五大连池地区深部结构探测研究

Volcanic eruptions are one of Earth's most dramatic and violent agents of change, can pose a big hazard to humans. Although there were no volcanic eruptions during the recent hundreds of years in China, the volcanism is quite active in Cenozoic time. The Quaternary Wudalianchi volcanoes were recorded to erupt in Chinese history. They are the best examples of the intra-continental monogenetic volcanoes. The mechanism of forming these volcanoes is still under debate. Several dynamic models were proposed to interpret this. The position and shape of the magma chamber and the channels of hot mantle materials play a key role to reveal the mechanism and dynamic processes of the Wudalianchi volcanism. Considering the distribution of seismic arrays deployed by the previous studies, we plan to deploy a 2-D array with the average station interval of 50 km for two year observation in this project. Combining the seismic data recorded by both this array and the previous arrays, we adopt multiple methods of seismic imaging to yield the seismic velocity models and the shapes of seismic discontinuities beneath the Wudalianchi volcanoes and surrounding regions. we focus on the shape of low velocity bulks and their spatial contact with the surrounding blocks. Based on the results of our seismic imaging, we discuss the details about the geodynamic processes of Wudalianchi volcanoes combining with the studies of geochemistry and geophysics.

火山喷发是严重破坏人类社会和自然环境的重大地质灾害。中国虽然缺少现代火山喷发，但是新生代的火山活动相当频繁。五大连池火山群是中国为数不多的有火山喷发文字记载的火山之一，是中国著名的第四纪火山群,也是世界陆内单成因火山的典型代表，对该火山成因及机制还存在着许多争议并提出了多种动力学模型。研究五大连池火山的成因及深部动力学机制的关键还要获得火山下方岩浆房和热物质通道的位置与深部形态。基于目前五大连池地区已有的地震台站分布情况，本项目拟布设一个平均台站间距为50公里的二维地震台阵开展为期两年的观测。利用采集的地震数据并结合该地区已有的地震资料，采用多种地震成像方法获取五大连池及邻近地区的深部地震波速度分布、间断面形态等地震学信息，重点关注低速体分布形态，及其与周围构造块体的深部接触关系。在此基础上，结合地球化学及其它地球物理观测结果，探讨火山活动的深部动力学过程。

WEK火山带包括五大连池火山、二克山火山和克洛火山，该火山带位于松辽盆地北缘，大兴安岭与小兴安岭造山带之间，南北方向分布且远离日本海沟。此火山带是研究中国东北第四纪板内火山的典型范例，但其成因机制一直存在争议。研究五大连池火山成因及深部动力学机制的关键是要获得火山下方岩浆房和热物质通道的位置与深部形态。基于此，项目组在WEK周边区域布设了42个宽频带流动地震台站，观测时间从2015年6月至2018年6月，称之为WDLC台阵。地震台站安装了3种类型的地震计，分别是Güralp CMG-3ESP, CMG-3T 和Nanometrics Trillium 120PA型号。通过2至3年的观测，我们获得了约900GB的原始记录数据和约520GB的事件数据。. 通过密集台阵的背景噪声成像和双平面波成像方法，我们构建了五大连池火山区岩石圈三维S波速度结构模型。结果显示五大连池火山区下方从Moho面到约90 km深度范围内呈现高达 5%的 S 波速度低异常区，该低速异常区很有可能是地幔内岩浆上涌的通道且为同源。 科洛火山、五大连池火山和二克山火山岩带下方在100-200 km 深度范围内的低速异常沿北西-南东分布，其方向与SKS的快轴方向一致。结合这种低速特征，我们认为当100-200 km深度范围内的低速异常体流动到五大连池火山区岩石圈界面附近时，由于五大连池火山区下方的岩石圈厚度存在明显而产生的软流圈剪切作用会引起岩浆的局部上涌，形成火山。