| The Tan‐Lu fault zone is a giant Meso‐Cenozoic tectonic active belt in eastern China, which is generally thought to begin south of Yangtze River of Guangji area in the south, extend northwards to thought Anhui, Jiangsu and Shandong province, across the Bohai Sea and the northeastern three provinces to the Far East region of Russia, in China more than 2400 km and trend NNE. It originated in the north and south block collision of transform faults mode, resulting in Sulu‐Dabie orogenic belt sinistral offset by 350km; in the early of late Jurassic activities again, it extended further northward and dislocated the northern boundary of the North China plate about 200 km, and a total of 550 km. It is extension from the early Cretaceous to Paleogene form of a large number of faulted basins and Eastern China’s "graben‐horst‐graben" structures. The fault zone has been suffered nearly NEE compression since Neogene due to the back‐arc spreading of the west Pacific Ocean and tectonic extrusion produced by the northward collision of the Indian Plate.Earthquake is one of the most important natural calamities in our country, which has caused heavy losses of life and property. The Tanlu fault is located in densely populated and economically developed of Eastern China, and it had occurred on 70 BC M 7(magnitude) Anqiu earthquake and 1668 M 8.5 Tancheng earthquake, if the fault occurs M≥7 earthquake again will result in more greater loss of life and property, and damage to our economy, even it could be lead to economic stagnation. Thus, over the years, all levels of government, responsible for seismic of department and geologists have been pay attention to it extremely. Since the Eighth Five‐Year plan period, the Tanlu Active Fault Geological Mapping group by large scale mapping of the system showed that the total length of the active faults from Weifang city in the north to Sihong County in the south is about 360 km, which is divided into three segments, Anqiu segment, Juxian‐Tancheng segment and Xinyi‐Sihong segment, respectively. In 2003‐2006 Researcher Song Fangmin etc. and 2009‐2013 Researcher Wang Zhicai etc. worked on Anqiu segment, and identified spatial distribution, geometry and the latest activity characteristics. In the 2013‐2015 years, at the implementation of Suqian city active fault detection and seismic risk evaluation project, Researcher Ran Yongkang and Xu Hangang etc. conducted a series of seismic profiles, drilling, trenching on Suqian section. Researcher Yao Daquan also have done a lot of field geological survey in the border of Jiangsu and Anhui provinces.On the basis of recent studies, the length of Tanlu seismic fault had expanded, only part of the onshore amounted to 460 km, and add Laizhou Bay section in south of the Zhangjiakou‐Bohai tectonic zone that is 140 km, a total length of 600 km. The early segmentation of earthquake rupture no longer meet the current need for understanding, also not meet the application needs of national economic construction. This thesis relies on the National Natural Science Fund Project: Study on initiation time of latest tectonic movement and its deformation style in the break‐up region, North China Craton. Researches carried out as follows:(1) on the basis of the indoor collating data, aerial and satellite images interpretation, and read and understood geological and topographic maps, we conducted field survey, mainly on verifing and measuring the dislocations of topography, confirming the location of the fault, choosing trenches point and excavation, drawing the fault profile, and collecting age samples; further, we analyzed the geometry and kinematics of the fault and overall understanded of the activities times. Based on above researches, we discussed seismogenic fault of the Anqiu and Tancheng earthquake;(2) we relocate a large number of small earthquakes that occurred in the Eastern China from 1990 to 2014 year using the double‐difference(DD) algorithm to improve relative location accuracy. The relocated seismicity mainly clusters around the Juxian‐Tancheng segment. Moreover, we obtain mechanism solutions of small earthquakes which it help us understand the regional stress field;(3) for the analysis of faults horizontal, vertical different physical properties, we collected the results of geophysical detection in the Eastern China to provide physical constraints on the segmentation of earthquake rupture;(4) the topography and structural characteristics in the adjacent segment boundary were analyzed. On the basis of previous studies, combined with the above analysis of the seismic fault, we conducted comprehensive segmentation;(5) in addition, we collected GPS data in the North China, and got GPS velocity profile of different segments, obtain current strain slip rate of each segment. In the last, based on recent activity era, small earthquakes characteristics and current strain slip rate of segments, we qualitatively analysis seismic risk for the different segments.The Tan‐Lu fault zone is divided into five earthquake rupture segments:(1) Laizhou Bay‐Changyi segment, includes the Changyi‐Nanliu section in the south and the Laizhou Bay section in the north, with a total length of 210 km, which the latest activities era is late Holocene. Through analysis and discuss, we suggest that the Changyi‐Nanliu section is seismogenic fault for the Anqiu earthquake. The segmentation is mainly according to difference recent activity times, and differences electrical in the transverse direction on both sides of fault.