Seismic techniques and suggested instrumentation to monitor volcanoes

TL;DR

This USGS Scientific Investigations Report chapter recommends seismic capabilities and instrumentation for monitoring volcanoes at four volcano levels, emphasizing broadband seismometers, on-scale recording, and detection/location of diverse volcanic seismicity. It provides tiered network and detection thresholds from Level 1 through Level 4.

Key findings

• Recommended network summaries: Level 1—five seismic stations within 200 km of the volcanic center, including two within 50 km; Level 2—five stations within 50 km, including two within 10 km; Level 3—seven or more broadband stations within 20 km, including at least two within 5 km; Level 4—12 to 25 broadband stations within 20 km, including at least 8 within 10 km and 4 or more within 5 km, plus at least one broadband or strong-motion station within 10 km.
• Detection and location goals: Level 1—detect >M1.5 and crudely locate >M3; Level 2—detect >M1, crudely locate >M2, and detect energetic seismic tremor; Level 3—detect >M0.5, accurately locate >M1, record and distinguish all event types, and detect tremor; Level 4—detect and accurately locate >M0, determine event type, detect and locate tremor, and provide on-scale recordings of energetic seismicity.
• Instrumentation guidance: broadband seismometers are preferred over short-period instruments; at least four seismic observations are required to locate earthquakes, with eight or more optimal; at least one station above the seismic source is critical for tracking depth; modern 24+ bit digitizers greatly increase on-scale recording range; minimum sample rate is 50 samples per second, with 100 samples per second an ideal goal; if >M4 earthquakes or large explosions are expected, at least one strong-motion sensor should be co-located with a broadband seismometer within 10 km of an expected vent.
• VLP and stress monitoring: VLP events can be detected with two broadband sensors within a few wavelengths (<5 km typically), located with at least three broadband stations within 5 km, and moment-tensor calculations typically use more than 10 stations; fault-plane solutions are poorly determined with fewer than 7 first motions and most reliable with 10 or more; spatial b-value monitoring requires event location to within 1-km accuracy.

Methods

• Design: a USGS Scientific Investigations Report chapter that synthesizes existing volcano seismology literature and prior instrumentation planning to recommend capabilities and instrumentation by volcano level 1–4.
• Data or sample: no new dataset is collected; the chapter draws on cited case studies and prior simulation work, including Thelen (2014), which simulated several thousand network configurations for Hawaiian shield volcanoes.
• Main analysis: qualitative synthesis and tiered recommendations for detecting changes in earthquake location, energy release, event type, source properties, localized stress fields, seismic source/path changes over time, and VLP/long-period events.

Limitations

• The paper states that emerging techniques and instrumentation such as distributed acoustic sensing and rotational seismometers are not considered because they still require maturation before general use in volcano monitoring.
• The paper states that localized recommendations for each unique volcanic system are beyond its scope and left to local experts; VLP moment-tensor calculation is difficult in real time because of complex volcanic topography and structure and is computationally intensive; dense station deployments within 5 km are only possible at a few volcanoes or for limited temporary deployments.
• Evident from the text: the recommendations are expert/consensus-based rather than validated by a new empirical test, and many thresholds depend on local geography, land-use restrictions, logistics, and volcano-specific conditions.