Yesterday morning, we finished acquiring data on our final profile and recovered all of the seismic gear, with beautiful morning views of Aleutian volcanoes, especially Great Sitkin. We have collected a really exciting dataset, and I can’t wait to start analyzing the data in more detail and finding out what they’ll teach us about the Aleutian volcanoes and subduction zone. After two months away, it is also great to start heading towards home.
Great Sitkin Volcano at dawn on October 5 as we began to recover seismic equipment.
We are so, so, so grateful to everyone who made this expedition possible during such an extraordinary time. We are particularly indebted to everyone onboard for their hard work, professionalism and positive spirit in frequently challenging conditions, and enduring the hardship of an extra 3-5 weeks away from home for pre-cruise COVID quarantining and due to other changes in schedule. It would not be possible to do this science without the Captain, crew and scientific technical staff on the Langseth, protected species observers, and the Ocean Bottom Seismic Instrument Centertechnical staff, and we are so thankful for everything you do. The Lamont-Doherty Earth Observatory Marine Office did an extraordinary job of managing the complex logistics of undertaking a cruise during the time of COVID in coordination with OBSIC, National Science Foundation and many others. It would not be possible to do this science without support from the National Science Foundation. Thank you thank you thank you!
We carefully monitor all of the data that we are collecting from the main lab, located in the bowels of the ship, below the water line. This is mission control for the scientific operations. A bank of 46 screens (!) display every conceivable kind of information about data that we are collecting, including the position and status of equipment towed behind the vessel and real-time displays of the raw data themselves. There are also 20 mice and 18 keyboards – good luck finding the right mouse the first time if you are not intimately familiar with the set up! Because we are collecting data 24 hours a day, 7 days a week, members of the R/V Langseth’s crew and scientific technical team and the science party work in shifts to monitor all aspects of the operations around the clock.
The main lab on the R/V Langseth.
If anything strange or unexpected occurs – for example, we see something odd in the data – we make detailed notes. On October 1 at 1:25 am local time, the watchstanders noticed some strange signals in the data recorded by the streamer and made a note of it. Today, we realized that a magnitude 5.3 earthquake occurred about 30 miles away at that time. The strange signals were actually sound waves generated by the earthquake! Although the main signal we intend to record are sound waves generated by the seismic source towed behind the ship, the sensitive recorders in the streamer will also detect any other sound waves. Our study area is a seismically active region that regularly produces earthquakes, so it is likely that we recorded quite a few earthquakes on the seismic streamer and the ocean bottom seismometers that we deployed during the first phase of this project. We also record ocean waves and whales. Thanks to Jim Gaherty (NAU) for bringing this earthquake to our attention! Every so often, it is useful having a seismologist husband….
Map showing location of all earthquakes during our study (red circles), including the one that occurred on Oct 1 (labeled with text). The orange star shows where the ship was at the time of the Oct 1 earthquake.Recording from seismic streamer showing the arrival of sound waves generated by the Langseth’s seismic source (first couple of seconds of record) and from a magnitude 5.3 earthquake (last ~5 seconds of record).
During the second phase of our cruise, we are collecting seismic imaging data using a long cable (6 to 9 km) filled with pressure sensors that we tow behind the ship, traveling very slowly. The streamer records sound waves emitted by our seismic source that have bounced off of layers in the Earth. The close spacing of many pressure sensors and the length of the cable allow us to record returning seismic waves in great detail, which we can use to reconstruct detailed pictures of geology below the seafloor, including sedimentary layering, faults and other features. It’s very powerful and cool data. What’s more, we can create preliminary images very soon after the data are collected to get a first peak, so we have been busy doing that as the data roll in (thus a gap in my blog entries…).
