Mantle transition zone structure beneath the Alaska-Aleutian subduction zone and its surroundings
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摘要:
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.
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关键词:
- 阿留申—阿拉斯加俯冲带 /
- SS前驱波 /
- 共反射点叠加 /
- 太平洋俯冲板片 /
- 地幔过渡带
Abstract: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.
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图 1
(a) 本文研究区域(蓝框)及所用台站(绿色三角形)和地震事件(红色圆形)分布图；(b) SS前驱波震相的反射点(黑色圆点)及四条测线(红色虚线)位置示意图
Figure 1.
(a) Map showing study area (blue box), seismic stations (green triangles) and selected events (red circles); (b) Map showing SS precursors bounce points (black dots) and seismic profiles (red dashed lines)
图 2
(a) SS波和前驱波震相的射线路径示意图，其中红色三角形代表台站，红色五角星表示地震事件；(b) 青海区域台网李家峡台站记录到的切向分量波形图
Figure 2.
(a) Sketch showing ray paths of SS and its precursors. Red triangles represent the seismic stations. Red star represents the event; (b) Transverse component waveform recorded by Lijiaxia station of Qinghai Regional Network
图 3
(a)、(b)分别为时差校正前、后的SS波及其前驱波震相的理论地震图
Figure 3.
Synthetic seismograms of SS and its precursors before (a) and after (b) moveout correction
图 4
(a)、(b) 分别为时差校正前、后的实际观测资料波形沿震中距排列的走时图
Figure 4.
Observed waveforms arranged by epicentral distances before (a) and after (b) moveout correction
图 5
(a) 沿AA′测线的SS前驱波共反射点叠加剖面图，其中顶部数字表示每个面元中叠加的波形数，粉色圆点表示地震分布；(b)和(c) 分别表示叠加剖面中410 km和660 km间断面对应的深度值分布
Figure 5.
(a) Depth section of stacked tangential components along profile AA′ aligned on the SS arrivals. The top numeral is the number of records within one cap. Pink dots represent earthquakes. The depths of the 410 km and 660 km discontinuities are shown in (b) and (c), respectively.
表 1
李家峡台站记录到的发生在墨西哥南部海岸的地震事件信息
Table 1.
Seismic event at the coast of southern Mexico recorded by the Lijiaxia station
Time/UTC Latitude/°N Longitude/°W Depth/km Magnitude/MW Distance/(°) 2012-03-20 18∶02∶47.6 16.47 98.37 19 7.5 124.426 
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