Slip Distribution of the 7 September 1999 Athens Earthquake Inferred from an Empirical Green’S Function Study by Z

Bulletin of the Seismological Society of America, Vol. 93, No. 2, pp. 775–782, April 2003

Slip Distribution of the 7 September 1999 Athens Earthquake Inferred from an Empirical Green’s Function Study by Z. Roumelioti, D. Dreger, A. Kiratzi, and N. Theodoulidis

Abstract The source process during the 7 September 1999 Athens (Greece) earthquake is investigated using broadband seismograms recorded at regional distances. Source time functions are estimated through an empirical Green’s function approach, and their shapes are inverted to reveal the spatial and temporal distribution of fault slip. The resulting slip-distribution pattern implies that about 50% of the total slip occurred at a depth greater than the hypocentral depth, indicating downward rupture propagation whereas 25% of the total slip was concentrated at a shallower patch. Forward calculations of the displacement field at the surface show that the shallower slip patch is possibly responsible for the asymmetry observed in the displacement field and it may have contributed significantly to the distribution of damage.

Introduction On 7 September 1999 a moderate-size earthquake (1990) for local earthquakes and extended to regional dis- (M 5.9) occurred within a few kilometers from the center of tances by Dreger (1994). The applied method involves es- the metropolitan city of Athens, the capital of Greece. De- timation of the source time functions (STF) of the mainshock spite its moderate size, the earthquake caused extensive dam- by deconvolving broadband body waves from a nearby age, mainly concentrated at the north–northwest suburbs of smaller event with similar focal mechanism (empirical the city. One hundred forty-three people were killed in 30 Green’s function). The resulting STFs are backpropagated collapsed buildings, both residential and industrial, and re- onto the fault plane to produce an estimate of the slip dis- ported injuries were as high as 2000 (Anastasiadis et al., tribution and kinematic parameters of the rupture. 1999; Papadopoulos et al., 2000; Gazetas, 2001). Although the earthquake triggered 15 accelerographs within 30 km from the epicenter, none of these instruments Data was installed within the meizoseismal area. Therefore, the actual strong ground motions in this area are only inferred The data used consist of broadband waveforms recorded from the distribution of the damage or other phenomena like by the Greek national broadband network, operated by the the toppling of heavy cemetery monuments (Gazetas, 2001). National Observatory of Athens. The stations of the network High levels of excitation, which can be attributed to are equipped with Lennartz LE-3D/20s sensors. At the time several factors, can only explain the extensive damage re- of the examined event, only the two horizontal components ported in the N–NW part of Athens. Among these factors, were recorded by these sensors, whereas the vertical com- local soil conditions and topographic relief are believed to ponent was recorded only by short-period sensors and was have signiﬁcantly contributed to the intensity and the geo- therefore not used in our study. These data were supple- graphical distribution of damage (Anastasiadis et al., 1999; mented by data from station SER, which is jointly operated Marinos et al., 1999; Bouckovalas and Kouretzis, 2001; by Charles University and the University of Patras. This sta- Gazetas, 2001). Furthermore, synthetic calculations of strong tion is equipped with a Guralp CMG-3T sensor and provided ground motion imply the occurrence of directivity effects the closest broadband waveforms of the Athens earthquake. toward the heavily affected area (Tselentis and Zahradnik, The azimuthal distribution of the stations used in this 2000; Roumelioti et al., 2002; Zahradnik and Tselentis, study, with regard to the epicenter of the examined event, is 2002). shown in Figure 1. In general, the station coverage has a In the present work we examine the contribution of the large bearing on the applicability of the method and espe- seismic source to the damage-distribution pattern of the Ath- cially on the ability to determine the relative amplitudes of ens earthquake. In particular, we investigate the source different subevents (Antolik, 1996). In the Athens main- process during the Athens mainshock, using a method of shock, the station coverage is satisfactorily good, because earthquake source inversion introduced by Mori and Hartzell the largest azimuthal gap is about 60Њ.

