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. All rights reserved. deformational characteristics of north-south zoning. Three rows of anticlines are developed from south to north, and mud volcanoes are mostly developed in the second and third rows of structural belts (Wang, 2000; Zheng et al., 2010; Nakada et al.,2011; Dai et al., 2012; Wan et al., 2013). The second structural belt row includes the Horgos anticline, the Manasi anticline and the Tugulu anticline, from east to west, with a near east-west trend. The third structural belt row includes the Shawan-Anjihai structural belt and the Dushanzi structural belt, which extend nearly east-west. Most of the mud volcanoes in Xinjiang are concentrated in the Piedmont upwelling zone of the North Tianshan Piedmont fold belt (Zhou, 2006; Ji et al., 2008; Chen et al., 2010;

Sha et al., 2011) or the western fold-fault tectonic belt (Chen et al., 2010). The tectonic system in the southern margin of the Junggar Basin is the result of multiphase tectonic activity, with tectonic movements in the Indosinian, Yanshanian and Himalayan periods playing a major role (Zhou, 2006). With the superimposition and transformation of multiphase tectonic activity, the tectonic deformation in the southern margin of the Junggar Basin has the characteristics of lateral zonation, longitudinal sectioning and vertical stratification. The multi-stage assemblages and superimposition of different tectonic systems and the roles of different tectonic systems in the same period caused the piedmont belt and its adjacent area in the southern margin of the Junggar Basin to eventually become a compound type dominated by Late Paleozoic and Meso-Cenozoic terrestrial deposits from overlapping basins (Bai, 2008). There is active neotectonic movement and earthquakes have frequently occurred in the southern margin of the Junggar Basin and Northern Tianshan area. The regional stress field of Northern Tianshan area is chiefly controlled by the near north-south horizontal compressive stress (Long et al., 2008). And because of the compression from the south , the Tianshan block is thrust over the Kazakhstan-Junggar and Tarim

This article is protected by copyright. All rights reserved. blocks at its south and north sides respectively, causing dip-slip earthquakes and leading to compressive rupturing (Feng, 1986). Most of the earthquakes in the Northern Tianshan area are located on the fringe between the Tianshan and Junggar basin (Xu et al., 2000). Since the 20th century, a series of earthquakes occurred on both sides of the Tianshan Mountain are closely related to active tectonics (Wang et al., 2001). These earthquakes showed a zonal distribution, which was interpreted as being related to the active faults or folds, such as the Qingshuihezi fault and the Horgos-Tuguuru fault (Long et al., 2008). From 1990 to August 2017, more than 100 earthquakes with magnitudes between 4 to 5 occurred, earthquakes of magnitude 5 (5≤M <6) occurred 14 times and earthquakes of magnitudes larger than 6 (including 6) occurred 2 times in the northern margin of the Junggar Basin and the northern Tianshan (43°-45°N, 83°-87°E) (data from the U.S. Geological Survey (USGS)).

2. Developmental characteristics of mud volcanoes on the southern margin of the Junggar Basin Many mud volcanoes are distributed in the southern margin of the Junggar Basin; most of them are located in the central part of the North Tianshan Mountains. The most representative four groups are the Horgos mud volcanoes, the Dushanzi mud volcanoes, the Baiyanggou mud volcanoes and the Aiqigou mud volcanoes (Figure 2, Table 1). All of the volcanoes are located on different anticlinal zones; the Baiyanggou mud volcanoes and the Aiqigou mud volcanoes are located on the Tuositai anticlinal axis, the Dushanzi mud volcanoes are located in the Dushanzi anticlinal shaft and the Horgos mud volcanoes are located in the Horgos anticlinal shaft (Yang et al., 2014; Wang, 2000). The Baiyanggou mud volcanoes are located in Baiyanggou Town, southwest of

This article is protected by copyright. All rights reserved. Wusu City, Xinjiang. The number of mud vents of these mud volcanoes has been gradually reduced to 20 from more than 200. There are big differences among the different vents; for example, the diameter of the largest vent is 3.5 m and the smallest one is only 80 cm, but all of them have relatively diluted, slurry viscosities (Wan et al., 2013; Yang et al., 2014; Wan et al., 2015). After the Mw 6.3 Xinyuan-Hejing earthquake of June 30, 2012, the temperature of the slurry was slightly lower below the shock, with an increase in the bubble diameter and rate of mud discharge (Du et al., 2013). The Aiqigou mud volcanoes are located in the foreland of the Aiqigou region and consist of two mud volcanoes distributed in the SN direction 10 m from each other. The crater diameter of the northern mud volcano is 5 m and the diameter of the southern one is approximately 1.2 m. The southern mud volcano had gradually dried up before the Xinyuan-Hejing earthquake of June 30, 2012 (Wan et al., 2013; Yang et al., 2014). No mud eruption occurred on the southern mud cone after the earthquake, and the vent remained dry. However, the northern one experienced a liquid level change process of ‘decline-turning-rising-turning-recovering background values’ before and after the earthquake. The temperature of the mud first rose and then dropped, and the rate of bubble discharge did not significantly change, although the slurry discharge rate increased after the Mw 6.3 Xinyuan-Hejing earthquake (Du et al., 2013). The Dushanzi mud volcanoes are located in the Dushanzi District of Karamay City, Xinjiang. There were 5 mud volcano vents in this area, but only two currently exist. One is a mud hole with a 0.08 m spout and a base diameter of approximately 10 m that is relatively more active than the other volcano, which has a larger vent of 1.6x1.1 m but is inactive (Yang et al., 2014; Wan et al., 2015). There were significant differences between the two spouts after the earthquake of June 30, 2012 (Du et al.,

