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Review of Soil and Topography Effects in the September 7, 1999 Athens () Earthquake

George D. Bouckovalas National Technical University of Athens, Greece

George Kouretzis National Technical University of Athens, Greece

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Recommended Citation Bouckovalas, George D. and Kouretzis, George, "Review of Soil and Topography Effects in the September 7, 1999 Athens (Greece) Earthquake" (2001). International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. 7. https://scholarsmine.mst.edu/icrageesd/04icrageesd/session14/7

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Review of Soil and Topography Effects in the September 7,1999 Athens (Greece) Earthquake

George D. Bouckovalas George Kouretzis National Technical University of Athens National Technical University of Athens Greece Greece g.b oarc~~.c~vviE.MP~la.~~ g?2~~4~~f~~,~~~f~~?.Plftla.gP

ABSTRACT

The earthquake that hit Athens, Greece on September 7, 1999 was an unexpected disaster. It originated fiom a previously unknown seismotectonic structure, at about 18 km to the west of the historical center, and left behind 143 casualties, about 100,000 homeless and 100 totally collapsed buildings. On the other hand, it provided a number of reliable strong motion recordings and well-defined patterns of damage at sites with known geological and geotechnical conditions. Evaluation of this information shows that the very stiff soils of the Athens basin amplified seismic ground motion by an average of 40% compared to nearby outcropping soft rocks. In addition, the 40m in height, 15-3O"inclination clis of Kifissos river canyon may have aggravated the seismic motion by an additional 50% on average. Modem seismic codes totally oversee the first of these effects and underscore the second.

INTRODUCTION results are presented fiom a site-specific analysis of mainshock strong motion recordings and the on-going On September 7, 1999 (11:56:50.5 GMT) a violent earthquake correlation of damages to local soil and topographical of Ms=5.9 was recorded at the western bounds of the greater conditions. More general information about the earthquake can metropolitan area of Athens, at 18 lun distance fiom the be found in the attached literature, as well as at a number of historical center. According to official accounts, it was one of relevant web sites. the most damaging events in the modem history of Greece, (e.g. F;ate??:/~~.cv.nein.b.noa.~l,~~://~~~.~~~~~. ~/K~~~~.~~, and certainly the most damaging for the city of Athens. About ~~~://~~~~~~.~~~~~.~~~~~.~/) 100 buildings collapsed totally, raising the number of casualties to 143 and the number of wounded to more than 2,000. During the first days after the shock, more than 100,000 SEISMOLOGICAL ASPECTS AND STRONG MOTION people were rendered homeless. Some historical monuments RECORDINGS suffered significant damage, like the Byzantine monastery of Dafni and the 5" century B.C. castle of Fili, while ancient Table 1 summarizes the focal parameters of the mainshock, monuments have resisted the earthquake impressively well. according to a number of independent fault plane solutions based either on teleseismic or near-field station recordings. In Most of the damage occurred within 15 km of the epicenter addition, Fig. 1 shows the prevailing view regarding the and 7 km from the fault rupture, which is identified for location of the mainshock epicenter, as well as the aRershock simplicity as the trace of Fili fault in Fig. 1, and concerned epicenters recorded during the period fiom September 8& to residential or factory buildings. The damage to lifelines was October 29& 1999 by the National Observatory of Athens insignificant while no major ground failures (liquefaction, (NOA). In general, there is consensus that the earthquake was slope failures, etc.) were triggered by the earthquake. The due to a normal fault rupture, striking N 110"-123" and distribution of damage within the affected zone was irregular, dipping 47"-56" S W. Most likely, the rupture originated at a creating speculation for rupture directivity and local site depth of 8-18 km and propagated fiom southwest to northeast amplification phenomena, as it will be discussed in latter and upwards (Papadopoulos et al. 2000, Tselentis and sections. The maximum intensity in Modified Mercali scale Zahradnik 2000, Sargeant et al. 2000) creating strong reached IX at the epicentral area and exceeded VI in the directivity effects on seismic ground motions. Apart from fault greater part of Athens. plane solutions, this view is confirmed by the analysis of regional broadband seismograms (Tselentis and Zaradnik, Following a brief seismological review, this paper reports on 2000) which revealed a shorter apparent source duration to the the geotechnical aspects of the Athens Earthquake. Namely, "E of the epicenter (4-5sec) and a longer one to the SSW (7-

