| The Haihe fault is the southern boundary of the middle segment of the Zhangjiakou-Bohai Fault Zone (ZBFZ). This fault plays an important role in adjusting the left-lateral shear movement and changing regional stress and strain status of ZBFZ. Meanwhile it also controls late Cenozoic tectonic evolution and moderate earthquake activity in the Tianjin area. The Haihe fault is an active fault in late Quaternary which directly crosses the downtown area of Tianjin and the Binhai new area. It is of important significance not only in science but also in earthquake prevention and hazard reduction to study the spatial distribution, geometric features, fault segmentation, characteristics of kinematics and dynamics, active features in late Quaternary and its deep structure characteristics as well as seismic risk of the Haihe fault. Through synthetic analyses the results of precedents and combining with the author's work in the project of urban active fault survey during the period of the ninth five-year and tenth five-year, the dissertation made a systematic research on the Haihe fault about its spatial extension, geometric feature, fault segmentation, kinematics and dynamics, characteristics of deep structure and so on. This work began with the Quaternary stratum in Tianjin and established the chronostratigraphic sequences for (late) Quaternary in the region, and then, analyzed the active features of the Haihe fault in late Quaternary profoundly. Based on the above analysis, seismic risk of the fault was studied. The new insights and progresses of this thesis are summarized below.1. Tianjin is located in the middle-northern part of the Jibo fault block depression and the southern part of the Yanshan fault block uplift. It spans several secondary tectonic units such as Jizhong depression, Cangxian swell and Huanghua depression. Due to the regional NEE-SWW trending of principal compressive stress, the NE trending Hebei Plain fault zone is intersected and conjugated sheared with the NW trending ZBFZ in the Tianjin area. Two fault zones play an important role in controlling Neotectonic movement and recent tectonic activity. They also control the occurrence of regional strong earthquakes. Regional strong shocks and most of the moderate earthquakes usually occur in the intersection place of the two fault zones. The NE trending fault zone controls the magnitude and source mechanism of the earthquakes while faults striking NW affect in large measure the locations and aftershock distribution of earthquakes.2. The Haihe fault is trending northwest and extends about 110km along the Haihe River in Tianjin. The fault intersects with the Cangdong fault, Tianjin fault and other faults in the area. It is a regional fault which crosses through the Huanghua depression, Cangxian uplift and Jizhong depression. Bounded by the Cangdong fault and Tianjin fault, the Haihe fault can be separated into three segments, of which spatial distribution characteristic, geometric structure and deep structure features are obviously different. The east segment is located east to the Cangdong fault, which stretches 50 km in a shape like letter"Y". The Main fault of the segment dips toward south and the secondary ruptures on the south side dips toward north, intersect and combine with the main portion, forming a 2 km wide fault zone. The segment can be subdivided into two secondary parts further: land part with 30 km length and sea part with 20 km length, which are arranged in a left-step manner. The Middle segment of the fault lies between the Cangdong fault and Tianjin faults with a length of 30 km, which are arranged mainly in parallel steps. The main rupture strikes south, dipping steeply at shallow depth and becoming gently downwards. The secondary ruptures of the fault zone disperse upward in the Cenozoic stratum with a space about 700-1000m, thus the width of the whole fault zone reaches 8-10 km. This segment also can be divided into two secondary parts bounded by the Dasi fault. The west segment of the Haihe fault is located between the Tianjin fault and the Xiaowangzhuang village, Wuqing district, which is about 30 km long and stretches in a manner of cluster that display as a series compact ruptures. The SW trending main rupture with shallow buried depth lies in the middle or south side of the fault zone. Secondary ruptures dip opposite or same to the main rupture and the space between them is very small, thus the whole width of the fault zone is only about 1 km. The fault zone begins to disperse near the Chenzui village at the west end of the fault, and the width of the fault zone increases evidently. Meanwhile, their trends become disordered but the predominant trends retain NNW and NW. The fault exhibits as branched light and shade belts in satellite remote sensing images.3. The sediment development histories of the Huanghua depression