This article is protected by copyright. All rights reserved. 2013). One spout experienced a slight decrease in mud temperature and less mud volume, and the vents even cracked after the earthquake. Additionally, the bubble discharge rate of the other mud volcano was significantly increased as displacement and mud volume significantly increased. Two mud volcanoes are located in Horgos in the southern region of Shawan County, Xinjiang. Gao et al. (2008) observed the slurry surface dynamically and continuously during 1990 to 2004 using a Red Flag-1 water lever gauge at one of the Horgos mud craters. The data showed that the observed Horgos mud volcano had a good relationship with seismic activity. When the earthquake occurred, the "pulse" phenomenon of mud volcanoes was very evident, the activity of the mud volcano obviously increased, the inrushing mud volume increased, the content of gases increased, and the liquid level also increased.

3. Data and methods

3.1 Coulomb stress simulation of earthquakes in areas with developed mud volcanoes To analyze the influence of seismic activity on the mud volcanoes in the Junggar Basin, we analyzed the stress field characteristics of seismic activity by simulating the Coulomb stress of these earthquakes. According to the Coulomb failure assumption, the rupture of an object can be caused by the interaction of shear and normal stresses applied to its cross-section. The definition of the Coulomb failure function(e.g., Stein et al., 1992; Reasenberg et al., 1992; Stein, 1999; King et al., 1994; Kilb et al., 2002; King, 2014 ) ΔCFF is:

ΔCFF=Δτ+μ（Δσn+ΔP）

where Δτ is the shear stress change on the fault, μ is the friction coefficient, Δσn is the

This article is protected by copyright. All rights reserved. normal stress change (which is positive if the fault is unclamped), and ΔP is the change in the pore pressure. However, the change in Coulomb failure stress is always rewritten as:

ΔCFF=Δτ+μ′ Δσn where μ' is the apparent coefficient of friction, which includes the effect of the pore fluid pressure change and the properties of the fault zone. We computed the static stress changes caused by earthquakes on the potential feeder dikes of the above mud volcano systems. We calculated the normal stress

change (Δσn), which is positive if the fault is unclamped. Normal stresses were computed in a homogeneous elastic half-space (Okada, 1992). The assumption of an elastic medium is an approximation to the real Earth that is normally invoked when researching the responses of magmatic volcanoes to earthquakes (e.g., Bautista et al., 1996; Nostro et al., 1998; Walter, 2007). According to Bonini et al. (2016), mud and magmatic volcano systems share some causal mechanisms and may be governed by similar processes; we extended this assumption to study mud volcanoes. We used uniform elastic moduli, with Poisson's ratio ν = 0.25, Young's modulus E = 8 × 105 bar (80 GPa), shear modulus G = 3.2 × 105 bar (32 GPa) and an apparent coefficient of friction μ' = 0.4. The normal stresses caused by a ‘source’ earthquake can be resolved on a specific ‘receiver’ fault defined by strike, dip, and rake. In particular, rake does not affect normal stress changes (Bonini et al., 2016). Because of the uncertainty of the internal structures of the mud volcanoes in the southern margin of the Junggar Basin, we could not know the internal channels of rising mud of each mud volcano. There are many uncertainties in a specific case of calculating normal stress changes that will have a direct impact on the results. Therefore, we assumed that all the accepted faults were vertical and that the strikes of receiver faults were similar to the strikes of the folds in the southern margin of the

This article is protected by copyright. All rights reserved. Junggar Basin, i.e., nearly 270°. However, we also calculated the stress changes in other directions (including strikes of 180°, 210° and 300°) to account for uncertainties in fault strike. We calculated the Coulomb stress change at a depth of 1 km since the source of the mud volcano could be extended vertically at least 1 km downwards (e.g., Davies et al., 2013 ). The normal stress changes were all calculated using the Coulomb 3.3 software (https://earthquake.usgs.gov/research/software/coulomb/).

3.2 Earthquake parameters We selected four earthquakes that were related to mud volcanoes in the southern margin of the Junggar basin according to previous statistics: the M5.5 Wusu earthquake of May 2, 1995, the M5.4 Shawan earthquake of January 9, 1996, the M5.3 Shihezhi earthquake of February 14, 2003 and the M6.3 Xinyuan-Hejing earthquake of June 30, 2012. The catalog of earthquakes came from the USGS, and the epicenter of these earthquakes came from the China Earthquake Networks Center (CENC) (Table 2). When calculating normal stress changes caused by an earthquake in a mud volcano region, the rupture length and width of the ‘source‘ fault are based on the empirical formula of Wells & Coppersmith (1994):

log(L) = a1 + b1*M

= 2 + 2 log(W ) a b *M where W is the width of the fault, L is the length of the fault, M is the magnitude, and a and b are constants that are not zero and have different values for different types of faults; the magnitudes of the above earthquakes are within the threshold of the empirical formula. The amount of seismogenic fault dislocation needed to calculate the Coulomb failure stress can be estimated by the definition of seismic moment (Aki and Richards,

= µΑ D Mo / where μ = 3.2×1010, Mo is the scalar seismic moment, A is the rupture area, and μ is the shear modulus of rocks surrounding the fault.

3.3 Continuous observations of the Horgos Mud Volcano Gao et al. (2008) dynamically and continuously observed the slurry surface during 1990 to 2006 using the Red Flag-1 water lever gauge at the one of the Horgos mud craters and recorded the number of days of abnormal pulses in this mud volcano from 1991 to 2004. The following results are for the abnormal pulse days of the slurry levels for 1991 to 2003 (Table 3).

