Vol.17 No.2 (230~243) ACTA SEISMOLOGICA SINICA Mar., 2004 Article ID: 1000-9116(2004)02-0230-14 Study on distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background∗ HAN Wei-bin (韩渭宾) JIANG Guo-fang (蒋国芳) Earthquake Administration of Sichuan Province, Chengdu 610041, China Abstract In the paper, the distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background, especially the relation to Sichuan-Yunnan and Sichuan-Qinghai crustal blocks have been studied. The main results are: a) Strong earthquakes in Sichuan-Yunnan area distribute mainly in Sichuan-Yunnan and Sichuan-Qinghai crustal blocks; b) Most of strong earthquakes of the two blocks distribute mainly along their boundary faults; c) A few strong earthquakes are not obviously related to active faults. It shows that the relation between strong earthquakes and geological tectonics can be very complex; d) There is a certain correlativity for seismic activities among boundary faults of the two blocks, but they have different features; e) There are some anomalous changes of velocity structures in the deep crust of boundary faults of the two blocks. Many boundary faults, especially Longmenshan fault, cut obviously the Moho discontinuity. The Xianshuihe fault, a typical strike-slip fault, has no obvious indication of cutting the Moho discontinuity, but has distinct low-velocity zone in different depths. Key words: Sichuan-Yunnan area; seismicity; crustal block; deep structure CLC number: P315.2 Document code: A Introduction Both Sichuan and Yunnan are provinces with more earthquakes. Based on catalogue of strong earthquakes in China compiled by the Prediction Department of China Earthquake Administration, there are 639 M≥5.0 earthquakes during 26 B.C.~A.D. 2001. Among them, 475 are M=5.0~5.9 events, 124 are M=6.0~6.9 events, 39 are M=7.0~7.9 events, and one is M=8 event occurred in Sichuan and Yunnan area. Here is one of the areas where seismic activities are most active in China. Sichuan-Yunnan area is located at the east edge of Qinghai-Xizang (Tibet) plateau, where is also the middle-southern section of North-South Seismic Zone in the central part of Chinese mainland. There are many favorable conditions for studying regional seismicity and its relation to geological structure, because of the particular geologic structures and more strong earthquakes ∗ Received date: 2002-12-24; revised date: 2003-04-09; accepted date: 2003-05-03. Foundation item: State Key Basic Research Development and Programming Project of China (G1998040700/95-13-02-03). No.2 HAN Wei-bin, et al: DISTRIBUTION CHARACTERISTIC OF EARTHQUAKES IN CHUAN-DIAN 231 here. After 1970, seismic organizations were set up in Sichuan and Yunnan Province. As a duty department of provincial government and under the leadership of China Earthquake Administra- tion (CEA), the overall regional seismic work was carried out by these seismic organizations or- derly. In the past more than 30 years, they set up more than 170 seismic stations, improved unin- terruptedly observational technologies, investigated systematically historical strong earthquakes, researched scientifically recent strong earthquakes, and developed successfully geological map- ping of 1:50 000 for active faults. CEA, ADC and The Ministry of Geology and Mineral Re- sources of China (MGMRC) completed 15 artificial seismic sounding profiles with a total length of 6 870 km in Sichuan-Yunnan area. In studying earthquake prediction, the prediction depart- ments of earthquake administration of the two provinces have accumulated knowledge about seismicity characteristics. Some authors have made the studies on this subject and obtained many valuable results. The authors of this paper attempt to study further distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background on the basis of predecessor′s work and the work of their own. The focal point in the paper is the study on the relationship between strong earthquakes and Sichuan-Yunnan and Sichuan-Qinghai crustal blocks, particularly on the relationship between strong earthquakes and crustal deep structures. 