A study on the predominant period of long-period ground motions in the Kanto Basin, Japan
© Yoshimoto and Takemura; licensee Springer. 2014
Received: 30 January 2014
Accepted: 19 August 2014
Published: 26 August 2014
We analyzed seismic waveforms of moderate to large shallow earthquakes recorded in the Kanto Basin, Japan, to clarify the characteristics of long-period ground motions in deep sedimentary basins. Our analysis revealed a tendency of long-period ground motions to develop in limited periods up to around 7 s. In addition, we found a specific relation between the predominant period of long-period ground motions and the depth of bedrock. The predominant period tends to increase in proportion to bedrock depth until it reaches approximately 7 s; however, it becomes almost constant for bedrock depths greater than 2 km. These observations, in conjunction with numerical experiments performed in this study using realistic sedimentary basin structure models, indicate that the characteristic of long-period ground motions, excited by the basin-induced fundamental Love waves, is controlled strongly by the shallow sedimentary structure, if the bedrock depth exceeds approximately 2 km. This result might also be important in understanding and evaluating the characteristics of long-period ground motions in other deep sedimentary basins of the world for moderate to large shallow earthquakes.
In deep sedimentary basins, long-period ground motions induced by large-amplitude seismic surface waves might cause damage to large-scale constructions, such as high-rise buildings and oil storage tanks (e.g., Koketsu and Miyake2008). Thus, it is necessary to study the characteristics of long-period ground motions in the corresponding areas for the purposes of estimation and mitigation of the damage from future destructive earthquakes. Since the analysis of velocity waveforms has been extensively carried out in these fields (e.g., Joyner2000; Hatayama et al.2004; Chen et al.2012), we deal with long-period ground motions in terms of velocity waveforms in this study.
In past studies, the dependence of the predominant period of long-period ground motions on bedrock depth has been studied by assuming simple sedimentary layer structures (e.g., Kudo1978). The conventional approach is effective and has advantages for calculation costs; however, a modeling using only few layers would be oversimplification of subsurface structure if the bedrock depth is deepened and the physical effects of the sedimentary structure on wave propagation become complex. In this study, we regard this effect as crucial for a deep understanding of the characteristics of the long-period ground motions in deep sedimentary basins and try to investigate the characteristics of the spatial variation of the predominant period in association with local sedimentary basin structure. We selected the Kanto Basin as the target basin because high-density seismic observation networks are deployed and the population density is very high.
Because seismic wave propagation in deep sedimentary basins might be sensitive to variations of seismic velocity with depth in the sediment, the characteristics of long-period ground motions should be investigated based on realistic sedimentary structure models. In this study, using realistic sedimentary velocity models constructed from vertical seismic profiling (VSP) measurements, we discuss the reasons why the predominant period of long-period ground motions in deep sedimentary basins is limited to several seconds to around 7 s for moderate to large shallow earthquakes.
Predominant period of long-period ground motions in deep sedimentary basins
Long-period ground motion is characterized by the effects of the source, path, and local site (e.g., Joyner2000). In relation to this, our observational results, which are obtained from seven earthquakes having different hypocenter distances and wide azimuthal coverage, confirm that the local site effect is an important factor to characterize the predominant period of long-period ground motions in the Kanto Basin. Thus, we focus our attention to the effect of the local site structure on surface wave excitation.
Seismic velocity structure of the sediment and surface wave modes
The seismic velocity structure of the Kanto Basin has been investigated by many researchers using a variety of geophysical techniques, such as seismic refraction experiments, H/V spectral ratio analysis, and microtremor array exploration (e.g., Koketsu and Higashi1992; Sato and Higashi2006; Yamanaka and Yamada2006). In addition, limited numbers of VSP measurements at deep boreholes (deeper than 1 km) have revealed very accurate in situ seismic velocity structures (Yamamizu1996,2004). Assembling the results from 14 VSP measurements, Yoshimoto and Takemura (2014) indicated that the depth variation of seismic velocity of the sediment in the Kanto Basin could be modeled satisfactorily by the exponential asymptotically bounded velocity function (Ravve and Koren2006) in the first-order approximation. It is worth noting that at most of the VSP measurement points, the S-wave velocity shows a continuous increase with depth from 0.5 km/s or less at the free surface to about 2 km/s at depths of 1.5 km (Yoshimoto and Takemura2014).
Discussion and conclusion
As seen in Figure 4, it is evident that the predominant period of long-period ground motion is affected by the bedrock depth of the sedimentary basin. A similar observation has been reported for the Osaka sedimentary basin, western Japan (Miyakoshi and Horike2006). Hence, it might be meaningful to investigate the relationship between the amplitude response of fundamental Love waves and the bedrock depth by modifying Iwatsuki's structure model, which shows a standard characteristic of the sediment of the Kanto Basin (Yoshimoto and Takemura2014).
We overlay the theoretical prediction for the fundamental Love wave excitation discussed above on Figure 4 (cross in the figure) to compare it with the observation. The theoretical prediction explains well the observed relationship between bedrock depth and the predominant period of long-period ground motions for shallow bedrock depths (<about 2 km), except for a slight overestimation. This result might indicate the effective excitation of Love waves in the basin edge area (e.g., Kinoshita et al.1992). One possible cause for the overestimation is the 3D geometrical effect of the Kanto Basin, which has not been taken into account in our theoretical consideration. As the Kanto Basin has a 3D shape in which the bedrock depth increases towards the center, it is likely that the observed Love waves at a certain station show the excitation effect from shallower parts of the basin if the incident wave for the Love wave excitation comes from outside the basin. A dipping bedrock interface might enhance the trapping of the incident wave energy in the sediment; thus, exciting the surface waves more effectively (e.g., Frankel1993). However, we do not discuss this effect in detail in this paper.
There are seismic observations from the Kanto Basin reporting the predominant period of long-period ground motions longer than 10 s (e.g., Tanaka et al.1979). In this paper, we noted the observation that the predominant period of long-period ground motion of moderate to large shallow earthquakes has the tendency to become almost constant (approximately 7 s) in the Kanto Basin. However, as for the large to great shallow earthquakes at local to regional distances, we should expect long-period ground motions with predominant periods longer than 10 s, in relation to the source spectrum having very rich energy in the long period. It is appropriate to leave the quantitative research on this topic for a future study.
In this paper, we reported the observation that the predominant period of long-period ground motion of moderate to large earthquakes shows the tendency to increase with bedrock depth in the shallow parts of the basin, whereas it is almost constant in the deeper parts of the basin. By using realistic sedimentary structure models and conventional surface wave theory, this observation was interpreted as indicating that the shallow velocity structure at less than about 2 km depth plays an important role in the excitation of basin-induced Love waves in the deep sedimentary basins. This perception and the associated seismic investigations could be very important in the evaluation of the development of surface waves, not only in deep sedimentary basins in land areas, but also in thick accretionary prisms in marine areas throughout the world.
The authors acknowledge the National Research Institute for Earth Science and Disaster Prevention, Japan, for the K-NET and KiK-net data, and the Earthquake Research Institute, University of Tokyo for the SK-net data. The authors would like to thank Dr. R. B. Herrmann and Dr. Y. Hisada for providing the computer program for the surface wave analysis. We also thank two anonymous reviewers for their thoughtful reviews of this article. Some figures in this article were drawn using the Generic Mapping Tools software package.
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