Earthquake Reaearch in China  2018, Vol. 32 Issue (4): 501-509
Anomaly Feature Extraction of the Gravity Field before the Jiuzhaigou Earthquake in 2017
Liu Fang1, Zhu Yiqing1, Zhao Yunfeng1, Liu Tao2
1. The Second Monitoring and Application Center, CEA, Xi'an 710054, China;
2. Shaanxi Yan Chang Petroleum(Group) Co., Ltd, Xi'an 710075, China
Abstract: The MS7.0 Jiuzhaigou earthquake occurred on August 8, 2017. The earthquake occurred in the vicinity of the Tazang fault, the Minjiang fault and the Huya fault, where the focal mechanism is of the strike slip type. The static and dynamic anomalies of the gravity field can provide important physical field information for studying the structural properties of deep crust. Multi-scale decomposition techniques are used to separate Bouguer gravity at different depths and give some explanation to gravity variations at different time space scales.The results indicate that the wavelet multi-scale results of the EGM2008 model and the measured gravity data are consistent. Through comparative analysis, it is found that the Jiuzhaigou earthquake occurred in the stress enhanced region. The variation of gravity field at different time scales has a certain scientific significance for further understanding potential earthquake risk trend.
Key words: Jiuzhaigou MS7.0 earthquake     Gravitational field     Wavelet decomposition     Multi-scale

INTRODUCTION

On August 8, 2017, a MS7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province (33.20°N, 103.82°E), at a depth of about 20km. The focal mechanism shows that it belongs to strike slip type (Yi Guixi et al., 2017), and the coseismic dislocation demonstrates mainly left-lateral strike-slip movement (Ji Lingyun et al., 2017). The epicenter of this earthquake is located near the Minjiang fault, Tazang fault and Huya fault, in the eastern margin of the Bayan Har block (Xu Xiwei et al., 2017). The Jiuzhaigou earthquake is another major earthquake that occurred around the Bayan Har block after the MS8.1 earthquake in the west Kunlun Mountain pass region, the Wenchuan MS8.0 earthquake and the Yushu MS7.1 earthquake.

Before the Jiuzhaigou earthquake, the China Earthquake Administration carried out multiple sections of mobile gravity observations along the North-South Seismic Belt, and observed obvious gravity anomalies near the epicenter. Zhu Yiqing et al. (2017) believed that the gravity variation in Sichuan, Gansu and Qinghai Provinces was severe and showed characteristics of four-quadrant distribution, with gravity differences of more than 100×10-8m ·s-2, and there is the possibility of M6.0-7.0 earthquakes in Diebu, Maqu and Xiahe of Gansu Province, Henan, Maqên and Tongde of Qinghai Province and Zoigê, Jiuzhaigou of Sichuan Province. In recent years, existing research results have shown that crustal deformation and changes in properties of the medium at the source region in the preparation process of an earthquake will cause changes in the gravity field around the earthquake zone, thus the anomaly of the gravity field is one of the reliable precursory physical parameters (Gu Gongxu et al., 1998; Zhu Yiqing et al., 2010a; Li Hui et al., 2011; Chen Shi, 2016). Bougu'er gravity anomaly is one of the important pieces of geophysical data for studying lithosphere and geological structures and can reflect obvious gravitational effects of lateral inhomogeneity in the distribution of material in the shallow and deep crust, which is related to the density anomaly in lateral density zone of crust materials. When the gravity anomaly is used to identify normal and abnormal states of gravity field variations, it is necessary to not only see the degree of inhomogeneity reflected in the pattern of gravity change, but also to analyze the evolution process and change trend of gravity change, and to pay attention to the relationship between gravity field changes and the background field of Bouguer gravity anomalies (Shen Chongyang et al., 2007; Zhu Yiqing et al., 2012). It is therefore of great significance to systematically and deeply analyze Bouguer gravity anomalies and time-varying gravity field data along the North-South Seismic Belt and study the gravity change before the Jiuzhaigou earthquake, so as to understand the variation of deep crustal environment and study the formation mechanism of the earthquake.

