关键词: 西藏, 懂错-安多地堑系, 地震烈度, SN向正断层, 震源机制解

Abstract: The M_S 5.6 earthquake occurred in Amdo county of central Tibet on March 7,2004.The epicenter is 31.68°N,91.31°E and 31.64°N,91.24°E from CENC(China Earthquake Netwosks Center)and NEIC(National Earthquake Information Center of America)respectively(Fig.1).It is the maximum magnitude earthquake near Golmud-Lhasa railway after the 2001 M_S 8.1 Kunlun earthquake.Based on field intensity susveying,the iso-seismal contours map of the earthquake was drawn.The map shows that iso-seismal lines for the earthquake are NNE-trending and the area of the same intensity is larger to north and west than to south and east(Fig.2).Its maximum intensity is Ⅶ and lies between Gongbatang and Jianong at the east of Dung Co Lake(Photo 1,2),so this earthquake is called the 2004 M_S 5.6 Dung Co earthquake.The intensity survey results suggest that the macroscopic epicenter is 31.70°N,91.26°E,which approximately coincides with the epicenter located based on instrument monitoring.The distribution of intensity indicates the earthquake probably results from faulting of NNE-trending fault zone along the eastern margin of the Dung Co basin.The results of active faults survey confirm that the fault zone controlling the earthquake is a main boundary normal fault zone of the Dung Co basin and called the eastern boundary fault zone of the NNE-trending Dung Co basin,which is a half graben about 10～15km wide and 40km long,connecting with the NNE-trending Peng Co basin to the south and adjacent to the NE-trending Tsona-Amdo graben to the north.The boundary fault zone of Dung Co graben is NNE-trending,about 40km long and composed of three minor normal faults,all dipping to west and forming a right-stepped en echelon array indicating the left lateral strike-slip component of the fault zone(Fig.3).Surface observation along the faults shows that the fault underwent prominent normal faulting during late Quaternary.The severe faulting is expressed by the prominent northwest facing triangular facets along mountain front and multiple generations of fault scarps in late Quaternary sediments(Photo 5～7).The results of in-suit measurement with level instrument and tape show that there are three sets of fluvial terrace surfaces and a piedmont slope that were vertically offset by 1～1.2m(terrace 0～1 and 0～2),2～3m(terrace 1)and 13～19m(or piedmont slope)along the northern segment of the fault zone,respectively;and there are three sets of fluvial terrace surfaces which were vertically offset by 0.5～0.7m(terrace 1),2～3m(terrace 2)and 12～15m(terrace 3)along the central segment of the fault zone,respectively.The offset of terraces all took place during late Pleistocene and Holocene,implying that the normal faulting has continued and the average rate of vertical displacement is estimated as 0.2±0.1mm/a and about 0.06mm/a on the northern and central segment respectively since late Pleistocene based on the relationship between the terraces evolution and climate change during late Quaternary and comparing the distributions of similar terraces in the adjacent areas of Dung Co.Along the northern segment of fault zone,a ～6km long surface rupture,which offset the terrace 0～1 and terrace 0～2 of late Holocene,was discovered,indicating the latest paleo-earthquake event since late Holocene.The vertical displacement is 1.0～1.2m across the surface rupture,which represents the minimum offset during the last paleo-earthquake event on the northern segment(Photo 6).In space,the central segment of the eastern margin fault zone of Dung Co graben coincides with the maximum intensity area of the 2004 M_S 5.6 Dung Co earthquake.It indicates that the earthquake results from the normal faulting along the central section of the fault zone.The results of field survey are in agreement with the fault plane solution from CENC,but are inconsistent with the focal mechanism solution from HRV.It suggests the results from CENC are reasonable for explaining the 2004 M_S 5.6 Dung Co earthquake.Studies on regional tectonics and distribution features of earthquake suggest that the NNE-trending Peng Co basin,Dung Co graben,and Amdo-Tsona basin make up apparently of a regular left-lateral en echelon distributed rift system trending N35°～45°E from south to north,about 120 long(Fig.1).This rift zone constitutes a NNE-trending seismic zone called Dung Co-Amdo earthquake zone.The magnitude of late Quaternary faulting varies remarkably along different segments of Dung Co-Amdo seismic zone.Based on field survey,the vertical slip rates on the northern segment of Dung Co fault zone and the western segment of northern boundary fault zone of Amdo-Tsona graben are much faster than that of the central-southern segment of Dung Co fault and eastern segment of Amdo-Tsona northern boundary fault,and there are records about paleo-earthquakes of late Holocene along the two fault segments,but recent earthquakes(M_S≥3)since 1971 are almost distributed around the central-southern segment of Dung Co basin and eastern segment of Amdo-Tsona graben.The prominent differences indicate that the southern segment of Dung Co fault zone and the eastern segment of northern boundary fault zone of Amdo-Tsona graben are the sites where future earthquake will possibly occur because the strain accumulating time is longer at these areas.

Key words: Tibet, Dung Co-Amdo rift system, earthquake intensity, North-south trending normal fault, focal mechanism solution