Bulletin of the Seismological Society of America, Vol. 88, No. 2, pp. 400412, April 1998

Source Model of the 1995 Hyogo-ken Nanbu Earthquake and Simulationof Near-Source Ground Motion
b y Katsuhiro K a m a e and Kojiro Irikura

Abstract

The 1995 Hyogo-Ken Nanbu earthquake struck the heavily populated
Kobe and adjacent cities in western Japan. More than 6400 people were killed, andmore than 150,000 buildings were destroyed. The characteristics of mainshock
ground motions in the heavily damaged area are needed to understand how buildings
and bridges performed and why they reached failure. Unfortunately, very few strong
ground motions were recorded in the heavily damaged area during the mainshock.
In this study, we attempt to estimate mainshock ground motions by using the empirical Green's function method (EGF method). First, we assume an initial source
model with the asperities based on the rupture process obtained by inversion ofstrong-ground-motion records. For simplicity; we consider each asperity as a subevent with uniform stress drop in a finite extent. Then, the initial model was improvedby matching the synthetic and observed ground motions using a trial-and-error procedure. The final model consists of three subevents: subevent 1 with stress drop of
163 bars, under the Akashi Strait around the rupture starting point; subevent 2 withstress drop of 86 bars, under the Nojima Fault in Awaji Island; and subevent 3 withstress drop of 86 bars, under Kobe. Finally, we estimate strong ground motions using
aftershock records at sites where the mainshock was not recorded. The near-source
motions in Kobe synthesized with the best-fit model are characterized by two large
pulses with a duration of 1 to 3 sec. The pulses are caused by forward rupture
directivity effects from subevents 1 and 3. Peak horizontal acceleration and velocityof the synthesized motions at the heavily damaged sites are about 1000 cm/sec 2 and130 crrdsec, respectively, while those at a rock site in the near-source region areabout 300 crrdsec 2 and 60 cm/sec.

Introduction
source directivity are the cause of the concentration of heavy
damage along a narrow zone (e.g., Kawase, 1996; Pitarka etal., 1998; Koketsu, 1997).
In order to discuss how buildings and bridges performed
during the earthquake and why they failed, we need to know
the main characteristics of the strong ground motion. Unfortunately, very few strong ground motions were recordedin the heavily damaged areas during the mainshock. Pitarka
et al. (1998) simulated the near-source ground motions by apurely deterministic approach based on the inverted source
model. They successfully explained the appearance of theheavily damaged belt, since ground motions were amplified
in a narrow zone in the sediment side due to the basin-edge
effects as well as the forward rupture directivity. However,
the amplitude of the synthesized motions are considerably
small compared with the observed ones, because surface soft
layers are not taken into account.
In this study, we attempt to estimate ground motion at
the sites inside the damaged area in Kobe city using after-

The 17 January 1995 Hyogo-ken Nanbu earthquake(Mw = 6.9) occurred just beneath Kobe, a megacity with apopulation of about 3 million, and caused the greatest earthquake damage in Japan since the 1923 Kanto earthquake. It
is one of the most important subjects to clarify the source
process in order to understand how the mainshock groundmotions were generated and why they caused such severe
damage. Various explanations related to the generation ofdestructive ground motions have already been reported.
Some geologists suggested that the fault rupture ran just beneath the narrow heavily damaged area, even though there
were few aftershocks located in this region (e.g., Shimamoto, 1995; Watanabe and Suzuki, 1995). By examining
particle motions at strong-motion observation sites in thenear-source area, Irikura (1995) has pointed out that thecausative faults run along the southern edges of the known
active faults and not beneath the narrow damage area. Manyseismologists believe that significant amplification due to thebasin-edge effect and local site effects as well as forward
400

401

Source Model of the 1995 Hyogo-ken Nanbu Earthquake and Simulation of Near-Source Ground Motion

