A system is in place to document seismic activity on the Nicoya Peninsula where experts expect a major earthquake soon, according to Marino Protti of the Universidad Nacional’s seismology institute.
The network consists of 19 GPS monitoring stations and 18 seismographs. Protti emphasized that it is not intended to predict the expected quake, just gather data for long-term analysis.
The seismographs record the usual sort of earth tremors that are part of any earthquake, while the GPS monitors measure the movement of the Cocos tectonic plate which is sliding under the Caribbean plate at a rate of about 90 mm (3.5 inches) per year. This movement has produced major earthquakes in Guanacaste in 1853, 1900, and 1950. By that measure one is somewhat overdue, and Protti said its anticipated magnitude is about 7.7 to 7.9.
The fault in question is about 100 kilometers (62 miles) long by 60 (37 miles) wide, and runs roughly up the Río Tempisque valley and then the northernmost coast of Guanacaste into Nicaragua. No tsunami is likely in Costa Rica since the fault is mostly under land. To generate a tidal wave requires significant displacement of the sea floor itself.
The development of GPS technology has revealed a previously unknown phenomenon which shows slow slip behavior along faults of this type. This is as if there was an earthquake but the energy is released over a period of days or weeks instead of minutes. The large Cascadia fault off Oregon and Washington has given researchers a demonstration of this behavior. It is also happening here in Costa Rica, Protti said.
The previous Guanacaste earthquakes were all in the 7.0 range. The strongest in living memory anywhere for most Costa Ricans was in 1991 which was about 7.5 magnitude. That had its epicenter in a lightly-populated area in the Talamanca mountains near the Caribbean coast, but still caused heavy damage to bridges and roads in the area, and resulted in about 120 deaths in the southernmost coastal area and adjacent Panamá. Protti said the anticipated Guanacaste quake is likely to be about twice as powerful as the 1991 event.
Protti mentioned a similar fault lies on the Peninsula de Osa, which also has a fast-moving subduction fault. This one produced earthquakes in 1904, 1941, and 1983. The fault is much shorter and produces smaller quakes than the Nicoya zone.
Protti is with the Observatorio Vulcanológico y Sismológico at the university.