Activity at the Soufriere Hills Volcano between 18 and 24 April continues at an elevated level, similar to the previous week. Only one significant pyroclastic flow was generated, which was, like previous ones, confined to the Tar River valley. Seismic activity at the volcano continues at an elevated rate in the form of frequent hybrid events. Rockfalls were perhaps slightly more frequent in the later part of the week.
Visual observations during the week have been restricted by low cloud, although some observations were possible on most days. The activity in the crater has been dominated by the continued growth of the dome and rockfalls.
A new spine, which had grown over a period of no more than 36 hours, was seen on 18 April . The top of the spine was measured at about 911 metres above sea level and its height above the top of the dome was 30 metres. It was inclined towards the northeast. A smaller spine was observed on the morning of 19 April, with a height of about 20 metres. The large spine appeared to fracture on 20 April and the debris fell to the base of the northeastern part of the old dome. On 22 April, the spine was seen to have fallen over and to be lying at the top of an unstable slope on the north-eastern flank. A new small spine was seen in the same location as the large one on 24 April.
There were vigorous steam emissions from the dome throughout the week, particularly from a small area at the base of the southwestern flank of the dome.
Rockfalls were observed throughout the week, with the largest ones producing significant ash clouds which drifted mainly westwards over the Gages Valley area or towards the northwest. The largest events were at 12:37 on 18 April, 15:11 on 21 April and 06:35 on 22 April. The 21 April event generated an ash cloud which rose to a height of 1,700 to 2,000 metres above sea level and a small pyroclastic flow which traveled about 300 m down the Tar River valley and generated an ash cloud which rose to about 1,300 metres above sea level. The 22 April event possibly deposited ash in the Richmond Hill and Fox's Bay areas.
Seismicity at the Soufriere Hills Volcano during the week continues the trend seen since 9 April, with intense seismicity dominated by hybrid seismic events.
The number of hybrid earthquakes increased noticeably on 18 April, from a level of about 1 event every two minutes to about two events per minute. The seismicity then declined gradually from 19 April until the 24 April, finishing the week at about one event every two minutes. The decline was not smooth, there being slight fluctuations in the rate.
Broadband seismic tremor was recorded at times during the week, on the Gages and Chance's Peak stations nearest the crater. The duration of individual tremor episodes was normally less than an hour. The longest period of tremor was 8 hours, between 17:00 on 23 April and 01:00 on 24 April.
Signals from rockfalls have been recorded throughout the week. The numbers of these varied, but there was a slight increase in the second half of the week.
Three VT earthquakes were recorded, on the 20, 22 and 23 April. The first two were located north of the crater, beneath Farrells mountain at 0.25 and 4 km depth. The third could not be located.
The northern, eastern and southern EDM triangles were occupied during the week. Only the eastern triangle shows any changes above the precision of the technique. The lines to the reflector below Castle Peak from both Long Ground and White's Yard continue to show the slow shortening of about 1 mm per day which has been observed since the end of November 1995.
Installation of the GPS monitoring network on the volcano is now almost complete. Two base networks have been established. The first is a relatively large-scale network with line lengths of about 7 km, essentially the same as the network set up by the University of Puerto Rico. The second is a denser network of 18 stations on the flanks of the volcano, with an average inter-station spacing of 2 km. This covers most of the volcano, except for the southeastern sector. All stations have been established and reoccupied at least once. No changes have yet been detected above the 1 cm precision of the technique.
MVO has started a programme to investigate the structure of the new lava dome at the Soufriere Hills Volcano. Accurate angular measurements of features on the dome have been combined with measurements made from photographs to build up a topographic model. This has been compared with a digital terrain model of the old English's Crater and gives a dome volume on 18 April of 9.5 +/- 0.5 million cubic metres. This volume gives a mean extrusion rate of approximately 70,000 cubic metres per day, since 30 November 1995. This is about one fifth of the mean rate for Unzen Volcano, Japan, in the 1990-1995 dome-building eruption.
The measurements and photographs will be repeated at intervals to monitor dome growth. Previous measurements and photographs will be analyzed in order to get accurate figures on past growth rates.
A new geological map of the dome has been drawn up. This shows where the areas of active growth are located and where the areas of instability are.
Origin of eruption columns
There has been some uncertainty as to whether some of the larger observed ash columns have been generated by explosions. Two observations indicate that the explosive component cannot be significant. The recent ash deposits are uniformly fine-grained, with no clasts above ash-size getting outside the crater. This is inconsistent with an explosive model, where larger ballistic clasts and deposition of lapilli might be expected. The second observation comes from a video of one of the smaller pyroclastic flows. This shows a sizeable thermally-convective column being generated when the flow hits the crater wall. Thus the evidence so far indicates that the ash columns are generated from the pyroclastic flows and rockfalls and not explosions.