[S-SS24_1PM2] Seismicity

[SSS24-06] A boundary of stress-field orientation in northwestern area of the Kanto plain

*Tomoko elizabeth YANO¹, Tetsuya TAKEDA¹, Katsuhiko SHIOMI¹ (1.NIED)

Keywords:Seismicity and tectonics

Kanto-Tokai area is particularly important in terms of seismic hazard and mitigating disaster since this area is having high potential to economic and social impacts. Despite the fact, the Kanto region is one of the most seismic active areas due to its complicated tectonics and has an active fault zone, containing Fukaya fault, in northwestern area of the Kanto plain, which has potential to the M8 class earthquake. Many studies and research projects have attempted to understand the seismic activity and stress field. However, reliable and high-resolution catalog is required for the detailed discussion.We have launched Japan Unified High-resolution Relocated Catalog for Earthquakes (JUICE) project since 2013. Events were relocated using the Double-Difference method for high-resolution hypocenter location to estimate seismogenic layer thickness, to evaluate active faults, and to understand the tectonic processes in Japan. We have completed for the first version of Catalog in the region of Kanto-Tokai area for the shallow (> 40 km) earthquakes between M0 and M6.5 from 2001 to 2012. Here, in this presentation, we introduce the result from JUICE focusing on the northwestern area of the Kanto plain that contains a sharp boundary in which pressure and tension axis dramatically change by 90 degrees.The JUICE catalog clearly shows a band of seismicity from Izu peninsula to the north. This seismic band has a nearly constant width of about 50 km. The focal mechanisms show that strike and thrust type dominate throughout this seismic band continuously, though there exists a area where pressure and tension axis dramatically change within this seismic band. While this “area” has been already recognized (e.g. Suzuki, 1989), JUICE helps to draw a precise “line” as a stress-field orientation boundary where happened to be close to Fukaya fault.Bouguer gravity anomaly and seismic exploration data imply structural changes at the stress-field orientation boundary. According to the Bouguer gravity anomaly (Komazawa, 2004), the boundary appears to be associated with the gravity-low zone. The gravity anomalies show a lineation that trends NW-SE, the same direction of the boundary. Seismic profile (Sato et al., 2003) displays changes in basement character showing the pattern of depression beneath Fukaya fault. The shape of depression corresponds to the pattern of seismicity beneath this area, and also the boundary sites beneath the lowest point of the depression.It appears to split into different regimes at the stress-field orientation boundary. We conclude that it is possible to have major tectonic boundary underneath this northwestern area of the Kanto plain. Moreover, we suggest that Median tectonic line (MTL) is a major candidate underneath this area. MTL runs parallel to the island arc through southwest Japan and divides different geological structures into outer (the forearc side) and inner arc (the backarc side). The trace of MTL disappears on the eastern side of Itoigawa-Shizuoka tectonic line, but Takagi et al. (2006) found an evidence of inner arc materials in the core sample obtained around this area. Therefore we assume that MTL is buried underneath the boundary. This finding may eventually impact on the research relates to hazard of Kanto area.