Imaging the Sources of Great Alaskan Earthquakes
Off the coast of New Zealand, there is an area where earthquakes can happen in slow-motion as two tectonic plates grind past one another. The Pacific plate is moving under New Zealand at about 5 centimeters per year there, pulling down the northern end of the island as it moves. Every 14 months or so, the interface slowly slips, releasing the stress, and the land comes back up.
Unlike typical earthquakes that rupture over seconds, these slow-slip events take more than a week, creating an ideal lab for studying fault behavior along the shallow portion of a subduction zone.
In 2015, Spahr Webb, the Jerome M. Paros Lamont Research Professor of Observational Physics at Lamont-Doherty Earth Observatory, and an international team of colleagues became the first to capture these slow-slip earthquakes in progress using instruments deployed under the sea. The data they collected from the New Zealand site, published this year by lead author Laura Wallace of the University of Texas, will help scientists better understand earthquake risks, particularly at trenches, the seismically active interfaces between tectonic plates where one plate dives under another. Members of the team are discussing their work this week at the American Geophysical Union (AGU) Fall Meeting.
“We don’t yet understand the stickiness of the interface between the two plates, and that is partly what determines how big an earthquake you can have,” Webb said. “In particular, we care about the stickiness near the trench, because when you have a lot of motion near a trench, you can generate big tsunamis.”
Previously, scientists thought that the soft sediments piled up near trenches were usually not strong enough to support an earthquake and that they would dampen the slip, Webb said. “We’re recently seen a lot of big tsunamis where there has been large slip right close to the trench,” he said.
One reason the 2011 Tōhoku earthquake in Japan was so devastating was that part of the interface very close to the trench moved a large distance, around 50 meters, pushing the water with it, Webb said. While the main part of the Tōhoku earthquake involved uplift of only a few meters, the part near the trench doubled the size of the tsunami, leading to waves almost 40 meters high at some points along the coast.
To be able to anticipate tsunami-producing earthquakes and more accurately assess regional risks, scientists are studying why some areas of trenches have these slow-slip events, why others continuously creep, and others lock up and build strain that eventually erupts as a tsunami-generating earthquake.
The Alaska Risk
Webb has his sights next on the Aleutian Trench, just off Kodiak Island, Alaska. It is one of the most seismically active parts of the world. A large tsunami-generating earthquake there could wreak havoc not only in Alaska but along the west coast of North America and as far as Hawaii and Japan, as the Good Friday earthquake did in 1964.
Lamont scientists, including Donna Shillington and Geoffrey Abers, who are also presenting their work this week at AGU, have spent years studying the structure of the Aleutian Trench and what happens as the Pacific plate dives beneath the North American plate. Webb and a large group of collaborators now want to find out where sections of the trench are sliding and where sections are locking to help understand what determines where it locks. Finding slow-slip earthquakes could help reveal some of those secrets.
To study the New Zealand slow-slip event, Webb and his colleagues installed an array of 24 absolute pressure gauges and 15 ocean-bottom seismometers directly above the Hikurangi Trough, where two plates converge. Absolute pressure gauges deployed on the seafloor continuously record changes in the pressure of the water above. If the seafloor rises, pressure decreases; if the seafloor moves downward, pressure increases due to the increasing water depth. When the slow-slip event began, the instruments recorded how the seafloor moved.
The scientists found that parts of the Hikurangi interface slipped and others didn’t during the slow-slip event. “It may be that much of the interface slips in these events but you have a few places that are locked, and those finally break and create earthquakes and tsunamis that cause damage,” Webb said.
Most of the instruments used in the New Zealand study were built at Lamont in the OBS (ocean-bottom seismometer) lab started by Webb.
In Alaska, Webb and his collaborators have proposed an experiment that would again use a large numbers of Lamont-built ocean-bottom seismometers and pressure gauges, this time to collect data near Kodiak Island. Alaska is a special challenge for seafloor measurements. The ocean is quite shallow south of Alaska before deepening near the Aleutian Trench, and seismic instruments on the seafloor can be moved by strong currents or damaged by bottom trawling. Webb and the team in the OBS lab at Lamont developed a solution: they built heavy metal shields that sink to the sea floor with the seismometers to protect them.