4. Results

4.1 Simulated results for static stress changes of the M5.5 Wusu earthquake of May 5, 1995 The M5.5 Wusu earthquake of May 5, 1995 had a depth of 42 km. The calculated moment was 1017.25N/m2, the length of the fault was 6.92 km, the width of the fault was 4.9 km, and the slippage was 0.175 m. We used the nodal plane Ⅰ listed in the Table 2 (Ji, 2014), where the strike was 198°, the dip was 82° and the rake was 33°. We assumed that the strikes of the vertical receiving faults were 180°, 210°, 270° and 300° (Table 4). The results showed that the different receiving faults had different normal stress change values (Figure 3). However, the normal stress changes (Δσn) of the four mud volcanoes on the receiving faults with different strikes were all less than 0.1 bar, and the normal stress changes of the Horgos and Dushanzi mud volcano areas were

4.2 Simulated results for static stress changes of the M5.4 Shawan earthquake of January 9, 1996 The M5.4 Shawan earthquake of January 9, 1996 had a depth of 33 km. The calculated moment was 1017.1N/m2, the fault length was 5.57 km, the width of the fault was 4.76 km, and the slippage was 0.158 m. We used the nodal plane Ⅰ listed in the Table 2 (Ji,2014), where the strike was 258°, the dip was 70° and the rake was 79°. We assumed that the strikes used to calculate normal stress changes on the vertical receiving faults were 180°, 210°, 270° and 300° (Table 5). The results showed that the normal stress changes in these mud volcano areas

were much smaller than 0.1 bar and that the Δσn values were negative in the Aiqigou and Baiyanggou mud volcanoes (Figure 4). The normal stress change was negative on the receiving fault, with a strike of 300° in the Horgos mud volcano, but it was positive on other receiving faults. The Dushanzi mud volcano area had negative Δσn values on the strikes of 270° and 300°.

4.3 Simulated results for static stress changes of the M5.3 Shihezi earthquake of February 14, 2003 The M5.3 Shihezi earthquake of February 14, 2003 had a depth of 23.9 km. The calculated moment was 1016.95N/m2, the length of the fault was 5.89 km, the width of the fault was 5.08 km, and the slippage was 0.099 m. We used the nodal plane Ⅰ listed in the Table 2 (Ji, 2014), where the strike was 283°, the dip was 62° and the rake was 105°. We assumed that the strikes of vertical receiving faults used to calculate normal stress changes on the vertical receiving faults were 180°, 210°, 270° and 300° (Table 6).

This article is protected by copyright. All rights reserved. The results showed that normal stress changes in these mud volcano areas were much smaller than 0.1 bar and that most of them were even less than 0.0001 bar (Figure 5). The Aiqigou and Baiyanggou mud volcano areas, which had a relatively

large epicentral distance, had negative Δσn values on the receiving faults with a strike of 210°.

4.4 Simulated results for static stress changes of the M6.3 Xinyuan-Hejing earthquake of June 30, 2012 The M6.3 Xinyuan-Hejing earthquake of June 30, 2012 had a depth of 18 km. The length of the fault was 30 km, the width of the fault was 20 km (Ji, 2014), the calculated moment was 1019 N/m2, and the slippage was 0.157 m. We used the nodal plane Ⅰ listed in the Table 2 (Ji, 2014), where the strike was 300°, the dip was 57° and the rake was 167°. We assumed that the strikes of the vertical receiving faults used to calculate normal stress changes on the vertical receiving faults were 180 °, 210°, 270° and 300° (Table 7). According to Table 7, the different modes of occurrence of the receiving faults had different normal stress change values (Figure 6). The Δσn values in the Horgos mud volcano were negative, and the Aiqigou mud volcano nearest to the epicenter had

larger Δσn values that were still less than 0.1 bar. The result of normal stress analysis showed that there were different values in these four mud volcano areas and that only the result in the Aiqigou mud volcano approximately reached the triggering threshold of 0.1 bar.

5. Discussion

This article is protected by copyright. All rights reserved. 5.1 Influences of earthquake intensity on the abnormal eruption of mud volcanoes The magnitude of an earthquake and the distance between a mud volcano and an earthquake’s epicenter have an influence on the triggering or regulation of a mud volcano. Studies by Wang et al. (2006) and Manga et al. (2009) showed that it is possible to trigger the eruption of mud volcanoes when the magnitude and epicentral distance satisfy certain empirical formulas. Using the focal mechanism solution data of CENC, we plotted the earthquakes that occurred during 1991-2003 with magnitudes of larger than 5 (including 5) in Xinjiang and the M6.3 Xinyuan-Hejing earthquake of June 30, 2012 on the Magnitude-Epicentral distance coordinate (Figure 7). We found that the magnitudes of the eight earthquakes that occurred during 1991-2003 were less than that the Magnitude-Epicentral empirical formulas required, which meant that these earthquakes may not have triggered mud volcano activity. Because mud volcanoes may have anomalous eruptions before or after an earthquake, we calculated the moving averages of the observed data to reflect the dynamic state of the mud volcano's surface (Gao et al., 2008) from 1991 to 2003, where the ‘days of anomalous pulses’ in a month was the average of the abnormal days from the first 3 months to 3 months later; then, we drew the graph of Figure 8. The term ‘anomalous pulse’ means that the mud volcano began to be active with the increase of fluids and gases and that the volume of bubbles began to increase. When the mud volcano was in a relatively quiet period, the fluid level in the observation well was relatively low and stable and was recorded as a smooth curve but not a “pulse curve’. We can see from Figure 8 that the Horgos mud volcano almost remained active during the period 1991 to 2004, although it is obvious that there were two low periods (December 1992 to December 1993 and September 1999 to October 2001). There is a relationship between a mud volcano’s activity and earthquakes. A