1 DATA AND METHODS

Gravity anomally is an abnormal response to the deviation of gravity values from normal gravity values caused by uneven distribution of crustal mass, which can directly reflect the distribution of geologic bodies, deep structure of the crust and the distribution of fault structures. The gravity field however, is a superimposed field which incorporates field source information from different layers of lithosphere. For example, regional field signals are superimposed with local field signals and deep field signals are superimposed with shallow field. Therefore, when using gravity data to study deep structure, effective separation of field source information is crucial (Zeng Hualin, 2005). Studies have shown that the rapidly developed wavelet transform method has become an important technological means for the decomposition of gravity anomalies. This method can divide gravity field anomaly into different components in the geometric sense, so as to extract local anomaly information of different scales in the object of study and achieve the purpose of potential field separation. Mallat S.G., (1989) first proposed the pyramid algorithm of wavelet multi-scale analysis. Hou Zunze et al. (1997) expounded the principles of wavelet transform and multi-scale analysis, and applied it to the study of Bouguer gravity anomalies in the Chinese mainland. Yang Wencai et al. (2001) discussed the problem that should be paid attention to in discrete wavelet transform and the multiple decomposition of gravity anomalies. In addition, many researchers have made some progress in the study of geological structure of gravity fields by using the wavelet transform method (Gao Dezhang et al., 2000; Fang Shengming et al., 2001; Jiang Wenliang et al., 2010; Li Dahu et al., 2014; Tan Hongbo et al., 2017). Therefore, when using gravity data to study tectonic characteristics, appropriate and effective data processing methods should be adopted to separate the gravity field anomaly information to extract information related to the research object and then analyze the local field anomaly caused by the field source.

According to the principle of wavelet multi-scale decomposition, the gravity anomaly can be decomposed as (Yang Wencai et al., 2001)

 $\Delta g\left({x, y} \right) = {A_i} + {D_i} + {D_i}_{ - 1} + \cdots + {D_1}$ (1)

where, Ai is the ith-order approximation of the gravity anomaly (i is an integer no less than 2), that is, the low-frequency components of the gravity anomaly; Di(i=1, 2, …i) is the detail of wavelet of each order obtained after the ith decomposition, that is, the high frequecy components of gravity anomaly. In order to effectively separate the gravity anomaly generated by the deep anomalous body from the total anomaly, "bior3.5" (Diao Bo et al., 2007), a biothogonal wavelet basis function of the two-dimensional gravity anomaly decomposition is used in Matlab to conduct fourth-order-scale wavelet decomposition of the Bouguer gravity anomaly and dynamic varation anomaly of the time-varying gravity field.

2 STUDY ON CHARACTERISTICS OF REGIONAL BOUGUER GRAVITY ANOMALY 2.1 Characteristics of Regional Bouguer Gravity Anomaly

The Bouguer gravity anomaly is the gravity anomaly presented by rock masses of different densities in the earth's crust, which can reflect directly the characteristics of deep structures and distribution of fault structures. EGM2008 is a global gravity field model with the highest precision and spatial resolution, which can provide spherical harmonic coefficients for up to 2160 orders of gravity anomaly field (Pavlis N. K. et al., 2008, 2012). The data can be downloaded from the world gravity map website (WGM2), which provides free-air gravity anomaly and Bouguer gravity anomaly data. The EGM2008 model has the overall accuracy for spatial anomalies in Chinese mainland of 10.5mGal (Zhang Chuanyin et al., 2009), and on the basis of free-air gravity anomaly, plate correction, correction for earth's curvature and terrain correction are respectively made to get 2.5'×2.5' regional Bouguer gravity anomaly (Fig. 1). It can be seen from Fig. 1 that the regional Bouguer gravity anomaly as a whole is negative, -560mGal- -80mGal, which is low in the northwest region and relatively higher in the southeast region and gradually increases from west to east. Among them, the Bouguer gravity anomaly is -560mGal- -280mGal in the Western Sichuan Plateau, -200mGal- -120mGal along Longmenshan fault zone and about -80mGal in the Sichuan Basin. It can be seen that the Bouguer gravity anomaly high gradient zone is formed along the vicinity of Longmenshan fault zone, indicating great density difference between the Western Sichuan Plateau and Sichuan Basin. Large earthquakes often occur at the junction of the gravity anomaly zone of abrupt changes and active fault zone (Zhou Zhipeng et al., 2014). The Jiuzhaigou earthquake occurred in the transition zone between the Bayan Har block and Sichuan Basin, within the gravity anomaly zone of abrupt gravity changes near the junction of Tazang fault zone and Minjiang fault zone.

