On 7 October 2014(UTC 13:49:39),a strong earthquake struck Jinggu county in Yunnan Province,China. The result from the China Earthquake Networks Center(http://www.cenc.ac.cn/publish/cenc/904/2014100721 5911163239744/index.html)shows that the surface wave magnitude of this seismic event is 6.6. The epicenter of the earthquake is located at 23.4°N,100.5°E,about 26 km southwest away from the city of Jinggu,and its focal depth is approximately 5 km. Two months later,on 6 December,two smaller earthquakes of Ms5.8 and 5.9 occurred in the seismic area. According to the USGS(http://earthquake.usgs.gov/earthquakes/eventpage/usb000sjim#scientific),the moment magnitude of the Jinggu earthquake is 6.1,and its focal depth is approximately 10.9 km. The magnitudes of the following two earthquakes are both Mw5.6. The USGS quick moment tensor solution indicates two candidate fault planes(I: strike 334°,dip 83°,rake –169°; II: strike 242°,dip 79°,rake –7°). The seismic intensity map from the China Earthquake Administration(http://www.cea.gov.cn/publish/dizhenj/464/ 478/20141011091458652940205/index.html)illustrates that the highest intensity degree of the earthquake is VIII and the longitudinal axis of the intensity zones strikes in NW. The relocated aftershocks of this event are located in a NW narrow zone with a steep dip angle,coincident with the characteristics of a typical strikeslip seismic event. Meanwhile,the inversion of the coseismic rupture process by Institute of Geophysics,China Earthquake Administration suggests that the rupture plane strikes in ~151° and dips at ~90°(http://www.ceaigp. ac.cn/tpxw/270908.shtml). Namely,the Jinggu earthquake could be a strike-slip seismic event occurred on a NW fault.

The southwest Yunnan block(SYB),where the Jinggu earthquake occurred,is one of the most seismically active blocks in China presently. The SYB,located at the southern segment of the China North-South trending seismic zone,has hosted several earthquakes greater than 6 since the 20th century,and is now in a newest episode of earthquake cluster(Deng et al.,2014). Historical earthquakes(China Earthquake Administration, 1995,1999)and instrumentally recorded earthquakes(China Earthquake Networks Center,2015)in this area are mainly located on the newly-generated Longling-Lancang fault zone within the SYB and at the southern part of the Sichuan-Yunnan block,leaving an ellipse without any earthquake greater than 5 and thus indicating a seismic gap(Fig. 1). It is in this seismic gap that the Jinggu earthquake happened.

Fig. 1

Fig. 1 Regional tectonic map around the Jinggu earthquake

In this seismic gap,there are several significant faults,such as the Nantinghe fault zone(NFZ),the Red River fault zone,the Lancangjiang fault zone,and the Wuliangshan fault zone. Among them,the NFZ is located 94 km northwest of the Jinggu earthquake and is at the outer area of the intensity zone VI(Fig. 1). The occurrence of the Jinggu earthquake has brought out significant questions such as how would the static Coulomb stress change on surrounding faults because of the earthquake,especially that on the NFZ,what is the influence on the seismic risk of the NFZ,and whether it would produce any destructive earthquake due to the failure of Coulomb stress. To solve these problems,this work calculated,through numerical simulation,the coseismic static Coulomb stress perturbation on surrounding faults triggered by the Jinggu earthquake,especially that on the NFZ. By integrating paleoseismological trenching results,the seismic potential of the NFZ was further analyzed.

The SYB is one of the active blocks within the Yunnan-Burma active tectonic block region(The top left inset in Fig. 1; Xu et al.,2003; Zhang et al.,2003). Within the SYB,there are primarily two sets of active faults, NE trending sinistral strike-slip faults at similar distances of approximately 150 km including the Longling-Ruili fault,the NFZ,the Menglian fault,and the Daluo fault,NW trending dextral strike-slip faults which intersect the formers such as the Longling-Lancang fault zone and the Wuliangshan fault zone(Deng,2007; Wang Y et al.,2014).