775 776 Z. Roumelioti, D. Dreger, A. Kiratzi, and N. Theodoulidis

RDO

KZN

JAN PRK

SER

ITM APE

ARG

Km VAM

Figure 1. Map showing the locations of the broadband stations used in the present study, with respect to the epicenter of the Athens earthquake.

Methodology In the following stage and prior to the inversion, the estimated STFs are normalized to unit area. This is done to In the frequency domain and in the case of a point avoid problems with amplitudes due to differences in the source, the far-field wave displacement, U(x) recorded at radiation patterns of the eGf and mainshock, and to ensure ␾ distance r and azimuth can be represented by that all of the STFs used integrate to the appropriate scalar seismic moment. This also tends to equalize the weighting .M˙ (x) G(x,r,␾) RI(x), (1) of individual STFs in the inversion ס (U(x The following step involves the inversion of the STFs, which are estimated as described previously. The inversion where M˙ is the moment rate function (source time function) technique (Mori and Hartzell, 1990) used is based on the of the event, G is the Green’s function response of the me- assumption that the variations in STF shape can be mapped dium along the wave path, which includes the effects of onto the spatial and temporal slip history of the event. The attenuation and geometrical spreading, R is the radiation pat- source is parameterized through a radially propagating rup- tern factor, and I is the response of the recording instrument. ture front, which expands with a constant rupture velocity. To retrieve the desired source time functions from the dis- Slip is confined to one of the nodal planes indicated by the placement records one has to remove the effects of the prop- focal mechanism of the event. Then the inverse method fits agation path and the recording instrument. A simple way to the STF of the examined event by summing contributions accomplish this is by deconvolving the waveforms of a from different subfaults, taking into account the time delay nearby smaller event with similar focal mechanism, herein due to wave propagation and to the propagation of the rup- referred to as an empirical Green’s function (eGf). In this ture front, with respect to the hypocenter. Distances to the approach the small event is assumed to be a point source in different subfaults are estimated using a half-space ray tra- both time and space, and to the extent that this assumption jectory approximation. The contribution from each subfault does not hold, the STF of the target event will be a relative can take the form of several synthetic time functions. Here estimate to the STF of the small event. Therefore, the eGf the subfault STFs have the form of boxcar functions. The should be small enough to be treated as a point source, but optimal values for the fault orientation, the rupture velocity, also large enough to ensure a satisfactory signal-to-noise and the dislocation rise time are found by performing a series level at the examined distances. of inversions testing the parameter space. Slip Distribution of the 7 September 1999 Athens Earthquake Inferred from an Empirical Green’s Function Study 777

