Smithsonian Institution Global Volcanism Network Bulletin v. 20, no. 10, October 1995 Poas (Costa Rica) High seismicity Poas Costa Rica 10.46N, 84.71W, summit elev. 2,708 m All times are local (= GMT - 6 hours) During October, tremor at Poas reached 101 hours; the last time tremor rose over 12 hours/month was May-September 1994, an interval when tremor ranged between 49 and 307 hours/month. The number of minor earthquakes, which were predominantly of low frequency, continued to climb during the month of October, reaching 9,838 events. This was a value ~8% larger than the total for September, the previous month with the most seismic activity in 1995 (see histogram, Bulletin v. 20, no. 9). The crater lake has risen consistently: by ~5 m during June- October (ICE), and by ~30 cm in the last month (OVSICORI- UNA). During October 1995, the fumarole on the W terrace appeared to have decreased its emissions compared to recent months (< 50-m-high steam plumes), and others on the lake~s NW and SW sides also had diminished output. Fumaroles on the S and SW crater wall produced steam columns reaching 100 m tall. During October, bubbling in the lake still continued. During October OVSICORI-UNA scientists measured the temperatures at several sites: pyroclastic cone, 93 degrees C; fumaroles on the S and SW sides of the crater, 95-97 degrees C; the lake in the inactive crater (Lake Botos), 15 degrees C; and the lake in the active crater, 30 degrees C. Head scarps of landslides that emanate from the dome and flow toward the lake displayed ongoing mass wasting; ICE workers mentioned that this mass wasting may have been triggered by recent heavy rains. In addition, ICE reported that on 17 September (at 0548) a M 3.9 earthquake struck; it had a depth of 5 km and an epicenter 1.6 km SW of the main crater. At the summit, the earthquake~s intensity was MM III-IV. One of the most active volcanoes in Costa Rica, Poas is located 35 km NW of San Jose. The summit area contains several eroded calderas and two crater lakes, one clear and the other, to the N, colored. Information Contacts: OVSICORI-UNA (see Arenal); Mauricio Mora, Escuela Centroamericana de Geologia, Universidad de Costa Rica; G.J. Soto, ICE (see Arenal).