Collaborative Research: Deployment of Seafloor Optical Fiber Strainmeters for the Detection of Slow Slip Events

地球表面深处的对流力量导致构造板块（大陆和海底）每年缓慢移动数厘米（几英寸）。在某些地方，板块边界必须相互滑动以适应这种运动。有时这一过程很顺利，但有时板块边缘由于摩擦而粘在一起，根本不会滑动，直到产生足够的应力，在地震中板块突然相互滑动。在过去的二十年里，人们研究了另一种不同形式的构造边界滑动，在这种情况下，积累的应力会像地震一样偶尔放松，但速度要慢得多，需要几天、几周、几个月甚至几年的时间才能逐渐滑动。这些事件被称为“慢滑事件”或“慢震”。由于这些运动是渐进的，它们不会像普通地震那样产生地震震动，因此危险性要小得多。然而，发生在摩擦粘滞区域附近的慢滑事件可能引发大地震，这一现象需要进一步研究。这项研究的重点是在卡斯卡迪亚俯冲带的近海浅层寻找慢滑事件，卡斯卡迪亚俯冲带位于美国西部沿海，从加利福尼亚北部延伸到加拿大边境北部。在这里，一个海洋构造板块与北美碰撞，每隔几百年就会产生非常大的破坏性地震和海啸。由于缺乏与慢滑事件相关的地震震动，使得这些事件难以探测，特别是在海上。在陆地上，它们在精确、连续的GPS记录中很明显，这些记录显示板块在几周内开始和停止几厘米的运动。由于GPS信号无法穿透海水，在海底观察到这种运动要困难得多。因此，必须设计替代方法来检测海上慢滑事件。这个研究项目将非常精确地记录在俄勒冈海岸附近海底延伸的光纤长度。如果在靠近光纤的区域发生慢滑事件，则每根光纤末端的电池供电激光系统将记录相关的光长度变化。在海上研究这些事件将有助于建立它们对大地震时间和地点影响的模型，并可能导致未来地震预报的进步。该项目支持培训一名学生。在过去的十年中，GPS传感器的广泛部署已经帮助识别了慢滑事件，特别是在卡斯卡迪亚、哥斯达黎加、日本和新西兰的俯冲带断层附近。了解sse为我们提供了一个机会，使我们对控制俯冲带和其他断层的锁定和解锁机制有了新的认识，这对于评估潜在的大地震和海啸所带来的危险程度可能很重要。特别是，发生在强锁定带的下倾极限处的sse可能有引发大地震破裂的危险。因此，在Cascadia（与大多数俯冲带一样）位于海上的锁定带底部寻找sse至关重要。在本研究中，将在Cascadia俯冲带上方的海底安装两台正交光纤应变仪来探测海上sse。最近一项利用陆上GPS数据对Cascadia的sse累积效应进行的研究表明，在锁定带的底部可能会发生海上慢滑，假设这种情况与陆上sse同时发生。然而，使用陆上GPS探测这种滑动的能力非常弱。确认卡斯卡迪亚离岸慢滑的存在与否，对于理解sse在影响下一次大地震的时间和位置方面的潜在作用非常重要。虽然GPS网络有足够的灵敏度来绘制陆上地震的位置，但它们不能覆盖海洋下的地壳部分，也不能区分次事件的时间，因为需要每天平均一次。相比之下，光纤应变仪可以部署在海上，并且在较短的周期内具有最佳的信噪比，可以在位置和频带上补充陆上GPS。由于sse以复杂的模式演化，表明应力前沿的传播，因此在尺度和时间上解析变形信号以更全面地了解破裂面的演化是很重要的。与GPS相结合，海上高灵敏度和稳定的应变仪的可用性将使这种表征成为可能。本研究中的测量将与预期的陆上SSE进行计时，以捕获假设的海上滑动，测试该模型和其他解决海上锁定区范围的模型。除了对sse的探测之外，随着这一海上应变仪的部署，许多其他研究将变得可行，包括调查地震波的应变和潮汐观测，以推断当地的地球结构。此外，这项工作将推进光纤传感器技术，并可能在其他学科中找到应用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Convective forces deep below Earth’s surface cause tectonic plates (continents and seafloor) to slowly move at many centimeters (several inches) each year. In places, the plate boundaries must slide past one another to accommodate this motion. Sometimes this occurs smoothly, but sometimes the plate edges are stuck together by friction and don't slide at all until enough stress builds to the point where the plates slip past each other suddenly in an earthquake. Over the past two decades, a different form of tectonic boundary slip has been studied in which built up stress is relaxed episodically, as in an earthquake, but at a much slower rate taking days, weeks, months, or even years to gradually slip. These events are called "slow slip events" or "slow earthquakes." Because the motions are gradual they do not generate seismic shaking (as normal earthquakes do), making them far less dangerous. However, slow slip events occurring near areas that are frictionally stuck may trigger large earthquakes, a phenomenon that requires further study. This study focuses on searching for slow slip events in the offshore, shallow part of the Cascadia subduction zone, which lies offshore the western United States stretching from northern California to north of the Canadian border. Here an oceanic tectonic plate is colliding with North America, producing very large destructive earthquakes and tsunamis every few hundred years. The lack of seismic shaking associated with slow slip events makes these events difficult to detect, especially offshore. On land they are evident in precise, continuous GPS records which show plate motions of several centimeters that start and stop over a few weeks. Such motions are much more difficult to observe on the seafloor because GPS signals do not penetrate sea water. Consequently