Using Ne(electron density) and Te(electron temperature) data of Langmuir Probe onboard DEMETER(Detection of Electro-magnetic Emissions Transmitted from Earthquake Regions) satellite,the paper studies the variations of electron density and electron temperature associated with strong earthquakes.The specific scientific objectives of Langmuir Probe are to map the bulk plasma parameters and to study their variations associated with seismic activity,volcanism and human activity and other sources of perturbations.Langmuir probe sweeps in voltage allow to obtain a current-voltage(Ⅰ-Ⅴ) characteristic every second.Bulk plasma parameters are obtained from the analysis of the Ⅰ-Ⅴ characteristics.The work principle,data format of Langmuir Probe onboard DEMETER is briefly introduced.We mainly studied the 2006 M_W 7.9 Tonga earthquake and the M_W 7.1 Taiwan earthquake in this paper.For each earthquake,we choose the orbits which passed over the area about 2000km around epicenter,including four months data before and two months data after the earthquakes.There are mainly three steps to reprocess the raw data,in order to avoid disturbances resulting from solar activities and geomagnetic activities.The result on the M_W 7.9 Tonga earthquake shows that at the equatorial region the electron density was relatively low before 28 April,while it began to increase from 28 April,and got the highest at 2 May one day before the earthquake.Until 4 May after the main shock,Ne began to decline.Based on comparison between revisited orbits,the orbit 097681(before shock) and 098721(after shock) observed certain disturbance.To further investigate the features of the preearthquake ionospheric anomalies,using 5 days as time window and 1 day as step size,we examined the temporal and spatial evolution of electron density within the area of 2000km around epicenter.Results show that the normal distribution of Ne kept at a relatively high level,except a relatively sharp reduction of Ne from 29 April to 2 May.The result on the M_W 7.1Taiwan earthquake shows that Ne changed randomly,while Te kept at a relatively high level,but dropped gradually before the quake.Currently we have no evidence to conclude that these changes are the results from the seismic activity,but based on data reprocessing,we can exclude the disturbances induced by solar activity and magnetic field at certain degree.If these changes of Ne/Te really resulted from earthquake,we may prudently conclude that ionospheric disturbance associated with seismic activity may possibly occur in a short time impending an earthquake with a short duration.

As the most important fault of Late Pleistocene in the Lhasa area,the Nalinlaka Fault is a left-lateral thrust fault,striking NWW,dipping SSW with a high dip angle,and extending over 33km.According to the studies on the latest strata on the Nalinlaka Fault zone,this fault zone has been obviously active since Late Pleistocene and the movement left behind some geomorphologic phenomena on the earth's surface,especially at the sites of the gully west of Cijiaolin and around Xiecun village.For example,some rivers,ridges and terraces are dislocated,forming beheaded gullies,fault escarps and so on.The horizontal displacements since Late Pleistocene at the above two places are 54～87m and 20～67m,respectively.Based on the studies on the 4 trenches along the fault using progressive constraining method,we conclude that there might have occurred 5 paleoearthquake events along the Nalinlaka Fault since 70ka BP,the ages of each paleoearthquake are 8.53,54.40,<41.23,21.96,and 9.86 ka BP,and the average recurrence interval is 14.67ka.Because of the limits of trenches and earthquake events exposed by each trench,no single trench revealed completely all the 5 events.So,there may be some errors in determining the upper and lower limits of some events in this article.

The 14 November 2001 M_S 8.1 Kunlun earthquake at northern Tibet is the largest earthquake that occurred in the Chinese continent since 1950.We apply a three-dimensional(3-D)finite element numerical procedure to model the coseismic displacement and stress fields caused by the earthquake dislocations,which are constrained by field investigations.We then further investigate the stress interaction between the M_S 8.1 earthquake and the intensive aftershocks.Our primary calculation shows that the coseismic displacement field is centralized around the east Kunlun Fault zone.And the attenuation of coseismic displacements on the south side of Kunlun Fault zone is larger than that on the north side.The calculated coseismic stress field also indicates that the calculated maximal shear stress field is centralized around the east Kunlun Fault zone,and the directions of the coseismic maximal principal stress are contrary to that of the background crustal stress filed of Tibet Plateau.It indicates that the earthquake relaxes the crustal stress state in Tibet Plateau.Finally,we study the stress interaction between the M_S 8.1 earthquake and its intensive aftershocks.The calculated Coulomb stress changes of the M_S8.1 great earthquake are in favor of triggering 3 aftershocks.