The NFZ extends southwestward from Yunxian,through Daxueshan and Gengma,and enters into Burma crossing the national border at Qingshuihe(Fig. 2). It divides the SYB into two sub-blocks,and is the longest one of NE striking left-lateral strike-slip faults in the SYB with a total length of ~380 km(Shi,2014; Wang Y et al.,2014). Along the fault trace,there are well-developed typical tectonic landforms of strike-slip faults (Yeats et al.,1997),such as linear ridges,pull-apart basins,sinistrally offset drainages illustrating that the NFZ is one of the most active faults within the SYB. The restoration of offsets of channels and geological masses suggested a left-lateral offset from several kilometers to tens of kilometers(e.g. Lacassin et al.,1998; Wang and Burchfiel,1997; Wang et al.,1998; Wang Y et al.,2014). The NFZ consists of an east and a west branch,the west branch is much longer,more continuous than the east branch with younger depositional basins developed along the fault trace. Such facts imply that the slip along the NFZ has been mostly accommodated by the west branch(Wang et al.,2006; Zhu et al.,1994).

Since the late Quaternary,the NFZ has been slipping left-laterally with a normal slipping component.Geological surveys determined a strike-slip rate of ~3.6±0.4 mm·a⁻¹ as well as a dip-slip rate of ~1.1±0.3 mm·a⁻¹(Shi,2014),coincident with the GPS measurements(~4.3±1.6 mm·a⁻¹ and ~1.2±1.5 mm·a⁻¹ for strike-slipping and dip-slipping rates,respectively; Shen et al.,2005; Wang et al.,2008). Wang Y et al.(2014) estimated the average slip rates on most active faults within the SYB through their total offsets acquired by geomorphic interpretation and estimated initiation time of around 5 Ma. It is implied that the slip rates of most of these faults are < 4 mm·a⁻¹,while the NFZ is slipping at a much higher rate of 4.2 mm·a⁻¹ than the formers.

Fig. 2

Fig. 2 The Nantinghe fault zone and its surrounding seismicity

The recent paleoseismological trenching excavation on the northern segment of the NFZ determined a strong seismic event that had generated surface rupture along this fault segment illustrating that it is active in Holocene. Radiocarbon dating of the associated sediments constrains the event occurrence between 900–1480 AD(Fig. 3). Within the 244 earthquakes around the world in the study of relationships between the parameters of coseismic surface rupture and magnitude by Wells and Coppersmith(1994),the magnitudes of earthquakes that produced surface deformation are generally greater than 6,even taken landslides and cracks into account. Meanwhile,the 1988 Lancang M7.6 and Gengma M7.2 earthquakes occurred along the Longling-Lancang fault zone in the SYB produced surface deformation zones of 50 km and 14 km,respectively(Yu et al.,1991). Furthermore,the Jinggu Ms6.6 earthquake did not generate any rupture on the ground surface except some tensional cracks without any tectonic implications induced by ground motion and gravity(Shi et al.,2014). Thus,it is implied that the magnitude of the paleoearthquake revealed by the trench investigation on the northern segment of the NFZ is no less than 7.

Fig. 3

Fig. 3 Paleoseismological trench wall on the northern segment of the Nantinghe fault zone

Additionally,on the basis of the length of the NFZ and scaling law between length of fault trace and magnitude(Blaser et al.,2010; Wells and Coppersmith,1994), Wang Y et al.(2014)concluded that the magnitude of an earthquake hosted by the NFZ could be up to 8.2,much larger than those on the other faults within the SYB. However,although the NFZ is slipping at a much higher rate,it is one of the active faults with fewer earthquakes recorded in the SYB. No destructive earthquakes were recorded in historical documents except an M ~7 earthquake in 1941 at the China-Burma border region and no earthquakes of M > 6 have been recorded by instruments along the NFZ since then(China Earthquake Administration,1995,China Earthquake 1999; China Earthquake Networks Center,2015). Thus,the NFZ,located in a seismic gap,is probably in the stage of stress accumulation for a large earthquake.

