New evidence from hybrid ground-based gravity observations of a regional gravity increase in southern Tibet before 2015

The 2015 Mw 7.8 Nepal earthquake occurred in the Himalayan tectonic belt, where the Indian Plate collides with the Tibetan Plateau. High-precision ground gravimetry can be used to detect transient deep mass changes before the earthquake but limited by sparse absolute gravity stations and the effects of complex surficial factors. In this study, we present new evidence from hybrid terrestrial gravity measurements, carried out in southern Tibet near the epicenter of the 2015 Nepal earthquake. Measurements from 9 relative and 3 absolute gravity stations are integrated using a novel adjustment method. The results confirm the significant regional gravity increase before the 2015 Nepal earthquake, with a rate of about or larger than 15 μGal/yr (1 μGal = 10 −8 m/s 2 ) at four stations during 2010–2013. We show that this gravity increase cannot be explained by vertical ground motion and/or local hydrological processes. Furthermore, due to inadequate spatial resolution, the observed gravity change could not be detected by the Gravity Recovery and Climate Experiment (GRACE) measurements. We suggest that the gravity increase could be caused by preseismic strain and mass (fluid) transfer in a broad seismogenic source region north of the Main Frontal Thrust. Absolute gravity observations show that the gravity increase stopped after the 2015 Nepal earthquake. Our results contribute to the exploration of possible precursors of large continental earthquakes and shed light on the potential mechanisms of large earthquakes in the Indo-Asian collision zone.