1 Strong earthquakes in Sichuan-Yunnan area distribute mainly in both Sichuan-Yunnan and Sichuan-Qinghai crustal blocks In the end of 1970s and the beginning of 1980s, KAN, et al (1977), LI and WANG (1977), HAN and XIA (1980) advanced the existence of Sichuan-Yunnan and Sichuan-Qinghai crustal blocks and their movements towards southeast. The main bases were: strong earthquakes distrib- uted mainly along the boundary fault zones of the two blocks, the focal mechanisms of strong earthquakes and dynamic nature of faults found by field investigation were consistent with the moving direction of the crustal blocks, earthquake activities of these boundary faults were interre- lated, and so on. In more than 20 years after, results from studies on distribution of strong earth- quakes, focal mechanism, and field investigation of active faults testified further this viewpoint. Recently, the authors of the paper summarized specifically the new evidences for Sichuan-Qinghai crustal block and their movement towards southeast (HAN, JIANG, 2003). Figure 1 shows Si- chuan-Qinghai and Sichuan-Yunnan crustal blocks and their movements given in that time (HAN, XIA, 1980). As an important basis for judging the two crustal blocks and their movement towards southeast, the fault-plane solutions and P axes of strong earthquakes determined in those years were displayed in the figure. In more than 20 years after, except Batang M=6.7 earthquake swarm in 1989, the focal mechanisms of strong earthquakes occurred on these boundary faults of the two blocks sustained also the above judgment (SU, QIN, 2001). Figure 2 shows the epicenter distribu- tion for M≥5.0 earthquakes in Sichuan-Yunnan area. There were 423 M≥5.0 events in the two blocks in the period of 26 B.C.~A.D. 2001. The events accounted for 66.2% of the total M≥5.0 earthquakes occurred in both Sichuan and Yunnan Province. Among them, 1 event was M=8, 29 events were M=7.0~7.9, and 90 events were M=6.0~6.9, making up 100%, 74.4% and 72. 6% of the total events with corresponding magnitudes in the two provinces, respectively. It shows that strong earthquakes mainly distribute in the two blocks, moreover, the higher the magnitude is, the larger the proportion of earthquake numbers of the two blocks to that of the two provinces. 232 ACTA SEISMOLOGICA SINICA Vol.17 Figure 1 Sichuan-Qinghai and Sichuan-Yunnan crustal blocks and their movements (HAN, XIA, 1980) 1 Xianshuihe fault; 2 Longmenshan fault; 3 Shuergan-Huashixia fault; 4 Huya fault (One segment of Minjiang faults) Seeing from Figure 2, the fault strikes change progressively to NE to the east of two blocks, but distribution direc- tion of moderately strong earthquakes are obviously NNW. With the increase of dis- tance to the east, earthquake strength de- creases. For example, Mingshan-Mabian- Zhaotong seismic zone is the nearest one in east of the two blocks. There are NNW or NS faults along this zone, and some NE faults intersect them here. Based on geo- physical research, here is a gravity gradi- ent belt. There were 47 M≥5.0 earth- quakes in this seismic zone, among them, 2 events were M=7.0~7.9 and 6 events were M=6.0~6.9. To further east, there is Renshou-Zigong-Yibin seismic zone with the strike of NW, too. There were 14 M=5.0~5.9 events and the largest one was the M=5.8 Fushun earthquake occurred in Zigong city in 1896. However, the strike of Huayingshan fault is NE here. We can see that the distribution of moderately Figure 2 Distribution of M≥5.0 strong earthquakes in strong earthquakes in the east of the two Sichuan-Yunnan area (26 B.C.~A.D. 2001) crustal blocks is also influenced by the No.2 HAN Wei-bin, et al: DISTRIBUTION CHARACTERISTIC OF EARTHQUAKES IN CHUAN-DIAN 233 lateral pressure of southeast moving of the two blocks. A few researchers attempted to divide blocks using Nujiang-Lancangjiang fault as a boundary, but there were no M≥5.0 earthquakes occurred in the fault. There were 8 M=7 events and tens of M=6 events along Tengchong-Lancang-Gengma seismic zone. However, no great faults with strike NW can be found here, but a series of NE-trending faults with equal intervals such as Ruili-Longling fault, Nandinghe fault, Menglian-Lancang fault, and Daluo-Jinghong fault (JIANG, 1993), which are all left-lateral strike-slip faults. How to divide crustal blocks in this region is still a question to be studied. 2 Most of strong earthquakes of the two blocks distribute along their boundary faults The east boundary of Sichuan-Yunnan block consists of Xianshuihe fault, Anninghe-Zemuhe fault and Xiaojiang fault. Its west boundary consists of Jinshajiang fault and Honghe fault. The boundary of Sichuan-Qinghai block consists of Xianshuihe fault, Minjiang fault, Longmenshan fault and East Kunlun fault. The Xianshuihe fault is the boundary between the two blocks. There were 380 M≥5.0 earthquakes along these boundary faults. The events amounted to 89.8% of the total M≥5.0 earthquakes occurred in the two blocks. Among them, 1 event was M=8, 27 events were M=7.0~7.9 and 84 events were M=6.0~6.9, making up 100%, 93.1% and 93.3% of the total corresponding magnitude earthquakes in the two provinces, respectively. Some moderate earthquakes and a few strong earthquakes occurred also inside the two blocks. From