阿留申—阿拉斯加俯冲带及周边地区地幔过渡带结构研究

Mantle transition zone structure beneath the Alaska-Aleutian subduction zone and its surroundings

利用国家测震台网固定台站和“中国地震科学台阵探测”项目在南北地震带北段布设的宽频带流动台阵记录到的极远震事件,通过SS前驱波震相研究,获得了阿留申—阿拉斯加俯冲带东段及邻区下方410 km和660 km间断面的埋深和起伏形态特征.为增强对SS前驱波震相的识别,我们采用了时差校正和共反射点叠加分析.叠加结果显示,毗邻阿留申俯冲带的白令海、阿拉斯加半岛、以及阿拉斯加中南部和东部地区下方,410 km和660 km间断面的埋深基本呈正相关关系,因而具有正常的过渡带厚度.这表明在阿留申—阿拉斯加俯冲带东段,北太平洋板块还没有俯冲到地幔过渡带深度范围内.其次,在阿拉斯加西部地区下方,660 km间断面出现明显下沉,而上覆的410 km间断面埋深接近于全球平均值,从而导致过渡带明显加厚.据此,我们推测在阿拉斯加西部地区下方地幔过渡带底部可能存在库拉残留板块.

The topographies of the 410 km and 660 km discontinuities beneath the Aleutian-Alaska subduction zone and adjacent areas are imaged by identifying SS precursors from teleseismic data recorded by the National Digital Seismic Network and a regional seismic array deployed in the northeastern Tibetan plateau operated by“ChinArray”project.We use the moveout correction and common midpoint stacking to enhance the SS precursory arrivals.Results show that the 410 km and 660 km discontinuities appear to be positively correlated beneath the areas adjacent to the Aleutian-Alaska subduction zone,such as the Bering Sea,Alaska Peninsula,and the southern and eastern Alaska.Thus a normal transition zone is inferred beneath these areas,indicating that the North Pacific plate has not yet subducted into the upper mantle transition zone.Beneath western Alaska,the 660 km discontinuity is strongly depressed and a thicker transition zone can be mapped,while the overlying 410 km discontinuity appears to be normal compared to the global model.Such observations suggest the presence of the remnant Kula plate lies just above the 660 km discontinuity beneath western Alaska.