LP earthquakes, Chouet Nature 1996

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Volcano Seismology - 25 Feb 2009

• LP earthquakes, Chouet Nature 1996
• Next Monday LP earthquakes in the laboratory
• Read Perspective by Burlini, L., and G. D. Toro
  (2008), Volcanic Symphony in the Lab, Science,
  322(5899), 207-208, doi10.1126/science.1164545
  
• and report by Benson, P. M., S. Vinciguerra, P.
  G. Meredith, and R. P. Young (2008), Laboratory
  Simulation of Volcano Seismicity, Science,
  322(5899), 249-252, doi10.1126/science.1161927.

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LP earthquakes, Chouet Nature 1996

• Source of LPs
• Eruption prediction based on LPs
• Introduction - VTs vs. LPs (and tremor)
• VTs
• are more spread out in space and time
• originate in the solid rock
• LPs
• are typically shallow
• Exceptions (e.g., Pinatubo, Mauna Loa) sometimes
  precede eruptions
  
• LPs and tremor have similar temporal and spectral
  components indicting a common source process
  
• Related to pressurization of magmatic and/or
  hydrothermal fluids and the coupling to the
  solid
  

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LP earthquakes, Chouet Nature 1996

• Source of LPs
• Common at volcanoes, but not recognized due to
  different names
  
• B type, screw (tornillo), butterfly, N-type,
  tremor-like earthquakes, single-frequency
  earthquakes
  
• Waveform characteristics had been commonly
  attributed to path affects, but studies show
  source affects are more important
  
• Artificial source studies show broadband
  characteristics
  
• Many types of events originate from same source
  region
  
• Originate in fluid
• Magma conduit
• Water/steam/gas-filled cracks adjacent to
  conduit
  
• VTs originate in rock - brittle failure events

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Example LP earthquakes

• All signals have a harmonic coda following a
  higher-frequency onset
  
• HF onset is absent at larger distances
• Note that some might call these hybrid events
  due to the HF onset
  
• Typical frequency range 0.2-2 Hz (0.5-5 sec
  period)
  
• h - source depth
• r - epicentral distance

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Example volcanic earthquakes

• Waveforms and spectrograms from Redoubt
• LP, hybrid and shallow VT occurred 1.4-1.7 km
  below crater
  
• LP
• Dominant f1.5Hz
• Broadband onset
• Hybrid (mixed 1st motions)
• Non-dispersive coda
• Shallow VT
• Broadband body waves
• Dispersive coda not obvious
• Deep VT
• Shorter coda (less efficient at generating
  surface waves)
  

• Tremor
• resembles LP spectral content
• Dominant f1.5Hz

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Source processes

• Classification based on source process offers a
  way to interpret seismicity in terms of processes
  in the fluid or solid
  
• Shear and tensile sources are in solid
• Related to response of volcano to pressure,
  cooling, etc.
  
• Volumetric sources in fluid-filled bodies
• Fluid (gas or liquid) may be magmatic or
  hydrothermal in origin
  
• Kilaua - basalt magma with gas
• Many other places steam seems most likely
• Hybrid events represent a class that involves
  both brittle failure and volumetric components
  
• Shear failure in conduit wall or faults
  connecting fluid-filled cracks
  
• High pressure (greater than lithostatic) fluids
  could induce fracture - hydrofrac
  

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Source processes

• Chouets source hypothesis
• Shallow LPs and tremor are manifestations of
  pressurization in a magmatic/hydrothermal system
  
• Offer a window into fluid dynamics
• If LP activity is more intense, pressure is
  greater - eruption potential is greater
  
• Sealed system may support strong pressurization
  and energetic LP swarm
  
• Redoubt (1980) - a few events per minute -
  sealed, pressurized system
  
• Galeras (1993) - 1-2 events per day - leaky
  system
  
• Mount St. Helens (2005) event every few minutes
• Crack model for LPs
• Acoustic signal consistent with emission for
  every event

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Event families

• Repeating similar events are commonly seen
• Suggests a common, nondestructive source process,
  such as
  
• Repeated excitation in a fluid-filled conduit
  capable of resonance

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Event families
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Chouets crack model

• Crack might be most reasonable shape given
  differential stresses - dikes seem common to many
  systems
  
• Two important parameters
• Crack stiffness, C
• Controls resonant frequencies
• Impedance contrast, Z
• Controls duration of radiated signal and affects
  frequency
  
• Fluid viscosity
• Related to energy loss and signal duration

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Chouets crack model

• Crack stiffness, C
• Velocity of crack wave decreases with increasing
  C
  
• As crack stiffness increases resonant frequency
  decreases
  
• Permits reasonable crack sizes for explaining
  observed resonant frequencies
  
• Related to
• aspect ratio (crack length aperture)
• Ratio bulk modulus fluidshear modulus crack

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Chouets crack model

• Impedance contrast, Z
• Solid density X solid ? fluid density X fluid
  acoustic velocity
  
• Increased bubble content increases Z by
  decreasing fluid density and velocity

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Chouets crack model

• Radiated seismic signal
• Spectrum related to ratio of crack width to
  length as well as C and Z
  
• Note that you dont need a large crack to have
  low frequencies (large C), just a large aspect
  ratio

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Synthetic LP earthquakes

• a) Galeras LP
• b) synthetic LP
• c) spectra of both
• Z15
• C100
• L/d3600

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Synthetic LP earthquakes

• Most spectral peaks due to path effects, but
  consistent 3.8 Hz peak associate with source
  resonance
  
• Other peaks vary according to receiver location

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Synthetic LP earthquakes

• Peaks associated with resonant source are
  independent of source-receiver location
  
• Peaks associated with path effects vary according
  to crack position
  
• Crack resonance is the same for both the
  homogeneous and layered medium
  
• Although there are significant differences in the
  spectra, LP and tremor have similar dominant
  peaks
  

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MSH LPs

• Stacking of spectra from receivers with distinct
  locations should enhance source-related spectral
  peaks and attenuate path effects

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MSH LPs

• 1.7 Hz peak always observed, but not always the
  dominant peak
  
• Why?

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LP earthquakes - Model for Mt. Redoubt

• Model for LP earthquakes and time history of
  LP and tremor intensity at Redoubt on 13-14 Dec,
  1989.