Earthquake Reaearch in China  2018, Vol. 32 Issue (1): 119-129
The NE Directed Seismicity Belt in Tibet after the MS8.1 Nepal Earthquake and Its Predictive Significance
Wang Shuangxu, Zhu Liangyu, Xu Jing, Ji Lingyun, Jiang Fengyun     
The Second Monitoring and Application Center, China Earthquake Administration, Xi'an 710054, China
Abstract: After the 2015 MS8.1 Nepal earthquake, a strong and moderate seismicity belt has formed in Tibet gradually spreading along the northeast direction. In this paper, we attempt to summarize the features and investigate the primary mechanism of this behavior of seismic activity, using a 2-D finite element numerical model with tectonic dynamic settings and GPS horizontal displacements as the constraints. In addition, compared with the NE-trending seismicity belt triggered by the 1996 Xiatongmoin earthquake, we discuss the future earthquake hazard in and around Tibet. Our results show that:the NE-directed seismicity belt is the response of enhanced loading on the anisotropic Qinghai-Tibetan plateau from the Indian plate and earthquake thrusting. Also, this possibly implies that a forthcoming strong earthquake may fill in the gaps in the NE-directed seismicity belt or enhance the seismic hazard in the eastern (the north-south seismic zone) and western (Tianshan tectonic region) parts near the NE-directed belt.
Key words: The 2015 MS8.1 Nepal earthquake     Qinghai-Tibetan plateau     NE-directed seismicity belt predictive significance    

INTRODUCTION

Seismic activity imaging is a visual representation of spatial seismicity distribution. The anomalous regional seismicity image, shown as a"gap"and"belt"before strong earthquake (Mei Shirong et al., 1993), or such kinds of special seismicity patterns in the related tectonic areas after a strong earthquake, may reflect the changes of regional tectonic dynamic environment and stress state of the crustal medium. Since the 1980s, experts and scholars at the earthquake institutions in China have carried out a large amount of systematic research on the characteristics of seismicity belts. Chen Zhangli et al. (1981) considered that the emergence of a small earthquake"belt"before the MS7.0 earthquakes has universality to a certain extent. Han Weibin et al. (1985), Liu Puxiong et al. (Liu Puxiong et al., 1989; Liu Puxiong, 1993) proposed methods for identifying seismicity belts and distinguishing abnormal belts, and analyzed the relationship between seismicity belts and large earthquakes. In recent 10 years, with the development of accurate earthquake location technology, seismic waveform data processing and large-scale computing technology, great progress has been made in 3-D numerical simulation on the mechanism of the precursory seismicity images in China (Jiang Changsheng et al., 2005; Lu Yuanzhong et al., 2007), the seismicity gap and belt identification (Liu Wenlong et al., 2006) and seismic wave identification (Liu Wenlong et al., 2006). The seismicity belt pattern is regarded as one of the most commonly-used methods in earthquake prediction research (Li Yingzhen et al., 2011). These results play an important role in the study and practice of earthquake prediction in China.

After the 2015 Nepal MS8.1 earthquake, a lot of research has been carried out on the background and characteristics of the seismogenic structure of the Nepal MS8.1 earthquake (Liu Jing, et al., 2015), the inversion of the rupture process of this earthquake (Zhang Yong et al., 2015), the historical seismicity in the Himalayan arc (Du Fang et al., 2016), and future earthquake tendency in the Himalayan orogenic belt (Zhao Genmo et al., 2015). However, the formation of the NE-directed strong and moderate earthquake belt at the north of the seismogenic structure in Tibet formed after the MS8.1 Nepal earthquake and its mechanism, especially its effects on future seismic activity in western China remain to be further studied. In this paper, using the basic characteristics of seismicity belt distribution and dynamic environment and geological structure in combination of numerical simulation with GPS horizontal displacements as the constraints, we explore the formation mechanism of the seismicity belt and its implications for predicting the trend of future strong earthquakes.

