effects of surface topography on seismic ground response in the egion
TRANSCRIPT
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Effects of surface topography on seismic ground response in the Egion(Greece) 15 June 1995 earthquake
G.A. Athanasopoulos*, P.C. Pelekis, E.A. Leonidou
Department of Civil Engineering, University of Patras, GR-26500Patras, Greece
Received 23 January 1998; received in revised form 24 July 1998; accepted 12 August 1998
Abstract
The Greek coastal town of Egion on 15 June 1995 was shaken by a strong, small epicentral distance, earthquake that caused heavy
damages to buildings and loss of life. The damages were concentrated in the central elevated part of the town whereas the at coastal regionremained almost intact. This non-uniform distribution of damage is studied in this article in terms of surface topography effects by
conducting seismic response analyses of a simplied 2-D prole of the town. A dynamic nite element code implementing the equiva-
lent-linear soil behavior (FLUSHPLUS) was used for the analyses and it was found that the step-like topography amplied greatly the
intensity of motion without affecting its frequency content. The analyses showed that the motion recorded by an accelerograph installed at
the center of the town is in agreement with the computed values; they also indicated a particularly intense amplication close to the crest of
the steep slope, where a multi-story RC residential building partially collapsed. In contrast, the level of motion was found to be low at the at
coastal zone of the town where the earthquake damages were insignicant. It is concluded that the characteristic surface topography of the
town played an important role in modifying the intensity of base motion. q 1998 Elsevier Science Ltd. All rights reserved.
Keywords: Seismic ground response; Topography effects; Site effects; Finite element method; Dynamic soil properties
1. Introduction
On 15 June 1995 a strong earthquake occurred in the
vicinity of the western end of the Gulf of Corinth in Central
Greece, Fig. 1. The epicenter of the earthquake was located
in the sea between the coastal towns of Egion (in Northern
Peloponnese) and Eratini (in Southern Sterea Hellas).
Although the earthquake damages were spread in a rather
extended area, the hardest hit town was that of Egion and for
this reason the particular earthquake has since been known
as the `Egion 1995 earthquake'. The occurrence of the
Egion 1995 earthquake was followed by the appearances
of almost all the phenomena usually studied under the
general heading of `seismic ground response' i.e. amplica-tion and attenuation of base motion, effects of surface topo-
graphy, ground ruptures, liquefaction and landslides. The
main shock as well as the major events of the aftershock
activity were recorded by an accelerograph installed at the
center of the town and by similar instruments installed in a
number of cities and towns surrounding the epicentral area.
The damage pattern in Egion was not uniform and included
partial collapse of buildings and loss of life.
Following the destructive Egion 1995 earthquake severalresearchers [15,4,16] expressed the suspicion that the
presence of the fault escarpment that runs through the
town might be responsible for some amplication of the
ground motion in the central part of the town. These suspi-
cions were based on the existing knowledge on the subject,
which is briey reviewed in the following.
The effect of surface topography to the seismic ground
response has been the subject of numerous studies during
the last 25 years [17]. These studies have examined the
cases of ridge-or valley- type surface irregularities in a 2-
D form whereas only a limited number of results are avail-
able for 3-D congurations of the problem (e.g. Shanchez-
Sesma et al. [18]). Pioneering work on the subject wasaccomplished by Aki and Larner [19] who introduced a
numerical method based on a discrete superposition of
plane waves; this method was later extended by other inves-
tigators [2022]. Useful results have also been reported by
Wong and Trifunac [23], Wong [24] and Sanchez-Sesma et
al. [25].
Aki [26] used a simple structure of a wedge-shaped
medium to illustrate the effects of topography, Fig. 2(a).
An exact solution exists for SH waves propagating normal
to the ridge and polarized parallel to the ridge axis, which
predicts a displacement amplication at the vertex equal to
Soil Dynamics and Earthquake Engineering 18 (1999) 135149
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@upatras.gr.
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2/v, where the ridge angle is np (0 , n, 2). Faccioli [27]
used this triangular wedge structure to model approximatelyridge-valley topography, as shown in Fig. 2(b). This simple
model predicts an amplication at the crest relative to the
base equal to v1/v2 and may be used for rough numerical
estimates of amplications at the crest of ridges or deam-
plications at the bottom of valleys or canyons.
The nite element method, which offers the advantage
of being able to model irregularities of arbitrary shape
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149136
Fig. 1. Map of the western part of the Gulf of Corinth (Greece) with the locations of Egion and other coastal towns and communities, the surface traces of the
ve major normal faults of the region, the routes of the four major rivers (or streams) of the area and the approximate position of the 15 June 1995 epicenter.
