Activity was at a low level during January with a total of 106 explosions (C-type earthquakes) recorded (figure 9). Daily maximum tremor amplitudes increased slightly. A significant increase in explosive (Strombolian type) activity occurred 11-14 February. A total of 377 explosion earthquakes were recorded in February; tremor remained at January levels.

Figure 9. Seismicity recorded at Arenal at station FOR, January-March 1987. Number of daily volcanic earthquakes (top) and maximum daily tremor amplitude (bottom). Courtesy of ICE.

The number of explosions decreased in March but their energy increased considerably; 231 events were recorded. Bombs were ejected at velocities of >100 m/s reaching 1,800 m NW and 1,000 m W of the active crater. Several explosions were recorded at seismic stations >100 km from the volcano. Daily maximum amplitudes showed a significant increase.

On 7 and 8 March, a shallow earthquake swarm was registered by National Seismological Network stations. The 15 tectonic-like events had epicenters 15 km E of Arenal. Three earthquakes were felt by residents of La Fortuna (figure 10) at MM II-IV. The strongest reached magnitude 4.2 (figure 9). On 18 March a large ash eruption occurred.

Figure 10. Sketch map of Arenal and the surrounding area, showing locations of seismic stations and epicenters of the 7-8 March 1987 earthquake swarm. Courtesy of ICE.

Geological Summary. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: R. Barquero and Guillermo Alvarado, ICE.