Development of a Plate-scale Distributed Strain Sensing System: A Candidate for Earthquake Early Warning

在许多大陆与其邻近的海底之间的边界上，海底构造板块在大陆构造板块之下滑动的过程中，应力不断增加。由于海底板块（有时被称为“下沉板块”）缓慢地俯冲到大陆下方（平均每年移动几英寸），沿边界的摩擦导致两个板块发生弹性变形。这种机制所产生的压力最终会在一场巨大的、具有潜在破坏性的地震中释放出来。这通常伴随着海啸——这两者的结合对许多沿海地区构成了重大威胁，包括美国西北太平洋地区。虽然利用永久的GPS站点很容易在陆地上观测到累积应力，但由于GPS使用的卫星信号无法穿透海水，因此很难观测到海底的变形。因此，必须采用其他方法来观察变形。该项目旨在通过一条长光纤电缆（长达100公里）与一系列较短的传感光纤电缆相连，以确定测量海底变形的可行性。通过光纤发送的光可以检测到电缆中非常微小的长度变化。如果小心地连接到海底，光纤电缆长度的变化表明下面的材料发生了变形。通过观察海底变形，研究人员希望有一天能够探测到由近海地震引起的突然变形变化，并在相关震动到达陆地之前将信息传输到陆地网络，从而促进地震的早期预警。这个项目中的技术方法已经在小范围内得到了验证。在相距几百米的两个海底锚之间拉紧一根光纤，用干涉测量法探测长度的变化。光纤电缆一端的固体激光器将光注入光纤分束器。一部分光沿着拉伸的光纤电缆到达远端的镜面，镜面将光反射回分路器。另一部分激光到达一段缠绕在固定玻璃芯轴上的参考光纤末端的镜面。当两个反射光束在分光器处重新组合时，它们会相互干扰，产生波动的光水平，可以通过处理来揭示纳米级长度的变化。在这个项目中，既定的方法将被扩展到包括一系列的干涉测量传感器，这些传感器连接在一条电缆上，其长度可以覆盖整个大陆架，那里是最危险的应力积聚发生的地方。时分复用方法假定能够探测沿长电缆分布的十个应变传感器，从而充分扩大覆盖范围，以监测100公里长的剖面中的应变变化。要进行的测试将在实验室中进行，测试适合这些地球物理测量的光纤长度的时分多路复用方法。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。

On the boundary between many continents and their neighboring seafloors, stress builds up from the process in which the seafloor tectonic plate slides beneath the continental tectonic plate. As the seafloor plate (sometimes called the “downgoing slab”) is slowly subducted below the continent (moving at a few inches per year, on average), friction along the boundary causes both plates to elastically deform. The stress built up by this mechanism can ultimately be released in a large, potentially devasting earthquake. This is often accompanied by a tsunami – the combination poses a significant hazard to many coastal areas, including the Pacific northwest portion of the United States. While the built-up stress is readily observed on land using permanent GPS stations, deformation of the seafloor is more difficult to observe because the satellite signals used by GPS cannot penetrate seawater. Consequently, other means must be used to observe the deformation. The project aims to establish the feasibility of measuring deformation of the seafloor with a long optical fiber cable (up to 100 km) connected to a series of shorter, sensing optical fiber cables. Light sent through the optical fibers can detect very slight length changes in the cable. If carefully attached to the seafloor, changes in the length of the optical fiber cable indicate deformation in the underlying material. By observing seafloor deformation, researchers hope to one day be able to detect sudden deformation changes caused by an offshore earthquake and transmit the information to a land-based network faster than the associated shaking reaches land, facilitating an early-warning to the earthquake.The technical approach in this project has been demonstrated on a small scale. An optical fiber, tensioned between two seafloor anchors separated by a few hundred meters is interferometrically probed to track length changes. A solid state laser at one end of the optical fiber cable injects light into an optical fiber beamsplitter. Part of the light travels along the stretched optical fiber cable to a mirror at its far end, which reflects the light back towards the splitter. Another part of the laser light travels to a local mirror at the end of a length of reference optical fiber wound onto a fixed glass mandrel. When the two reflected light beams are recombined at the splitter, they interfere, creating fluctuating light levels that can be processed to reveal changes in length at the nanometer level. In this project, the established method will be expanded to include a series of interferometric sensors attached to a cable whose length could cover the entire continental shelf, where the most hazardous stress buildup occurs. The method of time-division multiplexing in postulated to be capable of probing ten strain sensors distributed along the long cable, thereby expanding coverage adequately to monitor strain changes in a 100 km long profile. The tests to be performed are to be done in the laboratory testing the time-division multiplexing approach in optical fiber lengths appropriate for these geophysical measurements.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.