| The Tibetan Plateau is a region with intensive tectonic deformation and high level earthquake activity. On the basis of historical records, there have been 18 earthquakes with magnitude over 8 and more than 100 events with magnitude between 7 and 7.9 in the plateau. All of these events occurred either along plate boundary of the Himalaya or along boundaries among the intra-plate fault blocks as well as sub-blocks. Historical records show that large earthquakes are clustering along the boundary faults of active blocks in the Tibetan Plateau. Each phase of the cluster is characterized by events of magnitude over 8 and a number of events of magnitude over 7 to form an earthquake series, since instrument seismic records. This might imply that Large earthquakes on the boundary faults of active blocks with temporal and spatial migration patterns, which shows response characteristics of large earthquakes on the block boundary.More than ten large Mw≥6.5 earthquakes, including the devastating Wenchuan Mw 7.9 event of 12 May 2008, the Yushu Mw6.9 event of 14 April 2010 and the Lushan Mw 6.6 event of 20 April 2013 have struck the boundary of Bayan Har block in the most recent 50 years. The latest large earthquakes, on the Garze-Yushu and Longmenshan faults, were generated by the uneven eastward extrusion of the Bayan Har and Sichuan-Yunnan blocks, which suggests that the Bayan Har block is one of the most active blocks in the Tibetan Plateau. However, there haven’t any M≥6.5 earthquakes except the 2014 Ludian Ms 6.5 earthquake happen on the eastern and southwestern boundary faults of Sichuan and Yunnan block that is adjacent and has the same geodynamic background to the Bayan Har block. So, whether the Ludian Ms 6.5 earthquake on the southern tip of the Daliangshan fault suggests that the strong earthquake activities have transferred from boundary faults of Bayan Har block to the boundary faults of Sichuan and Yunnan block? Thus, the future strong earthquakes and their risk study on the boundary faults of Sichuan and Yunnan block becomes more urgent. It’s a very effective way to understand faulting characteristics from the perspective of historical earthquake activity. However, to learn the fault behavior on base of the seismic instrumental records and historical records data with transient problems, and may lead to limitations of understanding. Paleoseismic research can make up the transient and limitations of historical and instrumental earthquake records in a great degree, and enable us to understand long-term faulting behavior and estimate the risk of future earthquakes in several earthquake recurrence intervals.To enhance our understanding on the earthquake environment and rupture characteristics and response relation on the boundary faults of Sichuan and Yunnan block. We make a comparative research on the boundary faults of Sichuan and Yunnan block to study the latest rupture characteristics and activity history of the Baogunao-Xiaohe fault (new infancy fault zone of the southern tip on the Daliangshan fault), recurrence rule of Xiaojiang fault (maturity phase fault zone) and the activity (it is dead or not) of the middle and southern segments of the Red River fault (old age fault zone?), on base of high resolution remote sensing image interpretation, field geological landform reconnaissance, trench site selection and fine paleoseismic research, and so on. That’s to provide the reference on the seismogenic mechanism, seismic risk analysis and future large earthquake danger zone division on the boundary faults of Sichuan and Yunnan block. The following conclusions were drawn:1) The northwest striking Baogunao-Xiaohe fault ① is the seismogenic structure of the 2014 Ludian Ms 6.5 earthquake, with left-lateral strike slip patterns, and may belong to the southern termination of the Daliangshan fault. ②The Ludian Ms 6.5 earthquake caused several discontinuous segments of NW-striking surface ruptures, from Xiejiayingpan to Wangjiapo about 8 km long. These surface ruptures with 30-35 cm left-lateral dislocation and 20-25 cm dip-slip component, the maximum horizontal dislocation is about 62±5cm.③The Ludian Ms 6.5 earthquake caused numerous ground fissures, landslides and other secondary geological disaster not only along the northwest striking fault but also on the northeast trending Zhaotong-Ludian fault that is conjugating with NW-striking fault. Numerous northeast striking tectonic fissures, Landslides and other geological secondary geological disasters, the ground motion traces of strong motion station at Longtoushan and the source rupture process showed that the Ludian earthquake was a compound fault rupture event. Even if the focal mechanism, distribution characteristics of aftershocks and Intensity isoseismal line distribution characteristics support the result that the seismogenic structure is the northwest striking fault.③We opened a trench on the Xiaoyakou segment of the surface ruptures. Four earthquakes were identified and named E1 to E4 from the oldest to the youngest. We constrained the ages of the paleoseismic events at ～8950 BC,1000 BC～900 AD,910-1240AD and 2014 AD, respectively. The latest three earthquakes E2, E3 and E4 with good age constraint and average recurrence interval of 560-1500 yr.⑤Paleoseismic research shows that the Baogunao-Xiaohe fault is the Holocene fault and belong to the juvenile stage active faults on the southern termination of the Daliangshan fault. Due to the weak intensity of fault activity and relatively long term recurrence interval of strong earthquakes, faults revealed from the trench section were hard to distinguish, thus increasing the paleoseismic recognition difficulty, also increasing the possibility of earthquake leak down.