The Xiaojiang fault zone is located in the southeastern margin of the Tibetan plateau, the boundary faults of Sichuan-Yunnan block and South China block. The largest historical earthquake in Yunnan Province, with magnitude 8 occurred on the western branch of the Xiaojiang Fault in Songming County, 1833. Research on the Late Quaternary surface deformation and strong earthquake rupture behavior on the Xiaojiang Fault is crucial to understand the future seismic risk of the fault zone and the Sichuan-Yunnan region, even crucial for the study of tectonic evolution of the southeastern margin of Tibetan plateau. We have some new understanding through several large trenches excavated on the western branch of the Xiaojiang fault zone. We excavated a large trench at Caohaizi and identified six paleoseismic events, named U through Z from the oldest to the youngest. Ages of these six events are constrained at 40000-36300BC, 35400-24800BC, 9500BC-500AD, 390-720AD, 1120-1620AD and 1750AD-present. The Ganhaizi trench revealed three paleoearthquakes, named GHZ-E1 to GHZ-E3 from the oldest to the youngest. Ages of the three events are constrained at 3300BC-400AD, 770-1120AD, 1460AD-present. The Dafendi trench revealed three paleoearthquakes, named E1 to E3 from the oldest to the youngest, and their ages are constrained at 22300-19600BC, 18820-18400BC, and 18250-present. Caohaizi and Ganhaizi trenches are excavated on the western branch of the Xiaojiang Fault, the distance between them is 400m. We constrained four late Holocene paleoearthquakes with progressive constraining method, which are respectively at 500-720AD, 770-1120AD, AD 1460-1620 and 1833AD, with an average recurrence interval of 370~440a. Large earthquake recurrence in the late Holocene is less than the recurrence interval of~900a as proposed in the previous studies. Thus, the seismic hazard on the Xiaojiang Fault should be reevaluated. We excavated a large trench at Dafendi, about 30km away south of Caohaizi trench. Combining with previous paleoseismological research, it is found that the western branch of Xiaojiang Fault was likely to be dominated by segmented rupturing in the period from late of Late Pleistocene to early and middle Holocene, while it was characterized by large earthquakes clustering and whole segment rupturing since late Holocene.

Nine earthquakes with M≥6 have stricken the northern segment of the Red River fault zone since the historical records, including the 1652 Midu M7 earthquake and the 1925 Dali M7 earthquake. However, there have been no earthquake records of M≥6 on the middle and southern segments of the Red River Fault, since 886 AD. Is the Red River fault zone, as a boundary fault, a fault zone where there will be not big earthquake in the future or a seismogenic structure for large earthquake with long recurrence intervals?This problem puzzles the geologists for a long time. Through indoor careful interpretation of high resolution remote sensing images, and in combination with detailed field geological and geomorphic survey, we found a series of fault troughs along the section of Gasha-Yaojie on the southern segment of the Red River fault zone, the length of the Gasha-Yaojie section is over ten kilometers. At the same time, paleoseismic information and radiocarbon dating result analysis on the multiple trenches show that there exists geological evidence of seismic activity during the Holocene in the southern segment of the Red River fault zone.

Through the shaft reconstruction of the well under the condition of artifical automatic flow,we managed to keep the continuous measurement of concentration of chemical contents and water level by adopting shaft observation method.After observation for more than a year,it has been revealed that the crisis of halt of measurement of the well was removed.Moreover, it provides a new approach for technological reconstruction of high temperature artesian well.

The Huailai No.4 well is one of the national key observation wells. Obvious anomalies have been observed in this well before the occurrence of many earthquakes. Therefore,the well has played an important role in the monitoring of earthquake activity in Northwest Beijing region. This paper describes in detail the anomalies of underground fluid in the well before the occurrences of 5 moderate-strong earthquakes. In addition,we have also collected the earthquake-reflecting anomalies of the underground fluid observed in the Huailai No.4 well before the occurrence of earthquakes of magnitude above 4.0 in Huailai and its neighboring area since 1998. The research shows that the Huailai No.4 well may sensitively reflect M≥4.0 earthquakes that occur within the range of ≤260km around the well. The main features of precursory anomalies of underground fluid observed in the Huailai No.4 well can be summarized as follows: ⑴ The precursory anomalies of water mercury appear as abrupt variations. They occur several days to 2 months before the occurrence of an earthquake. The amplitude of variation might be several to tens times as high as the background value. The higher the amplitude of anomalies the greater the magnitude of earthquake will be. The earthquakes occur after the ending of anomalies; ⑵ The precursory anomalies of carbon dioxide in fault gas appear as sharp variations at many points. They occur 16 days to 47 days before the occurrence of an earthquake. The amplitude of variation might be several times to ten times as high as the background value. The higher the amplitude of anomalies,the greater the magnitude of earthquake will be. The earthquakes occur when the anomalies fall off; ⑶ The precursory anomalies of water radon appear as stepped variation. They occur several days to 2 months before the occurrence of an earthquake. The amplitude of anomalies might be of about 15% of the background value. The earthquakes occur during the growing period of anomalies; ⑷ The precursory anomalies of hydrogen escaped from groundwater appear as abrupt variations. The amplitude of variation of anomalies might be 3 times as high as the background value. The earthquakes occur after the ending of anomalies. All these results may shed light on the earthquake-related behavior of the underground fluids in this well,and may enhance the capability of earthquake prediction in the near future.

Based upon the far-field and near-field digital seismic records, the hypocenter locations of M_L≥3 earthquakes of the Zhangbei earthquake sequence occurred during January, 1998-March, 1999 have been re-determined by using relative locating method. The results show that the process of the earthquake sequence can be divided into three stages. The first stage occurred in January 1988, which is the stage of the occurrence and adjustment of the main shocks. The seismic activity in this stage occurred mostly along NWW-SEE direction. The second stage occurred during February ～August 1998, which is the active period of moderate aftershocks occurred mostly along NNE direction. The third stage was represented by the occurrence of M 5.6 earthquake on March 11, 1999, the hypocenter of which was aligned also along NNE direction, but was out of the hypocenter area of the main shock. It seems that the two hypocenters were not connected with each other. The rupture characteristics of the Zhangbei earthquake sequence is given on the basis of the relocation of the hypocenters and the revised focal mechanisms, as well as macroscopic intensity distribution data of this earthquake sequence. It is suggested that the earthquake sequence consists of one main left-lateral strike-slip rupture plane, and 2 right-lateral strike slip secondary rupture planes. The main rupture plane is NWW-trending, NNE-dipping at an angle of 44°, and 11.5km in length. The two secondary rupture planes are NNE-trending with a high dip angle. The main rupture plane is conjugate with the NNE-trending secondary rupture planes while the two NNE-trending rupture planes are aligned in right-step en ecelon. The three rupture planes occurred sequentially, the depth of the which is 1.4～7.6 km at shallow part of the crust. The present study shows that it is a doable approach to research the focal rupture of a strong earthquake sequence from three dimensional space by using exact hypocenter 1ocation, focal mechanism and macro-intensity distribution data in "tectonically stable area" where no active fault is found.