The following condenses scientific reports from Montserrat Volcano Observatory (MVO) for 12 April-10 May 1998.

Summary. Activity during the reporting period continued at low levels: there were no changes in dome morphology and only a few pyroclastic flows occurred. Seismicity was generally low, with occasional volcano-tectonic (VT) earthquakes being the predominant signals recorded by the seismic network. Rockfall activity was particularly low but showed an increase after heavy rains. Several mudflows were generated during the reporting period, most of them moving down Dyers River into the Belham River valley.

Visual observations. Fresh pyroclastic-flow deposits were seen along the N side of the Tar River Valley on 14 April. A small pyroclastic flow occurred during the morning of 19 April and was seen by the remote video camera at MVO as it traveled down the Tar River as far as the site of the Tar River Estate House. Another small pyroclastic flow coursed down the Tar River on the morning of 25 April, but could not be seen from MVO due to low clouds. Three more small pyroclastic flows traveled halfway down the Tar River Valley during 26 April and the morning of 27 April. All of these flows were believed to have originated on the steep upper flanks to the E of the old dome. Each event lasted 2 minutes and generated small ash clouds.

Rockfall activity was minor with small rockfalls occurring down the E and SW flanks. Some of these events are gradually carving deeper chutes on the Tar River side and S flank of the new Galway's dome. Minor rockfall activity also began near the top of the dome over Galway's wall and traveled down a chute on the S flank of Chances Peak.

During a brief clearing in the weather around the dome early on 6 May observers on a reconnaissance flight saw no evidence of new growth, suggesting a lack of significant extrusion since the growth of the summit spine around 10 March (BGVN 23:04). They did see moderate fumarolic activity coming from a point in the main chute on the upper E flank, and in several areas within the trench located between the scar of 26 December (BGVN 22:12) and the fresh growth within it. They also noted that the summit area appeared blanketed by over 5 m of tephra including both fine ash and blocks of glassy dome rock (up to 1 m diameter).

The temperatures of the pyroclastic flows deposited at Trant's during the 21 September collapse (BGVN 22:10) were measured on 28 April. A maximum temperature of 348°C was obtained at a depth of 2 m. They showed only very minor changes since they were last measured 2 weeks ago.

Seismicity. Over the reporting period, seismicity remained low. Volcano-tectonic (VT) earthquake activity continued to be dominate (table 28). VT earthquakes mainly occurred in groups too few in number to constitute swarms, but exceptions to this are shown in table 29, including a swarm of hybrids on 6 May. These were the first hybrids of high amplitude seen for many weeks, but were not followed by others of similar type.

Table 28. Earthquake counts at Soufriere Hills listed by type (based on signal character), 12 April-9 May 1998. These counts were of events that triggered the broadband network's event-recording system between 0000 and 0000 each day. The type "Dome RF" denotes a dome rockfall. The type "Long-period / Rockfall" signifies a Long-period earthquake followed by rockfall signal. "Hybrid / Rockfall" is a hybrid earthquake followed by rockfall signal. Courtesy of MVO.

Table 29. Swarms registered at Soufriere Hills during 12 April-10 May 1998. Courtesy of MVO.

Epicenters were located on the E of the volcano at focal depths tightly clustered from 2.5 to 3.5 km below the summit. Fault-plane solutions were calculated using P-wave first-motions detected by the 7 broadband stations along with first motions from the Lee's Yard and Jack Boy Hill stations of the short-period network. The calculated fault-plane solutions are consistent with a strike-slip fault mechanism. The number of recorded rockfall signals was very low. However, in many cases there was a correlation between occurrence of the rockfalls and periods of heavy rainfall.

Ground deformation. With respect to the Harris GPS measuring station, the stations at Dagenham, Old Towne, Lookout Yard, and Windy Hill showed height increases of 5, 5.5, 6, and 4 cm respectively since December 1996. These values are preliminary, as the height component is the least well constrained by GPS. It was judged more likely that the reference at Harris was actually sinking. Height differences between Harris and sites on the E (Long Ground, Tar River and Perches) all showed continued slow movement to the NE of around 7 cm in the last year; Whites and Roches have moved slightly less and in different directions.

A survey from Windy Hill measured the distance to the N crater wall reflector and found it had shortened by only 1 cm since the middle of March. The line to Windy Hill from Harris is stable, as confirmed by repeated measurements since December 1997 that gave site positions lying within a box 3 mm by 7 mm. In contrast, the survey point at Brodericks had shown accelerated movement: 3 cm to the N between November 1997 and January 1998. This coincided with the period of rapid extrusion in the S area of the dome during December, 1997. Subsequently Brodericks appeared to stabilize in its new position.

A new permanent GPS site was installed in the South Soufriere Hills. Telemetry equipment used by the station was installed by the University of Puerto Rico on Antigua and in the Centre Hills.

Volume measurements. A new theodolite site known as Fergus Ridge was set up on the high ridge of the W flank of South Soufriere Hills, to the N of Fergus Mountain, overlooking the White River Valley. Measurements from this site triangulated with measurements from Perches Mountain were obtained on 16 April. In conjunction with the combined photo and GPS data collected on 6 April, a revised total dome volume was calculated to be 113 x 10⁶ m³. This figure differed from the initial estimate of 120 x 10⁶ m³; however, the revised figure incorporated a greater number of theodolite, photo, and GPS points that improved constraints on both the summit area and the new dome on the SW sector of the complex.

Environmental monitoring. Generally, low volcanic activity and the number of rain showers kept aerosol levels low through the reporting period. Extremely wet weather, 14-15 April, produced the lowest aerosol levels since the heavy ash fall at the beginning of February. Rain also prevented the ash produced by the small pyroclastic flow of 19 April from being transported N by wind to any of the sites that were being monitored.

The volcano's small ash output left inhabited N island areas comparatively ash free. Each disturbance of ash by moving vehicles seemed to help the wind and rain remove more ash.

The three pyroclastic flows that occurred on 26-27 April had no effect on the measured levels of airborne ash and dust. On 1 May observers saw a very small venting of ash escape at the top of the Tar River Valley. Scientists working in the SW of the island over the next few days noticed a strong smell of rotten eggs (hydrogen sulfide). Following the hybrid swarm on 6 May dust levels remained low, but aerosol levels doubled. Heavy rain two days later once more reduced levels. Aerosol levels continued low later in this period despite drier weather, except in Salem, an area likely affected by ash blown W from the pyroclastic flows.

Geological Summary. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/).