Gravity variations preceding the large earthquakes in the Qinghai-Tibet Plateau from 21st century

Abstract

Since the beginning of 21st century, there have been 7 large earthquakes in the Qinghai-Tibet Plateau: The Kunlun Mountain Pass M s8.1 earthquake in 2001, the Wenchuan M s8.0 earthquake and the Yutian M s7.3 earthquake in 2008, the Yushu M s7.1 earthquake in 2010, the Lushan M s7.0 earthquake in 2013, the Yutian earthquake M s7.3 in 2014, and the Jiuzhaigou M s7.0 earthquake in 2017. Prior to these earthquakes, the China Earthquake Administration (CEA) had gathered mobile gravity observations over several periods near the epicenter region, obtaining reliable gravity change with time data. In this study, the relationship between gravity variation and these 7 large earthquakes was analyzed using the absolute and relative gravity observations. The results revealed that: (1) The gravity variation fields related to the occurrence of these earthquakes were in response to the movement and change of underground fluid distributed deep in the crust. An enhanced regional stress field leads to deep material migration and transformation, which results in large-scale regional gravity change. (2) Large earthquakes tend to occur in the high-gradient transition zones between positive and negative gravity anomaly regions associated with the boundary of tectonic active blocks. The inflection point of the gravity change isoline is the transition zone between increasing and decreasing material density. The difference in the motions of the increasing and decreasing material is intense, and shear stress easily occurs in the high-gradient zones of the gravity change isoline. This leads to rupture, which then induces earthquakes. (3) The 2001 Kunlun Mountain Pass M s8.1 earthquake occurred near the intersection of the high gradient zone of gravity change and the East Kunlun fault zone on the northern boundary of the Bayan Hara Block; the Wenchuan M s8.0 earthquake in 2008 and the Lushan M s7.0 earthquake in 2013 occurred in the high gradient zone of gravity variation along the Longmenshan fault zone on the eastern boundary of the Bayan Hara block; the 2 Yutian M s7.3 earthquakes in 2008 and 2014 occurred on the 0 value line of the high gradient zone and the inflection point of the gradient zone of gravity variation, which is basically consistent with the strike of the Kangxiwa fault on the western boundary of the Bayan Hara block; the 2010 Yushu M s7.1 earthquake occurred on the Ganzi-Yushu fault along the high gradient zone of gravity change and the southern boundary of the Bayan Hara block; and the Jiuzhaigou M s7.0 earthquake in 2017 occurred at the intersection of the Tazang fault and the Minjiang fault in the high-gradient zone of gravity change and the northeastern boundary of the Bayan Hara block. (4) Gravity data reliably reflect the 7 earthquakes with magnitude 7 or above that have occurred in the Qinghai-Tibet Plateau since 2001. Obvious regional gravity anomalies and high-gradient zones were observed in the epicenter area and its vicinity prior to the earthquakes, which may be the gravity precursor information observed during the process of earthquake preparation. The authors made medium-term forecasts before the Yutian M s7.3 earthquake in 2008, the Wenchuan M s8.0 earthquake in 2008, the Lushan M s7.0 earthquake in 2013, the Yutian M s7.3 earthquake in 2014, and the Jiuzhaigou M s7.0 earthquake in 2017, primarily focusing on earthquake location based on gravity anomaly variations.