and Cangxian swell in the coastal area of Tianjin have similarity as well as appreciable differences. Since 0.2 Ma BP, the Huanghua depression underwent several times of marine transgressions and formed the sediment with thickness about 60 m. Since late Pleistocene, there existed two sediment discontinuity surfaces; the ages of them are 68.2ka-112.2ka BP and 68ka -6.1ka BP, respectively. During the period of 0.2-0.78Ma BP, the sediments in the region were mainly terrestrial facies such as lake, river and some times accompanying with storm tide or marine deposit, the rate of sedimentation was relatively slow and the sediment thickness was about 50 m in total. While in the period of 0.68-1.2Ma BP, the rate of terrestrial sediment became high, related thickness reached about 100 m.Sediment thickness in the Cangxian swell is no more than 30 m in recent 0.2 Ma BP. It underwent two marine transgression events, and there developed three sediment discontinuity surfaces since late Pleistocene, which correspond to the periods of 92.0ka～122.2ka BP, 41.5ka～88.7ka BP and 10.0ka～41.5ka BP, respectively. The underlying strata of the discontinuity surfaces are all terrestrial deposit. During the period of 0.2～0.78Ma BP, sediment in the region was mainly terrestrial, of which the sediment rate was very little and related sediment thickness there was only about 20 m, indicating that the region was ever in a state of denudation in a long time or sediment discontinuity. From early to middle Quaternary, the sediment was also thin which indicates that the sediment environment did not change much.4. Research on Paleomagnetism in the region shows that the Quaternary chronostratigraphic sequences, represented by the BZ1, BZ2, and TN3 boreholes, respectively, have large differences between the Huanghua depression and Cangxian swell. There also exist large differences compared with the results of precedents. According to the previous research of other people, the standard stratum of Quaternary south to the Baodi fault was not subdivided, the stratum sequence of the Huanghua depression and Cangxian swell were considered as the same, that is, the buried depth of the low bottoms of Helocene series, upper Pleistocene series, middle and lower Pleistocene series are 20 m, 70-80 m, 180-210 m and 400 m, respectively. In this research, the author found that two chronostratigraphic sequences are obviously different, they represent two different sediment histories in Quaternary and have different sediment types and different sediment thickness, so it should be divided into two different sediment units. In addition, this work shows that in the Huanghua depression, the depth of low bottoms of Holocene series, upper and middle Pleistocene series are respectively 19 m, 45 m, 103 m, while in the Cangxian swell they are 13 m, 30 m and 56 m, respectively. Meanwhile the M/G interface which is considered as the bottom of Quaternary is only about 162 m.5. Research of this thesis also shows that late Quaternary active features of the Haihe fault vary obviously between its eastern and western segments. According to the results of 14C dating, the western segment of the Haihe fault was active since 36290±2680a B.P., but its latest active time was no later than 8415±115a B.P, so the main active period of the segment is late Pleistocene. However, the latest active age of the eastern segment of the Haihe fault is about 7200±140a B.P., implying its active age has entered into early or middle Holocene.Shallow acoustic survey reveals that upper-break point depth of the Haihe fault in Tanggu is 39.3-43.3 m below river water surface, while the borehole data shows that it is 16.4m below ground surface. So it is certain that the related active age has entered into Holocene. In the Bohai offshore, acoustic exploration result shows that the up-break point depth of the Haihe fault is 31.2 m below seabed and shallower than that in the land area, thus it is concluded that activities of the Haihe fault are strong in the east and weak in the west since late Pleistocene, and the activity in the sea area is stronger than that in the Tanggu terrestrial area.It has advantages of high resolution, high ability of resisting against interference to apply the shallow acoustic stratum survey to explore buried faults in freshwater riverway inland, which can obtain more detailed information about the shallow stratum. Combining with shallow seismic reflection survey and borehole log, the credibility of exploration results can be enhanced. 