulation of coseismic changes in Earth rotation and low‐degree gravity field based on the spherical Earth dislocation theory

In this paper we present analytical formulas to compute the coseismic Earth rotation change (polar motion and length of day) and low-degree gravity field change based on a spherical dislocation theory. Using the preliminary reference Earth model and our formulas, we calculate coseismic changes in Earths rotation and low-degree Stokes coefficients caused by the four largest earthquakes since 1960. The results verify that the present method is consistent with previous studies, but the method cannot be verified by the observations. A case study on the 2011 Tohoku-Oki earthquake (Mw 9.0) indicates that the coseismic Earth rotation change depends on the magnitude and source parameters, and results show a difference between a point source and finite fault model. We also investigate the effect of seawater redistribution on coseismic Earth rotation change, but the effect is small and can be neglected.

Truncated co-seismic geoid and gravity changes in the domain of spherical harmonic degree

A concept of truncated geoid and gravity changes is proposed in this study and corresponding truncated expressions are presented for investigating co-seismic deformations. Numerical investigations are carried out to observe whether or not co-seismic geoid and gravity changes are detectable by gravity satellite missions. Results of an individual harmonic degree or a summation to interested degrees are compared with the expected errors of the gravity missions, assuming a seismic source equivalent to the fault size of the Alaska earthquake (1964, mw = 9.2). Corresponding co-seismic deformations indicate that both the gravity and geoid changes are about two orders larger than the precision of GRACE. Based on these results, the minimum magnitudes of earthquakes detectable by GRACE are derived. The conclusion is that co-seismic deformations for an earthquake with a seismic magnitude above m = 7.5 (for the tensile sources) and m = 9.0 (for the shear sources) are expected to be detected by GRACE. Finally, a case study is made on the 2002 Alaska earthquake (m = 7.9). Results show that the co-seismic geoid and gravity changes are at or below the error level of GRACE, and are difficult to detect.

The potential of GRACE gravimetry to detect the heavy rainfall‐induced impoundment of a small reservoir in the upper Yellow River

Artificial reservoirs are important indicators of anthropogenic impacts on environments, and their cumulative influences on the local water storage will change the gravity signal. However, because of their small signal size, such gravity changes are seldom studied using satellite gravimetry from the Gravity Recovery and Climate Experiment (GRACE). Here we investigate the ability of GRACE to detect water storage changes in the Longyangxia Reservoir (LR), which is situated in the upper main stem of the Yellow River. Three different GRACE solutions from the CSR, GFZ, and JPL with three different processing filters are compared here. We find that heavy precipitation in the summer of 2005 caused the LR water storage to increase by 37.9 m in height, which is equivalent to 13.0 Gt in mass, and that the CSR solutions with a DDK4 filter show the best performance in revealing the synthetic gravity signals. We also obtain 109 pairs of reservoir inundation area measurements from satellite imagery and water level changes from laser altimetry and in situ observations to derive the area-height ratios for the LR. The root mean square of GRACE series in the LR is reduced by 39% after removing synthetic signals caused by mass changes in the LR or by 62% if the GRACE series is further smoothed. We conclude that GRACE data show promising potential in detecting water storage changes in this ∼400 km2 reservoir and that a small signal size is not a restricting factor for detection using GRACE data.

Is it possible that a gravity increase of 20 μGal yr−1 in southern Tibet comes from a wide‐range density increase?

With absolute gravimetric observations from 2010 to 2013 in the southern Tibet, Chen et al. (2016) reported a gravity increase of up to 20 μGal/yr and concluded that it is possible if there was a density increase in a disk range of 580 km in diameter. Here we used observations from the gravity satellites Gravity Recovery and Climate Experiment (GRACE) over 12 years to evaluate whether the model was practical, because a mass accumulation in such a large spatial range is well within the detectability ability of GRACE. The gravity trend based on their model is orders of magnitude larger than the GRACE observation, thus negating its conclusions. We then evaluated contributions from seasonal variation, lakes, glaciers, rivers, precipitation, and snowfall and concluded that these factors cannot cause such a large gravity signal. Finally, we discussed some possible explanations for the gravity increase of 40 μGal in two years.

Absolute gravity change associated with magma mass movement in the conduit of Asama Volcano (Central Japan), revealed by physical modeling of hydrological gravity disturbances

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.