Download Ground-shaking mapping for a scenario earthquake considering

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120 (DOI: 10.1002/eqe.207)
Ground-shaking mapping for a scenario earthquake considering
eects of geological conditions: a case study for the 1995
Hyogo-ken Nanbu, Japan earthquake
Kazuo Fujimoto∗; † and Saburoh Midorikawa
Department of Built Environment; Interdisciplinary Graduate School of Science and Technology; Tokyo
Institute of Technology; 4259 Nagatsuta; Midori-ku; Yokohama 226 8502; Japan
SUMMARY
In order to examine the applicability of ground-shaking mapping techniques to a near-eld earthquake,
a peak ground velocity map of the 1995 Hyogo-ken Nanbu, Japan earthquake computed from seismic
zoning methods that consider the eects of geological conditions is compared with the actual observed
intensity map. When computing the ground-shaking map, the site amplication at each site is calculated
in terms of the average shear-wave velocity of the ground estimated from the corresponding geomorphological conditions. This map shows a relatively good agreement with the observed intensity map.
However, the computations provide smaller values for certain disastrous areas of the earthquake, where
the eects on ground motion of a deep, irregular underground structure have been reported. The eect
of such structures on site response is examined implementing 2D FEM analyses, thereby being also
incorporated into the method. Results considering the eect of the irregular underground structure show
better agreement with the observed intensity map. Copyright ? 2002 John Wiley & Sons, Ltd.
KEY WORDS:
ground-shaking mapping; geological conditions; irregular underground structure;
Hyogo-ken Nanbu earthquake
INTRODUCTION
Seismic microzoning has been often conducted to identify seismic hazard and risk in urban
areas [1–3]. Since the seismic risk involving damage to buildings and infrastructure is triggered by seismic hazard (ground shaking, liquefaction and landslide), prediction of seismic
hazard is indispensable in seismic risk assessments. Especially, ground-shaking zonation is a
fundamental input to seismic hazard and risk assessments because liquefaction and landslide
are also induced by ground shaking.
Site amplication on ground shaking is recognized as the most signicant factor in ground
motion zoning because conspicuous dierence in site response has been observed over just a
∗
Correspondence to: Kazuo Fujimoto, Midorikawa Laboratory, G3 Building, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama 226 8502, Japan.
† E-mail: [email protected]
Copyright ? 2002 John Wiley & Sons, Ltd.
Received 26 October 2001
Revised 2 February 2002
Accepted 27 March 2002
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K. FUJIMOTO AND S. MIDORIKAWA
few hundred metres [4]. To evaluate site amplication characteristics, site response analyses
such as the propagator matrix method [5] and the equivalent linear response analysis [6] have
been frequently employed in the engineering practice. However, when a spatial distribution of
ground motion intensity is needed, the analyses require much more labour, time and a higher
cost for collecting soil data within the area of interest.
To evaluate site amplication at the regional scale, the use of surface geology for seismic
microzoning turns into a practical alternative because surface geological maps are available in
many regions. Thus, the relationship between surface geology and seismic intensity increment
has been investigated [7]. Later on Borcherdt and Gibbs [8] proposed the concept of relative
amplication (site amplication with respect to a reference outcrop or ground), founding a
strong correlation between the surface geology and the average spectral amplications in the
San Francisco Bay region. They illustrated that the average horizontal spectral amplications
for alluvium and bay mud are about 4 and 10 times greater than that for granite. Such
correlation between surface geology and site amplication is interpretative as follows: (1) the
shear-wave velocity of surface deposits plays a signicant role in site amplication, and (2) the
geological and physical properties for a given surface geological unit show strong correlation
with the shear-wave velocities of the ground [9]. Therefore, the average shear-wave velocity
of the ground to a certain depth (30 m in most cases) has been used to evaluate the site
amplication [1; 9; 10].