6. Deep structure detections and research results show that the crust beneath Tianjin has an obvious layered structure, with varied electronic and velocity features in different tectonic units. Some faults in North China have particular deep structure combinations: There exist shovel-shaped faults in shallow crust, which are compressive-shear faults due to the Yanshan movement in late Mesozoic and display as left-lateral slip shear faults under the recent stress field. The shallow faults do not stretch downwards below the depth of 6-8 km. In the middle crust, there exist near-vertical strike slip faults, which have the trend of developing upward. The deformation in the lower curst is ductile and then transforms into elastic deformation in middle crust. These deep vertical faults do not intersect with the shallow ones directly. They are usually separated by the layers with lower velocity. In the lower part of the middle crust or upper part of the lower crust there are high conductive (or low velocity) layers which play a role of decoupling between the ductile deformation in the lower crust and elastic deformation in the middle crust. These structures constitute the deep background of earthquakes. Earthquakes mainly occurred in high-velocity areas which are located above low-velocity layers, especially in the edge between them. Such kind of deep structural styles noted above also exists in the eastern and western segments of the Haihe fault.7. Results of numerical simulation for the seismogenic structure in the study region indicate that the portions, where the effective shear stress increases most rapidly, appear above the high-velocity part and deep vertical faults in the crust at a depth about 8～15 km. It means that the regional shear stress has already overcomes the confining pressure, rock will break and then earthquakes would occurr if the effective shear stress changes to positive in the latest load process. However, at larger depth in the crust, the confining pressure is too strong to surmount and effective shear stress retains minus, thus the crust will keep stable.According to the principle of effective shear stress, distribution of effective shear stress in the area of the Haihe fault zone and its vicinity areas is calculated by the numerical simulation method. Results show that areas with positive effective shear stress lie in the places south to the Shuangkou area which is near the western segment of the Haihe fault, and Dengshangu and Dagu areas in the eastern part of the Haihe fault. That is to say, these two places have a higher risk of strong earthquakes. Nevertheless, in the middle segment of the Haihe fault the effective shear stress keeps minus, so it maybe has lower earthquake risk.8. Based on the above analysis, seismic risk of the Haihe fault is studied and two seismic risk zones for both the Tanggu-Dagu area in the east segment of the Haihe fault and the south Wuqing area in the western segment of the Haihe fault are determined.9. Through detection and research on activity of Haihe fault, a set of approach to detect and research on buried fault in coastal area is fished out, which could be used for reference in detection and research on complete buried fault in similar area.Summarizing above statements, this work has obtained the following innovative results:1. Through chronological and paleomagnetic dating, test and analysis, chronostratigraphic of the Huanghua depression and Cangxian swell are studied. Quaternary standard stratum profile in Tianjin is established, and the boundaries of the layers within Quaternary are classified again. A new approach is provided to research magnetic strata and Quaternary chronostratigraphy profoundly in the region.2. Based on thorough survey and analysis, more information about the Haihe fault including its spatial extension, geometric structure, fault segmentation and active characteristics during late Quaternary have been acquired. Through profound and systematic analysis of the deep structure of the Haihe fault and its adjacent areas, this work reveals the deep structure features and deep seismogenic background of the fault for the first time.3. Based on the results of deep structure detections and researches, a seismogenic structure model for the Haihe fault is eatablished and applied to numerical simulation. Synthesizing all results of the research to the fault, seismic risk segments of Haihe fault are estimated, which have innovative significance in the quantitative research on seismic risks of the fault.4. Through detection and research on activity of Haihe fault, a set of approach to detect and research on buried fault in coastal area is fished out, which could be used for reference in detection and research on complete buried fault in similar area. Especially, shallow acoustic stratum surveying is applied to the practice of exploring buried faults in freshwater river way inland. Combining with shallow seismic reflection surveying and borehole log, it can provide a convenient and credible approach for exploring complete buried fault, determining fault activity and paleoearthquake research of faults in coastal area.
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