Abstract: Previous isostatic study shows that the crust around the middle and south segments of the Longmenshan fault zone is in great isostatic imbalance, which indicates an M≥7.0 earthquake in the future. We conducted a gravity and hybrid GNSS observation campaign in June of 2016 around the above great imbalance area, and improved spatial resolution of the gravity anomalies. We inversed the density structures around the Wenchuan M_W7.9 earthquake using the above and previous observation data. Our results show that the Longmenshan fault zone is the high gradient belt of crustal density change. The crust east of which is relatively thinner than that in the west. As a whole the upper, middle and lower crusts show the same characteristics. Moho depths in the eastern parts of the study area are 35~40 km. but those in the western parts are 60~65 km. Besides, we calculated the vertical tectonic stress of the lithosphere of the study area using gravity and GNSS data. We found that the crusts store up vertical tectonic stress of -30~-40 MPa around Lingqiang, Emei Mountain, and the area north to Wenhcuan M_W7.9 earthquake. However, the crust stores up vertical tectonic stress of 40 MPa around the middle and south segments of the Longmenshan fault zone. The maximum of vertical stress locates near the epicenter of the 2013 Lushan M_W6.6 earthquake. Statistical results show that the earthquakes often occur near the high gradient zones of vertical tectonic stress.

Key words: Bouguer gravity anomaly, Isostasy, Density structure, Vertical tectonic stress of the lithosphere, The 2008 Wenchuan M_W7.9 earthquake