Analysis and Modeling of Near-Field Tsunami-induced Tilt Signals at Coastal Broadband Seismometers at Stromboli Volcano, Italy

Volcanic landslides along the Sciara del Fuoco (SdF) flank of Stromboli frequently enter the sea, triggering near-field tsunamis. These tsunamis produce static pressure loads on the seafloor and coastal areas, inducing elastic ground deformation detectable by nearby broadband seismic stations as measurable ground tilt signals.

We present a comprehensive investigation of tsunami-induced ground tilt recorded at inland coastal seismometers to test the potential for early tsunami detection and modeling. Our analysis focuses on near-field coastal broadband seismic records generated by the tsunamigenic landslide event of 3 July 2019 at Stromboli. These records are dominated by tilt induced by static tsunami loading, exhibiting distinctive horizontal very-long-period (VLP) seismic signals ranging from about 70 to 120 seconds, polarized perpendicular to the coastline.

To establish the physical connection between tsunami generation and the observed coastal ground tilt signals, we implemented a model to compute the effect of elastic ground deformation induced by quasi-static tsunami loading, using outputs from the tsunami modeling of this specific event.

Tsunami generation and propagation were simulated using the Multilayer-HySEA hydrodynamic numerical model (Macías et al., 2021), which accurately reproduces the observed tsunami signals at three sea-level stations around Stromboli. Moreover, the tilt signals computed from the tsunami load model fit satisfactorily the observed seismically derived tilt signals at the coastal broadband seismic stations, capturing even the early tsunami phase. The analysis demonstrated the early detectability of the tsunami: clear tilt signals emerge ~0.5-1.5 minutes before tsunami arrival at the nearest offshore and coastal gauges, respectively. This highlights the potential of coastal seismic sensors as a tsunami detector and for providing short-term early warnings before the tsunami reaches the coast.

The recorded tsunami-induced tilt amplitudes range from 0.05 to 0.15 µrad, decreasing with the station’s distance from the coast and remaining detectable up to ~500 m. This underscores the importance of station proximity for effective tsunami detection.

We also establish an empirical scaling relationship between coastal tilt amplitudes and tsunami height, potentially providing a practical tool to estimate tsunami amplitudes for future events. After further testing, this approach may complement traditional tsunami monitoring and warning systems.