Uneven aftershock distribution of Wenchuan Ms 8. 0 earthquake and possible mechanism

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1 Geodesy and Geodynamics 211,2(2) : jgg9. com Doi:1.3724/SP.J Uneven aftershock distribution of Wenchuan Ms 8. earthquake and possible mechanism Li Zhixiong 1,Chen Zhangli 2, Zhao Cuiping 1, Shao Zhigang 1,Hua Wei 1, Wang Qincai 1 and He Ping 1 1 lnstituje of Eart/upuJJre Science, Chma Eart/upuJJre Administration, Beijing 136, China ' China Earthquake Administration, Beijing 136, Chma Abstract:The aftershock activity ofwenchuan Ms8. earthquake showed different spatial and temporal distributions along two different segments of the Longmenshan fault. This difference was likely the result of segmentation of the earthquake rupture process, which in turn may be the result of the fault ' s segmentation in its long-term geotectonic condition. Key words: Wenchuan earthquake ; aftershock activities; spatial and temporal unevenness ; mechanism study ; inheritance movement 1 Introduction The Ms8. Wenchuan earthquake occurred on May 12,28 along Longmenshan fault ( 31. N, E) in seismically-active Sichuan Province 32 years after the occurrence of a pair of Ms7. Songpan-Pingwu earthquakes in When examining the aftershocks of this earthquake, we found unevenness in their spatial and temporal distributions. In this paper, we show the aftershock data for a period of about 3 months after the earthquake, and discuss a possible mechanism for the observed unevenness. 2 Uneven aftershock distributions 2. 1 Spatial distribution Figure 1 shows the spatial distribution of the aftershocks from catslogue of China Earthquake Networks Received: ; Accepted: Corresponding author: Tel: lzx638@ seis. ac. en This work was supported by the Earthguak:e Science Joint Foundation ( A77 ) ond The Projret of China Eanthguake Adminititon ( ) Center and their frequency-of-occurrence distribution along a rupture-parallel cross-section. From figure 1 (a) and (b), we may see that the aftershocks of Ms;;. 2. extended from SW to NE for about 3 km along Longmenshan fault, and that the stronger aftershocks of Ms;;. 5. congregated mainly in two segments divided by a gap near Mianyang. Figure 1 ( c) and (d) show the same congregation of Ms;;.3. and Ms;;.4. aftershocks, respectively, near Wenchuan-Beichuan and Qingchuao Temporal distribution Figure 2 shows the time series of Ms ;;.5. aftershocks, from which we may see that the stronger aftershocks of Ms ;;.5. occurred frequently during a one-month period after the main shock, but became quiescent for a bout one month, and then became active again Spatial distribution of b-values Figure 3 shows the spatial distribution of Ml ;;.2. aftershocks and the spatially scanned result of b-values of the aftershocks. It may be seen that the b-values displayed a regional difference, being less than. 6 to the SW of Mianzhu and more than. 6 to the NE.

