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

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is that the Jiaocheng fault used to be active thousands of years ago,though there is no M≥7.0 earthquake recorded in historical documents.Evidence of new Jiaocheng fault zone activity during the Holocene is important for the earthquake safety assessment of Taiyuan city in the future.

Discussion of Some Problems in the Research into the Seismotectonics of Strong Earthquakes on the North China Plain

The tectonic characteristics and research problems of five earthquakes with M≥7.0 on the North China Plain over the last 300 years are addressed in the paper, including the cognition that there were no ground fractures in the 1966 Xingtai earthquake, the question caused by the thrust activity of the seismic fault of the Tangshan Earthquake and the discussion of the seismotectonic environment of the 1830 Cixian earthquake and the 1937 Heze earthquake. The author thinks that the main reason for the problems in research of strong earthquake tectonics in the region is that the status of activity of the main tectonics during the Late Quaternary are unknown. This affects the founding of discrimination criteria for seismotectonics of strong earthquakes on the North China Plain. Discriminating the Holocene active faults from the large number of faults is the most effective method for seismic hazard assessment in the area in future.

Discussion on the Seismogenic Fault of the 1976 Tangshan Earthquake

The opinions of two papers carried in the journal "Seismology and Geology" are discussed in the paper.One is that the Tangshan fault is a high-angle,west-dipping and thrust with strike-slip fault.The other is that the Fuzhuang-Xihe fault distributed on the east side of Tangshan city is the seismogenic fault that caused the Tangshan earthquake.For the former opinion,it needs to explain the relationship between the active style of the thrust Tangshan fault and the formation genesis of a Quaternary depression along the west side of Tangshan city.For the latter opinion,if the Fuzhuang-Xihe fault is the seismogenic fault of the Tangshan earthquake,it needs to explain the genesis relationship between this west-dip slip fault zone and the strike-slip surface fissure zone that extends through Tangshan city.And it needs more evidence exclude the possibility that the surface rupture belongs to the rupturing of a secondary structure.This paper suggests doing more work on the active fault that controls the Caobo Quaternary depression.

四川雀儿山错坝断裂活动特征

错坝断裂走向NW50°、长约26km、展布在川藏交界海拔4000m以上高山,属于甘孜-玉树断裂带的一条分支断裂。研究错坝断裂活动特征有助于了解甘孜-玉树断裂带东南段南西侧分支断裂的活动性。错坝断裂的断错地貌现象丰富,有基岩断崖、断层沟、坡中谷、冲沟水平扭曲。这些现象分布在拔河高度上百米的山体。实测17个地层热释光测年样品,其中7个样品断错了距今18~51ka地层,其余10个样品为地貌面地层年代样品。结合地层测年及地貌面分析,厘定错坝断裂的最新活动时代为距今18~16ka之间,属于晚更新世晚期活动断裂。错坝断裂晚更新世晚期的左旋走滑活动速率约为1.4mm/a,垂直活动速率约为0.3~0.7mm/a。该断裂在晚更新世时期至少有过3次活动。

鄂尔多斯地块周边断陷系6～6 3/4级地震时空分布特征及与活动构造的关系

一、前言鄂尔多斯地块周边断陷系是我国历史地震活跃区。据历史文字记载,近2000年期间该区曾发生4次8级地震,6次7~7 1/2级地震,28次6~6 3/4级地震。有关该区7级以上历史强震与活动断裂的关系,一直受到地震地质学者的关注。但对于该区6级地震的构造背景讨论不多。在Wells统计的全球19世纪下半叶以来发生的地震数据库中,在有地表破裂的122次地震中,震级等于或大于7级的地震54次,占有地表破裂地震总数的45%,震级6~6.9级的地震52次,占总数的42%。这些有地表破裂的6~6.9级地震。

辽宁海城NW向构造全新世活动特征及古地震研究

一、引言辽宁省海城位于我国东北下辽河平原与辽东山地交界地带,地质构造属于华北断块区下辽河块陷与辽东块隆的分界。中国东部著名的郯庐断裂带向北过渤海进入辽东湾后,大致沿下辽河凹陷东界向北继续延伸。70年代中期以前,NNE向断裂一直被认为是该地区活跃的构造。1975年海城地区发生的7.3级地震改变了人们的认识。

Evidence of multistage late Quaternary strong earthquakes on typical segments of Longmenshan Active Fault Zone in Sichuan, China

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.