shock records as empirical Green's functions. The advantageof the empirical Green's function method (EGF method) isthat it includes the propagation path and local site effects
and estimates basically broadband motions as long as theaftershock records are accurate enough in broad-frequency
band. Aftershock observations were carried out immediately
after the mainshock by a group from the Disaster Prevention
Research Institute, Kyoto University, and Izumi ResearchInstitute, Shimizu Corporation. Several sites were installed
in and around the heavily damaged area in Kobe (Iwata etal., 1995).
To apply the EGF method to simulate strong groundmotion from the mainshock, we need a source model that is
appropriate over a broad-frequency band of engineering interest (e.g., 0.1 to 20 Hz). Soon after the earthquake, Kikuchi(1995) proposed a source model consisting of three subevents by inverting teleseismic body waveforms. His result is
not useful for estimating near-source ground motions because the rupture zone is not precisely located and the wavelengths used for the analysis are too long (about 100 kin).
In addition, the rupture process of this earthquake was determined by the inversion of the strong-ground-motion records (e.g., Sekiguchi et al., 1996; Ide and Takeo, 1996) andalso from the joint inversion of strong ground motion, teleseismic, and geodetic data (Horikawa, 1996; Wald, 1996;
Yoshida et al., 1996). However, these models are also not
useful for estimating high-frequency strong ground motions
because they were derived from low-pass filtered ( < t.0 Hz)
velocity or displacement data. On the other hand, Kakehi etal. (1996) estimated the radiation process of high-frequency
seismic waves between 2 and 10 Hz by using the envelope
inversion of acceleration seismograms. This model also cannot be applied directly for estimating broad-frequency band
motions, considering its effective frequency range.
In this study, we determine a source model for estimating broadband motions using a forward modeling approachby the EGF kmethod. We assume that ground motions are
generated from several asperities, each of which has a uniform stress drop with a finite extent on the malnshock fault
plane and obeys an co-2 spectral scaling. Their initial locations are determined based on an inverted slip model. Fromthe initial model, we find the best model in the sense of the
fit of synthesized motions to the observed motions using aforward trial-and-error approach. Finally, using the bestsource model, we simulate the strong ground motions in theheavily damaged area of Kobe city.
Empirical G r e e n ' s Function M e t h o dSince Hartzell's pioneering study (1978), the semi-empirical method for synthesizing ground motions from a large
earthquake by using actual small-event motions as theGreen's functions instead of the theoretical Green's function
has been developed by many researchers (e.g., Irikura, 1986;
Dan et al., 1989; Ikeura and Takemura, 1990). Moreover,the idea of the EGF method was extensively applied for in-verting for the rupture process in source areas (Fukuyama
and Irikura, 1986). Irikura (1986) formulated the EGF
method to match the scaling of source spectra as well as thescaling of source parameters, in which both large and small
events follow the co-2 model. The ground motions from alarge event are expressed as a superposition of the recordsof small events as follows:
N N
U(t) = C ~ ~ r F(t - tij) * u(t),i = l j = l r~/

(1)

~ij
tiJ= rq -/~ r o + vR

(2)

where

Nl'z E1 (3)1)(N - 1 ) n ' ,F(t) = ~ (t) + --n' h-=l~
~ t - (k-and U(t) and u(t) are the ground motion for a large eventand the observed record of a small event used as the EGF,
respectively. The terms r, rij, and r 0 are the respective distances from the site to the hypocenter of a small event, fromthe site to the (i, j) subfault, and from the site to the starting
point of rupture on the fault plane of the large event. The
term ~;~ is the distance between the starting point and (i, j)
subfault, fl is the shear wave velocity, VR is the rupture velocity, T is the rise time of the large event, C is the stressdrop ratio for the large and small events, and n' is an arbitrary integer number to shift the artificial periodicity to afrequency higher than that of interest. The scaling parameter
N is obtained from equation (4):( Mot / 1/3
N = \C-~0,]'(4)

where, Mot and M0s are the seismic moments for the large
and small events, respectively.
This formulation can be easily extended to cases where
the target event consists of multiple shocks, each of which
follows the co-2 model. The omega-squared model used heremeans that the source displacement spectrum has a flat levelat low frequencies and an omega-squared decay at high frequencies beyond a corner frequency but not always a constant stress drop between large and small events.
In this study, we use this formulation to estimate asource model of the 1995 Hyogo-ken Nanbu earthquake byforward modeling. The inverted source models as shown
later have some separate regions with relatively large moment releases, so-called asperities. In simulating ground motions from the mainshock, we assumed that the ground motions are generated only from the asperities, each of which
has a uniform stress drop in a finite extent on the mainshock
fault plane. In other words, we synthesize mainshock groundmotions based on the assumption that the mainshock consists

402

K. Kamae and K. Irikura134" 30'
35" 00'135"00'135" 30'
35* 00'01/1713:05
02/0216:19 ~r34" 30'34" 30'134" 30'135" 00'

Figure 1. Map showing the station locations and epicenters of the mainshock (largestar) and the aftershocks (small stars) that areused as the empirical Green's functions. The
fault mechanism of the mainshock is determined using the initial motion.

