Bulletin of the Seismological Society of America, Vol. 88, No. 2, pp. 428-440, April 1998

Three-Dimensional Simulation of the Near-Fault Ground Motion for
the 1995 Hyogo-ken Nanbu (Kobe), Japan, Earthquake
by Arben Pitarka, Kojiro Irikura, Tornotaka Iwata, and H a r u k o Sekiguchi

Abstract

The 17 January 1995 Hyogo-ken Nanb u earthquake is a typical example
showing that the ground motions along basin-edge faults can be very destructive. In
this study, we simulate the near-fault ground motion from this earthquake based ona kinematic fault model and a simplified 3D velocity structure of the Kobe area. The
kinematic earthquake rupture and the wave propagation are modeled using a 3D
finite-difference method (FDM). Our simulation identifies the basin-edge effect as animportant factor that influenced the ground-motion amplification pattern in the Kobearea. We found that the coupling of the source directivity and basin-edge effects
causes impulsive ground motions with extremely high amplitude at periods greaterthan 1 sec and in a narrow zone offset less than 1 km from the basin edge. The
combination of these effects acted to create a fairly continuous band of amplificationthat extends about 30 km in an elongated zone parallel to the basin-edge boundary.
In some areas, localized site effects might have been as important as the abovementioned effects, but they cannot explain the continuity of the extended east-westzone of damage.

Introduction
Nagano, 1996) and aftershocks (Pitarka et al., 1996a, 1997),
based on different techniques, suggests that the cause of theground-motion amplification in the damage zone is the basin-edge effect. In these studies, it is argued that the widthof the damage zone is controlled by the interference between
the basin-edge diffracted waves and the primary S wave. Our
purpose is to analyze the extension of this phenomena along
the northern part of the basin and demonstrate the implication of the interaction between the basin-edge effect and thesource effects in the observed near-fault ground motions byusing 3D numerical simulation.
In general, near-source ground motions are controlled
primarily by the source radiation and the directivity effects.
However, for cases in which the earthquake occurs on abasin margin fault that forms the basin edge, as the Hyogoken Nanbu earthquake, the basin-edge effect becomes very
important. The Hyogo-ken Nanbu earthquake is a good example where the interaction of the source directivity andbasin-edge effect causes impulsive ground motions with particularly large-amplitude ground velocities. Since other large
cities are constructed on basin margins that are fault controlled, like Kobe, it is important to understand how the nearfault ground motions from Hyogo-ken Nanbu earthquake
were amplified by the basin-edge effect.
Since the pioneering work of Bard and Bouchon(1980a,b) the basin-edge effect has been demonstrated inmany 2D and 3D numerical simulations (e.g., Hisada and

The 17 January 1995 Hyogo-ken Nanbu earthquake wasone of the most destructive earthquakes in Japan eventhough of a moderate magnitude (M = 6.9). The earthquake
caused extreme damage in Kobe city and its surroundingarea, close to the northern edge of the Osaka basin (Architectural Institute of Japan, 1995). The area of the highest
seismic intensity (VII-JMA scale) lies mainly in a densely
populated part of Kobe (Fig. 1). The earthquake occurred ona system of predominantly strike-slip faults that form thenorthern boundary of the Osaka basin (Sekiguchi et al.,1996b).
Near-source strong ground motions from the earthquake, recorded at different sites in the city, and analysis of
the damage to the structures show that the damage can be
attributed to extremely large ground velocities (Told et al.,
1995; Irikura, 1995; Nakashima et al., 1995; Suzuki et al.,1995). The initial hypothesis of the cause of the damage belt
was based on the assumption of the existence of an unknown
buried fault beneath the damage zone. This hypothesis wasabandoned since no surface breaks have been found in the
Kobe city area. Furthermore, the fault rupture inversions(Wald, 1995, 1996; Ide et al., 1996; Yoshida et al., 1996;
Sekiguchi et al., 1996b; Horikawa et al., 1996) show thatthe slip beneath Kobe city is not concentrated in the shallow
part of the fault.
Two-dimensional numerical modeling of the groundmotion from the mainshock (Kawase, 1996; Motosaka and428

Three-Dimensional Simulation of the Near-Fault Ground Motion for the 1995 Hyogo-ken Nanbu (Kobe), Japan, Earthquake
135"00'135"15'

429

135"30'~ 7 Mainshock
•
O
.....

StationArea of Intensity VII (JMA-scale)'~'

Basin Edge'"

, Fault."'"
34' 45'34'45'

TKT

0

10 km

34' 30'34-30

Awaji Is.
135"00'135"15'135"30'Figure 1. Map of Kobe area and station locations. The rectangle shows the locationof the basin model used in this study. The dashed line P-P' depicts the profile used tO
display the seismic record sections. Also shown are the locations of the heavily damaged zone (intensity VII, JMA scale, digitized data from Koketsu, 1997) and fault
segments.

Yamamoto, 1996, and references therein). Ground-motion
modeling based on 2D techniques has been useful at identifying basin-edge effects (Zahradnik and Hron, 1987; Vidale and Helmberger, 1988; Moczo and Bard, 1993; Scrivher and Helmberger, 1994; Graves, 1995; Hatayama et al.,
1995; Pitarka and Irikura, 1996; Kawase, 1996; Pitarka et
aL, 1996a,b; Haase et al., 1996). Nevertheless, an accuratequantitative evaluation of the phenomena needs a full 3D
approach (e.g., Sanchez-Sesma and Luzon, 1995).
The finite-difference methods have been particularly effective in modeling ground motions by fully integrating the
3D velocity structure with the earthquake rupture on a finite
fault (Frankel, 1993; Olsen et al., 1995; Olsen and Archuleta, 1996; Graves, 1996b; Wald and Graves, 1998). In thisstudy, we apply a similar technique to simulate strong
ground motions in the Kobe area from a kinematic model ofthe fault rupture that generated the Hyogo-ken Nanbu earthquake.
Our approach is to simulate the near-fault ground-motion amplification pattern at long periods ( > 1.25 sec). This
period limit was chosen based on computational demands
and the source model that is valid only for periods longer
than 1 sec. We analyze the effects of the basin structure andthe source process by comparing the simulated ground motions with the observed damage distribution and the recordednear-source seismograms. The kinematic earthquake rupture

