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年的阿拉斯加俯冲带断层上，太平洋板块在北美板块上的阿拉斯加下方急剧滑动。在这次9.2级的地震中，太平洋板块进一步滑到阿拉斯加下面，释放出累积的压力，导致科迪亚克岛附近的地区突然向东南移动了18米。这导致了一场巨大的海啸，造成了巨大的破坏和许多人死亡。太平洋板块继续以每年6厘米的速度向西北方向移动，挤压阿拉斯加地区，积累的应变将在随后的地震中释放。科迪亚克岛上的GPS站点记录了向西北方向的运动和应变积累，但舒马金群岛上的站点记录很少或没有明显的运动，这表明了两种可能性--要么舒马金地区的太平洋板块与上覆板块分离，这样板块在该地区下方安静地滑动，要么太平洋板块与海沟附近的上覆板块锁定。第一种情况表明，阿拉斯加西部的俯冲带地震几乎没有地震和海啸风险，第二种情况构成重大风险和大海啸的可能性。 确定板块界面锁定在何处以及是否在近海发生的唯一方法是测量海沟附近海底的运动。该项目将在距离Shumagin岛至Kodiak区域的海沟以及俯冲带从未锁定界面到锁定界面的明显过渡处60公里处安装三个海底GPS声学（GPS-A）站点。如果断层被完全锁定，测量结果应该显示所有三个地点在此期间移动了大约12厘米。如果板块在舒马金地区解锁，预计两个相邻地点的运动很少或没有。 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，并将为海沟附近俯冲带断层的行为提供新的理解。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一名本科生进行研究，他们也将参加海上作业。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，以测量阿留申巨型逆冲断层沿着与应变积累有关的变形。 两个地点将连接舒马金地震空区，第三个地点将位于1938年8.2级地震的震源塞米迪段。这些地点将跨越一个区域，其中从陆上GPS观测得出的推断板间耦合从近0%增加到90%，代表推断地震和海啸危险的巨大变化。陆上观测不足以解决海沟附近的耦合，这对评估海啸危险至关重要。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，该平台在大地测量上与全球GPS网络联系在一起。用这种方法可以获得1厘米量级的水平位置精度。使用间隔两年的测量，我们可以测量每个站点的变形速度，精度为7毫米/年（或约10%的板块收敛速率）。沿着向观测将更好地约束Shumagin地震空区和1938年地震区耦合的变化。了解俯冲带内俯冲板块和上覆板块之间的耦合如何以及为什么会发生变化，对于了解俯冲过程以及俯冲巨型逆冲断层带来的地震和海啸危险至关重要。新的数据将是对最近重复1995年前的全球定位系统活动观测和正在进行的连续全球定位系统站点所获得的观测的重要补充。板块耦合的位错模型表明，观测结果应限制海沟附近的耦合，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.