Analysis of spatiotemporal variations in middle-tropospheric to upper-tropospheric methane during the Wenchuan &amp;lt;i&amp;gt;M&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;s&amp;lt;/sub&amp;gt; = 8.0 earthquake by three indices

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

doi:10.5194/nhess-19-2841-2019

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

Jing Cui, Jingfa Zhang + Show 3 more

Open Access

https://doi.org/10.5194/nhess-19-2841-2019

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Abstract

Abstract. 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.

Highlights

The great Wenchuan Ms = 8.0 earthquake, on 12 May 2008, occurred in the Longmenshan fault zone in western Sichuan Province, China
The focus of this study is to examine the spatiotemporal variation in CH4 in the middle to upper troposphere during the Wenchuan earthquake (12 May 2008) using Atmospheric Infrared Sounder (AIRS) retrieval data and to discuss the mechanism of the methane anomaly
These figures show that the CH4 concentration in the study area decreased significantly from the middle troposphere to the upper troposphere, and the average methane concentrations in each stratum were 1.794, 1.775 and 1.762 ppm for the middle, upper middle and upper troposphere, respectively

Summary

Introduction

The extent of the earthquake and aftershock affected areas in the northeast along the Longmen Shan fault, a thrust structure along the border of the Tibetan Plateau and the western Sichuan Basin (Fig. 1). This earthquake was one of the worst continental earthquake events to have struck China in recent decades, and it killed more than 10 000 people in several cities along the western Sichuan basin. The Longmenshan fault zone has a high dipping angle (more than 50–60◦) near the surface and a low angle at depth (15–20 km) This listric shape favors significant strain or energy accumulation, forming large earthquakes. This earthquake was characterized by slow strain accumulation, a long recurrence interval and significant damage power. It is a new type of earthquake that deserves further study (Zhang et al, 2008)

Results

Discussion

Conclusion