10:45 AM - 12:15 PM
[SSS07-P01] Differences in P-wave excitation between explosive and single-force sources
Keywords:single force source , 2022 eruption of Hunga volcano, Tonga
Teleseismic P-waves were observed worldwide when the 2022 eruption of the Funga volcano, Tonga. Some studies estimated of the source mechanism after the eruption, but the preliminary results were still confusing (e.g., Thurin et al. 2022, Garza-Girón et al. 2023). This confusion was partly a technical problem related to the numerical calculation of Green's function. It is now believed that the observed P waves can be explained by a vertical downward single force but that an explosive epicenter component is also required. The confusion could originate from the behavior of how the free surface boundary condition affects the excitation of far-field P waves by the explosion source and single force source. In this presentation, we will discuss whether it is possible to distinguish explosive sources and single force sources from far-field P-waves by evaluating the effects of free surface based on ray theory.
In this presentation, we consider P-wave excitation in a semi-infinite medium. First, for simplicity, we consider a fluid with a free surface. If the explosion source excites P waves at a sufficiently shallower depth than the wavelength, no P waves are excited to satisfy the boundary condition. In a framework of ray theory, let us consider the radiation of the P wave. P-wave is radiated both downward and upward in phase, and the reflection coefficient at the surface is -1. Because the downgoing P-wave and the surface reflection have opposite phases, they completely cancel out each other. On the other hand, in the case of a single-force source, the upgoing P-wave has the opposite phase to the downgoing wave. Therefore, the surface reflection and the direct wave are in phase. The same discussion can apply to an elastic medium. In the case of an explosive source, the excitation efficiency of far-field P-waves is low, and in the case of a single force source, far-field P-waves are significant. In the case of the 2022 eruption of Hunga volcano, Tonga, the observed waveforms show the significant amplitude of the far-field P waves, which is consistent with excitation by the vertical downward single force. It is important to separate the explosive source from the single force source when discussing the eruptive sequence of the eruption. The energy ratio of surface waves and P waves could be a proxy for the separation.
In this presentation, we consider P-wave excitation in a semi-infinite medium. First, for simplicity, we consider a fluid with a free surface. If the explosion source excites P waves at a sufficiently shallower depth than the wavelength, no P waves are excited to satisfy the boundary condition. In a framework of ray theory, let us consider the radiation of the P wave. P-wave is radiated both downward and upward in phase, and the reflection coefficient at the surface is -1. Because the downgoing P-wave and the surface reflection have opposite phases, they completely cancel out each other. On the other hand, in the case of a single-force source, the upgoing P-wave has the opposite phase to the downgoing wave. Therefore, the surface reflection and the direct wave are in phase. The same discussion can apply to an elastic medium. In the case of an explosive source, the excitation efficiency of far-field P-waves is low, and in the case of a single force source, far-field P-waves are significant. In the case of the 2022 eruption of Hunga volcano, Tonga, the observed waveforms show the significant amplitude of the far-field P waves, which is consistent with excitation by the vertical downward single force. It is important to separate the explosive source from the single force source when discussing the eruptive sequence of the eruption. The energy ratio of surface waves and P waves could be a proxy for the separation.