The trouble began 180 million years ago. In southern Asia, relentless movement of Earth’s crustal plates caused the Oceanic Plate to subduct beneath southern Tibet to the north. The collision of the two plates, which continues today, has buckled the crust and pushed the rock up to create one of the world’s great mountain ranges, the Himalaya. Today, the Himalaya dominates the landscape in northwest India, Nepal, Kashmir, Bhutan and southwestern China, including the former nation of Tibet.

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It’s not a gentle collision. Riding on the Oceanic Plate, the Indian subcontinent moves northward roughly 4.0 centimeters or 1.6 inches each year. Half of the motion is absorbed by the Himalaya, pushing the mountains up. The rest of the energy goes into squeezing the rock along the boundary, or fault, between the two plates. From time to time, the rock ruptures to release the accumulated strain, resulting in an earthquake. A major rupture occurred this past April 25, when the fault broke in central Nepal about 15 kilometers or 9.3 miles below the surface, roughly 80 kilometers or 50 miles northwest of Kathmandu. The resulting magnitude 7.8 earthquake killed more than 9,000 people, injured more than 23,000, and damaged or destroyed countless buildings and houses.

The April event was not the first quake to strike Nepal, and it unfortunately won’t be the last. The behavior of the quake — and the ways in which it could be studied — opens the door for new understanding of future earthquakes in the region.

The Well-Measured Fault

Earthquakes in Nepal are neither predictable nor unexpected, says University of Colorado geophysicist Dr. Roger Bilham. The earliest earthquake in Nepal recorded by humans occurred in 1255. Since then, there have been at least 10 quakes of magnitude 6.3 or more.

Scientists note the region’s earthquakes appear to occur at roughly consistent intervals with similar locations and behavior. Bilham explained that a magnitude 8.4 quake in 1934 appears to be a repeat of the 1255 event — the two quakes occurred in the same location and produced significant surface ruptures and extensive damage. Likewise, the 2015 event (known as the Gorkha quake) is very similar to one that occurred in 1833. The earlier quake, estimated as magnitude 7.7, originated at the same location as the Gorkha epicenter and produced 3.5 meters or 11.5 feet of slip, matching the slip of April’s magnitude 7.8 Gorkha event.

To make their analyses, Bilham and other scientists rely on arrays of sensors to capture data on the motion of the crust. Since the 1990s, a network of more than two dozen GPS continuously operating reference stations (CORS) has collected data on plate motion in Nepal. “Everything we know about historical quakes comes from damaged buildings or evidence of surface ruptures,” Bilham said. “Today, we have GPS and other ways to measure vibration and displacement.”

Seismic sensors can detect subtle motion and are very good at capturing relatively small movements at high frequencies, but deriving accurate displacement from acceleration data is an inexact science. Additionally, seismic sensors can become saturated by larger movement such as experienced in a great earthquake. By using GPS to directly measure displacement of centimeters and more, researchers have complementary sensors that provide a more complete picture of plate motion and the effects of earthquakes. In ideal cases, seismic sensors are collocated with GPS stations.

While it’s common to have GPS and seismic networks in earthquake-prone areas, the GPS network in Nepal provides unique advantages in measuring the effects of earthquakes along subduction faults. Bilham pointed to recent strong quakes in Japan, Chile and Sumatra. The fault ruptures in those areas occurred along coastlines where it’s not possible to use GPS to measure motion on both sides of the fault. But in landlocked Nepal, GPS sensors on the Indian and Asian plates could precisely measure the motion of the quake.

To provide a complete picture of the displacement, the GPS receivers in Nepal capture and store data at multiple recording rates. Data collected at 15-second intervals provides information on the normal, slow plate motion over months and years. The receivers in Nepal also captured high-rate data five times per second (5Hz), which could provide a detailed picture of the shaking during the quake itself. But when the Gorkha quake struck and the data was urgently needed, landslides and damage made retrieving the data nearly impossible. The person who could do it was on the other side of the planet.

High-Pressure Data Recovery

If you are looking for someone to visit remote sites to work with sophisticated electronics under difficult conditions, then John Galetzka is your guy.

Trained as a U.S. Army Ranger and equipped with a Bachelor’s Degree in geosciences, Galetzka has set up GPS networks around the world. During a stint with the U.S. Geological Survey (USGS), Galetzka played a key role in building the Southern California Integrated GPS Network. He then joined the California Institute of Technology (Caltech) Tectonics Observatory, where he worked on GPS networks in Sumatra, Nepal, Chile and Peru. Working in Nepal over a 10-year period, Galetzka had set up 28 GPS stations for Caltech and a 29th station shared with a French research agency. (Roughly 20 additional GPS CORS in Nepal are managed by other agencies. All the CORS are in collaboration with the Nepal Department of Mines and Geology.)

