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Effects of subsurface structures of source regions on long-period ground motions observed in the Tokyo Bay area, Japan
© The Author(s) 2017
Received: 21 December 2016
Accepted: 16 May 2017
Published: 22 May 2017
Events and observation stations used in this study
Event source parameters
March 12, 2011
November 22, 2014
In this study, we consider data from two networks. One is the broadband velocity seismometer network on the shore of Tokyo Bay, operated by Tokyo Electric Power Company Holdings (TEPCO). The green squares in Fig. 2 indicate the TEPCO stations. Servo velocity seismometers (Tokyo Sokushin VSE-355G3) are installed in TEPCO’s network. The full scale of the sensor is 2 m/s, and the sensor response is flat in the frequency range between 0.008 and 70 Hz. The data are recorded with 100-Hz sampling and 24-bit resolution. The other data are from the strong motion seismograph networks (K-NET and KiK-net) operated by the National Research Institute for Earth Science and Disaster Resilience, Japan (NIED) (Okada et al. 2004). The red and blue circles in Fig. 2 show the NIED stations used in this study. Feedback-type high-resolution accelerometers are installed at the K-NET and KiK-net stations. Because we primarily discuss the characteristics of velocity waveforms, the acceleration data were integrated into the velocity traces in the frequency domain.
The gray contours in Fig. 2 indicate the depth of seismic bedrock from the “Japan Integrated Velocity Structure Model Version 1” (Koketsu et al. 2012). The upper boundary of the seismic bedrock around Tokyo Bay is very deep, about 3000–4000 m. The contour also suggests that the velocity structures in the source regions of the two events are different. The depth of seismic bedrock in the source region of the 2011 event is deeper than the source region of the 2014 event.
Velocity seismograms observed around the Tokyo Bay area
Ground motions at the stations located outside of the basin
Effects of source region velocity structure
A fourth-order staggered grid FDM was used for the P-SV problem. The model size was 60 km in the horizontal direction and 30 km in the depth direction with a 50-m grid size. The bottom, left, and right sides of the model were set with absorbing boundary conditions (Cerjan et al. 1985). The Q value was taken into account according to Graves (1996), and the reference frequency was 0.2 Hz. The time step of calculation was 0.002 s. We set the same moment source based on a reverse fault with a dip of 45° at 6 km depth in both simulations. The Herrmann function (Herrmann 1979) with a 5-s period was used as the source function.
Effects of basin features on surface wave propagation
Yuzawa and Nagumo (2012) studied the shakability of long-period ground motion in Kanto Basin and pointed out that its variability is due to source region. They interpreted the amplitude variation as due to the distance that the surface wave travels through sedimentary layers. Our results indicate that the variation in shakability is not only affected by propagation distance in the basin but also by input wave characteristics.
A shallow M6.7 earthquake occurred in northern Nagano Prefecture, Japan, on November 22, 2014. The magnitude, focal depth, and source mechanism of this event were almost identical to an event that occurred near the border of Nagano and Niigata prefectures on March 12, 2011. However, the seismograms of these events observed from the Tokyo Bay area were quite different. Significant long-period later arrivals with a predominant period of 5 s were recognized in the traces of the 2011 event, but were not recognized in the traces of the 2014 event. Because the incident wave to the Kanto Basin is a controlling factor on long-period ground motion in the Tokyo Bay area, we examined the ground motion at an outside the basin site. A large wave packet with a predominant period of 5 s was clearly recognized in the velocity traces of the 2011 event. This wave packet was likely a Rayleigh wave. From a numerical simulation, we confirmed that the subsurface structure of the focal region affected the excitation of the Rayleigh waves. Therefore, the difference in long-period ground motion between the two events in the Tokyo Bay area was affected by excitation of Rayleigh waves in the focal regions. This suggests that it is important to consider the effects of both the local and source regions.
The source parameters for the earthquakes used in this study were provided by the Japan Meteorological Agency. The CMT solution for the F-net and strong motion data from K-NET and KiK-net were provided by the National Research Institute for Earth Science and Disaster Resilience, Japan. Generic Mapping Tools (Wessel and Smith 1998) was used to draw the figures. Editor Takuto Maeda, reviewer Yadab P. Dhakal, and an anonymous reviewer helped to significantly improve the manuscript.
The author declares that he/she has no competing interests.
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