The Athens (Greece) Earthquake of September 7, 1999: Preliminary Report on Strong Motion Data and Structural Response.
by Anastasiadis A. N., Demosthenous M., Karakostas C. H., Klimis N., Lekidis B., Margaris B.,
Papaioannou C. H., Papazachos C. and Theodulidis N.
Institute of Engineering Seismology and Earthquake Engineering (ITSAK)
P.O. Box 53, Finikas GR55102, Thessaloniki, GREECE
On September 7, 1999 at 14:56 local time (11:56 GMT) a strong earthquake with magnitude Mw=5.9, occurred close to the city of Athens in Greece. The coordinates of the earthquake were 38.09N, 23.63E and its focal depth, h» 11km. The fault plane solution proposed by Harvard University (f=114, d=45, ?=-73) indicate WNW-ESE trending, almost south-dipping normal fault.
More than 40 buildings collapsed, 143 deaths were reported while hundreds of injuries were attributed to the earthquake. The area affected by the mainshock belongs to zone II of the new Greek seismic code with an effective acceleration, ag=0.16g. The most serious damage caused by the earthquake was observed at the northern suburbs of Athens (Ano Liosia, Menidi, Metamorphosi, Thracomakedones etc.) that fall close to the epicentral area. The dominant construction systems in these suburbs are reinforced concrete frames and one or two-storey buildings with masonry walls. Most of the buildings were built according to the old code with seismic coefficient equal to 0.04, 0.06 and 0.08 for firm, medium and soft soils, respectively (Seismic zone I).
The Institute of Engineering Seismology and Earthquake Engineering (ITSAK) had installed long before the earthquake three analog accelerographs in the city of Athens, that recorded the mainshock. Moreover, within a few days another six digital accelerographs as well as a mobile multi-channel recording unit on a public building were installed. A preliminary damage investigation was also carried out in the strongly affected area.
Recorded accelerograms and response spectra
The earthquake of September 7, 1999, hit and caused serious damage to a number of northern suburbs of Athens close to the seismic fault. The most affected area by the mainshock is shown in Fig. 1, where Modified Mercalli intensity varies from VI to IX. Long before the mainshock three analog accelerographs (SMA-1 type), belonging to the permanent strong motion network of ITSAK, were installed, (i) in the Chalandri district (ground floor, 2-storey bldg, station code:ATH02), (ii) in the Kipseli district (groung floor, 3-storey bldg, station code: ATH04), (iii) in the Kallithea district (ground floor, 1-storey bldg., station code:ATH03), which recorded the mainshock. On September 8, another four digital accelerographs were installed in the Town-Hall of Metamorphosi suburb (basement, 2-storey bldg., station code:MET), in the Town-Hall of Menidi suburb (basement, 2-storey bldg., station code:MND), in the Town-Hall of Ano Liossia suburb (basement, 2-storey bldg., station code:ALS) and in the Town-Hall of Fyli village (basement, 2-storey bldg., station code: FYL). On September 16, another two digital accelerographs were installed, (i) in the Faran S.A. company (ground level, free-field, station code:FAR), (ii) in the Hand Ball field of Kamatero (ground level, shelter, station code:KAM). In addition, 6 channels of a mobile multi-channel recording system were also installed in the building of the National Telecommunication Organization (OTE) of Ano Liosia. The aforementioned deployment of the aftershock network was imposed by the limited number of available instruments and in order to acquire strong motion data close to the epicentral area where severe damage has been observed.
The accelerograms of the mainshock recorded at the aforementioned stations at epicentral distances of 15km to about 17km are shown in Fig. 2. The largest peak horizontal ground acceleration (PHGA) recorded at the ATH03 station, was 0.30g and appeared at a period of 0.25sec. The bracketed duration, that is, the time span between the first and the last peak with acceleration ³ 0.05g, was 5.5sec. The PHGA at the other two stations, ATH02 and ATH04, was 0.16g and 0.12g, respectively. Taking into account that at a distance of about 15km the observed peak ground acceleration was around 0.30g and using attenuation relation of peak ground acceleration proposed for the area of Greece (Theodulidis and Papazachos 1992) a normalized epicentral (R» 5km) peak ground acceleration of the order of 0.60g may be predicted.
In Table 1 (below), some parameters of the Athens 7/9/1999 earthquake in comparison with that of the Thessaloniki 20/6/1978 (THE78-1), Corinthos 24/2/1981 (COR81), Kalamata 13/9/1986, Kozani 1995 (KOZ95-1) and Aigion 15/6/1995 destructive normal faulting earthquakes in Greece, are given. A characteristic parameter of seismic motion which is included in this Table is the ratio of peak ground velocity to peak ground acceleration. This ratio is indicative of the strong motion frequency content and local soil conditions. According to Seed et al. (1976) values of this ratio up to 66± 7cm/s/g are indicative of rock soil conditions and up to 114± 18 of stiff soil conditions. Regarding the recording station ATH03, where the strongest peak ground acceleration was observed, its subsoil consists of very stiff to hard silty to sandy, marly clay and clayey marl with a depth ranging from 12m to 28m. This soil layer overlays the Athenian schist that could be considered, at a first stage, as the bedrock of the area. A preliminary estimate of the anticipated fundamental period of the soil layer would give values around 0.2sec, approaching thus the period at which the peak ground acceleration of 0.30g appeared. This agreement, although preliminary, indicates a possible influence of local soil conditions on strong ground motion.
