Seismic Observation of Tsunami at Island Broadband Stations

Abstract

Previous studies have reported seismic observations of tsunami recorded at island broadband stations. Coastal loading by the tsunami can explain them. For further quantification, we model tsunami propagation assuming an axisymmetric structure: a conical island with a flat ocean floor. The total tsunami wavefield can be represented by superposition between an incident tsunami wave and the scattering. The ground deformation due to the total tsunami wavefield at the center is calculated using static Green's functions for elastic half‐space with a first‐order correction for bathymetry. By fitting the modeled displacement to observed seismic data, we can infer the incident tsunami wave, which can be interpreted as the virtual tsunami amplitude without the conical island. First, we apply this new method to three components of seismic data at a volcano island, Aogashima, for the 2015 Torishima‐Oki tsunami earthquake. The estimated tsunami amplitude from the vertical component is consistent with the offshore array observation of absolute pressure gauges close to the island (1.5–20 mHz). The estimated incident azimuth from the three components is also consistent with ray theory. Second, we apply this method to seismic data at four island broadband stations in the Indian ocean for the 2010 Mentawai tsunami earthquake in Indonesia. Despite the limited observed frequency range from 0.5–2.0 mHz, the amplitudes and incident azimuths are consistent with past studies. These observations can complement offshore tsunami observations. Moreover, this method is applicable not only for a tsunami but also for background ocean infragravity wave activity.

Ultrabroadband Seismic and Tsunami Wave Observation of High‐Sampling Ocean‐Bottom Pressure Gauge Covering Periods From Seconds to Hours

“…A pressure change of Δ p = 1 hPa corresponds to a sea‐surface height change of Δ η = 1 cm (Δ p = ρ w g 0 Δ η , ρ w ∼ 1.03 g/cm 3 : seawater density, g 0 = 9.8 m/s 2 : gravitational acceleration); therefore, the tsunami amplitudes in PGs and tide gauges are comparable. Tsunami‐related tilt changes are also recorded with the coastal tiltmeter (e.g., Kimura et al, 2013; Nishida et al, 2019). The amplitudes of seismic waves and tsunamis in the PGs are almost comparable, whereas tsunamis recorded in the tiltmeter (orange trace in Figure 2c) have much smaller amplitudes than the seismic waves, by magnitudes of 10 2 .…”

Seismic Detection of Oceanic Internal Gravity Waves From Subaerial Seismometers

“…It is interesting to note that earthquakes can produce acoustic waves that propagate along the low velocity channel of the ocean (T‐waves), and travel times of T‐waves are impacted by the internal tide (Munk et al., 1981; Sugioka et al., 2005; Wu et al., 2020). Tsunami waves are a type of infragravity wave that have induced tilt signals due to loading on the seafloor observable on both island (Nishida et al., 2019; Yuan et al., 2005) and coastal (Boudin et al., 2013; Nawa et al., 2007) broadband seismometers; it is therefore possible for seafloor pressure perturbations to generate near‐field tilt that is observable onshore.…”