Basin-to-slab Seismic Imaging of the Alaska Subduction Zone

弧前盆地是俯冲系统的基本组成部分。库克湾前弧盆地是北美最大的活动前弧盆地，面积为200公里× 100公里，深度为7.6公里。在过去的160年里，沿着阿拉斯加南部边缘的辐合和增生，这个盆地一直是一个沉积和沉降的地方。该盆地是一个50公里厚的地壳的一部分，覆盖在一个自由滑动的板块界面上，正好是1964年Mw 9.2地震破裂程度的下倾。一个高磁、低重力、大陆尺度的信号被称为南阿拉斯加磁高压（SAMH），与库克湾盆地重合。这个信号的来源被认为是地壳内部的一个古老的岩浆弧，或者是地幔楔中的一个蛇纹石化带。我们假设与SAMH相关的构造体在地质时间尺度上推动了弧前盆地的形成。通过表征盆地-地壳-板块-地幔系统的三维地震结构，应该有可能分离出SAMH的起源，进而分离出全球重要的弧前盆地的起源。沉积盆地是地震成像的主要挑战。地震波的盆地反射使对俯冲系统的下部和邻近部分（包括下地壳、上地幔和俯冲板块）进行成像的工作复杂化。建立高质量的Cook Inlet地区地震结构模型需要三个要素：(1)地震成像能力，既能处理三维模型的结构复杂性，又能处理全地震波形内的细节；(2)最大分布局地地震，提高分辨率；(3)一组战略性放置的宽带地震仪。我们计划一个为期两年的EarthScope FlexArray部署30个地震台站，以瞄准(1)包含库克湾盆地的50公里厚的大陆地壳，以及(2)横跨盆地、穿过Redoubt火山、进入弧后区域的400公里长的俯冲带弧-法向样带。由于地壳和俯冲板块内的地震活动频率惊人，库克湾地区为俯冲构造的地震成像提供了一个世界级的机会。通过隔离来自盆地的地震信号，将有可能对下地壳、上地幔和俯冲太平洋板块内部的深层结构和过程进行成像。我们计划在基于伴随的成像技术中利用全波场模拟的准确性，该技术已在南加州证明是成功的。对构造环境的完整描述将来自于对局部和远震剪切波分裂测量的详细调查，以推断地幔流动模式，以及从地壳和板块内地震的数百个m3矩张量推断的构造应力的综合。

Forearc basins are fundamental components of a subduction system. The Cook Inlet forearc basin is the largest active forearc basin in North America, with dimensions 200 km by 100 km and a depth of 7.6 km. The basin has remained a site of sedimentation and subsidence over the past 160 Ma of convergence and accretion along the southern Alaska margin. The basin is part of a 50-km thick crust overlying a freely slipping plate interface, just downdip from the rupture extent of the Mw 1964 9.2 earthquake. A high-magnetic, low-gravity, continental-scale signal known as the Southern Alaska Magnetic High (SAMH) coincides with Cook Inlet basin. The origin of this signal has been proposed to result from either an ancient accreted magmatic arc within the crust, or a serpentinized zone within the mantle wedge. We assume that the structural body associated with the SAMH has driven forearc basin formation over geological time scales. By characterizing the three-dimensional seismic structure of the basin-crust-slab-mantle system, it should be possible to isolate the origin of the SAMH and, in turn, the origin of a globally significant forearc basin.Sedimentary basins pose major challenges for seismic imaging. Basin reverberations of seismic waves complicate efforts to image underlying and adjacent portions of the subduction system, including the lower crust, uppermost mantle, and subducting slab. Three components are needed to derive a high-quality seismic structural model of the Cook Inlet region: (1) seismic imaging capabilities to handle both the structural complexities of the three-dimensional model and details within full seismic waveforms; (2) maximal distribution local earthquakes to enhance resolution; and (3) an array of strategically placed broadband seismometers.We plan a two-year EarthScope FlexArray deployment of 30 seismic stations to target (1) the 50-km thick continental crust containing Cook Inlet basin, and (2) a 400-km long arc-normal transect of the subduction zone across the basin, through Redoubt volcano, and into the backarc region. With a phenomenal rate of seismicity within the crust and subducting slab, the region of Cook Inlet offers a world-class opportunity for seismic imaging of subduction tectonics. By isolating the seismic signals due to the basin, it will be possible to image deeper structure and processes within the lower crust, upper mantle, and subducting Pacific slab. We plan to harness the accuracy of full wavefield simulations within an adjoint-based imaging technique that has proven successful in southern California. A complete characterization of the structural setting will result from detailed investigations of local and teleseismic shear-wave splitting measurements to infer mantle flow patterns, in addition to a synthesis of tectonic stresses inferred from hundreds of M 3 moment tensors from earthquakes within the crust and slab.