In Fig. 3, the response spectra for 5% damping, of the horizontal components of the Athens 7/9/1999 mainshock are given. The acceleration response spectra of the ATH03 recording station exhibit amplitudes Sa>0.30g for a period range of 0.05sec to 0.70sec and reveal a peak of about 1.3g at 0.25sec. On the contrary the ATH04 station recording, results in acceleration spectral values of Sa£ 0.3g. Station ATH02 recorded acceleration spectral values of 0.3g£ Sa£ 0.6g for a period range 0.10sec to 0.30sec. A preliminary examination of the data shows that variation of strong ground motion is clearly observed although the epicentral distances seem to be comparable for the three accelerograph stations (about 15km). This fact gives an evidence of influence of local conditions on strong ground motion. However, source effects (e.g. directivity) could not be completely excluded since energy radiation pattern is not yet clear, expecting for detailed information on the fault mechanism and relevant source properties.
Table 1. Seismological and strong ground motion information on recent, normal faulting, disastrous earthquakes in Greece.
Place Date Mw R
Thessaloniki 20-6-78 6.4 29 0.15 16.7 3.4 111 6 0.13 II - 0.16 Corinthos 24-2-81 6.6 30 0.29 24.6 6.7 85 11 0.24 III- 0.24 Kalamata 13-9-86 6.0 12 0.27 32.3 7.2 120 4 0.28 III- 0.24 Kozani 13-5-95 6.6 19 0.21 8.8 1.5 42 7 0.14 I - 0.12 Aigion 15-6-95 6.4 18 0.54 51.7 7.5 96 6 0.43 III- 0.24 Athens-02
II - 0.16
(*) : Bracketed duration, time span between the first and last peak with ag³ 0.05g
(**) : Effective Peak Acceleration according to FEMA(1985)
(***): Seismic Code Zone - Proposed effective acceleration
Instrumentation of a public building Investigation of the dynamic behaviour and pounding effects under seismic excitations
One of the serious factors affecting the degree of damage of buildings during an earthquake is the pounding between adjacent buildings or structurally separate parts of the same building. Transfer of kinetic energy from one to the next (adjacent) building during impact, leads to substantial increases of inertia forces. Towards this end, a structural array consisting of six accelerometers and a data acquisition system was used for the instrumentation of a two-story, plus basement, public building (National Telecommunication Organization of Greece- Ano Liosia). The reinforced concrete building is separated in two parts by an expansion joint. Two accelerometers were installed at the basement, and the rest four at carefully selected places at the top of the building. The configuration of the installed array permits the investigation of the dynamic behaviour of the building during an earthquake, as well as the study of the pounding effect between its two separate parts (Photo 1).
Elastic design spectra and seismic code provisions
This earthquake, as it is apparent from the response spectra of the recorded motion, affected both low and middle-rise buildings (two to four-storeys) in the broader area of Athens. For buildings with five to ten or more storeys, corresponding to about T>0.4 sec, spectral accelerations decrease rapidly indicating that this particular event has not affected them severely. A comparison of the response spectra of the recorded motion with the elastic design spectra (soil category B soil conditions) of the New Greek Seismic Code based on ultimate strength design- indicates that the latter are quite lower at short periods (0.2sec<T<0.4sec) and much higher at longer periods (Fig. 3).
For the area of Athens the base shear coefficient, according to the 1959 Greek Seismic Code was e = 0.04, 0.06 and 0.08, for firm, medium and soft soils, respectively. This coefficient was constant, independently of the buildings period and applied uniformly to all type of buildings. Since the 1959 Code was based on the allowable-stress design method, the coefficient has been modified to account for a factor of safety 1.75, a 20% increase in allowable stresses for seismic design and a multi-degree of freedom effect expressed by the factor 0.85 (estimated for a 4 to 6-storey building) (Anagnostopoulos et al. 1987, Pitilakis et al., 1992). Thus, using a mean safety factor of 1.75/(1.20*0.85) = 1.72, the respective coefficients for ultimate strength design are found to be e' = 0.07, 0.10 and 0.14. Thus, the ductility demands imposed on the buildings of Athens, constructed according to 1959 code, were quite high (above 6). However, strength reserves of the existing buildings, that is their redundancy, over-strength of individual structural members and infill walls, contributed to a significant increase of their behaviour factor (Bertero 1989). In this way one can explain the survival of many buildings, although the spectral accelerations were high. According to the new Greek seismic code the city of Athens and its suburbs belong to the seismic zone II, with effective acceleration equal to 0.16g. The effective peak accelerations recorded, at least by ITSAKs strong motion network, were 0.11g, 0.14g and 0.25g for the stations ATH04, ATH02 and ATH03, respectively. Those values coincide almost to effective accelerations of the first three zones of the new Greek seismic code. The fact that almost all buildings, with exception of those very close to the epicentral area, performed satisfactorily in spite of high recorded effective peak accelerations, indicates at a first approach that the city of Athens reasonably belongs to the zone II of the new Greek seismic code. However, in the meizoseismal area where strong ground motion should be much higher (probable peak ground acceleration >0.50g) compared to the observed ones downtown, the degree of damage reached up to IX in modified Mercalli intensity.
