Complex Interaction, Striations, and Periodicity of Deep Tremor Surrounding a Tremor Gap in the Central Nankai Subduction Zone, Japan

• Published in: Journal of geophysical research. Solid earth 2023-03, Vol.128 (3), p.n/a
• Main Authors: Ohta, Kazuaki, Shiomi, Katsuhiko, Matsuzawa, Takanori
• Format: Article
• Language: English
• Subjects: Distribution; Earthquakes; Geophysics; Nankai subduction zone; Periodic variations; Periodicity; Physical properties; Plate motion; Plate tectonics; Plates (tectonics); Seismic activity; Seismicity; Slip; slow earthquakes; slow slip; Spatial distribution; Spatial variations; Striations; Subduction; Subduction (geology); Subduction zones; tectonic tremor; Tectonics; Tremors
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB025373

The Nankai subduction zone is characterized by a belt of deep tremor extending from western Shikoku to the Tokai region and a tremor seismicity gap beneath the Kii Channel between Shikoku and the Kii Peninsula. Tremor activities near the gap are not well understood because of poor seismic network coverage. In this study, we conducted an observation campaign in eastern Shikoku to investigate the detailed tremor activity across the tremor gap. The new tremor catalog shows high detectability and resolution, with tremor clusters in both eastern Shikoku and the western Kii Peninsula having distinct shapes. We discovered characteristic time intervals between tremor activity on both sides of the gap, suggesting the occurrence of undetected slow slip events (SSEs) within the gap that propagated approximately 10 km/day. We also found a relationship between the tremor activities and a shallower long‐term SSE. Our examination of the detailed distribution of tremor events showed a distinct striation pattern that may be associated with the history of the subducting plate motion and the plate geometry, and our examination of their periodicity showed spatial variations in the dominant period of tremor activity that likely reflect the heterogeneous physical properties of the plate interface. Plain Language Summary Slow earthquakes are imperceptible slip events occurring close to the generation areas of megathrust earthquakes in many subduction zones. The smallest member of a family of slow earthquakes is tectonic tremor, and it is thought that the occurrence of background slow slip events and the physical properties of the plate interface can be inferred from tremor activity. The Nankai subduction zone in southwest Japan, which is one of the most active slow earthquake regions in the world, is characterized by a gap in tremor distribution associated with a complicated plate geometry. To understand tremor surrounding this gap, we analyzed the tremor activity in the central Nankai subduction zone observed by a temporary dense seismic observation network. We discovered an interaction of tremor activities across the tremor gap, suggesting that undetectable slow slip events occur in the gap. These tremor activities also interacted with other geodetically observed slow slip events. We also revealed that the spatial distribution and periodicity of the tremor events were associated with the plate subduction history and the physical properties of the plate interface. Our findings