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Table 2 Timeline of the volcanic crisis from July 1975 – June 1977

From: Retrospective analysis of uncertain eruption precursors at La Soufrière volcano, Guadeloupe, 1975–77: volcanic hazard assessment using a Bayesian Belief Network approach

Date Eruptive outcome Key analytical & recorded observables Key scientific decisions Key official decisions
24 July 1975 – 7 July 1976: non-eruptive unrest phase: escalating seismicity (3189 recorded VTs, 64 felt VTs)
31 Jul 75   Seismic swarm 1 24-26/07 (30 VT, 1 felt; 3 stations) 2x 19 year baseline of 15VT/month, 0.2 felt/month Scientific vigilance – authorities informed  
Nov-Dec 75   Seismic swarm 2: 13/11 to 30/12 (296 VTs, 4 felt) 20x baseline 13/11: MF warns Préfet unrest could lead to eruption 13/11: Préfet requests Volcano Contingency Plan (VCP)
10 Jan 76   Decline in seismic activity to 2.6x baseline followed by increase 19/01: MF 2nd note to Préfet informing on increasing seismicity (number of VTs and energy) 10/01: First draft of VCP produced; 4/02: VCP to local authorities for confidential review
24-25 Mar 76   Onset seismic swarm 3: 220 VTs, 12 felt VTs in 24 hr. Total for swarm 3 from 1/03 to 07/07/76: 2713 VTs, 59 felt, 45x baseline   25/03: French government alerted; 1st local press article on unrest - spontaneous evacuation of 200 persons
29-30 Mar 76   March: marked increase in seismicity, 607 VTs, 22 felt, 41x baseline 30/03: HT: 48h visit with GA & GJ: HT gives reassuring assessment, uncertain unrest will lead to eruption, but given an eruption low probability that it will be “cataclysmic” 29/03: Préfet holds crisis response meeting; VCP officially issued defining risk area & evacuation centres; Préfet requests risk map from BRGM
1-12 Apr 76     1-12/04: Préfet reviews VCP with town officials
22-23 Apr 76   120 VTs, 7 felt in 48 hr 20/04: meeting gov Paris, enhance observatory capacity, HT: unrest not exceptional cannot exclude serious outcome, important precautionary measures needed; 23/04: MF reassuring, situation serious but not critical, nothing likely ≤ 4 months given current seismicity trend; 30/04: MF eruption probable July-August if unrest continues 23/04: Préfet press conference, VCP made public; 30/04: Overseas Minister visits Observatory with National Civil Protection to validate VCP, national VCP on file at Paris Civil Protection Office; 22/05: President of France visits, requests zero risk policy
April: seismicity still increasing, 747 VTs, 14 felt, 50x baseline
27-28 May 76   100 VTs, 4 felt in 48 hr;   
   May: 611 VTs, 9 felt, 41x baseline   
4-9 Jun 76   4/06: 169 VTs; 8-9/06: 65 VTs 10 felt in 24 h; 9/06: landslide on La Ty fault SE dome fissures on road base dome aligned La Ty fault 02/06: new BRGM risk map sent to Préfet; 14/06: MF note to Préfet, risk map exaggerates risk of PDCs, revision to include ashfall & mudflow risk  
02 Jul 76   June: 668 VTs, 14 felt, 45x baseline MF 2nd report to authorities on crisis upon return from Paris  
8 July – 10 November 1976: Phase 1: 17 explosions, escalating seismicity (11649 VTs recorded, 68 felt VTs), ash & gas venting, mudflows
08 Jul 76 EX1 8:55 1st and largest EX. Unexpected "blue sky" paroxysm, 48 min tremor, 20 min darkness, new 400 m fracture, cold PDC 1 km, aquifer resurgence, lahar 3.5km, ashfall. Escalating seismicity MF: reassuring assessment, stronger than 1956, phreatic, no immediate danger Spontaneous panic evacuation of 25,000, no official order. Traffic regulated. No implementation VCP 1st alert level.
