15:30 〜 16:30
[S07-P-26] Surface wave propagation and magnitude (Mj) overestimates in western Japan
It is recognized that the Mj of the Japan Meteorological Agency (JMA)'s scale sometimes shows large discrepancy between Mw. Typical examples were the Western Tottori earthquake in 2000 (Mj=7.3; Mw=6.7) and the Yamaguchi earthquake in 1997 (Mj=6.6; Mw=5.8), both are shallow, inland earthquakes in western Japan. Since the estimates of the Mj for shallow (h < 60 km) earthquake use the amplitude of the ground motion recorded by long-period (T0=5 s) seismometers, the propagation of the long-period ground motions might be different between northern and western Japan. In this study we examined the cause of such larger Mj by examining the long-period wave propagation from shallow inland earthquakes.
First we conducted regression analysis between Mj(from JMA catalog) and Mw(from GCMT catalog), for 47 inland earthquakes of shallow (h < 40 km) and large (Mj > 5.5) events occurred from Sep. 1994 to Nov. 2016 and obtained an equation of Mj = Mw + 0.16. We then selected the peculiar events which are largely deviated from the regression line, with larger Mj in Chugoku-Kinki area and a line area from south Fukushima to south Niigata, and smaller Mj in Tohoku area.
We then examined the strong motion record of the K-NET and KiK-net for the 2000 Western Tottori and the 2004 Mid Niigata earthquakes, and find that the attenuation of the long-period (5 s) ground displacement from the Mid Niigata earthquake is very strong with propagation in northern Japan, while very weak for the Western Tottori earthquake. Since the large ground motions appear in tangential motion, the observed large ground motion in western Japan is expected the fundamental-mode Love waves.
Therefore, the overestimates of the Mj in western Japan might occur due to the efficient development of the long-period (5 s) Love wave from shallow, strike-slip fault sources which propagates longer distances without significant attenuation and dispersion, probably due to the peculiarity of the shallow structure in that area.
First we conducted regression analysis between Mj(from JMA catalog) and Mw(from GCMT catalog), for 47 inland earthquakes of shallow (h < 40 km) and large (Mj > 5.5) events occurred from Sep. 1994 to Nov. 2016 and obtained an equation of Mj = Mw + 0.16. We then selected the peculiar events which are largely deviated from the regression line, with larger Mj in Chugoku-Kinki area and a line area from south Fukushima to south Niigata, and smaller Mj in Tohoku area.
We then examined the strong motion record of the K-NET and KiK-net for the 2000 Western Tottori and the 2004 Mid Niigata earthquakes, and find that the attenuation of the long-period (5 s) ground displacement from the Mid Niigata earthquake is very strong with propagation in northern Japan, while very weak for the Western Tottori earthquake. Since the large ground motions appear in tangential motion, the observed large ground motion in western Japan is expected the fundamental-mode Love waves.
Therefore, the overestimates of the Mj in western Japan might occur due to the efficient development of the long-period (5 s) Love wave from shallow, strike-slip fault sources which propagates longer distances without significant attenuation and dispersion, probably due to the peculiarity of the shallow structure in that area.