Volcano generated tsunami recorded in the near source

Tsunamis are predominantly (~80%) triggered by large earthquakes, but a significant minority (~10%) arise from aerial/submarine landslides and volcanic flank instabilities, which can produce extreme local wave heights and runup. Recent events (e.g., 2018 Anak Krakatau and Palu Bay, and 2022 Hunga Tonga atmospheric-driven tsunami) exposed gaps in conventional earthquake-centric warning systems. Volcanic islands often host weak flanks prone to large-scale failures capable of generating hazardous tsunamis over local to regional scales. Stromboli (southern Tyrrhenian Sea, Italy) features the Sciara del Fuoco, a steep (~35°) collapse scar extending subaerially and submarine for ~3 km. Paleo- and historical evidence indicates repeated flank failures and pyroclastic density currents (PDCs) capable of generating damaging tsunamis (e.g., 2002 event). Numerical models suggest Sciara-generated waves can impact Stromboli’s coasts within 3–4 minutes and nearby regions within tens of minutes, necessitating rapid, automated detection without human validation. This study leverages near-source instrumental tsunami records associated with 2019 PDCs to understand source dynamics, constrain models, and develop fast warning strategies when sources are very close to populated coasts.

The paper situates volcanic and landslide-generated tsunamis within broader tsunami science, noting their smaller global fraction but potentially greater local severity (e.g., Lituya Bay, Anak Krakatau 2018). It references: sector collapses in Hawaii, Canaries, Cape Verde, Krakatau, and Stromboli; historical volcanic tsunamis at Montserrat (1997, 2003) and Rabaul (1994); medieval tsunami deposits at Stromboli linked to ~180×10^6 m³ collapses; and the 2002 Stromboli tsunami from 10–30×10^6 m³ collapse. Prior numerical modeling for Stromboli (e.g., NHWAVE and shallow-water models) and empirical approaches (solid block vs granular flow) are reviewed, highlighting uncertainties in landslide dynamics, water interaction, and bathymetry, and limited 3D experimental constraints.

Field instrumentation and data: Two seabed pressure-sensor tsunami gauges mounted on elastic beacons (PDC at ~260 m from Punta dei Corvi; PLB at ~350 m from Punta Labronzo) measure sea level at 46–50 m depth with 125 Hz sampling, optimizing SNR for 50–200 s tsunami periods while attenuating shorter-period sea waves. A fixed visible camera (LBZ) provides georeferenced imagery of the Sciara del Fuoco slope to track PDC fronts. Additional geophysical networks operate at Stromboli (seismic, infrasound, deformation, gas, thermal/visible cameras). Image analysis: Videos were georeferenced; PDC front positions and velocities were extracted along the Sciara slope, determining impact times and locations relative to gauges. PDC dynamics modeling: A multiphase mass flow landslide model was used to fit observed front motion, with basal friction angle δ=20°, volume fraction α_s=0.56, deriving viscous drag coefficient β=0.0019 from terminal velocity u_t≈45.7 m/s. Analytical solutions show terminal velocity reached after ~15 s within first 400 m. Tsunami source location: A finite-difference nonlinear shallow-water model (FDTD) computed travel times from a grid of candidate sources (19×6 nodes, 100 m spacing) to PDC and PLB. Comparing observed inter-station delays (Δt_obs=26 s) with modeled Δt_theory identified the best source location and mean celerity. Empirical models: Two 3D empirical frameworks related tsunami amplitude to landslide parameters: (i) solid-block model (Panizzo et al.) deriving amplitude via dimensionless underwater propagation time t_r and geometry (width b, thickness h), Froude number F_r, slope θ; (ii) granular flow model (Mohammed & Fritz) expressing amplitude A as a function of water depth H_0, relative distance R=r/H_0, direction γ, F_r, relative slide thickness S=h/H_0, width B=b/H_0, and length L=V/(hbH_0). Slide geometry used a truncated hyperbolic secant footprint (elliptical planform) with volume V=0.3508 b w T. Numerical benchmarking: Empirical predictions were compared against prior 3D non-hydrostatic NHWAVE simulations for Stromboli landslides with specified parameter sets. Early warning algorithm: A STA/LTA detector tailored to Stromboli sea-state: LTA window 4500 s; STA 40 s; preprocessing via decimation and low-pass filtering; detection threshold STA/LTA=20, with logic requiring exceedance at both stations for ≥120 s. The algorithm was trained/validated on 5 years of data to ensure sensitivity (≥40 cm waves under worst sea conditions) and zero false alerts.

