3A-P11

The Joint Symposium of Seismic Hazard Assessment(Sendai Japan), 17-19 June, 2013

Modeling of the subsurface structure from the seismic bedrockto the ground surface for a broadband strong motion evaluation
Shigeki Senna, Takahiro Maeda, Yoshiaki Inagaki, Haruhiko Suzuki,
Naohiro Matsuyama, Hiroyuki Fujiwara#Ⅰ

THE OUTLINE OF THE STRUCTURE MODEL TO BUILD

In order to perform earthquake ground motion prediction for a broadband, it is important to get to know the foundation structure model from the seismic bedrock to theground surface. However, in the previous research, the structure model aiming at earthquake motion prediction of the broadband in a wide area is not created.
In this study, we are modeling of the subsurface structure from the seismic bedrock to the ground surface for a broadband strong motion evaluation with about 250m
intervals(the 7.5-arc-second JGEM).

Fig. 1 The relational figure of the S wave of velocity structure,and the periodic characteristic in the kanto plain in Japan.

ⅡFig.2 The key map of the subsurface structure from the seismic bedrockto the ground surface creation

COLLECTION OF DATA FOR STRUCTURE MODEL

We carried out microtremor observation of the single point (H/V) at intervals of about 2 km. On the other hand,
we carried out microtremor array observation at intervals of about 5 to 10km. Microtremor observation points
are always seismic observation point or school. We used an integrated microtremor observation unit (JU-215,Hakusan Corp.), equipped with two horizontal elements, one vertical element, and a data-logger (LS-7000XT).
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Fig. 3 The flow of formation of a structure model construction

INITIAL VELOCITY STRUCTURE MODEL CONSTRUCTINGⅢ

In the initial stage, we combined the two ground structure models (Shallow-structure and Deep-structure)
to form the seismic bedrock to the ground surface. To use the initial velocity structural model as the basis
for analysis, we adopted the existing deep structure model ( J-SHIS model ) deeper than engineering
bedrock, and created a shallow structure model from the geologic columns provided by the Chiba andIbaraki prefectures or ourselves.
The creation method of a shallow structure model and an initial model(deep structure and shallow structurecombine model) are shown.

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地盤モデル
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Fig.7 The key map of themodeling method by spaceinterpolation of theanalyzed structure models

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Fig.6 Microtrmor observational data of Chiba andIbaraki Prefecture( left: single observation points,right: array observation points )CHB021CORRECTION ANALYSIS OF A STRUCTURE MODELFirst, the depth of seismic bedrock was adjusted with theR/V spectrum of seismic observation record. Next, thereverse-analysis by the H/V spectral ratio and the phasevelocity was applied on the initial model.
In order to create a final velocity structure model, we werereverse-analyzed by the phase velocity created by theabove-mentioned velocity structure model, and microtremor
observation (H/V spectral ratio) result.
Reverse analysis is conducted in two steps. The laststructure model averaged the S wave velocity of the samegeological formation model, and was created.

Fig.11 The layer structure revision method in a joint inversion

Fig.10 The flow of reverse analysis

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Fig. 5 The seismographstation which collected
the data of Chiba andIbaraki Prefecture
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Fig. 4 Collected borehole datain the Kanto district (Data ofabout 100,000 points or more )CHB013

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THE CREATED STRUCTURE MODEL ( LAST RESULT)In addition, the spectral amplification rate obtained from our model is consistent with that of earthquake ground motion
recorded by strong ground motion observation point. As a result, the velocity structure model which can mainly explain
the excellence period of the range for 2.0 to 0.5 second was able to be created.In response to this result, modeling fromthe seismic bedrock for earthquake motion prediction to the ground is due to be performed in a wide area from now on.

Fig.8 Calculation of initial S wave
velocity (Vs(m/s))

R/V(Seismic Obs) +Phase velosity(microtrmor array)
Joint Inversion

Fig.9 The built initial structure model

H/V(Microtremor Obs) +Theoretical phase velosity(Initial model)
Joint Inversion( f < 0.5Hz )CR : Rayleigh wave theoretical phase velocity of structure model
H/V : H/V spectrum ratio (Rayleigh/Love ratio = 0.72 : 4thmode composition).
IR and IHV : data number.
Subscript m and S : observed and theoretical value.
W : weight function(WR=1.0 and WHV=1.0)
E : Residual
PV : Phase Velocity RV : R/V spectrum
Npv : the number of data of phase velocity.
Co(fj) is the observation phase velocity.
Cc(fj)is theoretical phase velocity.
W (f) is a weight function.

Fig.13 Joint inversion result

Fig.14 Joint inversion result by H/V spectrum and phasevelocity.(Brackline:microtremor(H/V),Blueline:Past
Study,Greenline:Initial model,Redline:This study )Ⅵ VERIFICATION OF THE CREATED STRUCTURE MODEL(REFERENCE INFORMATION)
Finite Difference Method(FDM)
About the structure model which we created by thisstudy, after dividing the short period and the longperiod, accuracy was verified.The site amplification
factor of the observation point by conducting
spectrum conversion analysis of earthquake groundmotion was calculated.

(f < 0.5Hz)

One dimensional multi-reflection theory( f ≧ 0.5Hz)

Fig.19 The earthquake (left figure) used for calculationof a finite difference method, and the observation point
which performed waveform calculation (right figure)Fig.15 Vs30 map and Vs300map from the 7.5-arc-second JGEM
(about 250m mesh) in Ibaraki and Chiba Pref.

Vs
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Q( f ) =⋅ fcb ( f ≥ fc )Q( f ) = ⋅ f ( f < f c )aa( b = 0.44, a = searches for 0.1 to 100 )Fig.16 Upper surface depth of the deep structure
model created by this study and previous study
Fig.17 Joint inversion result by H/V spectrum and phase velocity (J-SHIS) (Upper : Vs700(m/s) , Middle : Vs900(Blueline:Past Study,Greenline:Initial model,Redline:This study ) 1000(m/s), Lower : difference of two models).
(Standard deviation by structure models to the microtremor
observations shown in Fig.14)

Fig.18 Identification analysis result of Q-value, black:observation amplifying characteristic (spectrum inversion result), light blue: result of AVS100, blue: averagevalue (a=10.6), green: identification analysis result(15 Results of Ibaraki Prefecture)

Fig. The velocity waveform in two points of course-of-traverse A-A' of the Fig16(left figure), and comparisonof the Fourier spectrum. Observation record (black line:Obs) . The analysis result (red line: NEW) using thefoundation model of this research. The result (blue line:J-SHIS) of having used the J-SHIS model.