Collaborative Research: Measuring Strain Along the Aleutian Subduction Zone Trench to Better Constrain Seismic and Tsunami Hazard

美国历史上有记录以来最大的地震发生在1964年阿拉斯加俯冲带断层上，太平洋板块在北美板块的阿拉斯加下方急剧滑动。在这场9.2级的地震中，太平洋板块在阿拉斯加下方进一步滑动，释放出积聚的压力，导致科迪亚克岛附近地区突然向东移动了18米。这导致了一场大海啸，造成了巨大的破坏和许多人的死亡。太平洋板块继续以每年6厘米的速度向西北移动，挤压了阿拉斯加的部分地区，积聚了在随后的地震中释放出来的压力。科迪亚克岛上的GPS站点记录了向西北移动和应变积累，但舒马金群岛的站点记录了很少或没有明显的运动，这表明有两种可能性——要么是舒马金地区的太平洋板块与上盖板块分离，这样板块在该地区下方悄悄滑动，要么是太平洋板块与上盖板块在海沟附近锁定。第一种情况表明，俯冲带地震对阿拉斯加西部的地震和海啸风险很小，第二种情况则有很大的风险，可能发生大海啸。确定在海上板块界面锁定的位置和是否发生锁定的唯一方法是测量海沟附近海底的运动。该项目将在从Shumagin岛到Kodiak地区的海沟60公里处安装三个海底gps -声学（GPS-A）站点，并在俯冲带上从未锁定界面明显过渡到锁定界面。如果断层完全锁定，测量结果应该显示在这段时间内，所有三个地点都向西北方向移动了大约12厘米。如果板块在Shumagin区域被解锁，预计两个相邻的位置很少或没有运动。这些观测结果对于评估该地区的地震和海啸风险至关重要，并将提供对海沟附近俯冲带断层行为的新认识。该项目将在这些新的海底大地测量技术方面培训新的工程师和研究人员，并将支持一名研究生和本科生进行研究，他们也将参与海上作业。该项目还利用了新的波浪滑翔机技术，为更广泛的推广提供了机会。将在阿留申海沟附近建立三个GPS-A站点，以测量与阿留申巨型逆冲构造的应变积累相关的变形。​这些站点将跨越一个区域，在该区域，从陆上GPS观测推断出的板块间耦合从接近0%增加到90%，这表明推断出的地震和海啸危害变化很大。陆上观测不足以解决海沟附近的耦合问题，这对评估海啸危害至关重要。GPS- a方法使用声波传播时间测量，将在海底建立的永久基准与海面平台联系起来，该平台以大地测量方式与全球GPS网络相连。该方法可获得1 cm量级的水平定位精度。使用间隔两年的测量，我们可以测量每个地点的变形速度，精度为7毫米/年（或约10%的板块收敛率）。沿走向观测能较好地约束舒马金地震断裂带和1938年地震断裂带的耦合变化。了解俯冲带内俯冲板块与上覆板块之间耦合变化的方式和原因，对于理解俯冲过程以及俯冲特大逆冲带来的地震和海啸危害至关重要。新的数据将对最近重复1995年以前的全球定位系统运动观测和持续不断的全球定位系统站点所获得的观测结果起到重要的补充作用。板块耦合的位错模型表明，观测结果应该约束海沟附近的耦合，这对了解潜在的海啸危险至关重要。

The largest earthquake ever recorded in US history occurred in 1964 on the Alaskan subduction zone fault where the Pacific plate slides steeply beneath Alaska on the North American plate. During this magnitude 9.2 event the Pacific plate slipped further beneath Alaska releasing built up strain, causing regions near Kodiak Island to suddenly move SE as much as 18 meters. This resulted in a large tsunami, much damage, and many deaths. The Pacific plate continues to move northwestward 6 cm/yr, compressing regions of Alaska, building up strain that would be released in a subsequent earthquake. GPS sites on Kodiak Island record northwestward motion and strain accumulation, but sites on the Shumagin Islands record little or no apparent motion suggesting two possibilities - either the Pacific plate in the Shumagin region is uncoupled from the overlying plate, such the plate is slipping quietly beneath the region, or else the Pacific plate is locked with the overlying plate near the trench. The first scenario suggests very little seismic and tsunami risk in western Alaska from subduction zone earthquakes, the second scenario poses significant risk and the potential for a large tsunami. The only way to determine where and if plate interface locking is occurring offshore is to measure the motion of the seafloor near the trench. This project will install three seafloor GPS-Acoustic (GPS-A) sites 60 km from the trench spanning the Shumagin Island to Kodiak region and the apparent transition from an unlocked to a locked interface on the subduction zone. If the fault is fully locked, the measurements should show all three sites moving roughly 12 cm NW during this interval. If the plate is unlocked in the Shumagin region, little or no motion of the two adjacent sites is expected. These observations are critical to assessing seismic and tsunami risk in the region and will provide a new understanding of the behavior of subduction zone faults near the trench. This project will train new engineers and researchers in these new seafloor geodetic techniques and will support a graduate student and undergraduate students in the research who will also participate in the at sea operations. The project also makes use of novel wave-glider technology that offers opportunities for broader outreach.Three GPS-A sites will be established near the Aleutian trench to measure deformation associated with strain accumulation along the Aleutian megathrust. Two sites will bridge the Shumagin seismic gap and the third site will lie within the Semidi segment, the source of the 1938 M8.2 earthquake. The sites will span a region where the inferred interplate coupling derived from onshore GPS observations increases from near 0% to 90%, representing a large variation in inferred seismic and tsunami hazard. Onshore observations are inadequate to resolve coupling near the trench and this is critical to assessing tsunami hazard. The GPS-A method uses acoustic travel time measurements to tie permanent benchmarks established on the seafloor to a sea surface platform that is geodetically tied into the global GPS network. A horizontal positional accuracy of order 1 cm can be obtained with this method. Using measurements two years apart, we can measure the deformation velocity at each site to an accuracy of order 7 mm/yr, (or about 10% of the plate convergence rate). The along strike observations will better constrain the variation in coupling across the Shumagin seismic gap region and the region of 1938 earthquake. Understanding how and why coupling varies between the subducting plate and the overlying plate within subduction zones is critical to understanding subduction processes and seismic and tsunami hazard from subduction megathrusts. The new data will be a crucial augmentation to observations obtained with a recent repeat of pre-1995 campaign GPS observations and with ongoing continuous GPS sites. Dislocation models of plate coupling show the observations should constrain coupling near the trench essential to understanding the potential tsunami hazard.