• Near-source records of volcano-generated tsunamis: Two 2019 paroxysms at Stromboli produced PDCs impacting the sea and generating tsunamis recorded within <1.6 km.
• PDC kinematics: Both events had nearly identical front velocities u≈45.6–45.7 m/s, reaching terminal velocity within ~400 m. Impact occurred ~28 s (3 July) and ~34 s (28 August) after paroxysm onset, implying different initial conditions despite similar steady speeds.
• July 3, 2019 tsunami: First recorded at PDC 43 s after onset with peak-to-peak Δp_PDC=2.59 m and period 40 s, then at PLB 26 s later (69 s after onset) with Δp_PLB=1.03 m and period 55 s; dispersive stretching observed. Source inversion places impact ~150 m offshore in the SW Sciara sector at depth H_0≈105 m; mean celerity c≈39 m/s; distances to gauges ~610 m (PDC) and ~1590 m (PLB).
• August 28, 2019 tsunami: Recorded only at PLB (PDC damaged by flow) with positive onset, Δp_PLB=0.60 m, period 55 s; impact-to-detection time difference 31±1 s at r=1170 m gives c≈37.7±1 m/s.
• May 19, 2021 crater rim collapse: Tsunami at PLB, A_PLB=0.54 m, c≈36.7 m/s, r=1250 m; PDC front velocity assumed u_p≈50 m/s from prior thermal estimates.
• Near-field regime and waveform: With c≈39 m/s and T≈55 s, wavelength λ≈2145 m exceeds source-gauge ranges (r≤1590 m), indicating near-field (r/λ≤1). Waveforms are symmetric with positive onset, consistent with weakly nonlinear oscillatory (Stokes-like) waves at Froude number F_r≈1.43 and relative slide thickness S≤0.34 (h_r≤36 m), matching camera-derived front thicknesses (≤30 m).
• Robust waveform invariance: Tsunami period and waveform at PLB (T≈55 s) remained essentially unchanged despite ~2 orders of magnitude variability in landslide volume (~10^4–10^6 m³ in modeling comparisons) and different source locations/dynamics.
• Volume estimation via granular model: Using granular empirical relations with b≈150–200 m: • 3 July 2019: A_PDC=2.59 m and A_PLB=1.03 m imply h_t≈20±4 m and V≈(2.14±0.5)×10^5 m³. • 28 August 2019: A_PLB=0.60 m implies h_t≈10±1 m and V≈(1.05±0.21)×10^5 m³. • 19 May 2021: A_PLB=0.54 m implies h_t≈6.5±1 m and V≈(0.71±0.15)×10^5 m³, consistent with independent image-based estimate ~0.8×10^5 m³.
• Empirical vs numerical consistency: Granular model predictions match NHWAVE simulations within a few percent and outperform the solid-block model (which overestimates amplitudes by ~20–50%). A linear relationship between tsunami amplitude A and landslide volume V is supported (fit approximately V≈6.8×10^5 A − 3.9×10^5, SI units), enabling first-order volume estimation from near-source wave height.
• Early warning performance: STA/LTA detector flagged the 3 July tsunami 7 s (PDC) and 16 s (PLB) after onset, before peak amplitude; 28 August event detected 11 s after onset despite modest amplitude (~0.2 m). On 28 August, civil protection manually activated acoustic alerts 11 s after onset and ~<4 min before coastal impact. Since Sept 9, 2019, the automated system has operated without false alerts; on Dec 4, 2022 it automatically alerted for a 1.5 m tsunami from a PDC.

The study demonstrates that in near-field conditions at Stromboli, tsunami waveforms and periods are largely insensitive to variations in landslide volume, source position, and dynamics, indicating that local bathymetry and dispersion quickly regulate the wave to a characteristic form (T≈55 s at PLB). This invariance, coupled with the strong empirical link between amplitude and landslide volume using granular-flow scaling, enables rapid hazard assessment: near-source gauges can infer slide volume from early wave height, even when detailed source parameters are unknown. The source localization and celerity estimates confirm underwater continuation of PDCs to ~105 m depth, informing realistic source depths for modeling. The early warning algorithm, tailored to local sea-state and tsunami periods, detects events within seconds of onset, providing crucial lead time (minutes) for coastal alerts when propagation distances are short (<10 min to shore). Agreement between granular empirical models and NHWAVE simulations validates the use of simplified empirical tools for operational estimation while acknowledging their domain of applicability. Together, the integrated sensing, modeling, and detection pipeline addresses the challenge of mitigating volcanic tsunamis where conventional earthquake-based systems struggle.

This work provides unprecedented near-source records of volcanic tsunamis generated by PDCs at Stromboli and shows that, despite variability in landslide volumes and dynamics, tsunami waveform and period at near-field stations remain stable. A granular-flow empirical framework reliably converts early wave amplitude into first-order slide volume, consistent with advanced NHWAVE simulations. The bespoke STA/LTA detection system achieves rapid, robust tsunami identification, enabling civil protection to issue timely alerts in scenarios with very short coastal arrival times. The approach—combining near-source seabed pressure gauges, rapid source localization, empirical volume estimation, and automated detection—offers a practical template for other volcanoes and landslide-prone coasts. Future work should expand 3D experimental datasets, refine empirical relations across diverse bathymetries and source types, integrate precomputed inundation scenarios linked to real-time amplitudes, and extend networks to ensure redundancy and coverage for larger or more complex flank-failure events.

• Generalizability: Findings are derived from Stromboli’s specific geometry and bathymetry; waveform invariance and empirical relationships may differ elsewhere.
• Instrumentation exposure: The near-source PDC gauge was impacted/damaged during one event, reducing redundancy during critical windows.
• Limited event set: Results are based on a small number of near-field volcanic tsunamis; broader statistics are needed.
• Source parameter uncertainty: Some parameters (e.g., 2021 PDC front velocity) were assumed due to poor visibility; empirical models depend on estimates of h, b, F_r, γ.
• Empirical/model assumptions: Granular-flow equations and truncated secant volume shapes simplify complex physics; few 3D experiments exist to capture lateral variability; solid-block models overestimate amplitudes.
• Near-field focus: Deep-water attenuation and far-field dispersion behavior were not the primary focus; applicability to deeper deployments may be limited by strong attenuation of relevant periods.