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Long-period ground motions in a laterally inhomogeneous large sedimentary basin: observations and model simulations of long-period surface waves in the northern Kanto Basin, Japan
© Takemura et al.; licensee Springer. 2015
- Received: 19 July 2014
- Accepted: 31 January 2015
- Published: 1 March 2015
To conduct practical evaluations of the long-period ground motions (period of 4 to 8 s) in a laterally inhomogeneous large sedimentary basin, we constructed a three-dimensional (3D) sedimentary velocity structure model for the northern Kanto Basin in Japan using a simple velocity gradient function, where strong lateral variations of seismic velocities in the sediments were expected. The model construction employs waveform analysis and geophysical data from vertical seismic profiling and microtremor surveys in the target region. To validate the velocity structure model, we conducted large-scale 3D finite-difference method simulations of the long-period ground motions for two shallow moderate earthquakes: the northern Tochigi earthquake and the northern Ibaraki earthquake. The simulation results for both earthquakes accurately reproduced the observed long-period ground motions in terms of arrival times, amplitudes, and durations of surface waves. By detailed comparisons of the seismograms for observational and simulated data, we demonstrated that the lateral variation of the seismic velocities in the sediments determines the characteristics of the surface wave propagation in the northern Kanto Basin. Such analyses can provide a better understanding of the complex propagation characteristics of surface waves in laterally inhomogeneous, large sedimentary basins.
- Long-period ground motion
- Surface wave propagation
- Sedimentary S-wave velocity
- Kanto Basin
Long-period ground motions with dominant periods of several to 10 s are observed frequently during shallow local and/or regional earthquakes, especially in large sedimentary basins (e.g., Beck and Hall 1986; Shin and Teng 2001; Koketsu et al. 2005). Long-period ground motion has the potential to cause significant resonance in basins, and this can lead to severe damage of large-scale, man-made structures such as high-rise buildings, oil storage tanks, and suspension bridges (e.g., Koketsu and Miyake 2008). Therefore, to assist disaster mitigation efforts for future large earthquakes, it is necessary to evaluate the characteristics of long-period ground motions in relation to the three-dimensional (3D) heterogeneous structure of large sedimentary basins.
Extensive efforts to construct a 3D structure model of the Kanto Basin have been put forth by many researchers (e.g., Yamanaka and Yamada 2006; Koketsu et al. 2008, 2009; Fujiwara et al. 2009). Using available geophysical and geological information in Japan, Koketsu et al. (2008, 2009) proposed the Japan Integrated Velocity Structure Model (JIVSM), which is one of the latest and most widely used structure models for the evaluation of strong and long-period ground motions of large earthquakes (e.g., Iwaki et al. 2013; Maeda et al. 2013b), the estimation of rupture complexities for large earthquakes (e.g., Koketsu et al. 2011; Asano and Iwata 2012), and the simulation of seismic wave scattering (e.g., Maeda et al. 2014; Takemura and Yoshimoto 2014). The JIVSM represents the 3D heterogeneous subsurface structure using 23 constant-velocity layers including those of the crust, the mantle, the Philippine Sea (PHS) Plate, and the Pacific (PAC) Plate, and also the complex basement shapes of sedimentary basins. Despite such a detailed model of the subsurface structure, precise evaluations of the long-period ground motions for the Kanto Basin still have room for improvement, compared to the similar trials for other large sedimentary basins in Japan, e.g., the Osaka Basin (e.g., Iwaki and Iwata 2010) and the Nobi Basin (e.g., Horikawa et al. 2008).
Recently, Yoshimoto and Takemura (2014a) suggested that realistic modeling of the sedimentary velocity structure is necessary for the precise evaluation of long-period ground motions in the northern Kanto Basin. Using a simple velocity gradient function (Ravve and Koren 2006), they proposed the construction of a smooth-varying 3D velocity structure model of the sediments from the vertical seismic profile (VSP) measurements of 14 deep boreholes in the Kanto Basin. Although their 3D finite-difference method (FDM) simulations successfully reproduced the excitation and propagation of long-period surface waves in a limited area of the northern edge of the Kanto Basin, the Kanto Basin is too large to achieve a detailed modeling of the 3D velocity structure using data from only 14 boreholes.
In this study, we construct a new 3D velocity structure model for the northern Kanto Basin (area outlined by the red square in Figure 1), which can be used for the precise evaluation of long-period ground motions with periods of 4 to 8 s. The model construction employs waveform analysis and available geophysical data in this region. The model constructed in this study exhibits strong lateral variations of S-wave velocities in the northern Kanto Basin, which may have large effects on the long-period surface wave excitation and propagation. In order to validate this model, we conducted large-scale 3D FDM simulations of the long-period ground motion for two shallow moderate earthquakes in northern Tochigi and northern Ibaraki and confirmed that there was good agreement between our simulation results and the observed seismograms. Based on the simulation results, we discuss the characteristics of surface wave propagation in the laterally inhomogeneous sedimentary basin.