Once data from the instruments are collected, they will be made publicly available so seismologists across the country can begin to analyze the records in search of clues to the area’s earthquake behavior.
By detecting patterns of earthquakes, scientists can help regional engineers plan construction to better withstand worst-case earthquake scenarios, but predicting earthquake remains elusive.
“If we start seeing precursors based on the off-shore data, then maybe we’ll also get some predictive ability,” Webb said. “The hope is if you have better off-shore measurements, you’ll start to understand things better, and maybe there is some sign of motion happening before the earthquake that will provide some warning.”
Learn more about the work underway at Lamont-Doherty Earth Observatory, Columbia University’s home for Earth science research.
At 6:30 am on August 5, the R/V Langseth pulled into port in Dutch Harbor, marking the end of our very successful research cruise. Our steam into port from our study area involved a trip through Unimak pass and beautiful views of Aleutian volcanoes, including majestic Shishaldin.
Many things are required to make a research cruise successful, but one of the most important is the people. And we had great people in spades. The Langseth’s crew and technical staff are excellent: extremely competent, hard working and dedicated. Throughout our endeavor offshore Alaska, there were challenges: temperamental aging scientific equipment, tricky maneuvering very close to the coast line, subpar weather, etc. All of these obstacles (and more) were handled admirably and without complaints. Protected species observers cheerfully spent long, cold hours exposed to the elements on the observation tower watching for mammals to ensure that we operated responsibly. Our science party was also terrific; everyone worked hard and worked well together. And if you’re going to spend 38 days at sea with a group of people, it doesn’t hurt if they are nice and friendly in addition to being smart, competent and hard working. And it was a uniformly nice and friendly crowd aboard our cruise, MGL1110. Our efforts would also not be possible without support ashore from Lamont’s Marine Office and the National Science Foundation. The evening of our arrival in Dutch Harbor, we celebrated the completion of our successful cruise and toasted (repeatedly…) the people who made it possible at a post-cruise party at the Harbor View Bar and Grill.
Many people flew home after our arrival in Dutch Harbor, but not me! (At least not yet). Katie Keranen and I will recover the seismometers we deployed way back at the beginning of the summer. Hopefully these instruments recorded lots of earthquakes as well as our offshore experiment, and hopefully they were not disturbed or damaged by curious wildlife (including people!). An Anchorage-bound flight from Dutch Harbor dropped me off in Cold Bay on Aug 6, where I rendezvoused with Katie. After the plane landed, the stewardess asked for our “Cold Bay passenger” to disembark. Passenger. Singular. I filed past all the folks heading to Anchorage and beyond. Unlike them, I will linger a little longer on the beautiful Alaska Peninsula.
Although we still have ~3 days of data collection aboard the R/V Langseth to go before we pull in our equipment and head for port, we are already drowning in beautiful seismic data. Following each pulse from the air gun array, the two 8-km-long streamers listen for returning sound waves for 22 seconds. This is enough time for the sound waves to travel down through the water, sediments, crust and upper mantle and back again. Arriving sound waves are recorded on a total of 1272 separate pressure sensors along the streamers, producing about 60 Mb of data for each pulse. Repeat this every 25 seconds for 3 weeks, and you end up with a pile of data! We have already recorded over 2.5 terabytes (2500 gigabytes!) of raw seismic data. This does not include other large datasets that we are simultaneously acquiring, such as detailed bathymetric mapping of the seafloor.
