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

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

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.