09 Jul 76   Acid vapour jet, pH 1.1-3.2, strong H 2 S odour to coast, SO 2 ? 609 VT/month, 12 felt VT/month last 3 months (8/04-8/07)   9-10/07: Préfet evacuates hospitals (1400) and prison. 10/07: Préfet asks population to return to normal life
13 Jul 76   5 felt VT, 14/07: 14 ppm H2S ambiant air 3 km SW of crater;   
24/25/27 Jul 76 EX2, EX3, EX4 July: Seismicity escalating 1220 VTs, 13/07: HT + RB + 8 scientists arrive + BRGM; HT downplays risk; no magma, precursors to warn of paroxysm; probability of Pelean PDC extremely low, others disagree; 26/07: HT reassures Préfet and leaves for Ecuador, recommends JT to Préfet if activity increases again 22/07: virtually all evacuees back, normal life activities; 25/07: new spontaneous evacuation Matouba (St Claude) 29/07: financial compensation to farmers for ashfall
9 Aug 76 EX5 20 felt, 81x baseline
12 Aug 76 ash venting 40% fresh vesicular glass ash 10/08: JT: P(magma present) = 0.25 ; P(large eruption given magma present) = 0.5 Matouba, Papaye, St Claude evacuated
13 Aug 76 ash venting 1194 VT/month, 17 felt VT/month in 3 months (13/05-13/08) Identification of 40% fresh vesicular glass in ash 13/08: RB returns confirms MF + JT, serious escalating unrest, imminent eruption possible, magma at depth 17:30 Préfet evacuates ill, elderly, Baillif; 19:30 Préfet evacuates all S Basse-Terre except civil servants. Approx 33600 evacuated since 12/08
15 Aug 76    MF, RB, JT request urgent meeting of officials "irreversible process, directed PDC impact possible anywhere, risk area must be evacuated" 8:40 Préfet orders phase 2 of VCP & evacuation of 73600 in <24h; remaining 40000 evacuated by 21:00, no injuries. Approx 2000 refused & remain in danger zone
16 Aug 76   725 VTs in 24 h, strongest VT of the crisis, Md = 4.2 (Intensity VI), felt Pointe-à-Pitre 60 km away RB: magmatic gases present, expects EX of 30Mt Préfet orders 24h evacuation of Volcano Observatory, and states a probability of 1 in 10,000 that nothing happens
21/25 Aug 76 EX6, EX7 24 Aug: 1527 VTs 24 h Strong felt VT, Md = 3.9 (VI) Pointe à Pitre   
29 Aug 76   Installation of 2 tiltmeters (USGS 28/08). August: 5989 VTs, ≥ 41 felt, 440x baseline; over last 3 months 2473/month, ≥ 25 felt VT/month (1/06-29/08) 28/08: CA + GA: arrive; 29/08: HT arrives, downplays risk & contradicts RB  
30 Aug 76 EX8 2st largest New fracture splits dome SE, cold PDC 1 km, ballistics & ashfall EX while scientists (CA, HT, GA…) at summit, HT injured  
31 Aug 76   85% glass magmatic origin in 30/08 explosion tephra Installation of tiltmeters by Los Alamos team (LANL) HT reassuring, people can return to work during the day without immediate risks; RB + JT leave  
14 Sep 76 EX9 small laterally-directed blast to 1.5 km SE-S, cold PDC 0.5 km, ballistics & ashfall, aquifer resurgence and 8 km lahar 15/09: Official scientific report (GA + CA): evidence precludes any optimistic interpretation of evolving unrest, major events likely in next few months/weeks, dangerous outcome threatening population cannot be excluded; 6/09: official Volcanic risk map (BRGM) issued to Préfet, part evacuated area (Capesterre; Vieux Habitants) now considered much less threatened; 19/09: CA arrives, reports to scientific team absence fresh glass & to Préfet on 20/09; 21/09: CEA report shows no evidence new magmatic gas; 29/09: press conf RF states unrest evolution needs to be monitored carefully 3/09: Local authority CG states situation remains critical, recommends evacuation maintained for 30 days; 6/09: road blocks open 5h30-18h to allow people to return to work in all areas except Basse-Terre, Gourbeyre, St-Claude where individual authorisations are required; 15/09: Préfet designates safe area for day & night time access with new risk map (Capesterre; Vieux Habitants), school restarts in Vieux Habitants, parts of Capesterre; 28/09: installation of siren network where daily access permitted for authorised activities & partial reoccupation
   Mostly H2S in fumaroles, no or little SO2, CO2, low F
19 Sep 76   Los Alamos reports total absence of “fresh glass” in 8-July-76 ash
22 Sep 76 EX10 Sept: 1716 VTs, 4 felt, 114x baseline
2/10/30 Oct 76 EX11, EX12, EX13 No fresh glass in ash, no deformation, tilt not useful 5/10: HT against IPGP’s erroneous report fresh glass 8/10: Given decrease unrest & lack magmatic components in erupted ash, MF agrees opening large part safe area (E-SE: Capesterre; Trois Rivières, Gourbeyre, parts of Basse-Terre); 14/10: BM reports lack magma involved, stable gas chemistry and VT hypocentral depths; likelihood rapid onset major eruption decreased, advises continued caution. 26/10: LS favourable reopening Basse-Terre to Baillif area given new analysis and expert opinions since 5 Sept report; 27/10: Scientific Board of IPGP dismisses HT as Head of the Volcano Observatories office of IPGP. HT resigns from IPGP; LS: no evidence magma, no immediate elevated risk 1/10: Day & night access OK as of 4/10 in Capesterre, Vieux Habitants, Trois-Rivières & Gourbeyre, with some areas assessed on individual request; access 5h-18h only in Basse-Terre, St-Claude, parts of Gourbeyre & Baillif;. 8/10: Préfet request to the Minister of the Interior that a international group of experts analyse the situation and formulate recommendations; 11/10: Préfet announces his intention to reopen as of 8 Nov schools in Trois-Rivières and Capesterre and daily activities in Basse-Terre without authorisation; 27/10: Préfet reopens Baillif, Basse-Terre and lower part of Saint-Claude for daily access without authorisation
