[SSS14-P20] 歪変化の空間分布に基づく短期的スロースリップイベント発生源モデルの再考
キーワード：スロースリップ、歪、震源モデル
Various slow earthquakes detected in the subduction zone are modeled as fault slips, just like ordinary earthquakes, but the mechanism generating episodic slow slips is unclear. On the other hand, knowledge of the fault strength suggests that source region of the deep slow earthquakes locates a brittle-ductile transition zone associated with the depth dependence of temperature-pressure conditions. With the transition from brittle to ductile fracture (or ductile flow), plastic deformation increases in plate boundary rocks. The plastic deformation occurs as strain caused by three-dimensional deviatoric stress, and it may not be able to be modeled by using only fault slip. The additional deformation mode can be detected as the normal component to the fault plane. Therefore, in this study, we re-estimate the source model for the short-term slow slip event (S-SSE), which is one of the deep slow earthquakes.
In this re-estimation, the azimuthal coverage of observation points is important. For example, source model of the 2001 Tokai SSE, one of the long-term slow-slip events (Mitsui & Ito, 2019), was re-estimated as tensile deformation normal to the fault plane in addition to fault slip. At the observation point located in the direction of the rake angle (southeastward in the case of Tokai SSE), there was no difference in surface displacement whether the source has tensile deformation or not, while at the observation point located in the opposite direction (northwestward), a remarkable difference was observed mainly in the vertical component of surface displacement.
Although many deep S-SSEs have been reported in the Tokai region (e.g., AIST, 2018), all of the stations used to estimate the source model are located on the coast side (in the direction of rake angle). Therefore, we use additional data of strain meters and stress meters owned by Tono Research Institute of Earthquake Science (TRIES) located in Mizunami City and Toki City, Gifu Prefecture, which is the inland side of the S-SSE source. And we will re-estimate the source model to discuss the existence of non-fault slip component of deformation in the source.
As a preliminary study, we calculated synthetic strain changes at the observation points based on the source model of S-SSE, which occurred in November 2017 (AIST, 2018). We also used a model in which a part of the estimated fault slip amount is reduced and the reduced slip is given as tensile dislocation normal to the fault plane. As a result, the more tensile dislocation increases, the more compressive strain increases causing the change of the direction of the principal strain at the inland observation point. This result suggests that a more appropriate source model can be obtained by using data observed at inland side.
The strain changes due to S-SSE observed by TRIES strain meters and stress meters are consistent with the above synthetic strain changes, and these data are useful for estimating the S-SSE source model.
In this re-estimation, the azimuthal coverage of observation points is important. For example, source model of the 2001 Tokai SSE, one of the long-term slow-slip events (Mitsui & Ito, 2019), was re-estimated as tensile deformation normal to the fault plane in addition to fault slip. At the observation point located in the direction of the rake angle (southeastward in the case of Tokai SSE), there was no difference in surface displacement whether the source has tensile deformation or not, while at the observation point located in the opposite direction (northwestward), a remarkable difference was observed mainly in the vertical component of surface displacement.
Although many deep S-SSEs have been reported in the Tokai region (e.g., AIST, 2018), all of the stations used to estimate the source model are located on the coast side (in the direction of rake angle). Therefore, we use additional data of strain meters and stress meters owned by Tono Research Institute of Earthquake Science (TRIES) located in Mizunami City and Toki City, Gifu Prefecture, which is the inland side of the S-SSE source. And we will re-estimate the source model to discuss the existence of non-fault slip component of deformation in the source.
As a preliminary study, we calculated synthetic strain changes at the observation points based on the source model of S-SSE, which occurred in November 2017 (AIST, 2018). We also used a model in which a part of the estimated fault slip amount is reduced and the reduced slip is given as tensile dislocation normal to the fault plane. As a result, the more tensile dislocation increases, the more compressive strain increases causing the change of the direction of the principal strain at the inland observation point. This result suggests that a more appropriate source model can be obtained by using data observed at inland side.
The strain changes due to S-SSE observed by TRIES strain meters and stress meters are consistent with the above synthetic strain changes, and these data are useful for estimating the S-SSE source model.