Once we obtain the raw data, our eager scientific party cannot resist beginning some rudimentary analysis, thereby generating even more large data files that take up yet more disk space. In search of instant gratification, we use some quick and dirty processing steps to produce preliminary images from our data and get a first peek at the structures beneath the seafloor. This is standard procedure on cruises aboard the Langseth and other seismic ships. Often, such images reveal very little; careful analysis of seismic data to create clear and accurate images of earth structures takes years. But in our case, the data are of such high quality that spectacular features are evident even in these rough first images, including the plate boundary and other faults. This assures us that hard work on the data following the cruise will produce very exciting results.
One of the key shipboard tasks is determining the position of the gear in the water and combining this navigational information with the raw data. Our streamers are 12 m under the sea surface, so we cannot simply attach tons of GPS sensors to them to figure out where they are at any given time. Instead, the Langseth’s infatigable Chief Navigator, David Martinson, works out the locations of the streamers using GPS’s at the beginnings and ends, a series of compasses spaced along the streamers, and several “acoustic nets,” sets of instruments that give the distances between the streamers at key positions. He can determine the positions of our two unruly 8-km-long cables to within ~5 m or less at any given time – amazing!
We also produce initial images of seafloor topography from bathymetry data. At sea we begin the arduous task of manually editing vast quantities of the data, but the effort pays off. Careful analysis of these high-resolution data can reveal faults that cut through the seafloor, seamounts, and sedimentary features.
For the last nine days, we have been underway acquiring seismic reflection data to study a plate tectonic boundary offshore Alaska with the R/V Marcus G. Langseth. Now that the initial excitement of deploying all of our seismic gear and watching the first sound waves arrive on our two 8-km-long streamers has faded, we have settled into a routine of watches and standard shipboard data processing. Meals, sleep and leisure also take on predictable patterns. Each day resembles the one before, and they all start to blend together. This may sound rather humdrum, but an uneventful day at sea is normally a successful and productive one (as one of the undergraduate watchstanders noted). When something “exciting” happens, it is usually not good.
Happily, a large proportion of our nine days have been blissfully boring, but we have had our share of happenings. Excitement takes the form of equipment failures, bad weather and marine mammals. Acquiring marine seismic reflection data is a fantastically complex undertaking involving a lot of sophisticated, interdependent gear, so things can and do go wrong once in a while. A few nights ago, one of our streamers sank too deep, causing a “streamer recovery device” (a specialized airbag) to deploy and float the streamer to the surface. The next morning, a team used the workboat to visit the problematic streamer section and remove the airbag. On a few other occasions, I have received phone calls in the middle of the night summoning me from my cabin to the main lab to discuss other equipment hiccups – no one ever calls at 3 a.m. to let you know that everything is going swell.
Whales are beautiful and majestic, and we have been treated to numerous sightings, but we try to keep our distance. Since we are creating sound waves to image the earth, and marine mammals use sound to navigate and communicate with one another, our activities might disturb them. A team of protected species observers (PSO) watches for mammals, and we suspend operations if a mammal comes too close. Yesterday morning, we found ourselves surrounded by three species of whales, including a rare Northern Pacific Right Whale – an amazing sight, but it prevented us from collecting data for nearly four hours.
Of course there are notable exceptions to the “excitement is bad” maxim, the most important of which is the science! We use our new data to create very preliminary images of the structures below the seafloor as we go, and they have revealed some intriguing and surprising features. A regular sight in the main lab is a group of people gathered around a computer screen or a large paper plot, talking and pointing excitedly. We have a lot of hard work ahead after the cruise to obtain concrete results, but it’s exhilarating to glimpse faults, sediments and other structures in our data for the first time and ponder what they might be telling us about this active plate tectonic boundary. Even after spending a total of nine months at sea on ten research cruises over my career, the excitement of new data has definitely not worn off.
