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

Area of Science:

• Geophysics
• Seismology
• Gravimetry

Background:

• The eastern Tibetan Plateau is seismically active, experiencing a Ms7.0 earthquake in Jiuzhaigou on August 8, 2017.
• This region is monitored by a dense gravity observation network, crucial for detecting subtle geophysical changes.

Purpose of the Study:

• To analyze microGal-level gravity variations preceding the Jiuzhaigou earthquake.
• To investigate the potential link between gravity field changes and crustal processes.
• To assess the utility of gravity field changes as a precursor for earthquake prediction.

Main Methods:

• Utilized data from seven high-precision hybrid gravity surveys conducted between 2014 and 2017.
• Applied the modified Bayesian gravity adjustment method for signal processing.
• Employed the equivalent source model inversion in spherical coordinates to determine near-crust density variations.

Main Results:

• Detected significant gravity changes across the eastern Tibetan Plateau from southwest to northeast starting approximately two years before the earthquake.
• Observed a gravity gradient zone aligning with tectonic trends within the Jiuzhaigou focal area during 2015-2016.
• Found that known factors like crustal deformation and water variations could not fully explain the magnitude of the observed gravity changes.

Conclusions:

• The observed gravity anomalies suggest a possible role of fluid material redistribution within the crust prior to the earthquake.
• Regional gravity field changes demonstrate precursory significance for high-risk seismic zones.
• Gravity field monitoring could serve as a candidate precursor for annual, medium-term earthquake prediction.