Abstract
Geophysical processes of the pre-earthquake activities are difficult to be determined since less pre-seismic signal is observed directly. Crustal density changes derived from the periodical terrestrial gravimetry may provide meaningful deep information for the pre-earthquake cue. In this study, the crustal density changes following the 2016 MS6.4 Menyuan earthquake are estimated using ground-based gravity-change data from 2011 to 2015 in the northeastern Tibetan Plateau. The results show that negative density changes dominate the region between the South Longshou Mountain fault and the Daban Mountain fault except the southeast of this region (the seismic region) during 2011–2012. Positive density changes appeared in the middle crust near the epicenter during 2012–2013 and in the upper and middle crust east of the epicenter approximately 1.5 years before the earthquake (2013–2014), and then negative density changes appeared under and northeast of the epicenter approximately four months before the earthquake (2014–2015). The state of the crustal materials near the seismic region changed from convergence to expansion, in turn, indicating that the characteristics of the deep seismogenic process was corresponding to Amos Nur’s 1974 dilatancy-fluid diffusion model.
Highlights
Temporal–spatial gravity changes from terrestrial gravimetry have contributed to understanding of the tectonic movements including earthquakes[1,2]
The main factor contributing to gravity changes should be crustal mass redistribution, which generally appears in the forms of the crustal deformation and density changes (DCs)[2,5,16,17]
The epicenter locates at the northeastern Tibetan Plateau (NETP), between the east end of the North Qilian Mountain (NQM) fault and the west end of the Tertiary Menyuan basin
Summary
Temporal–spatial gravity changes from terrestrial gravimetry have contributed to understanding of the tectonic movements including earthquakes[1,2]. Geodetic observation can intuitively present the surface vertical deformation[18,19], while the crustal DCs could not be determined directly. Xuan et al.[20] suggested that the crustal DCs derived from the temporal-spatial gravity-change data may imply the dynamic information related to the pre-earthquake. The observed gravity changed from increase to decrease near the LLL fault and surroundings before the 2016 Menyuan MS6.4 earthquake[27,28,29], suggesting the dynamic information related to the deep process. The gravity changes following the Menyuan earthquake with time on a one-year scale in the NETP were used to determine the crustal DCs using 3D gravity inversion method. Based on the DC results, the change characteristics of crustal materials are investigated before this event as well as possible factors contributing to the earthquake
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