Seismicity
Seismic activity during the weekly review period continued at the same general level as during the past few weeks. A very low number of volcano-tectonic earthquakes, numerous long period earthquakes, episodes of broadband tremor (recorded on the Gages seismograph station) and frequent signals associated with rock falls and small eruptions were recorded by the local seismometer network. Numerous repetitive hybrid earthquakes were also detected at the beginning of the week.
The week began with the occurrence of numerous small repetitive hybrid earthquakes being recorded by the seismograph stations at Gages and Chances Peak. They occurred at a rate of about one every 90 seconds from around 19:30 on 21 February until 20:00 on 22 February and then decreased in both rate of occurrence and amplitude thereafter to the point that they were being recorded on the Chances Peak seismograph station only.
Three locatable volcano-tectonic earthquakes were detected by the seismograph network during the week under review. These occurred at 05:47 on 22 February, at 05:05 on 26 February and at 00:16 on 28 February; instrumental locations at depths of between 2 and 5 km beneath Long Ground were obtained. Further volcano-tectonic earthquakes were observed on 28 February but they were all too small to constrain an instrumental location.
The broadband tremor, recorded on the seismograph station at Gages, continued during the week but at a generally lower amplitude level than previously observed. Some episodes of increased amplitude tremor, however, were still seen on several occasions during the week. The longest of these episodes started at 17:00 on 25 February, lasting approximately 12 hours. The intermittent periods of increased amplitude tremor are thought to be due to an increase in steam venting from the Castle Peak dome area.
Long-period earthquakes were observed at a relatively high rate throughout the week. The majority of these earthquakes were too small to obtain an instrumental location as they were only recorded on the two nearest seismic stations to the volcano. These smaller long period events occurred approximately every minute at their most frequent. Larger earthquakes of this type, however, were detected at a rate of about two to three per hour on several occasions during the week. Instrumental locations were obtained for many of the larger (magnitude 1.0 to 1.8) long period earthquakes; they all occurred beneath the volcano in the depth range 0 to 3 km.
Throughout the week, numerous seismic signals generated by rock falls from the lava dome were recorded on the seismograph stations nearest to the volcano (Gages and Chances Peak). Some of these signals were visually correlated by scientists and MVO staff either from the helicopter or from around the crater rim. Larger rock fall events were also recorded on more distant seismometers, and several of these events produced small ash clouds which were clearly visible from the Observatory, the Airport and the Long Ground area. Light ash falls were reported in the Plymouth and surrounding areas after two of these larger rock fall events; at 23:55 and 03:31, on 23 and 24 February, respectively.
Deformation measurements
All of the 4 EDM triangles around the flanks of the volcano were measured at least once during the week. Deformation on all of the triangles continues to be slow but consistent, with daily shortening of lines to the fixed reflectors high on the flanks of the volcano of the order of several millimetres per day. This movement is within the error of the technique on a daily basis but has established a long-term trend which is clearly significant and suggests continued slow deformation of the edifice as a whole. Neither the Castle Peak nor Gages Wall reflectors show any greater movement than the reflectors further from the area of dome extrusion, indicating that the surface manifestation of activity is not causing local deformation in itself. The key occupation of the sites to the Gages Wall for the first time since 14 January on 27 and 28 February provided reassuring data indicating that the wall is moving only as part of the whole deformation field.
A leveling line in the Amersham area to the west of the volcano was established fully during this period; first measurements on this line will take place once the concreted pins have settled in their positions.
Visual observations
A series of days with good visibility enabled reasonable viewing conditions in English's Crater, although high steam and gas production throughout the week prevented shooting of stereo vertical photographs for photogrametrical analysis. A general vertical and outward growth of the dome can be noted on a weekly basis, and local growth areas are indicated by semi-continuous rock falls. The central, northwestern and, to a lesser extent, southeastern sectors of the dome are the most active areas at present; rocks falling from the dome are glowing at night and several large cracks in the central part of the dome are strongly incandescent. Rock falls on the dome at night are now visible from much of the Plymouth and Richmond Hill areas in addition to the eastern and central villages. Although accurate volume estimates remain difficult, dome growth rate appears higher at present than it has been at any time so far.
Larger rock fall events continue to produce sufficient ash to be noticeable within the steam plume, and a big fall from the southern part of the dome produced several light ash falls in Plymouth on 23/24 February.
Close visual inspection of the new dome was possible on 27 February during a gas sampling visit. Copious gas and steam production is occurring from areas both on and surrounding the new lava dome, with native sulphur deposits noticeable in several areas. The most active area of gas production is from one of the early areas of dome extrusion within the main phreatic (18 July) vent dating from earlier phases of activity. Gases were sampled at 720C from a vent here, and red-hot rock was seen about 2 m beneath the surface. Rock was sampled from three areas of the dome; no obvious differences in crystal content or number of mafic inclusions were noted for any of these areas. Inspection of the outer skin of one of the large spines extruded in early December 1995 revealed a coating of crush breccia with slickensides.
Gas and water sampling
Gas sampling took place at three of the major soufrieres around the volcano during the week as well as on the active dome. Soufriere gas compositions have changed very little through the course of the activity, although only the Galway's Soufriere has been sampled in recent months; results from these analyses is expected within a couple of weeks. Testing of rainwater continues on a weekly basis, with pH fluctuating between about 2.5 and 3.5 in the upper Amersham area directly beneath the volcanic plume. Testing of drinking water at source springs on the western flank of the volcano revealed little or no influence from acids from the volcano and the volcanic plume has been at a higher elevation during the week so that the gas haze has been absent from the Gages valley for much of the time.
Summary
Despite a relatively low level of seismic activity during the week, dome growth continues at a rate which is currently higher than at any time in the past. Steam and gas production remain high and rock falls common, the larger ones recorded on at least the four nearest seismometer stations. Deformation of the edifice remains extremely slow though continuous, and no local deformation has been noted for the two targets closest to the area of dome extrusion.