Earthquake Reaearch in China  2017, Vol. 31 Issue (2): 169-178
The North-South Seismic Belt: Vertical Deformation Velocity Gradient Research
Liu Liwei, Ji Lingyun, Zhao Qiang     
The Second Monitoring and Application Center, CEA, Xian 710054, China
Abstract: The vertical deformation gradient can reflect the rate of vertical change in unit distance, and the vertical deformation velocity gradient can reflect the strength of the earths crust tectonic activities. In this paper, using long period leveling data combined with GPS data, the vertical deformation gradient values are calculated. Leveling data and GPS data are two different means of monitoring deformation, but the result is approximately the same vertical deformation gradient. The results show that the spatial distribution of the vertical deformation velocity gradient and tectonic distribution has an obvious correlation. The most significant gradient anomalies along the North-South Seismic Belt are Xianshuihe fault, Longmenshan fault and Xiaojiang-Zemuhe fault, while the second gradient anomalies in the northeastern Qinghai-Tibetan plateau are Zhuanglanghe fault and Lenglongling fault. The Menyuan MS6.4 earthquake in 2016 occurred in this abnormal area. However, according to the vertical deformation high gradient area distribution, there is also the possibility of an earthquake occurrence in the Tianzhu and Jingtai area. The area of convergence of three major fault zones is the strongest tectonically active region of the North-South Seismic Belt.
Key words: Gradient     North-South     Seismic Belt     Vertical deformation     Strong Earthquake activty    

INTRODUCTION

The North-South Seismic Belt and its surrounding areas, bordered between the east and west tectonic systems in the Chinese mainland, characterized by significant relative movement and deformation of tectonic active blocks and frequent strong earthquakes, are sensitive to overall tectonic dynamic environment and stress field changes (Ding Guoyu et al., 1993). Therefore, the North-South Seismic Belt has become an area most frequently studied for the relationship between precursor anomalies and earthquake occurrence, and also a key area for earthquake prediction and risk tracking. Crustal deformation monitoring and research are effective means of earthquake prediction. Currently, crustal vertical movement velocity obtained by calculation with local leveling data and GPS data has only relative significance, because this velocity is always by reference to a standard velocity. For the selection of this standard velocity, although we can make assumptions based on prior knowledge to make it closer to actual conditions, this kind of standard is presently subjective due to lack of constraints from external conditions. Thus, the velocity value itself can hardly be used as an objective criterion to measure crustal movement. Therefore in the study of crustal movement and seismic activities, the change of velocity (in direction and size) might be more meaningful than the value itself. In the study of the relationship between crustal vertical movement and earthquake activities, its generally considered that earthquakes are likely to occur along deformation gradient zones, especially in the intersection of active tectonics, the transitional zone of the deformation gradient belt and areas with strong crustal differential movement. The deformation gradient is the ratio of the difference of vertical deformation between two adjacent points to the distance between the two points, reflecting the degree of deformation changes in unit distance, therefore, the gradient of vertical deformation velocity can reflect the intensity of crustal tectonic activities (Guo Liangqian et al., 2007; Zhang Yingzhen et al., 1992; Zhang Jing et al., 2013).

The vertical deformation gradient is directional, and gradient values vary in different directions at the same point. When calculating the gradient value at each point, we only take into account the larger gradient value, ignoring its direction (Zhang Yingzhen et al., 1992). Some achievements have been made on the research of strong earthquake activities in different tectonic regions in the Chinese mainland based on vertical deformation gradient results in different temporal and spatial scales. Zhang Yingzhen et al. (1992) and Guo Liangqian et al.(2007) respectively studied the relationship between high-gradient deformation belt (zone) and spatial location and tectonic conditions of an earthquake on the basis two-period vertical deformation velocity in the Chinese mainland, using the vertical deformation velocity gradient method. For shallow-focus earthquakes with M≥7.0 occurred in the Chinese mainland from 1965 to 2005, in areas controlled by the leveling network, the occurrence of all earthquakes is associated with the high-gradient belt (zone) except for the Yongshan earthquake, which occurred in a relatively low-gradient zone, indicating that the high-gradient belt of vertical deformation velocity has a good control effect on modern earthquakes (Zhang Yingzhen et al., 1992).

