Do Earthquakes Trigger Mud Volcanoes? a Case Study from The

Wan Zhifeng (Orcid ID: 0000-0002-0005-1129) Do earthquakes trigger mud volcanoes? A case study from the Southern margin of the Junggar Basin, NW China Siling Zhonga, Zhifeng Wana*, Benchun Duanb, Dunyu Liub, Bin Luob a School of Marine Sciences, Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Sun Yat-sen University, Guangzhou, China b Department of Geology & Geophysics, Texas A&M University, College Station, USA Abstract Mud volcanoes are significant indicators of neotectonic activity and have important research significance. Mud volcanoes can not only be used as an important index for the long-term evaluation of oil and gas fields but are also an important symbol for locating gas hydrates in the seabed. Additionally, the eruption of mud volcanoes will affect drilling, pipe laying and other projects, and the eruption of large amounts of methane gas can also cause greenhouse effects and climate change. The trigger mechanisms of mud volcanoes have always been a focus of debate among geologists. In recent years, many scholars have argued that mud volcanoes are triggered by earthquake activity. However, the stress and strain caused by earthquakes and their control mechanisms on the eruption of mud volcanoes still require further study. This paper is based on the calculated results of static stresses caused by the M5.5 Wusu earthquake of May 2, 1995, the M5.4 Shawan earthquake of January 9, 1996, the M5.3 Shihezi earthquake of February 14, 2003 and the M6.3 Xinyuan-Hejing earthquake of June 30, 2012 on the Horgos, Dushanzi, Aiqigou and *Corresponding author, E-mail: [email protected] This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/gj.3222 This article is protected by copyright. All rights reserved. Baiyanggou mud volcanoes distributed in the southern margin of the Junggar Basin, NW China. The calculated static stresses, the earthquake response characteristics of these four groups of mud volcanoes, and the continuous observational data of the Horgos mud volcanoes showed that static stresses from these earthquakes did not reach the triggering threshold, though the mud volcanoes exhibited a good relationship with earthquakes. We speculate that static stress may not be the main triggering mechanism for the mud volcanoes and that the mechanisms of earthquake triggering may be divided into two types: (1) For a mud volcano in a critical state before an earthquake, dynamic stress changes may trigger eruption of the mud volcano by increasing permeability and mobilizing magma; (2) for mud volcanoes that have not yet reached the critical eruption state before an earthquake, both static and dynamic stresses play roles in their activities. Key words: Mud volcano, Earthquake, Trigger mechanism, Static stress, Dynamic stress, Southern margin of the Junggar Basin Introduction Mud volcanoes are hummocky geological structures formed by the deposition of deep-water-bearing muddy matter in a basin under abnormally high pressure along high-permeability paths (e.g., faults) to the surface or seabed (Dimitrov, 2002; Milkov, 2000; Etiope et al., 2004; Kopf, 2008; Yan et al., 2014). Mud volcanoes are widely distributed throughout the world. There are more than 40 areas on land and more than 20 offshore regions in the oceans that have developed mud volcanoes. Over 2,000 mud volcanoes have been discovered around the world (Milkov, 2000). They are This article is protected by copyright. All rights reserved. mostly located in the Tethys tectonic belt and the Circum-Pacific belt (Kopf , 2002; Sun et al., 2010). Over 400 mud volcanoes (Huang et al., 2011) have been found in at least 13 areas in China, such as the Junggar Basin, which is located in Xinjiang, the Qiangtang Basin which is located in Tibet, and the Yinggehai Basin, which is located in the northern part of the South China Sea (Yin, et al., 2003; He et al., 2010; Dai et al., 2012; Yan et al., 2014; Chen et al., 2014; Zhu et al., 2009; Huang et al., 2011). Mud volcanoes have important significance. First, hydrocarbons and muddy materials that erupt from mud volcanoes can be used as an important basis for deep sedimentation and hydrocarbon accumulation studies (Link, 1952; Abrams, 2005; Rovere et al., 2014). Second, seabed mud volcanoes can be used as important landmarks in the search for deep-water petroleum and natural gas hydrates (Ginsburg et al., 1984; Chen et al., 2005; Sha et al., 2005; Egorov et al., 2010; Franek et al., 2015). Third, mud volcanoes release large amounts of methane during the eruption process, which accelerates the greenhouse effect and thus global climate change (Kopf, 2005; Dimitrov, 2002; Sauter et al., 2006). Finally, mud volcano activity affects the exploration and development of petroleum, pipeline laying and other engineering activities and may even cause the loss of life and property (Mellors