Seismicity and focal mechanisms of deep low-frequency earthquakes in the Iwate Volcano, Northeast Japan

Deep low-frequency earthquakes (DLFs) have been observed at a depth of about 30 km near the Moho around active volcanoes. Source process of DLFs is thought to be related to the deep magma supply system (e.g., Nakamichi et al. 2003) and the number of them sometimes increase several months before an eruption. Therefore, DLFs is a key information for understanding the magma supply process from deep to shallow (e.g., Shapiro et al. 2017). The Iwate volcano, which located in Northeastern part of Japan, has experienced a volcanic unrest such as abnormal crustal deformation and increase of seismicity including DLFs since February 2024. In this study, we investigated the seismicity and focal mechanisms of DLFs in the Iwate volcano based on the matched filter method (e.g., Gibons and Ringdal, 2006) and waveform inversion.

We performed matched filter analysis for the period from January 2019 to December 2025. We used 175 template events relocated by Kurihara and Obara, (2021). This analysis detected approximately 8000 events over a 6-year period. The Iwate volcano has three DLF clusters: 10 km depth just beneath the volcano, 30 km depth of northern and southern parts of it. Among them, seismicity of DLFs in the north cluster has extremely increased since August 2024 and the number of them has also increased in the shallow cluster since three months after the activation of northern cluster. This result suggests the possibility that magma supply from deep to shallow areas has continued.

Then, we estimated source time functions and moment tensor components for template events based on the procedure of Aso and Ide (2014). Obtained moment tensors have various orientations and significant compensated linear vector dipole component, which is consistent with the previous study of DLFs in the Iwate volcano (Nakamichi et al. 2003). However, comparing focal mechanisms and seismicity, we found that DLF activity is mainly composed of events detected from specific templates. This result suggests the existence of a dominant local stress field such as the shape of stagnated magma near the Moho. As a preliminary interpretation, activation of DLFs can be caused by the volumetric deformation associated with intrusion into the magma.