1 CHARACTERISTICS OF THE NE-DIRECTED STRONG AND MODERATE SEISMICITY BELT AFTER THE 2015 NEPAL MS8.1 EARTHQUAKE

According to the catalog of China Earthquake Networks Center, the April 25, 2015 MS8.1 Nepal earthquake happened in the collision area between the Indian plate and Eurasian plate. Within more than one year after the earthquake, more than 10 moderate-strong earthquakes happened at the north of the seismogenic structure of the MS8.1 Nepal earthquake in the Qinghai-Tibetan plateau region, namely, the MS5.9 Dingri, Tibet earthquake on April 25, 2015, the MS5.3 earthquake in Nyalam, Tibet on April 26, 2015, the MS5.2 earthquake in Madoi, Qinghai on October 12, 2015, the MS5.2 Qilian, Qinghai earthquake on November 23, 2015, the MS5.3 Amdo, Tibet earthquake on January 14, 2016, the MS6.4 Menyuan, Qinghai earthquake on January 21, 2016, the MS5.5 Dingqing, Tibet earthquake on May 11, 2016, the two MS5.3 earthquakes in Dingri and Dinggyê, Tibet on May 22, the MS6.2 Zadoi, Qinghai earthquake on October 17, 2016, and the MS5.1 Nyainrong, Tibet earthquake on December 5, 2016, etc. The epicenters of these earthquakes are mainly distributed in a narrow zone which starting from the southern Tibet, extending in the NE direction across multi secondary tectonic units in the interior of Tibet to the northeast edge of Qinghai-Tibetan plateau (Fig. 1, Table 1), appearing as a typical belt of group activity of medium-strong earthquakes.

Fig. 1 The NE-directed seismicity belt of strong and moderate earthquakes in Tibet after the 2015 MS8.1 Nepal earthquake

Table 1 Statistics of strong and moderate earthquakes in the NE-directed seismicity belt in Tibet after the 2015 MS8.1 Nepal earthquake

Preliminary analysis shows that the seismicity belt has the following basic characteristics:

(1) The spatial distribution of the main seismic activity is along the NE major dynamic direction of extrusion of Indian plate to the Qinghai-Tibetan plateau, and its shape is roughly similar to the NE-directed seismicity belt of MS≥6.0 earthquakes along xiatongmoin, Tibet-Baotou around 1996 (Zhao Zhencai et al., 1999; Li Yonglin et al., 2000) (Fig. 2).

Fig. 2 Distribution of seismic activity of MS≥6.0 earthquakes along the NE direction in the Qinghai-Tibetan plateau and its northeastern neighboring region around 1996

(2) During the development of the seismicity belt, the distribution of epicenters was characterized by jump variation, rather than expansion from south to north in the strict sense.

(3) The intensity of seismic activity in the belt was fluctuating with time, rather than a successive enhancement or attenuation (Table 1).

2 ANALYSIS OF THE TECTONIC DYNAMIC ENVIRONMENT AND MECHANISM OF THE NE-DIRECTED SEISMIC ACTIVITY BELT IN THE QINGHAI-TIBETAN PLATEAU 2.1 The Geological Structure and Dynamic Environment of the NE-directed Seismic Activity Belt in Tibet