Depth-varying properties of sedimentary S-wave velocity
Conversion of the layered structure to a smooth depth-varying structure
It may be useful to consider the conversion of the layered structure model to a smooth depth-varying structure model when constructing a 3D velocity model of sedimentary structure for long-period ground motion simulations. This is because many conventional structure models were constructed by assuming a layered structure and it is difficult to compile these models, which are often represented by different medium parameters and numbers of layers.
P-wave velocity, density, and attenuation structure
To construct velocity structure models for 3D FDM simulations, we adopted the modeling procedure used in Yoshimoto and Takemura (2014a). The P-wave velocity (V P ) and density (ρ) of sediment were assumed from the S-wave velocity using empirical relations reported by the Ministry of Education, Sports, Culture, Science and Technology (MEXT) (2007) and Shiomi et al. (1997), respectively.
P - and S -wave velocities, densities, and anelastic attenuation of the JIVSM
V P [km/s]
V S [km/s]
ρ [g/cm 3 ]
Sedimentary layer 1
Sedimentary layer 2
Sedimentary layer 3
Oceanic crust layer 2 of PHS
Oceanic crust layer 3 of PHS
Oceanic mantle of PHS
Oceanic crust layer 2 of PAC
Oceanic crust layer 3 of PAC
Oceanic mantle of PAC
Estimation of V 0 and α near the edge of the northern Kanto Basin
For modeling the velocity structure in the northern edge of the Kanto Basin where available geophysical data are sparse, we conducted 3D FDM simulations of the long-period ground motions during the northern Tochigi earthquake (Mw = 5.8) on 25 February 2013 and estimated the local values of V 0 and α. We supposed that the local values of V 0 and α at a certain station near the basin edge could be estimated from an existing velocity structure model if the simulated waveforms from the model successfully agree with the observed long-period ground motions.
The model used for the 3D FDM simulation covered an area of 135.6 × 82.8 km2 in the horizontal directions (red dashed rectangle in Figure 1) and 52.5 km in depth, which was discretized by grid intervals of 0.15 km in the horizontal directions and 0.075 km in the vertical direction. The wave propagation in each grid point was calculated by solving equations of motions using a staggered-grid FDM with fourth-order and second-order in space and time, respectively. With a minimum S-wave velocity (0.3 km/s) and grid size (0.15 km), the FDM simulations can examine seismic wave propagation for periods longer than 4 s with sampling of eight grid points per minimum S wavelength.
To perform an effective simulation, we employed a parallel 3D FDM simulation code based on a domain-partitioning procedure that utilizes a large number of processors with a message-passing interface (MPI) (e.g., Furumura and Chen 2004). We used a split-type perfectly matched layer (PML) absorbing boundary condition at the boundaries around the 3D model (e.g., Moczo et al. 2007; Maeda and Furumura 2013; Maeda et al. 2013a) to avoid artificial refractions from the 3D model boundaries, and a smoothing technique for layer interfaces near the PML boundaries was also applied for numerical stability (Maeda et al. 2013a).
The source depth of 8 km, focal mechanism of the strike/dip/rake = 168/86/−10°, and seismic moment of M 0 = 5.54 × 1017 Nm were assumed by referring to the F-net centroid moment tensor (CMT) solution (Fukuyama et al. 1998; Okada et al. 2004). A source time function represented by the asymmetric cosine function (Ji et al. 2003) with t s = 0.3 and t e = 2.7 was adopted for a point seismic source. After the calculation, the M 0 value was adjusted to 77% of the CMT estimation to explain the observed S-wave amplitude at rock site TCG011 (K-NET Kuzuu) (Yoshimoto and Takemura 2014a).
Local structure from VSP measurements
Estimated values of V 0 and α at 14 deep boreholes and one K-NET station
V 0 [km/s]
α [s −1 ]
Using these parameters, we constructed sedimentary seismic velocities, density, and the anelastic attenuation structure based on the procedure discussed in the previous section. For spatial interpolation of the values of V 0 and α, we used a gridding algorithm ‘surface’ in the Generic Mapping Tools (Wessel and Smith 1998) software with a tension factor of 0.3. To construct a 3D velocity structure model for whole depths, the structures beneath the sediments, including the topography of the bedrock, Moho, subducted oceanic plates, and mantle, were obtained from the JIVSM. Hereafter, we call this velocity structure model the ‘initial model,’ which was mainly constructed from the data of VSP measurements.
