Investigation of Offshore Fault Modeling for a Source Region Related to the Shakotan-Oki Earthquake

Abstract

The purpose of this study is to verify fault modeling in the source region of the 1940 Shakotan-Oki earthquake using active faults offshore of Japan. Tsunami heights simulated in previous studies are found to be lower than observed levels, which makes it difficult to explain historical tsunami records of this earthquake. However, the application of appropriate slip magnitudes in the fault models may explain these differences. In the “Project for the Comprehensive Analysis and Evaluation of Offshore Fault Informatics (the Project),” a new fault model is constructed using marine seismic data and geological and geophysical data compiled by the Offshore Fault Evaluation Group, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) as part of the Project for Fault Evaluation in the Seas around Japan (Ministry of Education, Culture, Sports, Science and Technology, MEXT). Single-channel and multichannel reflection seismic data were used that includes information from a new fault identified in previous surveys. We investigated fault geometries and their parameters using the above data. Here, we show that the geometric continuity of these faults is adjusted by increasing the magnitude of fault slip. Standard scaling laws are applied on the basis of strong ground motion of the fault parameters, and the validity of the fault model is examined by comparing tsunami heights along the Japanese coastline from historically observed records with tsunami height from simulation analysis. This verification quantitatively uses Aida’s K and κ scale and variance parameters. We determine that the simulated tsunami height determined using the new model approach the heights observed historically, which indicates that the model is valid and accurate for the source region.

Reexamination of Tsunami Source Models For the 20th Century Earthquakes Off Hokkaido and Tohoku Along the Eastern Margin of the Sea of Japan

“…The scaling relation of the "Recipe" has three stages at bending points M w 6.5 and M w 7.4.The fault models obtained from the tsunami waveform inversions for the 1940 off Shakotan Peninsula, the 1993 off the southwest coast of Hokkaido, and the 1983 Central Sea of Japan earthquakes are in good agreement with the third stage (solid circle, solid square, and solid triangle in Fig.22). The fault model ofOhsumi and Fujiwara (2017) for the 1940 off Shakotan Peninsula earthquake estimated from the coastal tsunami trace heights (open circle in Fig.22) also correlates to the scaling relation. Regarding the 1993 off the southwest coast of Hokkaido earthquake, the model ofTakahashi et al (1995a) estimated from the coastal tsunami trace heights (open square in Fig.22) is out of the scaling relation because a large slip amount was required to explain the coastal tsunami heights.…”

“…The fault models used for the tsunami numerical simulation of the 1940 off Shakotan Peninsula earthquake were: MS01, MS02, ST01, and ST02 of the JSPJ (Fig. 2), the Okamura et al (2005) model, and the HKD-2239 model of Ohsumi and Fujiwara (2017). The multiple fault model MS01 + MS02 + ST01 + ST02 (referred to as Model A) corresponds to the location of the fault model in previous studies (Okamura et al 2005; Ohsumi and Fujiwara 2017).…”

“…Okamura et al (2005) investigated the fault location from the traces of past earthquakes using sea oor diving surveys, and estimated the fault slip by tsunami waveform inversion. Ohsumi and Fujiwara (2017) compiled fault models based on seismic re ection survey data and validated a suitable model to explain coastal tsunami heights along the Hokkaido coast.…”