Fig. 2. Approximation of ridge/valley topography by triangular wedges
[27].
Fig. 3. Relative distribution of peak horizontal accelerations along a ridge
from Matsuzaki area in Japan [33].
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involving inhomogeneous and non-linear soil materials, has
also been used in studies of surface topography effects
[28,29,17]. Some hybrid methods, combining a particlemodel with the nite element model have also been used
for studying the surface topography effects [30]. In terms of
physical modeling a photographic recording of particle
motion in a 3-D foam rubber physical model has been
used for studying the effects of topography at the Pacoima
Dam site [31]. The results of the studies mentioned above
(and of many others) indicate that the theoretically predicted
values of amplication of motion in steep topography
depend on the relative size of the irregularity (compared
to the incident wave-length), the angle of incidence and
the type of incident wave, i.e. SV vs. SH. Amplication
values range from 3 to 4 in the spectral domain and areless than 2 in the time domain [32].
In addition to the theoretical predictions, the amplica-
tion of surface motion in ridge-or steep slope-type topo-
graphy has also been veried from measurements during
natural earthquake events. The diagram of Fig. 3 depicts
the variation of normalized peak recorded horizontal accel-
erations from ve earthquakes in Japan as a function of
elevation across a ridge [33]. The normalization in this
diagram is referred to the crest motion and in addition to
the mean values, the standard error bars are also included in
the graph. The measurements indicate an amplication at
the crest (relatively to the base) varying from 1.8 to 5.5 with
a mean value of 2.5. In terms of damage patterns, increasingdamages have been reported [34] along the slope and the top
of hills after the Chile 1985 earthquake. A characteristic
example of increased earthquake damages close to the
crest of a step-like topography has been reported by Castel-
lani et al. [28] for the case of the Irpinia 1980 earthquake
and is illustrated in Fig. 4. In this case the damages of an
Italian village sitting at the top of a hill, were concentrated
close to the crest of a steep slope whereas they were insig-
nicant in the direction away from the crest.
It is worth mentioning that when comparing observed and
theoretically predicted amplications of surface motions
due to surface topography, it is usually found that the
observed values are much greater than the predicted ones.
Thus, the observed amplications range from 2 to 20 in thespectral domain and from 2 to 5 in the time domain. The
difference between predicted and observed values is attrib-
uted to the inuence of 3-D effects but it may also be due to
the fact that the measured amplications are actually rela-
tive amplications between points with amplied and
diamplied motion [22].
The article presents the results of a study regarding the
possible effects of surface topography on the seismic ground
response of the central part of the town of Egion. The
ground motion was analysed by using a 2-D nite element
code capable of modeling the surface relief and the strati-
graphy of the area with the aim of explaining the contrast inearthquake damages between the central elevated part of
town and the low and at waterfront area. Before proceed-
ing to the presentation of the main subject, however, some
information is given on seismological, geological, tectonic
and geotechnical data as well as on earthquake damages.
2. Seismological data
The Egion earthquake occurred on 15 June 1995 at 3:16
a.m. local time with a magnitude Ms 6.1 (or 6.2) and an
epicenter lying in the Gulf of Corinth northeast of Egion,
Fig. 1. There are minor deviations in the location of theepicenter of the main shock as reported by different sources
[1 3]. Thus the value of the main shock-epicentral distance
for Egion ranges from 8 to 26 km depending on the reported
location of the epicenter. Also, the reported values of the
focal depth range from 14 to 26 km [3,2]. The strongest
aftershock of the sequence (ML 5.4) occurred 15 min
after the main shock with a much shallower focus and
smaller distance from the town of Egion, whereas the post
earthquake activity was continued for several weeks with a
trend of epicenters moving toward the Peloponnese coast-
line [2]. The analysis of seismic data indicated the presence
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Fig. 4. Effect of surface topography on damage distribution in the Irpinia (Italy) 1980 earthquake [28].
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of a normal causative fault with the following source para-
meters: seismic moment M0 3.6 1025 dyn cm, fault
length L 13.4 km, stress drop Ds 53 bars and average
displacement u 0.85 m (Chouliaras and Stavrakakis [1]).
