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Home arrow Archives   The American Surveyor     

Measuring a Caribbean Disaster Print E-mail
Written by John Stenmark, LS   
Saturday, 14 August 2010

A 1.904Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

On January 12, 2010, a magnitude 7.0 earthquake struck the city of Port-au-Prince, the capital and largest city of Haiti. Tens of thousands of buildings collapsed, and more than 200,000 people died in the disaster.

Earthquakes are not unexpected in Haiti. The country sits astride several fault lines, among them the Enriquillo-Plantain Garden fault zone (EPGFZ), one of the main faults between the Caribbean and North American plates. For several years, geophysicists have used GPS to monitor the region and measure the small, continuous movement of the ground above the fault. Based on historical reports, they knew that the strain on the fault zone in Haiti had been building for more than 240 years.

The January earthquake set into motion enormous response efforts in rescue and recovery. Within hours, aid from around the world began pouring into the country. The scientists were not far behind.

Rapid Response
Dr. Eric Calais, Professor of Geophysics at Purdue University, received an alert from the U.S. Geological Survey (USGS) soon after the earthquake occurred. Looking at the magnitude and location of the quake, Calais immediately recognized that it was a major disaster. His first thought was to get to Haiti. "There were several reasons I wanted to go," he said. "From my previous visits to Haiti, I know a number of people there, and I wanted to help them. From the scientific standpoint, it's very important to capture the coseismic ground motion, such as how much the ground moved during the earthquake." Recovery efforts and aftershocks can obliterate evidence of the quake's initial effects, so data must be collected quickly. Calais contacted the National Science Foundation (NSF) to request assistance, and within a week submitted a proposal for a project to acquire geophysical and geological data in Haiti.

While securing the funding, Calais assembled a team of four researchers for the trip. The team's primary objective was to recover and measure more than 30 survey markers that had been established as part of a geophysical project Calais initiated in 2003 with funding from NSF. To facilitate longer-term observations, they needed to establish a network of continuously operating GPS stations. Before the January quake, GPS infrastructure in Haiti consisted of a single reference station operated by the Haiti National Center for Geographic Information (CNIGS). That station—installed on a building that collapsed during the quake—had stopped operating. Calais needed to find some additional GPS equipment.

Help came quickly. For the reference stations, Trimble supplied six Trimble® NetRS® GPS Reference Stations with Trimble Zephyr GeodeticTM Antennas. To measure the survey markers, the team would use GPS equipment from UNAVCO, an NSF-funded university consortium based in Boulder, CO, that facilitates geoscience research and education using geodesy.

UNAVCO provided 10 GPS campaign kits for the project. Each kit contained a Trimble R7 GPS Receiver and Trimble Zephyr Geodetic Antenna together with batteries, chargers and other accessories. UNAVCO has worked with Trimble since the mid-1990s, and has more than 200 campaign kits containing Trimble receivers. According to UNAVCO Engineering Project Manager Jim Normandeau, the campaign kits are designed to be easily transported and to provide researchers at remote locations with everything needed to use GPS for precise measurements. To enable unattended operation over long periods, the kits included small solar panels to recharge the batteries. Before the equipment left UNAVCO, technicians tested the systems and built weather-resistant enclosures for the GPS receivers.

"The big problem is getting equipment to the country, and then moving it around within the country," said Normandeau. "You need small, compact systems. As the receivers for the permanent installations came in, I had to develop a portable design for the receiver enclosures so they could be installed at the remote sites."

The Purdue Department of Aviation Technology connected Calais with AeroService in Miami, who put him and the GPS equipment on an AmeriJet cargo flight directly to Port-au-Prince. The other team members flew to Santo Domingo, Dominican Republic, then traveled by truck to the border with Haiti, where they met up with Calais and a group of Haitian technical experts. In addition to the CNIGS, the Haitian Bureau of Mines and Energy, the Haitian Civil Protection Agency and the State University of Haiti provided staffing and logistical support. Once everything was in place, the researchers deployed into three teams.

The first task was to identify the geodetic framework for the GPS observations. There were a few continuously operating reference stations in the Dominican Republic, and 10-15 stations on other islands within 1,000 km (600 mi) of Port-au-Prince. With the only existing continuous station in Haiti knocked out by the quake, the team needed to replace it. They installed a Trimble NetRS Reference Station in Port-au-Prince, at the central hub for Voilá, a major cellular service provider in Haiti. Voilá had responded rapidly to the quake. It provided additional free cellular service to its customers, and was involved in numerous humanitarian efforts. The Voilá site was ideal, Calais said. "It's possible to put a GPS antenna far enough away from the cellular tower so that there won't be any masking or multi path. The site is guarded, and it has reliable electricity and Internet access."

Recovery and Measurement
In 2003, a series of points for geophysical observations had been placed in secure locations. In Haiti, that meant installing the marks on top of buildings. "It's sub-optimal," said Calais. "We have to take the risk, because it gets very difficult and costly to do this kind of work. We are tracking a few millimeters a year, and you could easily be seeing the settling or deformation of the building rather than the motion of the earth's crust. We would have preferred to have benchmarks on bedrock but that's the reality of what we can and can't do." In the end, building deformation proved not to be a major issue, contributing only about 10 cm (0.3 ft) of movement, in the worst case, in Léogâne. Farther from the quake's epicenter, all sites were undamaged.