Local structure from JIVSM
Based on this conversion procedure, we estimated V 0 and α of the JIVSM in northern Kanto, where bedrock depths are larger than 1 km. The estimated parameters were used to construct a smoothed sedimentary velocity structure model using the same interpolation technique mentioned in the construction of the initial model. Hereafter, we call the constructed smoothed sedimentary velocity structure model from the JIVSM the ‘equivalent JIVSM (E. JIVSM)’.
Local structure from microtremor surveys
3D velocity structure model by integration of local structures
We integrated the estimated parameters at all of the sites mentioned above (Figure 7a) to construct a 3D velocity structure model of the sediment in the northern Kanto Basin (Figure 7b). Figure 7c shows the spatial variations of V 0 and α estimated by the interpolation, which used a total of 190 local site models. In Figure 7c, we found very complex lateral variations of the two parameters compared to the initial model (Figure 3). For example, high surface S-wave velocities (V 0) appeared at the western side of the Kanto Basin and strong velocity increments against depth (α) were found in central Ibaraki. By adopting the same procedure that we used for constructing the initial model, we constructed a 3D velocity structure model for the 3D FDM simulation of long-period ground motions in this region. Hereafter, we call this model the ‘smoothed basin velocity structure model (SBVSM)’. Figure 7d shows a comparison of the local S-wave velocity structures of the JIVSM, initial model, and SBVSM at three sites.
Large-scale 3D FDM simulation of long-period ground motion in the northern Kanto Basin
Performance of the constructed model for the evaluation of long-period ground motion
To demonstrate the performance of the SBVSM for the evaluation of long-period ground motions, we conducted large-scale 3D FDM simulations of long-period ground motions for two shallow moderate earthquakes. The model used for the 3D simulation covered an area of 153.6 × 153.6 km2 in the horizontal directions (red solid square in Figure 1) and 60.0 km in depth. The scheme for the simulation was the same as that used for the previous simulations in this study.
One earthquake used in the simulations was the northern Tochigi earthquake, which was used for the parameter estimation near the basin edge. Here, we examine the propagation of the long-period surface wave in a wide area of the Kanto Basin, including the areas of Ibaraki, Saitama, Tokyo, and northern Chiba. Another earthquake was the northern Ibaraki earthquake (Mw = 5.8) that occurred on 19 March 2011 at a depth of 5 km. We employed these earthquakes to validate the SBVSM in Ibaraki and northern Chiba. For the latter earthquake, a focal mechanism of strike/dip/rake = 141/48/−94° and seismic moment M 0 = 6.35 × 1017 Nm were assumed by referring to the F-net CMT solution. A source time function represented by the asymmetric cosine function (Ji et al. 2003) with t s = 0.3 and t e = 2.7 was adopted. After the calculation, the M 0 value of this event was adjusted to 77% of the CMT estimation to explain the observed S-wave amplitude at rock site IBRH19 (KiK-net Tsukuba).
Surface wave propagation in the northern Kanto Basin
Since simulated waveforms of the SBVSM accurately reproduced the observed waveforms in the northern Kanto Basin (Figures 8 and 9), it was concluded that our simulations succeeded in reproducing the excitation and propagation of surface waves in this area. Thus, using these simulated and observed waveforms, we investigated the characteristics of the excitation and propagation of the long-period surface waves during two shallow moderate earthquakes in detail.
In an early snapshot of the northern Tochigi earthquake (t = 30 s; Figure 10a), strong surface wave excitation around the Gumma-Saitama border (mark A) was observed. The excited surface waves propagated towards the middle part of the Kanto Basin, and they had a complex propagation pattern. At a lapse time of t = 50 to 80 s, the surface waves near the Saitama-Chiba border (mark B) were propagating much slower than the surface waves propagating in the midst of Saitama (mark C). In the later frames (t = 70 to 80 s), very slow surface waves (mark B) propagated towards Tokyo and Chiba while preserving their large amplitudes and shapes of wave packets.
Figure 10b shows the seismic wave propagation during the northern Ibaraki earthquake. At a lapse time of t = 45 s, large-amplitude surface waves were excited at the Ibaraki-Chiba border (mark D), and consequently, these waves propagated towards central Chiba. There was a significant dispersion of surface waves at northern Chiba (mark E), and these showed a delay of the large-amplitude arrival, which was confirmed with the observed seismograms at CHBH13 (lower right part of Figure 9).
From the snapshots of both earthquakes, we determined that the excitation of the surface waves occurred effectively at the area with a bedrock depth of approximately 1 km (e.g., at mark A and mark D in Figure 10). The excited surface waves propagated in the laterally inhomogeneous sedimentary basin and displayed complicated behaviors of propagation, especially for the northern Tochigi earthquake (mark B and mark C in Figure 10a). A conventional Fourier analysis revealed that the dominant period of the surface waves was approximately 6 s.
