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.

地球表面深处的对流力导致构造板块（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，板块边界必须相互滑动以适应这种运动。 有时这会顺利地发生，但有时板块边缘因摩擦力而粘在一起，根本不会滑动，直到足够的应力积累到板块在地震中突然相互滑动的地步。在过去的20年里，人们研究了一种不同形式的构造边界滑动，即累积的应力会间歇性地释放，就像地震一样，但速度要慢得多，需要几天、几周、几个月甚至几年才能逐渐滑动。 这些事件被称为“慢滑事件”或“慢地震”。“因为运动是渐进的，它们不会产生地震震动（像正常的地震那样），使它们的危险性小得多。 然而，发生在被卡住的地区附近的缓慢滑动事件可能会引发大地震，这一现象需要进一步研究。这项研究的重点是寻找在近海，卡斯卡迪亚俯冲带，这是离岸美国西部从北方加州延伸到加拿大边境以北的浅部的慢滑事件。 在这里，海洋构造板块正在与北美碰撞，每隔几百年就会产生非常大的破坏性地震和海啸。缺乏与缓慢滑动事件相关的地震震动使得这些事件难以检测，特别是近海。 在陆地上，它们在精确、连续的GPS记录中很明显，这些记录显示了几个厘米的板块运动，开始和停止了几个星期。 在海底观测这种运动要困难得多，因为GPS信号无法穿透海水。 因此，必须设计替代方法来检测海上慢滑事件。 这项研究项目将非常精确地记录横跨俄勒冈州海岸附近海底的光纤长度。 如果在光纤附近的区域中发生缓慢滑动事件，则将通过在每个光纤的端部处的电池供电的激光系统来记录相关联的光学长度变化。 研究这些近海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。在过去的十年中，GPS传感器的广泛部署有助于识别慢滑事件（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SSEs提供了一个机会，以获得新的见解的机制，控制锁定和解锁的俯冲带和其他故障，并可能是重要的，在评估潜在的大地震和海啸的危险程度。 特别是，发生在强锁定区下倾极限的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，两个正交的光纤应变计将安装在卡斯卡迪亚俯冲带上方的海底，以检测海上SSE。 最近一项使用陆上GPS数据对卡斯卡迪亚SSE的累积效应进行的研究表明，锁定区底部可能存在海上缓慢滑动，假设锁定区与陆上SSE同时发生。 然而，使用陆上GPS检测这种滑动是非常薄弱的。 确认卡斯卡迪亚是否存在海上缓慢滑动，对于了解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。 虽然全球定位系统网络有足够的灵敏度来绘制陆上小规模地震的位置图，但它们并不覆盖海洋下的那部分地壳，而且由于需要对每天的时间段进行平均，它们也无法区分子事件的时间。 相比之下，光纤应变仪可以部署在海上，并在较短的时间内具有最佳的信噪比，在位置和频带上补充陆上GPS。 由于SSE演变的复杂模式表示传播的应力锋，重要的是要解决，无论是在规模和时间，变形信号，以更全面地了解破裂面的演变。 与全球定位系统相结合，高灵敏度和稳定的海上应变计的可用性将使这种特性。本研究中的测量将与预期的陆上SSE同步，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。除了检测SSE之外，在部署了这种海上应变仪之后，还可以进行许多其他研究，包括调查地震行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NSF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。