Imaging the Sources of Great Alaskan Earthquakes

A major tectonic boundary on the seafloor off Alaska has produced fatal earthquakes and tsunamis similar to the recent one in Japan. In 1964, the second largest quake ever recorded happened here, and other parts of the fault may be building energy for another event. Scientists from Lamont-Doherty Earth Observatory are aboard the research vessel Marcus G. Langseth to better understand what causes these quakes, which will help assess the hazard for Alaska and beyond. Follow Lamont seismologist Donna Shillington from the field or click here to read the blog of the Columbia undergraduate students on board.

Posted By: Kevin Krajick on July 31, 2017

Scientists probing under the seafloor off Alaska have mapped a geologic structure that they say signals potential for a major tsunami in an area that normally would be considered benign. They say the feature closely resembles one that produced the 2011 Tohoku tsunami off Japan, killing some 20,000 people and melting down three nuclear reactors. Such structures may lurk unrecognized in other areas of the world, say the scientists. The findings will be published tomorrow in the print edition of the journal Nature Geoscience.

The discovery “suggests this part of Alaska is particularly prone to tsunami generation,” said seismologist Anne Bécel of Columbia University’s Lamont-Doherty Earth Observatory, who led the study. “The possibility that such features are widespread is of global significance.” In addition to Alaska, she said, waves could hit more southerly North American coasts, Hawaii and other parts of the Pacific.

A tsunami can occur as ocean crust (brown area) dives under continental crust (orange), causing the ocean floor to suddenly moves. In one region off Alaska, researchers have found a large fault and other evidence indicating that the leading edge of the continental crust has split off, creating an area that can move more efficiently, and thus may be more tsunami-prone. (Anne Becel)

A tsunami can occur as ocean crust (brown area) dives under continental crust (orange), causing the ocean floor to suddenly move. In a region off Alaska, researchers have found a large fault and other evidence indicating that the leading edge of the continental crust has split off, creating a tsunami-prone area where the floor can move more efficiently. (Anne Becel)

Tsunamis can occur as giant plates of ocean crust dive under adjoining continental crust, a process called subduction. Some plates get stuck for decades or centuries and tension builds, until they suddenly slip by each other. This produces a big earthquake, and the ocean floor may jump up or down like a released spring. That motion transfers to the overlying water, creating a surface wave.

The 2011 Japan tsunami was a surprise, because it came partly on a “creeping” segment of seafloor, where the plates move steadily, releasing tension in frequent small quakes that should prevent a big one from building. But researchers are now recognizing it may not always work that way. Off Japan, part of the leading edge of the overriding continental plate had become somewhat detached from the main mass. When a relatively modest quake dislodged this detached wedge, it jumped, unleashing a wave that topped 130 feet in places. The telltale sign of danger, in retrospect: a fault in the seafloor that demarcated the detached section’s boundary landward of the “trench,” the zone where the two plates initially meet. The fault had been known to exist, but no one had understood what it meant.

The discovery was made near the end of the Alaska Peninsula. A tsunami from here could reach many land areas across the Pacific. (Anne Becel)

The discovery was made around the western end of the Alaska Peninsula and the eastern Aleutian Islands. (Anne Becel)

The researchers in the new study have now mapped a similar system in the Shumagin Gap, a creeping subduction zone near the end of the Alaska Peninsula some 600 miles from Anchorage. The segment is part of a subduction arc spanning the peninsula and the Aleutian Islands. Sailing on a specially equipped research vessel, the scientists used relatively new technology to penetrate deep into the seafloor with powerful sound pulses. By reading the echoes, they created CAT-scan-like maps of both the surface and what is underneath. The newly mapped fault lies between the trench and the coast, stretching perhaps 90 miles underwater more or less parallel to land. On the seafloor, it is marked by scarps about 15 feet high, indicating that the floor has dropped one side and risen on the other. The fault extends down more than 20 miles, all the way to where the two plates are moving against each other.

The team also analyzed small earthquakes in the region, and found a cluster of seismicity where the newly identified fault meets the plate boundary. This, they say, confirms that the fault may be active. Earthquake patterns also suggest that frictional properties on the seaward side of the fault differ from those on the landward side. These differences may have created the fault, slowly tearing the region off the main mass; or the fault may be the remains of a past sudden movement. Either way, it signals danger, said coauthor Donna Shillington, a Lamont-Doherty seismologist.

Seafloor images were collected aboard the research vessel Marcus G. Langseth, the nation's main ship for seismic research. (Anne Becel)

Seafloor images were collected aboard the research vessel Marcus G. Langseth, the nation’s main ship for seismic research. (Courtesy Lamont-Doherty Earth Observatory)

“With that big fault there, that outer part of the plate could move independently and make a tsunami a lot more effective,” said Shillington. “You get a lot more vertical motion if the part that moves is close to the seafloor surface.” A rough analogy: imagine snapping off a small piece of a dinner plate, laying the two pieces together on a table and pounding the table from below; the smaller piece will probably jump higher than if the plate were whole, because there is less holding it down.