135" 30'

Table 1
Locations of Strong-Ground-Motion Stationsof multiple events, which are located in the respective asperities.
Strong-Motion D a t a
In our simulation, we used velocity strong motions at
six stations obtained by the Committee of Earthquake Observation and Research in the Kansai Aea (CEORKA) and
one from the Research Reactor Institute, Kyoto University.
The locations of these stations are shown in Figure 1 together
with the epicenters of the mainshock and the aftershocks
used here as empirical Green's functions. Table 1 gives thestation abbreviations and locations as well as the site geology. Table 2 shows the locations and the magnitudes of theaftershocks. The original velocity seismograms at MOT and
AMG were clipped in amplitude larger than 40 cm/sec.
Therefore, we used the restored seismograms based on thesaturation characteristics of the sensor (Kagawa et al., 1996).
The restored records are available for velocity and displacement, but not for acceleration, because the differentiation of
the restored records is less reliable.
First, we estimate the source parameters of the aftershocks. Here, we estimate the seismic moment (M0) fromthe flat level (f~0) in the low-frequency range of the displacement source spectra, expressed by equation (5), and the corner frequency (fc) from the intersection of both asymptotes
in the low- and high-frequency ranges.

Mo = 4npRflSf~°,
Roe,

(5)

where p, R, t, and Roe, are density, hypocentral distance, Swave velocity, and radiation coefficient, respectively; Roe, istaken as 0.63 as the average value (Boore and Boatwright,
1984). The displacement spectrum is computed as vectorialsummation of two horizontal components that were recorded

StationName

Latitude(deg)Longitude
(deg)

Soil

KBU
MOT
AMG
SAK
TDO
KMT
CHI
KMC
FKI
ASY

34.72534.72534.71834.56434.48034.38234.43934.73034.71834.724135,240
135,281135.408
135.469
135.408135.354135.659
135.286135.281135.309rock
diluviumalluviumalluviumdiluviumdiluviumrock
rock
alluviumalluvium

Table 2
Empirical Green's Functions
Date
Aftershock 1 1995/01/17 13:05
Aftershock 2 1995/02/02 16:19
Aftershock 3 1995/01/23 06:02

Latitude(deg)Longitude Depth Magnitude
(deg)
(km)
(JMA)

34.68834,69534.530135.172
135.150134.907

15.0
17.9
15.0

4,7
4.2
4,5

at rock site. Since the resultant M 0 is estimated as an averageof calculations at several sites, the effects of radiation pattern
are expected to be smoothed out to some extent. Then, we
confirmed that the displacement source spectra of the aftershocks used here almost follow the to-2 spectral model. Another important parameter, static stress drop (Aa), is estimated from the seismic moment (M0) and the fault size (r)obtained from the comer frequency using the Bmne's model(1970, 1971),
Aa -

7M0
16 r 3"(6)

Source Model of the 1995 Hyogo-ken Nanbu Earthquake and Simulation of Near-Source Ground Motion

134" 48'135" 00'

135" 24'

135" 12'34"48'..*'.

sube~nt-3 ~ / " ~I~~

Ane~o~-e(o~oe~a:19)_\

403~ul" 48'

MKT,,"" /
KBU

.o "

KMC

34" 36'

J""'°°'/.... 0Ikm

134"48'135"00'135" 12'~pp8N45*E
2°

.

210km

135" 24'

N233*E" 7

II,-•N53*E, D I P 9 0 °.

BC...................

•

ll0km ¢ "DIP85 °'=. . . . . . . . . . . . . . . . . . .

.-%

E
t--.

•

e-~--

,¢

t--

t-"

1

,,

.,.

(Xl

Ik.

, , ..,~f'-.~............................
•

¢

j

,l

•

~i_r',, ,,,L. . . . . . . . . . . . . . . . . . . . . . . . . .
it

19.Skin,t••" ¢ ~ ~ d/x • rupture time (sec

13.5kin24.0kin @...

1.0m
Figure 2. Source model consisting of three asperities estimated from forward modeling using the empirical Green's function method. Our model (hachured) is superimposed on the model of Sekiguchi et al. (1996). Source parameters for each subeventare summarized in Table 4. We assumed that ground motions are generated only fromthe hachured zone in the fault plane. Respective number of subfaults (N) and stressdrop ratio (C) for subevent 1 in case of using aftershocks 1 and 2 are 5 and 3.6, 10 and
1.9; for subevent 2 in case of using aftershocks 1 and 3, are 9 and 1.9, 12 and 1.1; forsubevent 3 in case of using aftershocks 1 and 2 are 5 and 1.9, 10 and 1.0.