In 2013, Galetzka knew that funding for his job with Caltech would soon end. He spent the year installing fresh batteries and modernizing Caltech’s GPS stations in Nepal; the GPS equipment at the stations consisted of Trimble NetRS, NetR8 and NetR9 reference station receivers. With permission from the Nepalese government, Galetzka installed cellular modems to push GPS data to FTP servers in the U.S. The 15-second data could be sent via cellular modem, but the volume of 5Hz data was simply too much for the limited bandwidth of cellular connections. So, Galetzka configured the built-in storage of the Trimble receivers to store several weeks’ of 5Hz data. The remainder of the receivers’ memory would store the 15-second observations in case the cellular connection went down. “In the event of an earthquake, we could use the 15-second data to look at the motion over several weeks or months,” Galetzka explained. “But the 5Hz data accumulates very rapidly. If not downloaded soon after an earthquake, the data captured during the quake can be overwritten by newer data, and the really important data is lost.”

In the two years since Galetzka had last visited the CORS sites in Nepal, the cellular connections to many stations had gone silent. At the time of the quake, only nine stations were sending data; the status of the others was unknown. In addition to saving the 5Hz data from being overwritten, scientists needed to retrieve the 15-second observations to provide a complete picture of the fault’s behavior.

When the Gorkha earthquake struck, Galetzka was in Mexico on assignment from his current employer, UNAVCO, a non-profit consortium that facilitates geoscience research and education using geodesy. “I looked at my phone and saw all these small earthquakes in Nepal,” Galetzka recalled. “I scrolled down and finally found the big, main shock. At that time, I think the USGS called it a 7.9. I woke my colleague, Luis Salazar, and we were in shock at the size of the earthquake. After thinking about it for a few moments, I told Luis, ‘I’ve got to go to Nepal.’” Four days later, Galetzka arrived in Kathmandu.

While Galetzka worked his way to Nepal, global relief efforts got underway. Nations around the world sent rescue crews, medical supplies, food and shelter to the stricken region. In addition to humanitarian needs, the scientific community began to organize people and equipment to assist in securing important geophysical data. One of the most valuable responses came from Trimble, which provided funding for helicopter time needed to access remote GPS stations. The company also donated seven Trimble NetR9 GNSS reference station receivers to replace old or damaged equipment and perform post-seismic monitoring. Trimble’s Mike O’Grady, who had extensive experience in Asia, hand carried the equipment to Kathmandu and assisted in the effort.

In the days immediately after the quake, science took a back seat to human needs. Private and military helicopters were kept busy on humanitarian missions. Thousands of people were living under tents and tarps, not because their homes collapsed, but out of fear of another quake. The fears were exacerbated with an aftershock on May 12, a magnitude 7.3 tremor northeast of Kathmandu that killed more than 200 people and set off additional smaller shocks. Galetzka was pummeled by questions from friends and people on the street: “What’s going to happen today? Are we going to get more earthquakes, more aftershocks? Is the big one coming? What does the GPS data show?”

Long Days Ahead

Galetzka’s first few days in Nepal were a blur. A typical day started around 2 or 3 a.m. “I remember waking up because of jet lag,” he said, “but then just because of sheer excitement. An aftershock might wake you up, and then it’s just impossible to get back to sleep. You’re thinking about what needs to be done.”

Early each day, Galetzka and his colleagues would go to the airport to check aircraft status. When a helicopter was not occupied with humanitarian work, they could use it to go out for a few hours to visit a station and download data. If the station was not telemetering data, they would troubleshoot and get it back online. If they couldn’t get the receiver to respond, they simply swapped it for a new one and took it back to Kathmandu where O’Grady, working in a space provided by a Toyota dealer, could recover the data and update the receiver to make it available for the next mission.

The rest of the day would be spent planning for the next day or the next week, getting people lined up to do vehicle missions to download data where possible or helping other projects. For example, Galetzka was one of the few people familiar with a USGS strong-motion accelerometer installed at the American Club operated by the U.S. embassy. He was able to retrieve the data from that instrument, which proved important in analyzing shaking in Kathmandu. Others, including Bilham, conducted damage assessments and looked for surface evidence related to the quake, as well as helping to recover the GPS data.