and the wave propagation are modeled using a 3D staggeredgrid finite-difference method (FDM, hereafter).
Geological Structure and Velocity Modelof the Region
The area in and around Kobe city is characterized bysignificant geologic complexity including numerous active
faults. In this study, we simulate strong ground motions ina selected region located along the northern edge of the
Osaka Basin, which includes the Kobe city area and the fault
segments (Fig. 1).
Recent investigations of the subsurface geological structure in the Kobe area, including reflection and refraction
profiles, borehole information (Geological Survey of Japan,
1996; Iwata etal., 1996b) and Bouger anomaly analysis (Kobayashi et al., 1996), have shown that on a large scale thebasin edge beneath Kobe city has uniform horstlike underground structure. The basement rocks consist mainly of
granite and partly volcanic and sedimentary rocks. They arewidely exposed in the surrounding mountainous regions to
the north of the city. The shear-wave velocity in the rock is
about 3.2 km/sec (Kagawa et al., 1993).
The sedimentary cover overlying the basement rockconsists of the Osaka Group, upper Pleistocene, and Holocene alluvial deposits (Nakagawa et al., 1996). In the Kobe

430

A. Pitarka, K. Irikura, T. Iwata, and H. Sekiguchi

Figure 2. Bedrock topography in the Kobe
area used in the 3D modeling.
region, these deposits are represented by three distinctive
layers dipping slightly to the south. In the study area, themaximum thickness of the sediments is about 2.1 km (Kagawa et al., 1993). The Holocene alluvial deposits that occupy a large part of the city area have variable thickness andare composed of very soft materials with a high water content and a shear-wave velocity of 0.1 to 0.25 km/sec in theupper few meters (Iwata et al., 1996a).
The Osaka Group and the upper Pleistocene layers lie
below the lowland area. Their respective average shear-wave
velocities are estimated to be 0.55 and 1.0 kin/see (Kagawa
et al., 1993). Distinctive landform features of terraces andhills, where the Osaka Group and the upper Pleistocene areexposed sporadically, have been developed between the lowland and the mountainous region. The seismic intensity inthese regions has been ranked lower than that in the alluvial
deposits areas. The soil along the coast is mainly reclaimed
land that is susceptible to liquefaction or similar phenomena
due to nonlinear response (e.g., Aguirre and Irikura, 1997).
Based on the foregoing information, we constructed arough three-dimensional velocity model with a spatial resolution that satisfies the band-limited frequency content (0.1
to 0.8 Hz) of our finite-difference simulation. The topography above the sea level was not included. The model has
two sedimentary layers with the interface dipping toward thesouth. The representation of the shallow part of the basin bya single layer is due to the computational limitations. Whileit affects the amplitude of the ground motion (e.g., Bard andGariel, 1986), this simplification may have a minor impact
on the overall amplification pattern caused by the basin-edge
effect in the considered low-frequency range. The velocity
structure of the bedrock is modeled by three horizontal layers. The subsurface bedrock topography is depicted in Figure
2, and the parameters of the sediment layers and rock layers
are shown in Table 1 and Table 2, respectively.

determined by these studies is the large slip in the Awaji
Island side and the relatively small and deep slip beneath
Kobe city. The forward modeling of selected near-source
ground motion suggests a shallower slip beneath Kobe city
(Pitarka et al., 1995; Kamae and Irikura, 1996; Kamae etal., 1998); however, the slip inversion studies to date haveused few data from the near-fault ground-motion stations.
One reason is that most of the near-fault stations are locatedinside the basin. It is expected that they have recorded
ground motions that are strongly affected by the local heterogeneous structure at low frequencies (0.1 to 1 Hz) (e.g.,
Pitarka et al., 1996a; Kawase, 1996). In addition, most of
the near-fault ground-motion data have only recently become available.
The slip model used in this study is obtained by inverting the bandpass filtered strong-ground-motion velocity
data, assuming four fault segments (segments A, B, C mad
D hereafter, see Fig. 1) (Irikura et al. 1996). The fault segmentation is similar to the one used by Yoshida et al. (1996)
in their source process inversion study. In our inversion procedure, the faults are divided into subfaults with dimensionsof 2.05 × 2.05 km 2. Each subfault motion is obtained by
the response of a point dislocation source placed at the centerof the subfault. The subfanlt slip-time history is modeledusing eight time windows having a duration of 0.8 sec, each
0.5 sec apart. The parameters of each segment are summa-

Table 1
Parameters of Basin Layers
Layer

Vp
(kna/sec)

Vs(km/see)Density(g/cm3)

Q

First
Second

1.4
2.1

0.6
1.1

1.8
2.1

80
150

Table 2
Bedrock Velocity Model

Kinematic Fault Model and Modeling Parameters
The slip history of the 17 January 1995 Hyogo-ken
Nanbu earthquake has been studied by several authors, using
inversion techniques (Wald, 1995, 1996; Ide et al., 1996;
Yoshida et al., 1996; Sekiguchi et al., 1996b; Horikawa etal., 1996). One important common feature of the slip models

Top Depth
(kin)

Vp

V~

(kin/see)(kin/see)Density(g/cm3)

Q

Variable
4.
17.8

5.5
6.0
6.7

3.2
3.46
3.87

2.6
2.7
2.8

300
400
500

Three-Dimensional Simulation of the Near-Fault Ground Motion for the 1995 Hyogo-ken Nanbu (Kobe), Japan, Earthquake

431

rate function used in the original inversion. In the FD calculations, the bell-shaped function is preferable because ofa low content of high frequencies. The differences in thesimulated waveforms between the bell-shaped function andthe slip-rate function used in the inversion are small in thefrequency range discussed here (e.g., Cotton and Campillo,1994).
The velocity model is discretized with a grid spacing of
0.15 km, and the frequency resolution limit is 0.8 sec. We
apply absorbing boundary conditions to reduce the artificial
reflection at the boundaries (Stacey, 1988). The 3D-FDM
employed in this study solves the stress-velocity equations
for elastic wave propagation in a heterogeneous medium.
The staggered-grid scheme we use is an extension of thetwo-dimensional velocity-stress formulation proposed by
Levander (1988). Details of the 3D formulations are given
by Graves (1996a). The simulation was carried out on Sun
Ultra Sparc workstation using approximately 375 Mbytes ofcomputer memory and requiring 43 hours to compute synthetics for 2000 time steps.