The upper panel shows a cartoon of a subduction zone, where two tectonic plates converge and one is thrust beneath the other one. The plate boundary fault separating these two plates produces very large earthquakes. We use sound waves to create images of this fault zone and other structures; the sound waves (represented by red arrows) are generated by our seismic source, bounce off of geological features below the seafloor, and are recorded by our seismic streamer. The lower panel shows an example of a recording on the streamer. We can see many sound wave reflections, including from the plate boundary fault. Orange and black colors represent positive and negative wiggles (zoom of data from blue box shown in inset), which provide information on the physical properties of the fault zone.
Seismic streamer stored on gigantic reel on R/V Langsetb
The streamer is stored on huge reels on the stern of the Langseth. To deploy it, the science technical team on the Langseth unspools it into the ocean, which takes about ~12 hours. Ideally, we would like to tow the streamer 9 meters below the sea surface, straight behind the ship along our collection profile. We can control the depth using ‘birds’ attached to the streamer. However, we have almost no control on its position; it often gets pushed around by currents. Yesterday, as we passed through a particularly strong current that was nearly perpendicular to our data collection line, the seismic streamer had a ~90º bend – ouch!
We have collected over half of our planned dataset so far, and the data look excellent!
Navigation display of streamer location (purple line and text), ship (white circle/blue line and text) and our data collection line (yellow). The streamer has moved north away from the data collection line due to currents.
Weather rules supreme in maritime operations. All plans are shaped around the forecast. For the last week, we have been carefully monitoring forecasts that predicted a large storm would sweep through our field area on Sunday and Monday (Sept 20-21), bringing >45 mph winds and >20 ft seas. In response, we decided to cut some of our planned ocean bottom seismometer deployments (painful!) in hopes we could complete our data collection and pick up all of the OBS before the brunt of the storm arrived. Thanks to the perseverance and professionalism of everyone on board, we were able to do just that. Over the last ~8 hours in deteriorating conditions, we finished recovering all of the OBS. This required deft maneuvering by the bridge to navigate to each OBS and position the ship safely for deck work, and careful and skilled deck operations by the WHOI and Scripps OBS teams and Langseth’s crew to snag each OBS and bring it onboard. Kudos to everyone! Now we are headed east to Dutch Harbor (and away from this storm!) to drop off our OBS teams and pick up fresh fruit and veg to sustain us for Part 2 of our expedition.
Donna Shillington NAU
Map of wind speed and direction from windy.comlate on Saturday night as we finished OBS recoveries. The white star marks the approximate position of the R/V Langseth.
Map of progress. White dots show ocean bottom seismometers from two transects. Solid black line shows profiles were we have finished OBS data collection. Dotted lines show transects where we plan to acquire data.
Over a four day period, we picked up nearly all of the ocean-bottom seismometers from our first transect along the Aleutian volcano chain between Seguam and Gareloi Islands and put them back out again on a transect across the chain near Atka.
To summon the ocean bottom seismometers back from the seafloor, we steam to the location where we deployed them and send them an acoustic command to release from their anchor. Without the anchor, the instruments are buoyant because they have glass spheres (and sometimes syntactic foam ) attached to them. We wait as they rise up to the surface at a speed of ~120-150 ft per minute. On our first transect, water depths were generally less than 1500 ft, so we did not have to wait very long for them to make the trip to the sea surface. Once they reach the surface, the crew on the bridge can spot them because each OBS is outfitted with a flag and light. Remarkably, the bridge crew maneuver the ship so that the OBS is right next to the starboard side of the vessel. This is very impressive when considering how small the OBS is compared to the ship; imagine driving your car up next to a ping pong ball. Our OBS teams hook them using long poles with ropes attached and then pull them out of the water using a crane. It takes careful coordination and skill by everyone involved, and I never cease to be impressed by the entire operation.
WHOI OBS Team and Langseth crew deploy an OBS
Equally impressive, this bundle of electronics that we dropped into the ocean a few days ago usually comes back having recorded a trove of data. We are happily pouring over some of the data now to get a first impression, and they look very promising! I will write a future post with some examples.
