Long-period Ground Motion Characteristics ofthe Osaka Sedimentary Basin
during the 2011 Great Tohoku Earthquake
K. Sato, K. Asano & T. Iwata
Disaster Prevention Research Institute, Kyoto University, JapanSUMMARY:
Long-period ground motion characteristics due to the 2011 Great Tohoku earthquake in the Osaka sedimentarybasin are studied. Dense strong motion records of this event are collected in and around the Osaka basin. The
pseudo-velocity response (pSv) map at 7s shows clear relationship between the amplitude and the basin depth ofthe model. Peak period of H/V spectral ratio of later phase portion shows spatial characteristics and it coincides
with the peak period of R/V spectral ratio of the theoretical fundamental Rayleigh waves obtained from the 1D
underground velocity structure model for most stations. We also found some stations where the observed peak
period differs from the theoretical one located near the basin edge or the Uemachi elevation. Peak shape of theH/V spectral ratio is not clear and it would be caused by the contamination of other type of waves, Love andbody-waves.
Keywords: Long period ground motion, the 2011 Tohoku earthquake, Osaka sedimentary basin

1. INTRODUCTION
Seismic risk by long-period ground motion is one of the present important issues in mega-cities inlarge sedimentary basins because the resonances of long-period structures such as oil tanks, longbridges, and high-rise buildings would cause the seismic disaster. Osaka sedimentary basin, where
many long-period structures are existing, is the most populated area in western Japan and stands about
200 km from the source region of the anticipated Nankai and Tonankai mega-thrust earthquakes
whose high probability of occurrence are about 60–70% within 30 years (Headquarters for Earthquake
Research Promotion, 2011). Together with the source model, the seismic velocity structure modelfrom the source to the target area including the sedimentary basin is needed for quantitative evaluationof long-period ground motions. For example, Iwata et al. (2008) has constructed the three-dimensional
crustal and basin velocity structure models for ground motion prediction for future crustal andsubduction earthquakes in southwestern Japan, and their models have been used for ground motion
simulation studies (e.g., Iwaki and Iwata, 2010). They constructed these models by compiling a
number of geophysical exploration results such as reflection profiles, receiver function profiles,
seismic tomography, etc. We have to make constant efforts to validate how the existing
three-dimensional velocity structure models explain the observed waveforms by ground motion
simulations in the target periods. Especially, the confirmation of the Osaka sedimentary basin model isvery important for accomplishing the reliable ground motion prediction.
The observed seismograms from large to moderate earthquakes, which have enough signal-to-noise
ratios in the long-period range (ex. 2–10 s), are usually used for ground motion simulations. However,such events are not always uniformly distributed around the target region, and large events bringing a
number of data with good signal-to-noise ratios do not occur very frequently. The 2011 Great
Tohoku-Oki earthquake (Mw9.0) rocked all over Japan and its long-period ground motions are
well-recorded in and around Osaka basin, where the source distance is about 600 km. We collected
about 100 records at strong motion stations of many organizations and analysed propagation

characteristics in the Osaka basin. As the long-period ground motion in period as long as 50 s hasenough SN ratio, we could confirm the orientation of seismometer. After correcting the sensor’s
orientation, we map the pSv and discuss its relation to the basin depth. H/V spectral ratio for later
phases is compared to the R/V spectral ratio of the theoretical fundamental Rayleigh wave of the 1D
velocity structure model beneath the station.

2. GROUND MOTION DATA DURING THE GREAT TOHOKU EARTHQUAKE IN AND
AROUND OSAKA BASIN AND ESTIMATION OF SEISMOMETER SETTING
From 1990s, geophysical surveys such as seismic refraction and reflection, gravity anomaly,
microtremor, and borehole logging are conducted in the Osaka basin to construct the basin velocity
structure model. Several research groups proposed their velocity structure models. The seismic
bedrock depth of the model by Iwata et al. (2008), one of those researches, is shown in Fig. 2.1 (Left).
The basin is approximately 80 km x 60 km in size, covering the most populated area in western Japan,including the cities of Osaka and Kobe. The basin thickness is at most 3 km under the Osaka bay andapproximately 1.5 km at the coastal area of Osaka bay.

Figure 2.1. (Left) The bedrock depth in the Osaka sedimentary basin (Iwata et al., 2008). The EW cross section
at the centre of the basin is shown in bottom. (Right) Strong motion stations used in this study. The seismic
intensity meters of Osaka prefecture are shown in red colours.

