9.1 Seismic Monitoring and Interpretation

Volcanoes rumble and shake before they erupt. Seismic monitoring listens to these underground rumblings, helping scientists predict eruptions. By tracking different types of earthquakes and tremors, experts can tell when magma is on the move and pressure is building up.

Seismometers placed around volcanoes pick up ground vibrations 24/7. This data helps pinpoint where magma is moving and how close it is to the surface. As activity increases, it can provide crucial early warnings of impending eruptions, potentially saving lives.

Seismic Monitoring for Volcanoes

Principles and Importance

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• Seismic monitoring detects and tracks volcanic activity in real-time by recording ground motion caused by magma movement, rock fracturing, and explosions within a volcano
• Seismometers measure and record ground motion in three dimensions (vertical, north-south, east-west)
  • Deployed in networks around a volcano to triangulate the location and depth of seismic events
• Increases in the number, size, and type of earthquakes generated by a volcano indicate:
  • Magma rising to the surface
  • Pressurization of the volcanic system
  • Structural instability that may lead to an eruption
• Seismic data is transmitted in real-time to volcano observatories
  • Analyzed to detect changes in activity, assess hazards, and inform emergency management decisions regarding evacuations and mitigation efforts
• Provides an early warning system for impending eruptions by detecting volcanic unrest weeks to months before other signs of activity (deformation, gas emissions)

Volcanic Seismic Signals

Earthquake Types

• Volcano-tectonic (VT) earthquakes are high-frequency events caused by brittle rock fracture as magma forces its way through the crust
  • Indicates magma movement and pressure buildup in the volcanic system
• Long-period (LP) earthquakes are low-frequency events generated by fluid movement (magma, gas, or hydrothermal fluids) in cracks and conduits
  • Often associated with pressure changes and gas release
• Hybrid earthquakes have characteristics of both VT and LP events
  • Suggests a combination of rock fracture and fluid movement processes as magma ascends and interacts with the surrounding rock
• Explosion earthquakes are high-amplitude, short-duration events associated with volcanic explosions
  • Often followed by a low-frequency coda caused by the air shock wave and ejection of volcanic material

Other Seismic Signals

• Tremor is a continuous, rhythmic seismic signal that can last minutes to days
  • Caused by sustained fluid movement or resonance in magma conduits
  • Often indicates magma ascent and pressurization of the volcanic system
• Surface events (rockfalls, pyroclastic flows, lahars) generate high-frequency, emergent seismic signals as material moves down the volcanic edifice
  • Can be used to track the progress and size of these hazards

Interpreting Seismic Data

Inferring Volcanic Activity

• An increase in the number and size of VT earthquakes over time often indicates:
  • Magma ascending from depth
  • Pressurizing the shallow volcanic system
  • May lead to an eruption if the pressure exceeds the strength of the overlying rock
• The appearance and increase in LP earthquakes and tremor suggests:
  • Magma and gases are moving through the shallow volcanic system
  • An eruption may be imminent if these signals intensify or shallower
• Changes in the depth and location of earthquakes over time can track the movement of magma toward the surface
  • Identifies regions of the volcano experiencing increased stress and prone to failure

Quantifying Seismic Energy

• Seismic energy release can be quantified using metrics (Real-time Seismic Amplitude Measurement or RSAM)
  • Assesses the overall level of activity
  • Identifies patterns or changes that may indicate evolving unrest or eruption dynamics
• Explosion earthquakes provide evidence that a volcano is actively erupting
  • Can estimate the size and intensity of the explosions based on their amplitude and duration
• The location and frequency of surface events indicate areas of the volcano that are unstable
  • May collapse to generate rockfalls, pyroclastic flows, or lahars
  • Allows hazards to be tracked and mapped in real-time

Seismic Networks for Volcanoes

Network Design and Deployment

• Seismic networks consist of multiple seismometers deployed in a geometry that allows accurate earthquake location and depth determination using triangulation methods
• Seismometers are typically installed in shallow boreholes or vaults
  • Minimizes surface noise and ensures good coupling with the ground
  • Often co-located with other monitoring instruments (GPS, tiltmeters)
• Seismic stations must be spaced close enough to accurately locate earthquakes but far enough apart to cover the entire volcanic system
  • Typical network consists of 6-20 stations depending on the size and complexity of the volcano
• Telemetry systems transmit seismic data in real-time from remote stations to a central observatory
  • Uses radio, cellular, or satellite links
  • Allows data to be quickly analyzed and interpreted

Maintenance and Challenges

• Seismic networks must operate continuously in harsh volcanic environments
  • Requires robust power systems (solar, wind, generator), lightning protection, and rugged enclosures
  • Withstands extreme temperatures, acidic gases, and ash fall
• Regular maintenance is required to keep seismic stations operational
  • Replacing batteries, cleaning solar panels, repairing damage from volcanic activity or vandalism
  • Can be challenging in remote or hazardous areas
• Temporary seismic deployments using portable instruments supplement permanent networks during periods of unrest or to study specific regions in more detail
  • Requires additional logistics and personnel to install and maintain