The vertical deformation velocity gradient can reflect the spatial-temporal range of crustal tectonic activities, depending on spatial-temporal range of the observation data. Because there has been no large scale level monitoring data in recent years, we cannot get the latest achievement of gradient of vertical deformation velocity. The currently available result of the vertical deformation velocity gradient obtained from regional leveling data is calculated by the use of the national leveling velocity results from the periods 1951-1982 and 1951-1990 (Zhang Yingzhen et al., 1992; Zhang Zusheng et al., 1996; Guo Liangqian et al., 2001), and the leveling deformation data utilized was from 25 years ago, which may not be able to reflect the current situation of crustal deformation gradient changes. Since 2010, with the accumulation of data from the geophysical observation project of the China Earthquake Administration, we have obtained the latest leveling observation results in areas such as Yunnan, Sichuan, Shaanxi, Gansu and Ningxia, along with leveling data accumulated from routine earthquake monitoring along the northeast edge and the North-South Belt in recent years, which has to a great extent renewed the leveling data along the North-South Seismic Belt.

1 THE RESULTS OF THE VERTICAL CRUSTAL MOVEMENT VELOCITY GRADIENT

Since the 1990s, with the rapid development of GPS space surveying techniques and the continuous accumulation of monitoring data, the use of GPS technology has helped us get a wealth of horizontal crustal movement information in the Chinese mainland (Zhang Peizhen et al., 2002; Wang Min et al., 2003, Jiang Zaisen et al., 2003). The use of constantly updated GPS results makes it possible to study the relationship between dynamic changes of horizontal crustal movement and earthquakes. Current research related to earthquake forecasting mainly manifests in the studies of the relationship between the locations of strong earthquakes and horizontal crustal movement difference area, strain accumulation area and their dynamic changes (Guo Liangqian et al., 2011; Jiang Zaisen et al., 2007; Yang Guohua et al., 2007). With the continuous improvement of GPS observation and processing technology, the precision of GPS in calculated results of vertical deformation has also been significantly improved. Liang Shiming et al. (2013) calculated GPS vertical velocity field in the Tibetan Plateau and its east edge using GPS observation data from 1998-2013, but has not used the GPS vertical results to calculate the gradient of vertical deformation velocity results till now.

Vertical results obtained from regional leveling data and GPS data have their own characteristics: ① Difference in the spatial distribution of the observation points. The results of vertical deformation velocity obtained from leveling data are limited by leveling monitoring networks. Leveling monitoring networks are sparser in West China than that in the eastern region, while GPS stations are more densely distributed in the eastern margin of the Tibetan Plateau; ② Different accuracy. The first-order precision leveling observation can have vertical precision of 0.1mm, while although the precision of GPS in horizontal direction can reach the millimeter scale, its accuracy in the vertical direction is poor; ③ Different data accumulating time. GPS emerged in the late 1990s, and so far its accumulated data only has a history of more than ten years, while leveling observation in China started in the 1950s, and data is easily collected and arranged and can be traced back to the 1970s. Therefore, the combined use of spatial and temporal distribution characteristics of leveling data and GPS vertical results, complementing and verifying each other, to comprehensively explore the gradient of vertical deformation velocity along the north-south belt, enables us to have a more comprehensive understanding of the tectonic movement along the North-South Seismic Belt.

1.1 Data Application

In this article, the vertical movement velocity field used to calculate the gradient of vertical deformation rate is based on more than 40 years of (1970-2011) high precision leveling observation data, using vertical movement velocity results obtained from GPS stations in the study area as prior constraints, which in combination with the linear dynamic adjustment model is applied to deal with the acquired crustal vertical movement velocity field along the North-South Seismic Belt (Hao Ming, 2013). The vertical deformation velocity field obtained by GPS surveying takes ITRF2008 as the reference framework, and makes use of the GPS vertical velocity field in the Tibetan Plateau and its eastern margin calculated from GPS observation data from 1998-2013 (Liang Shiming et al., 2013). Both leveling and GPS results have excluded the effects of great earthquakes, and thus can reflect long-term crustal tectonic movement features, and leveling lines and GPS survey stations used in the study are spread almost all over the North-South Seismic Belt (Fig. 1).

Fig. 1 The North-Sorth Seismic Belt leveling, GPS roadmap
1.2 Calculation Method of Gradient of the Vertical Deformation Velocity

The vertical deformation gradient is dimensionless, reflecting the degree of deformation changes in unit distance. Vertical deformation gradient is directional, and gradient values vary in different directions at the same point. When calculating gradient values at each point, we only take into account the larger gradient value, ignoring its direction (Zhang Yingzhen et al., 1992). Specific calculation steps are as follows:

(1) Using the Kriging interpolation method, we interpolate the vertical movement velocity field calculated from leveling lines and GPS survey stations in Fig. 1, and get a velocity value of 10′×10′ grid points;

(2) Using velocity value of grid points to calculate the horizontal gradient of velocity by the following method. According to the scalar function z=(x, y), the horizontal gradient is defined as follows:

