Analysis of spatiotemporal variations in middle-tropospheric to upper-tropospheric methane during the Wenchuan <i>M</i><sub>s</sub>&#8201;=&#8201;8.0 earthquake by three indices

Cui, Jing; Shen, Xuhui; Zhang, Jingfa; Ma, Weiyu; Chu, Wei

This research studied the spatiotemporal variation in methane in the middle to upper troposphere during the Wenchuan earthquake (12 May 2008) using AIRS retrieval data and discussed the methane anomaly mechanism. Three indices were proposed and used for analysis. Our results show that the methane concentration increased significantly in 2008, with an average increase of 5.12×10-8, compared to the average increase of 1.18×10-8 in the previous 5 years. The absolute local index of change of the environment (ALICE) and differential value (diff) indices can be used to identify methane concentration anomalies. The two indices showed that the methane concentration distribution before and after the earthquake broke the distribution features of the background field. As the earthquake approached, areas of high methane concentration gradually converged towards the west side of the epicenter from both ends of the Longmenshan fault zone. Moreover, a large anomalous area was centered at the epicenter 8 d before the earthquake occurred, and a trend of strengthening, weakening and strengthening appeared over time. The gradient index showed that the vertical direction obviously increased before the main earthquake and that the value was positive. The gradient value is negative during coseismic or post-seismic events. The gradient index reflects the gas emission characteristics to some extent. We also determined that the methane release was connected with the deep crust–mantle stress state, as well as micro-fracture generation and expansion. However, due to the lack of any technical means to accurately identify the source and content of methane in the atmosphere before the earthquake, an in-depth discussion has not been conducted, and further studies on this issue may be needed.

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Cui, Jing / Shen, Xuhui / Zhang, Jingfa / et al: Analysis of spatiotemporal variations in middle-tropospheric to upper-tropospheric methane during the Wenchuan <i>M</i><sub>s</sub>&#8201;=&#8201;8.0 earthquake by three indices. 2019. Copernicus Publications.

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