Jiulong, Yajiang, Yanyuan, Ninglang, to Yongsheng and Lijiang, some M=5 or M=6 earth- quakes occurred and even a M=7.5 event in Yongsheng in 1515. Jinhe-Qinghe fault, Chenghai fault and so on are located here, which are mutation zones of geomorphology and crustal thickness. The Middle Yunnan plateau with an elevation of 2 500 m is on the southeast of these faults, its crustal thickness is 48~52 km. The West Sichuan plateau with an elevation of 4 500~5 000 m is on the northwest, its crustal thickness is more than 60 km (SU, et al, 1999). Some people consider it as a boundary between the two secondary blocks of Middle Yunnan and West Sichuan. In 1948, the Litang, Sichuan, M=7.3 earthquake occurred in Dewu fault, a left-lateral fault with strike NW. However, this is a remote mountainous district and field investigation is very difficult. In addition, its historical record is very short and there are fewer events occurred in the recent tens of years. As time goes on, some earthquakes might occur and field investigation might be carried out further, then, the faults could be considered as the boundaries of secondary blocks. Of course, there were a few moderate events inside the two blocks, for example, in Xiaojin, Rangtang, Huili, Wuding, Yao′an earthquakes and so on. 3 A few strong earthquakes are not obviously related to active faults. It shows that the relation between strong earthquakes and geological tectonics can be very complex For many strong earthquakes, for example, the M=7.6 Luhuo earthquake in 1973, M=7.8 Tonghai earthquake in 1970 and so on, their surface rupture belts are very obvious and related clearly to seismogenic faults. Whoever goes for field investigation, the conclusions are almost the same, so are the focal mechanisms made by different institutions. Moreover, one of nodal planes of their focal mechanisms is always consistent with the seismogenic fault and the long axis of iso- 234 ACTA SEISMOLOGICA SINICA Vol.17 seismic line. These strong earthquakes are related obviously to active faults. The events occurred in boundary faults of the two blocks, particularly in the lateral faults like Xianshuihe fault are of this kind. But there are a few exceptions, for example, the M=7.5 Diexi earthquake in Sichuan Province in 1933, M=7.1 Yongshan earthquake in Yunnan Province in 1974 and M=6.6 Batang earthquake swarm in Sichuan Province in 1989 and so on. The relation between these events and geologic structure is very complex. Figure 3 shows 4 types of isoseismic lines for the M=7.5 Diexi earthquake occurred in Si- chuan Province on August 25, 1933 obtained by different investigators in different institutions. The earlier investigator drew the isoseismic line in proximate EW direction and considered Canlingshan rupture zone as the seismogenic fault. The later careful investigation found that Canlingshan rupture zone is gravity landslide. Which fault is seismogenic fault? Is it in the direc- tion of EW, NS, or NW? Up to now, the geologists have still different viewpoints. Figure 3 4 types of isoseismals for the M=7.5 Diexi earthquake occurred in Sichuan Province on August 25, 1933 (from TANG, HAN, 1993) 1 Meizoseismal region; 2 Seriously damaged region; 3 Slightly damaged region; 4 Influenced region (a) From CHANG Long-qing; (b) From catalogue of earthquakes in China; (c) From Southwest Seismic In- tensity Team of CEA; (d) From Seismogeological Team of Earthquake Administration of Sichuan Province For the M=7.1 Yongshan earthquake occurred in Yunnan Province on May 11, 1974, CEA sent early or late two investigation teams to do field investigations. However, their results for iso- seismic line and seismogenic fault were quite different, one was NW and the other NE. Up to now, the scientists have still contentions on this question. From April 16, 1989, the M=6.6 Batang earthquake swarm occurred, which was composed of No.2 HAN Wei-bin, et al: DISTRIBUTION CHARACTERISTIC OF EARTHQUAKES IN CHUAN-DIAN 235 4 M=6 events and several M=5 events, it was interesting to notice that Jinshajing fault, in the vi- cinity of Batang earthquake swarm, consists of a series of NS-trending faults, while the 4 M=6 events, several M=5 events and a great number of aftershocks distributed nearly in the direction of EW. And there was always one nodal plane with the direction of about EW in the focal mecha- nisms of these M=6 and M=5 events (Seismological Bureau of Sichuan Province, 1994). Some people consider there are EW-strike hidden faults, while other people think that moderate earth- quakes occurred one after another in several NS-strike faults with small dip angles, and their af- tershocks jointed together to form a proximate