This article is protected by copyright. All rights reserved. mud volcano’s active period represents a period of frequent earthquakes. During the period of frequent earthquakes, the slurry of the Horgos mud volcano increased, the liquid surface rose and the bubble content increased. The Horgos mud volcano remained relatively calm after the earthquake of February 3, 1993. In other words, although the seven earthquakes (the Hejing earthquake of June 6, 1991, the Hejing earthquake of March 19, 1995, the Wusu earthquake of May 2, 1995, the Shihezi earthquake of January 9, 1996, the Xinyuan earthquake of June 4, 1997, the Luntai earthquake of September 23,1999 and the Shihezi earthquake of February 14, 2003) did not trigger the eruption of the Horgos mud volcano, the earthquakes did increase the discharge of gases, which may indicate that there was a good correspondence between the moderately strong earthquakes and mud volcano activity The Horgos mud volcano maintained a state of relatively calm after the earthquake of February 3, 1993. The magnitude of this earthquake was 5.4 and the Horgos mud volcano was 190 km from the epicenter, located in the area of “no triggering”. However, compared with the Hejing earthquake of March 19, 1995, the magnitude was larger and the epicentral distance was smaller. Additionally, compared with the Hejing earthquake of June 6, 1991, both earthquakes had similar epicentral distances but larger magnitudes. However, the mud volcano remained relatively calm after the earthquake.Therefore, it appears that the mud volcano was not affected by the magnitude and epicentral distance. According to the eruptive characteristics of mud volcanoes recorded by Du et al. (2013), the Baiyanggou, Aiqigou, Dushanzi and Horgos mud volcanoes showed characteristics responsive to the earthquakes, with increasing amounts of slurry and gas contents. However, one of the Aqigao mud volcanos that had ceased to overflow and gradually dried up still cracked when the M6.6 Xinyuan -Hejing earthquake of June 30, 2012 occurred. One of the mud holes at the Dushanzi Mud volcano also had

This article is protected by copyright. All rights reserved. a reduced amount of slurry, and the hole even cracked after the earthquake. Mud volcano eruptions show higher sensitivity to earthquakes (Manga et al., 2009), and Bonini (2009) showed that the strain induced by seismic waves can only activate mud volcanoes at critical conditions. Moreover, the Horgos mud volcano was already in a relatively static condition when the Hejing earthquake of February 3, 1993 occurred. We hypothesize that the Horgos mud volcano did not change its eruption state after the earthquake because the erupted mud volcano or mud volcano in the critical eruption state was more sensitive to the earthquake and the mud volcano that had weaker activity or had stopped erupting before the earthquake was not sensitive to the earthquake. After an eruption, mud volcanoes need some time to recharge (Mellors et al., 2007; Manga et al., 2009; Bonini, 2009). Mud volcanoes that reach this critical condition will have higher sensitivity to an earthquake (Manga et al 2009; Bonini, 2009). However, the appearance of the two low-value periods in the continuous observations may also mean that the Horgos mud volcano needed some time to recharge after the eruption and may erupt again under certain trigger conditions.

5.2 Influence of static stress caused by earthquakes on the eruption of mud volcanoes The static stress changes caused by earthquakes may have an impact on mud volcano eruptions. Static stress may influence mud volcanic eruptions by expanding or compressing reservoirs or unclamping mud ascent paths (Bonini et al., 2016); when the static stress changes caused by an earthquake reach a certain threshold (0.1 bar), they may affect a mud volcanic eruption. 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. We analyzed four earthquakes with magnitudes greater than 5.0 within 200 km in the southern margin of the Junggar Basin. The results showed that the normal stress caused by the M5.5 Wusu earthquake of May 2, 1995 was less than 0.1 bar or even negative in the Horgos and Dushanzi mud volcano areas. The absolute value of the normal stress caused by the M5.4 Shawan earthquake of January 9, 1996 was extremely small. The absolute value of the normal stress change in the Aqigou and Baiyanggou mud volcanos was less than 0.001 bar, whereas the normal stress changes in the Horgos mud volcano, with an epicentral distance of 46 km, was only slightly larger than 0.001 bar. The normal stress changes caused by the M5.3 Shihezi earthquake of February 14, 2003 were mostly positive. The epicenter was only 58 km away from the Horgos Mud Volcano, but the normal stress change of the Horgos mud volcano was only 0.001 bar, far less than 0.1 bar, and the stress change values of the Aiqigou and Baiyanggou mud volcanoes, whose epicenter distances exceeded 150 km, were even less than 0.0001 bar. The M6.3 Xinyuan-Hejing earthquake of June 30, 2012, which had the highest magnitude and an epicentral distance of less than 100 km except for the Horgos mud volcano, had a larger normal stress change. On the receiving fault, with a strike of 270°, the normal stress of the Dushanzi, Aiqigou and Baiyanggou mud volcanoes were 0.049 bar, 0.095 bar and 0.04 bar, respectively. The Horgos mud volcanoes were in the negative region. Static stress may influence mud volcanic eruptions by expanding or compressing reservoirs or unclamping mud ascent paths (Bonini et al., 2016). It is specified in the

Coulomb 3.3 calculation that a positive change in normal stress (Δσn) means that

"normal stress becomes less compressive", that is, when Δσn> 0, the occurrence of earthquakes may reduce the compressive stress of the ascending channel of fluid and broaden the fluid supply channel with the upwelling of bottom material, such as mud and gases, which leads to a mud volcano eruption.