Borcherdt [11] developed the method for estimating site amplication from the average
shear-wave velocity (input motion intensity-dependent, short- and mid-period amplication
factors in terms of the average shear-wave velocity to a depth of 30 m). This work was incorporated into the 1997 Uniform Building Code. As the available shear-wave velocity data
is limited, to estimate the distribution of site amplication using this method we need an
empirical equation expressing the relation between surface geology and shear-wave velocity. Because of this Park and Elrick [12] obtained the average shear-wave velocity map in
southern California by extrapolating discrete velocity proles using surface geology. Similarly, Midorikawa [13] showed the empirical relationship between the average shear-wave
velocity and the relative amplication factor for peak ground velocity using the Japanese
data, and later on Matsuoka and Midorikawa [14] proposed a simple relationship to estimate the distribution of site amplication from geomorphological conditions. However, the
correspondence between ground-shaking map computed from the empirical methods and spatial distribution of the recorded ground motion or the actual damage has not been fully
examined.
On the contrary, it has been pointed out that the average shear-wave velocity in the upper
30 m is insucient to account for the entire site response, and therefore additional information, such as eects of deeper structure, is necessary [15]. Hartzell et al. [16] examined the
variability of site response in the Los Angeles area using aftershock records at 231 sites. They
concluded that the variation in shallow shear-wave velocity is still a contributing factor to
site response, while the deeper structures (upper few kilometres) can play a primary role in
local site response.
In the case of the 1995 Hyogo-ken Nanbu, Japan earthquake, the ground motion records
were obtained in and around the disastrous area of the earthquake. In order to supplement the
instrumental observations, a multitude of detailed damage surveys (building damage, overturned tombstones and questionnaire intensity survey) were conducted in the aected area of
the earthquake in order to obtain a detailed intensity map [17]. This map provides a good
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
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opportunity to examine the applicability of ground-shaking mapping techniques to a near-eld
earthquake.
The objective of this study is to examine the applicability of the ground-shaking mapping
techniques to the Hyogo-ken Nanbu earthquake and the necessity for incorporating the eect
of a deep underground structure into the techniques.
SEISMIC INTENSITY MAP OF THE 1995 HYOGO-KEN NANBU EARTHQUAKE
The 1995 Hyogo-ken Nanbu, Japan earthquake (MW = 6:9) caused catastrophic damage in the
Hanshin and Awaji areas. In the earthquake, seismic intensity 7 (the highest rank in the Japan
Meteorological Agency (JMA) scale) was observed for the rst time. The area of the seismic
intensity 7 determined by the JMA is illustrated in Figure 1.
The location of the strong motion stations is also shown by solid circle in Figure 1. Although
many strong-motion records were obtained during the earthquake, few records were obtained
within the area of the JMA seismic intensity 7, and the spacing of the observation station is
not so dense to derive the detailed intensity map.
Figure 1. Map of Hanshin and Awaji areas.
Copyright ? 2002 John Wiley & Sons, Ltd.
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K. FUJIMOTO AND S. MIDORIKAWA
Seismic Intensity 6Seismic Intensity 6+
Seismic Intensity 7 (J.M.A.)
0
10
20
(km)
Figure 2. Observed seismic intensity map of the Hyogo-ken Nanbu earthquake.
For past earthquakes seismic intensity in the aected area has been commonly estimated
from the damage level of wooden houses, the overturned and non-overturned of simple bodies
(such as tombstones in Japan), and questionnaire intensity survey. In order to delineate the
isoseismal map of the earthquake, the authors examined the relation of the seismic intensities
with several damage indexes, such as the seismic intensity evaluated from the questionnaire
intensity survey [18], the overturning rate of tombstones [19] and the damage rates of buildings
[20]. For the earthquake the areas of seismic intensity 6− and 6+ are delineated by overlaying
the distribution of the instrumental seismic intensity and the seismic intensities converted from
damage indexes [17].
Figure 2 shows the area of seismic intensity 6−, 6+ and 7 in the JMA scale. According to
the empirical relationship between the JMA seismic intensity and ground motion parameters
[21], seismic intensity 6− or higher corresponds to a peak ground velocity of about 40 cm=s
or greater. Similarly, seismic intensity 7 roughly matches with a peak ground velocity of
about 100 cm=s. Also, peak ground velocities for JMA seismic intensities of 6−; 6+ and
7 correspond to those for Modied Mercalli intensities of 9, 10 and 11, respectively [22].
In Figure 2, the length of the regions with seismic intensity 6− and 6+ are about 85 and
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
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65 km, respectively. The areas with seismic intensity 6− and 6+ are about 390 and 130 km2 ,
respectively.
