Yellowstone Volcanic-Seismic Activity Status (1984-2025)

Technical Brief for Government Reference

Prepared by: GU Institute of Earthquake Prediction (GUIEP) / GEPC Team

Date: 2025-12-29

Contact: Professor Jicheng GU / Boston, MA USA / gujicheng@guiep.org / +1.617.497.1108

Data source: USGS ComCat (FDSN Event Web Service). Query: center 44.615°N, -110.6°W; radius 80 km; Mc = 1.5 / 2.0 / 2.5; depth all (max observed ~13.01 km); 1984-2025; swarms retained.

Executive Summary

Using a consistent spatial window centered on Yellowstone (44.615°N, -110.6°W; radius 80 km), we analyzed the Seismicity Index S time series from 1984-2025 at multiple magnitude thresholds and time scales (annual/monthly/weekly). S exhibits a persistent long-term increase that remains present when smaller earthquakes are excluded (M≥2.0 and M≥2.5). This indicates that the observed intensification is not solely an artifact of small-event catalog completeness.

Importantly, the frequency-domain structure (FFT) does not presently show a sudden collapse of energy into a single dominant low-frequency “main peak,” a spectral signature that often

accompanies late-stage, near-critical volcanic instability. The current pattern is best described as an “elevated and gradually intensifying background state” of the shallow volcanic-hydrothermaltectonic system, not evidence of imminent eruption. We recommend continued integrated monitoring and propose quantitative trigger thresholds for escalation.

1.  Data and Study Definition

Region: Circular window, 80 km radius, centered at 44.615°N, −-110.6°W (Yellowstone). Depth: All cataloged depths in the dataset; observed maximum depth ~13.01 km (shallow system).

Time window: 1984-2025 (modern continuous monitoring era).

Magnitude thresholds (Mc): Primary runs at M≥1.5, with robustness tests at M≥2.0 and M≥2.5.

Swarms: Retained (no declustering). This is appropriate for volcanic systems where swarms are a key signal, not “noise.” 

Method: We compute the Seismicity Index S(t) at annual/monthly/weekly resolutions and evaluate long-term trends and spectral properties via FFT.

2.  Key Findings

2.1  Long-term increase persists across magnitude thresholds

The annual S series shows a positive linear trend in 1984-2025 for all tested thresholds:

• M≥1.5: slope ≈ +0.012 / year 

• M≥2.0: slope ≈ +0.0189 / year 

• M≥2.5: slope ≈ +0.0079 / year 

Interpretation: The persistence of the trend at M≥2.0 and M≥2.5 demonstrates that the intensification is not driven solely by the increasing detectability of very small earthquakes. The magnitude-threshold consistency supports a genuine increase in moderate event activity and/or underlying system energy release.

2.2  System remains in an “elevated background” spectral regime

FFT analyses (annual and monthly) show distributed low-to-mid frequency energy without a single dominant low-frequency peak overwhelming the spectrum. This spectral pattern is consistent with a system where:

• multiple processes contribute (tectonic stress, hydrothermal fluid migration, fracture opening/closing),

• the system may be gradually intensifying,

• but has not entered a near-critical “spectral convergence” stage.

Working hypothesis: A late-stage pre-eruptive regime often exhibits rapid spectral focusing (energy concentrating into one or a few dominant low-frequency modes), along with accelerating deformation and gas anomalies. Current spectra suggest we are earlier than such a stage.

3.  Risk Interpretation (What This Does and Does Not Mean)

What the results support

• Yellowstone’s shallow volcanic-hydrothermal-tectonic system shows gradually increasing seismic activity intensity over multi-decade scales.

• The increase is robust against raising magnitude thresholds, making it less likely to be a catalog artifact.

What the results do NOT claim

• This analysis alone does not indicate imminent eruption.

• Seismicity trends without corroborating deformation/gas/thermal indicators are insufficient to conclude an eruption is near.