rized in Table 3. The fault is 42.5 km long and 20.5 km
down dip. The rupture initiates at a depth of 16.4 km andterminates at a depth of 1.3 km. The slip distribution on thefault is shown in Figure 3. This model shares similar featureswith the rupture models determined by other investigatorsfrom strong-motion data and multiple-fault segment models(e.g., Sekiguchi et al., 1996a,b; Wald, 1995, 1996; Ide et al.,
1996; Yoshida et al., 1996). The three asperities that havedominated the source process are located beneath, respectively, Akashi Strait, Awaji Island, and Kobe City (see Fig.
3). In this article, we refer to them as the first, second, andthird asperities.
For the forward simulation of the ground motion, we
retain the complexity of rupture in the slip model obtainedby the inversion. We use the same fault segmentation, subfault dimensions, slip-time history, and rupture velocity. We
have slightly modified the original slip model by reducing
the length of segment A (Awaji side) by 4 km on the westernedge and segment D by 2 km on the eastern edge where theslip is negligible. These parts of the fault were excluded fromthe simulation in order to reduce the required computer
memory.
The kinematic earthquake rupture with a variable slip
velocity is simulated by activating 250 subfaults each havingnine point sources mapped onto the predetermined finitedifference (FD) grid. The point source implementation in the
FD calculation is done following a procedure similar to that
proposed by Graves (1996a). Each point source within a
subfault has the same slip-time history that is lagged appropriately in time to simulate the propagation of the rupture
front across each subfault. We use a bell-shaped slip-ratefunction. Its shape is slightly different from that of the slip-

Simulation Results
We calculated velocity seismograms using the velocity
structure and kinematic fault model described in the previous
section. All the synthetics are bandpass filtered in the frequency range 0.1 to 0.8 Hz to exclude the numerical errors
in higher frequencies. We discuss the simulation results inthree steps.
In the first step, we analyze the main features of theamplification pattern obtained from the simulated velocity
seismograms. Snapshots of the wave field are used to inves-

Table 3
Fault Segment Parameters
Segment A

Segment B

Segment C

Segment D

Strike
(deg)

Dip
(deg)

Length(km)

Slrike
(deg)

Dip
(deg)

Length(km)

Strike
(deg)

Dip
(deg)

Length(kin)

Strike
(deg)

Dip
(deg)

Length
(km)

N45°E

75

16.4

N50°E

90

14.5

N233°E88

10.2

N219°E88

12.3

Noj irna

Suma

A

B

Suwayama G o s u k e b a s hCD~---;--.2_?-._f-..
~ ~r',. -~~, ~~-'~-.~--~i ', ~---. . . . ". . :--~--~-',;---.---.--~--~-r:', ~--~- ~--.- . . .". 2
L

i~

k/

e..~..~., c...

,

.
i

0

.

.

r

.
i

],- ,, ,- ~-e-e.-e-- ~- e- e- ,4~4--¢'-- *-¢'-- ~.._ ,r,. ~, ,, ,. ,............................

i. . . . . . . . . . . . . . . . . . . .

t_ . . . . . . . . . . . . .

±.................

1 . 0mFigure 3. Cross section of the dislocation
model used in this study, viewed from thesoutheast, showing the total slip vector distribution on the four fault segments.

432

A. Pitarka, K. Irikura, T. Iwata, and H. Sekiguchi

tigate the presence of the basin-edge effect. In the second
step, we investigate the robustness of the basin-edge effecton ground-motion amplification. This is done by comparing
seismic record sections resulting from the three-dimensional
basin structure used in this study, and a reference flat-layer
structure. Finally, we compare the synthetic and observed
ground motions at several near-fault stations. We show thatthe coupling of the source effects with the basin-edge effects
lead to the large-amplitude pulses observed at sites close to
the basin edge.
P e a k Amplitude Distribution
The peak velocity distribution of the simulated horizontal, fault-normal (N143°E), fault-parallel (N53°E), and vertical components of the ground motion are shown in Figure
4. The horizontal component at each time step is the squareroot of the sum the squares of the horizontal components.
Although the simulation indicates large-amplitude groundmotions in the Kobe area and in the northern part of the
Awaji Island, we concentrate our analysis on ground motions in the Kobe area.
The most important feature that characterizes the simulated peak amplitude distribution is the concentration of thehorizontal peak velocities higher than 40 cm/sec within anarrow elongated zone that is located south of the causative
fault and offset from the basin edge by about 1 kin. This
zone of maximum long-period ground-motion amplification
correlates with the damage zone (see Fig. 1) where, as suggested by the recorded data, the ground-motion amplitude
was large even at low frequencies (>0.4 Hz). The simulated
maximum amplification factor from inside to outside thezone is considerable. In general, it reaches the maximum
value of about 2.8 in the central part and decays gradually
toward the zone boundary. As a consequence of the directivity effects of the strike-slip faulting, the simulated peak
amplitude distribution is controlled by the fault-normal motion that dominates over the fault-parallel motion.
To understand the mechanism that causes the concentration of the amplification within a limited area, we examine
snapshots of the synthetic wave field on the free surface,taken at times between 1 and 12 sec after the origin time.
The snapshots are depicted in Figure 5. They show the faultnormal component of the velocity. The rupture propagatesradially from the starting point to both segments. The starting point is located at the western bottom edge of segment
B, coinciding with the hypocenter. At 2 sec into the simulation, the rupture is propagating on both segments A and
B. At this time, the first and the second asperities are activated. At 3 sec, the S waves generated by the activation of
the first asperity appears on the surface. The wave front of
S waves radiated from the first asperity contains two distinctive portions. The northern portion propagates in rockwith an apparent velocity that is close to the rupture velocity.
Therefore, this part of the wave front remains almost perpendicular to the fault during the rupture propagation, form-