SPL - 10.3 1 Fig. 1. Satellite photo of the epicentral area of the Athens 07/09/1999 earthquake. The white triangle corresponci3 to the epicenter of the main shock small black triangles indicate ajershock epicenters over the period of Sept 8thto Oct 29"' 1999, while white circles denote strong motion recording sites. The continuous white line represents the simpl@ed trace of Fili fault, while the dashed white line encircles the metropolitan area of Athens. (NOA, Papadopoulos et al. 2000)

Moment Depth Strike Dip Lat. Long ("1 k) (deg) (de& (degN) (degE) USGS 7.8e17 9 123 55 38.13 23.55 Harvard 1.2e18 15 114 47 38.02 23.71 GLUT 7.6e17 11+18 119 56 38.04 23.61 Stavrakakis et al. (2001) 5.66e17 10 117 52 - Papadopoulos et al. (2000) - 16 113 56 38.10 23.58 Voulgaris et al. (2000) 8 110 55 - 1 Tselentis&Zaradnik(2000) I - 1 10 1 117 1 52 I - I - 1

8 sec). Note that the fault plane geometry is similar to the a pre-existing fault plane, most probably the plane of Fili fault geometry of a number of regional tectonic faults. This (Fig. l), and has propagated all the way up to the ground observation, combined with geologic and seismotectonic field surface. However, this view is considered with skepticism by investigationsmainly reported by Pavlides et al. (1999, 2000) many seismologists (Rodoyanni, 2000, Elnashai & Ambraseys support the hypothesis that the earthquake rupture occurred on 2000, Voulgaris 2000, Tselentis and Zaradnik,

SPL - 10.3 2 1.2

1

0.4 0.8

0.2 -m h v m Io - 0.6 m d -0.2

-0.4 0.4

~~:~~~m -0.2 0.19g o.2

-0.4 0 o 2 4 6 8 1012 0.01 0.1 1 Time (sec) Period (sec)

Fig. 2. Acceleration time histories and elastic response spectrapom a typical ground su~acerecording (SPLB) of the main shock. (Distancefrom fault rupture r9 km, stifsoil conditiom.)

2000). In fact, the view prevailing today is that an extension of stations of the Athens subway system (METRO). Thus, it has the fhlt plane to the ground surface would produce a trace been possible to collect data fiom pre-existing geotechnical similar to that of Fili fault. investigations and define the local ground conditions. Additional estimates were obtained directly from the available The strong ground motion from the main shock of the Athens seismic recordings, based on the horizontal-to-vertical spectral Earthquake has been recorded by eighteen (18) ratio or HVSR. (Nakamura 1989, Dmitriou et. al. 1999, accelerographs, located as shown in the map of Fig. 1: Bonilla et al. 1997, Theodoulidis et al. 1996, Yamazaki and fourteen (14) within the central area of Athens and four (4) at Ansary 1997). In the present case, this technique is slightly the center of nearby towns of Rafina, Lavrio, Aliveri and modified in an attempt to improve the accuracy of predictions. Thiva. According to these records, peak ground accelerations More specifically, the normalized elastic response spectra (for within the city of Athens varied fiom 0.08g to 0.51g in the 5% damping) of the recordings were used instead of the horizontal direction and from 0.04g to 0.19g in the vertical Fourier spectra mostly used in the literature. This modification direction. To outline the basic features of recorded seismic will be referred briefly here after as Normalized Horizontal-to- ground motion, Fig. 2 shows the acceleration time histories Vertical Spectra Ratio or NHVRS. and the elastic response spectra of one typical recording (SPLB) obtained at the surface of stiff alluvial deposits, under It is noted that the site period estimated with NHVSR practically free field conditions. Observe the short apparent (TwsR) could be grossly verified against equivalent-linear duration and the relatively high frequency (low period) of the seismic ground response analyses based on the available motion. According to Tselentis and Zaradnik (2000), the fist geotechnical data (Bouckovalas, Kouretzis and Kalogeras, of these features supports an asperity-like model for the 2001). Consequently recording sites were classified in two source, characterized by a very short rise time (0.1 to 02sec) categories: and a nearly complete stress release, that led to the relatively high recorded accelerations. Furthermore, the second feature is (a) Rocksoft Rock formations, including all sites with interpreted as a Doppler effect created due to rupture TWsR between 0.08 and 0.22sec. The geological directivity. formations of this category are slightly to medim weathered phases of the Athens Schist (SGMA and ATH-04), metamorphosed schist and limestone (DMK STIFF SOIL AMPLIFICATION EFFECTS and LAIR), cohesive talus cones and medium-to-well cemented conglomerates (THVC) or neogene marls Analysis of strong motion recordings (RFN and ALIV).