The subfault STFs(B) are related to the observed STFs presented in Table 1, along with the parameters estimated (D) through a system of equations of the form for the mainshock from several authors. In Figure 2 we com- pare the horizontal displacement seismograms at three dif- m ferent stations for the mainshock and the chosen aftershock. מ ס ס Di (t) STFi (t) ͚ Bjijj(t s )w (2) Even though the waveforms of both the small and large j event show considerable complexity along the different regional paths, the aftershock seems to be satisfactorily iden- where s is the time delay due to wave and rupture propa- tical to the target event within the examined frequency range. gation, i is a station index, j is a subfault index, and w is a The source time functions of the Athens mainshock weight proportional to fault slip. In this system of equations were estimated using the broadband waveforms from the we also impose a positivity constraint to require all subfaults stations depicted in Figure 1. The original broadband data to have the same slip direction. A spatial derivative mini- were integrated to displacement and bandpass filtered in the mization constraint is applied to smooth the resulting slip frequency range 0.05–1.0 Hz. A 1% water level (Clayton model. As a result, equation (2) can finally be written in and Wiggins, 1976) was used to stabilize the deconvolution matrix form as process. To enhance the quality of the output, in most stations we independently used each one of the two horizontal BD .components and stacked the resulting source time functions (3) ,΅΄ ס w΅΄ kS 0 In this way, the required STF is better enhanced through a reduction of the background noise level. An example of the where S is the matrix of first spatial derivatives and k is a stacking procedure at one of the examined stations (APE)is constant controlling the weight of the smoothing equation. depicted in Figure 3. The slip weight vector is obtained by standard least The resulting STFs, normalized to unit area, for each of squares. Slip amplitudes at each subfault, uj, are finally ob- the nine stations used are shown in Figure 4. The distinctive tained on the basis of an independent seismic moment esti- differences among the shapes and amplitudes of the STF al- mate, M0, through the relation low us to draw some first conclusions with respect to the emergence of directivity effects. Source time functions at Mw0 j east-southeast (e.g., stations APE, ARG, PRK) present larger ס uj , (4) Al amplitudes and shorter durations (2–3 sec), which indicate propagation of the rupture in this direction. On the contrary, where A is the subfault area and l is the shear modulus, STFs estimated at west-northwest stations (e.g., KZN, JAN, -Pa. SER, etc.) present considerably lower amplitudes and dura 1010 ן usually taken equal to 3.5 tions as long as 4–5 sec. Application For the inversion of the STF shapes we used a planar .km discretized into 1-km2 subfaults 25 ן fault model of 25 The choice of the appropriate STF in the study of the The dimensions were chosen to be larger than those expected Athens mainshock was based on the published focal mech- for an M 5.9 earthquake to avoid errors at the boundaries of anisms (Papadimitriou et al., 2000; Papadopoulos et al., our model. The fault plane was oriented relative to the hy- 2000; Papazachos et al., 2001; Louvari and Kiratzi, 2001; pocenter at a depth of 8 km (Papadimitriou et al., 2000). Zahradnik 2002), and in addition, on the direct comparison Nevertheless, we ought to mention that significant discrep- of the mainshock waveforms with the waveforms of the larg- ancy exists among the published epicentral coordinates and est aftershocks. An aftershock of M 4.5 was identified as focal depths (Papadopoulos et al., 2000; Papazachos et al., suitable for our study. The parameters of this aftershock are 2001; Papadimitriou et al., 2000; USGS); therefore, our re-

Table 1 Source Parameters for the Athens Mainshock and the Aftershock Used as Empirical Green’s Function

Date Time Latitude Longitude Depth M0 Strike/Dip/Rake Nm) (Њ) Reference 1017 ן) Event (dd/mm/yy) (h:m:s) (ЊN) (ЊE) (km) M

(Papadopoulos et al. (2000 90מ/Main 07/09/99 11:56:51 38.08 23.58 16.8 5.9 113/39 USGS 84מ/123/55 7.8 9.0 23.55 38.13 CMT catalog 73מ/114/45 12.0 15.0 23.71 38.02 (Papadimitriou et al. (2000 80מ/105/55 17.0 8.0 23.57 38.11 38.06 23.57 14.5 Papazachos et al. (2001) (Louvari and Kiratzi (2001 80מ/115/57 9.22 10.0 (Zahradnik (2002 84מ/112/61 (Papadopoulos et al. (2000 30מ/eGf 7/9/99 20:32:27 38.12 23.63 18.2 4.5 105/45

Results from several researchers are listed for the mainshock. 778 Z. Roumelioti, D. Dreger, A. Kiratzi, and N. Theodoulidis