The status of stress on a fault zone is mainly influenced by the regional tectonic movement induced by plate motions. Besides,sudden tectonic events like earthquakes and volcanic eruptions in the nearby area could also change the accumulated stress. Therefore,for an active fault,the occurrence time of an earthquake in the future not only depends on the long-period and slow stress loading from the regional tectonic movement, but also is influenced by the perturbation of impulse-type tectonic events including earthquakes or volcanic eruptions(e.g. Jiang,2013; Stein,1999). Jinggu earthquake is one of the major seismic events occurred around the NFZ in recent years and thus its perturbations on the stress of the NFZ is of great significance in learning and assessing its seismic risk in the future.

Based on the rectangular dislocation theory in elastic half-space(Okada,1992)and two respective finite fault slip models provided by Zhang et al.(2014)(Peking University)and Wang W M et al.(2014)(Institute of Tibetan Plateau Research,Chinese Academy of Sciences),we calculated the coseismic static Coulomb stress changes(Fig. 4)triggered by the Jinggu earthquake on the optimally orientated 2-D fault planes(King et al.,1994)at a depth of 10 km. According to the regional tectonic stress map of the Jinggu earthquake (http://www.eq-icd.cn/showinfo.asp?ArticleID=23949),the regional tectonic stress field is set to be 104 kPa with an azimuthal angle of the maximum horizontal compressive stress of N20°E. Regarding of the strike-slip property of this rupture event,the effective friction coefficient and Lame coefficient are prescribed to be 0.3 and 3.2×1010 Pa(Parsons et al.,1999).

Fig. 4

Fig. 4 Two kinds of static Coulomb stress changes triggered by the Jinggu earthquake from the coseismic

In general,the results based on two slip models show a great similarity in the distribution of areas where stresses increased and decreased,respectively(Fig. 4). It is found that the northern segment of the NFZ,the central segment of the Heihe fault in the Longling-Lancang fault zone,the northern segment of the Wuliangshan fault zone,as well as several segments of the Jinggu fault and Lancangjiang fault near the epicenter of the Jinggu earthquake,outlined by black dashed rectangles and labeled with their respective values of stress changes in Fig. 4,are located in the areas where stresses increased. It could be concluded that the Jinggu earthquake affected the closer Jinggu fault and the Lancangjiang fault much more significantly on which the changes of stress could be up to 90 kPa,while it produced relatively less effect on the farther faults,namely the static Coulomb stresses on the NFZ and the central segment of the Heihe fault have increased by approximately 1.1 kPa and the increased stresses on the northern segment of both west and central branches of the Wuliangshan fault zone are approximately 6.8 and 4.5 kPa(Fig. 4).

To acquire more accurate changes of stress value and further analyze the seismic risk,we calculated the positive perturbation of the static Coulomb stress on the fault plane of the northern segment of the NFZ using different fault parameters. Considering the characteristic of left-lateral strike slipping with a component of normal slipping and the geometry of the NFZ,the width of the fault plane is set to be 15 km with the dip angle range from 70° to 90° and an effective friction coefficient of 0.3,and the rake angle is prescribed to be between 0° and –40°. Our result illustrates that the positive values of static Coulomb stress perturbation from the slip model of Wang W M et al.(2014)are generally slightly greater than those from the slip model of Zhang et al.(2014)(Table 1). Using either slip model in the calculation,when the dip angle of the fault remains unchanged,the stress change decreases in value with smaller rake angle. On the other hand,the stress perturbation increases in value with larger dip angle when the rake angle is between 0° and –20°,while it almost stays the same when the rake angle is –30° and decreases instead with larger dip angle when the rake angle is –40°. Furthermore,our results,using different fault parameters and both slip models,show that the maximum change of static Coulomb stress value on the northern segment of the NFZ occurs when the dip angle of the fault plane is 90° and the rake angle is 0°,namely 1.18 kPa on the east branch and 0.89 kPa on the west branch, respectively(Table 1).

Table 1

Table 1 Parameters for the target fault during calculation of static Coulomb stress changes and the results

Table 1 Parameters for the target fault during calculation of static Coulomb stress changes and the results

In general,the perturbations of stress on the fault planes of the east and west branches of the NFZ from both slip models are distributed in half circle and decrease in value along the strike and depth(Fig. 5). Moreover, the maximum of positive stress changes occurred near the ground surface at 24.15°N,which from the slip model of Wang W M et al.(2014)is slightly southwestward as compared to that from the model of Zhang et al. (2014).

