A Low-Power Global Positioning System-Acoustic Payload to study the subduction zones offshore the Pacific Northwest and Alaska

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Great subduction zone earthquakes offshore Japan, Chile, and earlier Sumatra caused extensive damage due the earthquake itself and the resulting tsunami. A similar type of subduction zone fault lies offshore the Pacific Northwest, the Cascadia Subduction Zone (CSZ). It last ruptured in 1700 long before the extensive establishment of populated communities. Since then the CSZ has been building up elastic strain in the crust at the rate of a few centimeters per year. In the future, this stored elastic energy will likely be released as a powerful earthquake and tsunami. Much of the elastic strain is accumulating the seafloor offshore the coast. By measuring the slow buildup of this strain offshore, a better understanding of the potential earthquake and tsunami risk can be established. By combining the Global Positioning System and an acoustic ranging system on a sea surface platform, and acoustic transponders on the sea floor, the elastic strain buildup can be measured. Small (surfboard-size) wave- and solar-powered platforms can now replace large ships for much data collection. However, this requires re-engineering the onboard electronic components to be smaller and lower power, which is the goal of this project. The Global Positioning System-Acoustic (GPS-A) approach has been adapted to a Wave Glider, a remotely controlled, wave- and solar-powered sea-surface vehicle. The Wave Glider is funded through 2017 to measure plate deformation at three seafloor sites in the Cascadia Subduction Zone. In Sept. 2014, we collected 30 hours of GPS and acoustic data at a 3000-m-deep site 90 Nm offshore central Oregon. Preliminary results validate the Wave Glider as a replacement for high-cost ships in Cascadia, and likely at other global sites. We found, however, the power requirements of the present GPS-Inertial Navigation System (GPS-INS) and the computer onboard the Wave Glider draw about 22 Watts(W). This limits data collection to 24-30 hours, before the 660 Watt-Hour battery bank aboard the Wave Glider is depleted. At this point, GPS-A operations must be suspended for 2-3 days, while the solar panels recharge the onboard batteries. Our research and that of Japanese researchers demonstrated that 4-5 days of GPS-A measurements are needed for centimeter-level positioning. Because data collection must be suspended after only ~24 hours to charge batteries for 3 days, it takes 16-20 days to collect the cumulative 5 days of GPS-A data at a single site. Adding 3-5 days for transiting between sites, only three to five GPS-A sites can be visited and measured over a 3 month summer weather window in Cascadia. This allows little contingency time for weather delays, mechanical problems, etc. As more seafloor sites offshore Cascadia are proposed and added, the inefficiency is magnified. This project will replace the existing GPS-INS and onboard CPU with low power versions that consume only 6-8 W. GPS-A data could then be collected continuously for up to 5 days. The onboard batteries are recharged over the few days as the Wave Glider transits between seafloor sites. This is an efficient operational approach for use in Cascadia, the Aleutians, and potentially other regions.