EW-trending belt. Which viewpoint is correct? Further study is needed. The seismogenic faults of these strong earthquakes were so unobvious that different investi- gators would give different conclusions. Or although there are active faults with a certain scale, corresponding seismic fractures could not be found, and the long axis of isoseismic lines, the long axis of aftershocks distribution and one of the nodal planes of focal mechanisms were not consis- tent with the faults. In Sichuan-Yunnan area, these kinds of cases are only a few, but attention should be paid. 4 Analysis and comparison between seismicities of several boundary faults of Sichuan-Yunnan and Sichuan-Qinghai crustal blocks 4.1 The strength of strong earthquakes in the east boundary faults of Sichuan-Yunnan block was higher than that in its west boundary faults, but the frequency of moderate earthquakes in the east boundary faults of Sichuan-Yunnan block was lower than that in its west boundary faults Based on catalogue of strong earthquakes in China, Table 1 shows the earthquake numbers for several magnitude sections, b value and its correlation coefficient and standard deviation in several periods for both east and west boundary zones of Sichuan-Yunnan block. It can be found from Table 1 that the M≥7.0 earthquakes on east boundary zone were more than that on west boundary zone, while the M=5 or M=6 moderate events on west zone were more than that on east zone. The largest event of east zone was M=8, while the greatest earthquake of west zone was only M=7.8. The difference of earthquake numbers of different magnitudes can be confirmed by the difference of b value that is the slope of relationship between earthquake numbers and magnitude. The b values of east zone were obviously lower than that of west zone. Of course, complete earthquake catalogue and enough samples are very important for getting reliable b value. However, it could not be ensured that the records of M=5 events are complete even if in the early 20th cen- tury. Therefore, statistical results of several periods are given in the paper, because the compara- tive conclusions obtained from these statistical results in different periods are consistent; the Table 1 Earthquake number N and b value in different periods and magnitude sections for east and west boundary zones of Sichuan-Yunnan crustal block Earthquake number N Correlation Standard Seismic zone Period b value M=5.0~5.9 M=6.0~6.9 M=7.0~7.9 M=8.0 coefficient r deviation Sb East boundary Total 83 32 12 1 0.61 0.97 0.04 zone 1700~2001 69 30 9 1 0.59 0.98 0.03 1900~2001 52 19 3 0.71 0.99 0.02 West boundary Total 154 41 11 0.78 0.98 0.04 zone 1700~2001 129 34 8 0.85 0.98 0.05 1900~2001 109 24 4 0.85 0.99 0.04 236 ACTA SEISMOLOGICA SINICA Vol.17 correlation coefficients for calculating b value are all more than 0.97; and the differences of b val- ues between east zone and west zone are more than their standard deviations. Therefore, the above characteristics obtained from the comparison should be reliable. The experiment results of Mogi (1962) indicated that b value is related to homogeneous de- gree of material and structure of rock samples. When rock samples are inhomogeneous, the b value is high. The experiment results of Scholz (1968) showed that b value is related to stress con- dition. If stress is high, the b value is low. The east boundary zone consisting of Xianshuihe, Anninghe, Zemuhe and Xiaojiang faults, in general, is a larger left-lateral fault. Although the strength and frequency of earthquake activities of each fault on east zone were slightly different, there were strong events in every fault. While the west boundary zone consisting of Jinshajiang and Honghe faults with right-lateral strike-slip was different. Strong earthquakes concentrated in some sections of these faults. For example, strong events of Jinshajiang fault concentrated only in the vicinity of Batang. Just as mentioned above, the relationship between Batang earthquake and Jinshajiang fault is very complex. Strong earth- quakes of Honghe fault distributed mainly at its two ends. The northwest Yunnan seismic zone at the north end of Honghe fault includes also the NS-trending Chenghai fault and NE-directional Lijiang-Jianchuan fault and so on; while the strong earthquakes at the south end occurred mainly in Qujiang and Shiping-Jianshui faults located on the north side of Honghe fault. Both Honghe and Jinshajiang faults have the longer sections without any strong earthquake. Therefore, we can say the material and structure of east zone are more homogeneous than those of west zone. Then it could be explained by the experiment results of Mogi (1962) why