$ \text{grad}Z=\frac{\partial Z}{\partial x}\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {i}+\frac{\partial Z}{\partial y}\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {j} $ (1)

Where, i, j denote the unit vector on the x, y axes respectively, so the module of horizontal gradient is

$ \text{ }\!\!|\!\!\text{ grad}Z|=\text{sqrt}\left[ {{\left(\frac{\partial Z}{\partial x} \right)}^{2}}+{{\left(\frac{\partial Z}{\partial y} \right)}^{2}} \right] $ (2)

Partial differential of function Z can be approximately expressed as:

$ {{\left(\frac{\partial Z}{\partial x} \right)}_{ij}}=({{Z}_{i+1, j}}-{{Z}_{i-1, j}})/2\Delta x $ (3)
$ {{\left(\frac{\partial Z}{\partial y} \right)}_{ij}}=({{Z}_{i+1, j}}-{{Z}_{i-1, j}})/2\Delta y $ (4)

That is, the gradient of grid point pij in x direction, ($\partial Z/\partial x$)ij, is calculated using the field value Zi-1, j and Zi+1, j of two adjacent grid points pi-1, j and pi+1, j in x direction. Similarly, we get the gradient of grid point pij in the y direction.

1.3 The Results of Vertical Deformation Velocity Gradient

According to the calculation method of the vertical deformation velocity gradient, we obtained vertical deformation velocity gradient results along the North-South Seismic Belt (Fig. 2, Fig. 3). To make the velocity gradient clearly visible, we divide the gradient change into three levels according to the numerical size, that is: the high gradient zone, with a gradient value of 40×10-9/a-97×10-9/a; sub-high gradient zone, with gradient value of 20×10-9/a-40×10-9/a; low gradient zone, with gradient value less than 20×10-9/a. Because the value of the vertical deformation velocity gradient reflects the intensity of crustal tectonic activity, in the investigations of correlation between the vertical deformation velocity gradient and faults, we mainly analyze the distribution of the high vertical deformation velocity gradient zone.

Fig. 2 The North-Sorth Seismic Belt vertical deformation velocity gradient (leveling)

Fig. 3 The North-Sorth Seismic Belt vertical deformation velocity gradient (GPS)

It is observed that the spatial distribution of the high gradient zone and slow gradient zone of the vertical deformation velocity gradient along the north-south belt calculated from leveling data and GPS data is basically consistent, and their distribution is highly correlated with tectonic distribution. In the interior of blocks are mostly slow vertical deformation velocity gradient change zones, for example, inside the Bayan Har block, the Sichuan Basin block and the Alxa block, velocity gradient values are all less than 15×10-9/a. Block boundaries are mostly in the high vertical deformation velocity gradient zone, and the spreading of the high vertical deformation velocity gradient zone is basically consistent with the tectonic strike. From the view of the overall North-South Seismic Belt (Table 1), faults located along high vertical deformation velocity gradient zones based on both leveling and GPS data include the Longmenshan fault zone, the southern segment of Xianshuihe fault zone, the Anninghe-Zemuhe fault zone, the Mabian-Yanjin fault zone and the Jinshajiang fault in the Sichuan-Yunnan region, and the Zhuanglanghe fault zone and the Liupanshan fault zone along the northeastern margin. Faults located in high gradient zones based only on leveling data are mainly concentrated on the northeastern margin, which are the Yellow River-Lingwu fault zone, the northern Qilian fault zone and the Guanzhong area in the Weihe basin. Faults located in high gradient zones based on only GPS data include the Honghe fault zone in Yunnan and the Riyueshan-Lajishan fault and Haiyuan fault zone on the northeastern margin, among which, the Haiyuan fault zone lies in a high GPS gradient zone and sub-high gradient zone based on leveling data.

Table 1 Faults located in the high gradient zone

The value of vertical deformation velocity gradient reflects the intensity of crustal tectonic activities. Slow vertical deformation velocity gradient zones are mostly located inside the blocks, reflecting weak relative tectonic movements inside the blocks. From the view of distribution of high gradient zones, the intersection of the Longmenshan fault zone, the south segment of Xianshuihe fault zone and the Anninghe-Zemuhe fault zone has the most intense tectonic movement along the whole North-South Seismic Belt, with a maximum deformation velocity gradient of 97×10-9/a, which is consistent with the results achieved by Guo Liangqian et al. (2007). Secondly, the junction of the Zhuanglanghe fault zone near the mid-east segment of Qilian Mountain and the Lenglongling fault zone has the most intense tectonic movement on the northeastern margin, with the maximum deformation velocity gradient of 60×10-9/a. The recent Menyuan MS6.4 earthquake on January 21, 2016 took place in the high vertical deformation gradient zone based on leveling data and on the margin of the high gradient zone based on GPS data.

