Waves from a tsunami generated by an 8.8-magnitude earthquake that struck Chile’s coast on Feb. 27 hadn’t even reached San Diego and Bruce Appelgate was already getting calls from researchers wanting to study the event.
Appelgate’s cell phone rang steadily as he drove to the Scripps Institution of Oceanography, UC San Diego Nimitz Marine Facility on San Diego Bay’s western lip. The director of ship operations at Scripps was on his way to the spot where the institution’s research fleet is docked. He needed to make sure the waves from a hemisphere away rippling through the Pacific Ocean at the speed of a jet aircraft would not cause any adverse effects on his patch of bayfront.
By the end of the day, the wave (less than a yardstick’s height) had come and gone and plans were in motion to redirect a Scripps vessel that had not been in its home port at the time of the quake. In fact, research vessel Melville had been on the open ocean about 100 kilometers (55 miles) away from the quake’s epicenter when the first tsunami waves radiated from the Chilean coast.
“The same day as the earthquake, we had the basic idea of what we wanted to accomplish and how we could do it,” said Appelgate.
GETTING TO THE SCENE
When an oceanographer or marine biologist or geophysicist needs to use a ship in the United States’ research fleet, known as the University-National Oceanographic Laboratory System, or UNOLS, he or she typically expects to start the application process at least a year in advance of actually boarding the vessel. Thanks, however, to a fast-tracking process used by the National Science Foundation (NSF) to clear research on the quick, Scripps geophysicist David Chadwell and oceanographer Peter Lonsdale redirected Melville 19 days after the quake struck. The scientists wanted to understand what caused the tsunami that had generated local waves as high as 60 meters (197 feet). They needed to get to the scene while evidence was fresh.
In the early morning hours when the quake struck, the ship was off the coast at a spot where the ocean water was 2,600 meters (8,530 feet) deep. A collection of geologists and marine biologists were using the ship for INSPIRE, which included an examination of the methane seeps created by the same forces of subduction that had released so much energy on Feb. 27. The biologists, led by Scripps graduate student Andrew Thurber, were interested in the unusual organisms that had adapted to life in an environment choked with methane gas.
The INSPIRE scientists and crew scarcely noticed the passing tsunami wave, which was lost among the three-meter (12-foot) -swells the ship’s science team had become accustomed to. The depth of the ocean at that location diluted the tsunami’s energy, though on land, it was a different story. Fellow researchers at the University of Concepcion campus in the coastal town of Dichato, 100 kilometers (62 miles) south of the epicenter, saw most of the city wiped out by a trio of waves spaced 30 minutes apart. They ranged in height from 10 meters (33 feet) to 18 meters (59 feet) and encroached nearly a mile inland. By the time the last wave hit, University of Concepcion research vessel Kay Kay II was resting in a patch of mud a kilometer (3,280 feet) inland from where it had been docked.
The Melville cruise continued uneventfully until March 17, when it returned to Valparaiso, its crew unaware that its shore leave was about to be scuttled completely.
A NEED FOR SPEED
Chadwell and Lonsdale needed to get to the scene as quickly as possible. Recently collapsed slopes will continue to settle for weeks and months and multiple earth-shaking aftershocks, some as large as magnitude-6.9, were continuing to reshape the seafloor.
Though from slightly different fields, the geodesist Chadwell and the geomorphologist Lonsdale were both interested in what caused the tsunami wave — whether it was triggered by a sudden upheaval of the seabed or by one or more landslides caused by the collapse of undersea slopes. They also wanted to know if further tsunami-generating slope failures were imminent.
To get answers, the researchers would image the seafloor and look for spots where it had either subsided or been violently pushed up from below. They hoped to compare it as the “after” to a set of similar “before” images made by German researchers a few years earlier. Teams of scientists had performed such surveys in the wake of modern history’s most famous tsunami — the Indian Ocean wave that claimed more than 200,000 lives in 2004. The scientists then found sites where slopes had collapsed but with little sense of when the collapses happened, they could draw few definitive conclusions.
“Because they had no prior map, it was really just guesswork as to whether the features they saw on the seafloor would have indeed caused a tsunami,” said Lonsdale.
Luckily the most important piece of equipment the Scripps team would need was already installed on Melville. A multibeam sonar unit affixed to its hull has been a stock piece of the ship’s equipment almost since Melville was delivered to Scripps in 1969. Chadwell and Lonsdale used a recently upgraded version to find evidence of undersea landslides, especially in the vicinity of coastlines battered by the tallest tsunami waves. The local nature of those waves could be an indicator that they were generated by very specific slope failures.
Melville zigzagged up and down the Chilean coast, guided in part by estimates of tsunami wave height and the extent of run-up made by research teams that surveyed the coastline. When the waves actually hit was vital information. Chadwell said there was some anecdotal evidence that they didn’t strike at certain locations until three hours after the quake, bolstering the case that either slope failure or a simple focusing of coastal wave energy had caused a delayed response. Such time estimates have been difficult to come by.
“The biggest problem is that when people hear that a tsunami is coming, they don’t find out what the timing is and write it down,” said Lonsdale. “They just run like hell.”
A few more pieces of equipment would enhance the cruise’s prospects for delivering a useful dataset. Chadwell had emailed NSF within 24 hours of the quake and within 72 had a response encouraging him to formulate a mission plan. The agency enabled the researchers to bypass the typical process of applying for ship time, waiting to have the merit of the proposed cruise judged by a review panel, and then being issued a time slot. In addition, Chilean scientists who would join Chadwell and Lonsdale on the cruise worked to obtain clearance from their government to allow Melville to spend seven more days in its territorial waters.
“That hurdle that normally takes months was solved with a few phone calls over a few days,” Chadwell said.
Lonsdale and Chadwell had final approval within seven days of the quake. The cruise, named Scripps Institution of Oceanography’s Survey of the Earthquake and Rupture Offshore Chile (SIOSEARCH) was underway and the race was on to crate and ship equipment and people to Chile. The equipment flown to Chile would be used to create a second record of the forces behind the tsunami.
“International shipping can always be difficult and then you try to do it in two weeks and then you try to ship to a country that’s just experienced a major earthquake,” said Chadwell, “so that was a hassle.”
Two crates made it to the relatively undamaged Valparaiso port where Melville awaited. To save time, the vessel had merely dropped anchor in the harbor rather than docking. The crates contained pressure sensors that would look for signs of tapering seafloor upheaval. They will remain on the seabed for a year and record pressure changes if further changes in elevation take place.
BACK ON LAND…
A few hours after the SIOSEARCH team disembarked, Melville was on its way to a scheduled cruise to study electromagnetics off the Central American coast — the crew still having had no shore leave. The researchers are now refining the first wave of data gathered in real time via sonar. In May, Chadwell presented an overview of SIOSEARCH at a conference in Santiago scheduled well before the February quake to commemorate the 50th anniversary of the 1960 magnitude-9.5 temblor that struck Chile, still the largest the world has experienced since 1900.
Though there was evidence of large-scale slope failure — defined as large if at least football field-sized portions collapsed — many of the places where the slumping happened were unfortunately not surveyed by the German team, making comparison difficult.
But submarine surveys now being proposed by other researchers might be able to determine when the slopes collapsed. The Scripps team, meanwhile, has come away with an unprecedented post-earthquake record of activity at one of the world’s major subduction zones — enough data to rate the cruise a success. It could take a year or more for the scientists to come up with hard conclusions about what happened in the moments after the earth moved. At least for this part of the task, emergency mode will not be required.