With respect to the dynamic action of the plate boundaries around China, the Indian plate is the most important to the Chinese mainland (Zhang Guomin et al., 1994). In recent years, strong earthquakes with magnitude greater than 7.0 occurred successively in the dynamic source area of the collision boundary between the Indian plate and west China, such as the MS8.1 Nepal earthquake on April 25, 2015, the MS7.8 Hindu Kush earthquake on October 26, the MS7.4 Tajikistan earthquake on December 27, the MS7.1 Afghanistan earthquake on April 10, 2016 and the MS7.2 Myanmar earthquake on April 13, which reflect the enhanced northward pushing of the Indian plate. In particular, the seismogenic structure of the April 25, 2015, MS8.1 Nepal earthquake was located in the middle of Himalayan orogenic belt generated by the collision between the Indian plate and the Eurasia plate. Because of the long-term northward extrusion of the Indian plate to the Qinghai-Tibetan plateau, the dynamic boundary zone has been subject to high stress accumulation. The low-angle thrust faulting of the MS8.1 Nepal earthquake soon triggered the Dingri MS5.9 earthquake (2015-04-25) and the Nyalam MS5.3 earthquake (2015-04-26) on the near-NS extensional formal faults in southern Tibet. This indicates that the closer it is to the above dynamic boundary zone, the more sensitive the response to the large earthquake event is.

More than ten medium-strong earthquakes happened in Tibet in more than a year after the MS8.1 Nepal earthquake, forming a NE-directed seismicity belt along the northeast direction of the main dynamic action, crossing from the southern Tibet to the northern Tibet the secondary tectonic units such as the Lhasa block, Qiangtang block, and Bayan Har block, etc. (Fig. 1). It is preliminarily thought that this phenomenon is related to the low-angle thrust faulting of the Nepal earthquake which led to the northward transfer of stress and strain energy on one hand, but also reflects the high stress level of the Qinghai-Tibetan block in the background of the enhanced northward pushing of the Indian plate on the other hand.

2.2 Numerical Simulation with the GPS Horizontal Displacements as the Constraints and Preliminary Study on the Mechanism of the NE-trending Seismicity Belt

We preliminarily analyzed the macroscopic characteristics and tectonic dynamic environment of the above seismicity belt along the NE direction in Tibet after the MS8.1 Nepal earthquake. The northward pushing of the Indian plate is the main dynamic source of tectonic deformation and earthquake activities in western China (Ding Guoyu, 1991), and from the average GPS horizontal velocity field data of the period 2009-2015 in the Qinghai-Tibetan plateau and its peripheries (Fig. 3, data is up to the date of the Nepal MS8.1 earthquake in 2015), it can be seen clearly that the movement direction of the central Qinghai-Tibetan plateau is in the NE direction, coinciding to the extension direction of the Qinghai-Tibetan plateau in the geological period and the aforementioned NE-trending seismicity belt. This brings us to the question of whether a NE-directed high stress/strain zone can be formed in the interior of the Qinghai-Tibetan plateau in the dynamic background of current tectonic movement in Tibet. In this paper, we adopt the 2-D elastoplastic finite-element method to simulate the crustal strain state of Tibet by using GPS horizontal velocity field data as the boundary constraint.

Fig. 3 Distribution of MS≥5.0 earthquakes in and around Tibet after the MS8.1 Nepal earthquake and the GPS horizontal velocity field of 2009-2015 (relative to Eurasia)

First of all, deformation in the Qinghai-Tibetan plateau is dominated mainly by active tectonic blocks, characterized by the coexistence of fault slip, block rotation and internal deformation (Zhang Peizhen et al., 2003, 2008). In addition to the frictional boundary model describing the characteristics of fault movement, the elastoplastic model is used to describe the intra-block deformation and block rotation. It is believed that there are differences between the yield strength of each sub-block, and these differences not only result in different crustal stress distribution but also in stress transfer direction between the blocks. The influence on the distribution of plastic strains inside the Qinghai-Tibetan plateau is analyzed through the plastic strength simulation test of the sub-blocks.

Secondly, a two-dimensional plane finite-element geometry model (Li Yuhang et al., 2014) is established according to the different grades of blocks in the Qinghai-Tibetan plateau. We assume that the surrounding area of the Qinghai-Tibetan plateau (the Indian plate, Tarim block, Alxa area, South China block and Sanda block) is of elastic medium with the same parameters, the elastic modulus is 90Gpa, Poisson's ratio is 0.25. The interior of Tibet is assumed to be the plastic deformation zone and divided into 5 second-grade blocks and 15 third-grade blocks. As the structure and crust medium of each block are different, and their yield strength also differs significantly, we accordingly analyze the influence of strength difference around the block according to the method of successive approximation from the whole to the local.

