During April 2002-May 2003, monthly visits were made to Masaya for observations and temperature measurements. This report summarizes the recorded activity.

Between April and October the volcano continued to emit large amounts of gas. Tremors stayed consistently above 40 RSAM units. Seismicity was low, with fewer than 50 total earthquakes during the observation period; temperatures generally remained constant.

Fumarole temperature measurements in the Santiago crater on 22 April 2002 showed only a slight variation from October 2001. On 9 May, however, temperatures showed an increase of 20°C since April; again on 4 June a 20°C increase from May was observed. Measurements by Jaime Cardenas of the National Park at El Comalito and San Fernando on 10 and 30 April also showed little change from previous measurements. Similarly, on 5 and 21 May and in June measurements at El Comalito and San Fernando showed no significant changes. The temperatures at El Comalito and San Fernando fumaroles remained constant through the rest of the year.

In July 2002 tremor stayed above 40 RSAM units, and the volcano continued to emit great amounts of gas. Seismic stations registered 20 earthquakes. On 7 July several rumblings were reported. During a visit to the volcano emissions of dark-colored gases were seen. Landslides were observed to have extended to the inner crater, which had a diameter of 20 m; the diameter was 10 m when the crater opened on 23 April 2001. Gas emanations were abundant; a plume rising more than 1,000 m was observed. Fumarole temperatures varied between 106 and 89.3°C.

In August 2002 gas emissions continued. Martha Navarro and Virginia Tenorio visited on 15 August and observed and clearly heard gases emanating from two locations in the inner crater. Gas columns mixed with vapor reached heights of up to 700 m. The emission of gases was lower than during the previous month, possibly due to decreased rainfall. The tremor continued to stay above 40 RSAM units, and 11 earthquakes were registered.

Navarro visited the volcano twice in September. Gas columns were low and there was little vapor on 13 September; on 30 September she observed greater gas emissions and within the inner crater she could hear with greater force the sound of gas emissions. Weeds within an area of 600 m had been affected by acid rain. A small collapse along the N and E walls was observed within the crater.

On 3 October park guards reported a small collapse from the W wall. Observations on 7 and 28 October showed more water vapor than in September, as well as greater gas emissions and louder sounds associated with them. Through September and October tremor remained above 40 RSAM units; no earthquakes were registered. On 6, 16, and 18 December fumarole temperature measurements were taken with an infrared camera at Santiago crater; measurements on those dates were 216°C, 230°C, and 205°C respectively.

Through December 2002 and January and February 2003 fumarole temperatures at El Comalito and San Fernando remained constant. The low level of gas and vapor exhalation continued; columns reached as high as 100 m at the mouth of the crater. RSAM stayed constant at 30 units, with frequency between 1.5 and 2 Hz. In both January and February two earthquakes were registered. During 25 and 26 February there was a small earthquake swarm in Masaya caldera, with earthquakes located under the lake. Six earthquakes registered in March, with 3 Hz frequency, and five registered in April, with 2.8 Hz. RSAM stayed at 20 units, with frequencies between 1.5 and 2 Hz, in March and April.

Gustavo Chigna (INSIVUMEH-Guatemala) reported that the sulfur-dioxide measurements obtained using COSPEC on 28 March yielded a flux of 840 t/d. Measurements by Glyn Williams-Jones (University of Hawaii) with a 2FlySPEC (gas measurement spectrometer) on 28 March showed a value of 849 t/d. On 8 and 22 May measurements at El Comalito and San Fernando showed little variation. The temperatures at the six fumaroles at El Comalito ranged between 59.5°C at fumarole 6 and 76.4°C at fumarole 3. At San Fernando temperatures ranged from 56.4°C at fumarole 4 to 63.6°C at fumarole 2. The seismic tremor stayed constant with 20 units RSAM, with frequencies of 1.5-2 Hz. Only one earthquake registered, with 3 Hz frequency. Pedro Pérez measured the fumarole temperatures in the Santiago crater at 175°C on 15 May.

Gas monitoring. A scientific and technical team from ITER, INETER, and WESTSYSTEMS (Italy) installed a geochemical station, developed by WESTSYSTEMS, for continuous monitoring of diffuse CO₂ and H₂S degassing at El Comalito. The observation site was selected after a 1999 diffuse degassing survey at Masaya. The station has been in operation since 15 March 2002.

Geological Summary. Masaya volcano in Nicaragua has erupted frequently since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold" until it was found to be basalt rock upon cooling. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of observed eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Recent lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Virginia Tenorio, Wilfried Strauch, and Martha Navarro, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Nemesio M. Pérez, Instituto Tecnológico y de Energías Renovables (ITER), 38611 Granadilla, Tenerife, Canary Islands, Spain; Giorgio Virgili, WESTSYSTEMS, Via Molise, 3 56025 Pontedera, PI (Italy) (URL: http://www.westsystems.com/).