1:45 PM - 2:00 PM
[SCG50-13] Spatio-temporal patterns of inland stress field: Characteristics revealed by a new method using focal mechanisms and interseismic plate coupling, and its tectonic implication
★Invited Papers
Keywords:inland stress field, spatio-temporal pattern, focal mechanism, interseismic plate coupling, tectonics
It is known that the activity of shallow inland earthquakes in the Kinki region, southwest Japan, increased from about 50 yr before to 10 yr after the occurrence of large interplate earthquakes along the Nankai trough (Utsu 1974; Hori & Oike 1999). We also know that inland seismic activity significantly changed before and after the 2011 Tohoku-oki earthquake. These observations indicate that the shallow inland earthquakes occur in response to the interseismic plate coupling, facilitating further research on the causal relationship between the inland earthquake activity and the interplate coupling state.
Imanishi and Noda (The 2020 SSJ Fall Meeting) proposed a method to investigate the inland stress field by examining whether the focal mechanisms of inland small earthquakes align with the stress field due to the plate coupling or not. The basic concept of this method is as follows:
(1) The total stress field is described as the background stress plus the temporally varying stress due to the plate coupling.
(2) There exist pre-existing faults with various orientations in the crust, most of which are close to a critically stressed state.
(3) Earthquakes will preferentially occur on faults whose slip direction is close or in agreement with the stress field due to the plate coupling, although earthquakes may occur on faults that are favorably oriented to the background stress field.
When an earthquake occurs in agreement with the stress field caused by the plate coupling, the misfit angle (i.e., the angle between the observed slip direction and the tangential traction predicted by the reference stress tensor) is expected to be a small value, and vice versa. Therefore, we can visualize the spatio-temporal patterns of inland stress field with respect to the plate coupling by taking the spatio-temporal average of the misfit angles.
We applied the method to inland earthquakes in southwestern Japan. We merged the JUNEC catalog (Ishibe et al. 2014) and the JMA catalog to produce the consolidated one, which results in a 35-year-long catalog. For the calculation of the stress tensor caused by the plate coupling, we adopted the slip-deficit model of Noda et al. (2018). The spatio-temporal distribution of misfit angles was obtained by taking a one-year moving average for each mesh at 0.25 ° intervals. The results indicate that the misfit angle fluctuates considerably both temporally and spatially. We observe that earthquakes with a magnitude of 6 or larger, including the 2016 Kumamoto earthquake, tend to occur when the misfit angle was small. This is consistent with the observation that inland earthquakes occur in response to the interseismic plate coupling. On the other hand, the misfit angle fluctuates at a high level in the Chugoku region compared to other regions. We infer that a stress loading by the San-in shear zone (Nishimura and Takada, 2017) dominates the stress field in this area. It is also noted that the misfit angles change with a period of a few years in some regions. Such a cyclic stress change has also been reported by repeated measurements of in-situ stress in northern Kinki (Tanaka et al., 1998). Because the present method evaluates the effect of the plate coupling, this cyclic fluctuation can be attributed to the change of the state along the plate boundary (e.g., occurrence of slow slip event).
In summary, the subtle spatio-temporal variation of inland stress field can be detected by this method, which will lead to the elucidation of the relationship between the subduction process and the stress field in the Island-arc and the mechanism of inland earthquake generation. In the presentation on the day, we will report the characteristics of inland stress field before and after the Tohoku-oki earthquake by applying this method to the Tohoku region.
Imanishi and Noda (The 2020 SSJ Fall Meeting) proposed a method to investigate the inland stress field by examining whether the focal mechanisms of inland small earthquakes align with the stress field due to the plate coupling or not. The basic concept of this method is as follows:
(1) The total stress field is described as the background stress plus the temporally varying stress due to the plate coupling.
(2) There exist pre-existing faults with various orientations in the crust, most of which are close to a critically stressed state.
(3) Earthquakes will preferentially occur on faults whose slip direction is close or in agreement with the stress field due to the plate coupling, although earthquakes may occur on faults that are favorably oriented to the background stress field.
When an earthquake occurs in agreement with the stress field caused by the plate coupling, the misfit angle (i.e., the angle between the observed slip direction and the tangential traction predicted by the reference stress tensor) is expected to be a small value, and vice versa. Therefore, we can visualize the spatio-temporal patterns of inland stress field with respect to the plate coupling by taking the spatio-temporal average of the misfit angles.
We applied the method to inland earthquakes in southwestern Japan. We merged the JUNEC catalog (Ishibe et al. 2014) and the JMA catalog to produce the consolidated one, which results in a 35-year-long catalog. For the calculation of the stress tensor caused by the plate coupling, we adopted the slip-deficit model of Noda et al. (2018). The spatio-temporal distribution of misfit angles was obtained by taking a one-year moving average for each mesh at 0.25 ° intervals. The results indicate that the misfit angle fluctuates considerably both temporally and spatially. We observe that earthquakes with a magnitude of 6 or larger, including the 2016 Kumamoto earthquake, tend to occur when the misfit angle was small. This is consistent with the observation that inland earthquakes occur in response to the interseismic plate coupling. On the other hand, the misfit angle fluctuates at a high level in the Chugoku region compared to other regions. We infer that a stress loading by the San-in shear zone (Nishimura and Takada, 2017) dominates the stress field in this area. It is also noted that the misfit angles change with a period of a few years in some regions. Such a cyclic stress change has also been reported by repeated measurements of in-situ stress in northern Kinki (Tanaka et al., 1998). Because the present method evaluates the effect of the plate coupling, this cyclic fluctuation can be attributed to the change of the state along the plate boundary (e.g., occurrence of slow slip event).
In summary, the subtle spatio-temporal variation of inland stress field can be detected by this method, which will lead to the elucidation of the relationship between the subduction process and the stress field in the Island-arc and the mechanism of inland earthquake generation. In the presentation on the day, we will report the characteristics of inland stress field before and after the Tohoku-oki earthquake by applying this method to the Tohoku region.