Abstract

To explore a local relationship between geological structures and the occurrence of very-low-frequency earthquakes (VLFEs), a particular class of slow earthquakes with characteristic periods of 10–100 s, we investigated three-dimensional (3D) structural features using reprocessed 3D seismic data from the Nankai Trough off Kumano, southwestern Japan. In this region, VLFEs have been observed along the subducting Philippine Sea Plate. Although the detailed source distribution of VLFEs was estimated by means of recent land-based and offshore seismic networks, the relation with geological features is not well understood. First, we reprocessed the 3D seismic data with advanced techniques and reinterpreted the fault distribution in the sediment layer of the accretionary prism and tracked two key horizons: a décollement and the oceanic crust surface. In the accretionary prism sediments, multiple continuous reflectors of basal detachments in the underthrust sequence and conjugate faults cutting the shallow imbricated thrust sequence were identified. In contrast to the gentle variation in the décollement surface, the topographic relief of the oceanic crust was prominent, with ridges and surface displacement due to faults in the oceanic crust. Then, we compared the structural features with the VLFE source locations. Most VLFEs were located deep in the underthrust sediments where the sediments may consist of underconsolidated muds. Furthermore, a high spatial correlation was observed between the VLFE distribution and the oceanic crust topographic relief. The maximum stress direction, which was inferred from the conjugate faults in the imbricated thrust zone, was consistent with the spatial relation between the VLFE localization and the oceanic crust central ridge. Oceanic crust ridges may cause strain accumulation in the underthrust sediments on the landward sides of the ridges, and low-angle slow thrust movements might be caused using weak slip planes in the underthrust muddy sediments. That is, the topographic relief of the oceanic crust may control the occurrence of shallow VLFEs in the Nankai Trough.

Highlights

  • Recent observations using land-based and offshore seismic and geodetic networks have revealed spatial variations in the occurrence of slow earthquakes, which are Shiraishi et al Earth, Planets and Space (2020) 72:72 and low-frequency tremor (LFT), and their occurrence times and locations are correlated (e.g., Ito et al 2007)

  • The topographic change in the décollement above the central ridge in the oceanic crust is small, while the topographic high associated with the steep slope of the décollement in the southwestern area corresponds to another ridge in the oceanic crust

  • We showed a local relationship between 3D structures and shallow Very-low-frequency earthquake (VLFE) in the Nankai Trough off Kumano

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Summary

Introduction

Recent observations using land-based and offshore seismic and geodetic networks have revealed spatial variations in the occurrence of slow earthquakes, which are Shiraishi et al Earth, Planets and Space (2020) 72:72 and low-frequency tremor (LFT), and their occurrence times and locations are correlated (e.g., Ito et al 2007). In the Nankai Trough, southwestern Japan, Ito and Obara (2006) detected VLFEs using a land-based seismic network. Takemura et al (2019) located VLFE sources from observations using the land-based seismic network of southwestern Japan in a wide area along the Nankai Trough during a long interval from 2016 to 2018. These studies suggest spatial variations in the distribution and their mechanism along the subducting Philippine Sea Plate boundary (Obara and Kato 2016)

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