It is fortunate that most of the recording sites lay in the vicinity of major public works, such as surface or underground SPL - 10.3 3 1 with distance fiom the fault rupture. These relations are shown in Figs. 3a and 3b for the peak horizontal and the peak vertical acceleration respectively. In each figure, the circles denote ground surface recordings while the triangles denote relevant analytical predictions obtained fiom subsurface recordings with the aid of equivalent linear 1-D wave propagation analysis. Furthermore, a distinction is made between the data points corresponding to Rock - Soft Rock and to Very StirSoil formations. The continuous curves drawn to show the average trend of the data points were produced by vertical translation of the attenuation relations proposed by Abrahamson and Silva (1997) for the hanging wall of shallow crustal earthquakes of M=5.9 magnitude and geological conditions similar to these of the recording sites.

I RockSoft Rock Observe that local soil conditions have relatively small effect on the vertical peak ground accelerations but differentiate grossly the peak horizontal acceleration values for the Rock- Soft Rock and the Very Stiff Soil formations. Namely, for a distance of 9-15 km fiom the rupture (Le. distance of most recording sites), the average attenuation curves for the latter

5

4 6 IO 30 60 f9 Distance from Rupture (km) %3 E Fig. 3. Attenuution of (a) horizontal and 0) vertical peak -% ground acceleration with distance JLom fault rupture, '/I2 for soft rock and stirsoil geologicalformations. 1 (b) Very Stiff Soil formations, including the remaining sites with TmsR between 0.24 and 0.32sec. In geological terms, the formations of this category include moderately 0 thick weathering products of the geological bedrock 5 (ATHA and KERA), alluvium deposits of medium to lligh density (ATH-02, ATH-03, PNT and SPLB) or ancient manmade fill (MNSA). 4

Note that the differences with regard to the seismic response characteristics of the above categories are not as significant as g3 their geological origin may imply. In fact, seismic code -% dehitions would not distinguish between the different sites '/I2 and categorize them in the same group. For example, all recording sites belong to Site Class C (soft rock / very stiff soil) of "-97. A possible exception applies only to 1 DMK and LAVR sites, which may be categorized as Site Class B (rock). 0 From Fig. 1, it may be observed that all recordings sites, 0.01 0.1 1 Period (sec) except from that in the town of Thiva, (THVC in the map of Fig. 1) lay to the east of the rupture zone and on the hanging Fig. 4. Range and mean value of normalized elastic response wall side of the fault. Furthermore, they correspond to a wide spectra (5% damping) @om suifiace strong motion range of fault distances (9 + 52 km) and consequently they recordings of the main shock on soft rock (Fig. 44 may provide evidence for the attenuation of seismic motion and stirsoil (Fig. 4b) geological formations.

