Seismology Flashcards
What is volcanic seismology?
The analysis of earthquakes caused by volcanic processes
How does volcanic seismology work?
Salvage et al., 2017; Iverson et al., 2006
As magma moves in the lithosphere towards the surface, it will cause changes in surrounding stresses
These pressure and stress perturbations commonly result in fracturing and thus lead to numerous small-magnitude earthquakes
These seismic events can then be picked up on surface seismometers, with deviations from the background level indicating possible unrest
What characteristics of seismicity can be used?
Can be based on waveform similarities, though it is traditionally based on a signal’s time and frequency characteristics
Outline the use of frequency
Chouet and Matoza, 2013
Different bands of frequency relate to different active source processes at depth
These can usually be split into high and low frequency events, though in reality there is a continuum across a spectrum
Outline high-frequency signals
Arciniega-Ceballos et al., 2003
High-frequency signals, also known as Volcano-Tectonic (VT) events, have energy concentrated in the frequency range 1-20hz
They are characterised by clear, impulsive P and S-wave arrivals, followed by a short coda
Such seismicity is usually attributed to brittle failure within the edifice, where magmatic processes create enough elastic strain to force the surrounding rocks to fail
Outline low-frequency signals
Chouet, 1988
Low-frequency signals occupy the spectral range of 0.2-5hz
Characterised by emergent P-wave onset’s and a lack of S-wave arrivals
Suggested that the occurrence of these events is linked to the resonance of seismic energy trapped at a solid-fluid interface either within a crack of volcanic conduit - although the exact mechanism is still debated
Outline hybrid events and very long period signals
Hybrid events are those which fall between the two end-member categories and are very common (Chouet and Matoza, 2013)
Very long period earthquakes occupy a spectral range below 0.01hz - these have only become detectable with the advent of broadband sensors (Dawson et al., 2011)
Outline the failure forecast method
Sparks, 2003
Based on the laws of material failure in relation to strain rate
Retrospective analyses of seismic energy release patterns from Montserrat indicated that eruptions could have been predicted within a few hours to days using this approach (Kilburn and Voight, 1998)
E.g. Boue et al., 2015 use FFM and Bayesian probability modelling to generate a model capable of predicting eruption - when tested on Volcan de Colima (Mexico) found to accurately predict 80% of activity
Outline RSAM
Sparks, 2003
Real-time seismic amplitude measurement
Allows for the automatic calculation of mean seismic amplitude
Requires only one instrument and enables rapid response
If funding is lacking, this is a great technique - especially since it has a quantitative element that can be used to define thresholds which tends to be easy to explain to decision-makers
RSAM tends to reach a maximum several hours before an eruption, which is interpreted as a weakening of the system as it approaches failure
Outline tracking hypocentre locations through time
The migration of hypocentres can track lateral propagation and the swallowing of dykes prior to eruption
E.g. in the 2014 eruption of Bardarbunga volcanic system, Iceland, tracking of seismic waves allowed for real-time observation of lateral dyke growth in a rifting event over 45km (Sigmundsson et al., 2015)
Outline the positives of seismology (3):
“Seismometers relatively low cost, proven reliability, wide availability, simplicity and keen insight make them a good choice for most volcanic applications and therefore they are often the first time of defense”
1) Seismometers are relatively cheap to build and easy to install (Dzurisin, 2003)
2) Provide real-time data that can be remotely analysed from only a few sensors being placed around the volcano (Salvage et al., 2017)
3) Seismic waves can easily be recorded from km away within seconds - this is not the case for geodetic or geochemical signals
Swanston et al., 1983
Dacite dome at Mt St Helens following 1980 eruption
Deformation was occurring, sometimes several meters per day, but because it was shallow it couldn’t be observed outside the crater
In contrast, thousands of shallow earthquakes were recorded by seismometers on the outer flanks
Outline the limitations of seismology (3):
1) Earthquakes can be indicative of a wide range of phenomena and thus it can be hard to determine their precise cause e.g. the occurrence of LF signals is related to the presence of a two-phase fluid in a resonating crack, but this fluid could be gas exsolved from fresh magma, a hyper saline brine derived from cooling magma, or gas-charged magma itself (Dzurisin, 2006)
2) Not all seismology means there will be an eruption - in fact most VT crises do not lead to an eruption (Newhall et al., 1988)
3) Some volcanic deformation can occur aseismically, especially when the source is relatively deep or the strain rate relatively low (Dzurisin, 2003)
