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A Scenario for Volcanic Unrest at Mt Taranaki

Jérôme Lecointre and Vince Neall

Weeks leading to volcanic unrest: unusual seismic activity detected on the Taranaki Volcano-Seismic Network (TV-SN). Data transmitted by radio to the Taranaki Emergency Management Office (TEMO) in New Plymouth, for analysis by GNS scientists and discussions with representatives from the Egmont Advisory Group.

Day 1: 17 events (2<M<3.6) recorded with a main swarm of 5 HF earthquakes felt in the western Taranaki region.

Day 12-13: Intense swarm recorded on the network. About 25 earthquakes concentrated on the western flank of the edifice, over an area delineated by the 900 and 1700 m contours. Isolated, HF (tectonic) signals reach M4.2. Fresh scars and a couple of large slips detected in the headwaters of the Stony River. On advice from GNS scientists, Alert Level raised to Level 1.

Irregular swarms of moderate magnitude (M2.5 to 4) continue to be recorded over the next couple of weeks. Media interest and concern in the local population increased.

Day 28: Swarm of 36 events originating from the same broad area on the cone. Shallow source detected at c.3.5 km below the volcano. Regular shaking affects populations located between Stratford, Okato and Opunake. Large landslips in the Stony River confirmed and increased turbidity noticed in the adjacent streams. Trampers at the Kahui Hut (altitude: c.900m) reported strong shaking of the ground, rock falls, and an increased water flow at local springs.

Radio talkback shows in the main town centres discuss the current situation at the volcano. Families with children attending isolated schools in the Okato-Pungarehu-Opunake sector are especially concerned.

Shallow signals (2.5<M<3) follow a short period of calm during the following week. Additional seismic stations set up on the volcano.

Day 35: At 3:45 am, an M5.2 quake is detected by the Taranaki seismic network. Panic triggered in the local population. Emergency Services and TEMO flooded with phone calls. Quake source identified beneath the volcano, but associated with fault movement at intermediate depth in the Taranaki Basin. Aftershocks follow during the next few days. Two other moderate seismic shocks widely felt between New Plymouth, Stratford and Hawera.

Day 36: Voluminous landslides reported by aerial surveys in the Stony River, Warea River and Oaonui Stream catchments. Other surficial changes observed in mountain springs (decrease of flow discharge) and damage on tracks reported. Surface deformation and hydrological changes are now monitored in detail. GPS survey initiated at key reference locations around the volcano.

Day 38 (Waitangi Day): Alert Level maintained at Level 1.

Day 39-40: Irregular swarms of M2-3 tectonic earthquakes recorded. No significant changes (temperature, resistivity, chemical composition) reported by hydrologists. Fresh landslides triggered by heavy rain overnight observed on the western side of the mountain.

Day 42: Seismicity dramatically increases overnight: >53 events recorded by seismic network. Large amplitude, LP signals detected for the first time, suggesting displacement of fresh magma at shallower depths.

Day 43: Major rock fall (>106 m3) in the headwaters of Stony River, below Hook Hill (1143m). Coarse debris accumulated across the riverbed, forming a partial dam. Avalanche witnessed from Holly Hut, east of The Dome. Loud rumbling was heard downstream by local farmers. Pyramid Track route cut by several large landslides.

Day 44: Swarms of low magnitude tectonic earthquakes recorded (c.65 events/day). Network of tiltmeters now installed at deformation stations covering the summit area and the western flank of the mountain (Puniho Road/Track, Kahui Track and Ihaia Track/Brames Falls Track). Two additional portable seismic stations set at Holly Hut and Lake Dive Hut.

Day 46: New fumaroles located along a fracture bordering the E-W ridge leading to the Three Sisters signalled by climbers. Trampers at the Waiaua Gorge Hut (altitude: 600m) indicate displaced abutments to the nearby swing bridge. Alert Level at Taranaki is officially raised to Level 2 (volcanic unrest).

Day 50: Short seismic lull. Rising temperature are recorded (+1-3°C) in streams crossed by the Pyramid Route. No significant increase in SO 2 emission detected by COSPEC survey over the summit. Thermal camera oriented towards the western flank of the volcano activated from Holly Hut.

Day 51: Significant change of seismic patterns. HF earthquakes less frequent while LP signals increase drastically. Aerial survey identifies minor cracks on upper slopes of the western flank (between Okahu Gorge and Big Pyramid). Fresh accumulations of rock debris litter the area. Water accumulated behind the Stony River dam. Observers at Lake Dive Hut note the lake level has significantly dropped.

Day 52: Tectonic quakes still recorded (c.50/day) and a dozen LF signals detected at more superficial levels. Booming noises reported. Base level stations along the coastline show significant variations on a GPS survey. Sudden flow discharge increases noted at several springs on the western slopes. Alert Level 2 is confirmed. Uplift in the Big Pyramid region (c.15 cm) recorded by laser-range distance meter and GPS stations. Hot spot covering several square meters in the same area revealed by thermal camera at Holly Hut.

Day 53: New fumaroles detected along fractures extending from the summit to the Big Pyramid-Turehu Hill region (between 2500 and 1300m). SO 2 flux increasing to 150 t/d.

Day 55: Swarm of micro-earthquakes detected on the western side of Fanthams Peak. No surface change noted in this sector of the volcano. Intense fracturing of the uplifted area (+ 0.8m) with sustained fumarolic activity, and development of patches with dying vegetation reported around fractured areas. Spot measurements at two sites indicate temperatures reaching 150°C.

