EAS 116 Earthquakes and Volcanoes - Purdue...

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EAS 116 Earthquakes and Volcanoes

J. Haase Forecasting Volcanic Eruptions

Assessment of Volcanic Hazard

•  Is that volcano active? – Mount Lassen: 12000 BP and 1915 – Santorini, IT: 180,000 BP, 70,000 BP,

21000 BP, 3600 BP, 197BC, 1620 AD, 1950 AD

– Yellowstone: 2 mya, 1.3 mya, 600,000 BC •  Can’t count on historical records

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Geological investigation Long term forecasting

•  Determine ages of previous eruption deposits –  Date using radioactive

decay of minerals in rock

–  C14 dating of charcoal preserved in ash

–  Date using paleomagnetic sequence

•  Determine long term recurrence interval

•  Determine spatial extent of ash fall and pyroclastic flows

Redoubt Volcano

Monitoring for short term forecasts

•  Seismic recordings •  Ground deformation monitoring •  Small phreatic eruptions •  Gas chemistry monitoring •  Webcams – visual monitoring •  Less common:

–  Infrared-temperature observations – Magnetic field measurements

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Volcanic precursors - Seismic

•  Earthquake locations – Magma forces its way through the rock and

cracks it – This cracking results in an earthquake – Locations of the earthquakes tell where the

magma is

•  Earthquake seismograms from Mt. St. Helens March 21-23, 1980

•  Note individual eq signals starts with small (dark) P-wave and then (lighter colored) larger amplitude S-wave

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•  Mt St. Helens, Mar 24-25, 1982

•  There are so many earthquakes, one can’t tell where one begins and the other ends


•  When magma moves it increases stress causing rocks to fracture on faults

•  Earthquake locations –  Show magma moving from mantle to crust –  Show magma moving into the magma

chamber –  Show magma moving from magma chamber

to eruption

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http://vulcan.wr.usgs.gov/Volcanoes/MSH/CurrentActivity/1995/summary_jan-sept.html

Earthquakes occur in and around the magma conduit system leading from the magma chamber to the lava dome.


•  Earthquake locations –  Deepest earthquakes (as much as 60 km deep in

Hawaii) are associated with magma moving from the mantle up into the crust

–  Intermediate depth earthquakes (6-10 km) are associated with magma moving into the magma chamber

–  Shallow earthquakes (less than 3 km to the surface) indicate rock fracturing as the magma forces its way to the surface to erupt

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Interior view of Kilauea •  Tremor indicates

magma moving close to surface as it erupts

•  Earthquakes indicate magma moving into magma chamber

Number of earthquakes •  Moderate earthquakes

–  Aftershocks increase in number after a volcanic event

–  In contrast, typically after a “tectonic” earthquake, the number of aftershocks decreases in number after the event.

–  (Mt St Helens, first M=4.1 earthquake on March 20, 1980)

http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/pre-eruption-0329.shtml

USGS

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Volcanic Precursors Number of earthquakes increases right before eruption

Number of earthquakes decreases over time

Magnitudes of earthquakes are larger => maintain high energy release Mount St. Helens

Seismic waves detect magma chambers

•  P wave velocities are slower in region with melt

•  (remember low velocity zone below lithosphere, and liquid core)

•  S wave velocities cannot travel through liquid

•  Seismic waves that travel through magma chamber arrive later than expected at the station.

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Volcanic Precursors - Seismic Volcanic tremor

•  Volcanic tremor or Harmonic tremor: – A rhythmic continuous ground motion

recorded by seismographs during or preceding an eruption

– Associated with the underground movement of magma

–  In contrast with sudden generation of energy during an earthquake followed by rapid decay

Volcanic precursors - Seismic •  Volcanic tremor from Mt. St. Helens


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•  Seismic recordings •  Earthquake locations, number of events, volcanic tremor,

seismic velocity anomalies

•  Ground deformation monitoring •  Small phreatic eruptions •  Gas chemistry monitoring •  Webcams – visual monitoring •  Less common:

–  Infrared-temperature observations –  Magnetic field measurements

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Mount St. Helens a few weeks before the 1980 eruption �

•  1 microradian tilt = 1 mile long board raised by placing a nickel under one end

•  Gradual inflation is due to filling of magma chamber

•  Abrupt deflation is due to magma moving out of chamber to vents for eruption

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•  Short period earthquakes occur just prior to eruption as magma cracks the crust (gray)

•  Long period earthquakes and tremor occur during the eruption as magma moves through crust during the eruption (pink)

•  Deformation (tilt) gives long term forecast •  Seismicity and deformation give short term forecast




•  Ground deformation monitoring •  Small phreatic eruptions •  Gas chemistry monitoring •  Webcams – visual monitoring •  Less common:


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Steam eruptions •  Phreatic eruptions

–  Magma makes contact with ground or surface water

–  Near instantaneous evaporation to steam

–  Produces an explosion of stream and rock (no magma)

