Distal Volcano-Tectonic Earthquakes (dVTs) are 1st in a Typical Seismic Progression for decades-dormant volcanoes Randall White, Wendy McCausland, John Power and John Pallister Volcano Disaster Assistance Program US Geological Survey

Distal VTs (dVTs) Occur in SWARMS  events increase in rate and size Largest events after middle of the swarm Several events within 0.5 M unit of largest dVTs die off after significant phreatic/ phreato-magmatic activity begins Total VT seismic moment PROPORTIONAL TO INTRUDED MAGMA VOLUME!!!

Rather Typical pattern of dVT seismicity and eruption onset UNZEN Unzen 1989-1995 (Nakada et al, 1999) major dVT seismicity (blue) occurs over large area 3-20 km W during intrusive period dVT seismicity (blue) dies out quickly with onset of proximal Hybrid and LF seismicity (red) as eruptive activity begins

Tacana 1985 – 1986 VEI 1 small phreatic eruptions in February, May, June 1986 dVTs (blue); then proximal Hybrid seismicity (red) Tacaná had small phreatic eruption in February, May, and June of 1986

Soufriere Hills 1995 VEI 3 Initial dVTs (blue); later Hybrids and LFs (red) dVT seismicity began 3 years prior to magmatic eruption Stopped when extrusion began

Nevado del Ruiz 1985 VEI 3 dVT seismicity (blue areas) began 12 months prior to magmatic eruption dVT magnitudes include M4.5 11 months prior, M4.1 ~2 months prior dVT energy died quickly after onset of phreatic activity and LF seismicity (red) additions from F. Gil (personal communication)

Popocatepetl, 1997 – 2003 VEI 3 a very hot dacite magma dVT seismicity (blue) preceded several vulcanian explosions (red) Same pattern between lulls in activity, 1997 - 2001

Guagua Pichincha 1998-99 VEI 3 dVT seismicity (blue) on tectonic fault 15 km NE and 15 km deep during 1 year prior to dome extrusions (red)

Rabaul 1994 VEI 4 dVTs (blue) 15 km outside large caldera, eruptions (red) Seismicity started in 1971, seismicity into 80s was in the caldera. Seismicity just before eruption at blue area outslde the Rabaul caldera, offshore. Also smaller swarms at same location prior to smaller later eruptions during following 3 years.

Pacaya 1980, Basaltic VEI 2 dVTs in 1979; continuous strombolian activity 1980-1986 dVT peak in 1979. A much smaller peak occurred during a smaller lull in DEc 1981-Jan 1982

Shishaldin, 1998 - 1999 Shishaldin 1999 VEI 4 Magma: basaltic andesite dVTs (blue) began 1 month before Sub-Plinean Eruption April 19, 1999 Then 6 weeks of Strombolian Fountaining subplinian eruption on April 19, 1999 that placed an ash cloud to 45,000 ft ASL (~13,700 m). The eruption style almost immediately changed to that of vigorous strombolian fountaining (Nye and others, 2002), which characterized the activity for the following six weeks.

Mauna Loa 1984 VEI 0 basalt (and 1975 VEI 0 ~identical)

Typical pattern of dVT seismicity Unzen 1989-1995 (Nakada et al, 1999) Major dVT seismicity (blue) occurs during intrusive period dVT seismicity dies out quickly with onset of magmatic activity (red)

Database of 65 eruptions during last 100 years which were preceded by dVT swarms Page 1 of 3

Almost all explosive eruptions are preceded by days to years of distal VT seismicity Volcano year VEI onset of distal VT’s Mt. Pinatubo 1991 6 14 weeks prior Mt. St. Helens 1980 5 10 weeks prior El Chichon 1982 5 2 years prior Bezymianny 1956 5 7+ weeks prior Shishaldin 1999 4 9 months prior Sheveluch 1966 4 20 weeks prior Karymsky 1996 3 35 weeks prior G. Pichincha 1999 3 1 year prior Rabaul 1995 3 10 weeks12 years prior Kliuchevskoy 1966 3 7 months prior Chaiten 2007 4 >2 days Kasatochi 2007 4 >5 days Possible Exceptions: volcanoes with eruptions in prior 30 years Reventador 2002 4 0 (4?) weeks prior Redoubt 1989 3 0-1 week prior