b value of east zone is lower than that of west zone. In addition, the slip rates of east boundary faults are higher than those of west one based on field investigation and GPS measurement. In the past twenty years, a number of geologists in China and the world (Seismological Bu- reau of Sichuan Province, 1989; WEN, et al, 1989; Allen, et al, 1991; LI, et al, 1997) have inves- tigated and researched carefully into Xianshuihe fault. The Seismogeological Team of Earthquake Administration of Sichuan Province has accomplished the active fault mapping of 1:50 000 for Xianshuihe fault. It is generally considered that the Recent average slip rate of Xianshuihe fault was 10~15 mm/a. The active fault mapping of 1:50 000 was also made for Anninghe, Zemuhe and Xiaojiang faults. The Recent average slip rate of Anninghe-Zemuhe fault was 6~7 mm/a (PEI, et al, 1998; DU, 2000) and that of Xiaojiang fault was 10 mm/a (SONG, 1998), while the Recent average slip rate of Honghe fault was much lower with an amount of 3.5 mm/a (Allen, et al, 1984; SU, et al, 1999). Based on the robust-Bayes least squares algorithm and dislocation model with several faults, the movement of boundary faults of Sichuan-Yunnan block was studied using the high-precision GPS data obtained in the period of 1991~1999. The calculating results show: a) Left-lateral slip rate of Xianshuihe and Anninghe faults was 30 mm/a and its dip-slip rate was 9~11 mm/a; b) Strike slip rates of Honghe, Chenghai and Heqing-Eryuan faults were 10 mm/a (right lateral), 11 mm/a (left lateral) and 13 mm/a (left lateral), respectively. Their dip-slip rates were 16 mm/a, 22 mm/a and 16 mm/a, respectively (SHEN, et al, 2002). Geologists study slip rate of active faults based on field investigation, analysis and explana- tion of satellite photograph and paleoseismic observation. Surveyors study slip rate of active faults based on data from GPS and other surveys. Because the time and space scopes in observations are quite different, we should not be surprised at the differences of their slip rates. What is important No.2 HAN Wei-bin, et al: DISTRIBUTION CHARACTERISTIC OF EARTHQUAKES IN CHUAN-DIAN 237 is that the average slip rates of east zone were always larger than that of west zone obtained from the relative comparison of their own. Then it could be considered that the stress of east zone was higher than that of west zone, so the seismicity in east zone was more than that of west zone. The experiment of Scholz (1968) could also explain this problem. 4.2 There is certain correlativity in both active and quiescent periods of east and west zones of Sichuan-Yunnan block, but there were respective characteristics in seismicity of every fault That the M≥6.0 earthquakes of east and west zones of Sichuan-Yunnan block change with time is showed in Figure 4. The ordinate of Figure 4 is latitude and its abscissa is time. There were alternately the most obvious active and quiescent periods in the north section of east zone, i.e., Xianshuihe fault. The periods of 1725~1816 and 1893~1982 were active periods. But there was not any M≥6.0 earthquake, even M≥5.0 event in the period of 1817~1892. It could be considered as a typical quiescent period of strong earthquake, because there was not any special matter for loss of his- torical records in this period. The alternation of active and quiescent periods in the middle and south sections of east zone was not as obvious as that in the north section. How- ever, it could be found there were only the M=8.0 Songming earthquake in 1833 and the M=7.5 Xichang earthquake in 1850, and not any M=6 event in the middle and south sections of east zone during the quiescent period of its north section; while during the active periods of its north section, there were so many M≥6.0 events in the middle and south sections of east zone. During the quiescent period of north section of east Figure 4 ϕ -t pattern of seismicity in east (a) and west zone, the number of M≥6.0 earthquakes in (b) zones of Sichuan-Yunnan crustal block the middle and south section of west zone was obviously smaller than that during the former and latter active periods. It seems there was a similar alternate feature in the middle and south section. In the north section (Jinshajiang fault), M≥5.0 earthquakes were so few that it is unable to divide active period and quiescent period. However, it was interesting that during the above quiescent period, only the M=7.5 Batang earthquake occurred in 1870 in Jinshajiang fault. Because there were no earlier historical records in the north section of both east and west zones of Sichuan-Yunnan block, Figure 4 was drawn using the data after 1700. Although the M=5 or even M=6 earthquakes might be lost in the 18th and 19th centuries, we have enough reasons to believe that the quiescent period is not due to the loss of historical records, because there were so many moderate and strong earthquakes in more than 100 years before the quiescent periods. If we draw Figure 4 using the data of M≥5.0 events, the above conclusion can also be ob- tained, but the image is not as clear as that obtained by using the data of M≥6.0 events. 