Seasonal gravity changes estimated from GRACE data

Since 2002, the GRACE program has provided a large amount of high-precision data, which can be used to detect temporal gravity variations related to global mass re-distribution inside the fluid envelop of the surface of the Earth. In order to make use of the GRACE data to investigate earthquake-related gravity changes in China, we first studied the degree variances of the monthly GRACE gravity field models, and then applied decorrelation and Gaussian smoothing method to obtain seasonal gravity changes in China. By deducting the multi-year mean seasonal variations from the seasonal maos,we found some earthouake-related gravity anomalies.

Far-field coseismic gravity changes related to the 2015 MW7.8 Nepal(Gorkha)earthquake observed by superconducting gravimeters in Chinese mainland

Using data from five SGs at four stations in Chinese mainland,obvious permanent gravity changes caused by the 2015 MW7.8 Nepal(Gorkha)earthquake were detected.We analyzed the gravity effects from ground vertical deformation(VD)using co-site continuous GPS(cGPS)data collocated at the Lijiang and the Wuhan station,and hydrological effects using GLDAS models and groundwater level records.After removing these effects,SG observations before and after the earthquake revealed obvious permanent gravity changes:−3.0μGal,7.3μGal and 8.0μGal at Lhasa,Lijiang and Wuhan station,respectively.We found that the gravity changes cannot be explained by the results of dislocation theory.

Three-dimensional admittance analysis of lithospheric elastic thickness over the Louisville Ridge

Using bathymetry and altimetric gravity anomalies, a 1°×9 1° lithospheric effective elastic thickness（Te） model over the Louisville Ridge and its adjacent regions is calculated using the moving window admittance technique. For comparison, three bathymetry models are used： general bathymetric charts of the oceans, SIO V15.1,and BAT_VGG. The results show that BAT_VGG is more suitable for calculating T e than the other two models. T e along the Louisville Ridge was re-evaluated. The southeast of the ridge has a medium Te of 10–20 km, while Te increases dramatically seaward of the Tonga-Kermadec trench as a result of the collision of the Pacific and IndoAustralian plates.

A new and high-precision gravity base network in the south of the Tibetan Plateau

A new gravity base network in the south of the Tibetan Plateau was established with a FG5X absolute gravimeter and three CG-6 gravimeters.The gravity base network consists of 10 absolute gravity points and 17 relative gravity points.Processing of the absolute data,pre-processing of the relative data and gravity network adjustment model are briefly described.Based a constrained weighted least squares,the combined adjustment of absolute and relative gravity measurements results in the gravity values with a precision of about±4.1μGal.

Gravity variation in the Tibet area before the Nepal Ms8.1 earthquake

This research utilized two periods of gravity monitoring results from 2010 to 2013 from the Continental Tectonics Environmental Monitoring Network of China, analyzed the corre- lation between gravity variation in the Tibet area and the Nepal Ms8.1 earthquake, and investigated the gravity variation mechanism in combination with the crust vertical movement and horizontal strain field observed by Global Positioning System （GPS）. The research results indicated that （1） the gravity variation exhibited apparent characteristics of a positive anomaly and high gradient zone in the Himalayan frontier. This observation is consistent with the existing recognition of the gravity anomaly and occurrence regularity of a strong earthquake; （2） the gravity variation exhibited apparent consistence with the spacious distribution of the vertical movement and the horizontal deformation field in that area. The crustal vertical movement was not the direct cause leading to the gravity vari- ation. It is assumed that the crust stress-strain accumulation in the Qinghai-Tibetan Plateau and its adjacent areas is the important factor that resulted in the variation of gravity.

Gravity change observed in a local gravity network and its implication to seasonal precipitation in Dali county, Yunnan province, China

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 gravi- meters are modeled within the observation function. Using this data-processing scheme, the mean point gravity pre- cision 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 pgal 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 mea- surements must be considered carefully.

Gravity variation before the Akto Ms6.7 earthquake, Xinjiang

The relationship between gravity variation and the Akto Ms6.7 earthquake on November 11, 2016, was studied by use of mobile gravity observation data from the China continental structural environmental monitoring network. The result revealed that before the Akto earthquake, a high positive gravity variation was observed in the Pamir tectonic knots region （within a maximum magnitude of approximately ＋80 microgal）, which was consistent with the existing knowledge of gravity abnormality and the locations of strong earthquakes. In view of the recent strong seismic activities in the Pamir tectonic knots region, as well as the strong upward crust movement and compressive strain, it is believed that gravity change in the Pamir tectonic knots region reflects the recent strong seismic activities and crust movement.