CALCULATION OF GROUND-SHAKING MAP
To examine the applicability of the seismic zoning method when estimating the ground-shaking
map of the Hyogo-ken Nanbu earthquake, the observed intensity map shown in Figure 2 is
compared with the computed one. In this comparison the peak ground velocity is adopted as
a ground motion parameter because this parameter shows good correlation with the seismic
intensity, and is a simple parameter to associate with damaging level of intensity [23]. Also,
when compared with the peak ground acceleration, the peak ground velocity does not show
a strong tendency to be aected by the non-linearity of the ground [24].
The ground-shaking map of the earthquake is calculated according to the following procedures. The peak velocity on seismic bedrock (VS = 3 km=s) is computed employing an
empirical method that considers the eects of fault rupture [25; 26]. The method consists of
the superposition of the eects of dierent pulses radiated from each of the individual subelements of the fault plane whose intensities are dened using an empirical attenuation law.
Based on the fault models proposed by several researchers [27], two fault planes are considered for the earthquake. Figure 3 shows a schematic diagram of the fault model used in this
study. Asperities are also shown by hatched rectangles in the gure, but they are not used
in the calculation of the ground-shaking map. According to this fault model, the moment
magnitude of the earthquake is estimated as 6.9. The peak velocity on seismic bedrock is
computed assuming that the shear-wave velocity on seismic bedrock is 3 km=s and the rupture
velocity on the fault is 2:8 km=s (Figure 4(a)).
The surface projection of the fault plane and the starting point of fault rupture are also
shown by a solid line and a star, respectively. The higher peak velocities are found around
both ends of the fault due to the eect of fault rupture propagation. Peak velocities on seismic
bedrock in Kobe City are about 35 cm=s, being at the same level of those calculated using
the variable rupture slip model [28] and deconvolved from the ground motion records in the
area [29].
Matsuoka and Midorikawa [14] proposed a simple method for estimating site amplication. The method is twofold: Firstly, the average shear-wave velocity of ground to a depth
Awaji
Harima
Kobe
Takarazuka
Depth (km)
0
Dip=90 deg.
A
-10
Starting point
of rupture
-20
-20
-10
B
0
C
10
20
30
40
(km)
Figure 3. Schematic diagram of the fault model.
Copyright ? 2002 John Wiley & Sons, Ltd.
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K. FUJIMOTO AND S. MIDORIKAWA
Peak Velocity
on Bedrock (cm/s)
40 -
0
10
20
35 - 40
(km)
30 - 35
25 - 30
20 - 25
- 20
(a)
Amplification Factor
0
10
20
(km)
2.5 2.0 - 2.5
1.5 - 2.0
1.0 - 1.5
- 1.0
(b)
Figure 4. Distributions of (a) peak velocity on seismic bedrock, and (b) site amplication.
of 30 m is estimated from the corresponding geomorphological conditions and elevation
(or distance from major river). The relation of the average shear-wave velocity with respect to elevation for dierent geomorphological conditions is expressed by the following
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
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CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
Table I. Regression coecients in Equation (1).
Geomorphological condition
Reclaimed land
Delta=back marsh D 6 0:5
D ¿ 0:5
Natural levee
Sand bar=dune
Alluvial fan
Gravel plateau
Hill
Mountain
Matsuoka and
Midorikawa [14]
This study
a
b
c
a
b
c
2.23
2.19
2.26
1.94
2.29
1.83
1.76
2.64
2.87
0
0
0
0.32
0
0.36
0.36
0
0
0
0
0.25
0
0
0
0
0
0
2.32
2.19
2.26
1.94
2.41
2.32
2.17
2.64
2.87
0
0
0
0.32
0
0.14
0.15
0
0
0
0
0.25
0
0
0
0
0
0
equation:
log AVS = a + b log H + c log D
(1)
where AVS is the average shear-wave velocity in the upper 30 m (m=s), H is the elevation
(m) and D is the distance from major river (km), respectively. The regression coecients
(a; b and c in Equation (1)) for the dierent geomorphological conditions considered are listed
in the left-hand side of Table I. Next, the site amplication factor is evaluated substituting
the average shear-wave velocity derived from Equation (1) into the following equation [30]:
log ARV = 1:83 − 0:66 log AVS
(2)
where ARV is the site amplication factor for peak velocity.
