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

合作研究：部署海底光纤应变仪来检测慢滑移事件

• 批准号: 2004259
• 负责人: Mark Zumberge
• 金额: $ 85.75万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-06-01 至 2024-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2004259&HistoricalAwards=false
• 关键词: Collaborative; Research; Deployment; Seafloor; Optical

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.

地面深处的连接力导致构造板（大陆和海底）每年在许多厘米（几英寸）中缓慢移动。在某些地方，板边界必须互相滑行以适应这一运动。有时会顺利进行，但是有时板边缘被摩擦粘在一起，并且根本不会滑动，直到足够的压力会产生到板块突然在地震中彼此滑倒的程度。在过去的二十年中，已经研究了一种不同形式的构造边界滑移，其中像地震一样，在情节上放松了构建的压力，但要花费数天，几周，几个月甚至几年才能逐渐滑动。这些事件称为“慢速事件”或“慢速地震”。由于对动作进行了分级，因此它们不会产生地震摇动（如正常地震一样），从而使它们危险得多。但是，在摩擦卡住的区域附近发生的缓慢滑移事件可能会触发大地震，这一现象需要进一步研究。这项研究的重点是寻找在海上，浅层俯冲区的慢速运动事件，该事件位于美国西部从北加州到加拿大边境北部的近海。在这里，一个海洋构造板与北美碰撞，每几百年就会一次大型破坏性地震和海啸。与慢速事件相关的地震震动缺乏使这些事件难以检测到，尤其是海上。在陆地上，它们是精确的证据，持续了GPS记录，这些记录显示了几厘米开始并停止几周的板块运动。由于GPS信号不会穿透海水，因此在海底上观察到这样的动作要困难得多。因此，必须设计替代方法来检测离岸慢速事件。该研究项目将非常准确地记录遍布俄勒冈海岸附近海底的光纤长度。如果在光纤附近的区域发生慢速事件，则相关的光学长度更改将由电池供电的激光系统记录在每个光纤末端。在海上研究此类事件将有助于建立其对大地震时间和位置的影响的模型，并可能导致地震预测的未来进步。该项目支持对学生的培训。在过去的十年中，全科医生传感器的各个部署有助于确定缓慢的滑移事件（SSES），尤其是在卡斯卡迪亚，哥斯达黎加，日本和新西兰的俯冲带靠近俯冲区的故障。了解SSE提供了一个机会，可以获得有关锁定和解锁俯冲带和其他断层的机制的新见解，并且对于评估潜在的大地震和海啸所带来的危害水平可能很重要。特别是，发生在强锁定区域的下降极限处的SSE可能会带来触发大地震破裂的风险。因此，搜索在锁定区底部发生的SSE至关重要，在卡斯卡迪亚（与大多数俯冲区域一样）位于海上。在这项研究中，将在卡斯卡迪亚俯冲带上方的海底安装两个正交光纤应变仪，以检测海上SSES。最近对Cascadia使用陆上GPS数据的SSE累积效应的最新研究表明，在锁定区底部的近海慢滑层可能仅在陆上SSES中就可以发生。但是，使用陆上GPS对此滑移的检测非常薄弱。确认卡斯卡迪亚的存在或不存在近海缓慢滑移对于理解影响下一次大地震时间和位置的SSE的潜在作用很重要。尽管GPS网络具有足够的灵敏度来绘制SSES陆上的位置，但它们不涵盖海洋下的地壳的那部分，也无法区分子事件的时机，因为需要在每日时间内平均。相比之下，光纤应变仪可以在海上部署，并在较短的时间内具有最佳的信号与噪声比，并补充了位置和频带中的陆上GPS。由于SSEs以复杂的模式演化，表明应力前线传播，因此在尺度和时间上解决变形信号是重要的，以便更充分地了解破裂平面的演变。与GPS结合使用，高度敏感和稳定的应变仪在海上的可用性将实现此类字符。这项研究中的测量值将与预期的陆上SSE进行计时，以捕获假设的海上滑动，并测试该模型以及其他解决海上锁定区域范围的模型。除了检测SSE外，在该海上应变计部署后，其他许多研究将变得可行，包括研究旅行地震波的应变以及推断局部地球结构的潮汐观察。此外，这项工作将推进光纤传感器的技术，并可能在其他学科中找到应用。该奖项反映了NSF的法定任务，并通过使用基金会的知识分子优点和更广泛的影响评估标准来通过评估来诚实地获得支持。