As the JMA seismic intensities are 2 or 3 in the Osaka area during the 2011 Tohoku earthquake, manystrong motion stations are triggered. We collected strong motion data of many organizations, NIED,CEORKA, JMA, BRI, and Osaka prefecture government. Osaka prefecture government newly
installed the JMA seismic intensity network in FY2010 and they observed this event records at 69stations. In Fig. 2.1 (Right), the strong motion station map is shown. Original records of K-NET andKiK-net of NIED, JMA, BRI, and Osaka prefecture are acceleration, and those of F-net (strongmotion) of NIED and CEORKA are velocity. All records have absolute timing corrected by the GPSsignal. Most of stations are triggered stations and record lengths are 5 minutes in average, while thoseat some stations are 2 or 3 minutes. Taking the amplitude spectra of those records, we recognized therecords have enough S/N ratios even at the period of 50 s both in velocity and acceleration records.
Using this long-period ground motion data, we firstly confirmed the orientation of seismometer for allstations used in this study. The procedure to find the orientation setting difference is as follows, [1]Taking band-pass-filtered acceleration records between 0.02–0.03 Hz (33–50 s in period), [2]calculating cross correlation (time-lag) between the fixed two horizontal components at OSKH04 ofNIED, as a reference, and the rotated (theta) two horizontal components at the target site for the lengthof 100s at the main portion of the ground motion, and [3] finding the time-lag and theta to give themaximum cross correlation value. We assume this long-period ground motion is approximately aplane wave. Using this value of theta, we got the difference of the orientation to the reference site. The
following analysis will be done after correcting the setting orientation.
3. GROUND MOTION CHARACTERISTICS IN THE OSAKA SEDEIMENTARY BASIN
DURING THE TOHOKU EARTHQUAKE
3.1 Pseudo velocity response distribution
On comparison between the Osaka basin velocity structure model and the observation, several
researches are reported. Miyakoshi and Horike (2008) compared the peak period of the observed
whole records of seismic ground motions of regional large earthquakes to the period of the Airyphases of Rayleigh (vertical component) and Love (horizontal component) waves of the 1D velocity
model beneath the observed station. They analysed about 10 broad-band strong motion station data in
the Osaka basin. Those observed peak periods of vertical and horizontal components arecorresponding to those Airy phase periods. Iwaki and Iwata (2010) obtained spectral ratios ofhorizontal to vertical component (H/V spectral ratio) of later part or coda part of observed seismic
ground motions at the sedimentary sites (ObsHV). They estimated the theoretical R/V spectral ratio of
the 1D fundamental Rayleigh wave at the observed stations (1DHV) and the H/V spectral ratio by thenumerical calculation for the 3D velocity structure model (3DHV). They concluded that both the1DHV and the 3DHV fit well to the ObsHV at sites in relatively flat basin bed area. On the contrary,the 3DHV fits better to the ObsHV than the 1DHV at sites whose basement is inclined or near theedge of the basin. Here we obtain the pSv and the H/V spectral ratios of this event records, and show
the characteristics.
Fig. 3.1 shows the comparison of velocity amplitude spectra of EW-component at ABU (rock site,north of the Osaka basin) and SKS (basin site) in the period range between 0.1 and 50 s. SKS islocated in the area where sediments thickness is the largest in the land area in the Osaka basin.
Amazingly, the maximum velocity amplitude spectra occurred at the period of about 40 s both at ABUand SKS. The velocity spectra both at ABU and SKS coincide to each other in the period longer than
20 s. The velocity spectrum at SKS is getting larger than that of ABU in the period range between 10to 20 s. In the period range shorter than 10 s, the amplitude at SKS is much larger than that at ABU
due to the basin effect. We mainly concern about the amplification characteristics observed shorter
than 10 s which related to the basin effect. Therefore, we took pseudo velocity response (pSv) in theperiod range shorter than 10 s. In this period range, we found that the spatial distribution of pSv at 7 sshows systematic characteristics. Fig. 3.2 shows the pSv distribution at 7 s. The maximum pSv wasobserved at SKS, where a high-rise building of Osaka prefectural government was shook as long asten minutes causing emergency stops of elevators and partial damages to the interior of the buildings(e.g. Osaka Prefectural Government, 2011). Larger pSv are observed along the coast and in south partof the centre of Osaka basin; those areas correspond to deeper bedrock area as shown in Fig. 2.1. This
spatial distribution is similarly observed during the 2004 Kii-Hanto-Nanto-Oki earthquake (Iwata andAsano, 2005). However, Iwata and Asano (2005) pointed out that this kind of distribution of pSv wasobserved at 6 s. The difference between the predominant pSv periods would occur by the difference of
the azimuthal direction of incident waves (E to ENE direction to the Osaka basin for the 2011 Tohoku
and SE direction for the 2004 Kii-Hanto-Nanto-Oki) and/or the difference of period characteristics ofincident waves. As shown in Fig. 3.1, wave energy at 7 s is stronger than the neighbour period range at
ABU and that would affect the dominant period of pSv in the Osaka basin. We need to consider thecause of 7 s predomination both in the source effect and the propagation-path effect outside of theOsaka basin.
In Fig. 3.3, we show the azimuthal pSv characteristics at SKS for horizontal component. Themaximum pSv occurred at 7s in the direction of about 10 degree, i.e. N010°E direction. The
back-azimuth to the source is about N068°E based on the CMT solution. The predominant direction ofpSv match neither the parallel to the source direction (as SV or Rayleigh waves) nor the perpendicular
to the source direction (as SH or Love waves). In Fig. 3.2, we showed pSv at 7 s for the EW and NScomponents. The response of NS component is larger than that of EW component at most of stations.
Those results show that the long-period ground motion in the predominant direction is generated by
the basin induced surface waves and investigation of the predominant direction of pSv is necessary forunderstanding ground motion characteristics at the basin site.