238 ACTA SEISMOLOGICA SINICA Vol.17 4.3 In several boundary zones of Sichuan-Qinghai block, the seismicity of Xianshuihe seismic zone was stronger than that of Songpan-Longmenshan seismic zone (Minjiang fault and Longmenshan fault), and the higher the earthquake magnitude, the larger the difference of earthquake numbers. The b value of the former was lower than that of the latter There are a few historical records for the East section of east Kunlun fault as the north boundary of Sichuan-Qinghai block. The comparison of earthquake number N and b value in sev- eral magnitude sections between Xianshuihe seismic zone as west boundary and Songpan- Longmenshan seismic zone as east boundary of Sichuan-Qinghai block are given in Table 2. Table 2 Earthquake number N and b value of east and west boundary zones of Sichuan-Qinghai crustal block Earthquake number N b Correlation Standard Seismic zone Period M=5.0~5.9 M=6.0~6.9 M=7.0~7.9 value coefficient r deviation Sb Xianshuihe Total 30 16 8 0.55 0.96 0.04 seismic zone 1700~2001 29 16 8 0.55 0.96 0.04 1900~2001 19 10 4 0.54 0.98 0.03 Songpan- Total 31 11 4 (including double events) 0.61 0.98 0.04 Longmenshan 1700~2001 24 8 4 (including double events) 0.59 0.99 0.04 seismic zone 1900~2001 22 7 3 (including double events) 0.58 0.99 0.03 It can be seen from Table 2 that the comparison for different magnitude sections in different periods shows that the earthquake numbers of Xianshuhe seismic zone were generally larger than those of Songpan-Longmenshan seismic zone; and the higher the earthquake magnitude, the larger the differences. The b values in each period of Xianshuihe seismic zone were lower than those of Songpan-Longmenshan seismic zone. These characteristics obtained from the comparison should be reliable because the differences of b values of both seismic zones were obviously larger than their standard deviations. Songpan-Longmenshan seismic zone consists of Minjiang and Longmenshan faults. Long- menshan fault is a huge thrust fault. In geotectology, it is a boundary of different geologic units: Songpan-Ganzi folded region and Yangtze platform. However, its seismicity was not very strong, there was not M≥7.0 earthquake up to now. And the moderate earthquakes distributed mainly in the middle and south section of the fault. To the north of Jiangyou, there were no M≥5.0 earthquakes occurred along Longmenshan fault, while along Minjiang fault on the north of Longmenshan fault, 4 M=7.0 and several moderate earthquakes occurred one after another. Minjiang fault consists of several not too big faults like Minjiang, Xuebaoding, Huya and Songpinggou faults and so on. It is noticeable that the seismicity strength and frequency of a very big fault as Longmenshan were much lower than those of the smaller Minjiang fault. The reason might be that Minjiang fault was younger than Longmenshan fault. In addition, the gradient zone of Bouguer gravity anomaly dis- tributes along the middle and south section of Longmenshan fault to Beichuan and Jiangyou, turns north along Minjiang fault to Gansu Province. Here is the geomorphic boundary (HAN, XIA, 1980). In summary, the medium structure of Songpan-Longmenshan seismic zone is more inhomogeneous than that of Xianshuihe fault, and Longmenshan fault was older than Xianshuihe fault, which might be the reason for the difference of seismicity strength and frequency between the two zones. 4.4 Alternation of active and quiescent periods of Xianshuihe and Songpan-Longmenshan seismic zones are of certain correlativity and different features Figure 5 shows there was an alternation of active and quiescent periods with a longer time scale in both Xianshuihe and Songpan-Longmenshan seismic zones, but the time lengths were not No.2 HAN Wei-bin, et al: DISTRIBUTION CHARACTERISTIC OF EARTHQUAKES IN CHUAN-DIAN 239 equal. There were not any M≥5.0 earth- quakes occurred in 1749~1899 in Song- pan-Longmenshan seismic zone. Although certain historical moderate earthquakes in this period might not be recorded, it could be believed that this was a quiescent period, because 1 M=7 event, 4 M=6 events and 3 M=5 events were recorded in 1600~1748, which was earlier than the period of 1749~1899. Its