The view from the helicopters was striking. “In the rural areas, the damage to the villages was incredible,” O’Grady said. “Clay and mud houses had collapsed. Most casualties occurred in the mountain villages.” Helicopter missions to GPS stations often included delivering food, medical supplies and tents. “The pilot knew the area and would land in places that needed help,” O’Grady said. “We would unload the relief aid and then go on to the GPS points.”

When the teams reached a GPS site, they found differing degrees of damage, but the integrity of the GPS data was consistently good. “The receivers got knocked around a bit, but none went down due to the quake,” said Galetzka. “There’s no evidence that the earthquake knocked out a receiver or somehow damaged an antenna. The short-braced monuments that we used are really solid and worked well in accurately measuring how the earth is moving. Overall, the network produced excellent results. There were stations directly over the fault rupture. We’ve never before seen or captured data like this.” Soon after a receiver’s data could be recovered, it was processed for initial analysis. Bilham said the quake started in the north, where the fault slipped as much as 5-6 meters (16-20 feet) deep underground. As the rock released the accumulated strain, the quake ran out of steam. By the time it reached Kathmandu, the slip had decreased to centimeter levels. Galetzka used the information to refine his strategy for recovering the GPS data. Because the quake had minimal motion in the western part of the country, he could give those GPS stations a lower priority.

Unexpected Results

Data results from both GPS and seismic sensors were put to work examining the quake behavior and effects. Analysis by Gavin Hayes at USGS used strong motion accelerometer and 5Hz GPS data to determine that, in less than 5 seconds, the Kathmandu valley heaved upwards by 60 centimeters (2 feet) and moved southwest by 1.5 meters (5 feet) at velocities of up to 50 cm/s (1.6 ft/sec). In the following 60 seconds, valley sediments oscillated laterally at four-second periods with amplitude of 20-50 centimeters (8-20 inches).

The shaking created fissured ground near the airport. Video captured during the quake shows pedestrians struggling to remain standing. Hayes’ analysis revealed that surfaces horizontal prior to the quake are now tilted down to the southwest, but by less than a single degree. Bilham noted the runway at Kathmandu’s airport lifted roughly 50 centimeters (20 inches) and tilted by 12 centimeters (5 inches). Scientists were surprised by one aspect of the Gorkha quake. According to USGS geophysicist Dr. Ken Hudnut, the shaking in Kathmandu was not as violent and severe as what would be expected based on the large amount of strain released in this fault rupture. Given the energy released in the quake and typical construction practices, damage to most buildings in the city was surprisingly light. Hudnut said that more work is needed to understand the surface motion associated with the quake and how the movement of smooth flat fault systems can translate to motions at the surface. He said it’s also important to know if the Gorkha quake put additional stress on other faults in the area, which could influence occurrence of future earthquakes.

Efforts on the ground by Galetzka and others will help provide the information Hudnut described. Because the GPS equipment at the existing stations was largely undamaged, teams could use the receivers donated by Trimble to establish several new monitoring sites. Galetzka said that the new GNSS-capable equipment allowed stations to be located in places previously difficult for GPS alone. “We couldn’t get on mountain tops,” he explained. “So we were forced to put stations in some very deep valleys. With GNSS capability, tracking not just GPS but also GLONASS, Galileo and BeiDou, we can track as many satellites as possible while still being in a deep valley. It should increase the data quality coming out of those stations.” Galetzka added that a team from Cambridge University is now working to install seismic sensors at many CORS sites.

The GPS and GNSS stations are also providing benefits for Nepal’s surveying and engineering communities. Prior to the quake, the nation’s geodetic framework was made up of control points based on conventional surveying. The surface displacements of the quake rendered all of the existing marks useless. Surveyors can use data from the CORS to remeasure the marks and establish new coordinates tied directly to the global reference frame. Nepal’s GPS network continues to monitor tectonic motion. The GPS data enables researchers to model the strain accumulating along the plate boundaries and estimate the strength of upcoming quakes. Emphasizing that the timing of quakes can’t be predicted, Bilham focused on increasing understanding of the accumulating strain. “This was a good dry run for future, larger quakes,” he said. “It affected a small part of the Himalaya and drove home the need to strengthen their homes and buildings. It was not the worst that could have happened, but it is the worst that will likely happen for a couple of decades.” Geophysicists can use information from the Gorkha quake to advise local authorities on the need for good building practices to mitigate future damage and loss of life.

Galetzka agrees. “Even in the face of this tragedy, I think Kathmandu dodged a huge bullet,” he said, “I believe people realize that. There’s a lot of tectonic energy still remaining in that part of Nepal; it wasn’t completely released in this earthquake. So, for me, it was urgent to understand what the earth did and what this means for the future for the earthquake hazard in Nepal.”

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