Each survey team had three or four sets of equipment and was assigned to recover and measure a subset of the points established in 2003. At each station, they set up the GPS equipment and configured it to collect data at 15-second intervals. The equipment operated on each point for at least three consecutive days. The team visited each receiver every day to download the previous day's data. Using UNAVCO's TEQC software, they checked the data to spot errors or inconsistencies that might cause trouble during processing and analysis. At the end of the three-day period, they packed the equipment and moved it to the next point.

Calais said there was no need to coordinate observation schedules between the teams to collect specific network cross vectors. "It's different from traditional surveying. We're using long sessions, precise orbits and advanced software for post processing," he said. "So there is virtually no difference between a 20 km (12 mi) baseline and 200 km (124 mi) baseline in our results. Given the complications in Haiti, we could not have a schedule requiring everybody to be recording at specific times. Logistically, it would be a nightmare."

Over a span of two weeks, the researchers completed observations on 35 different points. To optimize measurement of the deformation, points were placed densely (spacing of 1 to 5 km or 0.6 to 3 miles) near the fault zones. In areas away from the faults, the spacing increased to as much as 60 km (37 miles) between points.

When the measurements were completed, the teams reassembled in Port-au-Prince, where they consolidated and copied the field data for the flights back to the U.S. The researchers used GAMIT and GIPSY software (developed by MIT and NASA's Jet Propulsion Laboratory) to compute positions for the survey points. The long-observation data sets produced precision of roughly 2 mm (<0.01 ft) in the horizontal component and 5 mm (0.02 ft) in vertical. Calais explained the need for such high precision. "In an earthquake, you might question why we need two millimeters precision when the displacement is on the order of a meter. We have sites far away from the earthquake that moved by a centimeter or less. Those sites are important to define the mechanism of the earthquake. We need to know if the centimeter-level displacement we are measuring is true, or if it is just noise in the measurement. All the GPS measurements we do require high precision, whether it's after an earthquake, or before an event when we are working to determine how fast the strain is building up around a fault zone."

Shortly after Calais' team left Haiti, UNAVCO Field Engineer Sarah Doelger arrived in Port-au-Prince and worked with the Bureau of Mines and Energy and the State University to set up the five remaining continuous reference stations. Working at Voilá cellular facilities around the country, she installed the Trimble NetRS receivers and connected them to each site's power and Internet communications systems. The receivers collect data automatically, which is downloaded by UNAVCO for archiving and open distribution. The cooperation of Voilá is highly valuable, Normandeau said. In addition to geophysical research, the receivers will provide data for geodetic and surveying applications that will play a central role in rebuilding the areas devastated by the earthquake. Surveyors can use the data to establish local control needed for construction and cadastral surveys. In the future, the receivers may provide continuous data for use in surveying and real-time networks.

The Science
Interaction between tectonic plates in the Caribbean region is commonly described as one of two types of fault zones. Strikeslip faults involve plates that move laterally with respect to one another. Subduction faults move vertically, as one plate slides down and beneath the other. In Haiti, Calais compared the pre- and post-quake positions of the 35 points to develop a three-dimensional displacement vector at each location. Closest to the epicenter, they measured displacement of approximately 1 m (3.3 ft). At sites 200 km (120 mi) from the epicenter, the displacement vectors were less than 1 cm (0.03 ft). By analyzing the vectors, the geophysicists found that the quake included both lateral and vertical shifting. That makes the Haitian quake more complicated, as it occurred close to a known strike-slip fault. Without GPS, there would have been no way to discover the complex motion.

The experience in Haiti demonstrates the value of ongoing GPS measurements. Using data from the 2003 project, Calais' researchers had determined that the EPGFZ fault was accumulating elastic strain at a rate of about 7 mm (0.02 ft) per year. The last major earthquakes in Haiti had occurred in 1751 and 1770, and roughly 1.8 m (5.9 ft) of strain had built up over 250 years. The team computed that a quake that would result from releasing that strain would have a magnitude of 7.2. The January quake was magnitude 7.0. Their forecast was right on.

In the coming months, the researchers will rely on the newly-installed GPS receivers to study post-seismic deformation as the earth's crust recovers from the large earthquake and readjusts to a steady state. The data provides information about the mechanical properties of the crust. Periodic measurements will be made at the 35 survey sites as well. This data will help determine the longer-term buildup of strain on the fault, from which the scientists can create forecast scenarios for future events. Calais has already worked with the USGS to create hazard maps for Haiti, which show the probabilities of different levels of motion in the event of another major quake.

It remains impossible to predict exactly when and where an earthquake will strike. By helping to understand the forces that are at work far beneath the Earth's surface, GPS surveying plays a key role in assisting Haiti to mitigate damage when a quake does occur.

John Stenmark is a writer and consultant working in the AEC and technical industries. He has over 20 years experience in applying advanced technology to surveying and related disciplines.

Sidebar:
Understanding Seismic Motion

Geophysicists use survey-grade GPS with long baselines and extendedduration observation to obtain sub-centimeter results. They are interested in precise measurements of three different types of movement:
• Interseismic motion takes place in the time (often decades or centuries) between major earthquakes. The earth's crust is constantly moving along fault zones. This slow motion is an indicator of the buildup of energy stored as strain between the crustal plates.
• Coseismic motion is the displacement or deformation of the fault. It occurs rapidly at the time of the earthquake.
• Post-seismic motion takes place as the earth's crust adjusts and returns to a stable condition. Because this displacement can distort coseismic motion, it is important to take measurements as soon as possible following an earthquake. After the crust has stabilized, the interseismic motion continues, and the cycle repeats.

A 1.904Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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