This article is protected by copyright. All rights reserved. However, the above analysis shows that most of the static stress changes caused by these four earthquakes in the mud volcanoes were extremely small and even negative; the Aiqigou mud volcano, which had a normal stress change near 0.1 bar, did not have a stronger eruption than the Baiyanggou and Dushanzi mud volcanoes. Additionally, the eruption of mud volcanoes is intermittent (Mellors et al., 2007; Gao et al., 2008; Manga et al. 2009; Bonini, 2009), and intermittent eruptions may be affected by the reserves of the underground fluid and the internal stress conditions of mud volcanoes. However, the observational data showed that the Horgos mud volcano increased its activity within a few days to several months after earthquakes, with an increase of mud overflow and bubbles, and the active period of the mud volcano corresponded well with the period of frequent earthquakes(Gao et al., 2008). This shows that static stress is unlikely to play a leading role in triggering the eruption of mud volcanoes. In other words, the static stress change may not be the main mechanism to trigger the eruption of mud volcanoes, but earthquakes still play a decisive role in their eruptions.These results and observations show that static stress caused by the earthquakes we examined is unlikely to play a dominant role in triggering eruptions of the mud volcanoes in this area. We speculate that dynamic stress caused by these earthquakes, which can be much larger in amplitude than static stress, may have played an important role in their eruptions and activities.

5.3 Formation mechanisms of mud volcanoes The diapiric activity of deep-lying overpressured fluid, tectonic stress, and the triggering of earthquakes are considered to be the formation mechanisms of mud volcano eruptions (Manga et al., 2009; Mazzini et al., 2009; Bonini, 2012). Statistics show that the activity of a mud volcano will be enhanced within a few minutes, a few months or even years after an earthquake (Mellors et al., 2007). However, much

This article is protected by copyright. All rights reserved. controversy remains about how earthquakes trigger the eruption of mud volcanoes. Several changes caused by earthquakes, including liquefaction and loss of strength, increases in permeability and the nucleation and growth of bubbles, have been proposed as triggering mechanisms. Earthquakes may affect eruptions by increasing fluid pressure, causing the growth or nucleation of gas bubbles, or creating a hydraulic connection between the source and the surface by breaching the seal of overpressured reservoirs. However, the strain amplitude required for liquefaction is too large to account for the triggering response of remote mud volcanoes, whereas the nucleation or growth of bubbles requires high over-saturation conditions (Manga et al., 2009). Additionally, changes in groundwater levels have also been proposed for association with mud volcanic activity because groundwater levels may be affected by major earthquakes thousands of kilometers away (Roeloffs et al., 2003; Mellors et al., 2007). Dynamic stress may play a major role in triggering or regulating the eruption of a mud volcano (Mellors et al., 2007; Manga et al., 2009; Rudolph et al., 2010). Bonini et al. (2016) analyzed the eruptions of the mud volcanoes in nine regions of the world (including Romania, the Andaman Islands, Taiwan, Pakistan, Northern Italy, Indonesia, Azerbaijan, Southern California and Japan) and found that the dynamic stress caused by earthquakes may be the main triggering mechanism, although the change of static stress acting on the feeder dikes may have also played a role. Increases in mud volcanic activity in some areas in Romania, Taiwan and Italy were accompanied by changes of static stress and the unclamping of feeder dikes. However, eruptions of mud volcanoes in the Andaman Islands and Niikappu in Japan were related to the dynamic stresses produced by seismic waves. Moreover, dynamic stress may influence eruptions by changing permeability and nucleating or mobilizing magma (Bonini et al., 2016). Rudolph and Manga (2010)

This article is protected by copyright. All rights reserved. measured the discharge of gas at a field of mud volcanoes near the Salton Sea, Southern California, before and after the 4 April 2010 El Mayor-Cucapah earthquake and observed an increase in gas flux immediately following the earthquake and a subsequent recovery to pre-earthquake values. We thus favor the explanation that the increased gas flux was caused by a transient increase in permeability. Dynamic stress, which is caused by the passage of seismic waves, is a non-permanent stress. Although it can affect the subsurface structures within a range of fault lengths (Bonini et al., 2016), the dynamic stress will quickly disappear when the seismic waves pass. However, earthquakes can not only influence the activity of a mud volcano within a few minutes (for example, short-term triggers such as the Vrancea earthquake in Romania in 1977, the earthquake sequence in the Emilia region of Northern Italy in 2012 and the eruption of the LUSI Mud Volcano in Indonesia in March 2006, among others; Bonini et al., 2016) but can also affect the activity during months or even years after an earthquake (such as mud volcano eruptions caused by the 2000 Baku earthquake sequence; Mellors et al., 2007). For coseismic mud volcanic eruptions, the dynamic stress produced by the passage of seismic waves may be its triggering mechanism, whereas for delayed response mud volcanoes, dynamic stress may play an important role in the process of accumulating the energy of mud volcanoes. But both static stress and dynamic stress change may play a role in the eruption of mud volcanoes with delayed responses. Based on the above analysis, we believe that the earthquake triggering mechanisms of mud volcanoes cannot be simple. An earthquake may cause the eruption of mud volcanoes or regulate mud volcanoes that have erupted in a specific case. The southern margin of the Junggar Basin in Xinjiang was affected by the compressional stress in the north Tianshan Mountains and developed a fold-thrust belt (Yu et al., 2009). In other words, the Horgos, Dushanzi, Aiqigou and Baiyanggou mud