⑥ The Xiaoyakou trench showed that a earthquake with magnitude 6.5 could cause surface rupture and leave paleoearthquake remains in Yunnan Province.2)The Xiaojiang fault with a long history of formation and evolution. It’s one of the ingredients of the northsouth trending tectonic zone in China, and also the east boundary fault of the Sichuan and Yunnan block. The Xiaojiang fault is the strongest earthquake active zone in Yunnan Province since historical earthquake records.①We excavated two large trenches at the Ganhaizi trough on the west branch of the middle segment of Xiaojiang fault. Several paleoearthquakes were revealed from trench sections. Three Holocene paleoearthquakes were identified and named GHZ-E1 to GHZ-E3 from the oldest to youngest, and were constrained by the sequence of 4c dating samples and age correction at 3300BC～400 AD,770～1120 AD, and 1460AD～present, respectively.②We opened a large trench with 60m long at the northern edge of Caohaizi sag pond on the western branch of the Xiaojiang fault. Six paleoseismic events have been identified, and named E1 through E6 from the oldest to the youngest. Charcoal and woods are abundant,20 samples were dated to constrain the ages of the paleoseismic events at 40 000-36 300 BC,35 400-24 800 BC, 9 500 BC-AD 500, AD 390-720, AD 1120-1620 and AD 1750-present. ③The Ganhaizi trench and Caohaizi trench were opened on the western branch of Xiaojiang fault, the distance between the two trenches were 400m. We constrained four late Holocene paleoearthquakes with successive finite method at 500～720 AD,770～ 1120 AD, AD 1120～1460 and AD 1833, with a average recurrence interval of 370～440 yr. Large earthquake recurrence in the late Holocene is far less than the recurrence of 2000～4000 yr posed in previous studies.④We opened a trench at Dafendi trough on the western branch of Xiaojiang fault. Three paleoearthquakes were identified and constrained at 22930～22570BC,22750～22340 BC and 1833AD.Four trenches were opened by us and previous paleoearthquake research revealed that there were several large earthquake remains on the western branch of Xiaojiang fault. Faults were very clear in the trench sections, faulted layers and covered stratum were easy to be distinguished. Colluvial wedges and other paleoseismic identifications were obvious. Historical earthquake records and paleoseismic research show that the Xiaojiang fault is very active and a "maturity" active fault since late Quaternary. Combining previous and this article paleoseismic research from Dongguaying to Caohaizi segments, and a large number of 14 c dating results show that the Xiaojiang fault is likely to be ruptured in single segment from late Pleistocene to early Holocene, and large earthquakes were clustering and ruptured the whole western branch since late Holocene.3) The Red River fault zone:①We opened three trenches at Dayakou segment where deformation preservation well on base of digital image analysis and field reconnaissance on geology and landform. Three paleoearthquakes were identified and named E1 to E3 from the oldest to the youngest.20 samples were dated to constrain the ages of the paleoseismic events at ～1190 BC,341 ～126 BC and 1460～1784 AD. We associate the youngest event E3 with the M7 1652 earthquake. The recurrence interval between E2 and E3 is 1780-1990 yr. Paleoseismic research showed that there were several strong earthquakes happened on the middle segment of the Red River fault, and with Long recurrence interval; Furthermore, The 1652 Midu M7 earthquake caused surface rupture at Dayakou on the middle segment of the Red River fault, the surface rupture might extend to Xiangguolin area. ② We found fault troughs and drainage synchronization dislocation were obvious and showed more than ten km on the Gasa-Yaojie segment of the Red River fault zone, on base of topography and geomorphology investigation. This imply that the Southern segment of the Red River fault is still relatively active since late Quaternary. Two trenches were excavated at Daha village, faults and paleoearthquakes were revealed from the trench sections.14C dating results show that there have several strong earthquakes during Holocene on the southern segment of the Red River fault zone.③ Trenches excavated at Faqi village revealed three paleoearthquakes. We named them from FQC-E1 to FQC-E3 from the oldest to the youngest, and constrained at 4800～2000 BC,1700～650 BC, and 320～1660 BC. Combining historical earthquake record, we associated the youngest event FQC-E3 with the 1539 AD earthquake. We get 1770-3170 yr recurrence interval of the three paleoearthquakes. We also evaluated the slip rate is about 2.0～4.3mm/yr on the Gasa-Yaojie segment of the Red River fault zone since the middle and late Holocene. Assuming that the three paleoearthquakes have the same magnitude and with the same coseismic displacement, we evaluated the magnitude interval is 7.2～8.0 and with mean magnitude of 7.6.4) Comparative analysis of rupture characteristics research on different development period boundary faults of the Sichuan-Yunnan block in Yunnan Province shows that: ① The infancy Baogunao-Xiaohe fault is Characterized by geometric discontinuity in space. Small and strong earthquakes occupy the main activity of the infancy fault, because there are many small obstacles on the fault have not been broken through. The strong earthquake mainly rupture in single segment of the fault, the recurrence interval of strong earthquakes is about 500-1500 yr, and magnitude 7 is the maximum earthquake on the fault.②As the "maturity" structure, the Xiaojiang fault zone is characterized by continuous and linear features on the space, without small bump obstacle. Strong and large earthquakes play main role on the fault, recurrence interval of strong earthquakes generally less than 500 yr, and the maximum earthquake of the fault is to be magnitude 8; ③As the "old age" active fault, the Red River fault zone distributes continuously and with large scale tectonic size, dominated by large earthquakes on the fault. The recurrence interval of large earthquakes is about 1770～3170 yr. The maximum earthquake of the "old age" active fault can be up to magnitude 8. |
PDF Full Text Request