ing a radial curve away from the fault plane. The southern
portion, on the other side of the fault, propagates inside thebasin.
At 4 sec, the wave front has proceeded into the basin,
making an oblique line, as a part of a conical wave. Compared to its northern portion, the southern portion has larger
amplitude and delays during the propagation because of the
low apparent velocity of S waves in the basin sediments. At
5 sec, it divides into two parts. As demonstrated by 2D simulations using velocity models of cross sections normal tothe basin boundary (Kawase, 1996; Pitarka et aI., 1997), thefastest wave front represents the direct S wave, and the second one, the basin-diffracted wave that propagates with asmaller apparent velocity.
The waves diffracted from the basin edge make linear
wave fronts and proceed into the basin in a direction almost
normal to the basin edge. The wave fronts of the direct S
waves and those of the basin-edge-diffracted waves coincidealong a narrow zone that is offset from the basin edge. In
this zone where the two waves interfere, the amplitude of
the ground motion is relatively high. The width of the maximum amplification zone and its distance from the basinedge depends on the frequency contents of the incident
waves, basin-edge geometry, and the velocity contrast between the sediments and the rock (Kawase, 1996). Theoretically, the width of the amplification region and its distancefrom the basin edge increases when the frequency contentsof the incident wave decreases.
At 5 sec, a front of waves generated by the third asperity
begins to emerge. Again, in later snapshots, the amplification
due to the interference of different waves is remarkable in anarrow region close to the basin edge. The basin-diffracted
waves are followed by basin reverberations that transform
into surface waves as they propagate further into the basin.
After 9 sec, the amplitude of the wave fronts becomes
smaller due to the geometrical spreading and attenuation in
the sediments. The direct wave generated from the first asperity is followed by multiply reflected waves from the basin
structure. Even after 9 sec, the interference of different
waves remains effective at amplifying the ground motion ina narrow zone that extends to the east of the Kobe area.
Being consistent with damage distribution, this result helps
to understand the reasons why the belt of damage extends
in the eastern part of the city, away from the fault.
The cause for the eastward extension of the damage has
been a subject of several investigations (e.g., Irikura et al.,1996). To explain the large damage around this area, Shimamoto (1995) proposed the existence of a small fault segment along the coast line. Although the aftershock distribution does not show evidence of such a fault, Yoshida etal. (1996) and Irikura et al. (1996) checked the possibilityof its existence by using a five-segment fault model to analyze the rupture process obtained from the inversion ofstrong-motion waveforms and geodetic data. In both studies,
only small and deep slips were obtained on the fifth fault
segment. This result implies that even if the proposed fault

Three-Dimensional Simulation of the Near-Fault Ground Motion for the 1995 Hyogo-ken Nanbu (Kobe), Japan, Earthquake
3D-FD Maximum

V e l o c i t y (0.1-0.8 H z )Horizontal

g
v

10
5
0
0

5

10

15

20

25 30 35
N58° E (km)40

25 30 35
N58a E (kin)

40

25

40

45

50

55

Fault Normal-g 10 ¸
5
0

10

0

g
v

15

20

45

50

55

Fault Parallel

lO
5
0
10

0

15

20

30

35

45

50

55

N58° E (km)Vertical

g
v

10
5

0

5

t0

15

20

25 30 35
N58o E (kin)40

45

Loglo Velocity (cm/sec)
,..,
;~g~
0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

50

55

1,8

farther to the east and away from the fourth segment of thefault.
Another way to explain the extension of the damage tothe east is to analyze the contribution from each fault segment to the amplification pattern predicted by our modeling.
We calculated the ground motion from the four segments
separately. The result in terms of horizontal peak amplitude
is shown in Figure 6. In the Kobe area, the contribution to
the ground motion from segment A is not significant. The
amplification pattern indicates that the first asperity (segmentB) gives the largest contribution to the seismic energy in the
Kobe area. The ground-motion amplification caused mostly
by the basin-edge and the source directivity effects is visible
along the edge of the basin. Notably, it extends for about 30
km to the east of the fault. The amplification pattern seen in
Figure 6, second panel, shows that the effect of the basinedge diminishes to the east of the fault segment where theincidence of the direct waves becomes oblique.
As opposed to segment B, the contribution from segment C is concentrated in a limited area of the basin, close
to the fault. This is probably due to the fact that the asperityin this segment is small and relatively deep. Visibly, theground motion generated from segment D is largely amplified by the basin edge in the eastern part of the basin. Together with the ground motion generated from segment B,
which is affected by the basin edge, they dominate the amplification pattern in eastern Kobe. These analyses indicatethat the extension of the belt of damage in the eastern partof Kobe city is caused by the basin-edge and source directivity effects.
Coupling of Basin-Edge Effect with
the Source Effect~. ....
t.6

433

2.0

Figure 4. Simulated peak velocity distribution on
the surface for different components. The horizontal
motion is the mean square root of the fault-normal
and fault-parallel components. Note that the particle
velocities higher than 40 cm/sec are concentrated
along a narrow zone close to the basin edge. The
straight lines in each panel indicate the fault segment
locations. The diameter of the circles centered at thestation locations is proportional to the observed horizontal peak velocity.

segment ruptured during the earthquake, it may not havebeen the main cause of the damage in the eastern part of
Kobe city. In the snapshots of our simulation, it is interesting
to note that at the easternmost part of the considered area,
the two distinct wave fronts generated from the two asperities in segments B and C, respectively, propagate eastward
while the third one, generated from the deep slip in segment
D, and diffracted at the basin edge, propagates southward(T = 8 sec and T = 9 sec). The interference of these three
wave fronts causes the extension of the amplification zone

After finding evidence of the basin-edge effect, it is instructive to further discuss its coupling with the source effectand implication in the simulated ground-motion amplification pattern in Kobe area.
In Figure 7, we compare seismic record sections resulting from two simulations using, respectively, a reference
flat-layer model (see Table 1) and one including the actual
three-dimensional basin structure. The record section is
located along line P-P' shown in Figure 1. We refer to themas the reference-record section and the basin-record section.
The cross section of the three-dimensional structure along
the profile is also shown. The profile is 13.6 kin, trending
approximately north-south, and the receivers are spaced by
300 m. The flat-layer basin model corresponds to the onedimensional velocity profile taken from the three-dimensional FD velocity model beneath a site located on line PP', 1 km away from the basin edge.
The waveforms in the reference record section are directly related to the source process and are only affected by
ID resonance. The rupture directivity effect in the first andthird asperities (segments B and C) creates two pulses having
the largest amplitude in the fault-normal component. The

434

A, Pitarka, K. Irikura, T. Iwata, and H. Sekigucbi

Fault Normal Component
r=l.O (see)

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55

T=]~O #ec)

10

lo

'

50

45

50

55

T=12.0 (see)

"

0

55

N58 o E (kin)

0

5

10

15

20

25

30

35

40

45

50

55

N58 ° E (km)Figure 5. Snapshots depicting the absolute value of the particle velocity fault-normal component (N143°E), from 1 to 12 sec after the origin time of the rupture.

ground motion is fairly simple and is symmetrical on thetwo sides of the fault, which is expected for the predominantly strike-slip fault mechanism. The amplitude of the second pulse decays rapidly at receivers toward the south as thesource directivity effect decreases.
In contrast, the waveforms in the basin record section
are much more complex. Based on the analysis in the previous section (see Fig. 6), the amplification due to the basinedge effect is expected to be stronger for the second pulse(segment C) than the first pulse (segment B) at receivers
close to the basin edge. The comparison between the basinrecord section and the reference-record section shows thatat these receivers the amplification due to the basin-edge
effect is a factor of 1.6 for the first pulse in the fault-parallel
component and about 2.8 for the second pulse in both horizontal components. As shown in the next section, such differences in the peak amplitude between the two pulses canbe seen in the observed ground motion.