An analog accelerograph (SMA-1) installed at the ground
oor of a two-storey reinforced concrete building (with a
basement) in the center of the town, (herein denoted as OTE
site) recorded the strong motion of the main event. The time
histories of the three components of motion (transverse
longitudinalvertical) are shown in Fig. 5. The diagrams
of Fig. 5, in addition to the measured values of acceleration,
also include the calculated time histories of velocity and
displacement as well as the acceleration response spectra
(for 5% critical damping). The orientation of the horizontal
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149138
Fig. 5. The acceleration time histories (Transverse, Longitudinal and Vertical components) recorded at the accelerograph station in Egion (OTE site), the
calculated time histories of velocity and displacement, the corresponding acceleration response spectra (for 5% damping) and the orientation of the horizontal
components of the accelerograph.
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components of the accelerograph is shown at the bottom of
Fig. 5. The high intensity of the recorded horizontal ground
motion at the OTE site is remarkable: a peak acceleration
equal to 0.54 g in the T-component and 0.49 g in the L-
component (probably the highest recorded values in
Greece). However, the values of vertical acceleration
remained lower than 0.20 g. According to the time history
of recorded accelerations, the motion actually consisted of
only one or two cycles of strong motion. In terms of hori-zontal ground velocity, it may be seen from the calculated
time histories that the peak values were particularly high:
for both horizontal components they approached the value
of 50 cm/s. It may be further observed that the time history
of horizontal ground velocities consist of a high intensity
pulse involving an increment equal to 70 cm/s. It is worth
mentioning at this point that high ground velocity (and velo-
city increment) values are generated from shallow and near-
eld earthquakes. In such cases the time history of ground
motion bears the characteristics of the source and usually
reveals directivity effects [4]. Fig. 6 shows the in-plan
trajectory of accelerograph motion at the OTE site which
was obtained by combining the T- and L-components of thecalculated displacements. It may be seen that the horizontal
motion involved two major pulses: the rst in an approxi-
mate EW direction and the second in an approximate NS
direction.
According to the acceleration response spectra of hori-
zontal and vertical motion at the OTE site, shown in Fig. 5,
the peak spectral acceleration of the T-component reached a
value of 1.5 g with a predominant period of 0.5 s. It is
believed that this value of period reects the characteristics
of the source mechanism [4], whereas the period of a
secondary spectral peak ( , 0.25 s) is most probably
associated with the soil conditions at the site of accelero-
graph station being equal to the fundamental period of
ground at the OTE site. This value of the fundamental
period may also be derived from the stratigraphy of the
site (Fig. 13) as Ts (4 25)/400 0.25 s. The effective
horizontal acceleration determined in accordance with the
ATC (1978) provisions was found to be equal to 0.43 g
(Lekidis et al. [3]).
As mentioned in the introduction, the main shock of the
Egion earthquake was also recorded in a number of loca-
tions surrounding the epicentral region. Lekidis et al. [3]
have studied the accelerograms from nine accelerograph
stations installed at epicentral distances ranging from 18
to 84 km. They concluded that the earthquake energy was
anisotropically radiated from the source at a mean
frequency of 2 Hz and with attenuation rate depending on
the azimuth of the direction of propagation.
3. Geology and tectonics of the area
The area that was shaken by the Egion 1995 earthquakes
lies at the western end of the asymmetric Corinth graben.
This graben together with the Rio and Patras grabens form
an 140 km long and 40 km wide rift which separates the Pre-
Neogene folded basement of the Sterea Hellas and Pelepon-
nese [57]. A simplied geologic prole of the region is
shown in Fig. 7. The Pre-Neogene basement of the area
consists of Mesozoic carbonates (limestones) and ysch
and it is overlain by thick layers of marls deposited from
Upper-Pliocene to Lower-Pleistocene. The marl deposits
are in turn overlain by Quaternary alluvial fan depositsand fan delta deposits of considerable thickness (mostly
conglomerates). Finally, the surcial layers consist of Holo-
cene beach and river mouth deposits of variable thickness
(gravel, sand, silt and clays).
A number of north-facing WNW-trending active normal
faults have been mapped in NW Peloponnese by Doutsos
and Poulimenos [5]. Five of these faults are crossing the
wider Egion area and their surface traces are shown in the
map of Fig. 1 (Koukouvelas and Doutsos [8]). As shown in
the simplied NS cross-section passing through the town
of Egion, in Fig. 8, these faults have a curved listric geo-
metry and reach depths of about 10 km. The segmented
Egion fault has a total length of 12 km and its position ismarked by an escarpment 40100 m high along a 2 km long
segment which runs through the town of Egion [9]. This
escarpment forms a characteristic hill-front morphology
along the coast of Egion Bay as shown in the photograph
of Fig. 9. Some disagreement seems to exist regarding the
causative fault of the Egion 1995 earthquake. Tselentis et al.