This article is protected by copyright. All rights reserved. volcanoes are located in a compressive environment, whereas the Junggar Basin is a complex superimposed basin dominated by Late Paleozoic and Meso-Cenozoic continental facies (Bai, 2008). We consider the following factors: (1) A mud volcano that has a short-term response to an earthquake is in a state of critical eruption before an earthquake. The dynamic stress changes caused by the occurrence of the earthquake and the passage of the seismic waves make the fluid in an overpressure environment upwell because of the increase of permeability that exists above the mud source and then reaches the surface to form a mud volcano. (2) Mud volcanoes with long-term responses to earthquakes have not reached the critical eruption state before the earthquakes. The passage of seismic waves can promote the process of accumulating energy inside the mud volcanoes. After the volcanoes have accumulated energy for some time, the internal fluid pressure increases and the fluid reserve recharges or increases to meet the critical-state requirements of eruptions. In the meantime, because of the static stress acting on feeder dikes, mud ascent paths are unclamped, feeder dikes are broadened, and deep fluid rises along the paths to the surface to form a mud volcano (Figure 9). Bonini et al. (2016) also showed that the increased eruptions of mud volcanoes in Taiwan, Romania and Italy with short-term responses to earthquakes were accompanied by the increase in static stress and may have been affected by the change of static stress acting on the feeder dikes; dynamic stress may have also played an important role. We know that earthquakes may trigger the eruption of mud volcanoes or regulate mud volcano activity after eruption and that the abnormal changes in mud volcanoes may also predict the occurrence of earthquakes. However, because of the diversity of the factors that affect the earthquake response of mud volcanoes and the limitations of the above analysis, a clearer understanding of the correlation between earthquakes and the eruptions of mud volcanoes requires more systematic and

This article is protected by copyright. All rights reserved. comprehensive research and exploration. This may help to deepen the understanding of mud volcanoes, lay the foundation for the exploration and development of petroleum and natural gas hydrates associated with mud volcanoes and provide more accurate information for earthquake prediction.

6. Conclusions Based on the quantitative simulation of the Coulomb stress of typical earthquakes in mud volcanoes developed in the southern margin of the Junggar Basin and observational data on the eruption of mud volcanoes, we analyzed their formation mechanism. There was a good correspondence between mud volcanic activity and earthquakes, and the magnitude and epicenter distance of earthquakes were the key factors that affected the eruption of mud volcanoes. Static stress may not be the main trigger mechanism for the mud volcanoes. The results of Coulomb stress simulation showed that the normal stress change (Δσn) of mud volcanoes does not reach the threshold of mud volcano eruptions. The earthquake triggering mechanism of mud volcanoes is not simple; an earthquake may cause the eruption of a mud volcano or regulate mud volcanoes that have erupted in specific cases. Moreover, dynamic stress may play a major role in triggering a mud volcano or in regulating the eruption of a mud volcano.

Acknowledgments

This work was supported by National Nature Science Foundation of China (No. 41776056), Science and Technology Program of Guangzhou (No. 201607010214), the Fundamental Research Funds for the Central Universities (No. 17lgjc10), Geology Investigation Project of China Geological Survey (No. DD20160344, HD-JJHT-2017-100). B. Duan acknowledges support from NSF (grants 1254573 and 1524743).

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Table 1. The location of mud volcanoes in the southern margin of the Junggar Basin. HEGS means the Horgos mud volcanoes, DSZ means the Dushanzi mud volcanoes, AQG means the Aiqigou mud volcanoes and BYG means the Baiyanggou mud volcanoes. This table includes the latitude and longitude coordinates, vent sizes and characteristics of the four mud volcanoes (Wan et al.,

2013).

Table 2. Focal mechanism solutions of 4 typical earthquakes in mud volcano areas on the southern margin of the Junggar Basin.

Table 3. The abnormal ‘pulse’ days of slurry levels from 1991 to 2003 in the Horgos mud volcano

(Gao et al., 2008).

Table 4. The normal stress changes in mud volcano areas; ‘Distance’ is the distance from the mud volcano to the epicenter of the earthquake.

Table 5. The normal stress changes in mud volcano areas caused by the Shawan earthquake.

Table 6. The normal stress changes in mud volcano areas caused by the Shihezi earthquake.

Table 7. The normal stress changes in mud volcano areas caused by the Xinyuan-Hejing earthquake.

Figure 1. (a) The tectonic position of the mud volcanoes developed area, which is the Piedmont tectonic belt of the southern margin of Junggar Basin. Modified from Wan et al. (2015).The red triangles represent mud volcanoes (1:Horgos mud volcano; 2 : Dushanzi mud volcano; 3 : Aiqigou mud volcano; 4 : Baiyanggou mud volcano). (b) Geographic location of mud volcanoes and earthquakes in the southern margin of the Junggar basin and Northern Tianshan (the topographic map is from Google Earth). Red triangles represent mud volcanoes, stars represent the location of earthquakes, in which red stars represent earthquakes that we used to calculate normal stress changes in part 4 (1:M5.5 Wusu earthquake of May 5, 1995; M5.4 Shawan earthquake of January

9, 1996; M5.3 Shihezi earthquake of February 14, 2003; M6.3 Xinyuan-Hejing earthquake of June

30, 2012).

Figure 2. (a) The twin mud volcanoes in Aiqigou, (b) The vent photo of mud volcano in

Baiyanggou, (c) The vent photo of mud volcano in Dushanzi.