 Fig. 1 The Bouguer gravity anomaly and the distribution of major faults in the study area
2.2 Multi-scale Wavelet Decomposition of Bouguer Gravity Anomaly

The Bouguer gravity anomaly incorporates effects of gravity caused by all uneven density from deep earth to the ground surface. In order to analyze deep crustal structure characteristics for the Jiuzhaigou MS7.0 earthquake from varying scales and depths, multi-scale wavelet decomposition method is used in this study to decompose the Bouguer gravity anomaly, and it is found that the fourth-order wavelet approximation presents characteristics of a smooth regional field, so the fourth-order wavelet multi-scale decomposition is selected for field separation. Fig. 2 is a detailed drawing of 1st-4th order wavelet for Bouguer gravity anomaly. It can be seen from Fig. 2(a) that the details of the first-order wavelet show little change, within the range of ±10×10-8 m·s-2, and there is no obvious law of change. The overall range of the contour trap is small, indicating a small-scale gravity change, which mainly reflects the distribution of density inhomogeneous bodies in the shallow surface. There is a significant difference of positive and negative gravity anomalies on both sides of Tazang fault in the southeast of Jiuzhaigou, and in the meantime, there are regional gravity anomalies along the Xianshuihe fault zone. It can be seen from the details of the 2nd-order wavelet in Fig. 2(b) that the variation within the whole study area remains still around ±10×10-8m ·s-2, and on both sides of Barkam fault zone, centering on Xiaojin region, four quadrants of gravity change are presented. High gradient belts of gravity change have been formed on the southwest side of the Huya fault, as well as in the middle segment that is almost perpendicular to the Longmenshan fault zone in the Wenchuan region. It can be seen from the details of the 3rd-order wavelet in Fig. 2(c) that the overall range of isoline traps becomes larger, which highlights local density changes of deep materials, and many smaller details concatenate into larger anomalies. Near Jiuzhaigou, a high gradient belt of gravity change is formed along Minjiang fault zone, obvious four-quadrant characteristics of gravity change are presented between Wenchuan and Chengdu, and there are also significant density variations on both sides of Xianshuihe fault zone. It can be seen from the details of the fourth-order wavelet in Fig. 2(d) that a gravity negative anomaly area is formed centering on Songpan within a diameter range of 200km, which is consistent with the results of seismic tomography (Wang Weifeng et al., 2015). It is found that there is a large area of S-wave low velocity region in the middle-lower crust of the southeastern part of Songpan-Garzê block, indicating that it is a deformable area, a high gradient belt of gravity change is formed along Diebu-Bailongjiang fault zone and Tazang fault, and the Jiuzhaigou earthquake occurred in the bend area of this high gradient belt, while the Sichuan Basin presents a positive change of gravity anomaly.

 Fig. 2 Details of 1st-4th order wavelet for Bouguer gravity anomaly
3 CHARACTERISTICS OF DYNAMIC CHANGE OF GRAVITY FIELD MEASURED BY MOBILE GRAVITY SURVEY

In 2010, the China Earthquake Administration started an intensive gravity field change monitoring system for Comprehensive Observation of Geophysical Fields of China-Eastern Margin of Qinghai-Tibetan Plateau, a special scientific earthquake research project. Based on the national gravity basic network, which is taken as the overall framework, the network is used to optimize the existing seismic mobile gravity monitoring network along the North-South Seismic Belt by field and network formation. Gravimetric networks in Yunnan, Sichuan, Gansu, Ningxia, Hexi and Shaanxi are connected together to form a high-resolution monitoring network with a 60km-80km station spacing (Liang Weifeng et al., 2013). Relevant literature has performed research on early data and data after 2014 (Zhu Yiqing et al., 2010b; 2017), while this article mainly analyzes high-precision gravity observation data from 2010-2013. The results of gravity adjustment calculated by using absolute gravity control are good, with average precision of point value less than 10×10-8m ·s-2, reflecting reliable quality of gravity observation data. Fig. 3 shows 3 years of accumulated gravity variation during 2010-2013.