However, in Equation (2) the site amplication factor is dened as the ratio of peak velocity
on the ground surface to that on the engineering bedrock with a shear-wave velocity of
600 m=s. This value diers from the shear-wave velocity on seismic bedrock, and accordingly,
the amplication factor of this 600 m=s shear-wave velocity bedrock to that with 3 km=s is
evaluated as 1.46 taking into account the empirical relation between the amplication factor
and the average shear-wave velocity [13]. Hence, the peak velocity on the engineering bedrock
is identical to 1.46 times that on seismic bedrock.
For the geomorphological condition and elevation data, the Digital National Land Information (DNLI) compiled by the National Land Agency is used in the original method. The DNLI
is an extensive geographical database covering all over Japan with a grid of 1 km meshes.
Therefore, using the DNLI the ground-shaking map for a larger area can be drawn with a
resolution of 1 km. Since the distribution of the observed seismic intensity map is quite complex (see Figure 2), the distribution of site amplication with 1 km resolution is insucient
to delineate such a complicated distribution. Consequently, both, digital geomorphological
conditions and elevation maps with 250 m meshes are prepared to compute a detailed site
amplication distribution.
In the case of the geomorphological conditions map, the analogue maps compiled by the
National Land Agency at scales of 1=100 000–1=200 000 are digitized, transformed to a digitized polygon map, and nally converted to a digital map with a grid size of 250 m using a
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
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K. FUJIMOTO AND S. MIDORIKAWA
Geographic Information System (GIS). To obtain the digital elevation map, a Digital Elevation Model (DEM) with a grid size of 250 m compiled by the Geographical Survey Institute
is used.
Accordingly, the peak ground velocity is calculated by multiplying the peak velocity on the
engineering bedrock and the site amplication. However, the method for estimating the site
amplication [14] is based on the shear-wave velocity data obtained in the Kanto area, which
is almost 500 km east of the Hanshin area (see index map in Figure 1). The applicability
of the method to another region has not been fully examined. This implies that the shearwave velocity characteristics of a given geomorphological condition should be examined in
the Hanshin area.
CORRELATION OF SHEAR-WAVE VELOCITY OF THE GROUND WITH
GEOMORPHOLOGICAL CONDITION IN KANTO AND HANSHIN AREAS
In order to examine the correlation of the shear-wave velocity with the geomorphological
conditions in the Kanto and Hanshin areas, soil data on shear-wave velocity for these two
areas are collected. The total number of soil data compiled was 478 (the number of PS
logging and that of boring data is 69 and 409, respectively). In accordance with Matsuoka
and Midorikawa [14], the time-weighted average shear-wave velocity in the upper 30 m is
adopted. For the boring data, the average shear-wave velocity is estimated from the N -value,
depth and geotechnical conditions using an empirical relationship [31]. The average shearwave velocities estimated from the boring data show good correlation (r = 0:78) with the
measured values from the PS logging data.
The relation between elevation and the average shear-wave velocity for dierent geomorphological conditions is illustrated in Figure 5. Since the relations for Hill and Mountain in
the Hanshin area are similar to those for the Kanto area, the relations are not displayed herein.
The average shear-wave velocities for the Hanshin and Kanto areas are shown by solid and
open circles, respectively. The regression lines derived from the relations for the Kanto area
[14] are also shown by solid lines. Generally, the relations for the Hanshin area are consistent
with those for the Kanto area, although discrepancies are found in the relations for some
geomorphological conditions. For example, the average shear-wave velocity in alluvial fan
and gravel plateau for the Hanshin area is slightly higher than that for the Kanto area. This
is probably due to the locality of the geomorphic evolution process.
In order to evaluate the site amplication factor more appropriately in the Hanshin area,
the original method is partially modied, and regression analyses are applied to the relations for the Hanshin area. The resulting regression coecients are shown in the right-hand
side of Table I and the relationships for the Hanshin area are shown by broken line in Figure 5. The distribution of site amplication calculated by using these relationships is shown
in Figure 4(b). The site amplication distribution is complicated due to the variability of the
geomorphological conditions and the elevations.
