Estimation of subsurface geology
based on microtremor array
exploration: Application to
Kashiwazaki City
Kentaro MOTOKI (Kobori Research Complex Inc.),
Hiroaki YAMANAKA (Tokyo Institute of Technology)
and Hiroaki SATO (Central Research Institute of
Electric Power Industry)
Background and Objective
S-wave velocity data is one of the
important parameters for an estimation
of the effects of surface geology.
Microtremor array exploration can be
addressed as one of the inexpensive
methods, because of low cost for
measurements. In this study, actual
applications of the microtremor array
explorations are introduced, especially
focusing on a use of the resultant S-wave
profiles from the microtremor array
explorations in Kashiwazaki City, Japan,
to understand the amplification of Swaves during the aftershocks of the 2007
Niigataken Chuetsu-oki earthquake.
Effects of
Surface
Geology
Source
Bedrock
MT array measurement
Application of microtremors in Earthquake
Engineering
1. Direct estimation of ground vibrational characteristics
・Spectra, Spectral ratio, H/V spectra at single site
・Predominant period and Amplification factor
・Easy and inexpensive
・Not always applicable
Source of Microtremors
Seawave
Wind
Natural Sources
Traffic noises
Human Activities
Artificial Sources
2. Estimation of subsurface structure
・Determination of Vs and thickness
by array technique
⇒ Numerical estimation of transfer function
←Microtremor array
・Theoretical background was already developed
・Need of many obs. data
addresses here
・Assumption : microtremors are composed of surface waves
⇒Vertical motions are composed of Rayleigh waves
3. Structural vibration
・Natural period and damping of structures
(translation and torsion)
Procedure of microtremor array exploration
[OBS]
Array observation of microtremors
Estimation of phase velocity
(F-k method, SPAC method)
[1D array]
Inversion of phase velocity(GA, SA)
A
C
Wave propagation
time
1D Vs-profile
calculation of site effects
B
v
L AC
t AC
distance
Evaluation of phase velocity with MT-array
F-k Method
SPAC Method
Phase vel. (km/s)
T=3.38(s)
4
Ky(rad/km)
2
0
1.65km/s(0.30Hz)
4
N
3
2
1
0
E
Kx=-0.98, Ky=-0.59
-2
-4
-4
-2
0
Kx(rad/km)
2
4
Advantage: Arbitrary array shape
can be applicable
Disadvantage: Narrow coverage
for wave length
Advantage: Applicable for longer
wave length
Disadvantage: Array needs to be on
a concentric circle
Example of microtremor array exploration
0
3
NGT
Depth (km)
1
2
3
Vs
OBS
INV
1
2
3
Vs (km/s)
L5
L4
2
-1.5
-1
L3
L2
S3
-0.5
S7
S2
1
S4
0
NGT
S5
S6
S1(L1)
0.5
NGT
4
0
L6
L7
-2
NS(km)
Phase velocity (km/s)
-2.5
4
0
0
1
2
3
Period (s)
4
Vertical comp.
Calculation of
amplification
2
1.5
1
0.5
EW (km)
5
0
-0.5
-1
Applied to the central part of Kashiwazaki City damaged
during the 2007 Niigataken Chuetsu earthquake
Some of wooden
houses were collapsed
and this damage was
concentrated in the
central part of
Kashiwazaki City during
this earthquake.
The central part of
Kashiwazaki City
AIJ(2007)
Aftershock observation
Sand
Levee
Alluvial
Lowland
Reference
reference
Earthquake motions on sand levee sites and alluvial
lowland sites are amplified comparing to the reference
site on the foot of the mountain.
Result of Microtremor array explorations
In this case, we focus on the
subsurface structure to the Vs
500m/s layer. The sensors are
arranged with a radius of
several tens meters. Analysis is
performed with SPAC method.
Comparison of amplification factors between
aftershock motions and the estimation with the
model from MT-array
The amplification factors based on 1D model Estimated by MT-array
exploration correspond to the
experimental site effects evaluated
from horizontal spectral ratios of
aftershock motions against reference
site. The experimental site effects is
larger than the amplification factors
with 1-D model, because the
experimental ones include 2-D
and/or 3-D effect of soft soils.
Applied to Kashiwazaki-Kariha site
MT-array were performed in the
Kashiwazaki-Kariha site(KK site). In
this case, the structure model to
the seismic bedrock is estimated.
The boundary of the seismic
bedrock around this area is made
clear very deep and the subsurface
structure below KK site is irregular
by fold structure by previous
surveys. Large size array (Max. dist.
3km) were applied and the phase
velocity was evaluated with F-k
method.
The result of MT-array in KK site
Previous
下高町-1
survey
の地質構造
Inverted model
0
3.5
F-K法
逆解析結果による
理論位相速度
3
899
上部寺泊層
1634
2.5
2
2
KK-L
KK-Ｍ
西山層
椎谷層
（上部寺泊層）1.7km/s
1
Depth (km)
Phase velocity (km/s)
0
222
（西山層） 0.75km/s
（椎谷層）1.2km/s
KK-S
1.5
1
下部寺泊層
（下部寺泊層）1.9km/s
2579
七谷層
グリーンタフ
3
（七谷層・ｸﾞﾘｰﾝﾀﾌ） 2.6km/s
4
0.5
0
（基盤）
Seismic
bedrock
メインアレイ
5
0
0.5
1
Frequency (Hz)
1.5
2
6
0
0.5
1
1.5
2
2.5
3
3.5
4
S-wave velocity (km/s)
Simulated annealing is used as a inversion technique and 9 layer model were used in
this case for subsurface model. The boundary of inverted model is consistent with the
result of the deep borehole for petroleum exploration near KK site.
For irregular subsurface structure
0
0
西山
以降
0.2
0.2
0.4
椎
谷
Depth(km)
Depth(km)
0.4
0.6
0.8
寺
泊
1
西山
以降
0.6
0.8
椎
谷
1
Phase velocity (km/s)
2.5
1.2
1.2
2
0
1
2
3
(km/s)
4
0
1
2
3
(km/s)
4
1.5
1
0.5
0
0
0.5
1
Frequency (Hz)
1.5
2
For irregular subsurface structure, 2 MT-arrays
were performed near the PS logging points. The
inverted models are good agreement with the
result of PS logging. This is an example that the
irregular subsurface successfully are explored with
MT-array.
Conclusions
The methods and examples of microtremor array exploration were introduced.
･The amplification factors in the central part of Kashiwazaki City with 1-D
model explored using MT-array can be explain the experimental
amplifications using aftershock observation of the 2007 Niigataken Chuetsu
earthquake, generally.
･The deep structure to the seismic bedrock could be evaluated below KK site.
For irregular subsurface structure, the different depth of sedimentary layer
in Vs profiles of adjacent 2 points were successfully evaluated with MTarray.
The MT-array can be regarded as one of useful and inexpensive exploration
methods for S-wave velocity profiles of shallow and deep sedimentary sites.
But since MT techniques including MT-array are observed natural phenomena,
the frequency characteristics of sources cannot be controlled.