Seismicity
The level of seismicity has increased over the last week, although it is still at a low level. The seismic record continues to be dominated by rockfalls from the growing dome. Long-period earthquakes, up to magnitude 1.0, continue at a slightly increased rate of up to 28 per day. These events have a dominant frequency of 2 Hz, and their emergent character means they cannot be located. Since the evening of 25 March, hybrid earthquakes have reappeared, occurring at a rate of up to one every 5 minutes. These events are small (magnitude < 0.5) and recorded only at the closest stations, and thus they cannot be located. They are of similar character to previous hybrid swarms that were located at shallow depths (less than 2 km) beneath the crater. Two large rockfall signals were recorded at 08:23 on 23 March and 06:42 on 27 March. These were associated with rockfalls from the east of the dome into the upper Tar River / Hot River area. The 06:42 event on 27 March was the largest rockfall event recorded so far. It lasted for 28 minutes, and was composed of about 10 pulses. A later, smaller event at 17:25 may have been caused by the collapse of a spine. Six volcano-tectonic earthquakes have been located, scattered in locations to the north and east of the volcano mostly at shallow depths (less than 3 km). An unusual volcano-tectonic earthquake of magnitude 2.5 occurred at 5 km beneath Centre Hills on 27 March.
Deformation measurements and observations
All of the four EDM triangles were measured at least once during the week and, in addition, the line from Tar River to Castle Peak was also re-occupied. The lines continue to show slow deformation, at rates of 0.5 to 1 mm/day, consistent with trends that were established in late November. The lines to the Gages Wall were measured for the first time in 18 days on 21 March and showed rates of shortening similar to those being measured on other triangles. This rate of deformation supports the conclusions stated in the previous Scientific Report after visual observations that the Gages Wall is not under significant threat of collapse at present; laboratory strength tests will be undertaken to further quantify this conclusion.
A visual inspection and sampling of material from the crater wall above the Galway's Soufriere was undertaken on 26 March; this wall appears to be potentially more unstable than the Gages Wall and has already undergone some denegration during heavy rains in mid-December 1995. It is not thought that this wall is in imminent danger of collapse at the present time though.
The Long Ground electronic tiltmeter continues not to show any deformation events.
Visual observations
Visual observations were possible throughout the week from the helicopter and from various viewpoints around the flanks of the volcano and on Chances Peak. Dome growth has been ongoing in a number of areas, concentrating in the east for most of the week. Numerous ash clouds have been generated by rock falls within the crater area, and the first significant ash flow was generated by a rock fall at 06:42 on 27 March, having a run-out distance of about 1 km down the Hot River to the east of the crater.
Early in the week, rock falls were concentrated in the south and east parts of the crater, with no growth noted in the previously active area close to the Gages Wall in the northwest. Material being extruded in the east appears to be more friable than material elsewhere, which leads to rapid breaking up of the talus and generation of relatively large amounts of ash during rock fall events. Prior to 27 March, the most notable ash-producing event during the week occurred at 08:23 on 23 March; during this event, a channel was eroded by falling rocks down the east side of the old Castle Peak Dome to the north of Castle Peak itself and this channel has enabled rocks to fall progressively further into the upper reaches of the Hot River through the remainder of the week.
There has been no vertical growth of the high point on the dome during this period, although a tall spine on the east side of the dome did reach a greater height before collapsing. However, there are now several areas of equal height both to the east and south of the old high point above the Gages Wall and it is anticipated that much of the central part of the dome will rise to about the same height over the next few weeks assuming growth continues.
The major events of the week occurred on 27 March, when a major rock fall from the steep eastern flank of the dome at 06:42 produced a block and ash flow which travelled c. 1 km down the Hot River past the Tar River Soufriere. The ash generated during this event was blown westwards by the wind and caused darkness in Plymouth for several minutes. The ash fell along with rain, and the distribution axis was due west of the volcano. The rock fall was produced by a partial collapse of an unstable area of the eastern dome and involved an estimated 10 to 15 thousand cubic metres of material. This collapse and subsequent smaller rock falls partially undermined a tall spine atop the eastern dome, which subsequently collapsed, probably at 17:25 the same day. This second event also produced significant ash fall in Plymouth which was in the form of small (<3 mm) accretionary lapilli but was not accompanied by rain.
The early event deposited coarse blocks on a talus fan at the base of the old Castle Peak Dome at the head of the Hot River, with finer grained material flowing down the river channel itself. Burning logs were carried to the front of the flow but were probably set alight by coarse incandescent blocks close to source; no evidence was seen for charring of trees by ash further than the west end of the Tar River Soufriere, about 400 m from the source of the rock fall. Rock fall events later in the day produced ash which singed trees on the northern crater wall west of the Soufriere. Material from the 17:25 collapse is thought to have been cooler than that from the earlier events and did not generate a true block and ash flow, despite being of similar volume to the first event of the day.
Steam and gas production appears to have been continuing at a constant rate throughout the week, although covering of the main steam vent by new dome material has led to a more disrupted pattern of steam production from a number of small vents.
Summary
Dome growth at the Soufriere Hills volcano has continued throughout the week at a rate at least the same as that of the past two to three weeks. Activity has been concentrated on the east side, and the lack of a retaining crater wall led to generation of the first true block and ash flow from a large rock fall event, running out 1 km down the Hot River. Seismicity has been dominated by signals associated with rock falls, although a few volcano-tectonic earthquakes as well as long-period and hybrid events continue to be recorded. Deformation continues at a low rate.