[2] suggested that the main shock was probably generated in
a normal fault at the north side of the Gulf of Corinth,
dipping towards SSE (see Fig. 8) with a dip angle approxi-
mately 708. The main aftershock was then generated in
another normal fault (probably the Egion fault) dipping
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149 139
Fig. 6. The in-plan trajectory of the horizontal ground motion at the OTE
site during the main shock of 15 June 1995 Egion earthquake.
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towards NNE to the southern side of the Gulf. Koukouvelas
and Doutsos [8] have reported the results of eld observa-tions of surface ruptures (with sizes of a few centimeters)
and of geodetic measurements and suggest that the Egion
fault was reactivated during the 1995 earthquakes. It has also
been suggested that the focus of the earthquakes was hosted
by a low angle normal fault (like the one depicted with dotted
line in Fig. 8) cutting under the Gulf of Corinth and the town
of Egion at depths ranging from 10 to 25 km [3,4,10].
4. Geotechnical data
It was mentioned in the previous section that a segment ofthe fault of Egion runs through the town of Egion in
approximately EW direction and forms a characteristic
escarpment. The map of Fig. 10 shows the Egion town limits
and the horizontal extent of the escarpment. The major part
of the town has been built south to the escarpment on the
elevated region comprising the footwall of the fault. A
representative soil prole of this elevated region and
measured values of Standard Penetration Test blowcount
(NSPT) are shown in Fig. 11, based on borings at the OTE
site [11]. It may be seen that the depth to the conglomerates
at this site is 22 m, whereas the overlying soil layers are
stiff/dense clays, silts and gravels characterized by high
values of NSPT. It should be noted that the water table was
not encountered up to the explored depth of 45 m in this site;
however some perched water tables have been found to exist
in some areas of the elevated region of the town. Regarding
the thickness of the conglomerates it is believed that at the
OTE site might be greater than 150 m.
As shown in Fig. 10 a relatively small part of the town has
been built on the soft deposits of the coastal area lying to the
north of the escarpment. The soil prole of this low-elevation at region is rather variable but the stratigraphy
at the BH10 site [12] depicted in Fig. 12 may be considered
as a representative case. The characteristic feature of this
prole is the existence of a very soft clay layer character-
ized by very low NSPT values encountered at a depth of
10 m and having a thickness of 7 m. Silty sand-gravels are
encountered above and below this soft layer and they are
characterized by rather high values of NSPT. The explored
depth at this site reached only 30 m from the ground surface
so the depth to the conglomerates could not be established
with certainty. It is believed, however, that this depth may
range from 50 m to 60 m from the ground surface. Thewater table in this region of the town is high and it is
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149140
Fig. 7. Geologic prole of the Egion area.
Fig. 8. Simplied NS cross-section passing through the town of Egion and showing the geologic prole and the geometry of major faults of the area [5].
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encountered approximately 1.0 m below the surface of the
ground.
5. Earthquake damages
It was mentioned in the introductory section that thedamages from the Egion 1995 earthquake were spread in
an extended area around the epicenter of the earthquake. At
least 20 communities located close to the north coast of
Peloponesse to the east, south and west of Egion were
affected, whereas some damages also occurred in the
towns of Eratini and Itea located at the north coast of Gulf
of Corinth across Egion (see map of Fig. 1). The hardest hit
area, though, was that of Egion. The buildings of Egion area
are either reinforced concrete structures with one to nine
oors or bearing masonry structures with one to three oors.
Fardis [13] has presented preliminary information regarding
the distribution of damages among buildings with different
age, type of construction and number of oors. A correlation
of damage pattern with the local soil conditions has not yetbeen reported with the exception of the observation of a
strong contrast in damages between the waterfront and the
central area of the town. The absence of damages in the
waterfront area of the town becomes more impressive
when it is noted that in this area the buildings are very old
(some already ruined) and without any seismic resistance
provisions [13]. However, a large number of buildings
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149 141
Fig. 9. Photographic view of the coastal region of Egion looking southward.
Fig. 10. A simplied map of Egion showing the town limits, the extent of the elevated region and the location of points of interest and of the cross-section
A AH
.