Three-Dimensional Simulation of the Near-Fault Ground Motion for the 1995 Hyogo-ken Nanbu (Kobe ), Japan, Earthquake
3 D - F D Maximum Velocity (0.1-0.8 Hz)
Segment A

435

forms is prominent in a relatively narrow zone offset fromthe basin edge by less than 1 km. It diminishes noticeably
at sites to the south where both the basin-edge and source
directivity effects are much less significant.

10
5

C o m p a r i s o n of the Observed and Simulated
Near-Fault Ground Motion

0
0

5

10

15

20

25 30 35
N58° E (km)40

25 30 35
N58°E(km)

40

45

50

55

Segment B

10500

g
v

10

15

20

I0
5

0-~
0

g

5

5

10

15

20

25 30 35
N5~ E(km)

45

50

55

Segment C

!1!i
40

45

50

55

Segment D

10
5

0

5

10

t5

20

25 30 35
N58° E (km)

40

45

50

55

Loglo Velocity(cm/sec)-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Figure 6. Horizontal peak velocity distribution
calculated for each of the four fault segments indicated by a straight line. Note the large contribution to
the ground motion in the Kobe area from the fault
segment B.

As demonstrated by Kawase (1996) and Pitarka et al.
(1997), the amplification is caused by the constructive interference between the horizontally propagating basin-edgediffracted wave with the primary S wave, coming from thebottom of the basin. Our simulation suggests that this variable amplification is caused by the variable effectiveness ofthe three-dimensional structure at amplifying waves comingfrom different directions (the third asperity that generated
the second phase is located immediately beneath the area
considered). It might also be a consequence of the constructive interference of the waves originated by the third asperity
with the multiple reflections of the waves generated from thefirst asperity.
The coupling of the basin-edge and source directivity
effects characterized by the two large pulses in the wave-

The Hyogo-ken Nanbu earthquake is unique in that it
occurred on a basin-edge fault and was also recorded at nearfault sites. Most of the stations that recorded the near-fault
ground motions are located in the basin. Because these sites
are affected by the basin response in a broad frequency
range, they are not used in the inversions of the ground motion for the source process. We simulated the ground motion
recorded at six near-fault sites in the Kobe area (Table 4).
The station locations are shown in Figure 1, and the recorded
velocity seismograms ban@ass filtered (0.1 to 0.8 Hz) are
shown in Figure 8. Two sites (TKT and FKA) are locatedinside the damaged zone where the ground motion is dominated by the direct shear waves and basin-edge-diffracted
waves. KBU is a weathered rock site, a few hundred meters
north of the intersection of the basin bounding fault planewith the Earth's surface. The ground motion at this site ischaracterized by two distinctive long-period pulses createdby the rupture directivity effect. The other sites are on sediments along the basin margin. The main characteristic of
the recorded ground motion is the existence of two distinctive long-period pulses, with the second one being muchlarger at the basin sites.
The small number of stations located inside or close to
the heavily damaged zone and their spatial aperture make it
difficult to delineate the spatial distribution of the groundmotions from the mainshock in that zone. Nevertheless, we
can use them to check the simulated ground motions and tovalidate our conclusions about the factors that caused theamplification of the ground motion within the narrow zone.
The comparison of the fault-parallel and fault-normalcomponents of the observed and synthetic ground velocity
is shown in Figure 9. Both the synthetic and recorded waveforms are filtered at 0.1 to 0.8 Hz. They are aligned basedon the arrival time of the direct S wave in the synthetic
seismograms, except for KBU where the absolute time wasavailable.
Qualitatively, the simulation agrees with the observation except for TKT. The fault-normal component in thesimulation reproduces the two observed long-period pulses
generated by the forward rupture directivity in the two main
asperities and amplified by the constructive interference ofwaves. The similarity between the synthetic and the recorded
waveforms at the rock site, KBU, suggests that the model ofrupture process obtained by the inversion is adequately represented in the simulation. The small difference in the main
phases between the synthetic and the observed waveforms
at this station may be due to the differences in the shallow
part of the rock models used in the inversion and the 3D

436

A. Pitarka, K. Irikura, T. Iwata, and H. Sekiguchi

Depth (km)Basin Model
Fault-Normal

Fault-Parallel

U-D

to

Fault

g
0o

6,0 sec

Depth (km)Flat Layer Model
Fault-Parallel

Fault-Normal& rb '.X o

U-D

o
r~
4=.
Fault

g
co

3

6.0 seo

Figure 7. Comparison of two seismic record sections along a selected line (P-P', inFig. 1) resulting, respectively, from the three-dimensional basin model (upper panel)and flat-layer basin model (lower panel). The vertical cross sections of the velocity
models along line P-P' are also shown.

Table 4
Station Locations
Station
Name

Lat.
(deg)

Lon.
(deg)

KBU
MOT
KOJ
FKA
TKT
KOB

34.72534.72534.68934.69534.64934.690135.240135.281135.217135.211135.138
135.180modeling. The simulation at KBU indicates that the two lowfrequency pulses, caused by the source directivity effect,characterize the basin input motion. The "killer" pulses amplified by the local geological structure have probably
caused most of the damage to the structures (Heaton et al.,1995).
Quantitatively, the simulation predicts well the amplitude of the first pulse but underestimates that of the second
pulse by a factor of 1.6 to 2.5 depending on the site. In
general, the waveforms recorded at the closest sites to thefault, where the effect of the 3D structure is not preponderant, are better modeled. Whereas at TKT and FKA, locatedinside the damaged zone, and KOJ located farther to thesouth, the simulation fails to reproduce the second pulse. It
is important to note that the second pulse seen in the recorded ground motion is significant at all the stations. Originating from the asperity in segment C beneath KOB, it isobserved in a rather broad near-fault area that includes TKTand MOT. Our simulation shows a rather small contribution
at TKT from the segment C.
The slip models for the Hyogo-ken Nanbu earthquake,
including the one used in this study, are obtained using very
few near-fault ground-motion data. This suggests that complexities in the spatial and the temporal variation of the slip
distribution on the fault may not be completely resolved. It
is obvious that the near-fault ground motion is sensitive tothe details in the source process. However, at present, we do
not know how such complexities in the slip model will affectthe simulated near-fault ground motion. In their study of thesource process of the 1995 Hyogo-ken Nanbu earthquake,
Ide et al. (1996) found that the slip model's overall features
are not influenced by the ground motion at KOB, which is
the only near-fault station they used. But the comparison
between the observed and calculated synthetic seismograms
at KOB (Fig. 12 in their article) for two slip models that wereobtained using strong-motion data sets, both with and without KOB, reveals that the second pulse in the waveforms is
slip-model dependent. For the N-S component where we ex-Three-Dimensional Simulation of the Near-Fault Ground Motion for the 1995 Hyogo-ken Nanbu (Kobe), Japan, Earthquake

