A method for extracting the pre-seismic gravity anomalies over the Tibetan Plateau based on the maximum shear strain using GRACE data

The occurrence of earthquakes is closely related to the crustal geotectonic movement and the migration of mass, which consequently causes the changes of gravity. GRACE satellite can be used to detect gravity changes of large earthquake. However, previous GRACE satellite-based seismic gravity changes studies focused more on co-seismic gravity changes than on pre-seismic gravity changes. Moreover, it is difficult to eliminate the noise of north-south stripe in GRACE data, thereby resulting in the loss of some gravity information related to tectonic activities. To explore the pre-seismic gravity anomalies in a more refined way, this study firstly proposes a method of characterizing gravity variation based on the maximum shear strain of gravity, inspired by the concept of crustal strain. Then, the offset index method is adopted to describe the gravity anomalies, and the spatial and temporal characteristics of gravity anomalies before earthquakes are analyzed from the scales of fault zone and plate, respectively. In this paper, experiments are carried out on the Tibetan Plateau and its surrounding areas, with the following findings obtained: (1) From the observation scale of the fault zone, it is detected that large area gravity anomalies occur near the epicenter many times about half a year before the earthquake, and these anomalies are distributed along the fault zone. (2) From the observation scale of the plate, it is found that when an earthquake occurred on the Tibetan Plateau, a large number of gravity anomalies also occur at the boundary of the Tibetan Plateau and the Indian plate. Moreover, the above experiments also confirm that the proposed method can successfully capture the preseismic gravity anomalies of large earthquakes with magnitude less than 8, which provides a new idea for the application of gravity satellite data to earthquake research.