The source parameters for three aftershocks are summarizedin Table 3. Furthermore, we confirmed that the fault mechanisms of three aftershocks are similar to that o f the mainshock as shown in Figure 1 (Nakagawa and Iwata, 1997).

Table 3
Source Parameters for Aftershocks

Source Model and Synthetics
Several researchers have determined the rupture modelof the 1995 Hyogo-ken Nanbu earthquake from the inversion o f strong-ground-motion records, some including other

Aftershock 1
Aftershock 2
Aftershock 3

M0 (N'm)fc (Hz)S (krn2)A~r (bars)

MrMA

7.1 × 1015
1.7 × 10 I5
4,5 × 1015

1.5
3.2
2.1

2.4
0.6
1.2

46
86
82

4.7
4.2
4.5

404

K. Kamae and K. Irikura
KBU (N-S Component)

KM'I" (N40E Component)o.

el

EGF (Aflershock-2)me~Synthetic (Subevent-2)qSynthetic (Subevenl-2)
el

EGF(A~mm~k-2)

Synth~c(Subevent-3)

Figure 3. Separate contributions of subevents 1, 2, and 3 to the synthetics at KBU
and KIVlT.Aftershock data used as EGF for each subevent are depicted above eachsynthetic.

Table 4
Source Parameters for Three Subevents
M0 (N'm)

Subevent 1 3.4 × t0 ~Subevent2 1.0 x 1019
Subevent 3 1.8 × 10~8

L (kin) X W(km)

Aa Oars)

Depth of Top
(km)

8× 8
11 x 16
8× 8

163
86
86

8
0
5

information (e.g., Sekiguchi et al., 1996; Wald, 1996; Yoshida et al., 1996; Ide and Takeo, 1996). On the other hand,
Kakehi et aL (1996) estimated the radiation process of highfrequency seismic waves by the envelope inversion of acceleration seismograms between 2 and 10 Hz. The generalfeatures of those models are very similar to each other. These
inverted source models have three separate regions with relatively large moment release, hereafter referred to as asperities. The first asperity is located near the rupture nucleation
point under the Akashi Strait; the second, in the shallow partof the Nojima Fault in Awaji Island; and the third, under
downtown Kobe city. Comparing those models in detail,however, we can see that there are some differences in thelocation of the third asperity. Hartzell et al. (1996) have alsopresented that the sites of low-frequency asperities are notalways consistent with the regions of significant high-fie-

quency radiation from the waveform and envelope inversions for the 1994 Northridge earthquake. The total slip vector distribution obtained by Sekiguchi et al. (1996) is shownin Figure 2. The horizontal extent of the ruptured area isabout 45 kin, from 15 km southwest of the epicenter to 30
km northeast of the epicenter, and the vertical extent is fromnear-surface down to about 20 kin. One of our objectives inthis study is to determine a source model capable of explaining broadband motions containing low- and high-frequency
components. To accomplish this, first, we assume a simplified source model for the initial model, considering the inverted rupture process based roughly on the Sekiguchi et aL(1996) model, because they carefully located the fault segments from the investigation of particle motion diagrams
observed in the near-source region. Next, we adjust the locations, sizes, and stress parameters of those three asperities
to fit the simulated motions to the observed using a forward
modeling approach. We assumed that the ground motions
are generated only from the three asperities that correspondto subevent 1 under Akashi Strait, subevent 2 along the Nojima Fault, and subevent 3 under Kobe, respectively.
We assumed an S-wave velocity of 3.5 km/sec along thewave propagation path and a rupture velocity of 2.8 kin/see
on the fault plane. The rupture started from the hypocenter
under the Akashi Strait and propagated to both Kobe andSource Model of the 1995 Hyogo-ken Nanbu Earthquake and Simulation of Near-Source Ground Motion

