We are back on the R/V Revelle, and just left Auckland port to begin the second and final leg of our Hikurangi subduction zone survey! Bon voyage!
After 29 days of 24 hour operations, we have finally reached the conclusion of our grand voyage at the Hikurangi subduction zone. We successfully completed all of the planned data acquisition: 170 OBEM receiver deployments and 128 recoveries, as well as 500 line-km of SUESI tows (which is 100 line-km more than we set out to collect). Only three instruments came back with no data. A marvelous achievement given the inherent difficulty of collecting data at the bottom of the ocean. One that was only possible thanks to the dedication and hard work of the science party, the marine EM lab at Scripps Institution of Oceanography, and the captain and crew of the R/V Revelle. I am struggling to digest the overwhelming range of emotions now that our journey has reached its end.
Onshore celebrations in Wellington await.
Stay tuned for more blog posts during the second, shorter (and more relaxed) voyage beginning February 21st, when we will set sail to recover the 42 OBEM receivers left behind to collect five+ weeks worth of MT data.
In the first figure below, I have plotted the processed MT response functions from three sites on the southern line of receiver deployments. Site s03 was located on the incoming plate, s14 was near the trench axis, and s33 was on the Hikurangi fore-arc. The data show the apparent resistivity and phase of the transverse-magnetic (TM) and transverse-electric (TE) modes. The TM and TE modes represent the two geomagnetic field polarizations that induce electric currents parallel and perpendicular to the line of receivers, respectively. They have sensitivity to different electrical conductivity structures, which will become important when the data are modeled using geophysical inversion methods. Details aside, what is most important here is that the data look smooth, have reasonable error bars, and therefore are fairly high quality. The data shown here are generally representative of the quality of the processed MT for the southern line of sites (have yet to process the MT data from the other profiles).
In this next example, the top panel in the figure shows two repetitions of the 4-sec long waveform that is continuously transmitted by SUESI during deep-tow operations. The bottom panel shows a real snippet of the time-series from site s14, when the transmitter was within range of the receiver. There is a clear resemblance between the transmitted and recorded waveforms, but the recordings contain much more structure. This is because the transmitted waveform is being distorted by the electrical structure of the seafloor, which is what we hope to disentangle from the data with inversion modeling.
PI Samer Naif led a large team that included Lamont graduate students Christine Chesley, Bar Oryan, and Daniel Blatter onboard the R/V Roger Revelle in the waters east of North Island, New Zealand to carry out a marine electromagnetic (EM) survey of the Hikurangi subduction zone. The project includes over 168 seafloor EM receiver stations and over 400 line-km of transmitter deep-tow tows, making it the largest marine EM experiment at a subduction zone yet. The data will allow the team to assess the role that fluids play in both generating subduction zone earthquakes, and in controlling the seismic behavior of faults more generally.
I’m delighted to report that we have succesfully completed our final SUESI tow of HT-RESIST. We managed to collect nearly 20% more controlled-source EM data on top of the ~400 line-km originally planned. That may not seem like a big deal, but in the marine research world, it’s an achievement to celebrate (and celebrate we will, likely with oysters and karaoke). It’s all thanks to the 29 days of continuous and very hard work put in by the science party and the crew of the R/V Revelle.
Our work schedule will mellow out for the remainder of the cruise, and I should be able to post a few more blurbs on how the data are turning out, and what is left for us to do before returning to Wellington port.
Ever since Kerry Key was knighted as an ‘Earthquake Hunter’ on San Diego’s local Fox News station back in 2010 (during the SERPENT research cruise), I too have always wanted to claim this title. After perusing through some of the time-series data from this survey, I am happy to report that we recorded several earthquakes.
The OBEM receivers are equipped with induction coil magnetometers, which are magnetic field sensors that moonlight as seismometers since they are highly sensitive to tilt motions (check out the recent paper by Barak et al on the publications webpage for more insight). The screenshot below shows the signal generated by a magnitude (Mw) 5.0 earthquake on the magnetic field data of three OBEM receivers. This particular earthquake occured over 1100 km away!
We deployed 24 OBEM receivers across the central Hikurangi subduction zone, oriented parallel to the trench axis. After email discussions with a GNS Science colleague, I decided to extend the profile length considerably. The middle of the profile is now centered on the central tow line from about 10 days ago (the original plan was to tow only the southern half of the profile with 18 receivers). If all goes well and the weather doesn’t get in the way, it should take us about 48 hours to tow SUESI over the ~125 km long transect. Then back to navigating and recovering receivers. At this stage, we have now deployed the OBEM receivers nearly 150 times! Our crew are a hardworking bunch..
We completed OBEM recoveries for the northern transect early this morning. Although the SUESI operations proceeded without a hitch, this time around we hit a snag with logger setups. Three OBEM receivers were programmed to wake up and begin recording on the wrong date (Dec 2019 vs Dec 2018), and so were recovered with no data on them. Unfortunately, these were deployed consecutively and were centered on the trench axis. I was quite irritated by this all day yesterday, but such is life. Besides that, the remainder of the sites were recovered with plentiful high quality data, including the sites where we performed the additional crossline tows. Overall, very proud of our science team’s performance. It also helps that Poseiden must have sensed my frustration, because he gifted us with a wonderful display of dolphin magic that same afternoon, and I actually managed to snap a few pics of some bow surfing action.
Currently deploying more OBEM receivers and getting ready to start ops for our central crossing transect, the final SUESI tow of our voyage!
We’ve ironed out all the wrinkles from our first two SUESI tow lines, with extra starch, leading to a nearly perfectly executed performance on our northern profile (except for a few nail biting run-ins with extremely steep topography that left me in a panic; SUESI dropped down to just 25 meters above the seafloor at one point, which may seem like a lot, but it most certainly is not and scared me half to death). An added bonus, we are ahead of schedule. It doesn’t hurt that the seas have been flat with almost no wind over the last two days.
To make use of some of the extra science time we have accrued, we are collecting a handful of crossing profiles along a relatively flat portion of the fore-arc seafloor. After the prior close encounter, I was in no mood for any more drama. Plus, one of the crossing lines is very near to site U1519 from IODP drilling expeditions 372/375, which should complement those suite of data nicely.
We will be done towing in about eight hours, then begin OBEM recoveries for the second to last time. The most difficult aspects of the survey operations are now nearly completed, let’s hope things settle down a bit for the remainder of our voyage.
For your viewing pleasure, a photo of today’s sunset, and an even brighter night sky. In the words of one Kiwi fisherman, ‘cool bananas’!
We completed receiver recoveries for the central profile, and have maintained our 100% recovery rate. We are currently working on the northern profile, and expect to begin SUESI ops in less than 24 hours time. Our science team is now a well oiled machine, cranking out receiver deployments every half hour. The saga continues..