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Bringing 20 year volcanic crisis perspective – Recapturing: VOLCANO THIS WEEK

 

Bethel Methodist Church - demolished by the June 25 pyroclastic flow

Bethel Methodist Church – demolished by the June 25 pyroclastic flow

July 4, 1997

MVO reports

This week has seen the most devastating episodes produced by the volcano up to the present. In recent times pyroclastic flows have no longer been limited to the Tar River side of the volcano, but have been traveling in Mosquito and Tuitt’s Ghaut and over Gages into Fort Ghaut.

The seismicity during the early part of the week was dominated by hybrid swarms, which would intensify in both number and magnitude, culminating in continuous tremor, which would gradually subside into background noise. These swarms would last from three to as many as eight hours. There were few Long-Period events, almost no Volcano-Tectonic events while rock fall signals continued to be relatively abundant.

Dome views revealed a blocky surface with a number of spines. At night, glowing was sometimes observed. Material near the top was identified to be its source. At times there was strong steaming coming from the dome.

Pyroclastic flows continued to occur down Tar River, some of which reached to within 1 km of the delta. After an absence of just over a week, a flow occurred in Mosquito Ghaut on the 23rd.

The tiltmeter on Chances Peak, which was installed on May 23, and which had been showing a long period, low amplitude cyclic behaviour, superimposed on a relatively steady level background, moved to a higher frequency cycle, superimposed on a background that was moving down.

SO2 levels were found to range from 438-1170 tonnes/day. Background levels range from 200300 tonnes/day.

On the morning of the 25th seismicity continued at a high level with large hybrid swarms. There were several small pyroclastic flows in Mosquito Ghaut. At about 10:00 am an intense hybrid swarm started and merged into continuous tremor by 12:15 p.m. At about 1:00 p.m. major pyroclastic flow activity began in Mosquito Ghaut. The ash cloud quickly reached 30,000 feet. The flow traveled into the Farms River affecting Dyers, Streatham, Farrell’s, Harris’ and Bethel. Down river of Bramble, it fanned out between Trants and Spanish Point stopping just short of the sea. Windy Hill was affected by the surge. Material extended to within 1.5 km of Belham Bridge. A total of about 4 km2 was covered by pyroclastic flow and surge. The scar left by the collapse was spoon-shaped with a steep, back wall situated on the lower flanks of the dome above Mosquito Ghaut. The tilt had just about reached its lowest point and was on the down turn of a short cycle when the episode occurred. Unfortunately, given the presence of persons in the evacuated zone, it was clear that there was not only property damage but, also loss of life.

Windy Hill and Streathams have been dessimatedAbove and below – Windy Hill and Streathams have been dessimated

Windy Hill and Streathams have been dessimatedaThe following day the hybrid swarms continued but were reduced in duration and magnitude. The tilt dipped to its lowest position since installation and then began to rise. The frequency of the small cycles continued to be high and the amplitude also increased.

The following two days were each punctuated by significant events. On the 27th, there were two steam explosions around mid-morning. The second was larger and generated a 20,000 ft. ash cloud. Small rock fragments up to 8 mm in length were showered on Lover’s Lane, Dagenham, Richmond Hill and Foxes Bay. Because of the increased risk from pyroclastic surges Cork Hill, Weeks, St. George’s Hill, Delvins, Foxes Bay and Richmond Hill

On the 28th, there was a strong seismic signal at 03 :51 a.m. inferred to have been associated with a pyroclastic flow over Gages. Around 11:30 a.m., a hybrid swarm began and consisted of events larger than those seen in the recent swarms. This escalated into eruptive activity lasting over 11/2 hrs, producing a large volume of ash and several small pyroclastic flows in Mosquito Ghaut and a moderate flow in Fort Ghaut. This latter flow was later determined to have traveled as far as the vicinity of the hospital. The mass of this flow was confined to the ghaut but the surge clouds of hot ash covered the immediate areas adjacent to the channel resulting in the ignition of dried wood and rubbish close to the street.

ppPlymouth under siege – pyro flows are now beginning to affect the deserted capitol

Continuous pyroclastic flow activity resulted in deposition of a large amount of material in the ghauts mentioned earlier resulting in them becoming shallower with each new activity. This situation is likely to contribute to possible over-spilling of larger pyroclastic flows into some areas which hitherto had not been affected by the flows. In the past, a significant episode such as that of the 25th was followed by weeks, even months, of relative inactivity. So far this has not been the case with this latest episode. Under the circumstances, it is advisable that continued vigilance be maintained given the high state of instability, which is being demonstrated at the mountain.