SPL- 10.3 4 -1 00 u) 0) .r 75 .-P a a 50 cu 0) 25 e $0

0

20

h vE 5, 40 6

60

80

Fig. 5. Distribwion of structural damage and local soil conditions along a North-South section through municipality. formations yield values 40% higher than those of the former. Analysis of damage distribution This amplification is definitely unexpected since it comes from fairly stiff soils, which are often taken as seismic Additional evidence for stiff soil amplification of the seismic bedrock in practical applications. For example, according to ground motion is deduced indirectly, fiom the analysis of NEHRP 97, both geological groups belong to Site Class C and damages reported in the meizoseismal area of the earthquake. consequently they are assigned the same value of site The case study presented herein concerns the municipality of coefficient. An0 Liosia, at 1-3 km distance fiom the rupture zone (Fig. l), that was literally devastated during the earthquake: 35% of the The effect of local soil conditions on the fiequency content of buildings collapsed or had to be demolished, while another ground motion is analyzed in Figs. 4a and b. More 30% required major repair. specifically, Fig. 4a summarizes the normalized acceleration spectra (range and average) for 5% damping, obtained fiom all To assess the role of soil conditions in the irregular available ground surface recordings on Rock-So3 Rock and distribution of damages, a thorough geotechnical investigation Fig. 4b summarizes the same data for StzF Soil sites. The was carried out before reconshction of the area, including design spectrum for Class Site C of NEHRP 97 has been downhole measurement of shear wave velocity V, at fourteen added to both figures for comparison. In an attempt to (14) locations. Consequently, damages at different regions of minimize the effects of distance fiom the fault rupture, the municipality were correlated to the local soil conditions. A observations are performed on the basis of average normalized typical example is shown in Fig. 5, for a north-to-south section response spectrum. This exhibits a peak at a period passing through the center of the densely populated area. The approximately equal to 0.lOsec for Rock-Sop Rock sites and top part of this figure shows the percentage of buildings with 0.25sec for Very StzflSoil sites. In addition, the ratio of the severe (black bar), moderate (gray bar) and minor (white bar) two spectra suggests that Very StzifsSoiI sites deamplify the structural damages in seven (7) regions crossed by the section, lower period (T=0.03-0.17) and amplify the higher period while the lower part shows the associated geological and V, (T=O. 17-1.00sec) components of the seismic ground motion. profiles. In relative terms, the maximum de-amplification is about 35% and the maximum amplification is about 45%. Firstly, observe that subsoil is dominated by loosely to well cemented conglomerates up to a minimum depth of about

SPL - 10.3 5 5, To explore whether these dramatic changes can be attributed to the variation of soil conditions, the seismic ground response was estimated analytically for borehole sites CHT-2 (in the northern region of least damage) and DH-5 (in the severely damaged southern region). The analyses used three different, high fiequency excitations as input: two mainshock simulations of the ground motion at rock-like formations within the epicentral area (FYL, PAR) (Makropoulos et al. 2000), as well as, the mainshock recording at the rock site of Dimokritos (DMK) scaled to the peak acceleration of the simulated motion. The elastic response spectra of input excitations are shown in Fig. 6. Furthermore, Fig. 7 shows the ratios of spectral amplification for the two borehole sites, defined as the ground motion-to-input excitation ratio of elastic response spectra. It is evident that the seismic response 0.01 0.1 1 of the two borehole sites is indeed different, despite their Period (sec) similarity in terms of geology and shear wave velocity values. For borehole DH-5, soil amplification exceeds systematically Fig. 6. Elastic response spectra of input excitationspom the 50% for the period range of 0.00-0.40sec where most 1-D ground response analyses pe$ormed at borehole buildings of the area belong. On the contrary, soil sites CHT-2 and DH-5. amplification is negligible for borehole CHT-2, which behaves essentially as seismic bedrock.

TOPOGRAPHY EFFECTS

2 The evidence for topography effects stems essentially fiom observations that a large percentage of collapsed buildings were at close distance to the crest of relatively steep slopes of m" (I) 30-40m height. This evidence emerged vividly in the first days 2 1.5 following the September earthquake, and was substantiated (I) later, after completion of the damage recording. For instance, Fig. 8 shows the location of collapsed buildings, which are 1 associated with fatalities and have become the subject of a - 'CHTP thorough judicial investigation. - - IIIIIIIIIIIIIII

Fig. 7. Elastic response spectru ratio (ground su@ace over outcropping bedrock) rendeyed @om I-D ground response analyses of borehole sites CHT-2 and DH- 5.

25m, and by stiff mark at greater depths. Shear wave velocities are generally high, in excess of 500 dsec and show a clear increase with depth. The average shear wave velocity at the top 30m of depth (V,,,,,) ranges between 700 m/s and 500 mls, with a clear tendency to decrease gradually from the 0 1 2km northern regions of the municipality (borehole CHT-2) to the i? :Y southern (borehole DH-5). These formations could be considered as more or less uniform, at least in the premise of seismic codes (e.g. Site Class C of "-97). Still, the damage profile changes systematically: the percentage of buildings with severe damage increases fiom 3% in the north to 50-60% in the center-south, while the percentage of buildings with minor damage decreases fiom 75% to 5-10% Fig. 8. Locale of collapsed buildings (with fatulities) within respectively. the meizoseismal area of the Athens Earthquake.