VELOCITY
Band-Pass Filtered 0.1 _ 0.8 HzU~

70.7 ~

KB.~,N..~

33.8

KOJ . . ~ . . , . . ~

89.3

~
~

~

F 1 ~ ~ 1 1 2 . 1

~

KOB ~

51.9

N143
I

I

I

0.00

10.00

20,0o

30.00

Figure 8.

19.1

11.0

82.0

21.8
.

15,2

~

N53E

Time(sec)

43.2

24.5

CM/S

I

,36.0

~

I

I

I

o.0O

tO.00

20.00

30.00

85.7

..,.,,aVv.v.....,~

9R.4

~

!1.9

_
U--D

CM/S

I

~

Time(sec)

C~US

1

I

I

I

o.00

10.0o

20.oo

~0.oo

Time(sec)

Recorded velocity seismograms bandpass filtered at 0.1 to 0.8 Hz.

VELOCITYBAND--PASS FILTERED 0.1 _0.8 Hz V

44.6

. ~ ~ ¢ : ~

MOT

KBU

-

~

-

~

~

36.0

33.8

9.8
19.1

36.3
89.3

38.5
26,6

25.9

~

KOJ

33.5

70.7

~

44.1

FKA

112.1

"v""-

42.5
51.9
55.9
120.2

KOB

TKT

~~

=~l~:~=,~ 4..3.7
24,5

~
~

26.4
15.2
16.8
49,0

~
~

CM/S

N53E

c~

I

I

I

I

I

I

I

I

I

I

I

I

1

I

0.00

4.0O

&O0

12.00

lS.O0

20.00

24.00

0.00

4.tin

e.oo

12.00

le.co

20.t~

24.00

Time(sec)

Time(sec)

Figure 9. Comparison of the fault-normal (N143°E) and fault-parallel (N53°E) components of the observed and synthetic velocity seismograms, bandpass filtered at 0.1
to 0.8 Hz. They are aligned based on the arrival time of the direct S wave in the synthetic
seismograms, except for KBU where the correct time was available. The heavy traces
show the recorded seismograms, with the station name to the left and the peak amplitude (in cm/sec) to the right. The light traces show the synthetic seismograms with thepeak amplitude to the right.

437

438

A. Pitarka, K. lrikura, T. Iwata, and H. Sekiguchi

pect much of the directivity effect from the asperity immediately below the station, the peak amplitude of the second
pulse in their second model is about 2.8 time smaller thanin their first model. This result suggests that even smallchanges in the slip models may substantially affect the nearfault ground motion. Considering the mismatch in the amplitude between the observed and the simulated second
pulse, and our analysis of the simulated ground motion fromeach fault segment, we concluded that the rupture process
in the fault-segment C is not well resolved. As shown by
simulations using the empirical Green's function method(Kamae et aL, 1998), shallower slip in fault segment C improves the waveform fit at sites considered in this study.
The success of simulating the near-fault ground motion
at stations that are located close to the basin edge depends
also on the quality of the velocity structure. The 3D model
we use represents the underground bedrock topography well,while it assumes a simple two-layer structure for the basin
sediments. We showed that the extension of the damage zone
for a distance of about 30 km can be explained by the effectof the deep basin-edge structure. Our approximation to theshallow part of the basin structure consists of slightly reducing the velocity gradient in the surface sediments, resuiting in a higher shear-wave velocity at the surface. Basedon the comparison between the synthetic and observed
ground motion at TKT and KOJ, it appears that the simplifications in the structure of the shallow sedimentary layers
reduces the amplitude of the basin-induced surface waves.
Analysis and modeling of aftershock ground motions arenecessary in order to understand the effect of the soft-surface
sediments on the amplification of the ground motion and its
dependence on the direction of the incident wave in the Kobe
region.

Conclusions
Our simulation of the ground motions from the 17 January 1995 Hyogo-ken Nanbu earthquake, based on a kinematic fault model and a simplified 3D velocity structure,suggests the basin-edge effect as an important factor that
influenced the ground-motion amplification pattern in the
Kobe area. It also indicates that the ground motions recordedat near-fault stations were strongly affected by the basin
structure.
The fault-normal ground motions with high-amplitude
long-period pulses, observed in a narrow zone offset lessthan 1 km from the basin edge, were caused by the couplingof source and basin-edge effects, and such effects were responsible for the eastward extension of the damage zone.
Similar basin-edge effects led to amplified ground motions
and associated damage in the Santa Monica area during the1994 Northridge earthquake (Graves et al., 1997). However,unlike the case of Kobe, the Northridge event ruptured
northward away from the basin edge, thus minimizing thecontribution from source directivity effects. Nonetheless, thepotential for other Kobe-like scenarios still exists for many
fault-bounded basin structures throughout the world.
Our study is a first attempt to simulate 3D near-fault
ground motion from a basin-margin fault based on the kinematic earthquake rupture model. We have pointed out thedifficulties we face to match the simulated with the observed
ground-motion peak amplitude that relate with the fact thatthe near-fault ground motion in the basin sediments is very
sensitive to the details of the source process and the basinedge structure as well.
Our simulation demonstrates that even moderate size
(M < 7.0) earthquakes can cause near-fault ground motions
with extremely large amplitudes, when generated by basin
margin faults.
Acknowledgments
We thank D. Wald and R. Graves for helpful discussions and the reviewof the manuscript. We benefited from discussions with H. Kawase, J. Zahradnik, and T. Kagawa. Two anonymous reviewers provided comments thatsignificantly improved the article. This study was partially supported by aGrant-In-Aide for Scientific Research, Number 08248111, from the Ministry of Education, Science, Sports, and Culture, Japan.