Figure 3 Normal stress changes caused by the M5.5 Wusu earthquake of May 2, 1995. Red triangles represent mud volcanoes. (a) The receiving fault has a strike of 180° and Δσn less than

0.1 bar in the four mud volcano areas. The Horgos mud volcano is 94 km from the epicenter, with small and negative values of normal stress changes. (b) The receiving fault has a strike of 270°, and Δσn is less than 0.1 bar in the four mud volcano areas.

Figure 4 Normal stress changes caused by the M5.4 Shawan earthquake of January 9, 1996. (a)

The receiving fault has a strike of 180° and Δσn is less than 0.1 bar in the four mud volcano areas.

The Horgos mud volcano is 46 km from the epicenter and has the largest normal stress change. (b)

The receiving fault has a strike of 270°, and Δσn is less than 0.1 bar in the four mud volcano areas.

Figure 5 Normal stress changes caused by the M5.3 Shihezi earthquake of February 14, 2003. (a)

The receiving fault has a strike of 180°, and Δσn is far less than 0.1 bar in the four mud volcano areas. The Horgos mud volcano is 58 km from the epicenter and has the largest normal stress change of only 0.001 bar. (b) The receiving fault has a strike of 270°, and Δσn is far less than 0.1 bar in the four mud volcano areas.

Figure 6 Normal stress changes caused by the M6.3 Xinyuan-Hejing earthquake of June 30, 2012.

Red triangles represent the locations of mud volcanoes. It can be seen from the figure that as the epicentral distance increases, the changes of normal stress significantly decrease, and the Aiqigou mud volcano is the nearest to the epicenter (fault ruptured center). (a) The receiving fault has a strike of 180°, and Δσn values are negative in BYG, AQG and DSZ. (b) The receiving fault has a strike of 210°, and Δσn values are close to 0.1 bar in AQG and BYG but are negative in HEGS and

DSZ. (c) The receiving fault has a strike of 270° and has good symmetry of the normal stress changes, and Δσn in AQG is larger than in DSZ and BYG but has a negative value in HEGS. (d)

The receiving fault has a strike of 300°, Δσn in these mud volcano areas are negative and HEGS also has a negative value of normal stress.

Figure 7 The horizontal axis represents the magnitude of the earthquake, the ordinate indicates the epicentral distance between the mud volcano and the epicenter of the earthquake. The data shown in green rectangles come from Manga et al. (2009). Purple triangles indicate the Horgos, Dushanzi,

Aiqigou and Baiyanggou mud volcanoes, which are related to the M6.3 Xinyuan-Hejing earthquake of June 30, 2012. Orange diamonds represent the Horgos mud volcano associated with the eight moderate-strong earthquakes that occurred from 1991-2003. Solid and dashed lines indicate the empirical relationships between magnitude and epicentral distance, from Wang et al.

(2006).

Figure 8 The total days of abnormal eruption in each month from 1991 to 2003 in the Horgos mud volcano. When the activity of the mud volcano increased, the amount of inrush slurry increased, and the rate of bubble discharge increased with the formation of the original water level “pulse”.

The pulse was counted with the day as the smallest unit and the total days in a month were then calculated; the monthly abnormal value was then obtained, as represented by the vertical axis. The horizontal axis represents the time of recorded abnormal days from April 1991 to September 2003.

Figure 9 Internal structure model of mud volcano on the southern margin of Junggar Basin(Take

Dushanzi mud volcano as an example),referenced from Wan et al. (2015) and Xia et al. (2008).

Plastic source containing saturated slurry and gases may come from Jurassic Period (Yang et al.,

2014).Gray arrows on the right picture represent migratory direction of fluid and green arrows means “Horizontal compressive stress” on the feeder dike, which can be roughly expressed the direction of static stress. As for dynamic stress, it may play a role in changing the physical properties (e.g. the increase of permeability) of the channels and its surrounding rock (Harris,

1998; Kilb et al., 2000). And the passage of seismic waves which is the cause of dynamic stress may promote the process of accumulating energy inside the mud volcanoes.

(a)

9, 1996; M5.3 Shihezi earthquake of February 14, 2003; M6.3 Xinyuan-Hejing earthquake of June

30, 2012).

Figure 2. (a) The twin mud volcanoes in Aiqigou, (b) The vent photo of mud volcano in

Baiyanggou, (c) The vent photo of mud volcano in Dushanzi.

(a)

(b)

Figure 3 Normal stress changes caused by the M5.5 Wusu earthquake of May 2, 1995. Red triangles represent mud volcanoes. (a) The receiving fault has a strike of 180° and Δσn less than 0.1 bar in the four mud volcano areas. The Horgos mud volcano is 94 km from the epicenter, with

This article is protected by copyright. All rights reserved. small and negative values of normal stress changes. (b) The receiving fault has a strike of 270°, and Δσn is less than 0.1 bar in the four mud volcano areas.

(a)

Figure 4 Normal stress changes caused by the M5.4 Shawan earthquake of January 9, 1996. (a)

The receiving fault has a strike of 180° and Δσn is less than 0.1 bar in the four mud volcano areas.

The Horgos mud volcano is 46 km from the epicenter and has the largest normal stress change. (b)

The receiving fault has a strike of 270°, and Δσn is less than 0.1 bar in the four mud volcano areas.

(a)

(b)

Figure 5 Normal stress changes caused by the M5.3 Shihezi earthquake of February 14, 2003. (a)

(a)

(b)

(c)

(d)

Figure 6 Normal stress changes caused by the M6.3 Xinyuan-Hejing earthquake of June 30, 2012.

