Volcanic monitoring Flashcards
What is required for risk reduction (Mitigation)?
Detailed understanding of the hazard – probability of the event (e.g. frequency, magnitude) and potential processes/impacts
What is needed by geoscientists to form an idea for mitigation?
Long term and short term information
What are some examples of long term info that can be gathered from volcanoes?
Longer term behaviour and past eruptions allowing for an idea of worst case scenario and what typically happens
Where can short term volcanic info be derived from?
Monitoring of system allowing immediate understanding and decision making
What is often needed for a volcanos behaviour to be understood?
Individual study due to the uniqueness of volcanic settings
What is key to risk besides the process (hazard)?
Area impacted and the exposure and vulnerability of what lies within it
What areas will monitoring efforts usually be prioritised for?
Areas with the greatest exposure
Why must frequency vs magnitude be consiedered for mitigation/ monitoring?
Even if big events have occured in that area it takes 10,000s of years to build up magam for these events while monitoring will focus on a decadal scale in most cases
What is needed to aid monitoring systems to help reduce uncertainty?
A historical record of that volcanos behaviour
How does urbanisation affect mitigation?
With hard to mointor volcanoes in clusters and a greater population living in their proimity due to urban sprawl a greater % of the pop is at risk
What will evacuation plans be like in populated regions?
Cautious and based upon on worst likely scenario
WHat is greatly intergrated with volcanic fataility rate?
Exposure and vulnerability which exacterbate the record and mean size is not the only factor for death i.e. Ruiz 85 was typical eruption but lahars had big deaths
WHat is a potential precursor to eruptions?
Magma ascension to the surface (this has many measureable effects)
What might the warning timescale for volcanic eruptions be?
Vary from hours to years
Does unrest from volcanoes always result in eruption?
No an indepth understanding of the signals is required
What is the most difficult part to infer from signals pre-eruption?
Style and magnitude (where past events aid)
What sre the 3 main monitoring approaches for volcanoes?
Siesmicity (energy from magma movement)
Deformation (subsidence or uplift)
Gas emissions and other chemical or physical changes
WHat is made difficult once an eruption begins?
ground based monitoring impossible and not safe
AT what points of eruption is monitoring important?
before during and after (looking at fallout and ash dispersal for airspace)
WHat can volcanic seismicity be derived from?
magma, fluid or gas movement
What info can you get from volcanic seismicity?
likelihood, timing and location
What can and cant you do from only having 1 seismometer?
Movement; no. of events; changes in type (but cannot
distinguish non-volcanic)
WHat is the benefit of a multi-station seismomter network?
Precise locations; amplitudes;
fault plane solutions (as different processes have distinct signals)
What is common with volcanic siesmicity?
Swarms of volcano seismicity are common – most are not followed by eruptions
What are high frequency volcanic tectonic hazards?
Brittle failure due to magma movement, short, distinct S and P wave onsets
What are low frequency long period earthquakes?
Pressure changes in fluid-filled cracks
What will volcanic tremours look like on seismographs?
more continuous signals
How can event and type be distinguished on seimographs?
Event type (and process) distinguished by their frequency content
What is Infrasound?
Low frequency acoustic waves, propagating through atmosphere which can detect near surface and aerial activity (e.g. explosions) at distance
What are the main constituents of volcanic gas emissions?
Water (90%), but more deteacble constituents like co2, so2 and halogens also present
Why are gas emisisons not always the greatest monitoring tool?
Some volcanoes do not degas until eruption (when a path to the surface has been created) while others might have continuous degassing
What does degassing give ideas on?
Frequently active volcano
Magma levels in the conduit
If fresh magma has been supplied
What do changes in volcanic output preceed?
New or intensified activity
How can ground based gas emisison monitoring be carried out?
Spectrometer absrobs reflect light from underneath volcanic plume with different gas species having different frequencies and concentrations (intensity)
WHat might volcanic deformation be like?
Small changes in the shape of the ground surface can record magma movement or pressurisation in the subsurface
What can be found out by interpreting the deformation patterns?
location and magnitude (e.g. volume change) of these subsurface changes
WHat ground based methods are there for looking at deformation?
EDM - Electronic Distance Measurement
Levelling survey
Tilt meters
How does EDM work? (deformation)
Repeated measurements between a network of fixed points indicates relative horizontal displacement between sites (at a resolution of mm over km distances)
How do levelling surveys work?
Use a fixed network of benchmark sites
Repeated measurements showing changes in the relative angle (and therefore height) between fixed points, thus indicating vertical displacement patterns
How do tilt meters work?
Installed locally around a site of deformation (e.g. on the volcanic edifice) operate in the same way as a spirit level – sensitive to very local slope changes
How can tilt meters be usesful?
Can show relative flank movement and potential instability and thus landslide risk
What are the positives and negatives of GPS?
High precision cm scale
Location repeated meaurements give ground movement info, will tell where ground point moving relative to other ground points plotted as series of vectors
Highly effecitve widely used but don’t give complete spatial picture
Can give the when of an event
What are the positives and negatives of InSAR?
Measure shape of ground surface and looks at shape between 2 points of time, interference between 2 bits of time . The difference between images for Insar is called phase shift and if the difference in time it takes of a wave to reach the same point. Disadvantage is that it is between 2 fixed time points so don’t know when occurred so no detail of rate of change.
What can inhibit the use of InSAR?
Vegetation, ice, and atmospheric processes
What is needed for GPS to be effective?
Individual stations
What monitoring ight be done for infrequently active volcanoes?
small seismometer networks, perhaps coupled with periodic regional INSAR (deformation) surveys
What might the monitoring be like at more persistent/ active volcanoes?
more comprehensive seismic networks, long-term gas monitoring, video cameras, photogrammetry
What is the problem with the use if lots of monitoring for volcanoes?
Such systems are resource intensive
Any country is only likely to comprehensively monitor a few volcanoes
The majority of volcanoes have limited/no monitoring
Why is the geological record important for mitigation of volcanic hazards?
GIve info on events before the human written record
What information on volcanic behaviour can be derived from past events?
Frequency (but preservation is often limited to larger explosive eruptions)
Magnitude/style
Spatial impact
Why was chaiten not actively monitored and what was the effect of this?
Based on surface deposists chaiten had not erupted for 9500 years until 2008 however, lake sediemnts showed there had been several eruptions in the last 1000yrs it has now been made a top priority
Why is past activity often hard to reconstruct?
Dating problems (esp. effusive events)
Burial/erosion – small but frequent events are usually
absent from record, and their frequency and impact may be poorly known
Conversely, hazard maps based on historical activity may miss the rare but extreme event