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sitting in the elevated region of Egion, suffered heavy
damages including the partial collapse of a six-storey rein-
forced concrete residential building located at a small
distance behind the fringe of the Egion fault escarpment
(DESP site in Fig. 10). Partial collapses of reinforced
concrete buildings also occurred in the village of Valimitika
(four-storey Hotel ELIKI), about 7 km to the east of Egion
and at a site 2 km west of Egion (three-storey Administra-
tion Building of Hellenic Weapons Industry and a nearby
two-storey residential building). Twenty-eight people lost
their life in the earthquake whereas the total cost of damages
was estimated to be $600 million [14]. It is worth mention-
ing that the Egion 1995 earthquake did not result in damages
to the infrastructure of the wider area (roadways, bridges,
retaining walls, port facilities, etc.) It did result, however,
limited soil liquefaction at several coastal sites of Egion
Bay, along the banks of the Selinuntas and Kerynitis river
and along the coastal zone of Rizomylos, Fig. 1. The conse-
quences of soil liquefaction to buildings and other structures
were not signicant and no further mention to soil liquefac-
tion will be made in this article.
6. Surface topography effects
As was mentioned in the introduction, the main objective
of this study was to investigate the possibility of explaining
the differentiation of motion between the coastal area and
the elevated region of the town of Egion by the effects of
surface topography. The authors would like to make clear,
though, that surface topography is only one of the factors
that may be responsible for this differentiation of motion.
According to the seismological data presented in a previous
section the town of Egion is located in the near-eld of the
event and the radiation pattern and directivity of motion
have certainly affected the characteristics of the ground
motions. However, it should be taken into considerationthat the Egion fault was (most probably), not the causative
fault of the earthquake. Any attempt, then, to explain the
differentiation of motion in the two regions of the town in
terms of up-thrown and down-thrown blocks, may be ques-
tionable. In view of the above uncertainties the authors
believe that an investigation of surface topography effects
is worthwhile and could provide some useful insights in the
phenomenon.
7. Surface topography at the town of Egion
In order to study the effects of surface topography on theground seismic response at the town of Egion it was decided
to use a 2-D ground prole which was established by
considering a cross-section of the northern part of the
town along the NS direction. This cross-section was
made along the line AAH
shown in the map of Fig. 10.
The line AAH
was selected in such a way as to intersect
in a right angle the trace of the escarpment and to pass close
to the accelerograph station (OTE site), close to the site of
partial collapse of the RC building (DESP site) and through
a site of the port area where the earthquake caused no
damage (PORT site). The free surface prole along the
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149142
Fig. 11. Geotechnical soil prole and corresponding values ofNSPT at the
OTE site.
Fig. 12. Geotechnical soil prole and corresponding values ofNSPT at the
BH10 site.
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line A AH
was constructed by reading distances and surface
elevations from an 1:5000 topographic map of the regionand is shown in a simplied manner in the plot of Fig. 13.
The soil stratigraphy along the 2-D prole was established
by utilizing the geotechnical data presented in a previous
section. As shown in Fig. 13 the established 2-D prole has
a length of about 500 m and a height of 140 m below the
OTE site and 70 m below the PORT site.
7.1. Dynamic properties of soil materials
The dynamic soil properties that are needed in an equiva-
lent-linear type ground response analysis are the low-ampli-
tude shear wave velocity, Vs0 and the G/G0gc and Dgccurves describing the degradation of soil shear stiffness
with increasing amplitude of cycle shear strain, gc (G0
low-amplitude shear modulus, i.e. for gc # 1025, G
higher amplitude shear modulus). Values of Vs0 vs. depth
at the DESP site were obtained by applying the Spectral
Analysis of Surface Waves (SASW) method [35]. As
shown in the diagram of Fig. 14, a great depth of penetration
of surface waves was achieved in this site by utilizing the
drop of a heavy weight (5 kN) on the ground surface.
According to the diagram of Fig. 14, the measured shearwave velocities are remarkably high indicating the great
shear stiffness of soil formations at the elevated region of
the town. For comparison purposes, the diagram of Fig. 14
includes also plots of Vs0 versus depth from crosshole
measurements conducted at the OTE site by the Central
Laboratory of Public Works (CLPW) of the Ministry of
Public Works and Environment [11]. Although the DESP
and OTE sites are about 150 m apart, a good agreement
seems to exist between the two Vs0 vs. depth proles. To
check the reliability of an empirical Vs0NSPT correlation
established by Athanasopoulos [36,37], the diagram of
Fig. 14 includes also a plot ofVs0 vs. depth curve, estimatedby entering the values ofNSPT taken from Fig. 8 into Eq.(1)
Vsom=s 107:6NSPT0:36
: 1
The comparison shows that the agreement is good, espe-
cially when considering the empirical nature of Eq. (1).
Based on the Vs0 vs. depth curves of Fig. 14 the soil strati-
graphy and corresponding Vs0 values for the elevated region
of the town of Egion were established as shown in the 2-D
prole of Fig. 13.