Figure 3.1. Comparison of velocity amplitude spectra of EW components between ABU (rock site) and SKS(basin site).

Figure 3.2. Map of pSv distributions of EW (left) and NS (right) components at 7s. Contours show the pSv
distribution calculated from the observation.

Figure 3.3. Azimuthal pSv at SKS. NS and EW directions are 0 and 90 degrees, respectively.

3.2 H/V spectral ratio
We took an H/V spectral ratio, a receiver function in seismology, to find the relationship between
ground motion characteristics and sedimentary basin model, as Iwaki and Iwata (2010) studied. They
analysed coda part (starting from 60 s after the time of S-wave onset) of regional large event records.
For the 2011 Tohoku earthquake, as the source distance is larger than their cases and most of records
are triggered finite records length, we analysed later part of observed records after the main groundmotions. Fig. 3.4 shows an example of the analysed part in the record. We use a 163.84s time window
to take a Fourier spectrum with 10% smoothing window of each component. We calculated the H/Vspectral ratio by the ratio of the vector summation of NS and EW components to UD component. Fig.
3.5 shows examples of the H/V spectral ratios. We picked up the period band that shows dominantH/V spectral ratio together with the period of peak H/V ratio (Tp). We compared the observed Tp tothe peak period of R/V spectral ratio of the fundamental Rayleigh waves computed using the 1D
velocity structure model at the site, which is extracted from the modified version of the 3D velocity
structure model (Iwaki and Iwata, 2011) from that by Iwata et al. (2008). Fig. 3.6 shows thecomparison of the observed Tp and the theoretical Tp. The observed Tp is in good agreement with thetheoretical one except for some stations. This result shows our basin velocity structure model isreasonable in this period range (2-10s) at the area covered by this network. At some stations, theobserved Tp differ from the theoretical Tp. We found those stations were located near the edge of thebasin or near the Uemachi elevation. These areas seem to be modelled insufficiently by thesedimentary velocity model, because that model uses the B-spline function to describe the boundary
shape which is not easy to explain the sharp boundary shape well.

Figure 3.4. Example of the observed velocity waveforms and analysis portion for H/V spectral ratio, at FKS, thebasin site. Blue-shaded area indicates the analysis window.

Figure 3.5. Example of the H/V spectral ratios. We picked the peak period as indicated by the open circle. Blueshaded area show larger H/V ratios. In the case of obvious two peaks in the H/V spectral ratio, We also took thesecond peak area indicated as the red shaded area as shown in the right figure.

Figure 3.6. Comparison between the observed peak period of H/V spectral ratio and the theoretical peak periodof R/V spectral ratio of the fundamental Rayleigh wave under the assumption of 1D underground velocity
structure at the site. Open circles show the peak period. Blue and red bars indicate dominant period area of thefirst and the second peak, respectively. Solid and dashed lines indicate 1:1 and 20% difference, respectively.

4. DISCUSSION AND CONCLUSION
Long-period ground motion characteristics due to the 2011 Great Tohoku earthquake in the Osaka
sedimentary basin are studied. We collected about 100 strong motion records of this event. Using
long-period (more than 30 s) ground motion records, which has sufficient S/N ratio, we estimated theorientation of seismometers. This process is very important for discussing ground motion generationand propagation in the basin. At this moment, we obtained pSv and H/V spectral ratios of this record.
The pSv map at 7 s shows clear relationship between the amplitude and the basin depth of the model.
Peak of H/V spectral ratio of later phase portion showed spatial characteristics, and it is related to thepeak of R/V spectral ratio of the theoretical fundamental Rayleigh waves obtained from the 1D
underground velocity structure model (Iwaki and Iwata, 2011; Iwata et al., 2008) at most stations. The

basin velocity model can be confirmed for the long-period (2–10 s) ground motion simulation.
However, the peak shape of the H/V spectral ratio is sometimes complex, e.g. two distinctive peaks, orflat dominant portion of H/V. It would be caused by the contamination of other type of waves, Loveand body-waves and we will further think about this effect. Detailed comparisons of thesecharacteristics due to the 2011 Tohoku earthquake and other events will be future works forinvestigating generation and propagation of ground motions in the Osaka basin and discussing theperformance of the basin model.

ACKNOWLEDGEMENTS
We used strong motion data obtained by K-NET, KiK-net and F-net of NIED, CEORKA, BRI, JMA,and Osaka prefecture. JMA unified hypocentre catalogue was used. Figures are drawn using theGeneric Mapping Tools (Wessel and Smith, 1998).
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