Explosivity of eruption versus Duration of distal VT seismicity

This shows distance and depth below summit for all dVT swarms dVT swarms seem to occur at the base of the brittle zone. The precise location of the swarms is not relevant for eruption forecasting. dVT swarms occur on pre-stressed local fault(s) and may move from one fault to another near the volcano

Total Seismic Moment (almost all from dVTs) is proportional to intruded volume Red triangle are volcanoes having deformation data for determining intruded volume. Blue triangles are wells with measured volumes of injected H2O. (See abstract for Moment vs Volume formula)

Conclusions: Distal VT earthquakes are exceptionally useful for eruption forecasting dVT’s are very easy to monitor and use in real-time In “closed” systems (repose >30 years): dVTs are earliest seismic precursor to eruptions dVT’s die off when LP and VLP seismicity begins If intrusion rate is high, dVT’s correlate with inflation; dVT’s give days to months of warning for large explosive eruptions Duration of precursory dVT seismicity appears roughly inversely related to explosivity Total VT seismic moment is proportional to intruded volume

How has VDAP used dVT characteristics for forecasting How has VDAP used dVT characteristics for forecasting? Sulu Range, PNG 2006 VEI 1 July 6 – a few earthquakes first reported felt. July 10 – dVTs increase significantly; the first M5 dVT. Many M>4 events picked up by NEIC Booming noises heard at Kaiamu and Silanga. July 13 to 15 - a local seismograph recorded continuous VT earthquakes; Earthquakes felt every 2-3 minutes; July 19 - seismicity peaks with M6.4 and M5.9 dVTs 25-30 km offshore Explosive emissions of mud and steam from Silanga, Mato and Bakama hot springs. VT seismicity dies off quickly as 1/t

Sulu Range, Papua New Guinea 2006 VEI 1 Upper: location of volcanoes (red triangles) and M>5 dVTs, times and focal mechanisms. Note dVTs proceed 30 km WNW L-band InSar indicates 0.7-1.0 km intruded volume (Wicks 2008) as forecast from total seismic moment. Lower: aftershock dVT’s; dVTs decayed as 1/t; locations by temporary local network

Cosiguina, Nicaragua 2002 dVT swarm indicates an intrusion in 3 pulses, But total seismic energy indicated intrusion too small to reach surface (VDAP and V.Tenorio)

Cartoon of seismic pattern

June 14 Pre-climatic LP & tremor Mount Pinatubo, 1991 VEI 6 3rd stage of typical Seismic Progression for long- June 7 Hybrids dormant systems June 12 LP’s June 14 Pre-climatic LP & tremor June 14 >LP Seismic swarms Common progression: HF: dVT’s = Activation of existing faults LF Tremor = boiling off hydrothermal system Hybrids = degassed carapace magma rising to surface by stick-slip Very large-scale Tremor = Eruption Post-eruption dVT seismicity = Crustal response to magma withdrawal

March - 30 May: distal VT swarms 5-6km NW March - 30 May: distal VT swarms 5-6km NW. Magma ascends, pressurizes regional faults 31 May - 3 June: Phreatic eruptions – fluid-moves in cracks - LP’s and LP tremor June 12: explosive eruptions 03:41 & 08:51. LP’s, tremor, eruption signals June 12-14: LP events continue … & increase in magnitude; RSAM increases 2 April: small steam explosion Magma … …mixes, intrudes overlying.. dacite in reservoir, breaks.. through crust and erupts.. ..lava dome on June 7 Pre-climactic phase 14-15 June: …mix of VT, LP, tremor, eruption signals… RSAM steps down after 6/14 13:04 eruption then ramps up 15 June: caldera collapse & eruption of >5 km3 magma… seismic net destroyed… post eruption VT earthquakes outline magma reservoir Distal VT swarm from Augustine Gas rich basalt intrudes…from great depth (35 km)… LP tremor Linking seismic monitoring, observations and petrology to improve understanding of volcanic “plumbing” Proximal Hybrid swarm precedes extrusion of dome on June 7. June 8: pressure released & 30 hrs seismic quiet Based on Harlow et al (1996), White (1996), Mori et al. (1996) and Pallister et al. (1996)

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