Xizhang trench is located 10 km northwest of Taiyuan city,Shanxi Province,in front of a NNW-trending scarp of 4.6m height on the northern segment of the Jiaocheng fault zone.The dimensions of the trench are 108m in le...Xizhang trench is located 10 km northwest of Taiyuan city,Shanxi Province,in front of a NNW-trending scarp of 4.6m height on the northern segment of the Jiaocheng fault zone.The dimensions of the trench are 108m in length,8m in width,and 10m in depth.There are 18 horizons revealed in the trench.The upper strata are sandy loam;the upper strata of the downthrown block of the fault are gravels,the lower ones are an interbed of brown loam and sandy loam.The strata on the upthrown block of the fault are sandy loam containing gravel.The trench shows 3 fault planes,and the upper offset point on the fault plane is 1.5m below the ground surface,the newest dislocated stratum is(3.74±0.06)ka BP.The trench reveals a lot of deformation traces,such as fault planes,dislocated strata,colluvial wedges and formation tilting.The relationship between strata and faults in the trench shows that 3 paleoearthquake events have occurred at the Jiaocheng fault zone since the Early Holocene,they are about(3.74±0.06)～(3.06±0.26)ka BP,(8.35±0.09)ka～(3.74±0.06)ka BP,and(10.66±0.85)～(8.35±0.09)ka BP.The average interval among the events is 2.6～3.6ka.The minimum coseismic vertical displacements of the 3 events are 3.0m,2.5m and 3.2m,respectively.The significance of Xizhang trench is that the Jiaocheng fault used to be active thousands of years ago,though there is no M≥7.0 earthquake recorded in historical documents.Evidence of new Jiaocheng fault zone activity during the Holocene is important for the earthquake safety assessment of Taiyuan city in the future.展开更多
Investigation of offset landforms and trench excavation are important means to acquire the evidence of multistage activities of active faults. Here we present the result of fault trough investigation in Beichuan Count...Investigation of offset landforms and trench excavation are important means to acquire the evidence of multistage activities of active faults. Here we present the result of fault trough investigation in Beichuan County and the Pingtong Town of Pingwu County along the Longmenshan Central Fault Belt, as well as the result from trench excavation at the platform foreslope in Hanwang Town of Mianzhu County on the Longmenshan Front Range Fault Belt. These results show that at least three fault activity events, including the Wenchuan earthquake, occurred in the Beichuan fault trough, at least two, including the Wenchuan earthquake, at Pingtong fault trough, and 2-3 paleoearthquakes in the Hanwang trench. Among these three localities, the times of the last strong earthquake prior to Wenchuan earthquake at Beichuan fault trough and Hanwang trench are close, approximately 6000 years ago, i.e., greater than 5.8 ka and smaller than 6.63 ka ago. This provides the evidence of synchronous activity of the Central Fault Belt and the Front Range Fault Belt of the Longmenshan Active Fault Zone during the previous strong earthquake activities prior to Wenchuan earthquake.展开更多
基金sponsored by the program of "ActiveFault Test Detectionin City" of the National Development and Reform Commission,PRC.
文摘Xizhang trench is located 10 km northwest of Taiyuan city,Shanxi Province,in front of a NNW-trending scarp of 4.6m height on the northern segment of the Jiaocheng fault zone.The dimensions of the trench are 108m in length,8m in width,and 10m in depth.There are 18 horizons revealed in the trench.The upper strata are sandy loam;the upper strata of the downthrown block of the fault are gravels,the lower ones are an interbed of brown loam and sandy loam.The strata on the upthrown block of the fault are sandy loam containing gravel.The trench shows 3 fault planes,and the upper offset point on the fault plane is 1.5m below the ground surface,the newest dislocated stratum is(3.74±0.06)ka BP.The trench reveals a lot of deformation traces,such as fault planes,dislocated strata,colluvial wedges and formation tilting.The relationship between strata and faults in the trench shows that 3 paleoearthquake events have occurred at the Jiaocheng fault zone since the Early Holocene,they are about(3.74±0.06)～(3.06±0.26)ka BP,(8.35±0.09)ka～(3.74±0.06)ka BP,and(10.66±0.85)～(8.35±0.09)ka BP.The average interval among the events is 2.6～3.6ka.The minimum coseismic vertical displacements of the 3 events are 3.0m,2.5m and 3.2m,respectively.The significance of Xizhang trench is that the Jiaocheng fault used to be active thousands of years ago,though there is no M≥7.0 earthquake recorded in historical documents.Evidence of new Jiaocheng fault zone activity during the Holocene is important for the earthquake safety assessment of Taiyuan city in the future.
基金Supported by Fundamental Research Fund of Institute of Crustal Dynamics, China Earthquake Administration (Grant No. ZDJ2008-33)
文摘Investigation of offset landforms and trench excavation are important means to acquire the evidence of multistage activities of active faults. Here we present the result of fault trough investigation in Beichuan County and the Pingtong Town of Pingwu County along the Longmenshan Central Fault Belt, as well as the result from trench excavation at the platform foreslope in Hanwang Town of Mianzhu County on the Longmenshan Front Range Fault Belt. These results show that at least three fault activity events, including the Wenchuan earthquake, occurred in the Beichuan fault trough, at least two, including the Wenchuan earthquake, at Pingtong fault trough, and 2-3 paleoearthquakes in the Hanwang trench. Among these three localities, the times of the last strong earthquake prior to Wenchuan earthquake at Beichuan fault trough and Hanwang trench are close, approximately 6000 years ago, i.e., greater than 5.8 ka and smaller than 6.63 ka ago. This provides the evidence of synchronous activity of the Central Fault Belt and the Front Range Fault Belt of the Longmenshan Active Fault Zone during the previous strong earthquake activities prior to Wenchuan earthquake.
