10:45 AM - 11:00 AM
[SSS07-01] Constraints on Shallow Earth Structure in Alaska and California by the Compliance Method
★Invited Papers
Keywords:Compliance method, Shallow Earth structure, Atmoapheric Lamb waves, Volcanic eruption
The compliance method is based on the analysis of data from co-located pressure and seismic sensors. It has been shown to provide constraints on shallow elastic structure, primarily the vertical shear modulus structure. The method was applied to observations on the ocean floor (e.g., Crawford et al., 1991) and on land (e.g., Tanimoto and Wang, 2019). These studies analyzed ambient seismic noise at low frequencies (< 0.1 Hz), where the coherence between surface pressure changes and ground motions can be high. Selecting such highly coherent frequency intervals ensured that we obtained information on both the cause of deformation (surface pressure) and its result (ground deformation).
Most of these analyses relied on ambient seismic noise. In this paper, we demonstrate that a similar compliance approach can be used when large atmospheric pressure waves, excited by a volcanic eruption, propagate around the Earth. Specifically, we examine the eruption of Hunga Tonga-Hunga Ha’apai (hereafter referred to as Hunga-Tonga) on January 15, 2022. This eruption has been widely reported to have generated atmospheric pressure waves (Lamb waves) that traveled around the globe (e.g., Matoza et al., 2022). These pressure waves were accompanied by ground deformations in the solid Earth, which are an integral part of Lamb waves in the coupled Earth system. In this study, we compute Lamb waves within a coupled Earth model by extending the approaches of Pekeris (1948), Press and Harkrider (1962), and Harkrider (1964) to include the elastic, solid Earth.
As a compliance method, our approach analyzes the ratio between vertical displacement and surface pressure observed when the Lamb wave passes a co-located station. Since pressure and vertical displacement are part of an eigenfunction of a Lamb wave mode, we first compute the eigenfunctions and obtain the depth sensitivity kernels for the compliance ratio through numerical differentiation. These kernels exhibit primary sensitivity to the near-surface shear modulus, with additional sensitivity to the shallow bulk modulus at hard rock sites. In this study, given the Lamb wave phase speed of approximately 310 m/s and the high-coherence range limit of around 0.01 Hz, the depth range of reliable resolution was determined to be the upper crust (approximately the uppermost 15 km).
Based on this analysis of Lamb wave signals from Hunga-Tonga, and by combining these results with findings from ambient noise analysis, we will summarize our findings about shallow structures in Alaska and California.
Most of these analyses relied on ambient seismic noise. In this paper, we demonstrate that a similar compliance approach can be used when large atmospheric pressure waves, excited by a volcanic eruption, propagate around the Earth. Specifically, we examine the eruption of Hunga Tonga-Hunga Ha’apai (hereafter referred to as Hunga-Tonga) on January 15, 2022. This eruption has been widely reported to have generated atmospheric pressure waves (Lamb waves) that traveled around the globe (e.g., Matoza et al., 2022). These pressure waves were accompanied by ground deformations in the solid Earth, which are an integral part of Lamb waves in the coupled Earth system. In this study, we compute Lamb waves within a coupled Earth model by extending the approaches of Pekeris (1948), Press and Harkrider (1962), and Harkrider (1964) to include the elastic, solid Earth.
As a compliance method, our approach analyzes the ratio between vertical displacement and surface pressure observed when the Lamb wave passes a co-located station. Since pressure and vertical displacement are part of an eigenfunction of a Lamb wave mode, we first compute the eigenfunctions and obtain the depth sensitivity kernels for the compliance ratio through numerical differentiation. These kernels exhibit primary sensitivity to the near-surface shear modulus, with additional sensitivity to the shallow bulk modulus at hard rock sites. In this study, given the Lamb wave phase speed of approximately 310 m/s and the high-coherence range limit of around 0.01 Hz, the depth range of reliable resolution was determined to be the upper crust (approximately the uppermost 15 km).
Based on this analysis of Lamb wave signals from Hunga-Tonga, and by combining these results with findings from ambient noise analysis, we will summarize our findings about shallow structures in Alaska and California.