One of the core objectives of our project is to image the part of the plate tectonic boundary that locks up and then ruptures to produce great earthquakes. In the Aleutian subduction zone, the Pacific plate is being thrust northwards underneath the North American plate. To examine deep parts of the interface between these plates, we need to go as far north (and as close to the coast) as possible. This is easier said than done. We are towing a lot of scientific equipment behind the ship, including two 8-km-long cables (streamers) filled with pressure sensors, so approaching the coast and making turns is complicated and requires special attention to safeguard our gear. The southern edge of the Alaska Peninsula is rugged and flanked by lots of small jagged islands and shallow features just below the surface of the ocean. Currents and water density can vary locally near the coast, which could affect the positions and depths of our streamers behind the ship. And there is more fishing activity close to the coast, and thus increased risk of tangling seismic gear with fishing lines and nets. To reduce the risk, we scouted all of the trickiest parts of our survey ahead of time before we deployed the streamers, and we monitor the currents and fishing as we approach the coast. Captain Jim O’Loughlin, Chief Science Officer Robert Steinhaus, and the Langseth’s other crew and technical staff have a tremendous amount of experience and skill in maneuvering in tight spots while towing seismic equipment.
We recently completed one of our closest approaches to land near Unga, one of the Shumagin islands. At the apex of the turn, our 8-km-long (5-mile-long) streamers came within less than a mile of the coast. Due to some early difficulties with our equipment and an abundance of marine mammals, we had to make several attempts to collect data on the landward part of the line (and thus several passes near the shoreline). I held my breath and watched our third (and final) pass from the bridge. After the ship and gear passed safely through the most harrowing part of the turn, the captain turned to me and asked, “We’re not going to do this again, are we?” Thankfully not! At least not here. But there are several other important parts of our survey ahead that will require close approaches to the coast to image critical parts of the plate tectonic boundary. As with this near-shore encounter, we will rely on the skill and experience of the mates and the technical staff, as well as a little luck.
On July 11, we marked the successful completion of the first phase of our project and embarked on the second. Part 1 involved deploying ocean bottom seismometers and recording air-gun-generated sound waves. We successfully retrieved all of the OBS’s, and the data that they recorded look very exciting at first blush (and contain some surprises!). Part 2 involves towing two 8-km-long cables (or streamers) filled with pressure sensors behind the R/V Langseth, which will also record sound waves from the Langseth’s airgun array. Changing gears in terms of scientific activities also involved changes to our science party; we swapped personnel in Sand Point on a beautiful sunny evening. The excellent OBS team from Scripps departed on the Langseth‘s zodiak, and we were joined by new reinforcements. The newcomers included five undergraduate students from Columbia University, who are also blogging about their experiences at sea.
Just two hours after taking on our new personnel, we started deploying seismic gear – a very quick transition! Our seismic streamers are stored on gigantic spools, which unreel cable off the back of the ship into the ocean. A large buoy is affixed to the end of the streamer, and ‘birds’ are attached along its length, which can be used to control the depth of the streamer. Large paravanes hold the streamers apart; these are like large kites flying in the water off the back corners of the ship.
Deploying miles of streamer and the other attending gear is an impressively long and complicated undertaking. We started over two days ago, and have been working around the clock in shifts ever since. Many repairs and adjustments are made to the gear as it’s deployed. The streamer is divided into 150-m-long sections connected by modules; both sections and modules can fail and need to be replaced. Replacing a 150-m-long section of cable is an arduous task involving major manual labor by teams of ~5-6 people. But we are nearing the finish line; as I write, the last kilometer of the second streamer is going over the back of the boat. Fingers crossed that the deployment will soon be complete and the data collecting can begin!
After leaving our seismometers on the seafloor offshore Alaska for a few days to record sound waves generated by the air guns of the R/V Langseth, we returned to collect them. The recovery of OBS always involves a certain amount of suspense. Despite all of the advanced engineering and planning that goes into these instruments, it is an endeavor with inherent risk, and things can and do go wrong sometimes: one or more of the glass balls that make the OBS float could implode; the acoustic communication with the instrument could fail; it might be stuck on the seafloor for one reason or another; it could have been accidentally dragged off by trawlers. All of these thoughts ran through my mind at each site as we waited for the instrument to come to the surface.
