[S-CG60] Shallow Fault Zone Structure and Seismic Hazard Assessment

[SCG60-10] An attempt to predict strong ground motions by a blind fault beneath the coastal plain: In the case of the southern Sendai Plain blind fault

*Tomomi Okada¹, Shinsuke Okada² (1.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 2.Faculty of Science and Engineering, Iwate University)

Introduction
When a fault exists under soft ground in a coastal plain, it is difficult to identify it due to the topography and geology. Therefore, the existence of a fault is confirmed by a reflection seismic survey or gravity survey. Recently, Okada, S. et al. (2017, 2022) confirmed the existence of a blind fault in the southern Sendai Plain. The length of the fault is 25 km, equivalent to 7.2 on the JMA Magnitude, according to Matsuda (1975). The strike is approximately south and west trending. On the other hand, since the soft surface layer has a large amplification factor of seismic waves, faults and earthquakes under soft ground are assumed to cause large amplitude and seismic intensity at the ground surface compared to their magnitude. In this study, as an example of strong ground motion prediction for a blind fault, we attempted to predict seismic intensity for the Southern Sendai Plain Blind Fault.

Method
A blind fault was set up under the southern Sendai Plain according to the assumed fault of Okada, S. et al. (2017, 2022). The fault length was set to 25 km. The procedure followed the "simplified method” as follows. Each parameter was set according to the "National Earthquake Motion Prediction Map Technical Report (https://jishin.go.jp/main/chousa/09_yosokuchizu/g_model.pdf). The fault length of 25 km was converted to Mw 6.7. As with the settings for the nearby active fault, the Futaba Fault Zone, the lower and upper limits of the seismogenic layer were assumed to be 20 km and 2 km, respectively, also referring to the hypocenter distribution of Okada, T. et al. (2022). The dip angle was assumed to be 45 degrees. The fault width was assumed to be 18 km. We used the seismic motion estimation tool based on the distance attenuation relationship formula (Sen'na and Fujiwara, 2011), which is available from the National Research Institute for Earth Science and Disaster Prevention.

Results
The JMA seismic intensity exceeding 6 was approximately 70 km from north to south from the vicinity of Sendai City to the vicinity of Minamisoma City, Fukushima Prefecture, and approximately 20 km in a wide area from east to west. The seismic intensity of above 6 on in the southern Sendai Plain and in the basin near Kakuda City, Miyagi Prefecture, west of the fault. Between the surface trace of the fault and the western mountainous boundary, a zone of intensity 7 was also observed.
This distribution of seismic intensity is dependent on assumptions. Currently, there is a large uncertainty, and for a more accurate estimation or confirmation of the width of the estimation, it is necessary to estimate the dip angle of the fault and the upper and lower limits of the depth at which fault slip occurs and to estimate the range of the estimation. It is also necessary to consider the relationship with the Futaba Fault Zone, which is an active fault in the vicinity of the fault and partially parallel to it.

Acknowledgment
We used the seismic motion estimation tool based on the distance attenuation relationship formula (Sen'na and Fujiwara, 2011). We appreciate the suggestions from Dr. Y. Fukushima and Dr. H. Nakahara and the discussion with Dr. M. Ichiki and Dr. R. Takagi.