References
Aguirre, J. and K. Irikura (1997). Nonlinearity, liquefaction, and velocity
variation of soft soil layers in Port Island, Kobe, during Hyogo-ken
Nanbu earthquake, Bull Seism. Soe. Am. 87, 1244-1258.
Architectural Institute of Japan (MJ) (1995). Preliminary reconnaissancereport of the 1995 Hyogoken-Nanbu earthquake, English edition, Architectural Institute of Japan, Tokyo, Japan.
Bard, P.-Y. and M. Bouchon (1980a). The seismic response of sedimentfilled valleys. Part 1. The case of incident SH waves, Bull. Seism. Soc.
Am. 70, 1263-1286.
Bard, P.-Y. and M. Bouchon (1980b). The seismic response of the sediment-filled valleys. Part 2. The case of incident P-SV waves, Bull
Seism. Soe. Am. 70, 1921-1941.
Bard, P.-Y. and J.-C. Gariel (1986). The seismic response of two-dimensional sedimentary deposits with large vertical velocity gradients,
BulL Seism. Soc. Am. 76, 343-346.
Cotton, F. and M. Campillo (1994). Application of seismogram synthesis
to the study of earthquake source from strong motion records, Ann.
Geof. XXXVII, no. 6, 1539-1564.
Frankel, A. (1993). Three-dimensional simulation of ground motions in the
San Bernardino valley, California, for hypothetical earthquakes on
the San Andreas fault, Bull. Seism. Soc. Am. 83, 1042-1063.
Geological Survey of Japan (1996). Reports on surveys of active fault in
1995, 85 pp. (in Japanese).
Graves, R. W. (1995). Preliminary analysis of long-period basin responsein the Los Angeles region from the 1994 Northridge earthquake, Geophys. Res. Lett. 22, 101-104.
Graves, R. W. (1996a). Simulating seismic wave propagation in 3D elastic
media using staggered-grid finite-differences, Bull Seism. Soc. Am.
86, 1091-1106.
Graves, R. W. (1996b). Simulating realistic earthquake ground motions inregions of deep sedimentary basins, Proc. of the l l th World Conference on Earthquake Engineering, Acapulco.
Graves, R. W., A. Pitarka, and P. Somerville (1997). Effects of basin structure on long period strong ground motion during the 1994 Northridge
earthquake (abstract), EOS 78, F430.
Haase, J. S., E. Hauksson, V. Vernon, and A. Edelman (1996). Modelingof ground motion from a 1994 Northridge aftershock using a tomo-Three-Dimensional Simulation o f the Near-Fault Ground Motion for the 1995 Hyogo-ken Nanbu (Kobe), Japan, Earthquake

graphic velocity model of the Los Angeles basin, Bull Seism. Soc.
Am. 86, S156-S167.
Hatayama, K., K. Matsunami, T. Iwata, and K. Irikura (1995). Basin-induced Love waves in the eastern part of the Osaka basin, J. Phys.
Earth 43, 131-155.
Heaton, T. H., J. F. Hall, D. J. Wald, and M. W. Hailing (1995). Responseof high-rise and base-isolated buildings to a hypothetical Mw 7.0
blind thrust earthquake, Science 267, 201-211.
Hisada, Y. and S. Yamamoto (1996). One-, two-, and three-dimensionalsite effects in sediment-filled basins, Proc. of the 11th World Conference on Earthquake Engineering, Acapulco, Mexico.
Horikawa, H., K. Hirahara, Y. Umeda, M. Hashimoto, and F. Kusano(1996). Simultaneous inversion of geodetic and strong motion data
for the source process of the Hyogo-ken Nanbu, Japan, earthquake,
J. Phys. Earth 44, 455-471.
Ide, S., M. Takeo, and Y. Yoshida (1996). Source process of the 1995 Kobe
earthquake: determination of spatio-temporal slip distribution by bayesian modeling, Bull. Seism. Soc. Am. 86, 547-566.
Irikura, K. (1995). Causative faults, strong ground motions and damages
from the 1995 Hyogo-ken Nanbu earthquake, Butsuri-Tansa (Geophys. Exploration), 47, 463-489 (in Japanese with English abstract).
lrikura, K., T. Iwata, H. Sekiguchi, A. Pitarka, and K. Kamae (1996). Lesson from the 1995 Hyogo-ken Nanbu earthquake: why were such
destructive motions generated to the buildings? J. Natural Dis. Sci.
17, 99-127.
Iwata, T., K. Hatayama, H. Kawase, and K. Irikura (1996a). Site amplification of ground motions during aftershocks of the 1995 Hyogo-kennanbu earthquake in severely damaged zone--array observation ofground motions at Higashinada Ward, Kobe city, Japan, J. Phys.
Earth 44, 553-561.
Iwata, T., K. Irikura, K. Told, T. Sasatani, K. Kudo, K. Seo, and T. Yokoi(1996b). Seismic exploration in and around Kobe city, Reports onanalysis and verification of Hanshin-Awaji great disaster, granted
from the Japanese Ministry of Education, Science and Culture (No.
07300005), Chap. 2, 21-49 (in Japanese).
Kagawa, T., S. Sawada, Y. Iwasaki, and A. Nanso (1993). Modeling ofdeep sedimentary basin structure beneath the Osaka basin, Proc. 22th
JSCE Earthquake Symposium, 199-202 (in Japanese).
Kamae, K. and K. Irikura (1996). A fault rupture model of the 1995 Hyogoken-nanbu earthquake (Mjma = 7.2) estimated by empiricalGreen's function method, J. Natural Dis. Sci. 16, 31-40.
Kamae, K., K./rikura, and A. Pitarka (1998). A technique for simulatingstrong ground motion using hybrid Green's function, Bull Seism. Soc.
Am. 88, 357-367.
Kawase, H. (1996). The cause of the damage belt in Kobe. "The basinedge effect," constructive interference of the direct S wave with thebasin-induced diffracted/Rayleigh waves, Seism. Res. Lett. 67, 25-35.
Kobayashi, S., S. Yoshida, S. Okubo, R. Shichi, T. Shimamoto, and T. Kato
(1996). Two-dimensional analysis of gravity anomaly across the
Rokko fault system, J. Phys. Earth 44, 357-372.
Koketsu, K. (1998). An overview of the rupture process, Report on the
Hanshin-Awaji Earthquake Disaster, Common Series, Vol. 2, Maruzen, Tokyo (in press).
Levander, A. R. (1988). Fourth-order finite-difference P-SV seismograms,
Geophysics 53, 1425-1436.
Moczo, P. and P.-Y. Bard (1993). Wave diffraction, amplification and differential motion near strong lateral discontinuities, Bull Seism. Soc.
Am. 83, 85-106.
Motosaka, M. and M. Nagano (1996). Analysis of ground-motion amplification characteristics in Kobe city considering a deep irregular underground structure--interpretation of heavily damaged belt zone
during the 1995 Hyogo-ken Nanbu earthquake, J. Phys. Earth 44,
577-590.
Nakagawa, K., K. Shiono, N. Inoue, and M. Sano (1996). Geological characteristics and problems in and around Osaka basin as a basis forassessment of seismic hazards, Soils Foundations Jan., 15-28.
Nakashima, M., T. Fujiwara, Y. Suzuki, M. Brnnean, S. Iwai, and A. Ki-