SPL - 10.3 6 Hotel DEKELIA Vs (mkec) 0 400 800 0 400 800 0 400 800 0 400 800 0

10 I Section a-a 20

h 30 5 Adames Q +< 300 m- ;40

50

60

70 DEKELIA Adames Adames Adames Site I Site 2 Site 3

Fig. 9. Cross section across the ais of Kijhos river canyon Fig. IO. Typical shear wave velocity proJles at Dekelia and at (a) Hotel Dekelia and (6) Adames. Adames (sites 1,2 &3),

In fact, nine (9) out of the thirty-one (31) fatal building 3 collapses are aligned along Kifissos river canyon, formed in I] Section a-a I the northern-central part of the Athens basin.

To explore this coincidence, a thorough geotechnical investigation combined with 2-D seismic ground response analyses was undertaken for two locations along the canyon: the site of Hotel Dekelia (section a-a) and the site of Adames (section b-b) in Fig. 9. Athanasopoulos et al. (2001) and Kallou et al. (2001) present the detailed data for these studies in separate papers of this conference. In the following, part of 0.5 their findings is jointly recapitulated in an attempt to outline 3 the possible extent and the practical implications of topography effects. n 2-5 h7 Figure 9 shows the cross section of the topographic relief at $2 the above sites. At Dekelia Hotel, the canyon is V-shaped with 5 hCG 1.5 an approximate depth of 40m and 15" inclination of the cliffs m relative to the horizontal. At Adames, the riverbed is wider, 9. 1 and the east bank is considerably lower than the west bank which is the focus of interest of the relevant analyses. Here the 0.5 maximum depth of the canyon is also 4Om, but the cliff is 0.01 0.1 1 twice as steep. In both sites, the canyon is essentially cut Period (sec) within very stiff to hard pBo-Pleistocene formations (sandy clays, sandstones, conglomerates and marls) except fiom the Fig. 11. (2-D)/(l-D) ratio of elastic response spectra top 5-8 m consisting of recent terestrial deposits, mostly sandy predictedfor the crest of KiJisos river canyon clays with gravel.

The shear wave velocity (Vs)profiles at both sites are show Table 3. Attalytical predictions and seismic code provisions in Fig. 10. At Hotel Dekelia, the data come fiom SASW for topography ampIification (the respective period range is measurements performed at one site nearby the collapsed given in parenthesis). building, while at Adames they come fi-om crosshole measurements at three borehole sites: two at about 250 HOTELDEKELIA ADWS distance fi-om the crest (Adames-1 & 2) and another one at I I I I about 50m fiom the crest (Adames-3). The average shear I AFPS(1995) I 1.oo I 1.14 (wide) I wave velocities over the top 30 m of depth (VS,,,)at the above sites range between 365ds and 475m/q as in the lower shear I EC8 (2000) I >1.00+1.20 (wide) I 21.20 (wide) I wave velocity range for Site Class C (Very Stiff Soils) of 1.50 (T=0.20+0.50sec) 1SO (wide) NEW97. medictions

