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Description: 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 the offshore array observation. 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.