9:45 AM - 10:00 AM
[SVC26-04] Source model of deep low-frequency earthquakes beneath Zao volcano
Keywords:Zao, deep low-frequency earthquake, source model, tensile-shear crack, volcano seismology, deep magma plumbing system
Deep low-frequency earthquakes (DLFs) beneath volcanoes are possible evidence for deep-seated magmatic activity in the mid-to-lower crust and uppermost mantle. After the 2011 Tohoku-oki earthquake, the number of the DLFs beneath Zao volcano increased. The hypocenters of the DLFs form two clusters at shallower (20–28 km) and deeper (28–38 km) depths located near the side and lower parts of the high Vp/Vs zone, respectively (Okada et al., 2015). Ikegaya and Yamamoto (2021) estimated that the focal mechanism of the DLFs in both clusters had large isotropic (ISO) and deviatoric components using the S/P spectral ratio. Ikegaya and Yamamoto (2023, VSJ) further indicated that the source process of the DLFs consisted of the initial double couple (DC) and the long-lasting non-DC components by the temporal change of the S/P ratio within the waveform. In this presentation, we discuss the source model of the DLFs, and the dynamic process of deep magma.
We adopted the tensile-shear crack model, which is composed of the oscillation of the tensile crack coupled and the fault slip, as the most plausible model of the DLFs to explain the observed characteristics. To discuss whether the single or multiple tensile-shear crack model is more preferable, we examined these two models in terms of the following two aspects: First, we investigated the area occupied by these two models in the source type plot of Hudson et al. (1989); Second, we compared the decrease in the temporal change of the S/P ratio between these models. In the first examination, we assumed the moment tensor composed of a tensile crack and a co-planer oblique shear crack at 45 degrees, representing the single tensile-shear crack model. For the multiple tensile-shear crack model, we assumed a combination of two moment tensors representing both a DC mechanism and randomly oriented multiple tensile cracks. The principal axis orientations of the two moment tensors are shared. In the second examination, we calculated the temporal change of the S/P ratio for both models assuming initial DC and oscillating non-DC components. We set the different ratios of the ISO and CLVD components for the single (5/4) and multiple (0.57/0.26) tensile-shear crack models. Green’s function was calculated using the frequency-wavenumber integration method (Saikia, 1994) and the 1-D velocity structure JMA2001 (Ueno et al., 2002).
It was found that the area of the focal mechanism estimated from the observed DLFs in the source type plot could be explained only by the multiple tensile-shear cracks with various orientations. The variation in the crack orientation was estimated to be larger than 20° for the deeper cluster and larger than 42° for the shallower cluster, respectively. The temporal change of the S/P ratio by the multiple tensile-shear cracks showed a larger decrease of the S/P ratio than the single tensile-shear crack and explained the observed temporal behavior of the S/P ratio better. Thus, from these two aspects, we concluded that the multiple tensile-shear crack model was preferable. In addition, considering the concentration of the DC component near the onset and the large ISO component, the quasi-simultaneous fracturing of the multiple tensile-shear cracks by the external stress should be more plausible than the successive fracturing by pressure propagation in the fluid-filled crack or dynamic stress change. Ikegaya and Yamamoto (2021) reported the migration of the activity from deeper to shallower clusters, which may indicate the magma supply to the deep magma reservoir. Thus, the magma supply would be the source of the external stress triggering the occurrence of the DLFs.
In this study, we revealed the dynamic behavior of deep magma by utilizing the temporal variation of the S/P ratio. Conducting similar examinations of the DLFs beneath other volcanoes would further enhance the understanding of the relationship between the occurrence of the DLFs and the deep magmatic activity.
We adopted the tensile-shear crack model, which is composed of the oscillation of the tensile crack coupled and the fault slip, as the most plausible model of the DLFs to explain the observed characteristics. To discuss whether the single or multiple tensile-shear crack model is more preferable, we examined these two models in terms of the following two aspects: First, we investigated the area occupied by these two models in the source type plot of Hudson et al. (1989); Second, we compared the decrease in the temporal change of the S/P ratio between these models. In the first examination, we assumed the moment tensor composed of a tensile crack and a co-planer oblique shear crack at 45 degrees, representing the single tensile-shear crack model. For the multiple tensile-shear crack model, we assumed a combination of two moment tensors representing both a DC mechanism and randomly oriented multiple tensile cracks. The principal axis orientations of the two moment tensors are shared. In the second examination, we calculated the temporal change of the S/P ratio for both models assuming initial DC and oscillating non-DC components. We set the different ratios of the ISO and CLVD components for the single (5/4) and multiple (0.57/0.26) tensile-shear crack models. Green’s function was calculated using the frequency-wavenumber integration method (Saikia, 1994) and the 1-D velocity structure JMA2001 (Ueno et al., 2002).
It was found that the area of the focal mechanism estimated from the observed DLFs in the source type plot could be explained only by the multiple tensile-shear cracks with various orientations. The variation in the crack orientation was estimated to be larger than 20° for the deeper cluster and larger than 42° for the shallower cluster, respectively. The temporal change of the S/P ratio by the multiple tensile-shear cracks showed a larger decrease of the S/P ratio than the single tensile-shear crack and explained the observed temporal behavior of the S/P ratio better. Thus, from these two aspects, we concluded that the multiple tensile-shear crack model was preferable. In addition, considering the concentration of the DC component near the onset and the large ISO component, the quasi-simultaneous fracturing of the multiple tensile-shear cracks by the external stress should be more plausible than the successive fracturing by pressure propagation in the fluid-filled crack or dynamic stress change. Ikegaya and Yamamoto (2021) reported the migration of the activity from deeper to shallower clusters, which may indicate the magma supply to the deep magma reservoir. Thus, the magma supply would be the source of the external stress triggering the occurrence of the DLFs.
In this study, we revealed the dynamic behavior of deep magma by utilizing the temporal variation of the S/P ratio. Conducting similar examinations of the DLFs beneath other volcanoes would further enhance the understanding of the relationship between the occurrence of the DLFs and the deep magmatic activity.