(1) In this paper, the finite-element geometry model is established using the boundary of the Grade Ⅰ block in Fig. 3, thus the yield strength of the plastic deformation of the Qinghai-Tibetan plateau can be roughly estimated by measuring only one yield strength parameter.

(2) The geometric model is established using the second-grade block boundary.

(3) The geometric model is established using the third-grade block boundary to study in detail the influence of the strength difference of the grade Ⅲ blocks on the whole tectonic deformation.

Thirdly, considering that the plastic parameters in the plastic deformation zone generally depend on the yield criterion, in order to reduce the influence of the model parameters, this paper adopts the Von Mises stress yield criterion which has the minimum number of parameters. This criterion assumes that when the stress state at a certain point reaches a certain value, the material is in a plastic state, and the stress state remains constant (Wang Ping et al., 2006). Therefore, in the simulation process, we just need to set the critical stress state when simulating the plastic deformation. The Von Mises stress yield criterion is generally expressed as the second invariant of stress tensor:

$ {{J}_{2}}=\frac{1}{6}[{{({{\sigma }_{x}}-{{\sigma }_{y}})}^{2}}+{{({{\sigma }_{y}}-{{\sigma }_{z}})}^{2}}+{{({{\sigma }_{z}}-{{\sigma }_{x}})}^{2}}+6(\tau _{xy}^{2}+\tau _{yz}^{2}+\tau _{zx}^{2})] $ (1)

Where, σx, σy, σz, τxy, τyz, τzx are the stress components. After the critical stress state (the second stress invariant) is set, the yield strength of the block is determined and the plastic deformation can be simulated. In setting up the model, boundary conditions of Grade Ⅰ block boundary are obtained by interpolating the 2009-2015 GPS velocity fields data. The area outside the grade Ⅰ block is set to be the free boundary, and the sliding speed of the model boundary is calculated by the Quaternary Euler poles of the active grade Ⅰ block determined by the geological method (Xu Caijun et al., 2002).

Through a lot of numerical simulation tests, we found that under the contemporary dynamic environment of the border of the Qinghai-Tibetan plateau, for the second-grade blocks inside the plateau, when the plastic strength of Songpan-Garzê block is weaker than all other second-grade blocks, the total equivalent plastic strain distribution direction is NE in the central Qinghai-Tibetan plateau (high plastic strain in Fig. 4(a)), which is identical to the relatively low velocity of the Songpan-Garzê block obtained by Huang Jinli et al. (2006) using the P-wave velocity structures of the Chinese mainland acquired using seismic tomography technology. Therefore, we can think that the relatively weak strength in Songpan-Garzê block plays an important role in the NE migration of the internal material of the Qinghai-Tibetan plateau, and it is likely to be one of the main factors controlling the formation of the high strain belt of the present-day strain field, and also explains to some extent the emergence of the NE-directed seismicity belt of strong and moderate earthquakes after the MS8.1 Nepal earthquake in Tibet.

Fig. 4 The total equivalent plastic strain based on the 2-D elasto-plastic finite-element model, and distribution of MS≥5.0 earthquakes in the study area during the period from April, 2015 to December, 2016 Note: In Fig. 3, the white lines with different thickness represent the boundaries of blocks of various grades in and around Tibet; the red italics represent the Grade Ⅰ blocks, and there are 5 Grade Ⅱ and 15 Grade Ⅲ blocks in the interior of Tibet(Li Yuhang et al., 2014); black arrows represent the GPS horizontal velocity field of 2009-2015. In Fig. 3 and Fig. 4, the faint yellow rectangular region A and B represent the main earthquake migration area in the central and eastern Tibet, respectively