Other parts of the Aleutian subduction zone are already known to be dangerous. A 1946 quake and tsunami originating further west killed more than 160 people, most in Hawaii. In 1964, an offshore quake killed around 140 people with landslides and tsunamis, mainly in Alaska; 19 people died in Oregon and California, and waves were detected as far off as Papua New Guinea and even Antarctica. In July 2017, an offshore quake near the western tip of the Aleutians triggered a Pacific-wide tsunami warning, but luckily it produced just a six-inch local wave.

As for the Shumagin Gap, in 1788, Russian colonists then living on nearby Unga Island recorded a great quake and tsunami that wiped out coastal structures and killed many native Aleut people. The researchers say it may have originated at the Shumagin Gap, but there is no way to be sure. Rob Witter, a geologist with the U.S. Geological Survey (USGS), has scoured area coastlines for evidence of such a tsunami, but so far evidence has eluded him, he said. The potential danger “remains a puzzle here,” he said. “We know so little about the hazards of subduction zones. Every little bit of new information we can glean about how they work is valuable, including the findings in this new paper.”

In rural Alaska, infrastructure tends to cluster along the coast, making it vulnerable to tsunami. Waves generated here could reach to Hawaii and beyond. (Anne Becel)

In rural Alaska, infrastructure tends to cluster along the coast, making it vulnerable to tsunami. Here, a community on Kodiak Island. Waves generated in this region could reach to Hawaii and beyond. (Matthias Delescluse)

The authors say that apart from Japan, such a fault structure has been well documented only off Russia’s Kuril Islands, east of the Aleutians. But, Shillington said, “We don’t have images from many places. If we were to look around the world, we would probably see a lot more.” John Miller, a retired USGS scientist who has studied the Aleutians, said that his own work suggests other segments of the arc have other threatening features that resemble both those in the Shumagin and off Japan. “The dangers of areas like these are just now being widely recognized,” he said.

Lamont seismologists have been studying earthquakes in the Aleutians since the 1960s, but early studies were conducted mainly on land. In the 1980s, the USGS collected the same type of data used in the new study, but seismic equipment now able to produce far more detailed images deep under the sea floor made this latest discovery possible, said Bécel. She and others conducted the imaging survey aboard the Marcus G. Langseth, the United States’ flagship vessel for acoustic research. Owned by the U.S. National Science Foundation, it is operated by Lamont-Doherty on behalf the nation’s universities and other research institutions.

The other coauthors of the study are Spahr Webb, Mladen Nedimovic and Jiyao Li of Lamont-Doherty; Matthias Delecluse and Pierre-Henri Roche of France’s PSL Research University; Geoffrey Abers and Katie Keranen of Cornell University; Demian Saffer of Penn State; and Harold Kuehn of Canada’s Dalhousie University.

RELATED: Ancient Faults and Water Are Sparking Earthquakes Off Alaska


Posted By: Stacy Morford on December 15, 2016


This map of large earthquakes outlines some of the most active tectonic plate boundaries. Slow-slip earthquakes create an ideal lab for investigating fault behavior along the shallow portion of subduction zones. Map: USGS

Off the coast of New Zealand, there is an area where earthquakes 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.

 Lamont-Doherty Earth Observatory

A shipboard crane lifts a Lamont-built ocean bottom seismometer attached to a protective trawler shield. Photo: Lamont-Doherty Earth Observatory

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.


Posted By: Donna Shillington on August 08, 2011

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.

Shishaldin volcano

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.

The R/V Langseth and fishing boats in Dutch Harbor

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.

Group photo from cruise MGL1110

Posted By: Donna Shillington on July 31, 2011

Distraction from data analysis: Rainbow over the snow capped mountains of the 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.

Seafloor faults imaged by bathymetric mapping

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.

Posted By: Donna Shillington on July 24, 2011

Andy, Hannah and Mike in the main lab

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.

Workboat returns from repairing seismic streamer

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.

Humpback observed from marine mammal observation tower on Langseth (picture courtesy of PSOs)

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.

Other "good" excitement: beautiful views. Castle rock by Chignik Bay.

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.

Posted By: Donna Shillington on July 18, 2011

Seismic gear towed off the port side coming very close to the Unga shore

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.

Navigation screen showing the Langseth and her streamers (blue) approaching the Unga coastline (green)

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.

Posted By: Donna Shillington on July 14, 2011

Two reels of seismic streamer on the Langseth

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.

Columbia grad student Jiyao carries "bird" for seismic streamer

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!

Seismic streamer with a "bird" being deployed into the ocean

Posted By: Donna Shillington on July 08, 2011

Ernie reaches to tag an OBS

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.

OBS returned with surprise stowaway: an octopus

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…)

Recording from an OBS. An earthquake (large event) overwhelms the regularly spaced pulses from our air guns.

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.

Posted By: Donna Shillington on July 04, 2011

Mark, Ron and Mladen deploy an OBS

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!

Rugged Alaska Peninsula seen from the sea

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…

Posted By: Donna Shillington on June 30, 2011

View of Kodiak from the ship

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.

The R/V Langseth in port in Kodiak

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…