 Fig. 3 3 years of accumulated gravity variation during 2010-2013 Black dots are gravity observation points
3.1 Variation Characteristics of Regional Gravity Field

According to three years of accumulated gravity variation during 2010-2013, the overall variation trend shows changes from west to east and from negative to positive, which is similar to the spatial distribution of Bouguer gravity that also shows trend changes from west to east and from negative to positive. This indicates that the gravity change is affected by the regional stress field and controlled by activities of deep-seated large faults, and a large-scale high gradient belt of gravity change appears along the Tongren-Luqu-Maqu-Zoigê-Barkam region. In the meantime, centering on Songpan, gravity change shows four-quadrant variation features, and the gravity variation is greater than 100μGal. The Jiuzhaigou earthquake occurred near the center of the four quadrant of gravity change, which is in the bend area of the high gradient belt of gravity change along the Luqu-Jiuzhaigou-Wudu region, indicating that hazardous locations for strong earthquakes are related to the four-quadrant pattern, high gradient belt of regional gravity field and its turning and intersecting parts (Li Hui et al., 2011; Zhu Yiqing et al., 2015; Chen Shi et al., 2016).

3.2 Multi-scale Wavelet Decomposition for Regional Gravity Field Change

Multi-scale decomposition of the 1st-4th order wavelets for 3 years of accumulated gravity variation is performed. Fig. 4 (a) shows the 4th-order wavelet approximation of the Bouguer gravity anomaly, and Fig. 4 (b) shows the 4th-order wavelet approximation of accumulated gravity change. The 4th-order wavelet approximation mainly reflects the gravity effect caused by materials the of lower crust and upper mantle. As can be seen from Fig. 4, the characteristics of anomaly partition are very obvious, and the anomaly gradually increases from west to east in the region. The Sichuan Basin has high gravity anomaly values in both maps, the Songpan-Garzê block has low gravity anomaly values, and the strikes of the dividing lines for high and low values are pretty much the same. The geometric shape of the regional gravity field's anomalous changes is so closely related to the spatial distribution of the Bouguer gravity anomaly, which further proves the existence of exchange and dynamic action of material and energy during material migration in deep crust and mantle in the Western Sichuan Plateau and its vicinity (Teng Jiwen et al., 2008), indicating that the eastward motion of Bayan Har block is blocked by the hard Sichuan Basin, and the Jiuzhaigou earthquake occurred in the transition zone, which may be related to the material migration in the crust of this region.

 Fig. 4 Map of 4th-order wavelet approximation (a) The Bouguer gravity anomaly; (b) Accumulated gravity change
4 CONCLUSION AND DISCUSSION

Based on the wavelet multi-scale decomposition method, static gravity field (Bouguer gravity anomaly) and dynamic gravity field (mobile gravity) are separated in this paper to further analyze density change of regional deep crustal material before the Jiuzhaigou MS7.0 earthquake on August 8, 2017, and the following conclusions were reached.

(1) The Jiuzhaigou earthquake occurred in the transition zone between the Bayan Har block and the Sichuan Basin, and it may be related to the migration of material within the crust of this region.

(2) The wavelet multi-scale method is used to separate Bouguer gravity anomaly and accumulated gravity variation, and from the details of the 1st-4th order wavelet, it can be seen that the distribution law becomes more obvious with the increase of the order of the wavelet. The 4th-order wavelet approximation reflects regional anomaly in this region. Therefore, it is considered that the wavelet multi-scale analysis method can be used to study gravity field.

(3) The static and dynamic anomalies of the gravity field can provide important physical field information for the study of properties of deep crustal structures. It is found that the Jiuzhaigou earthquake occurred at the center of the four-quadrant distribution of the regional gravity field, and it has been shown by some earthquake examples that earthquakes occur in the process of gravity anomalous change which shows significant four-quadrant distribution characteristics (Zhu Yiqing et al., 2014). Gravity tectonics believes that gravity is the main driving force for tectonic movement of the earth, and the gravity field contains tectonic force information. The analysis of its variation characteristics can provide a certain basis for the study of regional crustal dynamics and the seismic environment, which is of certain scientific significance for further understanding potential seismic risks.

ACKNOWLEDGEMENTS

Grateful acknowledgements to all of the staff involved in the gravity survey. Many thanks to anonymous reviewers for their beneficial and constructive comments on this article.

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2017年九寨沟地震前重力场异常特征提取

1. 中国地震局第二监测中心，西安市西影路316号 710054;
2. 陕西延长石油（集团）有限责任公司研究院，西安 710075