2 8 Geodesy and Geodynamics Vol. 2 Legend 33'N 'N Legend l~~. \ 'N ~ \ \.. ~ 1. ~ 2..;3 3. g A Dujiangyan 14'E 15'E 12'E 14'E 16'E (a) aftershocks (Ms;;.2.) (b) aftershocks (Ms;;.5.) Wenchuan Mianzhu 15. Beichuan Pingwu Qingchuan (c) frequency of Ms;;.3. aftershocks along cross-sectiona-b (using 1 krn statistical radius, and 1 krn step); B km (d) frequency of Ms;;.4. aftershocks. Figure 1 Spatial distribution of aftershocks and the frequency distribution ( up to August 5, 28 ) 2. 4 Difference between SW and NE segments of the earthquake-rupture zone Based on the above-mentioned distribution, we divided the rupture zone into two segments[!], namely, Dujingyan-Wenchuan segment to the SW of Mianyang and Beichuan -Qingchuan segment to the NE. The time series of decay coefficient ( P value ) calculated with Omori formula for these sections ( up to August 5, 28) are shown in figure ~ Time (day) Figure 2 Time series of aftershocks with magnitude equal or above 5. ( up to August 5, 28) Figure 3 Spatial distribution of b-values (scanning radius is 5 km, spatial sliding step is. 1, up to August 5, 28)

3 No.2 Li Zhixiong, et al. Uneven aftershock distribution of W enchuan M s 8. earthquake and possible mechanism Beichuan Mianzhu Wenchuan Dujiangyan I sw ~.8 ~.., A-EN sequence -----SW sequence ~ -5 ]-! ~-15 t-2 c:::l OL---L---~--~---L--~ L L ~_ ()6..() {) Day/Month Figure 4 Decay coefficient (P-value) of Wenchuan aftershock sequence As shown in this figure, the aftershocks in the SW segment decreased rapidly until June 1, when the P value reached 1. 1; subsequently the P value stayed around This result means that these aftershocks reached normal attenuation at that time, and they had the feature of main-aftershock type. On the other hand, the P value of the NE segment increased relatively slowly, remaining below 1. until August 5. This means that these aftershocks had the feature of an earthquake swarm. 3 Possible mechanism for the uneven aftershock distribution The above-mentioned uneven spatial distribution of aftershocks was possibly due to the earthquake-rupture process and its stress-field background, as discussed below. 3.1 Mainshock rupture The rupture process of the mainshock was studied by Zhao, et al [ 4 J, using teleseismic P-wave and SH-wave data recorded by 32 stations from the IRIS data center. Their inversion results showed that the earthquake was composed of five sub-events ( Ms7. 3, 7. 6, 7. 4, 7. 5, and 7. 4), and it had an obvious segmentation feature in the rupture movement with a borderline near Mianzhu. As shown in figure 5, the rupture began at the hypocenter and extended northeastward for about 3 km to southwestern part of Yinxiu-Beichuan fault, causing a surface rupture of about 25 km in the northern part. The rupture had two distinct segments: From the initial thrust displacement in the southwestern part of Distance along the strike (N231 E) on the plane of fault Figure 5 Distribution of static slips on rupture plane of W enchuan earthquake ( The star indicates the hypocenter) Yinxiu-Beichuan fault, it turned into right-lateral slip towards NE near Mianzhu and continued to Beichuan and Qingchuan with a maximum displacement of 6. 5 m in Dujingyan-Mianzhu. The SW segment ruptured to a depth of about 3 km with a fault displacement of up to 8. m; the displacement was completely upthrust at a depth less than 15 km, but was upthrust and right-lateral at a depth more than 15 km. In contrast, the NE segment ruptured only to a depth of 1 km with a maximal displacement of 6. m. The energy released by the SW segment of the earthquake was larger than that by the NE segment Regional stress background and the earthquake rupture' s effect on aftershock occurrence We calculated the earthquake-related static stress changes both with and without considering the background stress field, using Coulomb stress method by Okada [SJ and average stress data provided by Professor Xie Furen ( Fig. 6). By comparing the spatial distributions shown in figure 6 ( a) and ( b), we found that the results of calculated range of influence of the earthquake were different: H the background was not considered, the stress change was only positive in Yingxiu Wenchuan-Lixian-Heishui area and should have almost no influence on the NE segment ; this result cannot account for the aftershock of M s5. or above that occurred there ( Fig. 6 ( b ) ). However, if the background -stress field was considered ( Fig. 6 ( a ) ), the stress change showed a distinct spatial segmentation pattern, being positive ( having loading effect ) in W enchuan-maoxian -Songpan-Heishui-Lixian, Beichuan Pingwu-Qingchuan, and Yingxiu-baoxing areas, and negative (unloading) in Maoxian-Huya-Nanping Wenx-

