Neotectonics Active Modes And Deep Background Of The Bohai Segement Of The The Tan-Lu Fault Zone

The Bohai segment of the Tan-Lu Fault Zone is also called the Yingwei Fault Zone.Previously comprehensive studies about Quaternary active faults in the fault zone werelimited, leading to controversy on many aspects. On the basis of a series of seismicprofiles, neotectonic activities of the fault zone are analyzed in this thesis. The resultsshow that activities of the fault zone are extensive and intense since Quaternary. Activefaults in the zone mostly inherit Neogene faults, and are composed of eastern andwestern major faults. Each of the two major faults shows left-stepped, en echelonarrangement. Bohai earthquake focal mechanism solution, present stress measurementresults and GPS data all show that ENE-WSW compression predominated in theneotectonic period. Their steeply-dipping fault planes in the Quaternary sediments,common Quaternary anticlines and ENE-WSW regional compression stress field allindicate that the fault zone is reverse dextral slip since Quaternary. Many seismicprofiles reveal that active faults in the Bozhong area, center of the Bohai Sea, are activesince Holocene whereas Holocene, Late Pleistocene and Quaternary active faults arealternated in the Weibei, Laizhou Bay and Liaodong Bay areas. On the whole, activetime of the fault zone is most late along center segment and relatively early along thenorthern and southern segments since Quaternary.Strong and frequent earthquake activities appeared the central segment of the Tan-LuFault Zone, that consists of the Bohai and Shandong segments, so it becomes animportant seismic belt in eastern China. Earthquake distribution shows that seismicactivity is obviously stronger in the eastern graben than the western one of the YishuFault Zone and exhibits weak-strong-weak feature from the south to the north of theYingwei Fault Zone. The fault zone shows local strong earthquakes, inhomogeneousdistribution of weak earthquakes and the presence of non-earthquake parts. On the basisof five magnetotelluric sections, comparison analyses are made for deep backgroundtriggering earthquakes in the central segment of the fault zone in this thesis. The resultsindicate that the fault zone cuts through the whole crust and shows highlyheterogeneous crust textures. The common presence of alternation of steep low-andhigh-resistivity zones in the crust sections leads to intense neotectonic activities alongthe fault zone. Three examples of crust electrical textures for≥Ms7earthquakes in thefault zone reveal that the strong earthquakes occurred when middle and lower parts ofupper crust have a rigid, high-resistivity layer connecting a rigid high-resistivity zone on one side or two rigid zones on both sides. In this case, the large, connecting rigidhigh-resistivity zone can accumulate big stress and trigger a strong earthquake. Weakearthquake regions in the fault zone show alternation of steep soft low-resistivity andrigid high-resistivity zones, and the latter can accumulate limited stress and lead to weakearthquakes. Aseismic regions in the fault zone correspond to the presence of anextremely thick, soft low-resistivity layer that becomes a totally creeping zone. In thiscase, a rigid high-resistivity zone by the side of the extremely soft low-resistivity layeris difficult to accumulate any stress and cannot cause any earthquakes. The abovecomparison analyses suggest that seismic activities along a large active fault zone areclosely related to its upper crust electrical texture and rheology.