To recover the OBS, we return to the place where we deployed it and communicate with it acoustically. We send it a command to release from its anchor and float back to the surface. The OBS rises through the water at 45 meters per minute, so the wait can be long if the water is deep. Some of ours were 5500 m below the surface! The instruments can also drift away from their original deployment location on the way down or the way back up due to ocean currents. When they arrive at the surface, we can spot their orange flags and strobe lights; they also send out radio signals.
Despite all the technology required to place a seismometer many miles below the ocean on the seafloor and summon it back to the surface, many aspects of actually plucking an OBS out of the ocean and pulling it on deck are remarkably low tech (yet still very impressive). Once we have spotted the OBS floating on the surface, the ship drives alongside. It is akin to driving your car up next to a ping-pong ball. People lean over the starboard side of the Langseth and attempt to attach a hook with rope to the bars on top of the OBS using a long pole. Its not always easy since the OBS is bobbing up and down in the waves. Once we hook it, we can attach a rope to the winch and haul the OBS onboard. Sometimes, OBS’s bring back surprises – an octopus returned with one of our OBS’s! He was alive and healthy, so we returned him to the sea (though some lobbied to keep him for lunch…)
Happily, we recovered 100% of our OBS’s and have started to (briefly!) pore over the data they recorded while they were on the seafloor. We can see the arrivals of sound waves from our air guns as well as lots of earthquakes, some very close and others far away. It would be delightful to dig into the analysis of these data immediately, but it must wait – there is more data to collect! We’re currently deploying OBS’s along our second profile.
On July 2, we finished deploying over twenty ocean-bottom seismometers as a part of our marine expedition to study a major tectonic boundary offshore Alaska. Ocean bottom seismometers (OBS’s) are autonomous instruments that sit on the seafloor and record sound waves traveling through the earth and the water. Floats made from glass balls and syntactic foam make each OBS buoyant, but an anchor holds it on the seafloor during the study. We communicate with each OBS acoustically, allowing us to send it a command to release from its anchor when we are ready to recover it.
For our project, we are placing OBS’s from Scripps Institution of Oceanography on the seafloor along two lines that span the major offshore fault zone. Immediately prior to deployment, we assemble the main components of each OBS on deck while the ship transits between sites. When we arrive at the deployment site, the ship slows down, and the OBS is lifted over the side of the vessel and into the water with a large crane. We release it, and it sinks to the sea floor. Thanks to the skill and hard work of the Scripps OBS team and the ship’s crew, we were able to deploy one OBS every hour, which is very efficient!
The larger the distance between the sound source (earthquakes or air guns) and the seismometer, the deeper into the earth the recorded sound waves travel. OBS are very sensitive and not attached to the vessel, so they can record sound waves generated very far away by earthquakes or air guns (commonly >200 km). Because we want to examine deep fault zones that cause large earthquakes off Alaska, OBS are a critical part of our effort.
In a few days, after we steam back over the OBS’s generating sound waves with our air guns, we will return to retrieve them. Even after ten years of working with ocean-bottom seismometers, it never ceases to amaze me that we can throw a bundle of very sophisticated electronics over the side of the ship and hope to pick it up and retrieve useful information from it. We are very excited about the new insights that will be provided by the data recorded on these instruments…
Yesterday evening, we left Kodiak aboard the R/V Marcus G. Langseth and began our 38-day-long research cruise offshore Alaska. As we left port, we were treated to clear skies, calm seas and spectacular views of Kodiak – dark grey mountains tipped with snow emerging from the lush green landscape.
Although Kodiak offered beautiful sights and delicious seafood (like locally caught halibut and scallops), our science party was eager to leave for sea. We have been waiting for the opportunity to collect these data for a long time. Our expedition was originally planned for September 2010, but there were delays in the Langseth’s schedule that would have required us to conduct our offshore study later in the fall, when the weather deteriorates. Rough seas make some marine operations more dangerous and can also reduce the quality of the data. We opted to postpone until the summer of 2011 to secure a better part of the limited weather window in this remote and northerly region.