SPL - 10.3 7 Based on the wealth of analytical predictions presented in the do not take into account the frequency content of the seismic aforementioned studies, Fig. 11 shows the ratio of 2-D versus excitation and class@ the sites according to geological- l-D predictions of elastic response spectra for the crest of the geotechnical criteria alone. cliff. As a crude approximation, this ratio is used to isolate topographical from soil amplification and is directly (b) The second issue has to do with the topographical comparable to the Topographic Aggravation Factor computed amplification of the seismic motion at the crest of steep cli&. by Kallou et ai. (2001) in terms of the corresponding Fourier The evidence for this effect is indirect, based on the systematic amplitude spectra. concentration of severely damaged reinforced concrete buildings along the crest of Kifissos river canyon. The two Comparison of the two different studies shows clearly the independent theoretical studies reviewed in this paper seem to significance of the cliff inclination. In the case of Adames support the significant contribution of local soil and with 30” angle of inclination, the average topographic topography conditions to the distribution of damages, contrary amplification is about 50% for a wide range of periods (as to the provisions of seismic codes, which would predict a high as 1 sec). On the contrary, at Hotel Dekelia with 15” rather moderate effect. inclination, similar values of topographic amplification are computed for the period range 0.30 to 0.50 sec alone. Seismic In concluding this brief review on the Athens Earthquake, it codes are rather hesitant to account for topographical would be misleading to leave the impression that our amplification. For instance, Table 2 compares the analytical understanding is complete regarding the reasons that led to predictions to the provisions of two prominent seismic codes this unprecedented (for local standards) destruction. In fact, (AFPS, 1995 and EC8, 2000) for the site conditions of Hotel the opposite is true! There is a number of seismological and Dekelia and Adames. At best, code provisions provide a lower geotechnical issues where the answers are pending (e.g. bound of the computed factors. Kallou et al., 2001, Tselentis and Zaradnik, 2000, Bouckovalas et al., 2001). For example, how did the directional fault rupture and the complex tectonic regime of Athens basin affect the propagation of seismic waves? What is DISCUSSION AND CONCLUSIONS the appropriate seismic bedrock definition for more or less uniform geological formations extending to a few hundred The Athens earthquake (Ms=5.9) of September 7, 1999 bears meters depth? What is the role of the vertical component of at least two characteristics commonly present in most recent motion in irregular topographies or what is the topographic destructive earthquakes. It was one more example of rather aggravation for obliquely incident and surface waves? The moderate size seismic activity that causes destructive study of the Athens Earthquake is still in progress and there is consequences due to its close proximity to urban areas. In hope that some at least of these questions can be investigated addition, it was certainly unexpected, in the sense that it in more depth. originated fiom a previously &own seismotectonic structure hidden among other well-established active faults. Unfortunately, this is practically the case with many recent REFERENCES earthquakes in Greece [e.g. Kalamata 1986 (Ms=6.2), Pirgos 1993 (Ms=5.5), Aegion 1995 (Ms=6.2)], which have Abrahamson, N.A. and W. Silva 119971. “Empirical Response challenged conventional thinking and methods to predict Spectral Attenuation Relations for Shallow Crustal seismic risk. Earthquakes ”. Seismological Research Letters, Volume 68, From a geotechnical point of view, this earthquake will not be Number 1, JanuaryFebruary. remembered for any spectacular ground hilures (e.g. liquefaction, lateral spreading or landslides), as the two French Association for Earthquake Engineering [19951. practically concurrat earthquakes of Kocaeli (M~7.4)in Recommendations AFPS95 for the reduction of rules relative Turkey and Chi-Chi (M~7.3)in Taiwan. On the other hand, it to the structures and installations built in regions prone to provided a number of reliable strong motion recordings and earthquakes, Saint-Remy-les-Chevreuse, France. well deiined patterns of damage distribution at sites with known geological and geotechnical conditions. In this way, we Ambraseys, N. [1994]. “Mbterialfor the investigation of the could draw practical clues on two issues of practical interest: seismicity of CentraZ Greece” in Historical Investigation of the seismicity of European Earthquakes, Albini and Moroni (a) The first is that VeyStiffsoil formations within Athens (Eds), V012,l-10 basin have amplified the seismic motion by an average of 40% relative to Rock-Soft Rock formations. From a theoretical Athanasopoulos G. A., Pelekis P. C. & Xenaki V. C. [2001]. perspective this is certainly not surprising since the seismic “Topography Eflects in the Athens 1999 Earthquake: The excitation from the Athens earthquake was rich in medium-to- Case of Hotel Dekelia”. Proc. of 4fi Intern. Conf. on Recent high eequencies (3- 15 Hz), which were selectively amplified Advances in Geotechnical Earthquake Engineering and Soil by the stiff soil profiles. However, stiff soil amplification Dynamics, San Diego CA, March 28-3 1, (CD-ROM I). phenomena are rarely evidenced in actual seismic recordings and this may be one reason for being overlooked by modern seismic codes. A second possible reason is that current codes SPL - 10.3 8 Bonilla F.L., Steidl J.H., Lindley G.T., Tumarkin A.G., Marinos P., Bouckovalas G. D., Tsiambaos G., et al. [2001] Archuleta RJ. [1997]. “Site AmpliJcation in the San “Ground Zoning Against Seismic Hazard in Athens, Greece’’, Fernando Valley, CA: Variability of Site Effect Estimation Int. Journal of Engineering Geology. (accepted for using the S-wave, Coh and WV methods”. BSSA, Vol. 87, publication) pp. 710-730. Nakamura Y. [1989]. “A Method for enamic Characteristics Bouckovalas G. D., Kouretzis G. & Kalogeras Y. [2001]. Estimation of Subsurjhre using Microtremors on the Ground “Site-SpeciJc Analysis of Strong Motion Data fvom the Sur$ace”. RTRI, Quarterly Report, Vol. 30, No. 1, Japan. 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