17:15 〜 19:15
[SCG45-P32] Repeating earthquake swarms following bursts of shallow tectonic tremor near the Japan Trench, Off Fukushima
キーワード：低周波微動、海底地震計、日本海溝
Despite of contrasting rupture processes and separation in the source regions, activities of slow and fast earthquakes often synchronize in space and time. Due to limited observations of the synchronization between slow earthquake and large or moderate-sized fast earthquakes, some researches focus on interaction between slow earthquake and microseismicity that are potentially occurring more frequent. Yamamoto et al. (2022) reported shallow SSEs in the Nankai subduction zone frequently accompany earthquake swarms. Similar synchronous activity also occurs in the Japan Trench subduction zone. Obana et al. (2021) reported bursts of shallow tremor often synchronized with earthquake activity in the Off Fukushima section, where is the southern proximity of the coseismic slip region of the 2011 Tohoku-Oki Earthquake (Iinuma et al., 2012; Kubota et al., 2023). However, their observation was made by short-term (four months) Ocean-Bottom Seismometer (OBS) observation. Because of the limited observational period, it was not clear how frequent such synchronous activity of slow and fast seismicity is occurring in the Off Fukushima.
In order to investigate the correlation of the slow and fast earthquake in Off Fukushima near the Japan Trench, we compare the long-term (six years; 2016 - 2022) catalogs of tectonic tremors and fast earthquakes. These two catalogs are constructed based on OBSs comprising S-net (Aoi et al., 2020). In particular, we chose eight OBSs that located around a known slow earthquake source region of Off Fukushima (Nishikawa et al., 2019; 2022; Matsuzawa et al., 2015; Ohta et al., 2019). Tectonic tremors are detected and located using a modified envelope correlation method (Mizuno and Ide, 2019). The tremor catalog constructed by the envelope correlation method is further refined by removing fast earthquake signals and airgun signals generated during active source seismic surveys, as well as space-time clustering (e.g. Wech and Creager, 2008). Fast earthquakes are detected by the Earthquake Transformer (Mousavi et al., 2020), which is a deep learning-based module to detect a fast earthquake from a continuous waveform. Detected earthquakes are phase- associated by the REAL (Zha et al., 2019) and located by hypomh (Hirata and Matsu’ura, 1987) using a local 1D velocity model.
Based on the constructed catalog, we identified at least four sources of earthquake swarms which activities correlated with recurring bursts of tectonic tremors. All of them are located in the surrounding region of the tectonic tremor sources. Especially, the swarm source located 50 km apart from the tectonic tremor source region repeatedly activated after tremor bursts. This swarm source is embedded in the region where seismicity is low relative to nearby region. During the analyzed period, at least five earthquake swarm sequences occurred in this region. While the tremor bursts itself recur every ~ three months, the earthquake swarms synchronous with the tremor bursts occur almost every ~ one year. This result indicates that even though tremor burst does not always accompany seismicity activation, episodic synchronization of slow and fast earthquakes taking place in the shallow part of the Japan Trench subduction zone.
In order to investigate the correlation of the slow and fast earthquake in Off Fukushima near the Japan Trench, we compare the long-term (six years; 2016 - 2022) catalogs of tectonic tremors and fast earthquakes. These two catalogs are constructed based on OBSs comprising S-net (Aoi et al., 2020). In particular, we chose eight OBSs that located around a known slow earthquake source region of Off Fukushima (Nishikawa et al., 2019; 2022; Matsuzawa et al., 2015; Ohta et al., 2019). Tectonic tremors are detected and located using a modified envelope correlation method (Mizuno and Ide, 2019). The tremor catalog constructed by the envelope correlation method is further refined by removing fast earthquake signals and airgun signals generated during active source seismic surveys, as well as space-time clustering (e.g. Wech and Creager, 2008). Fast earthquakes are detected by the Earthquake Transformer (Mousavi et al., 2020), which is a deep learning-based module to detect a fast earthquake from a continuous waveform. Detected earthquakes are phase- associated by the REAL (Zha et al., 2019) and located by hypomh (Hirata and Matsu’ura, 1987) using a local 1D velocity model.
Based on the constructed catalog, we identified at least four sources of earthquake swarms which activities correlated with recurring bursts of tectonic tremors. All of them are located in the surrounding region of the tectonic tremor sources. Especially, the swarm source located 50 km apart from the tectonic tremor source region repeatedly activated after tremor bursts. This swarm source is embedded in the region where seismicity is low relative to nearby region. During the analyzed period, at least five earthquake swarm sequences occurred in this region. While the tremor bursts itself recur every ~ three months, the earthquake swarms synchronous with the tremor bursts occur almost every ~ one year. This result indicates that even though tremor burst does not always accompany seismicity activation, episodic synchronization of slow and fast earthquakes taking place in the shallow part of the Japan Trench subduction zone.