Using remote sensing (RS) data, the major active faults of the eastern margin of Altai Mountains——the Hovd Fault and the Har-Nuur Fault, are studied, the detailed rupture character is analyzed from a viewpoint of topographic features, and the fault geometry and kinematics characteristics are revealed. Preliminary researches indicate the scale, slip-rate and strong seismic activity of active faults along the eastern margin of Altai Mountains are not weaker than those of the southwest’s. The Hovd Fault is of NNW-direction right-lateral strike-slip, with the length of 600 km, the maximal right-lateral displacement of river system of 9km since mid-Pleistocene(Q_p²), the possible slip-rate of 3.8～12.3mm/a,and the approximate average slip-rate of 7.8mm/a; The Har-Nuur Fault is of NNW-direction right-lateral strike-slip, with the length of 480 km. The fault is obviously active since Holocene(Qh), and there are signs of the latest faulting on the alluvial fans of Quaternary. So, the intense neotectonic movement and seismic activity on the eastern margin of Altai Mountains are related to the collision of India-Eurasia plate and maybe to the regional kinematic process.

Recognition of major tectonic events and tectonic cycles in Quaternary is an important topic that attracts more and more attention of many geologists. Recently, the research in this aspect has focused mainly on Quaternary sedimentation and geomorphologic evolution, while little attention has been paid to the study of fault evolution, which is directly related to tectonic cycle or tectonic event. This is mainly because of the lacking of geological evidence and the limitation of dating technique. Nevertheless, significant progress has been made in loess study, especially the characteristics and dating of loess-paleosoil sequences, which provide a time scale for regional correlation and timing of loess deposits. In this paper, an attempt has been made to discuss the migration and variation of Quaternary activity of fault zone through the analysis of loess deposition along the zone by using this time scale. The main purpose of this study is to provide direct evidence for the division of tectonic cycles in Quaternary. The present study deals mainly with the Lintong-Chang'an Fault and the Lishan mountain front fault on the southern margin of Weihe basin, as well as the Kouzhen-Guanshan Fault on the northern margin of the basin. The loess deposits along these fault zones have been studied in detail, while the main unconformities in loess sequences were identified in the geological sections across the fault zones and dated by using the loess-paleosoil time scale. The results show that tectonic unconformity presents broadly along the Lintong-Chang'an Fault and the Lishan mountain front fault, appearing as discordant contact of the S 8 paleosoil layer with the underlying strata. The underlying strata are offset significantly by the fault, but the overlying strata of S 8 are offset inconsiderably. Along the Kouzhen-Guanshan Fault zone, the S 1 paleosoil layer discordantly contacts with the underlying strata, which are significantly offset by the fault, but the overlying strata of S 1 layer are inconsiderably offset. In term of the loess-paleosoil time scale, the following conclusion can be drawn from the result of this study: The activity of the Lintong-Chan'an Fault zone on the southern margin of the Weihe basin was markedly changed at 800～900ka B.P. At the same time, the migration of activity occurred along the Lishan mountain front, while strong activity started to occur along the Weinan Yuan front Fault and the whole Weinan Yuan began to be uplifted. At 120ka B.P., the activity of the Kouzhen-Guanshan Fault zone began weakened. Thus, the variation of fault activity in this area may indicate two major tectonic events in mid-late Quaternary. This result may provide basic material for regional correlation of Quaternary tectonic events and for the research of the tectonic manifestation of tectonic events.