Figure 6 shows the computed peak ground velocity map. Since the seismic intensity of
6− or greater corresponds to the peak ground velocity of about 40 cm=s or greater [21], the
computed map shows a relatively good agreement with the observed one. Figure 7 shows the
comparison between the observed and computed peak ground velocities at the sites shown
by open circles in Figure 6. The relation between the observed and computed values shows
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
: Relationship for Kanto area
(Matsuoka and Midorikawa, 1995)
: Relationship for Hanshin area
: Hanshin Area
: Kanto Area
103
103
Reclaimed Land
Delta/Back Marsh
Average shear-wave velocity (m/s)
102
102
10-1
100
10 1
-1
Distance from Major River (km) 10
10
3
10
100
101
102
103
102
103
102
103
3
Natural Levee
Sand Bar/Dune
102
102
10-1
10
2111
100
101
102
10 3
3
10-1
10
101
Gravel Plateau
Alluvial Fan
102
10-1
100
3
102
100
101
102
10 3
10-1
100
101
Elevation (m)
Figure 5. Relationships between elevation and average shear-wave velocity.
good one-to-one relationship. However, the computed values in Kobe and Takarazuka Cities
are smaller than the observations in several sites (Takatori (#1), Fukiai (#2) and Takarazuka
(#3) in Figures 6 and 7). As shown in Figure 6, the long and narrow-shaped area of the
JMA seismic intensity 7 is found in Kobe City in the observed intensity map, however, this
is not apparent in the computed map. Although Takarazuka City is included within the area
of the JMA seismic intensity 7, and a peak ground velocity of about 80 cm=s was observed,
our computed peak ground velocities show smaller values.
EFFECT OF IRREGULAR UNDERGROUND STRUCTURE ON SITE RESPONSE
In Kobe City, the basin edge structure with a vertical oset of 500–1000 m is distributed
nearly along the Rokko Fault System, which is located about 1 km north of the area of
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
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K. FUJIMOTO AND S. MIDORIKAWA
Takarazuka
#3
#2
Harima
Kobe
#1
Awaji
Osaka
Seismic Intensity 6Seismic Intensity 7 (J.M.A.)
strong motion station
0
01
20
(km)
Peak Ground Velocity
100 - (cm/s)
75 50 - 50
Figure 6. Computed peak ground velocity map.
JMA seismic intensity 7. Simulations of strong ground motion carried out in Kobe City
using a three-dimensional basin structure [32] indicate that the disastrous area in this area
appears due to the eect of the irregular shape of the underground structure, the so-called
‘basin edge eect’. The existence of irregularities in the underground structure ascribed to the
movement of the Arima-Takatsuki Tectonic Line is also reported in Takarazuka City [33]. As
the computed peak ground velocity map shown in Figure 6 does not consider the eect of
an irregular underground structure on ground motion, the computed values may be somewhat
underestimated in these areas.
In order to examine the eect of the irregular underground structure in Kobe City, Kawase
[34] considered the local 2D ground model in the fault-normal direction along the A-A’ line
in Figure 8(a) for FEM analysis. The section of the model plane used for the analysis is
shown in Figure 8(b). The bedrock motion deconvolved from the ground motion observed at
the Kobe JMA station (JMA in Figure 8(a)) is used as an input motion to the model and
then the responses along the surface of the section is calculated. The distribution of the peak
ground velocity along the surface of the 2D FEM model section is shown in Figure 8(c).
The horizontal (fault-normal) component of the peak ground velocity has a prominent peak,
with 150 cm=s at a distance of about 800 m from the basin edge. Around the peak, the peak
velocity exceeds 120 cm=s for a region about 600 m in width. Beyond 2000 m away from the
edge, the velocity levels converge to a nearly uniform value that should correspond to the
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
Computed PGV (cm/s)
1:
1
Kobe
Takarazuka
Harima
Osaka
Other
150
2113
100
#2
#1
#3
50
PGVcomp.=0.97PGVobs.
(r=0.82)
0
50
100
Observed PGV (cm/s)
150
Figure 7. Comparison between observed and computed peak ground velocities.
one-dimensional response of the basin. Motosaka and Nagano [35] also conducted a similar
analysis, but applied to another model section (B-B’ line in Figure 8(a)). Both results are quite
similar. Kawase [34] concluded that the amplication of the ground motions about 1 km away
from the basin edge is caused by the coincidental interference of the vertically incident Swave inside of the basin with the basin-induced diracted=surface waves, which are generated
at the basin edge and are radiated horizontally into the basin.
