Since the last report of activity through May (BGVN 22:05), and through September, seismic and explosive pyroclastic activity remained vigorous. Blocky lavas continued to emerge from the active summit crater, Crater C, although a minor pause took place. Crater C continued to aggrade as lava and pyroclastic deposits accumulated on its floor. An estimate during June put the aggradation rate at ~1 m/month.

The passing of a tropical storm during June brought a shift in winds and caused ashfall at Fortuna, a settlement ~6.5 km E of the summit. N-flank lavas emitted in May traveled NW. During June these lavas advanced down to the 1,100-m elevation, and during July to 850 m. Around noon on 23 June incandescent avalanches descended the N flank to 850 m elevation. Associated ash blew W.

During August newly erupted lavas were absent, though pyroclastic flows were frequent. One, on 19 August, traveled S and another, on 20 August, was heard heading NW for a duration of 50 seconds. August also brought acidic rain and rain-triggered mass wasting. On 5 and 7 September observers noticed W-flank avalanches that traveled downslope to the 950-m elevation. In their upper reaches, these avalanches carved a small notch in the crater's western border. During the last week of September escaping lavas followed this avalanche track to 1,450 m elevation.

Survey of the distance network during June disclosed a monthly contraction of 2-4 ppm. This change was consistent with that seen over the past four years.

During the last week of June, seismic signals had variable amplitudes, periods of under 3 Hz, and continued unbroken for durations up to 400 seconds. These signals accompanied puffing and the expulsion of gas and ash. The same seismic signals repeated again in July but these had even longer unbroken durations of up to 800 seconds.

Unusually vigorous seismicity during the first half of June included an explosion detected at OVSICORI- UNA seismic stations over 150 km away. Such a seismically energetic explosion had not occurred during the past 3 years.

Gerardo Soto (OSIVAM) noted that many Arenal eruptions were relatively mild and quiet "mute events" that sent ash clouds to 0.5-km altitudes. During July, explosions occurred every half hour, whereas during early August, they occurred every ten minutes or less. Soto also confirmed the above-mentioned absence of NNW-flank lavas escaping in late August. OSIVAM found that temperatures and acidity in the springs around Arenal had remained stable.

The OSIVAM seismic system (at Fortuna Station, 3.5 km E of Crater C) measured the average numbers of earthquakes and hours of tremor/day (table 20). During this interval, 78 earthquakes/day occurred on average, about one every 18 minutes. Tremor prevailed ~34% of the time.

Table 20. Daily averages for earthquakes and tremor duration registered by OSIVAM, January-June 1997. Courtesy of G. Soto.

Soto also noted that during July, lava flows issued from the same vent high on the N slope that fed lava during the past year. Finally, Soto presented annual estimates on the rates and volumes of erupted products from Arenal during 1992-96 (table 21).

Table 21. Volume and rate of lava effusion at Arenal, 1992-96 (Soto, in press).

References. Soto, G.J., 1996, La actividad del volcán Arenal durante 1995: Boletín OSIVAM, San José, Costa Rica, v. 8, no. 15-16, p. 56-61.

Soto, G.J., in press, La actividad del volcán Arenal durante 1996: Boletín OSIVAM, San José, Costa Rica, v. 9, no. 17-18.

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: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Saenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.