alternative methods must be devised to detect offshore slow slip events. This research project will record very precisely the lengths of optical fibers stretched across the seafloor near the Oregon coast. If a slow slip event occurs in the region near the optical fibers, the associated optical length change will be recorded by a battery powered laser system at the end of each optical fiber. Studying such events offshore will help to build models of their influence on the timing and location of great earthquakes, and may lead to future advances in earthquake forecasting. The project supports the training of a student.Widespread deployments of GPS sensors in the past decade have helped identify Slow Slip Events (SSEs), especially near subduction zone faults in Cascadia, Costa Rica, Japan, and New Zealand. Understanding SSEs presents an opportunity to gain new insights into the mechanism governing locking and unlocking of subduction zone and other faults, and may be important in assessing the hazard levels presented from potential great earthquakes and tsunami. In particular, SSEs occurring at the downdip limit of the strongly locked zone may pose a risk of triggering large earthquake ruptures. It is therefore critical to search for SSEs occurring at the base of the locked zone, which in Cascadia (as in most subduction zones) lies offshore. In this study, two orthogonal optical fiber strainmeters will be installed on the seafloor above the Cascadia subduction zone to detect offshore SSEs. A recent study of the cumulative effect of SSEs in Cascadia using onshore GPS data indicates possible offshore slow slip at the base of the locked zone which is hypothesized to occur simultaneously with onshore SSEs. However, the detection of this slip using onshore GPS is very weak. Confirming the presence or absence of offshore slow slip in Cascadia is important for understanding the potential role of SSEs in influencing the timing and location of the next great earthquake. While GPS networks have sufficient sensitivity to map the location of SSEs onshore, they do not cover that portion of the crust under the oceans, nor are they able to distinguish the timing of sub-events because of the need for averaging over daily periods. In contrast, optical fiber strainmeters can be deployed offshore and have their best signal to noise ratio at shorter periods, complementing onshore GPS both in location and frequency band. Because SSEs evolve in complex patterns indicative of propagating stress fronts, it is important to resolve, both in scale and time, the deformation signals in order to understand more fully the evolution of the rupture plane. In conjunction with GPS, the availability of highly sensitive and stable strainmeters offshore will enable such characterizations. The measurements in this study will be timed with an expected onshore SSE to capture the hypothesized offshore slip, testing this model and others that address the extent of an offshore locked zone. In addition to the detection of SSEs, a number of other studies will become feasible following the deployment of this offshore strainmeter, including investigation of strain from traveling seismic waves and tidal observations for inferring local Earth structure. In addition, the work will advance the technology of optical fiber sensors and likely find applications in other disciplines.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.