Stress field in central Japan estimated by multiple inverse method from focal mechanisms
Abstract
In central Japan, active faults with strike-slip type are dominated. In addition small fractions of reverse faults are observed in Nigata Kobe Tectonic Zone (NKTZ) and other places. However large part of focal mechanism solutions in this region shows reverse fault type. Thus relation between existent fault and stress field are essentially important problem. To obtain precise heterogeneous stress field, we apply multiple inverse method (Otsubo et al., in press) and K-means clustering (Otsubo el al., 2006) to the shallower focal mechanism solutions (NIED F-net focal mechanisms catalogue: depth <30km) in this region. The multiple inverse method can treat heterogeneous stress field and have no requirement for strict a priori data selection which can be assumed under homogeneous stress field. First we extract two stress solutions within each 1° × 1° (lat, lon) region. Most of the stress solutions show strike-slip or reverse fault type with stress ratio (φ) between 0 and 0.66. Then we divide input fault parameters into four groups using misfit angles between the observed and computed slip directions for detected stress solutions in each region. The misfit angle is 30°. We estimated the spatial changes of stress field from the focal mechanisms of which observed slip directions are consistent with only one stress solution. These selected dataset are useful to map spatial heterogeneity of the stress field. Solutions with medium stress ratio (φ = 0.33 to 0.66) and strike-slip regime appear in central Japan enclosed by Itoigawa Shizuoka Tectonic Line and Median Tectonic Line. Stress solution with reverse fault regime and medium stress ratio are seen in northeast region and Japan Sea side. In contrast, solutions with low stress ratio (φ) between 0.00 and 0.33 (s1 >> s2=s3) appears along high strain rate zone (NKTZ). It may show that the high strain rate zone correspond to stress concentrated region toward Shmax direction.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T53D1994K
- Keywords:
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- 8123 Dynamics: seismotectonics;
- 8164 Stresses: crust and lithosphere