–  Magma must be within a few km of the surface

Mt St Helens phreatic eruptions (USGS) from Mar 27-May 18




•  Ground deformation monitoring •  Small phreatic eruptions •  Gas chemistry monitoring •  Less common:


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Volcanic gases

•  Increased release of SO2 sulfur dioxide and HCl –  There is a fresh magma source –  It’s a high temperature magma –  It’s not just steam from warming ground water

Correlation Spectrometer (COSPEC)


•  Small eruption on Mount St. Helens March 19, 1982

•  Monitoring of –  Earthquakes –  Deformation of dome –  Tilt of ground surface –  SO2 emissions

•  Led to successful prediction

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Debris landslide

http://www.youtube.com/watch?v=bgRnVhbfIKQ

Initial Eruption of Mount St. Helens�

Hyndman & Hyndman, Fig. 6-1

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8:27:00

8:32:47

8:32:53

8:33:03

First minute of Mt. St. Helens eruption �

Effect of Mount St. Helen lateral blast �

Hyndman & Hyndman, Fig. 6-24

Lateral blasts are pyroclastic flows (high-density mixtures of hot, dry rock fragments and hot gases) that move away from the vent at very high speeds due to the explosive force of the eruption. (Distinguished from normal pyroclastic flows that move downslope due to gravitational force.)�

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Mount St. Helens Eruption�May 18, 1980 �

Eruption cloud

Ash cloud

Ash Fall Distribution �

Hyndman & Hyndman Figure 6-9

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Ash Fall Distribution �

Hyndman & Hyndman Figure 6-9

2000 km

1500 km 1600 km

500 km 80 km

Mount St. Helens, Washington �1980 Eruption �

Hyndman & Hyndman Figures 6-6, 6-7

Before Eruption

After Eruption

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Products of Volcanic Eruptions�

Eruption volume •  It is very difficult to predict

the size of the volcanic eruption

•  Estimates are based on historic eruptions

•  Estimates are based on size of magma chamber from seismic velocities

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Yellowstone National Park

Old Faithful Geiser

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Yellowstone Hotspot Hot spot track

leaves trail of calderas where lava flows and low elevations from collapse distinguish it in the topography

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http://pubs.usgs.gov/fs/2005/3024/

Eruption volume •  It is very difficult to predict

the size of the volcanic eruption

•  Estimates are based on historic eruptions

•  Estimates are based on size of magma chamber from seismic velocities

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Kilauea Caldera

•  A Caldera goes through inflation and deflation cycles: –  Eruption –  Collapse after

magma is erupted –  Resurgent doming of

surface –  Lava dome extrusion

http://volcanoes.usgs.gov/About/What/Monitor/Deformation/TiltKilauea.html

1km

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Yellowstone Caldera

•  60 km wide •  Nearly flat

summit •  Gently

sloping sides

60 km

Kilauea Caldera

•  Caldera walls

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http://volcanoes.usgs.gov/yvo/2006/royalsoc.pdf Lowenstein et al., 2006

•  Seismicity indicates where tectonic stresses are high from magma intrusion

•  Seismic velocities indicate size/location of magma chamber

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Seismic velocity anomalies

•  Gas emissions from Yellowstone

http://volcanoes.usgs.gov/yvo/2008/ElementsJBL_SH.pdf Lowenstern et al 2008

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•  Currently large emissions of CO2 are found in the caldera

•  Measurements of ground deformation

•  Uplift 1976-1984

•  Subsidence 1985-1992

http://vulcan.wr.usgs.gov/Volcanoes/Yellowstone/Publications/OFR95-59/OFR95-59_inlined.html Dzursin et al, 1995

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•  Resurgent domes

•  2 areas of uplift within caldera

•  GPS measures vertical ground deformation at points

•  INSAR from satellite measures vertical deformation everywhere

•  Since 2004 there has been VERY fast uplift

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•  A study of the data indicates The giant Yellowstone "supervolcano" is rising upward faster than ever observed. The red arrows pointing up represent uplift of the Yellowstone caldera, or volcanic crater, while the downward red arrows show sinking of the land near Norris Geyser Basin. The black arrows indicate lateral or horizontal ground movement.

University of Utah

Yellowstone

•  Yellowstone is a caldera •  It erupts every ~700,000 years •  The ash from the eruptions covers an area

almost as big as the US •  There is currently seismicity and a very large

magma chamber •  Recent deformation since 2004 is occuring at

a rate of 7 cm/year, three times any previously observed rate.

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Review of Volcanic Precursors

•  For a review of precursory activity leading to an eruption, see

•  Mount St. Helens: •  http://www.fs.fed.us/gpnf/mshnvm/education/

teachers-corner/library/pre-eruption-0329.shtml •  Hawaii: •  http://pubs.usgs.gov/gip/hawaii/page16.html •  Yellowstone (potential eruption): •  http://pubs.usgs.gov/fs/fs100-03/ •  Hyndman & Hyndman p.165-177

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