By Matt A. Barr
At 8:42 am local time on September 5, 2012, a magnitude 7.6 earthquake struck the Nicoya peninsula on Costa Rica’s west coast. The ground shook as far away as Nicaragua, El Salvador, and Panama, with the eruption of Nicaragua’s largest volcano resulting three days later. Tsunami warning systems were triggered along the region’s Pacific coastline, a stark reminder that earthquakes in fault zones like this one also awaken the oceans. It would take several months and millions of dollars before the region recovered.
Dr. Andrew Newman saw it coming.
Dr. Newman regularly travels with students between his office at Georgia Tech’s School of Earth and Atmospheric Sciences and Nicoya, Costa Rica to work with a team of international scientists studying the fault there.
Seismometers located at strategic points throughout the region measure the energy released by almost any tectonic motion in this subduction zone, while GPS equipment plots the corresponding movement of land. Using records dating back to the 19th century and observations recorded by their instruments, careful analysis using geometric maps of the fault and precise deformation models has been conducted by Dr. Newman and his colleagues. This analysis gave them astounding insight into the tectonic activity in Nicoya and allowed them to forecast with reasonable accuracy the maximum likely magnitude and location of the next big quake. Timing, however, is difficult to pin down in earthquake science. With the team settling on a recurrence interval of about 50 years, and the last magnitude 7.0+ earthquake to hit the region occurring in 1950, another event was overdue as of 2012.
In almost every subduction zone, the most seismically active part of the fault lies far offshore. These faults, known as subduction megathrusts, are typically the largest faults on Earth. Consequently, they produce the most powerful earthquakes (the tsunami that caused the Fukushima disaster in 2011 resulted from a magnitude 9.0 earthquake that occurred at a subduction megathrust zone.) The “Ring of Fire” around the Pacific Ocean indicates the locations of many of these faults, which are created when tectonic plates collide and one subducts beneath another. When this subduction occurs, the top plate is pushed upward, often creating habitable land, while the bottom plate founders into the earth’s mantle. The volcanic activity characteristic to the Ring of Fire is due in part to the melting crust of the lower plate that results from the plate’s insertion into the hotter mantle. This interaction between plates also creates a deep trench where the beginning of associated subduction zones can be found.
The Nicoya subduction zone is unique in that the active megathrust fault is located directly beneath land rather than offshore. This allows for land-based seismic and GPS equipment to be deployed as opposed to ocean-based instruments, which can be cost-prohibitive and imprecise. These instruments allowed Dr. Newman and his team to conclude that the likelihood was high for an earthquake possibly reaching magnitude 7.8 to strike the Nicoya subduction megathrust zone. Their findings were published in July of 2012, just two months before the magnitude 7.6 earthquake struck the region on September 5th. It was later determined by Newman’s team that the size of the locked portion of the fault – the portion where the earthquake’s energy is stored – was slightly smaller than what their data showed prior to the earthquake. This is a possible cause for the 0.2 difference in their forecast. Monitoring of the region is ongoing.
The wealth of knowledge stemming from Dr. Newman’s research is one reason that he advocates for more resources being spent on developing better ocean-based earthquake monitoring technology. If it’s possible to forecast an earthquake like the one that struck Nicoya on September 5th for “little more than the cost of gas to get out there,” according to Dr. Newman, it stands to reason that similarly precise forecasting could be achieved for offshore subduction zones with the properly allocated resources.
More recently, Dr. Newman has been studying the Arenal volcano located in Costa Rica’s Guanacaste Province. Arenal is experiencing considerable subsidence on its western flank as the result of a 1968 eruption. This subsidence is alarming not only because of its potential to trigger landslides, but a rapid de-pressurization of volcanic magma chambers due to flank collapse may lead to a substantial and surprising eruption.
A flank collapse occurred under similar conditions at Mt. St. Helens in 1980, entering the history books as the largest landslide ever recorded and leading to the eruption there. It remains to be seen whether the subsidence at Arenal is stabilizing or destabilizing its western flank, but should the latter be the case, neighboring villages along with one of Costa Rica’s most productive hydroelectric dams could be destroyed.
Special thanks to undergraduate Sage Kemmerlin for her contribution to this article. She accompanied Dr. Newman to the Arenal volcano in Costa Rica this past spring to assist in his research there.