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Bethel Methodist Church - demolished by the June 25 pyroclastic flow

Bethel Methodist Church – demolished by the June 25 pyroclastic flow

July 4, 1997

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MVO reports

This week has seen the most devastating episodes produced by the volcano up to the present. In recent times pyroclastic flows have no longer been limited to the Tar River side of the volcano, but have been traveling in Mosquito and Tuitt’s Ghaut and over Gages into Fort Ghaut.

The seismicity during the early part of the week was dominated by hybrid swarms, which would intensify in both number and magnitude, culminating in continuous tremor, which would gradually subside into background noise. These swarms would last from three to as many as eight hours. There were few Long-Period events, almost no Volcano-Tectonic events while rock fall signals continued to be relatively abundant.

Dome views revealed a blocky surface with a number of spines. At night, glowing was sometimes observed. Material near the top was identified to be its source. At times there was strong steaming coming from the dome.

Pyroclastic flows continued to occur down Tar River, some of which reached to within 1 km of the delta. After an absence of just over a week, a flow occurred in Mosquito Ghaut on the 23rd.

The tiltmeter on Chances Peak, which was installed on May 23, and which had been showing a long period, low amplitude cyclic behaviour, superimposed on a relatively steady level background, moved to a higher frequency cycle, superimposed on a background that was moving down.

SO2 levels were found to range from 438-1170 tonnes/day. Background levels range from 200300 tonnes/day.

On the morning of the 25th seismicity continued at a high level with large hybrid swarms. There were several small pyroclastic flows in Mosquito Ghaut. At about 10:00 am an intense hybrid swarm started and merged into continuous tremor by 12:15 p.m. At about 1:00 p.m. major pyroclastic flow activity began in Mosquito Ghaut. The ash cloud quickly reached 30,000 feet. The flow traveled into the Farms River affecting Dyers, Streatham, Farrell’s, Harris’ and Bethel. Down river of Bramble, it fanned out between Trants and Spanish Point stopping just short of the sea. Windy Hill was affected by the surge. Material extended to within 1.5 km of Belham Bridge. A total of about 4 km2 was covered by pyroclastic flow and surge. The scar left by the collapse was spoon-shaped with a steep, back wall situated on the lower flanks of the dome above Mosquito Ghaut. The tilt had just about reached its lowest point and was on the down turn of a short cycle when the episode occurred. Unfortunately, given the presence of persons in the evacuated zone, it was clear that there was not only property damage but, also loss of life.

Windy Hill and Streathams have been dessimatedAbove and below – Windy Hill and Streathams have been dessimated

Windy Hill and Streathams have been dessimatedaThe following day the hybrid swarms continued but were reduced in duration and magnitude. The tilt dipped to its lowest position since installation and then began to rise. The frequency of the small cycles continued to be high and the amplitude also increased.

The following two days were each punctuated by significant events. On the 27th, there were two steam explosions around mid-morning. The second was larger and generated a 20,000 ft. ash cloud. Small rock fragments up to 8 mm in length were showered on Lover’s Lane, Dagenham, Richmond Hill and Foxes Bay. Because of the increased risk from pyroclastic surges Cork Hill, Weeks, St. George’s Hill, Delvins, Foxes Bay and Richmond Hill

On the 28th, there was a strong seismic signal at 03 :51 a.m. inferred to have been associated with a pyroclastic flow over Gages. Around 11:30 a.m., a hybrid swarm began and consisted of events larger than those seen in the recent swarms. This escalated into eruptive activity lasting over 11/2 hrs, producing a large volume of ash and several small pyroclastic flows in Mosquito Ghaut and a moderate flow in Fort Ghaut. This latter flow was later determined to have traveled as far as the vicinity of the hospital. The mass of this flow was confined to the ghaut but the surge clouds of hot ash covered the immediate areas adjacent to the channel resulting in the ignition of dried wood and rubbish close to the street.

ppPlymouth under siege – pyro flows are now beginning to affect the deserted capitol

Continuous pyroclastic flow activity resulted in deposition of a large amount of material in the ghauts mentioned earlier resulting in them becoming shallower with each new activity. This situation is likely to contribute to possible over-spilling of larger pyroclastic flows into some areas which hitherto had not been affected by the flows. In the past, a significant episode such as that of the 25th was followed by weeks, even months, of relative inactivity. So far this has not been the case with this latest episode. Under the circumstances, it is advisable that continued vigilance be maintained given the high state of instability, which is being demonstrated at the mountain.