439

tahara (1995). Damage to engineering building from the 1995 Hyogoken-nanbu earthquake, J. Natural Dis. Sci. 16, no. 2, 71-78.
Olsen, K. B. and R. J. Archuleta (1996). Three-dimensional simulation ofearthquakes on the Los Angeles fault system, Bull Seism. Soc. Am.
86, 575-596.
Olsen, K. B., R. J. Archuleta, and J. R. Matarese (1995). Three-dimensional
simulation of a magnitude 7.75 earthquake on the San Andreas fault,
Science 270, 1628-1632.
Pitarka, A. and K. Irikura (1996). Basin structure effects on long period
strong motions in the San Fernando valley and the Los Angeles Basinfrom the 1994 Northridge earthquake and an aftershock. Bull. Seism.
Soc. Am. 86, S126-S137.
Pitarka, A., K. Irikura, and T. Kagawa (1995). Source complexity of theJanuary 17, 1995 Hyogoken-nanbu earthquake determined by nearfield strong motion modeling: preliminary results, J. Natural Dis. Sci.
16, no. 2, 31-37.
Pitarka, A., K. Irikura, T. Iwata, and T. Kagawa (1996a). Basin structure
effects in the Kobe area inferred from the modeling of ground motions
from two aftershocks of the January 17, 1995 Hyogoken-nanbu earthquake, J. Phys. Earth 44, 563-576.
Pitarka, A., D. Suetsugu, and H. Takenaka (1996b). Elastic finite-difference
modeling of strong motion in Ashigara Valley for the 1990 Odawara,
Japan, earthquake, Bull. Seism. Soc. Am. 86, 981-990.
Pitarka, A., K. Irikura, and T. Iwata (1997). Modelling of ground motionin the Higashinada (Kobe) area for an aftershock of the 1995 January
17 Hyogo-ken nanbu, Japan earthquake, Geophys. J. Int. 131, 231239.
Sanchez-Sesma, F. J. and F. Luzon (1995). Seismic response of three-dimensional valleys for incident P, S and Rayleigh waves, Bull. Seism.
Soc. Am. 85, 269-284.
Scrivner, C. W. and D. V. Helmberger (1994). Seismic waveform modelingin the Los Angeles Basin, Bull Seism. Soc. Am. 84, 1310-1326.
Seldguchi, H., K. Irikura, T. Iwata, Y. Kakehi, and M. Hoshiba (1996a).
Determination of the location of faulting beneath Kobe during the
1995 Hyogo-ken Nanbu, Japan, earthquake from near-source particle
motion, Geophys. Res. Lett. 23, 387-390.
Sekiguchi, H., K. Irikura, T. Iwata, Y. Kakehi, and M. Hoshiba (1996b).
Minute locating of fault planes and source process of the 1995 Hyogoken-nanbu, earthquake from the waveform inversion of strong
ground motion, J. Phys. Earth 44, 473-487.
Shimamoto, T. (1995). Earthquake disasters in Kobe and presumed earthquake-generating faults, Kagaku 65, 195-198 (in Japanese).
Stacey, R. (1988). Improved transparent boundary formulations for theelastic-wave equation, Bull Seism. Soc. Am. 78, 2089-2097.
Suzuki, Y., T. Fujiwara, M. Nakashima, M. Brineau, S. Iwai, and A. Kitahara (1995). Overview of building damage and damage to woodhouses from the 1995 Hyogoken-nanbu earthquake, J. Natural Dis.
Sci. 16, no. 2, 79-88.
Toki, K. Irikura, and T. Kagawa (1995). Strong motion data recorded in
the source area of the 1995. Hyogoken-nanbu earthquake, January 17,
1995 Japan, J. Natural Dis. ScL 16, no. 2, 23-30.
Vidale, J. E. and D. V. Helmberger (1988). Elastic finite-difference modeling of the 1971 San Fernando, California earthquake, Bull Seism.
Soc. Am. 78, 122-141.
Wald, D. J. (1995). A preliminary dislocation model for the 1995 Kobe
(Hyogo-ken Nanbu), Japan, earthquake determined from strong motion and teleseismic waveforms, Seism. Res. Lett. 66, 22-28.
Wald, D. J. (1996). Slip history of the 1995 Kobe, Japan, earthquake determined from strong motion, teleseismic and geodetic data, J. Phys.
Earth 44, 489-503.
Wald, D. J. and R. W. Graves (1998). The seismic response of the Los
Angeles Basin, California, Bull. Seism. Soc. Am. 88, 337-356.
Yoshida, S., K. Koketsu, B. Shibazaki, T. Sagiya, T. Kato, and Y. Yoshida(1996). Joint inversion of near- and far-field waveforms and geodetic
data for the rupture process of the 1995 Kobe earthquake, J. Phys.
Earth 44, 437-454.

440

Zahradnik, J. and F. Hron (1987). Seismic ground motion of sedimentary
valleys: example La Molina, Lima, Peru, J. Geophys. 62, 31-37.
Woodward-Clyde Federal Services
566 E1 Dorado Street
Pasadena, California 91101
axpitar0@wcc.com(A.P.)

A. Pitarka, K. Irikura, T. Iwata, and H. Sekiguchi

D.P.R.I.
Kyoto University
Gokasho, Uji, 611, Japan
Tel/Fax: +81-774-33-5866(K.I., T.I., H.S.)

Manuscript received 2 October 1996.