13:45 〜 15:15
[SEM14-P21] Three dimensional resistivity structure around Fukuoka city and its relationship to the faults around the 2005 earthquake (M7.0)
キーワード：resistivity structure、fault、Kego fault、earthquake
The 2005 West off Fukuoka Prefecture earthquake (M7.0) occurred beneath the sea northwest off from Fukuoka city. The earthquake fault was a left-lateral strike-slip with a WNW-ESE strike direction (Fig. 1). The rupture of the 2005 earthquake did not extend to the central part of the Fukuoka City where 1.5 million people live. On the southeastern extension of the 2005 earthquake rupture zone, approximately 25 km length Kego fault (KF) is located in the densely populated area. Moreover, there are faults with similar strike direction on this region (Fig. 1). There is a concern for future earthquakes around Fukuoka city.
Recently, it was proposed that low-resistivity zones in the crust arrest the seismic rupture and control the final magnitude of earthquakes (Ichihara et al. 2017; Aizawa et al. 2021). Moreover, it was also proposed that ruptures that initiate along the outer edge of low-resistivity zones tends to become large earthquakes (Aizawa et al. 2021). Therefore, high resolution resistivity structure may provide valuable information for assessing the locations and maximum magnitudes of future earthquakes.
In 2021-2022, we conducted broadband MT observations on the 60 x 60 km area around Fukuoka city (Fig. 1). Research interest is the spatial relationship between the resistivity structure and the faults. In particular, we aim to have an insight why the rupture of the 2005 west off Fukuoka earthquake stopped at the northwestern edge of the Kego fault (Fig. 1). The time series data was acquired 7 to 21 days per one site because artificial noise is severe on this region. The obtained MT data was strongly contaminated by artificial noise, but the noise due to the leakage electric current from railways significantly decreases from 0:00 a.m. to 5:00 a.m. By using nighttime data, robust-estimation code (Chave and Thomson 2004), and remote-reference processing with the magnetic data recorded 200 km south of this region, we have obtained good quality MT response functions.
We estimated the 3-D resistivity structure by using the femtic code (Usui, 2015). Input data are impedances and tipper in the period range between 0.0125 to 1000 s. Ocean is included in the model. The preliminary inversion shows two low-resistivity zones at the depth greater than 10 km, one at around the northwestern side of the KF and the other at around the southeastern extension of the KF. KF and other NW-SE trending faults are located relatively resistive zone sandwiched by the two low-resistivity zones. This result is similar to the 2016 Kumamoto earthquake (Aizawa et al. 2021), implying that low-resistivity zones control the length of the faults (i.e., maximum size of the earthquakes). The bottom of the seismicity correspond to the top of the low-resistivity zones, suggesting that high-temperature (>400 degree Celsius) condition in the low-resistivity zones.
Acknowledgement
We thank D. Muramatsu, S. Aniya, A Okubo, and A Jodoi for the help of field survey. We are greatly indebted to the landowners for their permission to conduct the MT observations. This work was supported by MEXT under Earthquake and Volcano Hazards Observation and Research Program, and Earthquake, and Research Institute Joint Usage Program, the University of Tokyo.
References
Aizawa et al. (2021), Electrical conductive fluid-rich zones and their influence on the earthquake initiation, growth, and arrest processes: observations from the 2016 Kumamoto earthquake sequence, Kyushu Island, Japan. Earth Planets Space 73, 12. https://doi.org/10.1186/s40623-020-01340-w
Chave AD, Thomson DJ (2004) Bounded influence magnetotelluric response function estimation. Geophys J Int 157(3):988–1006. https://doi.org/10.1111/j.1365-246X.2004.02203.x
Ichihara et al. (2018), A 3D electrical resistivity model around the focal zone of the 2017 southern Nagano Prefecture earthquake (MJMA 5.6): implications for relationship between seismicity and crustal heterogeneity. Earth Planets Space, 70, 182. https://doi.org/10.1186/s40623-018-0950-1
Yoshiya Usui (2015), 3-D inversion of magnetotelluric data using unstructured tetrahedral
elements: applicability to data affected by topography,Geophys. J. Int. (2015) 202, 828–849. https://doi.org/10.1093/gji/ggv186
Recently, it was proposed that low-resistivity zones in the crust arrest the seismic rupture and control the final magnitude of earthquakes (Ichihara et al. 2017; Aizawa et al. 2021). Moreover, it was also proposed that ruptures that initiate along the outer edge of low-resistivity zones tends to become large earthquakes (Aizawa et al. 2021). Therefore, high resolution resistivity structure may provide valuable information for assessing the locations and maximum magnitudes of future earthquakes.
In 2021-2022, we conducted broadband MT observations on the 60 x 60 km area around Fukuoka city (Fig. 1). Research interest is the spatial relationship between the resistivity structure and the faults. In particular, we aim to have an insight why the rupture of the 2005 west off Fukuoka earthquake stopped at the northwestern edge of the Kego fault (Fig. 1). The time series data was acquired 7 to 21 days per one site because artificial noise is severe on this region. The obtained MT data was strongly contaminated by artificial noise, but the noise due to the leakage electric current from railways significantly decreases from 0:00 a.m. to 5:00 a.m. By using nighttime data, robust-estimation code (Chave and Thomson 2004), and remote-reference processing with the magnetic data recorded 200 km south of this region, we have obtained good quality MT response functions.
We estimated the 3-D resistivity structure by using the femtic code (Usui, 2015). Input data are impedances and tipper in the period range between 0.0125 to 1000 s. Ocean is included in the model. The preliminary inversion shows two low-resistivity zones at the depth greater than 10 km, one at around the northwestern side of the KF and the other at around the southeastern extension of the KF. KF and other NW-SE trending faults are located relatively resistive zone sandwiched by the two low-resistivity zones. This result is similar to the 2016 Kumamoto earthquake (Aizawa et al. 2021), implying that low-resistivity zones control the length of the faults (i.e., maximum size of the earthquakes). The bottom of the seismicity correspond to the top of the low-resistivity zones, suggesting that high-temperature (>400 degree Celsius) condition in the low-resistivity zones.
Acknowledgement
We thank D. Muramatsu, S. Aniya, A Okubo, and A Jodoi for the help of field survey. We are greatly indebted to the landowners for their permission to conduct the MT observations. This work was supported by MEXT under Earthquake and Volcano Hazards Observation and Research Program, and Earthquake, and Research Institute Joint Usage Program, the University of Tokyo.
References
Aizawa et al. (2021), Electrical conductive fluid-rich zones and their influence on the earthquake initiation, growth, and arrest processes: observations from the 2016 Kumamoto earthquake sequence, Kyushu Island, Japan. Earth Planets Space 73, 12. https://doi.org/10.1186/s40623-020-01340-w
Chave AD, Thomson DJ (2004) Bounded influence magnetotelluric response function estimation. Geophys J Int 157(3):988–1006. https://doi.org/10.1111/j.1365-246X.2004.02203.x
Ichihara et al. (2018), A 3D electrical resistivity model around the focal zone of the 2017 southern Nagano Prefecture earthquake (MJMA 5.6): implications for relationship between seismicity and crustal heterogeneity. Earth Planets Space, 70, 182. https://doi.org/10.1186/s40623-018-0950-1
Yoshiya Usui (2015), 3-D inversion of magnetotelluric data using unstructured tetrahedral
elements: applicability to data affected by topography,Geophys. J. Int. (2015) 202, 828–849. https://doi.org/10.1093/gji/ggv186