time length was 151 year and is longer than that of 76 year (1817~1892) for the quiescent period of Xianshuihe zone. The relatively active and quiescent time sections could be divided in the second active period of two seismic zones. The beginning of the second active period and its relatively active time sections of Xianshuihe seismic zone were slightly ear- lier than those of Songpan-Longmenshan Figure 5 Alternation of active and quiescent periods seismic zone, see Figure 2 in HAN and XIA with a longer time scale in Xianshuihe (a) and (1980). Songpan-Longmenshan (b) seismic zones 5 There are anomalous changes in velocity structures in deep crust in boundary faults of Sichuan-Yunnan and Sichuan-Qinghai crustal blocks In recent more than 30 years, 15 explosive seismic sounding profiles have been carried out by China Earthquake Administration (CEA), Ministry of Geology and Mineral Resources of China (MGMRC) and Chinese Academy of Sciences (CAS). Their arrangements are shown in Figure 6. Some authors (LIU, et al, 1989; CHEN, et al, 1990; SUN, et al, 1991; WANG, et al, 2002) studied the 3-D velocity model in Sichuan-Yunnan area using the earthquake data. The results obtained from the project 8301 of explosion seismology with Tangke-Langzhong- Pujiang triangular sounding profiles by CEA (CHEN, et al, 1988) and from Heishui-Shaoyang and Huashixia-Jianyang sections by MGMRC (CUI, et al, 1996) have shown that Longmenshan fault cut obviously the Moho discontinuity. Although the data obtained from different sections are slightly different, they all exhibit that the crust on the east of Longmenshan fault is thin with a thickness of more than 40 km, and the vPn is more than 8.0 km/s; while the crust on the west of Longmenshan fault is thick with a thickness of about 50~60 km, and the vPn is less than 7.5 km/s. Similar results have also obtained from studies on 3-D velocity structure. WANG, et al (2002) studied in detail the 3-D velocity structure using a large amount of seismic stations and data and indicated that Longmenshan fault displayed a dividing feature of different velocity structures in 1 km, 10 km, 30 km and 50 km deep, that is, the velocities on the west of this fault are lower than those on its east. 240 ACTA SEISMOLOGICA SINICA Vol.17 In the east section of East Kunlun fault in Sichuan Province, there is no special explosive seismic section, but it can be seen from the Fig- ure 5 of the paper ″A Preliminary Study on the Crustal Velocity Structure of Maqin-Lanzhou- Jingbian by Means of Deep Seismic Sounding Profile″ (LI, et al, 2002) that the Moho discontinuity undulates greatly near Maqin in the boundary between Sichuan and Qinghai Province. We can consider that the east section of East Kunlun fault might cut the Moho discontinuity. The results of Lijiang-Xinshizhen profile show that Anninghe fault cut the Moho discon- tinuity near Xichang (CUI, et al, 1987). It can be seen from Figure 7 that a few deep faults marked Sikai and Xiaojiang faults cut the Moho discontinuity, too. Maybe different units named the faults different names. In this profile, the crust on the east of Sikai and Xiaojiang faults is Figure 6 Sketch map of artificial seismic sound- thin with a thickness of about 48 km, the crust ing profiles in Sichuan-Yunnan area 1. Tangke-Langzhong; 2. Tangke-Pujiang; 3. on the west of Anninghe fault is thick with a Langzhong-Pujiang; 4. Heishui-Shaoyang; 5. thickness of more than 60 km, and the crust Huashixia-Jianyang; 6. Lijiang-Xinshizhen; 7. thickness in the transition zone is 54~60 km. Laza-Changheba; 8. Lijiang-Zhehai; 9. Xichang- Mouding; 10. Eryuan-Jiangchuan; 11. Zhefang- We have noticed since 1980s that after the Binchuan; 12. Simao-Malong; 13. Simao- seismological network of Sichuan Province was Zhongdian; 14. Zhubalong-Zizhong; 15. Tangke- Benzilan built, the seismicity of Anninghe fault where Profile 1, 2, 3 are Project 8301 of CEA; 4, 5, 6, many historical strong earthquakes occurred is 7 are project of MGMRC; 8, 9 are projects of very weak, while along Shimian, Yuexi, Zhao- CAS; 10, 11, 12 are project 82 of CEA; 13 is project 86-87 of CEA; 14 and 15 are Project jue, Butuo, to Ningnan, small earthquakes are Eastern Tibet-Western Sichuan of CEA frequent in a belt distribution. It was called Puxionghe seismic zone at that time. In recent years, the geologists of Geological Institute of CEA and Earthquake Administration of Sichuan Province found some paleoseismic traces and Holocene active evidences here in field investiga- tions, then it was named Daliangshan fault. Maybe we should combine Anninghe fault with Dali- angshan fault as a section of east boundary of Sichuan-Yunnan block. Lijiang-Zhehai