For Takarazuka City, the 2D FEM analysis is performed to evaluate the eect of the irregular shape of the underground structure. Several groups estimated the underground structure
in the area from explosion experiment [36], reection survey [33] and the microtremor array
observation [37]. According to the results, the underground structure in the area consists of
two sedimentary layers above the bedrock and spreads two dimensionally along the fault-strike
direction of the Tectonic Line. Therefore, the 2D FEM model orthogonal to the strike of the
Arima-Takatsuki Tectonic Line is considered. The FEM model is shown in Figure 9(a).
The ground motion record at the KBU site, located on weathered granite about 15 km
away from Takarazuka City (see Figure 1) is used as outcrop motion because there is no
record on sti soil or rock conditions in this area. Considering the geometrical spreading
eects on seismic waves and the conversion from surface motion to bedrock motion, the time
history at the KBU site with an amplitude of 0.4 times is used as input motion, as shown in
Figure 10(a). Two dominant phases (phases I and II) are found in the waveforms. According
to the fault model of the earthquake [27], three large asperities are found on the fault plane.
These locations are shown by hatched rectangles in Figure 3. The phases I and II originate
from the asperities B and C shown in Figure 3, respectively. Since the eect of the asperity
C is considered to be more critical to the ground motion in Takarazuka City than that of the
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
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K. FUJIMOTO AND S. MIDORIKAWA
lltt
aauu
aa FF
hhiiyy
s
s
A
A
KBU B
ulltt
uulltt
Fa
FFaa
maa
iikkii
aam
y
y
b
b
a
a
nnoo
ww
Su
N
Nuu
A
ulltt
FFaau
maa
m
a
a
eeyy
EEgg
B'
Motosaka and Nagano (1996)
JMA
A'
Kawase (1996)
J.M.A. Seismic Intensity 7
0
2
4
(km)
(a)
Suwayama Fault
VS =450 m/s
VS =550 m/s
VS =650 m/s
VS =1000 m/s
VS =2500 m/s
3960 m
(b)
A
A'
Peak ground velocity (cm/s)
150
Horizontal comp.
(Fault-normal)
Vertical comp.
100
50
0
(c)
1000
2000
Distance (m)
3000
4000
Figure 8. Calculated peak ground velocity along the 2D model, after Kawase [34].
asperity B, the phase II in the input motion is used for the analysis, which is shown by solid
line in Figure 10(a).
Figure 9(b) shows the distribution of the computed peak ground velocity along the surface
of the model section. In the fault-parallel component, prominent peaks in the distribution of
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
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Basin Edge
(Arima-Takatsuki Tectonic Line)
JRT
0
Depth (m)
Sediment
(a)
-200
VS =1000 m/s
-400
Bedrock
-600
-800
VS = 600 m /s
Sediment
VS =2500 m/s
0
2000
4000
6000
50
(b)
0
Damage rate (%)
Peak ground velocity (cm/s)
100
Fault-normal comp.
Fault-parallel comp.
2000
4000
6000
2000
4000
Distance (m)
6000
30
20
10
0
(c)
Figure 9. Calculated peak ground velocity and damage rate along the 2D FEM model.
the peak ground velocity are not found. On the contrary, in the fault-normal component, the
ground motion is amplied at the edge of the basin, reaches the peak about 1 km away from
the basin edge, and nearly converge to the one-dimensional response beyond 2 km from the
edge. The distribution of the rate of collapsed or heavily damaged low-rise buildings [20]
along the model section is also shown in Figure 9(c). The region of higher peak ground
velocity almost corresponds to that of the higher rate of the damaged buildings. The strong
motion record was obtained at the Takarazuka Station, Japan Railway Co. (JRT), which
is located on the sedimentary side in Takarazuka City (see Figure 1). Figure 10(b) shows
the comparison between the observed and computed velocity time histories at the JRT site.
The computed time history agrees well with the observation. Consequently, the eect of the
irregular underground structure on ground motion is signicant in Takarazuka City.
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
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K. FUJIMOTO AND S. MIDORIKAWA
VEL. (cm/s)
20
Phase I
Phase II
Fault-normal
10
0
-10
-20
0
5
10
15
20
Fault-parallel
10
0
-10
-20
0
5
(a)
10
15
Time (sec)
VEL. (cm/s)
100
Fault-normal
50
0
Obs.
Cal.
