On April 12, 2018, Princeton Professor Frederik J. Simons and graduate students Joel D. Simon and Alex Burky, along with their international collaborators, boarded the Fukae Maru for a two-day training cruise. The goal of the cruise, led by Kobe University Professor Hiroko Sugioka and JAMSTEC scientist Dr. Masayuki Obayashi, was to learn instrument deployment and data recovery procedures for the newest-generation MERMAID float, manufactured by the French engineering company, OSEAN. Lead engineer Yann Hello, from GéoAzur in Sophia Antipolis, called the shots. After steaming out of Osaka Bay a MERMAID programmed to dive down to 500 m below the surface was deployed into the Pacific.
Two MERMAIDS are loaded onto the Fukae Maru on April 12, 2018. Photo by Joel D. Simon (Princeton).
MERMAID, an acronym for Mobile Earthquake Recording in Marine Areas by Independent Divers, is an autonomous, freely drifting oceanic seismometer designed to remotely record seismic data in, and transmit from, the world's oceans. The goal of the MERMAID project is to fill data gaps in oceanic regions where seismic stations are sparse, especially in the southern hemisphere. MERMAID is an adjustable-buoyancy glass sphere fitted with batteries, GPS, an Iridium modem, and a hydrophone. It dives to parking depths down to 2000 m, where it passively drifts and listens, recording the acoustic wave field. When the onboard detection algorithm identifies earthquake activity, the instrument surfaces and transmits the seismogram via satellite along with a timestamp and its location. MERMAID is promoted by the international consortium EarthScope-Oceans as the solution in seismology to gather seismic data from the oceans in near real-time.
Planning the deployment. From left to right: Frederik J. Simons (Princeton), Guo Zhen (SUSTech), Hiroko Sugioka (Kobe University), Masayuki Obayashi (JAMSTEC), Yann Hello (GéoAzur), Alex Burky (Princeton). Photo by Joel D. Simon (Princeton).
The first MERMAID prototype was developed over a decade ago by Professor Emeritus Guust Nolet and then post-doc Frederik J. Simons, and designed and built by Dr. Jeff Babcock and his group at the Scripps Institution of Oceanography. The original floats were reprogrammed SOLO floats that had been fitted with a hydrophone. However, unlike the divers in the Argo or SOCCOM programs that conduct profiling measurements and surface at regular intervals, MERMAID stays submerged at depth until it detects an earthquake, and then it surfaces. Ten years on and some twenty deployments later, MERMAID has matured into a tested and proven platform, having returned nearly 1200 seismograms from the oceans that were matched to known earthquakes. Additionally, MERMAID has sent over 2200 seismograms that do not appear in any seismic catalog. Hence, these seismograms are the only data revealing earthquakes that are either too small, or too distant, to be recorded by any other seismic station on Earth.
Sketching out MERMAIDs mission plan. Photo by Joel D. Simon (Princeton).
Since 2012, MERMAIDs have been deployed into the Mediterranean Sea, the southern Indian Ocean, and the Pacific Ocean both near the Galápagos Islands and Japan. Our training cruise in Japan was in preparation for the most exciting deployment yet, when sixteen brand new MERMAIDs, purchased with the help of the Princeton Geosciences Department A. H. Phillips Fund, will be deployed in the South Pacific. From September 2018 onwards, our floats will return volumes of data, from earthquakes near and far, their ray paths sampling geometries never before recorded. The data will enable high-resolution seismic imaging of the mantle plume thought to underlie the region around Tahiti — some fifty years after Professor Emeritus Jason W. Morgan formulated his seminal theory.
MERMAID at the surface waiting to be recovered. Photo by Joel D. Simon (Princeton).
Back onboard the ship on the morning following the deployment, we made bets on when MERMAID would send its first GPS location as we anxiously waited for it to surface. Under normal circumstances, MERMAID would be deployed and then left to its own devices, happily surfacing to send data and then promptly sinking back to depth. As ours was a test deployment, however, we forced it to surface, in order to understand the ins and outs of programming the device. We did our best back-of-the-envelope estimates on the timing of the various stages of descent, parking, and ascent, working backwards from our ideal surfacing time of 8:30 am local, understanding there would inevitably be some play in the combined ~3 hours of the ascent and descent, plus the minutes it could take for MERMAID to obtain a GPS fix once at the surface. To our delight, at 8:36 am MERMAID sent us its first GPS location 2.6 km from where it had been deployed. Even more exciting, it had recorded a magnitude 4.9 earthquake that occurred seven degrees to the northeast, while staying submerged as instructed!
The location of the MERMAID float (red triangle) and the earthquake it recorded (gold star) on April 12, 2018.
The seismogram MERMAID recorded from a magnitude 4.9 earthquake that occurred 824 km to the northeast, bandpass filtered between two and four Hz. The red vertical line marks the theoretical arrival time of the first arriving P-wave, as computed using the earth model ak135.