Anelastic attenuation of sediments is an important factor for the amplification of long-period ground motion in sedimentary basins. In this study, we employed Q values estimated by a simple empirical relation (Brocher 2008) and a frequency-dependent Q −1 model for P and S waves with a central frequency of f 0 = 1 Hz based on the formulations for FDM simulations of Robertsson et al. (1994). Here, we discuss the effect of the anelastic attenuation of the sediments on the modeling of long-period ground motions.
To investigate the optimal Q S values for long-period ground motions, we conducted additional FDM simulations of long-period ground motions using the central frequencies of f 0 = 0.5, 1, and 2 Hz. Figure 14b shows a comparison of the simulated and observed waveforms of the transverse component (period of 4 to 8 s) at SIT003 (red triangle in Figure 1) during the northern Tochigi earthquake. The simulation results demonstrated that the central frequency of Q S affected the amplitude of the later part of the surface wave packet (period of approximately 4 s), whereas no apparent effects appeared for the earlier part and dispersion feature of the Love wave. By comparing these results with the observed seismogram, the most preferable value of the central frequency was 1 Hz, which was used in the 3D FDM simulations in this study.
Although we proposed a new 3D velocity structure model for the evaluation of the long-period ground motions in the northern Kanto Basin, it may not be acceptable for short-period (<1 s) seismic wave propagation because of the lack of small-scale velocity heterogeneities. The original S-wave velocity structure obtained by VSP measurements has rich shorter wavelength components of the velocity heterogeneities (Yamamizu 1996, 2004; Shiomi et al. 1997). To achieve precise simulations that are capable of reproducing shorter-period seismic waves, such as for the evaluation of the peak ground acceleration and scattering in the Kanto Basin, we should revisit the original results from the VSP measurements and the small-scale velocity heterogeneities should be embedded in the model.
In this study, we used only one earthquake for the estimation of the local structural parameters (V 0 and α) near the basin edge. It would have been better to use more earthquakes distributed in a wide azimuthal range for precise estimations. However, there were very few relevant moderate earthquakes (Mw approximately 6) that occurred near the basin edge. Hence, it would be useful to validate and reconstruct the SBVSM using various azimuthal events in future studies.
In the Kanto Basin, the sedimentary structure of the JIVSM was mainly constructed using geophysical information from refraction surveys (Koketsu et al. 2008, 2009). However, the refraction surveys were not densely carried out in the northern part of the Kanto Basin compared with the middle and southern parts. Thus, the JIVSM did not reproduce the observed long-period ground motion very well, especially for excitation near the basin edge. In contrast, by introducing dense geophysical data around the northern Kanto Basin (see Figure 7a), the SBVSM was able to reproduce the observed excitation and propagation of long-period surface waves. We believe that the incorporation of data from denser microtremor surveys has the potential to improve the reproducibility of the observed long-period ground motions in the Kanto Basin. In addition, precise information on the bedrock depths, which was not discussed in this study, should be required for more confident evaluations in future studies.
For practical evaluations of the long-period ground motions (period of 4 to 8 s), we constructed a 3D velocity structure model of the northern Kanto Basin using a simple velocity gradient function. Our structure model SBVSM, which was constructed from a waveform analysis and dense geophysical data from VSP measurements, the JIVSM, and the microtremor surveys showed strong lateral inhomogeneities in this basin. The large-scale 3D FDM simulations using the SBVSM model accurately reproduced the observed long-period ground motions during two moderate earthquakes that occurred in the northern Kanto region. Hence, these results show the adequacy of our modeling procedure.
By comparing the simulations and observations, it was found that the observed long-period ground motions were generated by the long-period surface waves excited at the basin edge, where the bedrock depths are approximately 1 km. The excited surface waves propagated towards the central or southern parts of the Kanto Basin, and they showed complicated propagation patterns with large-amplitude, distinct, long-period wave packets. Our results strongly indicate that it is necessary to consider the strong lateral variation of the sedimentary velocity structure to better understand the complex propagation characteristics of the surface waves in this basin.
We would like to thank two anonymous reviewers and the editor, H. Takenaka, for constructive comments that improved an earlier draft of this manuscript. We acknowledge the National Research Institute for Earth Science and Disaster Prevention, Japan (NIED) for providing the KiK-net and K-NET waveform data and the CMT solutions from the F-net. We also used SK-net waveform data that were provided by the Earthquake Research Institute, the University of Tokyo. The FDM simulations of seismic wave propagation were conducted on the computer system of the Earthquake and Volcano Information Center at the Earthquake Research Institute, the University of Tokyo. One of the authors, ST, is grateful for the financial support provided by a Grant-in-Aid from JSPS (the Japan Society for Promotion of Science; No. 24.5704). All figures in the present study were drawn using the Generic Mapping Tools software package developed by Wessel and Smith (1998).
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