Seismological Study On Present Activity Of The Arcuate Tectonic Belt In Southeast Yunnan

The Qujiang fault, Shiping-jianshui fault and Red River fault are located alongsoutheast margin of the Sichuan-Yunnan rhombic block, sequentially arranged fromnorth to south with the top of the arc convex to the SW. This fault system is known asthe arcuate tectonic belt or the wedge tectonic region of Southeast Yunnan. Thearcuate tectonic belt is adjacent to the South China block, intersected by the Xiaojiangfault to the east, on which tectonic activities is intense and frequent. Five M≥7earthquakes have happened according to the historic record, such as the TonghaiMs7.7earthquake on January5th,1970killing more than15,000people with anepicenter intensity of X degree. Using the plentiful data of small earthquakes toinvestigate structure of the ATB and kinematics can not help understand thegeneration and rupturing processes of strong earthquakes, but also provide evidencefor explore the activity dynamic models of the southeast edge of Tibetan plateau,especially the southeast region of Yunnan.Relatively speaking, little work focused on the arcuate belt has been publisheduntil now. Based on the work of predecessors, two different opinions on the currentactivity characteristics of the arcuate tectonic belt can be summerized as follows: Onepoint considers that the arcuate tectonic belt shows significant crustal shortening witha large thrust component. The left-lateral slip movement of the Xianshuihe-Xiaojiangfault zone is blocked by the arcuate tectonic belt. The Qujiang fault andShiping-jianshui fault have absorbed and converted the left-lateral slip of the westwall of the Xiaojiang fault belt with dextral strike-slip, horizontal shortening andthrust deformation. Another point of view regards crustal shortening deformation ofthe arcuate tectonic belt almost non-existent, or even exhibits a certain extensionalcomponent. Chunyong Wang and Lide Chen derived the focal mechanism of TonghaiMs7.7earthquake which shows us a normal strike-slip character of seismogenicstructure widely accepted as the Qujiang fault. This view point matches the result revealed by GPS data and the implication of the asymmetry of aftershocks and themeizoseismal area. In order to explore this scientific divergence, this work attemptedto use some seismology methods, namely, small earthquakes relocation and theirmechanisms, to study the present-day behaviours of the arcuate tectonic belt with thegeometry and kinematics characteristics of the related faults.The seismic phase data of12536events in the study area during1990to2011were collected from CENC (China Earthquake Networks Center) and TechnicalSystem Construction of Data Backup Centre for China Seismograph Network. First ofall, the phase data were carefully screened and checked.Because of the large errors of original location, it will bring a negative effect onprecision of the relocation results. Therefore, Hypo2000program was used forrelocating all the earthquakes to obtain more reasonable locations and depths. Afterrelocation, the horizontal errors of75%events were reduced to less than20km, thevertical errors less than10km, the average rms is0.78s, and all the events have depthsrecord.Finally, this work selected1448events in the study area from the absolutelocation results mentioned above. The double-difference method was used todetermine the more accurate location of the1448events. An one-dimensional velocitymodel referenced the results inversed by artificial seismic sounding data, regionalwaveforms data and preliminary waveforms data. By HypoDD,1009small eventswere relocated with the average errors and residual rms in E-W, U-S, and U-D as0.58km,0.57km,0.78km, and0.10s respectively. After precise relocation, the residualrms and location errors have been deduced notably. Thus it changes the diffusecatalog locations into sharp images of seismicity and reveals horizontal lineations ofhypocenters that define the narrow regions on the faults where stress is released bybrittle failure.The results show that earthquakes arre mostly distributed among the Qujiangfault, Shiping-jianshui fault and Honghe fault, like an inversed triangle on the cross section. Two trend surfaces towards to the SW corresponded to the Qujiang fault andShiping-Jianshui fault. Meanwhile, earthquakes in the south side of the Qujiang faultis significantly more than that of north side, the same as the Jianshui-Shiping fault.The Red River fault and Ailaoshan fault can be recognized from the two planestowards NE in the south of the cross section. Earthquakes are mainly distributed in thenorth of the two faults, which reflects the two faults extend towards to NE. All of theprofile characteristics are consisted with the morphological characteristics of normalfaults. Consequentily, it is inferred that the arcuate tectonic belt is dominated bynormal strike-slip activity at present.In total,177408pieces of binary digital waveforms (100s before P waves,600safter P waves) were collected from Data Backup Centre for China SeismographNetwork between2007and2012in the study area. This work selected9240piecesof vertical waveform records with M>2.0, using theSAC(http://www.iris.washington.edu/software/sac) program to do the pretreatment toderive the P wave first motion, and maximum magnitude of vertical component of Pand S waves.The program used to obtain the focal mechanism in this work is based on Liang’salgorithm by means of synthetic seismograms of a point source of dislocation in aplane layered medium, using the maximum amplitude ratios of vertical component ofP and S waves recorded by a regional network. In total,148focal mechanismsolutions were derived in arcuate tectonic belt. After a statistical analysis for all of thesource parameters, the result suggests that almost all the small ruptures were causedby oblique-slip, pure shear slip rarely seen. The focal mechanism planes were dividedinto four categories based on the standard of different combinations of slip and dipangles. Then it was found that the number of normal-strike planes is almost twicethat of inversed-strike planes. The plentiful normal strike-slip nodal planes indicatethat ruptures mainly occurred in a shear-tensional environment, which is coincidentwith the result revealed by GPS data. In sum, some preliminary conclusions as follows:1) The similar characteristics between the profile of focal depth and cruststructure associated with terrain elevation show that the Qujiang fault andShiping-Jianshui fault are inclined to the SW, the Red River fault inclined to NE.2) Focal mechanisms covering different parts of the arcuate tectonic belt. Theamount of normal strike-slip planes are almost twice that of reverse strike-slip planes,which indicates that present-day activity of the arcuate tectonic belt is dominated bynormal strike-slip consistent with the geometric characteristic mention above.