Therefore, it is believed that under the unique internal structure and external dynamic action of the Qinghai-Tibetan plateau, the high plasticity strain bands in the NE direction inside Tibet have the background of high seismic activity. As mentioned above, the three basic characteristics of the NE trending seismicity belt in Tibet formed after the MS8.1 Nepal earthquake, especially the jumping back and forth of the earthquake locations and the intensity fluctuations in the seismicity belt reflect the enhancement of tectonic dynamic environment and the complexity of the superposition effect of large earthquake in the dynamic boundaries on the changes of the tectonic stress fields in the interior of Tibet. Thus, this NE seismicity belt is not only a statistical phenomenon, but also a reflection of the geodynamic process in the Qinghai-Tibetan plateau.

In addition, from the active fault distribution on the eastern margin of Tibet, near-EW trending strike-slip faults are mainly developed to the north of Garzê-Yushu-Xianshuihe fault zone, while to the south of this fault zone, the near NS trending strike-slip faults are developed (Tapponnier P. et al., 2001), which also indicates that there are certain differences in strength between the western Sichuan block and Songpan-Garzê block on both sides of the fault zone, and because the Songpan-Garzê block is relatively more ductile, it stopped the northward extension of the NS-trending faults in eastern Tibet. This is similar to the results of this paper, in which the simulated high plastic strain belt extends in the NS direction only to the Songpan-Garzê block. In addition to the extension along the NE direction, a high value plastic strain zone is also distributed in the Sichuan and Yunnan border region on the north-south seismicity belt in the eastern Qinghai-Tibetan plateau, (the yellow rectangle in B area of Fig. 4), suggesting that a high plastic strain background exists in the region, and under the conditions of large earthquakes in the Himalayan collision belt and stress disturbance, it is more likely to trigger a strong earthquake.