To establish the Vs0 vs. depth variation for the low-eleva-
tion at coastal region of the town SASW measurements
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149 143
Fig. 14. Vs0 versus depth prole at the OTE site with results of SASW and
crosshole measurements compared with values obtained from Eq. (1).
Fig. 13. Two-dimensional soil prole along the direction A AH
with the Vs0 values used in the seismic ground response analyses.
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were conducted at the PORT site whose location is indicated
in the map of Fig. 10. The diagram of Fig. 15 depicts the
results of measurements. The penetration of surface waveswas smaller in this site and reached a depth of approxi-
mately 90 m. It may be observed from Fig. 15 that the
surcial soil layers at PORT site are characterized by low
Vs0 values. Beyond a depth of 10 m, however, the values of
Vs0 show an increasing trend with depth, whereas at a depth
of about 50 m, an abrupt increase of Vs0 is observed (Vs0 .
1000 m/s). Based on this abrupt increase, it may be assumed
that the depth to conglomerates in this site is equal to 50 m.
For comparison purposes the diagram of Fig. 15, also
includes a Vs0 versus depth curve estimated from Eq. (1),
by using the NSPT values taken from Fig. 12 for the BH10
site, which is about 700 m to the west of PORT site. The
rather large distance between the PORT and BH10 sitesseems to explain the deviations between the two curves.
Based on the results of measurements depicted in Fig. 15
the soil stratigraphy and corresponding Vs0 values for the
coastal region of the town of Egion were established as
shown in the 2-D prole of Fig. 13.
As mentioned previously the description of the non-linear
behavior of the soil materials of the 2-D model requires the
knowledge of the G/G0gc and Dgc curves which are
usually obtained through laboratory cyclic loading tests.
No such experimental data are, however, available for the
soils of Egion area and for this reason it was decided to
resort to the empirical relations reported recently by Ishiba-
shi and Zhang [38]. These relations allow the determination
of G/G0gc and Dgc curves in terms of the plasticity
index, Pl, and the mean effective normal stress, s0H
, of a
soil element. By taking into consideration two different
mean depth levels, two sets of curves were determined for
the PORT site and two more sets for the OTE site. The
determinations were accomplished by using the recently
developed PC program NOLISM [39]. All sets of curves
(OTE1, OTE2 and PORT1, PORT2) are shown in graphic
form in Fig. 16. It should be noted that the OTE2 set of
curves refers actually to rock material and was established
by utilizing the values frequently used for rock material in
the 1-D seismic response program SHAKE91 [40].
7.2. Seismic response analyses
The seismic response analyses of the 2-D ground prole
of Fig. 13 were performed by the 2-D dynamic nite
element program FLUSHPLUS [41]. This PC programmodels soil as linear viscoelastic material and simulates
the non-linear aspect of behavior by the iterative equivalent-
linear method. It should be mentioned that one of the objec-
tives of this study was to take into consideration the non-
linear behavior of surcial soil layers which affects signi-
cantly the ground response in the case of strong motion (as
in the case of Egion earthquake). The input motion is
applied at the rigid base of the model in the form of a
time history of acceleration; this means that the program
can only analyze the vertical propagation of shear waves
from the seismic bedrock to the surface of the ground.
The consideration of vertically propagating SV wavesconstitutes a simplication of the actual phenomenon, espe-
cially in the case of near-eld events involving complex
wave elds. In the case of Egion earthquake, though, the
focal region of the main event seems to lie at a horizontal
distance of about 20 km and at a depth of a similar magni-
tude. By applying Snell's law and utilizing the shear wave
velocities of soil strata (Fig. 13), it may be shown that at
least a portion of the seismic waves arrived at the site inves-
tigated herein following an approximately vertical direction.
One of the advantages of the FLUSHPLUS code is its
capability to use viscous dampers at the lateral boundaries
of the mesh and thus avoid undesirable reections of
outgoing waves. This feature makes possible the use ofnite element meshes with smaller lateral dimensions. The
discretisation of the 2-D ground prole into nite elements
is shown in Fig. 17. In the mesh of Fig. 17 the size of the
nite elements (in particular the vertical dimension) was
selected following the maximum size criteria suggested in
the manual of the program. By using these criteria, the size
of the nite elements remains (appropriately) smaller than
the wavelengths which are expected to be developed during
the passage of seismic waves through the particular soil
formations comprising the 2-D prole of Fig. 13. It should
be mentioned, however, that even in the case of conformance
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149144
Fig. 15. Vs0 versus depth prole at the PORT site with results of SASW
measurements compared with values obtained by applying Eq. (1) with
NSPT values of the BH10 site.
