龙门山断裂带晚第四纪的大地震活动--来自古地震研究的资料

Late-Quaternary fault activity of the Longmen Shan fault zone——Evidence from paleoseismic trenching

2018年5·12汶川地震后,龙门山断裂带的大地震活动特征一直是倍受关注的科学问题.而探槽古地震研究是最直接揭示活断层带晚第四纪大地震活动规律的重要途径.本文通过系统总结龙门山断裂带近十年来探槽古地震研究成果,全面分析了龙门山断裂带不同段落的晚第四纪大地震活动与复发特征.结果显示:龙门山断裂带的大地震活动具有明显的分段性,北川—映秀断裂和江油—灌县断裂中段(包括小鱼洞断裂)是全新世以来,最为活跃的段落,约距今6000年以来,发生过3次位移量近似相等的事件,分别发生在5920—5730cal BP和3300—2300cal BP,大地震活动具有3000年左右的准周期复发特征;北川—映秀断裂的北段具有独立破裂的能力,该段汶川地震前一次事件发生在大约665—1030AD,可能是历史上记载的942AD地震事件,另一次事件发生在8240—7785BC;江油—灌县断裂的南段(大川—双石断裂)发震能力明显弱于断裂带中段和北段.现有古地震数据表明,沿北川—映秀断裂,除汶川地震以外,并未显示出不同段落间在全新世期间存在级联破裂的证据,这可能意味着2008年汶川M8地震是龙门山断裂带上罕见的巨大地震事件.另外,青川断裂上探槽揭露的古地震变形事件不像是断裂直接位错的结果,更可能是被动响应北川—映秀断裂右旋位错的现象.

The fault activity of the Longmen Shan fault zone has been focused since the Wenchuan earthquake occurred on May 12,2018.Paleoseismic trenching has been suggested to be the mosteffective way to reveal Late-Quaternary fault rupture behavior.This paper integrates about 10-year paleoseismic studies on the Longmen Shan fault to further reveal its Late-Quaternary rupture behavior and recurrence characteristics.Herein we suggest that rupture behavior of the Longmen Shan fault shows segmentation characteristics.Fault activity of the middle segment of the Yingxiu-Beichuan and Jiangyou-Guanxian fault（including the Xiaoyudong fault）has been the most intense part since Holocene.Specifically,the segment ruptured three times since about 6000 years with a similar coseismic offset,the penultimate and third oldest event occurred at 3300—2300 cal BP and 5920—5730 cal BP,which indicates an average recurrence interval of about 3000 years.The northern segment of the Yingxiu-Beichuan fault had independent rupture behavior,the penultimate event on the segment ruptured at about 665—1030 AD that is associated with historical record 942 AD earthquake.The third oldest event occurred at 8240—7785 BC.The southern segment of the Jiangyou-Guanxian fault（Dachuan-Shaungshi fault）shows a weaker seismic potential than that of the middle and northern segments.The present paleoseismic data except for the 2018 Wenchuan earthquake that are associated with the Yingxiu-Beichuan fault suggest that no evidence has supported a cascading rupture behavior of the Yingxiu-Beichuan fault,which indicates that the 2018 Wenchuan earthquake might be an unusual great seismic event.Moreover,deformation evidence revealed from trenching on the Qingchuan fault might not be related to fault offset,but more possibly be related to responding deformation due to coseismic right-laterally offset of the Yingxiu-Beichuan fault.