The Langshan range-front fault (LRF)is a Holocene active normal fault that bounds the Langshan Mountain and Hetao Basin at the northwest corner of the Ordos Plateau. Paleoseismic trenching research at three sites, Dongshen Village trench (TC1), Qingshan trench (TC2)and Wulanhashao trench (TC3)from north to south was performed in this study to reveal the seismic hazard risk in Hetao Basin. The paleoevents ED1, ED2, ED3 from TC1 can be constrained to have occurred (6±1.3)ka, (9.6±2)ka and (19.7±4.2)ka respectively, while the paleoevent EQ1 from TC2 occurred about (6.7±0.1)ka and the paleoevents EW1, EW2, EW3 at TC3 took place about (2.3±0.4)ka, (6±1)ka and before 7ka respectively. In combination with paleoseismic results of previous researchers, the Holocene earthquake sequence of the LRF could be established as 2.3~2.43ka BP (E1), 4.41~3.06ka BP (E2), 6.71~6.8ka BP (E3), 7.6~9.81ka BP (E4), and (19.7±4.2)ka BP (E5). Although the possibility of missing events cannot totally be ruled out, based on the analysis on faulted geomorphology at Wulanhashao site, we argue the paleoearthquake history of the LRF during Holocene may be complete with an average recurrent interval about 2500 yrs. The apparent displacements associated with events E1, E3 and E4 are significantly larger than that of event, E2, that suggests that they might be great events with magnitudes 7.5 to even over 8 that ruptured the entire LRF, while the event E2 may be a smaller event that only ruptured a segment of the fault. The magnitude of event E2 might be about M7. This poses a significant seismic hazard to the area of the Linhe depression in the western Hetao graben region. With the further limitation of previous radiocarbon dating result near our trench site at Wulanhashao, the slip rate at Wulanhashao should be not smaller than, but close to 0.66mm/a since 15ka BP. And the slip rate at Qingshan site is supposed to be about 1.4~1.6mm/a since 6.8ka BP. Both our combined most recent paleoseismic cognition and current tectonic geomorphologic research results supports to reveal that the Langshan range-front fault now is an unsegmented fault, preferring to rupture the whole fault in a surface-rupture event. Considering the most recent event E1 and fault slip rate obtained above, the accumulated strain on the LRF could be estimated as about 1.52~3.94m. Given the ~2500a recurrent interval, we argue that the elapsed time since last major quake, E1, is approaching or even over the recurrence, and the seismic risk for another major quake is imminent, at least cannot be ignored.

The geodatabase of scientific investigation of Wenchuan earthquake stores many field investigation data,such as the data of field geological survey sites,the surface rupture belts,the Quaternary faults in the earthquake region,paleoseismological trenches,folds,etc.Moreover,it includes some collection data,such as strata data,the catalog of the Wenchuan mainshock and aftershocks,the relocation catalog of aftershocks of the Wenchuan earthquake,the catalog of historical strong earthquake.The field data are recorded,edited,analysed,mapped and output with ArcGis,a powerful function in Gis software.The geodatabase of scientific investigation of M_S8.0 Wenchuan earthquake was initially established,which includes the basic seismological information and realizes the integrated management of spatial location and attribute information.Data can be queried,analyzed and processed for the need of the correlation analysis of the data.Seismic tectonic map and surface rupture map can be drawn with the geodatabase,which can provide the basis for the determination of safety distance from the risky seismic rupture zones in post-disaster reconstruction and for the construction of the Wenchuan earthquake geographic information system.

In the historical rocords,there have been no comparable earthquakes with the May 12 Wenchuan earthquake in Chengdu and Longmenshan mountain region.Then,whether the ancient earthquake traces with comparable magnitude in the geological records can be found has become an important scientific issue.The authors and other members of Wenchuan earthquake geological investigation team did fieldwork in the earthquake region for more than one month.Four trenches and one geologic section were excavated along the middle segment of surface faulting of both the central fault and the mountain front fault.And geomorphologic surfaces of deformation were measured.In this paper,we discuss the fact that there is prehistory large earthquake along the seismogenic fault of Wenchuan earthquake from analyzing the accumulated deformation of old and young fault scarps or geomorphologic surfaces,trenching,and comparing the activities of related faults and so on.The result shows that whether at Xiaoyudong,Leigu located on the central fault or Bailu,Hanwang on the mountain front fault,or at other places along the surface faulting of the the Wenchuan earthquake,the height of the fault scarp on the second terrace has a multiple relation with that on the first terrace after the Wenchuan earthquake.The 4 trenches reveal that the dislocation of the marker strata of the second terrace on both sides of fault is twice of that of the Wenchuan earthquake,which shows that there was an earthquake event with the same scale of surface deformation of this M_S8.0 Wenchuan earthquake between the formation time of the second terrace in Lomhmenshan area and the May 12 Wenhucan earthquake.