Preliminary damage report
Almost all buildings of Athens, both in the center of the city and in the southwest- southeast part of it, did not suffer damage. Meizoseismal area was in Menidi, Ano Liosia and Chelidonou industrial area where modified Mercalli intensity reached values up to IX. In photo 2 collapse of a 2-storey RC building, with pilotis, in Ano Liosia is shown. In photo 3 failure of column, due to short column effect, of a 5-storey RC building in Ano Liosia, which was built in 1997 according to the new Greek seismic code, is shown. A mean modified Mercalli intensity around VIII, for the suburbs of Ano-Liosia, Menidi, Thrakomacedones and at the industrial area of Chelidonou, is estimated. Even though, for the time being, there are no detailed information on the factors which controlled the damage in these areas, there are evidences based on information collected during the fieldwork that site effects, among others, should have played an important role. In Fig. 1 sites with modified Mercalli intensity equal or greater than VIII for the north-northwest suburbs of Athens are depicted.
Significant attenuation of macroseismic intensity towards south was observed in the area of Halandri -Kipseli where modified Mercalli intensity reduces to about VI. A few collapses of buildings were observed at the suburbs of Nea Philadelpheia, Nea Erythrea and Metamorfosi, while in some cases damage was also observed on buildings which suffered minor damage due to the February 1981, Corinthos earthquake.
The Athens 1999 earthquake of moderate magnitude (Mw=5.9) hit the capital of Greece, caused heavy damage to both residential and industrial buildings and killed143 people. To our knowledge, from personal communication with other Greek Organizations that had also installed accelerographs in Athens, maximum recorded peak ground acceleration down town, at an epicentral distance R» 15km, was around 0.30g (with an exception at a particular site in Monastiraki where in one horizontal component peak ground acceleration of 0.53g has been recorded). Unfortunately, no accelerograph was installed neither at the northern suburbs of Athens nor at the industrial area close to the epicentral area. However, normalizing for distance one may predict maximum peak ground acceleration in the near field (R» 5km) of about 0.60g.
The small distance of Ano Liosia, Menidi, Fyli, Chelidonou, Zephiri, Kamatero, Thracomakedones and Metamorphosi from the epicentral region gave rise to strong ground motion which most probably produced high spectral accelerations. The duration of strong ground motion phase, however, was relatively short and judging from the damage distribution, its intensity attenuated rapidly. Had the strong motion lasted somewhat longer, the catastrophe would have been much greater.
Once again torsional effects, lack of infill walls in pilotis and lack of ductility of buildings played an important role in damage caused by the earthquake. In spite of the high recorded ground accelerations and resulting high spectral values, the overall damage was not as severe and widespread as one might have expected, despite the relatively poor design and construction practices applied in the past. Therefore it should be emphasized once again that conservative seismic design coefficients alone cannot by themselves ensure the safety of structures if they are not part of proper seismic design criteria and provisions.
Anagnostopoulos S., Rinaldis D., Lekidis V., Margaris V. and Theodulidis N., "The Kalamata, Greece, earthquake of September 13, 1986", Earthquake Spectra, 3, 365-402, 1987.
Bertero, V., "State-of-the-art report: Ductility based design", Proc. 9th World Conf. on Earth. Engin., V. VIII, Tokyo-Kyoto, Japan, pp. 673-686, 1989.
FEMA, NEHRP recommended provisions for the development of seismic regulations for new buildings, Build. Seismic Safety Council, Part 2-Commentary, Washington D.C., 1985.
Pitilakis K., Margaris, V., Lekidis, V., Theodulidis, N. and Anastasiadis, A., "The Griva northern Greece earthquake of December 21,1990," Europ. Earthq. Engin., 6, 20-35, 1992.
Seed B., Murarka R., Lysmer J. and Idriss M., "Relationships of maximum acceleration, maximum velocity, distance from source, and local site conditions for moderately strong earthquakes", Bull. Seism. Soc. Am., 66, 1323-1342, 1976.
Theodulidis, N. and Papazachos, B., "Dependence of strong ground motion on magnitude, distance, site geology and macroseismic intensity for shallow earthquakes in Greece: I, Peak horizontal acceleration, velocity and displacement," Soil Dyn. & Earth. Engn., 13, 317-343, 1992.