   12/10: report by Los Alamos shows tilt data is difficult to interpret, confirms lack of magmatic components in the ash, but the increase of sulfur in gases needs to be monitored carefully
1/6/7/10 Nov 76 EX14, EX15, EX16, EX17 Oct: 2315 VTs, 3 felt, 154x baseline
11 November 1976 – 4 January 1977: Phase 2: No Explosions, ash & gas venting, lower seismicity (968 recorded VTs, 9 felt VTs), partial reoccupation of evacuated zone
15-18 Nov 76    17/11: LS declares end crisis, low seismicity & ash emissions; 15-18/11: International Scientific Committee (ISC) validates Aug evacuation, phreatic or deep magma intrusion, estimates low direct risk, recommends return of population 18/11: Overseas secretary declares safe to open all areas including St Claude as of 1 December. Progressive return of ill & administration, Basse-Terre schools to reopen 2/1/77 – only 7000 evacuees left in relocation centres
30 Nov 76   Nov: 1040 VTs, 0 felt, 69x baseline; 1843 VT/month last 3 months, 0 felt (30/08-30/11)   Population from St Claude & Basse-Terre unwilling to return given frequent ashfalls & degassing
1-15 Dec 76   Strong gas jet at summit, gas temperature 184°C, ejection blocks; Dec: 399 VTs, 6 felt, 27x baseline   15/12: End of evacuation, partial reoccupation
5 January – 1 March 1977: Phase 3: 9 explosions, ash & gas venting, declining seismicity (475 recorded VTs, 7 felt VTs), total reoccupation of evacuated zone
5/13/14 Jan EX18, EX19, EX20 Frequent ashfalls on upper Saint-Claude, lapilli size ballistics to 1.3 km SW on 29/01; strong roaring; gas (H2S; SO2) odour; Jan: 312 VTs, 4 felt, 21x baseline   29/01:Matouba & Papaye villages closest to volcano to SW isolated overnight due to ash on road, people worried
15/17/29 Jan EX21, EX22, EX23
13/15 Feb EX24, EX25 Ashfall on upper Saint-Claude; Feb: 179 VTs, 3 felt, 12x baseline   
01-Mar 77 EX 26: Last & 3 rd largest Ashfall on upper Saint-Claude, Basse-Terre, and Baillif; 4/03: potable water contamination by ash. SO4-2 > maximum accepted; F- almost to maximum accepted. 297 VT/month (20x baseline) 4.3 felt VT/month (1/12-28/02)   
2 March – 15 June 1977: Post-eruptive unrest phase: no explosions, no ash venting, declining seismicity (212 recorded VTs, 5 felt VTs)
28-Mar-77   Reported stinging in nose from SO2 at base of dome; March: 153 VTs, 4 felt, 10x baseline   
15-Jun-77   April: 32 VTs, 0 felt; May: 19 VTs, 1 felt; June: 15 VTs, 0 felt, pre-crisis baseline level. Sustained degassing at summit & periphery   Eruption officially ends, clinics and hospitals reopen
  1. Dates in bold are pivotal dates when probabilities of eruptive outcome was queried using the BBN (Figure 5). Seismic data within each phase of the eruption is discontinuous and given only on specific key dates, the total seismicity is given in the heading of each phase. Detailed data is available in references below. EX: phreatic explosion; PDC: pyroclastic density current; RB: R. Brousse, Paris based, Expert 1 Head Government appointed Scient. Com.; HT: H. Tazieff, Paris based, Head IPGP Volc Obs, Expert 2 requested by Préfet; MF: M. Feuillard, IPGP, Head Soufrière Volc. Obs., JT: J Tomblin, Head SRU Trinidad, arrives 3/08/76 invited by Préfet (intern. expert suggested by HT on 29/07/76 given HT’s departure to Ecuador); GJ: G. Jobert; Dir. IPGP from 01/08/71 till 1/08/76; CA: C. Allègre; Dir. IPGP as of 08/08/76; BRGM: Bureau de Recherches Géologiques et Minières; GA: G. Aubert, Assistant Dir. INAG, Nat. Institute of Astronomy & Geophysics (CNRS); RF: Richard Fiske, expert from USGS; LS: L. Steinmetz, seismologist, head of IPGP scientific team in Guadeloupe from 29/09/76 to 02/12/76; CG: Conseil General Guadeloupe, local government; International Scientific Commission (ISC): Chair: F. Press; members: S. Aramaki, F. Barberi, J. Coulomb, R. Fiske, P. Gasparini, C. Guillemin, G. Sigvaldason); BM: B. Minster, head of IPGP scientific team in Guadeloupe from 03/09/76 to 17/09/76; PDC: pyroclastic density current (nuée ardente). Data compiled from: Dorel and Feuillard, 1980; Feuillard et al. 1983; Préf. Guadeloupe 1977; De Vanssay, 1979; Lepointe, 1999; Leguern et al. 1980; IPGP, 1956–2013; Smithsonian Inst. 1976, 1977; Sheridan, 1980; Feuillard, 2011; Boudon et al. 1988; Loubat and Pistolesi-Lafont 1977; Komorowski et al. 2005; Beauducel, 2006.