Awaji Island. The rupture of subevent 3 located under Kobecity starts from the southwest bottom after the rupture
reaches that point. We adopted the rise time of 0.6 sec foreach subevent using a trial-and-error approach.
We used the aftershock records bandpass filtered between 0.2 and 20 Hz, depending on the signal-to-noise ratios, as the empirical Green's functions. Near-source groundmotions from the mainshock at three stations, KI3U, MOT,
and AMG, were synthesized using two aftershock records(aftershocks 2 and 3) to represent paths from each subeventto the receivers. The motions from subevents 1 and 3 aresynthesized from the records of aftershock 2 occurring between subevents 1 and 3. Those from subevent 2 are done
from aftershock 3 occurring in the Awaji Island. Groundmotions at stations, SAK, TDO, KMT, and CHI, several tensof kilometers away from the source, are synthesized using
the records of only one aftershock (aftershock 1) occuning
around a middle point of the entire mainshock fault plane.
From our experience (e.g., Irikura and Kamae, 1994), thesimulation event using one aftershock record is feasible fora long distance site because the propagation path effects
from any point on the fault plane to the site are not significantly different compared with those for a near-source site.
Frankel (1995) also successfully simulated the mainshock
ground motions during the Loma Prieta earthquake using
only one aftershock data.
After several trials, we obtained the best source model(Fig. 2). The source parameters for each subevent are summarized in Table 4. Here, the sizes and stress drops for subevents 1 and 3 were determined to match the predominantperiod and velocity amplitude of two large pulses seen at
KBU and MOT. For subevent 2, these parameters were determined from the comparison between the synthesized andthe observed at KMT and CHI. As presented later, the contribution of subevent 2 to the waveforms at KBU and MOT
is negligible. In contrast, the waveforms at KMT, CHI, SAK,
and TDO, far away from the source area, are strongly influenced by the contribution from subevent 2. Subevent 3 plays
an important role of forming near-source ground motions in
Kobe. The location of the asperity corresponding to subevent
3 is slightly different between the inverted source models as
mentioned before. Therefore, we determine the location andstress drop of subevent 3 by fitting the synthesized motions
to the observed ones at KBU and MOT. The location of subevent 3 in our best model is shallower than the part of relatively large slip under Kobe inverted by Sekiguchi et al.
(1996), as shown in Figure 2. Our location of subevent 3
matches well the slip distribution in the Kobe side by Yoshida et aL (1996). The inverted source model recently retried
by Irikura et al. (1996) using additional strong-motion data
showed that large slips corresponding to subevent 3 appeared shallower than those by Sekiguchi et al. (1996). Furthermore, we assumed a rupture delay of 0.5 sec after the
rupture reaches to the asperity of subevent 3 and before it
starts to break, in order to explain the time lag between the
two large pulses seen at KBU.

405

Separate contributions of subevents 1, 2, and 3 to thesynthetics at KBU and KMT are shown in Figure 3. The contributions of subevents 1 and 3 to the waveforms at KBU arevery strong because of the forward rupture directivity effectin addition to the short source distance, making two large
pulses. The separate contributions at MOT are almost similar
to those at KBU. In contrast, the main contributions to theground motion at KMT are from subevents 1 and 2.
Finally, the mainshock motions are obtained by thesummation of respective motions generated from the threesubevents. The synthesized motions at KBU, MOT, and KNIT
are compared with the observed ones in Figure 4. We findthat the velocity and displacement synthetics at KBU and
MOT, which are located close to the causative faults, show
good agreement with the observed motions. The synthetics
at KMT located about 45 km from the causative fault also
show good agreement with the observed in terms of peak
amplitude and duration, although the matching of waveforms is not as good as for KBU and MOT.
Figure 5 shows the comparison between the synthetic
and observed pseudo-velocity response spectra (PVRS) for a
damping factor of 0.05. The synthesized PVRS agree wellwith the observed ones over a broad period range for bothhorizontal components in almost all of the stations we analyzed. The comparison in peak velocity and acceleration issummarized in Table 5. We find that the synthesized values
agree well with the observed values, suggesting that thesimplified asperity model is useful for estimating groundmotions at stations ranging from a few kilometers to about
50 kin.
The synthetic accelerations at soft-soil sites (SAK andTDO) contain higher frequencies than the observed ones.
This might be due to the nonlinear behavior of soft-soil layers that is not considered in our simulation (Aguirre andIrikura, 1995). Since the synthetic acceleration at KBU nearrock site slightly overestimates the observed one, anotherreason might be the difference of fmax between the mainshock and the aftershock. The fm~ of the source spectrumof the mainshock estimated from observed seismograms atrock sites, KBU and CHI, is around 4 Hz, while that of theaftershock is higher (around 10 Hz). However, we cannotemphasize the possibility of the source-dependent fmax because of such a discrepancy not appearing in all sites weanalyzed.
Estimation of Strong Ground Motion in the H e a v i l y
D a m a g e d Area in K o b e
We attempt to simulate ground motions from the mainshock at the FKI, ASY, and KMC stations in Figure 6, using
the EGF method based on the same source model (Fig. 2) asshown in the previous section. FKI and ASY are located ina heavily damaged area, while KMC is near rock outcrop(hereafter, we call KMC simply on rock). We have two mainshock recordings at TKT (Nakamura et al., 1996) and FKA(Osaka Gas Company) located very near the damaged area