But for some members of our science party, the wait has been much longer. In 2003, my colleagues Mladen Nedimović, Spahr Webb and the late, great John Diebold first conceived the idea for this study. Although many other scientists in our community and the National Science Foundation were very supportive of this project, it was scuppered by limited science funding and the temporary lack of a US academic seismic vessel between retiring the R/V Ewing and acquiring the R/V Langseth. But sometimes good things come to those who wait, and at long last we are setting out…
Seven days and eleven flights after we arrived in Alaska, we finished deploying our seismic stations onshore. Our final constellation of stations differs a little from our original plan (as always happens with field work), but achieves our main goal of instrumenting the part of the Alaska Peninsula that is nearest to our planned offshore work on the R/V Langseth. We installed our final seismic station yesterday in aptly named Cold Bay. This town sits next to a large bay with the same name and is famous for its wind. The most common damages sustained by cars and trucks here are jack-knifed doors from the wind (as I learned the hard way!).
As luck would have it, we finished deploying our seismometers just in time to catch a large earthquake (magnitude 7.2) that occurred farther west in the Aleutians around the Fox Islands. Of course we would love to immediately look at the recordings of this event on our stations, but we must wait patiently until August when we return to recover them. Many permanent seismic stations are telemetered, so data are transmitted back to scientists in near real time. But for temporary deployments like ours, the data are just written to a local disk and thus must be downloaded in person at the station.
We did have the chance to take a sneak peak at some of the data recorded at our station in Nelson Lagoon during the first few days of our deployment. Reassuringly, we saw evidence for several local earthquakes in these data, including a magnitude 3.1 near Sand Point.
Now that the onshore deployment is finished, Katie and Guy departed for home, and I soaked in some sunshine in Anchorage and started looking ahead to our upcoming research cruise. Tonight I fly to Kodiak to await the arrival of the R/V Langseth and our shipboard science party…
Every field location comes with logistical hurdles, and the Alaska Peninsula is no exception. Weather, wildlife and modes of transport pose the greatest challenges. We are hardly the first scientists to encounter these: Lamont-Doherty Earth Observatory has a long, rich history of collecting seismic data in this region (e.g., Shumagin Seismic Network, which ran for >20 years), and many groups continue to collect geophysical data here today.
The Alaska Peninsula is too rugged and wild for a network of roads, so planes, helicopters or boats are the only transportation options. We opted for planes, which immediately imposed a restriction on the locations of our seismic stations: they must be near airstrips. Happily, the Peninsula is sprinkled with small communities and lodges with airstrips, most of which lie close to the Pacific or Bering coasts rather than in the remote interior. Back in the office before our deployment, we chose the most ideally located airstrips for our stations and connected the dots between them with the most efficient possible flight plans. But, our plans quickly changed once we were in the field. The weather dictates when and where you can fly each day, and it varies dramatically. We have been lucky enough to have several clear days (even saw some blue skies and sunshine!), but other days we have been grounded by weather and wiled away the time indoors at the inn in Nelson Lagoon.
Once we arrive in each location, we need a quiet, safe place to install our equipment and a ride from the airstrip. On both counts, local communities have been unwaveringly helpful and friendly. The two school districts here kindly granted us permission to install our seismic stations at any of their schools, and we also obtained permission to place equipment at various lodges and village offices. Residents volunteered to take our gear and us from the airstrip to our sites. In one town, our pilot made a general plea over the radio: “Is anyone listening on Channel 3? I’m here at the airstrip with scientists who need a ride to the school”. Someone answered immediately and picked us up 5 minutes later.
Many of our sites are in spectacular places near remote lodges or in towns nestled between mountains and the ocean. All of them are home to impressive wild life that poses a risk to our equipment, particularly bears. We can protect the equipment against curious small animals but fully bear-proofing a station for a short (two-month-long) deployment is not feasible. Instead, we hope that placing our stations in villages (rather than in the wild) will provide some protection, but we will also need good luck. Fingers crossed…