Gravity Observations and Apparent Density Changes before the 2017 Jiuzhaigou Ms7.0 Earthquake and Their Precursory Significance.

Jinling Yang,Linhai Wang,Shi Chen,Jiancang Zhuang,Hongyan Lu,Bei Zhang

doi:10.3390/e23121687

Abstract

An Ms7.0 earthquake struck Jiuzhaigou (China) on 8 August 2017. The epicenter was in the eastern margin of the Tibetan Plateau, an area covered by a dense time-varying gravity observation network. Data from seven repeated high-precision hybrid gravity surveys (2014–2017) allowed the microGal-level time-varying gravity signal to be obtained at a resolution better than 75 km using the modified Bayesian gravity adjustment method. The “equivalent source” model inversion method in spherical coordinates was adopted to obtain the near-crust apparent density variations before the earthquake. A major gravity change occurred from the southwest to the northeast of the eastern Tibetan Plateau approximately 2 years before the earthquake, and a substantial gravity gradient zone was consistent with the tectonic trend that gradually appeared within the focal area of the Jiuzhaigou earthquake during 2015–2016. Factors that might cause such regional gravitational changes (e.g., vertical crustal deformation and variations in near-surface water distributions) were studied. The results suggest that gravity effects contributed by these known factors were insufficient to produce gravity changes as big as those observed, which might be related to the process of fluid material redistribution in the crust. Regional change of the gravity field has precursory significance for high-risk earthquake areas and it could be used as a candidate precursor for annual medium-term earthquake prediction.

Highlights

High-accuracy time-variable gravity measurement is useful for understanding the changing Earth
Using data from seven repeated high-precision hybrid gravity observation surveys from 2014 to 2017, gravity values before the occurrence of the Jiuzhaigou Ms7.0 earthquake were obtained with field source resolution better than 75 km based on the modified Bayesian gravity adjustment (MBGA) method
With consideration of the regional vertical crustal deformation, near-surface water variation, and a regional crustal velocity structure model, we proposed a possible mechanism of crustal material migration before the occurrence of the Jiuzhaigou earthquake

Summary

Introduction

High-accuracy time-variable gravity measurement is useful for understanding the changing Earth. Gravity observations acquired at different temporal and spatial scales have been widely used both for investigating earthquake precursor signals and for monitoring masstransport processes. A study using GRACE satellite data revealed large-scale gravity and mass changes throughout three tectonic plates and connected slabs prior to the occurrence of the 2011 Tohoku Oki Mw9.0 earthquake [12]. In previous studies on the Tibetan Plateau, high-accuracy absolute and relative gravity measurements were used to detect anomalies [13,14,15,16,17]. Spatiotemporal crustal density changes at different depths were observed in repeated relative gravity data acquired before the 2016 Menyuan Ms6.4 earthquake in the northeast of the Tibetan

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Discussion

Conclusion