-50
-100
0
5
10
15
100
Fault-parallel
50
0
Obs.
Cal.
-50
-100
(b)
0
5
10
15
Time (sec)
Figure 10. Velocity time histories of (a) input motion, and (b) output motion at JRT site.
RE-CALCULATION OF GROUND-SHAKING MAP
To obtain a more reliable ground-shaking map of the earthquake, the eect of the irregular
underground structure on ground motion is included in the method. According to the results
obtained from 2D FEM analyses, the distribution of the peak ground velocity in the faultnormal component along the model section in Kobe City (solid curve in Figure 8(c)) is similar
to that in Takarazuka City (solid curve in Figure 9(b)). Thus, the eect of the irregular
underground structure on the peak ground velocity in these areas is simply modelled as a
function of the distance from the basin edge. The function of amplication for the peak
ground velocity with respect to the distance from the basin edge and the locations of the
basin edge are shown in Figure 11.
Accordingly, the peak ground velocities are computed again considering the eect of the
irregular underground structure. The re-computed peak ground velocity map is shown in
Figure 12. Comparison of Figures 6 and 12 shows that in Kobe and Takarazuka Cities the
peak ground velocities of around 100 cm=s or more are distributed as long and narrow, which
show better correspondence with the area of the JMA seismic intensity 7. Figure 13 shows the
comparison between the observed and computed peak ground velocities at the strong-motion
stations shown by open circles in Figure 12. The re-computed values, especially at the Takatori
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
Depth
Amplification
CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
2117
Basin edge
1.5
1.0
Bedrock
Sediment
Bedrock
Takarazuka
Sediment
0
1000
2000
3000
Distance from basin edge (m)
Basin Edge
Bedrock
Sediment
Kobe
0
5
10
(km)
Figure 11. Function of amplication for peak ground velocity with distance from
basin edge and location of basin edge.
(#1), Fukiai (#2) and Takarazuka (#3) sites, show a better one-to-one relationship when considering the eect of the irregular underground structure. As a result, the correlation coecient
of the regression line is increased from 0.82 to 0.89. Since the basin responses are dependent
on the geometry of the basin edge [38], the function shown in Figure 11 will be applicable
to the other area having a rectangular shape of basin edge.
CONCLUSIONS
In order to examine the applicability of ground-shaking mapping techniques to a near-eld
earthquake, a peak ground velocity map of the 1995 Hyogo-ken Nanbu earthquake computed
from seismic zoning methods that consider the eects of geological conditions is compared
with the actual observed intensity map. When computing the ground-shaking map, the site
amplication at each site is calculated by means of the average shear-wave velocity of the
ground estimated from the corresponding geomorphological conditions and elevation. The
method for estimating the site amplication is partially modied based on the regional soil
data.
The ground-shaking map shows a relatively good agreement with the observed intensity
map. However, the computations provide smaller values for certain disastrous areas of the
earthquake, where the eects on ground motion of a deep, irregular underground structure
have been reported. The eect of such structures on peak ground velocity was examined
implementing 2D FEM analyses, thereby being also incorporated into the method, which
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
2118
K. FUJIMOTO AND S. MIDORIKAWA
Takarazuka
#3
#2
Harima
Kobe
#1
Awaji
Osaka
Seismic Intensity 6Seismic Intensity 7 (J.M.A.)
strong motion station
0
20
10
Peak Ground Velocity
100 - (cm/s)
(km)
75 50 - 50
Figure 12. Re-computed peak ground velocity map.
1
Kobe
Takarazuka
Harima
Osaka
Other
1:
Computed PGV (cm/s)
150
100
#1
#2
#3
50
PGVcomp.=1.04PGVobs.
(r=0.89)
0
50
100
150
Observed PGV (cm/s)
Figure 13. Comparison between observed and re-computed peak ground velocities.
modelled such eects as a function of the distance from the basin edge. Results considering
the eect of the irregular underground structure show better agreement with the observed
intensity map. Consequently, it is signicant to consider appropriately both, the eects of
surface geological conditions and the deep underground structure on ground motion.
Copyright ? 2002 John Wiley & Sons, Ltd.
Earthquake Engng Struct. Dyn. 2002; 31:2103–2120
CASE STUDY FOR THE 1995 HYOGO-KEN NANBU EARTHQUAKE
2119
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