profile crosses Anninghe fault between Panzhihua and Huili. Its results indi- cate that the south section of Anninghe fault could not cut the Moho discontinuity (XIONG, et al, 1986). According Simao-Malong profile, KAN and LIN (1986) considered that in the deep Xiao- jiang fault, the seismic phases P3 and P4 discontinued, P2 was active, and the Moho discontinuity was deeper on the east of east branch of this fault. In the map of 3-D velocity structure given by WANG, et al (2002), the negative anomalous zone of velocity or the transition zone of velocity anomaly distribute also along Anninghe fault, Zemuhe fault, to Xiaojiang fault. These data of deep structures have shown further why only the north section of Anninghe fault is considered as the No.2 HAN Wei-bin, et al: DISTRIBUTION CHARACTERISTIC OF EARTHQUAKES IN CHUAN-DIAN 241 east boundary, which extends to the south from Xichang and turns northwest to link Zemuhe fault and Xiaojiang fault. Figure 7 Crustal structure of Lijiang-Xinshizhen profile (from CUI, et al, 1987) 1. Surface layer; 2. Granitic layer; 3. Low velocity layer; 4. Basaltic layer; 5. Fault The results of Laza-Changheba profile (CUI, et al, 1987) and Xichang-Mouding profile (YIN, XIONG, 1992) along Anninghe fault indicate that the Moho discontinuity is gentle, but it is slightly shallow in the south and deep in the north. Both Zhefang-Binchuan and Simao-Zhongdian profiles cross Honghe fault. KAN and LIN (1986) considered that there are a few deep faults in the north and middle sections of Honghe fault. The southern Yuanjiang shot is near Honghe fault. The arrival times of seismic phase P40 on both sides of shot differ clearly, and the Moho discontinuity is shallow in the south and deep in the north. The energy of seismic wave attenuates obviously crossing Honghe fault, and the depth of Moho discontinuity has a dislocation of several kilometers. In the map of 3-D velocity structure (WANG, et al, 2002), Honghe fault is clearly displayed; in particular, in the map of anomalous distribution of P-wave velocity in the depth of 30 km, Honghe fault is a transition zone between positive and negative anomalies. Negative anomalous is clearly showed to the north of Honghe fault, while to the south of Honghe fault, there are mainly positive anomalies. Therefore, the crustal average velocity to the north of Honghe fault is lower than that to its south, which is iden- tical with the results of artificial seismic sounding profile (HU, et al, 1986). Lancangjiang fault cuts the Moho discontinuity, which is shallow on its south and deep on its north (KAN, LIN, 1986). However, there are no M≥5.0 earthquakes in this fault up to now. The strong earthquakes in Tengchong-Gengma-Lancang seismic zone occurred often in the low-velocity region of the upper crust (SU, et al, 1999). The map of 3-D velocity structure (WANG, et al, 2002) displays also some differences of deep structure between this seismic zone and Sichuan-Yunnan block. The results of Zhubalong-Zizhong profile indicate that the crust thickness in the intersection region of Xianshuihe, Anninghe and Longmenshan faults is obviously different with more than 40 km on its east and about 60 km on its west. But it is not known which fault cuts the Moho discon- tinuity. Tangke-Benzilan profile crosses Xianshuihe fault near Daofu, but the results show that the depths of the Moho discontinuity on both sides of the fault are not obviously different (WANG, et 242 ACTA SEISMOLOGICA SINICA Vol.17 al, 2003). However, the results from the study on 3-D velocity structure by WANG, et al (2002) indicate that there are clearly anomalous zones of low velocity in different depths of Xianshuihe fault. Maybe this is the characteristic of strike-slip fault and the traces of cutting the Moho discon- tinuity are not obvious, but there are velocity anomalies at different depths inside the crust. The above results and discussions provide further the background of crustal deep structure for dividing crustal blocks in Sichuan-Yunnan area. Generally, the boundary faults of Si- chuan-Yunnan and Sichuan-Qinghai blocks are deep faults and most of them cut the Moho discon- tinuity. The dip-slip faults, such as Longmenshan fault, cut the Moho discontinuity more obviously. Xianshuihe fault, as a typical strike-slip fault, has not any traces of cutting the Moho discontinuity, but has clearly anomalous zone of low velocity. These boundary faults are often boundary or tran- sition zones between positive and negative anomalies, or negative anomalous zones themselves. Moreover, their variation could be different in different depths of crust. 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