Less than a day after we finished retrieving the OBS from the first line, we started putting them back into the water on our second line. Sadly, we could not deploy as many seismometers as we had hoped because a storm is forecast to arrive in our field area in a few days, and we want to finish with the OBS work before then. So it goes, but we are still excited about what will be recorded by the OBS on their second trip to the seafloor.
My main exercise on the ship is running on the treadmill, when it’s calm enough. During a run the other day, it occurred to me that my pace on a treadmill at sea (with the ship motion) was similar to the speed that the R/V Langseth steams when we are towing gear and acquiring data (5-6 miles per hour). Of course I couldn’t keep up with the Langseth when she’s going full speed (~11-13 miles/hr), but I did wonder how we compared in total distance traveled if she was stopping and starting, as we do during OBS deployments and recoveries. So, on my last two long runs on the treadmill, we raced:
The panels on the left compare my average running speed (blue lines) with the ship’s speed (red) during the same time period for two different runs. The panels on the right compare the cumulative distance that I traveled during my run to the distance covered by the ship. Run 1 happened when we were towing gear and steaming at a slow speed. Run 2 happened during OBS recoveries when the ship was stopping and starting.
It’s a draw. Although I only have the tolerance for the boredom of treadmill running for a maximum of ~75 minutes, the Langseth keeps moving 24 hours a day, 7 days a week thanks to our ship’s crew… So, happily, she will win the long race.
Seismic source elements on the ship before deployment.
During this cruise, we are using sound waves to image geological structures deep below the seafloor. Determining the structure of Earth’s interior is one of the main types of seismology, and seismologists use a wide range of techniques and types of seismic waves to examine different depth intervals and resolutions. Many of these approaches use distant or local earthquakes as the source of sound waves. The special thing about the type of seismology we are employing on this expedition (‘active source seismology’) is that we create our own sound waves, so we can control their positions, timing and character. This allows us to obtain much higher resolution images of the upper part (~60 km) of the Earth than is possible with other methods.
We are towing a seismic source array behind the Langseth that creates sound waves by emitting pulses of highly compressed air. By using a combination of different source elements with different sizes, we can create a nearly ideal seismic source signal: a sharp, simple burst that is short in duration and contains a range of frequencies.
Seismic source array towed behind the ship (right side of picture) near Atka Island. The seismic source elements are hanging 9 m below the black floats that can be seen at the surface. Each float has a GPS to record its location.
We have spent the last 2.5 days steaming along our first transect VERY slowly (~5-5.5 miles/hour) emitting sound waves to be recorded by the ocean bottom seismometers (OBS). The OBS are autonomous instruments that are recording data to internal disks, and we will not know what they recorded until we retrieve them in a couple of days. However, we were delighted to learn that seismometers deployed on the Aleutian Islands by the Alaska Volcano Observatory for volcano monitoring recorded our signals! Unlike our ocean bottom seismometers, data from seismometers on land can be retrieved remotely and monitored in real time; these instruments are very important for monitoring earthquake and volcanic activity. The image below show the amplitude of signals at different frequencies that were recorded over a one-hour time period when the ship was near the seismic station. If a strong signal is recorded at some frequency, it shows up in lighter colors. The regular high amplitude blips on this recording are the Langseth’s seismic source! Encouraging! Many thanks to Matt Haney (USGS/AVO) for sharing these plots with us.
The top panel shows an example of recordings of our seismic source on an AVO seismic station on Kanaga volcano onshore. One hour of data are displayed from September 10, when the ship was close to Kanaga. Our source shows up as regular, light-colored blips in this spectrogram. The lower panel show a map of the islands, volcanoes (red triangles), AVO seismic stations onshore (orange squares) and ocean bottom seismometers (dark blue circles). The ship track is shown with a blue line, with our location every hour labeled with a circle and text. The dark red line shows where the ship was during the hour of recording shown in the upper panel.
