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: 2127938
• 负责人: Noel Jackson
• 金额: $ 29.84万
• 依托单位: University of Kansas Center for Research Inc
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-02-15 至 2025-05-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2127938&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.

地球表面深处的对流力量导致构造板块(大陆和海底)每年以许多厘米(几英寸)的速度缓慢移动。在某些地方，板块边界必须一个接一个地滑动，以适应这种运动。有时这是平稳的，但有时板块边缘由于摩擦而粘在一起，根本不会滑动，直到足够的应力积累到地震中板块突然彼此滑动的地步。在过去的二十年里，人们研究了一种不同形式的构造边界滑动，在这种形式下，累积的应力会像地震一样间歇性地松弛，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“慢地震”。由于运动是渐进的，它们不会像正常地震那样产生地震震动，因此它们的危险性要小得多。然而，发生在摩擦卡住区域附近的慢滑事件可能会引发大地震，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部寻找慢滑事件，该俯冲带位于美国西部近海，从加利福尼亚州北部延伸到加拿大边境以北。在这里，一个海洋构造板块正在与北美相撞，每隔几百年就会产生一次非常大的破坏性地震和海啸。与慢滑事件相关的地震震动的缺乏使得这些事件很难被检测到，特别是在近海。在陆地上，它们在精确的、连续的GPS记录中显而易见，这些记录显示板块运动在几个星期内开始和停止几厘米。在海底观察这种运动要困难得多，因为GPS信号不会穿透海水。因此，必须设计替代方法来检测近海慢滑事件。这项研究项目将非常精确地记录俄勒冈州海岸附近海底延伸的光纤长度。如果在光纤附近区域发生慢滑事件，则相关的光学长度变化将由电池供电的激光系统在每根光纤的末端记录。研究近海的这类事件将有助于建立它们对大地震发生时间和地点的影响模型，并可能导致未来地震预报的进步。该项目支持对一名学生的培训。在过去十年中，GPS传感器的广泛部署帮助识别了慢滑事件(SSE)，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提供了一个机会，以获得对俯冲带和其他断层的锁定和解锁机制的新见解，并可能对评估潜在的大地震和海啸带来的危险级别具有重要意义。特别是，发生在强锁定带下倾极限的SSE可能会构成触发大地震破裂的风险。因此，在卡斯卡迪亚(与大多数俯冲带一样)位于近海的锁定带底部寻找SSE至关重要。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，以检测近海SSE。最近利用陆上GPS数据对卡斯卡迪亚SSE累积效应的研究表明，可能在锁定带底部发生离岸缓慢滑动，这被假设为与陆上SSE同时发生。然而，使用陆上GPS对这一滑移的检测非常弱。确认卡斯卡迪亚近海慢滑的存在或不存在，对于理解SSE在影响下一次大地震的时间和地点方面的潜在作用是重要的。虽然全球定位系统网络有足够的灵敏度来测绘陆地上SSE的位置，但它们不能覆盖海洋下地壳的这一部分，也不能区分子事件的时间，因为需要对每天的周期进行平均。相比之下，光纤应变仪可以部署在海上，在较短的时间内具有最佳的信噪比，在位置和频段上都与陆上GPS互补。由于SSE以复杂的模式演化，表明应力前锋的传播，因此在尺度和时间上解析变形信号对于更全面地了解破裂面的演化是很重要的。与全球定位系统相结合，近海高灵敏度和稳定的应变仪的可用性将使这种特征成为可能。这项研究中的测量将与预期的陆上SSE计时，以捕捉假设的离岸滑动，测试该模型和其他解决离岸锁定区范围的模型。除了SSE的探测外，在部署这种近海应变仪后，还将有许多其他研究成为可能，包括调查行进地震波的应变和潮汐观测，以推断当地的地球结构。此外，这项工作将促进光纤传感器技术的发展，并可能在其他学科中找到应用。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。