MRI: Offshore Earthquake Monitoring at Subduction Zones Using Autonomous Underwater Vehicles and High-Speed Optical Telemetry For Data Retrieval

地球的俯冲带，一个构造板块覆盖另一个板块，是地球上最大和最危险的地震的地点。最近的例子是2004年苏门答腊岛9.1级地震和2010年日本东北9级地震。除了地面震动造成的大规模破坏外，这两次地震都引发了毁灭性的海啸。这些事件提醒我们，地球上最大的地震中的断层运动发生在大陆架下的近海，这是一个具有挑战性的监测区域。这些大地震的震源区缺乏地震仪器，限制了地震界回答基本科学问题和实时评估灾害的能力。对俯冲带进行持续的近海监测，不仅有助于更好地了解这些断层，而且可能有助于查明短期风险增加的情况。在2010年东北地震和最近的2014年智利皮萨瓜9级地震之前，位于遥远近海的广泛前震序列。这些序列表明，在几周的尺度上存在可检测到的时间段，此时俯冲带更有可能产生大的破裂。为了充分了解这一基本的断层行为，并可能利用对其的了解来降低风险，需要快速获取大地震正上方的高质量地震数据。地震界经常在海上部署地震仪，但数据访问需要舰载仪器恢复，这是一项漫长而昂贵的任务，在潜在的前震序列期间，这可能是不可能或不可取的。地震学是一门快速发展的科学：地震在几秒钟内被检测到，震级估计在几分钟内在Twitter上发布，断层面在几个小时内确定，研究文章往往在大地震后的几周内提交。只有将近海地震数据迅速纳入数据中心，才能充分发挥这些数据的潜力。伍兹霍尔海洋研究所在光学遥测和海洋机器人方面的最新进展，再加上商业现成的低功率地震传感器、数据记录器和原子钟的提供，使海底地震仪阵列(OBS)能够多年部署，能够将高频、精确计时的地震数据传输到岸上，数据延迟从几小时到几天，而不需要OBS恢复。该项目将通过将世界卫生组织设计的能够以20兆比特/秒的遥测速率与OBS和远程(长达286海里)Remus自主水下航行器(AUV)相结合的光学调制解调器来开发这样一个系统。这些经过验证的速率允许在几分钟内遥测4个通道(地面运动和压力)上一周的高速(100赫兹)地震数据(或在不到2小时内遥测一年的数据)。此外，准确的授时对于地震定位是至关重要的，但OBS缺乏GPS授时的好处。该光链路还将允许测量OBS时钟相对于AUV携带的GPS同步时间信号的偏移量，精度为~1微秒。这些技术将使按需数据卸载和时钟检查的OBS阵列的多年部署成为可能，而无需每年的恢复/重新部署巡航，从而为每次试验节省数十万美元。这种能力将特别适用于监测俯冲区所需的密集近岸阵列。

The earth's subduction zones, where one tectonic plate overrides another, are the sites of the planet's largest and most dangerous earthquakes. Recent examples are the 2004 magnitude 9.1 Sumatra and the 2010 magnitude 9 Tohoku, Japan earthquakes. In addition to massive destruction brought about by ground shaking, both of these earthquakes generated devastating tsunamis. These events are a reminder that fault motion in our planet's largest earthquakes happens offshore under the continental shelf, a region that is challenging to monitor. The scarcity of seismic instrumentation in the source regions of these great earthquakes has limited the seismological community's ability to answer fundamental scientific questions and to evaluate hazards in real-time. Sustained offshore monitoring of subduction zones would not only lead to improved understanding of these faults but also potentially allow the identification of increased short-term risk. Extensive foreshock sequences located far offshore preceded both the 2010 Tohoku event and the recent 2014 magnitude 9 Pisagua Chile earthquake. These sequences demonstrated that there are detectable time periods on the scale of weeks when subduction zones are more likely to produce a large rupture. To fully understand this basic fault behavior and possibly utilize an understanding of it to reduce risk, rapid access to high quality seismic data from offshore directly above the great earthquakes is needed. The seismological community routinely deploys seismographs offshore, but data access requires ship-based instrument recovery, a lengthy and expensive task that may not be possible or desirable during a potential foreshock sequence. Seismology is a fast moving science: earthquakes are detected within seconds, magnitude estimates are tweeted within minutes, fault planes are determined within hours, and research articles are often submitted within weeks of a major earthquake. The full potential of offshore seismic data can only be realized if these data are rapidly ingested into data centers. Recent advances at Woods Hole Oceanographic Institution in optical telemetry and marine robotics, coupled with the availability of commercial off-the-shelf low-power seismic sensors, data loggers and atomic clocks offer the capability for multi-year deployments of arrays of ocean-bottom seismographs (OBS) that are capable of delivering high-frequency, accurately-timed seismic data to shore with data latencies of hours to days without OBS recovery. This project will develop such a system by integrating a WHOI-designed optical modem capable of telemetry rates of 20 Mbits/second with an OBS and with a long-range (up to 286 nautical miles) REMUS Autonomous Underwater Vehicle (AUV). These proven rates allow telemetry of a week of high-rate (100 Hz) seismic data on 4 channels (ground motion and pressure) in minutes (or a year of data in less than 2 hours). Moreover, accurate timing is critical for earthquake location but OBS lack the benefit of GPS timing. The optical link will also allow measurement of the offset of the OBS clock relative to a GPS-synchronized time signal carried by the AUV to a precision of ~1 microsecond. These technologies will make possible multi-year deployments of OBS arrays with on-demand data offload and clock-check without the need for annual recovery/re-deployment cruises, saving hundreds of thousands of dollars per experiment. This capability will be particularly suitable for the dense near-shore arrays needed to monitor subduction zones.