Red triangles represent the locations of mud volcanoes. It can be seen from the figure that as the

This article is protected by copyright. All rights reserved. epicentral distance increases, the changes of normal stress significantly decrease, and the Aiqigou mud volcano is the nearest to the epicenter (fault ruptured center). (a) The receiving fault has a strike of 180°, and Δσn values are negative in BYG, AQG and DSZ. (b) The receiving fault has a strike of 210°, and Δσn values are close to 0.1 bar in AQG and BYG but are negative in HEGS and

DSZ. (c) The receiving fault has a strike of 270° and has good symmetry of the normal stress changes, and Δσn in AQG is larger than in DSZ and BYG but has a negative value in HEGS. (d)

The receiving fault has a strike of 300°, Δσn in these mud volcano areas are negative and HEGS also has a negative value of normal stress.

Figure 9 Internal structure model of mud volcano on the southern margin of Junggar Basin(Take

Dushanzi mud volcano as an example),referenced from Wan et al. (2015) and Xia et al. (2008).

Plastic source containing saturated slurry and gases may come from Jurassic Period (Yang et al.,

1998; Kilb et al., 2000). And the passage of seismic waves which is the cause of dynamic stress may promote the process of accumulating energy inside the mud volcanoes.

2013).

Latitude Longitude Vent size Eruptive characteristics (N) (E) (Diameter)

The bubbles have different size and rate at different position, about 40-70 times per BYG 44°10′ 84°23′ 3.5 m~0.8m minute. A large gas bubble can be accompanied with 3-5 small bubbles,the maximum diameter is about 15cm.

Bubbling points mainly located at the south AQG 44°11′ 84°29′ 5 m of the vent has a diameter of about 16 cm.

One is There are two bubbling points, but only one 0.08 m and of them has the diameter of 5cm which DSZ 44°18′ 84°50′ the other is erupts intensely with the rate up to 70 times 1m per minute.

There are two bubbling points, but the mud HEGS 44°18′ 85°45′ 1m never overflow.

Table 2. Focal mechanism solutions of 4 typical earthquakes in mud volcano areas on the southern margin of the Junggar Basin.

Lat. Lon. Strike Dip Rake Depth Time Epicenter M (N) (E) (°) (°) (°) (km）

1995.05.02 Wusu 43.776 84.66 198 82 33 5.5 42

258 70 79 1996.01.09 Shawan 43.697 85.647 5.4 33 108 23 118

283 62 105 2003.02.14 Shihezi 43.897 85.923 5.3 23.9 73 32 64

Xinyuan- 300 57 167 2012.06.30 43.433 84.7 6.3 18 Hejing 37 79 34

Table 3. The abnormal ‘pulse’ days of slurry levels from 1991 to 2003 in the Horgos mud volcano

(Gao et al., 2008).

Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sept. Oct. Nov. Dec.

1991 0 6 13 9 9 23 18 13 16 15 12 18

1992 7 29 13 16 31 23 13 15 0 0 0 0

1993 0 0 0 1 0 0 1 0 0 0 0 0

1994 5 0 0 7 12 11 7 0 4 16 6 18

1995 4 4 0 9 18 5 11 11 8 5 17 8

1996 0 14 22 24 9 5 25 9 11 17 22 9

1997 12 15 1 4 2 12 22 11 4 2 6 1

1998 3 0 0 15 0 1 14 0 0 0 0 0

1999 0 2 1 0 6 10 0 0 0 0 0 0

2000 0 0 0 0 0 0 0 0 0 0 0 0

2001 0 0 0 0 0 0 2 0 1 0 0 0

2002 0 2 15 25 6 11 9 8 5 6 9 7

2003 3 7 12 2 0 0 0 0 0 2 0 0

Table 4. The normal stress changes in mud volcano areas; ‘Distance’ is the distance from the mud volcano to the epicenter of the earthquake.

normal stress changes（bar） Distance Location strike180, strike210, strike270, strike300, /km dip90 dip90 dip90 dip90

HEGS 94 -0.002 -0.001 -0.0006 -0.001

DSZ 49 0.001 -0.001 -0.005 -0.003

AQG 39 0.009 0.023 0.024 0.01

BYG 47 0.01 0.015 0.006 0.001

Table 5. The normal stress changes in mud volcano areas caused by the Shawan earthquake.

normal stress changes（bar） Distance Location strike180, strike210, strike270, strike300, /km dip90 dip90 dip90 dip90

HEGS 46 0.001 0.004 0.003 -0.00005

DSZ 102 0.00004 0.00002 -0.0002 -0.00014

AQG 133 -0.00004 -0.0005 -0.00008 -0.00007

BYG 148 -0.00005 -0.00005 -0.00007 -0.00005

Table 6. The normal stress changes in mud volcano areas caused by the Shihezi earthquake.

Location Distance normal stress changes（bar）

dip90 dip90 dip90 dip90

HEGS 58 0.001 0.0005 0.001 0.001

DSZ 124 0.00005 0.00003 0.00007 0.00009

AQG 158 0.000006 -0.000002 0.00003 0.00004

BYG 175 0.000002 -0.000003 0.00003 0.00003

Table7. The normal stress changes in mud volcano areas caused by the Xinyuan-Hejing earthquake.

normal stress changes（bar） Distance Location Strike180, Strike210, Strike270, Strike300, /km dip90 dip90 dip90 dip90

HEGS 103 -0.007 -0.001 -0.012 -0.018

DSZ 75 0.008 -0.002 0.049 0.059

AQG 67 0.02 0.078 0.095 0.037

BYG 72 0.034 0.071 0.04 0.004