Study on Holocene Paleoearthquakes in Xizhang Trench on the Jiaocheng Fault Zone,Shanxi Province

摘要

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is that the Jiaocheng fault used to be active thousands of years ago,though there is no M≥7.0 earthquake recorded in historical documents.Evidence of new Jiaocheng fault zone activity during the Holocene is important for the earthquake safety assessment of Taiyuan city in the future.