3 THE PREDICTIVE SIGNIFICANCE OF THE NE-DIRECTED SEISMICITY BELT IN TIBET FOR FUTURE STRONG EARTHQUAKES

According to existing research, typical seismic activity patterns, such as tectonic related seismicity belts, have the meaning of the whole"field"(Chen Zhangli et al., 1981; Liu Puxiong, 1993), which reflects the change of the tectonic dynamic environment and regional stress field. In recent years, in the dynamic source region of the northward pushing and collision boundary of the Indian plate adjacent to western China, there occurred in succession the Nepal MS8.1 earthquake in the central part of the Himalayan arc on April 25, 2015, the Hindu Kush MS7.8 earthquake near the top of the arc in the western Himalaya on October 26, 2015, the Tajikistan MS7.4 earthquake on December 27, 2015 and Afghanistan MS7.1 earthquake on April 10, 2016, and the Myanmar MS7.2 earthquake near the eastern Himalaya on April 13, 2016. These earthquakes reflect that the adjacent areas of western China are in an intensive tectonic dynamic environment under the background of enhanced northward pushing and collision of Indian plate. The stress accumulation in the dynamic collision boundary between the India and Eurasian plates-the top of the eastern and western Himalayan arc and the whole Himalayan arc can trigger the NE-directed seismicity belt from Xaitongmoin Tibetan to Baotou Inner Mongolia, triggering seismicity in the north-south earthquake zone, the Xinjiang Tianshan seismic zone and the Qinghai-Tibetan region (Li Yonglin et al., 2000). If we compare the NE-directed strong seismicity belt formed in the Qinghai-Tibetan plateau after the MS8.1 Nepal earthquake with the MS≥6.0 Xietongmen-Baotou NE-direction seismicity belt in Tibet formed around 1996 (Fig. 2), we can find that the two belts are similar in forms to some extent, both extending along the NE main tectonic dynamic direction of the collision boundary of Indian plate; but there is a difference; the scale and magnitude of the belt in 1996 are larger than the recent one. Specifically, before the formation of the NE-direction seismicity belt of strong earthquake activities along Xaitongmoin, Tibet to Baotou, Inner Mongolia around 1996, strong seismicity was basically concentrated in the influencing area of the top of the eastern Himalayan arc. For example, the MS7.3 earthquake in July, 1995 in Myanmar border, the Wuding, Yunnan MS6.5 earthquake in October 1995, the Yao'an, Yunnan MS6.5 earthquake in January, 1996 and the Lijiang MS7.0 earthquake in February, 1996. After the formation of the seismicity belt, strong earthquake activity migrated to its west, where strong earthquakes occurred successively, such as the magnitude 7.1 earthquake in Karakoram in November, 1996, the MS7.5 Mani, Tibet earthquake in November, 1997, the MS6.8 Jiashi and MS6.9 Atushi, Xinjiang earthquakes in 1998 and the MS8.1 earthquake in the west of Kunlun Mountains pass in 2001, etc. This reflects that the typical seismicity belt pattern formed along the NE direction of the main tectonic dynamic direction, which has a certain extent of significance for predicting the trend of future strong earthquakes. Considering also the research results by Zhao Genmo et al. (2015), after the Nepal MS8.1 earthquake in 2015, the future strong earthquakes may possibly migrate in the longitudinal direction along the Himalayan belt, and larger earthquakes may occur in the eastern Himalayan belt, or migrate northward along the vertical direction to the Himalayan belt (i.e., lateral migration), subsequent strong earthquakes in southwest China may also occur following the large earthquakes in the Himalayan belt, so we should pay close attention to the trend of the seismic regime in this triangle area and in the Pamir-Baikal zone. Thus, the trend of future earthquakes in western China may exhibit"gap-filling characteristics"by strong and moderate earthquakes near the aforementioned NE-directed belt in Tibet (compared with the seismicity belt around 1996, the intensity of seismicity in the latter NE belt is not yet strong enough), as well as the characteristic of enhancement of strong earthquake activities to the east (the north-south seismicity belt) and west (western Qinghai-Tibetan plateau-Tianshan tectonic zone) of this belt.

4 PRELIMINARY CONCLUSION

In summary, the following preliminary conclusions can be drawn:

(1) After the Nepal MS8.1 earthquake on April 25, 2015 in the north boundary of the Indian plate, an earthquake activity belt was formed in central Tibet spreading along the NE-directed main dynamic action of extrusion of the Indian plate to the Qinghai-Tibetan plateau, with its distribution and morphology similar to that of the NE-directed seismicity belt of Xietongmen-Baotou around 1996.

(2) The above-mentioned NE direction seismicity belt is not only a statistical phenomenon, but also a reflection of the geodynamic process in the Qinghai-Tibetan plateau. It is the tectonic response caused by the NE-directed tectonic stress changes of Tibet at the north of the dynamic boundary under the background of enhanced northward subduction of Indian plate, relating to the low-angle thrust faulting of the Nepal MS8.1 earthquake and the northward transfer of crustal stress and strain. At the same time, there are high plasticity strain belts within the plateau, which have higher stress levels.

(3) After the MS8.1 Nepal earthquake, the typical seismicity belt image formed along the NE direction of the main tectonic dynamic process in the internal Qinghai-Tibetan plateau has certain significance for predicting the trend of strong earthquake activity in central and western China.

Future earthquake activity in western China may have the characteristics of "gap filling"by strong earthquakes near the NE-directed strong seismicity belt in the Tibet and enhancement of strong earthquake activity in a wide range on both east and west sides of the belt. In future, close attention should be paid to the occurrence of strong earthquakes in the north-south seismicity belt, the western Qinghai-Tibetan plateau and Tianshan tectonic zones in Xinjiang.

It is important to note that these findings are preliminary due to the level of our research. However, the typical seismicity patterns formed in the relevant tectonic units in inland China following strong earthquakes occurring in the dynamic source areas in the plate boundaries of China's continent and its predictive implications remain to be further explored.

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