The Seismogenic Fault Rupture Character And Recurrence Model Of Mani Earthquake And Yushu Earthquake

Since 55 ~ 60 Ma, the Indian-Eurasian collision formed the Tibetan Plateau, the youngest and highest plateau of the world, GPS data show that the convergent rate between the two plates still reach 35 ~ 42mm/a in recent days, based on this situation,there are numerous earthquakes happened in the interior plateau and its adjacent area.Previous studies have shown that there are series of large-scale active fault zones in the Tibetan Plateau from south to north: the Himalayan fault zone, Karakorum- Jiali fault zone, Garze Yushu – Xianshuihe fault zone, East Kunlun fault zone and Altyn tagh fault zone etc., which divide the Tibetan Plateau into Lhasa block, Qiangtang block, Bayan Har block, Qaidam block, etc., the surface rupture produced by high-magnitude earthquakes usually along the block boundary where faults distributed, the surface rupture distribution, the rupture pattern, the offset distribution alone the rupture are the direct evidence of seismogenic fault activity, the mechanical characteristics of the fault, earthquake recurrence model. These real samples also can verify the reliability of the numerical simulation results. Recent high-resolution remote sensing technology offers the possibility to obtain these data, combined with the quantitative research method of active tectonics, such as long-term fault slip rates obtained by the offset landforms, the paleoearthquake method for obtaining seismic sequence, we can make a synthetic analysis based on these information, we can also get the further crustal deformation behavior of the Tibetan Plateau, and testing various kinematic model of the Tibetan Plateau.Based on tectonic geomorphology, paleoearthquake, chronology and the region’s surface topography change constrainted by climate change and remote sensing techniques, this thesis systematically analysed the fault earthquake recurrence model and long-term slip behavior related to the 1997 Mani earthquake and 2010 Yushu earthquake. The surface rupture characteristics, structure implications, long-term slip behavior and recurrence model of these two earthquakes are discussed and obtained the following conclusions:We mapped the surface rupture zone of the 1997 Mani earthquake in details based on the recent high-resolution satellite images(0.5m) obtained after the earthquake. The surface rupture initiated from the west end point(35 ° 3’12.81 "N,86 ° 10’52.36" E) to the east end point(35 ° 24’7.62 "N, 88 ° 0’1.81" E), the total length is 170 km, the general strike is 76 °NE. According the rupture azimuth change,we divided the rupture into seven sections, such as the west Margai Caka section, the Margai Caka section, the East Margai Caka section, the Chaoyang Lake section, the East Chaoyang Lake section, the East Shuangduan Lake section, the easternmost section. Along rupture also the development of a wealth of surface rupture pattern,indicating different tectonic implications: in the Margai Caka section, due to the strike of the fault obvious deflection, combined with fault sinistral strike-slip, forming a local releasing bend, Margai Caka Lake which is located in the basin, at the other side of the fault the Suijia mountain rised relatively and a series of normal faults scarp located on the lake bank. The 1973 Mw 6.9 earthquake’s epicenter was also located in this region, involved extensional component is consistent with the local tectonic setting. In Chaoyang Lake section which near the epicenter of the 1997 Mani earthquake, ground shaking and landslides made the thrust deformation at the piedmont, this type of deformation is mainly caused by permafrost. In the east Chaoyang Lake section we can see a series of secondary rupture, the angle between the primary and secondary rupture increases gradually from west to east, this cracks located in zhe extension quadrant of the rupture. In the easternmost section the rupture forming a very complex rupture pattern, near the triple junction the azimouth of the rupture turned counter-clockwise nearly 90 ° then along the pre-existed north banch propagating and formed the end structure like a horsetail at the east end of the rupture, but not to continue along the original eastward propagation, combined with numerical simulation results had done by Nubuki K. the reason for the rupture of such style, is subject to pre-existing fracture distribution, principal stress directions and rupture propagating speed and other factors working together.According to the tomographic reconstruction we obtained 210 measurements along the fault, the amount of displacement ranging from 2m to 249.8m, which includes both co-seismic offset of 1997 earthquake and cumulative offset caused by paleoearthquake. Compare with the minimal offsets along the fault obtained from high-resolution satellite images reconstruction and the coseismic offset distribution obtained by In SAR, we found two methods of distribution are consistent, use the 1997 earthquake coseismic offset distribution as the standard, compared to the offset distribution of previous earthquakes, can found a maximum 7 meters characteristic slip, according to river terraces offset and climate change, we can estimate the long-term slip rate of Margai Caka fault is about 8mm/a, this rate is slightly lower than other parts of the East Kunlun fault may be due to the part of the deformation participated to the whale lake segment of the Kunlun fault to the north side;Combined maximum of about 7 meters offset of Mani earthquake, suggesting that the recurrence time of earthquake is 850 ~ 900 a.According to the high resolution remote sensing image combined with field survey we found that the total length of 2010 Yushu earthquake’s surface rupture is about 65 km, the west end is near the south area of Long Bao town(96.43863 ° E,33.22611 ° N), east end is near the Changu temple at the east coast of Baqu river(97.04354 ° E, 3.94768 ° N), the overall trend of the rupture is about 300 °, is a sinistral strike-slip, slightly thrust component seismic event, the maximum sinistral displacement is 2.4m. Formed shear cracks, transtensional cracks, transpressional cracks, tension cracks, mole-tracks and unique ice cracks developed in permafrost zone. The overall Yushu earthquake surface rupture zone can be divided into Jielong subordinate surface rupture zone for about 15 km long and Jiegu subordinate surface rupture zone for about 31 km, the two subordinate rupture zone are distributed in left step-over, the maximum amount of sinistral strike-slip displacement is 0.66 m and2.4m, between the two subordinate surface rupture there is no ruptured for about17 km, corresponding to Mw6.9 and Mw6.4 two secondary seismic events, theco-seismic displacement distribution pattern is consistent with the results of In SAR and seismic rupture process inversion by seismology wave.Three river terraces offset landform studies near Ganda village in Yushu County and Manzhen village in Shiqu County, show that: Ganzi-Yushu fault’s long-term slip rate in Yushu segment is about 9.7mm/a, it is conformity with other segments of north boundary of Sichuan-Yunnan block, combined with trenches paleoearthquake record of seismic events: there are 9.5~9.1ka、7.5~5.3ka、4.8~3.9ka、2.7ka four events and varitions in magnitude, suggesting the recurrence behavior of Yushu segment is closer to the variable slip model, the slip rate along the fault is consistent, but each time the offset of the earthquake is not same, earthquake recurrence interval is more random,which may be due to the complex tectonic setting, many factors can trigger the earthquake, but in view of the paleoearthquake displacement distribution data obtained is still limited, we initially speculate that the recurrent behavior of Yushu segment is closer to the variable slip model.