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to the above mentioned maximum size criteria, the results of
the analyses may still be affected by the density of the nite
element mesh. The best way to determine the inuence of
the nite element discretisation on the results of the analyses
would be to conduct a convergence type of study in which
the mesh is progressively rened until the results of the
analyses do not change appreciably. A limited convergence
study of this type was conducted herein by using two differ-
ent F.E. meshes. It was found that a two-fold increase in the
number of elements resulted in a 15% change in the
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149 145
Fig. 16. G/G0gc and Dgc curves used in the seismic ground response analyses of the 2-D soil prole of Fig. 15.
Fig. 17. Finite element discretization of the 2-D soil prole.
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response values without affecting their relative magnitudes.
This change was considered as acceptable for the needs of
the present investigation and it was therefore concluded that
the nite element mesh of Fig. 17 provides reliable results.
The nodes corresponding to the positions of PORT, DESP
and OTE sites are also indicated in the mesh of Fig. 17.
It should be mentioned that the surface topography effects
were investigated in this study in terms of a horizonal base
excitation only, although the FLUSHPLUS code allows the
input of both horizontal and vertical components of base
motion. This simplication was based on the fact that the
peak value of vertical component of motion recorded at
OTE site was a small fraction (37%) of the peak value
corresponding to the vertical direction. Further, the majority
of studies on surface topography effects are conducted in
terms of horizontal base motions.
One of the objectives of the study was to establish a base
motion that when propagated through the 2-D model gener-
ates a response at the OTE site similar to the one recorded
during the main shock of 15 June 1995. In the case of 1-Danalyses this task (deconvolution) can be conveniently
accomplished by using the SHAKE91 code (or other similar
codes) which takes into account the non-linear behavior of
soil materials. In the case of 2-D analysis of the present
study the task of determining the base motion from the
recorded surface motion at OTE site should be accom-
plished by using a 2-D code offering the capabilities of
the SHAKE91 code. Since such a code was not available
to the authors at the time of writing the article the objective
was accomplished by trial-and-error i.e. by providing at the
base downscaled time histories at the recorded horizontal
acceleration (T-component) at the OTE site. The frequencycontent of the input motion was kept identical to the
frequency content of the recorded motion on the premise
that the recorded accelerogram at the OTE site reects the
characteristics of the source and was not modied in
terms of frequency content by the soil stratigraphy and
topography. By following this trial-and-error procedure it
was found that a downscaled base motion with a peak hori-
zontal acceleration equal to 0.14 g, was necessary in order to
produce a response at the OTE site that approximately
matched the recorded motion (T-component). A comparison
of the computed and actually recorded time histories of
accelerations at OTE site is shown in the diagram of Fig.
18. This comparison indicates a good agreement betweenthe two time histories. A further comparison of the corre-
sponding acceleration response spectra, however, shows a
deviation of the computed value of peak spectral accelera-
tion which is approximately 50% higher than the recorded
one. There is no deviation, however, regarding the values of
predominant period at computed and recorded horizontal
motion. It should be noted that despite a large number of
trials in some of which not only the intensity but also the
frequency content of input motion was varied it was not
possible to establish a horizontal base motion that produced
a surface motion matching the recorded motion at the OTE
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149146
Fig. 18. Computed seismic response at the OTE site from 2-D analyses
compared to the recorded values in the 15 June 1995 Egion earthquake.
Fig. 19. Computed seismic response at the DESP site from 2-D analyses.
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site in terms of both the time history and the response spec-
trum. It was, therefore, decided to accept and use as an input
motion the recorded horizontal acceleration time history at
the OTE site (T-component) downscaled to a peak value of
0.14 g. Based on the above results it is concluded that the
motion at OTE site was greatly amplied with respect to thebase motion (amplication factor < 3.80). This amplica-
tion of horizontal motion cannot be attributed to 1-D reso-
nance, since the fundamental period of soil prole at OTE
site is approximately equal to 0.25 s, whereas the predomi-
nant period of input motion is 0.5 s. It seems, therefore,
appropriate to assume that the 2-D topography of the area
has resulted in the great differentiation of motion.
By using the base input motion established above, the
response at the DESP site was estimated and the results
are shown in graphical form in Fig. 19. A very strong ampli-
cation of motion is indicated at this site: the peak horizon-
tal acceleration is equal to 0.79 g, whereas the spectral
acceleration approached the value of 5 g with a predominantperiod equal to 0.5 s. Although these results may have been
amplied by articial resonance effect an inherent
problem in equivalent-linear seismic ground response
analyses they still indicate a tendency for a strong ampli-
cation of horizontal ground motion close to the crest of the
slope. This trend seems to be in agreement with the results
of eld observations reported in the Introduction (Fig. 4) and
with the damage pattern observed in this region of the town
partial collapse and heavy damages of RC buildings and
discussed in a previous section.