2 THE RELATIONSHIP BETWEEN THE VERTICAL DEFORMATION VELOCITY GRADIENT AND STRONG EARTHQUAKE ACTIVITIES

Previous research shows that the high vertical deformation velocity gradient zone might be a potential strong earthquake risk zone (Guo Liangqian et al., 2007;Zhang Yingzhen et al., 1992;Zhang Zushenget al., 1996;Guo Liangqian, Bo Wanju et al., 2001). Although there has been leveling data since 1970, considering the differences of data accumulating time and monitoring range in time and domain, in the analysis of correlation between vertical deformation velocity gradient and earthquake activities, we do not analyze earthquakes from 1970-1985 with leveling data, and do not analyze earthquakes occurred before 2000 with GPS data.

In combination with more than 20 strong earthquakes with MS≥6.0 occurring in the study area from 1985-2013 (Fig. 2, Fig. 3), it can be seen that within the scope of time and space controlled by leveling and GPS data, earthquakes with normal or thrust features in focal mechanical solutions are all located in an abnormal zone of a high vertical deformation velocity gradient based on both leveling and GPS data, such as the Wenchuan MS8.0 earthquake in 2008 and the Lushan MS7.0 earthquake in 2013 which occurred along the Longmenshan fault zone. The Minxian MS6.6 earthquake in 2013 occurred in the sub-high gradient zone based on both leveling and GPS data. Both the Yiliang MS6.0 earthquake in 2012 and the Ludian MS6.6 earthquake in 2014 took place in a high vertical deformation velocity gradient zone based on GPS data, and due to a lack of surveying points for both the Yiliang earthquake and Ludian earthquake in the leveling network, their epicenters are located on the margin of a high vertical deformation velocity gradient zone based on leveling data. The Minle MS6.1 earthquake in 2003 occurred on the margin of a high gradient zone based on leveling data, while the reaction of GPS to this earthquake is not obvious. The Panzhihua MS6.1 earthquake in 2008 occurred in the sub-high gradient zone based on leveling data and high gradient zone based on GPS data. The two MS≥6.0 earthquakes in Puer, Yunnan Province in 2007 and 1993 respectively took place in the sub-high gradient zone based on leveling data. The Lijiang MS7.2 earthquake in 1996 occurred on the margin of sub-high gradient zone based on leveling data. The Jingtai MS6.6 earthquake in 1990 took place on the margin of high gradient zone based on leveling data. The Gonghe MS7.1 earthquake in 1990 was outside the control of the leveling monitoring network, but rather close to survey lines, therefore this earthquake occurred on the margin of high gradient zone based on leveling data. The Lancang-Gengma earthquake in 1988 occurred on the margin of a high gradient zone based on leveling data, but leveling survey points are sparse near the epicenter and compared with the occurrence time of the earthquake, and the data accumulation time was quite short. The Ninglang MS6.2 earthquake in 1998 (normal fault type) did not occurr along a high or sub-high vertical deformation velocity gradient zone.

Strike-slip earthquakes, such as the Kangding MS6.3 earthquake which occurred along the Xianshuihe fault zone in 2014, were located in the sub-high gradient zone based on both leveling and GPS data. The Yushu MS7.1 earthquake in 2010 occurred on the margin of a sub-high gradient zone based on leveling data. The Yingjiang MS6.1 earthquake in 2014 took place on the margin of a high vertical deformation gradient zone based on GPS data, but its epicenter was located outside the leveling monitoring area. The Yongsheng MS6.0 earthquake in 2001 occurred in a sub-high gradient zone based on both leveling and GPS data. From 2000-2009, earthquakes with magnitude of 6.0-6.4 occurring near Yaoan and Dayao did not respond to both leveling and the GPS vertical deformation velocity gradient, and the Wuding MS6.5 earthquake in 1995 also did not occur inside or on the margin of a high or sub-high gradient zone based on leveling or GPS data.

