09:30 〜 12:00
[S09P-13] Slow Earthquakes in the Japan Trench
Slow earthquakes are episodic slow fault slips at plate boundaries. They are fundamental components of interplate deformation processes along with fast, regular earthquakes. Recent seismological and geodetic observations have begun to reveal detailed slow earthquake activity along the Japan Trench, the subduction zone where the 11 March 2011 moment magnitude (Mw) 9.0 Tohoku-Oki earthquake occurred.
Here we review observational studies on slow earthquakes along the Japan Trench. Compiling the observations of slow earthquakes (e.g., tectonic tremors, very-low-frequency earthquakes, and slow slip events) and related fault slip phenomena (e.g., small repeating earthquakes, earthquake swarms, and foreshocks of large interplate earthquakes), we present the integrated slow earthquake distribution along the Japan Trench. Slow and megathrust earthquakes are spatially complementary in distribution, and slow earthquakes sometimes trigger fast earthquakes in their vicinities. An approximately 200 km-long along-strike gap of seismic slow earthquakes (i.e., tectonic tremors and very-low-frequency earthquakes) corresponds with the huge interplate locked zone of the central Japan Trench. The 2011 Mw 9.0 Tohoku-Oki earthquake ruptured this locked zone, but the rupture terminated without deeply propagating into the slow-earthquake-genic regions in the northern and southern Japan Trench. Slow earthquakes are involved in both the rupture initiation and termination processes of megathrust earthquakes in the Japan Trench.
We then compare the integrated slow earthquake distribution with the crustal structure of the Japan Trench (i.e., interplate sedimentary units, subducting seamounts, petit-spot volcanoes, sediment thickness at the trench, horst-and-graben structures, residual gravity, seismic velocity structure, and plate boundary reflection intensity) and describe the geological environment of the slow-earthquake-genic regions (i.e., water sources, pressure–temperature conditions, and metamorphism). As a result, we find some correspondences between the slow earthquake distribution and crustal structure (e.g., fluid-rich interplate sedimentary units, subducting seamounts entraining fluid-rich sediments, and strong plate boundary reflection suggesting the existence of a thin layer of aqueous fluids) and between the slow earthquake distribution and geological environment (e.g., the site where opal and smectite dehydrate).
The integrated slow earthquake distribution enables us to comprehensively discuss the roles of slow earthquakes in the occurrence process of the 2011 Tohoku-Oki earthquake. The correspondences of the slow earthquake distribution with the crustal structure and geological environment provide insights into the slow-earthquake-genesis in the Japan Trench and imply that highly overpressured fluids are key to understanding the complex slow earthquake distribution. Furthermore, we propose that detailed monitoring of slow earthquake activity can improve forecasts of interplate seismicity along the Japan Trench.
Here we review observational studies on slow earthquakes along the Japan Trench. Compiling the observations of slow earthquakes (e.g., tectonic tremors, very-low-frequency earthquakes, and slow slip events) and related fault slip phenomena (e.g., small repeating earthquakes, earthquake swarms, and foreshocks of large interplate earthquakes), we present the integrated slow earthquake distribution along the Japan Trench. Slow and megathrust earthquakes are spatially complementary in distribution, and slow earthquakes sometimes trigger fast earthquakes in their vicinities. An approximately 200 km-long along-strike gap of seismic slow earthquakes (i.e., tectonic tremors and very-low-frequency earthquakes) corresponds with the huge interplate locked zone of the central Japan Trench. The 2011 Mw 9.0 Tohoku-Oki earthquake ruptured this locked zone, but the rupture terminated without deeply propagating into the slow-earthquake-genic regions in the northern and southern Japan Trench. Slow earthquakes are involved in both the rupture initiation and termination processes of megathrust earthquakes in the Japan Trench.
We then compare the integrated slow earthquake distribution with the crustal structure of the Japan Trench (i.e., interplate sedimentary units, subducting seamounts, petit-spot volcanoes, sediment thickness at the trench, horst-and-graben structures, residual gravity, seismic velocity structure, and plate boundary reflection intensity) and describe the geological environment of the slow-earthquake-genic regions (i.e., water sources, pressure–temperature conditions, and metamorphism). As a result, we find some correspondences between the slow earthquake distribution and crustal structure (e.g., fluid-rich interplate sedimentary units, subducting seamounts entraining fluid-rich sediments, and strong plate boundary reflection suggesting the existence of a thin layer of aqueous fluids) and between the slow earthquake distribution and geological environment (e.g., the site where opal and smectite dehydrate).
The integrated slow earthquake distribution enables us to comprehensively discuss the roles of slow earthquakes in the occurrence process of the 2011 Tohoku-Oki earthquake. The correspondences of the slow earthquake distribution with the crustal structure and geological environment provide insights into the slow-earthquake-genesis in the Japan Trench and imply that highly overpressured fluids are key to understanding the complex slow earthquake distribution. Furthermore, we propose that detailed monitoring of slow earthquake activity can improve forecasts of interplate seismicity along the Japan Trench.