Now that we have finally arrived in our field area, we have spent the last two days steaming along a ~300-mile-long stretch of the Aleutian island chain between Seguam and Gareloi islands and deploying ocean bottom seismometers (OBS) from Woods Hole Oceanographic Institution and Scripps Institution of Oceanography on our first transect (see this post for a map). Our hard-working OBS teams ‘install’ them by lowering them over the starboard side of the ship with a crane and releasing them into the ocean. Because they have a weight attached to the base, they sink to the seafloor. The engineering that goes into ocean bottom seismometers is amazing. Each OBS is equipped with very sensitive instruments that can detect tiny ground motions and changes in water pressure, which they record on an internal computer. They can withstand high pressures at deep water depths. We can send the OBS basic commands using acoustic signals . They will stay on the seafloor for the next few days, and record sounds emitted from our seismic source array. Afterwards, we will pick them up – more soon on all of that!
Tanaga Island
The last two days of work has also allowed us to get a first look at our field area, especially the volcanoes that are the focus of our project. They are spectacular! Snow-capped mountains with a green fuzz of vegetation rise out of the ocean. They are lined by impressive sea cliffs and the occasional water fall. Amazing! I’m glad that we will get several more looks at them in the coming days as we steam up and down this transect collecting data and recovering instruments.
Maps showing our progress each day traveling from Ketchikan (yellow star) to our study area in the Aleutians (red lines) with a yellow line.
To travel from Ketchikan, Alaska to our field area in the western Aleutians requires us to steam 1700 miles – nearly the entire width of Alaska (and Alaska is big). By any mode of transportation, this is a long trip, but especially by ship. Research ships generally go a maximum speed of 10-11 knots (11.5-12.5 miles per hour). Although we are moving slow, at least we are able to keep traveling around the clock thanks to the mates and ship’s crew. Progress can be helped or hindered by currents and winds, so that even if the ship is going 10 knots through the water, our speed over the ground may be slower or faster. We have already experienced a wide range of weather conditions on our journey from flat calm and sunny yesterday to 15 foot seas and 55 knot winds today (definitely prefer the former).
But the long trip is not all bad! For one, it allows everyone to get their sea legs and organize for the work ahead, including testing equipment. It also gave us a chance to do a little underway science. We were able to map the seafloor just offshore of Ketchikan and image the shallowest sediments below the seafloor. This is done by sending out high-frequency pings from an instrument mounted to the hull of the ship and recording returning signals that have bounced off of the seafloor and layers below the seafloor. We crossed part of a spectacular system of channels in the seafloor that carry sediments eroded from the steep mountains flanking the coast out into the sea, where they form huge sediment deposits on the deep seafloor called sediment fans. The sediment fans offshore Alaska are some of the largest of their kind in the world. But the sediment transport system that creates the channels and sediment fans is disrupted by plate tectonics (Walton et al., 2014). As the Pacific Plate moves north at ~44 mm/yr with respect to North America (Elliott et al., 2010), these seafloor channels are severed from their sediment supplies onshore and carried north, as shown by the interesting work of Maureen Walton and collaborators.
Lower panel shows mapping of the seafloor, which is colored by water depth. The deepest water in this plot (in blue) marks seafloor channels. The upper panel shows a cross-section through the upper part of the earth below the seafloor and shows the channels and the sediments in which the channels have formed.
We continue west and expect to arrive in our study area on Monday morning.
Donna Shillington, NAU
Elliott, J.L., Larsen, C.F., Freymueller, J.T., Motyka, R.J., 2010. Tectonic block motion and glacial isostatic adjustment in southeast Alaska and adjacent Canada constrained by GPS measurements. J. Geophys. Res. 115, doi:10.1029/2009JB007139.
Walton, M.A.L., Gulick, S.P.S., Reece, R.S., Barth, G.A., Christeson, G.L., Van Avendonk, H.J.A., 2014. Dynamic response to strike-slip tectonic control on the deposition and evolution of the Baranof Fan, Gulf of Alaska. Geosphere 10, 680-691, doi: 610.1130/GES01034.01031.