It can be seen that since 1980, almost all MS≥7.0 earthquakes have been related to spatial distribution of a high or sub-high vertical deformation velocity gradient zone, but were also associated with the duration of data accumulating time and spatial distribution of survey lines or survey stations. Earthquakes near survey lines or survey stations with longer data accumulating times basically took place in the high gradient zone, such as the Wenchuan earthquake and the Lushan earthquake. Earthquakes near the margin of leveling networks with short data accumulating time or sparse survey points occurred in the margin of high or sub-high gradient zones, such as Langcang-Gengma earthquake, Lijiang earthquake and the Gonghe earthquake. Except for the above-analyzed MS≥7.0 earthquakes, regardless of whether there were strike-slip earthquakes or thrust or normal earthquakes, of the 15 earthquakes with magnitude of 6.0-6.9 along the North-South Seismic Belt we analyzed, 11 took place inside or in the margin of a high or sub-high deformation velocity gradient zone. However, not all MS≥6.0 earthquakes are related to the high vertical deformation gradient, such as the Yaoan earthquake, the Dayao earthquake, the Ninglang earthquake and the Wuding earthquake, of which, except for the Ninglang earthquake, which is normal type, the other three are all strike-slip earthquakes.

3 CONCLUSIONS

Vertical deformation velocity gradients along the North-South Seismic Belt calculated by using two different measuring methods, regional leveling and GPS surveying, are basically consistent. By comparison with MS≥6.0 earthquakes, it is believed that spatial distribution of MS≥6.0 earthquakes along the North-South Seismic Belt is in good coincidence with the regional high deformation velocity gradient zone in space, especially MS≥7.0 earthquakes, as according to the distribution of survey points and survey lines, most earthquakes occurred inside or on the margin of the high gradient zone, and for MS≥6.0 earthquakes, the vertical deformation velocity gradient still shows good anomaly features before earthquakes.

Although leveling and GPS data used in this article is from before 2013, the time span of leveling data used in the calculation of vertical deformation velocity gradient is 43 years, and the time span of GPS data 15 years, therefore both observation results reflect the long-term trend of crustal movement, which not only has an indicating meaning for earthquakes within the data service time, but also a certain significance for earthquake prediction. For example, the Kangding MS6.3 earthquake in 2014 occurred on the margin of the high gradient zone based on both leveling and GPS data; the Ludian MS6.5 earthquake in 2014 took place in the high gradient zone based on both leveling and GPS data; the Jinggu MS6.6 earthquake in 2014 occurred in the sub-high vertical deformation velocity gradient zone based on both leveling and GPS data, while the 2014 Dehong MS6.1 earthquake in Yunnan took place in the high gradient zone based on GPS data (the epicenter located outside the area of the leveling monitoring network). Vertical deformation velocity gradients obtained from leveling and GPS data both reflect intense tectonic activities near the Longmenshan-Xianshuihe fault zone and the Anninghe-Zemuhe fault zone, but the spatial distributions of the high gradient zones are slightly different. The high gradient zone based on leveling data is distributed in the junction of the three major faults, closer to the Anninghe-Zemuhe fault zone, while the high gradient zone based on GPS data mainly is distributed along the Longmenshan fault zone, with small ranges along other fault zones. Predictably, strong earthquakes along the North-South Seismic Belt are most likely to happen in the junction of the three major faults or its surrounding areas in the future. On the northeastern margin, the high gradient of vertical deformation velocity is most notable from the Lenglongling fault to the Maomaoshan fault, near the Laohushan fault and the Zhuanglanghe fault zone. Although the Menyuan MS6.4 earthquake and the Minxian MS6.6 earthquake have already taken place, judging from the distribution of the high gradient zone of vertical deformation velocity, the area from Tianzhu to Jingtai also has a seismic background of strong earthquakes, moreover along the Liupanshan fault zone there is also a seismic risk of the occurrence of moderate-strong earthquakes.

This paper has been published in Chinese in the journal of Earthquake, Volume 37, Number 1, 2017.

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南北地震带垂直形变速率梯度与强震活动研究
刘立炜, 季灵运, 赵强     
中国地震局第二监测中心, 陕西 西安 710054
摘要:垂直形变梯度能够有效反映区域构造运动的垂直差异变化程度。本文利用长时间段的水准资料结合GPS资料计算垂直形变梯度值,虽然水准资料和GPS资料是两种不同的监测手段,但是垂直形变梯度结果大体相同。计算结果显示垂直形变速率梯度的空间分布与构造分布具有明显的相关性,南北地震带垂直形变速率梯度异常最显著区域在鲜水河断裂带,龙门山断裂带和小江-则木河断裂带附近,其次是青藏高原东北缘地区的庄浪河断裂带与冷龙岭断裂带交汇区域,2016年门源6.4级地震就发生在这一异常区域内,但是从垂直形变速率梯度的高梯度区域分布来看,在天祝至景泰一带也有发生强震的背景。三大断裂交汇区域是整个南北地震带构造活动最为强烈的区域,应该注意该地区未来强震发生的危险性。
关键词梯度    南北地震带    垂直形变    地震