Recommended risk statement: 

“Elevated background activity with gradual intensification; continue enhanced integrated monitoring; no basis for imminent-eruption warning from seismicity alone at present.”

4.  Proposed Monitoring Triggers for Escalation

We propose a two-tier threshold framework that is quantitative, auditable, and suitable for government decision support.

Tier A - Seismicity Index Acceleration Triggers (S-based)

Trigger A1 (Acceleration):

• The 5-year rolling trend of annual S exceeds the long-term slope by ≥2× and persists ≥24 months (monthly series may be used for earlier detection).

Trigger A2 (Sustained elevation):

• Annual S remains above the historical 90th percentile for ≥3 consecutive years (computed within the 1984-present baseline, and separately for M≥2.0 and M≥2.5).

Trigger A3 (Cross-threshold coherence):

• Anomalies occur simultaneously in M≥2.0 and M≥2.5 series (not only in M≥1.5), indicating intensification across moderate events.

Tier B - Spectral Convergence Triggers (FFT-based)

Trigger B1 (Peak dominance):

• The dominant low-frequency peak amplitude / median background amplitude exceeds a preset ratio (e.g., ≥3-5×) and remains elevated for ≥6-12 months (monthly analysis).

Trigger B2 (Spectral narrowing / increasing Q):

• Energy becomes concentrated into one/few bands with decreasing bandwidth (objective metric: decreasing spectral entropy or increasing peak sharpness).

Tier C - Integrated Confirmation Triggers (multi-parameter)

Escalate only when seismic triggers (Tier A/B) coincide with independent anomalies:

• Deformation: accelerating uplift/subsidence patterns (InSAR/GPS),

• Gas: sustained changes in CO₂/SO₂/He ratios or flux,

• Thermal/hydrothermal: persistent changes in heat output or hydrothermal activity, •    Migration: systematic hypocenter migration patterns.

Policy guidance: 

• Tier A alone → enhanced monitoring, technical briefings.

• Tier A + Tier B → elevated advisory level, interagency review.

• Tier A/B + Tier C → formal alert consideration.

5.  Recommendations

1. Maintain routine monitoring while establishing an S-based dashboard updated monthly and quarterly.

2. Adopt the above trigger framework as a living protocol, refined as more data are accumulated.

3. Ensure intercomparison across magnitude thresholds (M≥1.5/2.0/2.5) to reduce catalog bias concerns.

4. Integrate seismic indicators with deformation and gas data to avoid false positives.

Technical Notes / Caveats

• Seismicity-only indicators are necessary but not sufficient for eruption warning.

• Very long “periods” inferred from FFT near the record length should not be overinterpreted; operational decisions should rely on shorter, well-resolved bands and on timedomain acceleration tests.

• The protocol is designed for risk staging and monitoring escalation, not for deterministic eruption timing at this stage.  

YelloeStone Annual Sesimicity S Variation

Figure 1. Annual Seismicity Index S, Yellowstone, 1984-2025, M≥1.5, 80 km radius window. Red line: annual S; dashed blue line: linear trend.

YellowStone Anual Seismicity Varivation

 Figure 2. Annual Seismicity Index S, Yellowstone, 1984-2025, M≥2.0, 80 km radius window. Red line: annual S; dashed blue line: linear trend. 

Yellow Stone Annual Seismicity S Variation

Figure 3. Annual Seismicity Index S, Yellowstone, 1984-2025, M≥2.5, 80 km radius window. Red line: annual S; dashed blue line: linear trend. 

     Figure 4a.:  FFT-based amplitude/power spectra of annual S series (M≥2.5). Current spectra show  distributed energy without dominance of a single low-frequency peak. 

Figure 4b.: FFT-based amplitude/power spectra of monthly S series (M≥2.5). Current spectra show distributed energy without dominance of a single low-frequency peak. 

Fig 5： Yellowstone Volcano area （from USGS）