Fig. 5

Fig. 5 Distribution of the static Coulomb stress perturbation on the Nantinghe fault zone triggered by the Jinggu earthquake

Stress triggering is the changes of static Coulomb stress on geological structures caused by tectonic events nearby like earthquakes and volcanic eruptions. Once the change is positive,it is possible for a stress failure which could produce following earthquakes and thus increase the seismic risk,or the seismic risk would decrease. Usually,increasing of the Coulomb stress by 10 kPa is deemed as a threshold of stress change that begins to affect seismicity significantly and brings faults to failure(e.g.,Stein,1999). Our results have revealed that the positive stress changes triggered by the Jinggu earthquake are less than 10 kPa on the NFZ,the Longling- Lancang fault zone,and the Wuliangshan fault zone,implying that the Jinggu earthquake does not necessarily produce a sudden acceleration on the seismic risks of such faults. However,the latest rupture event on the northern segment of the NFZ is constrained to be between 900 A.D. and 1480 A.D.,indicating an elapsed time of 535–1115 years. On the basis of a strike slip rate of 3.6±0.4 mm·a⁻¹(Shi,2014),it is estimated that the slip accumulated on the northern segment of the NFZ could be 2.8+1.5/–1.0 m using the Monte Carlo simulations (Amos et al.,2007,2010; Thompson et al.,2002; Li et al.,2013). Further using the empirical scaling laws between magnitude and displacement of strike-slip faults(Wells and Coppersmith,1994),an earthquake with a magnitude of 7.5+0.1/–0.2 would be generated if all the accumulated slip is released. Although the stress perturbation on the NFZ is not enough to bring a stress failure,the Jinggu earthquake did cause a positive stress change on the NFZ. Therefore,under the consideration that the northern segment of the NFZ has accumulated seismic energy equivalent to M ≈ 7.5,it is suggested that a high seismic risk exists on this fault segment.

Our result from the paleoseismological trench excavation on the northern segment of the NFZ reveals that the NFZ is active and has produced a strong earthquake that generated surface rupture in Holocene. The age of the rupture event is constrained to be between 900 A.D. and 1480 A.D. by radiocarbon dating. The Jinggu earthquake is a strong earthquake occurred in the SYB,and had caused positive stress perturbations on surrounding faults. The static Coulomb stress changes on faults near the epicenter could be up to ~90 kPa,while those on farther faults are less than 10 kPa. Further computed using different fault parameters of the northern segment of the NFZ,the distribution of the stress perturbation on the fault plane illustrates that the maximum change of stress occurs near the ground surface at 24.15°N and decreases along the strike and depth. The maximum change of static Coulomb stress on the west branch of the NFZ is 0.89 kPa and the value on the east branch is 1.18 kPa. Although the stress changes on the NFZ triggered by the Jinggu earthquake are less than the threshold that affect seismicity significantly and implies that this earthquake does not necessarily produce a sudden acceleration of the seismic risk of the NFZ,under the consideration of positive stress perturbation from the Jinggu earthquake and that the northern segment of the NFZ has accumulated seismic energy equivalent to M ≈ 7.5,it is suggested that this fault segment will probably be in a high seismic risk.

This work was jointly supported by the China Active Fault Survey Project-The South-North Seismic Zone(201108001),the Basic Scientific Funding of Institute of Geology,China Earthquake Administration (IGCEA1403),the National Natural Science Foundation of China(41404012),and the China Postdoctoral Science Foundation(2014M560102). We thank Dr. Fang L H(Institute of Geophysics,China Earthquake Administration)for providing the relocated aftershocks,Dr. Zhang Y(Peking University)and Wang W M (Institute of Tibetan Plateau Research,Chinese Academy of Sciences)for providing the coseismic slip distribution models of the Jinggu earthquake,and Dr. Cui X F(Institute of Crustal Dynamics,China Earthquake Administration)for providing the regional tectonic stress map. Our thanks also go to reviewers for constructive suggestions which improved this manuscript greatly.