Quantitative study of active tectonics needs to get a series of deformation parameters,and fault displacement is one of the most basic active parameters. Alluvial fan and terrace around active fault zones can record the information of time and strength of fault movement. River terrace as a most common landform contains structural information. It is meaningful for quantitative tectonic movement study to ascertain river terrace. In the article we use photogrammetric software virtuo NT to extract high resolution DEM. Based on DEM,the method and program for terrace analysis are built on the platform of ARCGIS. In the light of the distribution characteristics of slope and height,we generate the slope classification map and height classification map. And by multiplication of the two maps,the terrace landform map is generated. In the Zimakua region of Sichuan province,we applied the method to extract terrace information and got its distribution. We compared the terrace extracted based on DEM with the terrace interpreted in the filed,and the result shows that they have good consistency. The terrace boundaries record the information of fault dislocation. By measuring the dislocation of terrace boundary,we get the average dislocation of secondary fault to be 85.4m,which is in accord with the filed surveys. The results indicate that the method of terrace extraction based on DEM has the properties of high precision, high efficiency,visualization and so on.

The Anninghe Fault is an important active fault along the eastern boundary of Sichuan-Yunnan active tectonic block,and the study of its distribution feature of displacement rates in the late Quaternary is of fundamental importance for understanding the dynamic theory on Chinese continent,boundary dynamic process of the active block and the recurrence interval of large earthquakes. The active Anninghe Fault in late Quaternary has the length of 160km and consists of two segments. Zimakua is an area where the latest neotectonic rupture trajectories are relatively simple and the offset geomorphologic sequences are well developed. Using the methods of detailed geomorphic and geological survey,digital image analysis,total station instrument survey,excavation of trench and dating,the paper makes an analysis on the geomorphologic sequences of the strata and obtains some new results as follows: The terrace(step)T₃ consisting of moraine deposits was formed at about 20ka BP and offset by the Anninghe Fault with a left-lateral displacement of (84±3)m and a vertical displacement of 18m. On the base of the moraine deposits overlies terrace(step)T₂ of mainly alluvial deposits formed at 10～7ka BP with the maximum left-lateral displacement of about 40m,an average of (36±4)m,and a vertical displacement of about 11m. Terrace T1 is composed mainly of the alluvium and partial dammed-pond deposits which were formed during 3～2.2ka BP. The earliest event that offset the T1 happened at 1.7kaBP.Three events have been discovered since 1.7kaBP and the cumulative left-lateral displacement is 10.5m and the vertical displacement 2.3m. So,the left-lateral displacement rates are 6.2mm/a,3.6～4mm/a,3.8～4.2mm/a and the vertical rates are 1.4mm/a,1.1mm/a,0.9mm/a(at least)on average in about 2.6ka,10ka and 20ka,respectively. The proportion of horizontal to vertical displacements is about 4:1. That means the vertical rate on Anninghe Fault is about 25% of horizontal slip rate. The left-lateral slip rate in late Holocene is well consistent with GPS measurement. The change of the left-lateral rates in different stage is also consistent with recurrence interval of paleoearthquakes. It means that there is alternation between strong and weak activities of the Anninghe Fault.

Xishan Fault group is distributed in the transition zone between the fold-reverse fault system along the front of the north Tianshan Mountains to the west and the thrust tectonics of Bogeda to the east.It is a tectonics that thrusts from the basin in the north to the mountains in the south,consisting of 4～5 faults that are more than 10 to about 30km long,showing low angles near surface and converging on the detachment surface at about 11km deep.We discovered that the activity of Xishan Fault group is distinct during late Quaternary by doing field investigation of geology and geomorphology,excavating trenches along faults and analyzing deep structure of the fault group.The faults offset the second and above terraces of Wanjiagou creek and created fault scarps of 0.5～5.4m high on the terraces.And traces of paleoearthquakes can be found easily.The younger two events on F₁,F₂ and F₃ are confined in(22.7±5.2)ka and 40ka BP by OSL samples dating,respectively and the traces of the youngest event on F₄ and the front fault of Xishan are covered by deposits whose ages of OSL samples are about(31.1±3.2)ka and(37.9±3.8)ka BP,respectively.It means that there was grouped faulting in late Quaternary in the Xishan Fault group.F₁,F₂ and F₃ or F₄ and the front fault of Xishan might rupture in a same event on near surface.Event traces on the Xishan Fault group and other reverse faults of low angle show that the deposits along the front of fault scarp,the offset relation between fault and deposit bed,and the abrupt increase and diminution of displacement on difference markers or unconformable surfaces on both sides of fault are important identification marks of paleoearthquakes along surface rupture-type reverse fault.The deposits along the front of fault scarp on reverse faults of low angle are much more different from those on normal fault.For ideal mode,the deposit in front of fault scarp of reverse fault of low angle is characterized with that the original structure of the collapsing thrust sheet front is not broken entirely on the lower part and the sloping deposit on the upper part may exist covering on both sides of the fault.We think that it is very important for reducing uncertainty of paleoearthquake identification to seek for evidences as many as possible and analyze the different influencing factors,such as tectonics,climate,environment and anthropic activities.