The response at the PORT site is shown in graphical form
in Fig. 20 The results indicate a much lower level of hori-
zontal motion: the peak horizontal acceleration is equal to
0.25 g and the maximum spectral acceleration is equal to
0.70 g with a predominant period at 0.5 s. These results are
again in agreement with the impressive lack of damages in
the waterfront area (including the undamaged Port facilities)
of the town which was mentioned in a previous section. It is
worth mentioning that the fundamental period at the PORT
site may be estimated (from the stratigraphy shown in Fig.
13) to be approximately equal to 0.5 s a value that almost
coincides with the predominant period of the base motion.
In case the wave eld was governed by a 1-D propagation, a
resonance should have been developed and the surface
motion at the PORT site could have been amplied (with
respect to the base motion) to a greater degree compared to
the OTE site. The fact that this trend was not observed in the
results of response analyses of this study seems to indicate,
again, that the surface response is governed by a 2-D propa-
gation of seismic waves i.e. the surface topography has
played a rather signicant role in modifying the groundmotion of the area.
The results of the 2-D seismic response analyses that were
presented above, seem to indicate a signicant surface topo-
graphy effect on the ground response during the Egion 1995
earthquake. Before reaching the nal conclusions, however,
it would be appropriate to examine the capability of 1-D
response analysis to duplicate the results obtained by the
2-D analyses. To accomplish this check the program
SHAKE91 was used to estimate the 1-D ground response
at the sites of OTE and PORT. The dynamic soil properties,
soil stratigraphy and depth to the base of each site were
identical to the ones used in the 2-D analysis. By usingthis approach the peak horizontal acceleration at the OTE
site was found to be equal to 0.69 g, a value that could be
accepted as not being too far from the results of the 2-D
analysis. At the PORT site, however, the surface accelera-
tion reached a value equal to 0.53 g. This value is more than
twice the value obtained by the 2-D analysis and it demon-
strates the inability of 1-D analyses to predict the surface
motion in this part of the town. It becomes, therefore, possi-
ble at this point to summarize the ndings of this study and
state the following conclusions.
8. Conclusions
The effects of surface topography in the Egion 1995
earthquakes was studied by establishing a 2-D ground
prole along a cross-section of the town. This section cuts
through the central part of the town, has a NS direction and
crosses in a right angle the fault escarpment that runs
through the town in the EW direction. The response of
the ground surface along the direction of the cross-section,
was analysed by a nite element code and implementing the
equivalent-linear method. It was found that the peak hori-
zontal acceleration of the seismic base of the area does not
G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149 147
Fig. 20. Computed seismic response at the PORT site from 2-D analyses.
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need to be greater than 0.14 g in order to generate the
recorded surface motion at the OTE site. This base motion
was greatly amplied (290%) at the elevated region of thetown as shown schematically in Fig. 21(a) whereas at sites
close to the fringe of the slope (DESP site) the amplication
was even greater: 460%. This behavior seems to be in agree-
ment with both theoretical results and eld measurements
and observations presented in reviewing the existing knowl-
edge on the subject as well as with the damage pattern in the
town. By normalizing the surface motion with respect to the
recorded motion at the OTE site Fig. 21(b), it may be seen
that close to the fringe of the elevated region of the town the
motion is amplied by 47% whereas it is deamplied by
57%67% at the waterfront low-elevation at region. It is
therefore concluded that the characteristic topography of the
town played an important role in modifying the intensity ofbase motion.
Acknowledgements
The authors express their thanks to Drs G. Stavrakakis
and I. Kalogeras of the Geodynamics Institute of the
National Observatory of Athens, Greece, for providing
accelerograph records for the Egion area in digital form.
Thanks are also expressed to the Hellenic Earthquake Plan-
ning and Protection Organization for the nancial assistance
during the in situ measurement of dynamic soil properties in
Egion area as well as to Mr P. Theodorou and to the Civil
Engineering students at the University of Patras, D.
Kostouros and N. Roumeliotis for their assistance during
the in situ tests and in collecting maps of the Egion area.
Finally, thanks are expressed to the anonymous reviewers of
the article whose comments helped the authors to address
some important issues and clarify some aspects of their
approach of examining the surface topography effects.
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