3D Geological Modeling And Strong Ground Motion Prediction Of Zhangzhou And Its Adjacent Regions

Strong ground motion study is very important in seismic hazard research. Strengthening thestudy on strong ground motion, especially near-fault strong ground motion and evaluating theprobable earthquake damage to a certain city are very important for city seismic disaster mitigation.Zhangzhou and its adjacent regions are always an important monitoring area for seismic disasterprevention and reduction in China. Although lots of research have been done about earthquakestructure and seismic hazard in this area, intensity is still used as the input parameter for seismichazard evaluation until now. This can't meet the need of city planning and engineering. It is aemergency requirement to study strong ground motion in Zhangzhou and its adjacent regions.Regional 3D (three dimensional) geological and geophysical models must be available beforestudying strong ground motion in Zhangzhou and its adjacent regions. Because there is no suchmodels in this area, 3D geological and geophysical models must be constructed at first. Then studyof the strong ground motion will be started as an application target. Results of this thesis supply abasic 3D model for future structural geology and the study of strong ground motion.By using geological and geophysical exploration data in the study area, a model of the crustabove Moho interface, a model of the quaternary basin and models of the earthquake faults wereconstructed. Then a computation model is created based on these models. Near fault strong groundmotion is predicated by using 3D finite difference and PEXT methods. Results are presented below.1. The 3D geological and structural modeling has been developed upon the 3D visualizationtechnology and the geological modeling theory. It breaks through the traditional 2D (twodimensional) presentation style for geological information and helps study geological problems from3D view. The 3D geological modeling is an integration of geological, geophysical and other relateddata. The modeling process includes data digitizing, spatial adjustment, data format translation, etc.The 3D model is created by using 3d modeling software, such as GOCAD, MVS and PETEL. Thedata processing and visualizing of the geological model is based on numerical interpolation,geological body construction methods and the 3D visualization study.2. Strong ground motion predication is a key problem in seismic hazard study. The ordinarymethods for numerical ground motion simulation include the empirical relation method, thenumerical Green function method and the empirical Green function method. The last two methodscan reflect the influence of local basin structure and are adopted widely as more about the local basinstructure is known . A finite fault source model must be adopted when computing near-fault strongground motion. The finite fault source model can be divided into two types, dynamic source modeland kinematic source model. Because the complexity of rupture process, the dynamic model is stillin developing. Kinemitic models are generally adopted to compute ground motion, including thedeterministic model, the stochastic model and the composite model.3. Zhangzhou and its adjacent regions are located in the southeast of Fujian province, where thenortheast trending Changle-Zhaoan fault zone and northwest trending fault belt cut each other andcreate the base structural style of this region. The Zhangzhou basin was formed since Pleistoceneunder the oblique slip action of regional normal faults. Modern earthquake distribution indicates thatthe near sea fault zone is the main earthquake fault and most earthquakes occurred in the area wherethe near sea fault zone and the Changle-Zhaoan fault belt intersect with the Yongan-Jinjiang faultbelt and the Jiulongjiang fault belt. Earthquake source depths of moderate and small earthquakes areabout 11Km to 15km.Crustal structure has the same style of the North China basin in this region.The historical earthquake data and crustal structure indicate that earthquake of Richter magnitude 6to 6.5 will occur in Zhangzhou and its adjacent regions in the future.4. Basd on six profiles which were got by deep seismic sounding since the "Eighth Five-yearPlan", a 3D crustal model over the Moho interface of Zhangzhou and its adjacent regions isconstructed. Analysis of the 3D model and the six profiles indicate that the structure and charactersof the crust are: G is the crystal basement face. Its depth is from 2km to 5km and the averagethickness above G is 4km. Velocity above G interface is from 4km/s to 6.1km/s.C1 is the interfacebetween upper and middle crust. Its depth is from 8km to 12km and the average thickness from G toC1 is 8km.Velocity from G to C1 is 6.1km/s to 6.15km/s. C2 is the interface of the low velocity layerin the middle crust. Its depth is from 12km to 20km and the average thickness from C1 to C2 is5km.Velocity from C2 to C1 is 6.0km/s to 6.1km/s. C3 is the interface between lower and the middlecrust. Its depth is from 16km to 24km and the average thickness from C2 to C3 is 4km.Velocity fromC2 to C3 is 6.35km/s to 6.45km/s. M is the Moho interface. Its depth is from 30km and the averagethickness from C3 to M is 6km. Velocity from C3 to M is 4km/s to 6.1km/s.5. Analysis of the 3D crustal model and seismic profiles indicate that there are following varietyrules of crustal interfaces beneath Zhangzhou and its adjacent regions: crustal thickness and depthsbecome larger from the Taiwan strait to the land and become smaller from Quanzhou toZhangzhou.The near sea fault belt is located in the area where crustal thickness and depth suddenlychange. The Qunzhou basin lies in the area where crustal thickness and depth change slowly. TheXiamen-Zhangzhou steped changing crustal belt lies in the same area as the Jiulongjiang fault. TheYongan-Jinjiang falut shows a clear sign on the interfaces C1 and G, but not so clear on C2 and C3,which indicates that vertical displacement of this fault is small. No clear sign of the Jiulongjiangfault can be seen on the crustal interfaces.6. The Zhangzhou basin formed since early Quaternary. The main structure style is thatnorth-east striking faults cut north-west trending faults each other. The North-west trending faults,including the Daishanyan-Hengkeng fault, the Zhukeng Fault, the Fuchuanshan-Kangshan fault , theJiulongjiang blind fault and the north-east trending Gutang-Dameixi fault are distributed in theZhangzhou basin. The sequence stratigraphy method is used to divide Quaternary strata of theZhangzhou basin into the Longhai Formation, Dongshan Formation and Changle Formation. Morethan 300 boreholes are used to build a sequence-structure framework under such a sequence dividingscheme and a database is built. MVS is used to construct the 3D basement model and thicknessmodel of every sequence. The irregular fault belts, distribution of the faults, activity of the faults andevolution of the Zhangzhou basin were then studied. These work supply a fundamental 3D faultmodel and basin media model for earthquake hazard evaluation and strong ground motionprediction.7. The shallow seismic investigation data are imported to GOCAD after spatial adjustment andprojection transformation. The 3D models of the Jiulongjiang fault, Gutang-dameixi fault andZhukeng fault are constructed by using GOCAD and the seicmic profile. Activity of these threefaults are studied and the Jiulongjiang fault is predicated to be the most active fault in the future. Anearthquake fault model is created with more detailed study on spatial distribution and activity of theJiulongjiang fault. This is a fundamental work for getting the finite fault parameters for strongground motion predication.8. Based on the regional 3D geological and structural models, the earthquake fault model, the3D finite difference method and the PEXT method are used to predict the future strong groundmotion distribution of the Zhangzhou basin induced by a near-field Richter magnitude 6.5earthquake. A double point source model is adopted when the 3D finite difference method is used tocompute ground motion and finite fault source model was adopted when PEXT method was used tocompute ground motion. The results got by the two methods are comparable. The absolute PGAvalues are close, the maximum values of PGA are 396cm/s2 and 360cm/s2. This indicates that thePGA values are referential. By using the finite fault model, the results got by the PEXT methodreveal the characters of near-fault strong ground motion distribution. Therefore, the results got byusing the PEXT method are recommended to be used in city planning and construction.