406

K. Kamae and K. lrikurar, ~KBU (N-S Component)~§

Synthl)tic

KBU (E-W Component)

MAX= 311.8

Synthetic

MAX. 266.3

ObunmdSyntt~lc

MAX,, SI.8~

MAX= 32.0

Syntl~ttlc

MAX- 3L8

5ynth~tlc

MAX.. 9.7'

l

S

y

n

t

h

e

l

l

~

SEC

$EC

MOT (N-S Component)

Oblmrved

MOT (E-W Component)

MAX= 384.1

SynthMIc

1.

oObllerVld

MAX- 720.3

MAX~ 839,9

8

~

Q

't

E~

ObN~edaSynthetic

MAXs 43.3

Oblerved

MAX- I5.1

Synthetic

MAX= 11.6

a" 2" if,' ~1"o f2 1'4 1'6 ~'a to ~ ~4-~ ~ ~o
5EC

S

$]
S~ J

y

n

t

~

ob4.,.,~

MAX. 2 u

Sy~U~etic

MAX- 23.S

SEC

F i g u r e 4.
Comparison between the synthetics and observed motions at KBU, MOT,
and KMT (continued on next page).

407

Source Model of the 1995 Hyogo-ken Nanbu Earthquake and Simulation o f Near-Source Ground Motion

8~

KMT (H40E Component)

Obmrv~

Ot~mrved

Synthetic

MAX= 185.8

Synthwttc

Syntheticd2" 4

KMT (N130E Component)~

MAX: 2.1

I~II1(1 12Syn~Observed

MAX= 13.3

Synthetic

MAX: 7.1

Observed

MAX: 1.7

Synthetic

MAX: 0.7

111 1"8 :~0 22 24
SEC

Iii

SEC

Figure 4.

(Fig. 2), although the aftershock records there were not obtained. Then we could not examine the synthesized motions
with the observed ones at these two stations. Nevertheless,these data are useful in validation of our simulation in theheavily damaged area. We used only the records of aftershock 2 as the empirical Green's functions, primarily because of restrictions in the aftershock data set. We consider
that the ground motions reproduced using only this aftershock are reliable because the main contributions to sites in
Kobe are due to subevents 1 and 3, as shown in Figure 3.
Iwata et al. (1995, 1996) showed that peak horizontal accelerations of this aftershock in the damaged area (FKI and
ASY) were several times larger than those at KMC on rock,
implying that the observed records reflect the site characteristics.
The synthesized ground motions at KMC on rock and at
FKI and ASY in the heavily damaged area are shown in Figure 7. The strong ground motions synthesized here as wellas those observed at KBU and MOT in the near-source area
are composed of two large pulses each with a duration of 1
to 3 sec. The large pulses dominate in the horizontal component, normal to the fault plane. Similar near-source
ground-motion characteristics were observed during the1994 Northridge earthquake and the 1989 Loma Prieta earthquake, even though these earthquakes had different faulting
mechanisms. These pulses are indicative of the forward rup-

Continued.

ture directivity focusing on near-source ground motion(Heaton et al., 1995; Somerville et aL, 1995). Heaton et aL(1995) has warned that these ground motions would cause
large deformation and possible collapse of steel-framedbuildings and the base-isolated buildings.
The synthetic velocity motions at KMC are very similar
to the observed ones at rock site KBU. The peak horizontal
acceleration and velocity of the synthetic motions at KMC
are about 300 cm/sec 2 and 57 cm/sec, respectively, almost
the same level as those observed at KBU. In contrast, peakhorizontal accelerations of the synthetic motions at FKI and
ASY in the heavily damaged area are about 900 and 1000
cm/sec 2, respectively, a little larger than the observed onesat FKA (810 cm/sec a) and at TKT (666 cm/sec2). The peakhorizontal velocities of the synthetic motions at FKI and ASY
are about 130 and 115 cm/sec, respectively, comparable tothose observed at FKA (121 cm/sec) and at TKT (138cm/sec).
Kawase et al. (1995) has reproduced ground motions
for the same sites in the heavily damaged area using equivalent linear or effective stress analysis with soil models thatwere derived from aftershock records. They used the mainshock record at KBU as an input motion to the base rock andconcluded that the ground motions in the heavily damaged
area must have reached at least the level of 600 cm/sec 2 inacceleration mad 130 cm/sec in velocity. Their results seem

408

K. Kamae and K. Irikura( CMISEC

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h=0.m I

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PERIOD ( S E C )10( CM/SEC

lO

0.1

i

lO *

lO

10 3

10 2

lO 2

lO

10 2

lO

10

1.0

1.o

0.1

1,.

