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, doi:10.1126/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, doi:10.1126/science.1161927.
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
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
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
Example volcanic earthquakes • Waveforms and spectrograms from Redoubt • LP, hybrid and shallow VT occurred 1.4-1.7 km below crater • LP • Dominant f=1.5Hz • 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 f=1.5Hz
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
Source processes • Chouet’s 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
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
Chouet’s 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
Chouet’s 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 fluid:shear modulus crack
Chouet’s 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
Chouet’s crack model • Radiated seismic signal • Spectrum related to ratio of crack width to length as well as C and Z • Note that you don’t need a large crack to have low frequencies (large C), just a large aspect ratio
Synthetic LP earthquakes a) Galeras LP b) synthetic LP c) spectra of both Z=15 C=100 L/d=3600
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
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
MSH LPs • Stacking of spectra from receivers with distinct locations should enhance source-related spectral peaks and attenuate path effects
MSH LPs 1.7 Hz peak always observed, but not always the dominant peak Why?
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.