(2) Anqiu segment, is comprised of Anqiu‐mengtuan section and Qingfengling section with the total length of 100 km, which the latest activity during the late Late‐Pleistocene to the early Holocene.(3) Juxian‐Tancheng segment, is about 130 km long and composed of five sub‐segment in right‐step en echelon arrangement, namely from north to south, the Tuling‐lingyang section, Shijing‐Dadian section, Zuoshan section, Tengma‐Zhonghuashan section and Huaqiao‐Yaoshang section, respectively. In the field, it is easy to see the late Cretaceous sandstone thrust above the Holocene sediments, faulted gullies, and fresh gouge in the fault plane. This segment is 1668 Tancheng 8? earthquake rupture, and early studies suggest that it have three paleoseismic events, 3500 a, 6000‐7000a and 10000 a, respectively. The relocated seismicity mainly clusters along this segment. There is 8km wide left‐step stepover between it and the Anqiu segment, and it is also boundary between Subei‐Jiaoliao Block and Jiaonan block. There is 3km wide left‐step stepover between it and Xinyi segment in the south, and they are dislocated by late Pleistocene NWW‐trending of the Cangshan‐Nishan fault. To the south, the height of the landform ridge was significantly lower by more than twenty meters. There are also differences electrical in the transverse direction on both sides of fault.(4) Xinyi segment, is about 80 km long and extend from the Maipo village of the Tancheng County to Suqian City. Recent studies suggest that five paleoseismic events occurred in this segment, which ages are 3.8ka, 6.2‐13.4ka, 33.9‐45.3ka, 46.2 ‐50.4ka and 66.5‐83.9ka, respectively. There are few of small earthquakes. The segmentation is mainly base of difference activity times, the Middle Pleistocene large Wuxi‐Suqian fault cutting, and differences electrical substances on the lateral sides of the fault.(5) Sihong segment, extends from Suqian southward across western slope of the Chonggangshan to north of the Huaihe River, is about 80 km, which the latest event occurred in the early Holocene. The modern small earthquakes is lack of along the fault segment.Finally, we qualitatively analyze the future earthquake risk of the Tanlu earthquake fault. Based on scientific hypotheses, as follows: the longer of the time elapsed, the greater the likelihood of future earthquake; the more frequent small earthquakes, the more release of strain energy; the greater the GPS strain slip rate, the less strain energy accumulated. Using the latest activities era of the segments, we constrained elapsed time of the segments. The Anqiu and Sihong segments have the longest elapsed time, up to 10 ka. Combined with relocated small earthquakes data show that these segments located in locked state, thus, suggesting that the future of these have large seismic risk. If strain slip rate obtain from GPS velocity field is reliable, we have reason to believe that Sihong segment bein relatively higher lockout degree, of course, more dangerous.Problems: Less study in paleoearthquake sequence of each segment and the age sample results are uncertain, so that the paleoearthquake sequence constraint is not enough; due to the serious remould in the Eastern China, there are less reliable offset geomorphology data. Thus, study of displacements and slip rates is also insufficient; as for fault segmentation, due to the relatively little understanding of the Laizhou Bay section, under the current conditions, we only can Laizhou Bay‐Changyi as one segment with a total length of 210 km. With the development of detection in the sea, this segment may be further divided in the future; in addition, the mark of segmentation between Xinyi and Sihong segments are relatively few, we cannot rule out a large earthquake rupture resulting in cascaded broken, and the stability of segmentation requires verification; the current seismic risk only is qualitatively evaluated, if different segments of the seismic fault can be research by quantitative geomorphology, paleoearthquake sequences, etc., so that earthquake recurrence model will be recognized, which will provide quantitative data to support future earthquake risk assessment. |
PDF Full Text Request