et al., 2007; Manga, 2009; Normile,2008). For example, the eruption of the Lusi Mud Volcano on Java Island in March 2006 caused the destruction of 13 villages and displaced nearly 50,000 people. Previous studies of mud volcanoes have involved many aspects, including the geochemical analysis of ejecta (Kopf, 2005; Sauter et al., 2006; Etiope et al., 2009; Dai et al., 2012; Yang et al., 2014), the geological structures of mud volcanoes and geophysical analyses (Yin et al., 2003; Mellors et al., 2007; Kopf, 2008; Manga, 2012; Du et al., 2013), the numerical simulation of the mud volcano eruption process (Murton et al., 2003; Zoporowski et al., 2009) and the study of the relationship This article is protected by copyright. All rights reserved. between oil and gas seepage in mud volcanoes and hydrate accumulation (Milkov, 2000; Wu et al., 2010; Nuzzo et al., 2012). However, the formation mechanisms of mud volcanoes remain controversial. Some scholars believe that mud volcanoes are caused by deep over-pressure fluid mud diapirism that pierces overlying strata and breaks through the surface or seafloor, whereas other scholars believe that they are formed by deep muddy fluid that spurts out through fractures as a result of compression, strike slips, extension or other structural stresses. At present, many scholars believe that mud volcanoes are triggered by earthquakes (Manga et al., 2009; Mazzini et al., 2009; Bonini, 2016; Feseker et al., 2014). Mud volcano activity can be enhanced within a few minutes, months, or even years after an earthquake (Mellors et al., 2007). For example, shortly after the 2013 Balochisatan earthquake, a 200-meter-wide and 20-meter-high island that was 383 km from the epicenter was formed on the coast of Gwadar (Bonini et al., 2016). However, already-erupting mud volcanoes or mud volcanoes in near critical condition appear to be much more sensitive to earthquakes than static systems (Manga et al., 2009). Additionally, there is a strong spatial connection between the locations of mud volcanoes and faults (Manga et al., 2009). Sites of mud volcanoes and subsurface fluid discharge are typically located on various types of fragile structures (e.g., faults and joints) (Bonini, 2012), and triggering can be enhanced when mud volcanoes and earthquakes are located in the same fault structure (Mellors et al., 2007). However, the issue of how earthquakes trigger mud volcanoes requires further study. The stress changes caused by earthquakes may have some effect on the eruption of mud volcanoes, as with magma systems. Static stress may influence the eruptions of a mud volcano by expanding or compressing reservoirs or unclamping mud ascent paths. However, static stress rapidly decays with increasing epicenter distance and becomes insignificant beyond a few fault lengths (Bonini et al., 2016). This article is protected by copyright. All rights reserved. Previous research has also shown that dynamic stress may play a major role in triggering mud volcanoes or regulating the eruption of mud volcanoes (Mellors et al., 2007; Manga et al., 2009; Rudolph et al., 2010). The analysis of mud volcanoes in Xinjiang is limited to the description of eruption characteristics (Gao et al., 2008; Du et al., 2013; Wang et al., 2014), geochemistry (Dai et al., 2012; Yang et al., 2014), the qualitative descriptions of the relationships between mud volcanoes and earthquakes (Wang et al., 2000) and pre-earthquake detections and forecasts (Gao et al., 2015), among other topics. However, the analysis of stress changes in mud volcano areas after earthquakes and the influence of magnitudes and epicenters on mud volcanoes are lacking. In this study, we quantitatively simulate the static stress changes in mud volcanoes after earthquakes in the southern margin of the Junggar Basin in Xinjiang and analyze the impact of magnitude and epicentral distance on mud volcano eruption. Then, we combine continuous observational data from 1991 to 2003 for the Horgos mud volcano to explore the trigger mechanisms of mud volcanoes. 1. Geological background of mud volcano development in the southern margin of the Junggar Basin The Junggar Basin is a large mountainous superimposed basin surrounded by fold-mountain systems during the Hercynian and Himalayan campaigns(Chen et al., 2001; Zhao et al., 2003; Zheng et al., 2007; Li et al., 2012, 2017 ). The southern margin of the Junggar Basin is located at the juncture of the Junggar Basin and the Tianshan orogen, and its deformational features reflect the coupling between the Tianshan orogen and the Junggar Basin (Figure 1). Faults are well developed in the southern margin of the Junggar Basin; most are reverse faults, and there are also some strike slip faults. The southern margin of the Junggar Basin shows the tectonically This article is protected by copyright.

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