0.1

10 4

1.0
PERIOD (SEC)

o.1

10

( CM/SEC

l

0.1

1( 4

" CHI-NS

h=0.05I

1.0
PERIOD (SEC)

1( 3

10 2

1(

10

h--O.~

1.o

i

0.1

"
0.1

i

i

i IIiii

!

i

I

1.0
PERIOD (SEC).1i I iiii

10i

i I ilia

I

I

I IIIII

1.0
PERIOD ( S E e )I0

KMT-EWi0.1

il I ll,lh=0.05ii

1.0
PERIOD (SEC)i Iiiiii

10( CWSEC )h=0.05

CHI-EW

10 3

I lllll10( CM/SEC
th---O.05

10 3

10i I Iiii

g

0.1

10 4

:'- K M T - N S

h=0.05- TDO-EW

I

1.0
PERIOD (SEC)

11

o.1

i I iiii( CM/SEC

10 4- TDO-NSiti I iii+.:- S A K - E W

1(I IIIII

i

=-_

1(

I

h--0.0~( CM~EC )- SAK-NS

10 2

=-_

i

o.1

( CM/SEC

h=O.05
I( a

I

3

1.o

1( 4

~ AMG-EW

10 a

0.1

( CM/SEC- MOT-EW

1c

1.0

0.1

14

h---O.O

2

1(
%0

1.(]

0.1

+

0.1

J

J l+J.

,o.1

J ~ IJHJ

1.0
PERIOD (SEC)

10

I

0.1

i

iiiiiiI

i

1,0
PERIOD (SEC)i IIiii

10

Figure 5. Comparison of the pseudo-velocity response spectra (PVRS)with damping
factor of 0.05 of the synthesized motions and those of the observed motions. Solid thinline shows the PVRS of the synthesized motions. Solid thick line shows the PVRS ofthe observed motions.

to be consistent with our results in peak velocity amplitudes,
although they estimate smaller peak acceleration because of
the nonlinear effects.
Figure 8 shows the pseudo-velocity response spectra of
the synthetic ground motions in the heavily damaged area,plotted with a number of observed spectra. The spectra withlarger amplitude in two horizontal components are shown inthis figure. The response spectra at periods of 1 to 3 sec at
FKI, ASY (the reproduced) and TKT, FKA (the observed) aresignificantly larger than those at KOB (JMA station) in theless-damaged area as well as at the Tarzana station without
any damages around there, in spite of its very large acceleration amplitude (about 1.8 g) during the 1994 Northridge
earthquake.

409

Source Model of the 1995 Hyogo-ken Nanbu Earthquake and Simulation of Near-Source Ground Motion
135"1Z

135"18'34" 42'34" 42'135" 12'Figure 6. Location of array observation stations (KMC, FKI, and ASY) in Higashinada
Ward, Kobe city. Gray zones show the heavily
damaged area (JMA Intensity 7) in Kobe city
(Koketsu, 1997). Solid triangles (KMC, FKI,
ASY) indicate temporal stations for aftershockobservation.

135" 18'

Table 5Comparison of Peak Amplitudes of the Synthesized Motions
and Those of the Observed Ones
Observed

Synthetic

Synthetic/Observed

Acceleration

Velocity

Acceleration

Velocity

Acceleration

Velocity(crn/sec2)(cm/sec)(cm/sed)(cm/sec)(cm/sec2)(crrdsec)

Station

N-S

E-W

N-S

E-W

N-S

E-W

N-S

E--W

N~S

E-W

N-S

E-W

KBU
MOT*
AMG*
SAK
TDO
KNIT
CHI

270.3
--149.7
287.7
219.2
88.7

303.0
--121.1
186.4
164.8
109.2

52.7
48.1
40.9
14.2
24.2
13.9
5.0

32.0
75.0
53.5
13.0
15.0
13.3
4.2

311.8
839.9
202.3
364.6
413.6
185.8
75.2

266.3
647.6
260.4
302.4
279.1
134.9
93.0

51.8
43,3
28.1
16.5
17.4
14.1
3.3

31.8
68.0
49.3
10.8
19.7
7.1
3.6

1.15
--2.44
1.44
0.85
0.85

0.88
--2.50
1.50
0.82
0.85

0.98
0.90
0.69
1.16
0.72
1.01
0.66

0.99
0.91
0.92
0.83
1.31
0.53
0.86

*Peak velocity amplitudes of the restored seismograms by Kagawa et al. (1996).