Day 57: Development of a bulge beneath the western upper flank of the volcano confirmed by nclinometry data. Limited deflation recorded towards the headwaters of the Mangahume Stream. Widening of main cracks noted below summit by late afternoon. Intensified fumarolic activity observed and loud noises heard during the evening.

Day 58: E-type (explosion) earthquakes appearing on the seismic network. Suspected vent opening phase in the summit region. Direct observations limited due to darkness. No ash fall reported yet. Large hot spot detected by thermal camera (steam plume?) over the new vent. Very shallow tectonic earthquakes (>150) accompanied by dozens of tremors and LP signals. Alert rose to Level 3.

Day 59: Fractured bulge clearly visible from a distance. Monitoring instruments disturbed by severe rattling. Critical power lines severed by rock falls and soil slips. Fine ash deposition reported in Stratford.

Day 60: 346 superficial tectonic earthquakes and 27 LP events recorded on the network by 5pm. At 5:10pm, HF signals saturate the network. Decreasing tremor activity in the evening. Drastic changes in fluid circulations noted all around the volcano. Fumarole temperatures in the deformed area reach 235°C. At 11:25pm, a M4.7 quake violently shakes the region. Panic reactions in the population. Rumblings soon followed by 2 violent explosions, strongly felt south of the Pouakai Range. Emergence of a dense mass flow reported from the area where fumaroles were very active. The avalanche passes the Big Pyramid towards the Stony River. Light cloud escaping towards the Okahu Gorge. Alert Level at Taranaki is officially raised to Level 4 .

Day 61: The early morning light reveals a 12 km high plume towering above the volcano. The sustained column is bent by the westerlies, carrying ash towards Stratford and further east. Contact with recording equipment located on the Pyramid Track is now totally lost. Observers at Pouakai Hut describe the repeated passage of pyroclastic flows across the Pyramid route. Most of the laterally directed, dense clouds travel down the steep slope contained between the Turtle, Big Pyramid and Puniho Hill, reaching feeding tributaries of the Stony River. One small nuée flowed at the base of Carrington Ridge and escaped towards the timely evacuated Holly Hut. Pyroclastic flows are also channelled into the upper Okahu Stream. Alert Level is raised to 5.

Day 62: Bush fires reported in vegetated areas. Wide, smoky amphitheatre identified on the upper slopes of the volcano, below the Turtle. Unstable rims of the depression shaken by tremors, leading to rock avalanching over the west-facing steep slopes. Ash column reaches 18 km in altitude. Umbrella expanding eastwards and ash fall signalled in Strathmore, Makahu (Forgotten Highway). Essential services disrupted in Stratford by corrosive ash. Eltham records only a fine dusting while Hawera seems to be unscathed by the events.

Day 63-64: Local farmers no longer able to access the Kahui Road Airstrip (altitude: 400m). Pumiceous deposits reach the western boundary of the Park and lahars are reported downstream (Okahu and Kapoaiaia Streams; Warea River). Ash column becomes unstable, and then collapses to generate small pyroclastic flows (upper flanks of the volcano). Ash plume reaches a distance of nearly 150 km east of Stratford (visible on SPOT satellite imagery). Seismic activity now characterised by swarms of tremors, interrupted by explosion earthquakes. Seismic network partially destroyed on the western flank of the edifice. Deflation of the southern flank recorded by inclinometers and GPS survey. Rock fragments and sand pile up in the muddy waters of the Stony River. New monitoring equipment set up along the upper channel of the river.

Day 65: Decreasing seismic activity recorded overnight. Main vertical plume reaching an altitude of 10 km. Life rendered difficult in Stratford due to continuous ashfall. At 4:35 pm, a M3.6 earthquake is recorded on the seismic network. One hour later, booming noise heard from Saunders Road as waters rise suddenly in the turbulent Stony River. Rock debris and trees carried by large volume lahar reaching the coast. Decreasing pyroclastic activity on the upper cone, but debris and ash further remobilised downslope by heavy rains.

Day 67: Coarse lapilli no longer fall in Stratford. Colour change noticed in the ash plume. Bluish haze detected on the southern slope of the cone, heading down the headwaters of the Mangahume Stream. Ash dispersed towards Inglewood and Waitara, as wind direction changes. Regional flight paths severely affected by the development of the ash column. Only 5 LP quakes recorded today. Farming activities disrupted by numerous landslides and slips cutting road accesses.

Day 68: Fresh lava dome reaching the surface of the fractured, upper western flank. Small volume, incandescent block-and ash flows produced at regular interval. Small cloud of fine ash accompanies the extrusion.

Day 69 and beyond: Dome growth continues, accompanied by partial gravity collapse of the viscous extrusion and the associated generation of small pyroclastic flows (BAFs). Similar dome-forming eruptions in Japan, Indonesia, Mexico and West Indies have been documented in detail (e.g. Mt. Unzen; Merapi; Popocatlepelt and Montagne Pelée) and have lasted 1 to +10 years in average. Such extended periods of volcanic activity, alternating violent explosions with quieter episodes of dome growth, put enormous pressure on civil defence and emergency management authorities, as they have to deal with sustained periods of uncertainty and economic disruption in the affected region.


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    Note: for current New Zealand Scientific Alert Levels for Egmont Volcano, refer to the previous publication, p.3.