4 1 Geodesy and Geodynamics Vol. 2 A BIMPa N ' ' (a) with consideration of average stress-field background 29N ' '---' (b) without consideration of average stress-field background -.2 Figure 6 Coulomb static stress and distribution of Ml ;l!: 5. aftershocks ian-pingwu and Maoxian -Chengdu-Anxian-Beichuan areas. The positive stress change here was more than. 5MPa, which is much more than. OlMPa needed to cause aftershock occurrence, as estimated in several previous papers[?-jol_ Thus we may attribute the observed aftershock occurrence in the NE segment to the stress-field adjustment caused by stress-field adjustment induced by the main shock Segmentation shown by larger aftershocks The same segmentation was also observed by Wang, et al [ ll] who estimated the stress field along the rupture zone by using FMSI[ 12 ' 13 l inversion method based on the moment-tensor solution inversion for aftershocks above Ms4.. Their calculated maximum principle stress directions shown in figure 7 are different between the same two segments as ours. confined to shallower depth ( Fig. 8 ( b) and ( c ) ). Figure 9 shows the temporal distribution of stress drops of the aftershocks along the entire fault [ 14 l. In the NE segment, most aftershock energy was released 4 days after the main shock Tectonic background The uneven slip distribution of the earthquake rupture might be related to the tectonic background of Longmenshan fault, which extends from Songpan -Ganzi orogenic belt northeastward to Y angzi block ( Fig. 1 ). The fault ' s movement was influenced increasingly by themotion of Indian plate since the Quaternary period Rupture-related uneven aftershock activity Figure 8 shows hypocenters of Ms ~ 3. aftershocks during a total period of 6 days after the earthquake ( determined with a precision of 2 km vertically and 1. 2 km horizontally). During the first 2 days, many aftershocks concentrated in the southwestern segment of the rupture zone around the hypocenter to a depth of 2 km ( Fig. 8 ( a) ) ; many aftershocks also occurred along the NE segment where co-seismic fault movement was smaller. During the subsequent two 2-day periods, the aftershocks along the NE segment tended to be deeper, while those along the SW segment tended to be 12E 13E 14E 15E 16E 17"E Figure 7 Spatial distribution of Ms ;l!:4. aftershocks and stress background

5 No.2 Li Zhixiong, et al. Uneven aftershock distribution of W enchuan M s 8. earthquake and possible mechanism 11 Dujiangyan Wenchuan Mianzhu Beichuan ~1.,9 il'... >.. 2. "' (a) Distribution ofm1;,.3. aftershocks within 2 days after main shock (Distance along the strike (231 o ) (ian) ) ~ 1 t Dujiangyan Wenchuan Mianzhu Beichuan Qingchuan I I I I I time (day),--,--~-,~~~~~--~~---,--~-,_l~~~~~,-~--,--, ~ 2 I_. o al9 4 1o 8 4 l ea o cf~ d' eo 1:9~ I \ ' ~,.,l " & a "'-~.;' t :.,~ :~.,._~ o ~ "' a. Il l+.~ ~ ~ ' ~ ' ~ :.. ~o~ :~ o ~: o o e o e o 1P ~~...,;. I fb ~ 8 a OJ G l&io., ' t C» (b) Distribution of Ml;,.3. aftershocks from 2-4 days after the main shock (Distance along the strike (ian) ) Dujiangyan Wenchuan Mianzhu Beichuan Qingchuan I I I I I time (day).. 6 o. ac ~. o : ;.. 81 ~... o ~.le ~a..,}ij-* ~~ Q>Q a. i.. IP o i T a ~ %.~ o ~ ~~ : ::: 55 o ~ ~ e ~. 