Near-source ground motions in Kobe were characterized by two large pulses due to two asperities lying under
Akashi and Kobe and the forward rupture directivity. The
difference in peak amplitude between the damaged sites suchas FKI and ASY and less-damaged sites KOB and MOT issignificantly due to the relative location from the basin edge.
Pitarka et al. (1996, 1997) showed that the ground motion
from an aftershock are largely amplified at FKI and ASY in
the damaged area offset by about 1 km from the basin edge
by the superposition of the direct S waves and the basinedge-diffracted waves. That is, in the heavily damaged area,
the large pulses due to the forward rupture directivity effects
could be furthermore amplified due to the basin-edge effects
and the local site effects. The large response at one to three
periods may be related to the damaging effects of groundmotions on buildings and bridges (Minami and Sakai, 1996).

aged sites in Kobe city where the mainshock was not recorded, using aftershock records. First, we confirmed thatthe source model consisting of three asperities on the fault
plane is valid to explain the main features of the observednear-source ground motions by comparing synthetic and observed seismograms. We found that the synthetic nearsource ground motions in Kobe were characterized by twolarge pulses caused by the forward rupture directivity of two
asperities lying under Akashi and Kobe. The peak horizontal
acceleration and velocity of the simulated ground motions
at the heavily damaged sites are about 1,000 crn/sec2 and130 cm/sec, respectively, while those at a rock site in thenear-source area (less than 1 km from the fault) are about
300 c m ] s e c 2 and 60 cm/sec, almost the same level recorded
at KBU. We conclude that the large response amplitudes
around 1 to 3 see caused the heavy damage to buildings andbridges.

Conclusions
To investigate why Kobe and adjacent cities were soseverely devastated by the 1995 Hyogo-ken Nanbu earthquake, we estimated strong ground motions at heavily dam-Acknowledgments
We thank the Committee of Earthquake Observation and Research in the
Kansai Area for providing us with the velocity records, and Mr. Takao

K. Kamae and K. Iriktzra

410

.~}

KMC (N-S Component)~iE

MAX= 19,1~]

KMC (E-W Component):tMAX- ~os.4

MAX= 5.O

SEC

FKI (N'S Component)§

ASY (N-S Component)

MAX- 95.0

FK! (E-W Component)~§

uAx- 4~.2

5EC

Figure 7.

MAX= 107.0

ASY (E-W Component)~=

MAX- 3s.s

SEC

Synthesized ground motions (acceleration, velocity, and displacement) at
KMC near rock, FIG, and ASY in the heavily damaged area.

Source Model o f the 1995 Hyogo-ken Nanbu Earthquake and Simulation o f Near-Source Ground Motion

10 3' h=O: 05'
#"

ILl
O3

_. _/L"

10 2

",lb

%,"

;TrtL-, '';

=E
V

>
O_

f
101

.........
.......
.

.

.

KOB
FKI

.

. . . .

TAR
I

i

I

T i

i

lo0

0-1

PERIOD(SEC)

F i g u r e 8.
Comparison of the PVRS with damping
factor of 0.05 of the synthesized motions in horizontal
E - W component at FKI and ASY in the heavily damaged area as well as the observed at TKT and FKA in
the near-heavily damaged area, and those of the observed motions at the less damaged site KOB as well
as at Tarzana station during the 1994 Northridge
earthquake.

Kagawa for the restored velocity record at MOT and AMG stations. We
would like to thank the Railway Technical Research Institute and Osaka
Gas Company for providing us with the respective mainshock motion at
stations TKT (Serial No. R-032) and FKA, and K. Koketsu for providing uswith the digitalized data for JMA Intensity 7 area. We are grateful to T.
Iwata and H. Sekiguchi for their valuable discussions. We greatly appreciate the useful review by two anonymous reviewers. This study was partially supported by a Grant-In-Aide for Scientific Research, Number
08248111, from the Ministry of Education, Science, Sports, and Culture.

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Research Reactor InstituteKyoto University
Noda, Kumatori-cho
Sennarl-gun, Osaka, 590-04, Japan
Tel: + 81-724-51-2369; Fax: + 81-724-51-2603;
E-mail: kamae @kuca.rri.kyoto-u.ac.jp(K.K.)

D.P.R.I.
Kyoto University
Gokasho, Uji
Kyoto, 611, Japan
Tel/Fax: +81-774-33-5866;
E-mail: irikura@ egmdpri01.dpri.kyoto-u.ac.jp(K.I.)

Manuscript received 28 May 1997.