1.., ~ 3> a.?," 5.,9 <> II II. ~ G II Q il' (c) Distribution of Ml;:.3. aftershocks during 41-6 days after the main shock (Distance along the strike (km) ) 4 Figure 8 Distribution of Ml~3. aftershocks along the strike for three different periods However, the level of activity in different regions has been different. Based on analysis on topography, geological structure, Bouguer gravity anomaly, and earthquake activity, etc., we found that the SW and NE segments were divided by Huya and Leidong faults. The movement activity was strong in the SW segment since late Pleistocene epoch, but a little weaker in the NE segment. During the earthquake, the rupture started from the hypocenter in the SW segment, where the tectonic energy might be more completely released; it propagated towards NE and might be hindered by Huya and Leidong faults, resulting a relatively incomplete energy release in the NE segment, which was subsequently made up by the aftershocks. Thus, the uneven spatial distribution of Wenchuan aftershocks was consistent with the segmentation of Longmenshan fault m its long-term geotectonic environment. 4 Conclusions Based on the above-mentioned observations and analysis, we tentatively conclude : 35rr~~-.~~~-.~~~-.~~~-.~ A2 3.. I I. ~ 25 ] ';; 2 ~ 15 a AI Cb Time/day Stress drop (MPa) Figure 9 -Beichuan.4 -Qingchuan -Wenchuan -Yingxiu Temporal distribution of stress drops of the aftershocks along the entire rupture

6 12 Geodesy and Geodynamics Vol. 2 References 33"N 32"N 31"N Figure 1 Heishui 13"E 14"E 15"E Legend ~ Quaternary l's:...:..ll rifted basin ~ uplift j:;g fault 16"E Geological setting of Longmenshan fault (Chen, et al[is]) 6km 1 ) Mtershocks of W enchuan earthquake during the initial two months showed different spatial and temporal distributions along two different segments of Longmenshan fault located, respectively, to the SW and NE of Mianzhu. 2 ) This segmentation of aftershock activity may be the result of a similar segmentation of the mainshock rupture process. 3) The segmentation of the rupture process, in turn, may be the result of a similar segmentation of the fault in its long-term geotectonic environment. Acknowledgments We are grateful to Wan Huimin and Lii Meimei coming from China Earthquake Networks Center for providing earthquake catalogue. We thank Professor Jiang Haikun for his help in the calculation of decay coefficient P value of earthquake sequence. Meanwhile we also thank the anonymous reviewers for their constructive comments for improving our manuscript. [ 1 ] Li Zhixiong, Shao Zhigang, Zhao Cui ping, et al. A segmentation study on the activity of Wenchuan Ms8. earthquake sequence. Earthquake, 29,29 ( 1) : (in Chinese) [ 2] Utsu T. A statistical study on the occurrence of aftershocks. Geophys Mag., 1961, 3: [ 3 ] Utsu T and Matsu' ura R. The centenary of the Omori formula for a decay law of aftershock activity. J Phys Earth., 1995, 45 : [ 4] Zhao Cuiping, Chen Zhangli,Zhou Lianqing,et al. A study on focus rupture processing and segmentation feature of W enchuan Mw8. earthquake. Scientia Sinica, 29,54(22) : [ 5] Okada T. Internal deformation due to shear and tensile faults in half-space. Bull Seism Soc Amer., 1992,82: [ 6] Wei Hua, Chen Zhangli, Li Zhixiong, et al. A study on Wenchuan M s8. earthquake triggering and spatial distribution of its aftershock activity.earthquake,29,29(1): (in Chinese) [ 7] Hardebeck J L, Nazareth J J and Hauksson E. The static stress change triggering model : Constraints from two southern California aftershock sequences. J Geophys Res., 1998, 13: [ 8] Reasenberg P A and Simpson R W. Respone of regional seismicity to the static stress change produced by the Lorna Prieta earthquake. Science, 1992,255: [ 9 ] Harris R A. Introduction to special section: Stress triggering, stress shadows, and implications for seismic hazard. J Geophys Res., 1998,13 : [ 1 ] Stein R S. The role of stress transfer in earthquake occurrence. Nature,1999,42: [ 11] Wang Qincai, Chen Zhangli and Zheng Sihua. Spatial segmentation feature of focus mechanism and stress direction in W enchuan earthquake aftershock sequence. Science Bulletin, 54 ( 16) : [ 12] Gephart J W and Forsyth D W. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to San Fernando earthquake. J Geophys Res., 1984, 89: [ 13] Gephart J W. Stress and the direction of slip on fault plane. Tectonics, 1994,(4) : [ 14] Wei Hua, Chen Zhangli and Zheng Sihua. A study on segmentation feature of focus parameter of W enchuan M s8. earthquake sequence in 28. Acta Geophysica Sinica, 29, 52 ( 2) : [ 15 ] Chen Guoguang, Ji Fengju, Zhou Rongjun, et al. Preliminary study on segmentation feature of Longmenshan fault activity in late quaternary. Seimology and Geology,27,29(3) :