RDO Station APE 0.04 0.00 Component EW 2 -0.04 Mainshock EW 10 0 0 -10 Egf EW

cm 0.04 4 0.00 Component NS -0.04 Mainshock NS 2 1 0 0 -1 Egf NS [Mo]egf/s 0 50 100 150 200 250 -2 Seconds Stack 2 APE 0 0.05 0.00 -0.05 Mainshock EW 0 5 10 15 20 4 seconds 0 Egf EW Figure 3. Source time functions at station APE -4 derived from the deconvolution of the east–west (up- 0.05 0.00 per part) and north–south (middle part) EGF com- -0.05 Mainshock NS ponents from the mainshock are compared to their 4 stack (lower part). 0 -4 Egf NS 0204060 80 100 120 140 Seconds Although the rupture velocity and the dislocation rise time are ﬁxed during the inversion procedure, several values JAN were tested to reveal the optimum combination. The values examined lie within the range from 0.1 to 1.0 sec for the 0.02 0.00 dislocation rise time and from 1.6 to 3.4 km/sec for the rup- -0.02 Mainshock EW ture velocity. Although the applied methodology, as has 2 been shown repeatedly for other events, is able to identify 0 the causative fault plane from the two possible nodal planes, -2 Egf EW as well as the rise time and rupture velocity, the magnitude

cm0.02 cm 0.00 is relatively small for the Athens event and therefore the -0.02 Mainshock NS source dimensions do not allow a clear distinction. We were 2 only able to identify a shallow minimum in variance for a rise 0 -2 Egf NS time of 0.4 sec and a rupture velocity of 2.5–2.7 km/sec. The 0 50 100 150 200 250 300 slip weights, derived from the STF inversion, were scaled ן ס Seconds based on the value of seismic moment (M0 9.22 1017 N m) estimated by Louvari and Kiratzi (2001) and the Figure 2. North–south and east–west component resulting slip-distribution pattern is presented in Figure 5. km region of 10 ן displacement seismograms, corresponding to three The activated area is conﬁned to a 10 different regional paths, are compared for the Athens mainshock and the selected aftershock (EGF). the modeled fault surface, but the rupture seems to have stopped at a depth of ϳ5 km below the ground surface. This in part explains the absence of clear surface ruptures (Pav- sults are subject to the error included in the employed lides et al., 2002) and is in good agreement with the results location. In more detail, the proposed epicentral locations from studies regarding the aftershock distribution (e.g., Pa- differ from each other from 3 to 16 km, whereas the hypo- pazachos et al., 2001). The average slip across the part of central depth ranges from 8 to 17 km. However, this uncer- the fault model that ruptured is about 16 cm, a value that is tainty affects the absolute location of slip and not its hypo- consistent with the values estimated from empirical relation- center-relative position, because the examined stations are ships applicable to Greece or globally (e.g., Papazachos and located at regional distances, and therefore, the differences Papazachou, 1997; Wells and Coppersmith, 1994) and close in the hypocenter parameters are not large enough to alter to the value determined by spectral analysis of teleseismic their relative position with respect to the fault plane. data (Louvari and Kiratzi, 2001). Locally the slip appears to Slip Distribution of the 7 September 1999 Athens Earthquake Inferred from an Empirical Green’s Function Study 779

1 1 1 RDO - 026° APE - 122° SER - 287°

0 0 0 1 1 1 PRK - 060° VAM - 169° JAN - 308° x [Mo]egf/s 0 0 0 1 1 1 ARG - 115° ITM - 237° KZN - 329°

0 0 0 0123 45 0 12345 0 1 2 3 4 5

Seconds Figure 4. Far-field source time functions obtained by deconvolving the broadband waveforms of an M 4.5 aftershock from the corresponding waveforms of the Athens mainshock. The STFs are normalized to unit area. The name and azimuth of each station is noted on the top right. be significantly larger and the peak value (ϳ95 cm) is ob- field at the surface by successively examining the effect of served very near the hypocenter location. the entire slip model and the contributions of the shallower In general, the slip appears to be concentrated in two slip patch and the deep slip patch in the overall field. The patches. The prevailing slip patch concentrates about 50% resulting displacement maps are shown in Figure 6, along of the total slip and is located at a depth greater than the with the interferogram of Kontoes et al. (2000), which is hypocentral depth, indicating downward propagation. An- shown for comparison. In the latter work, two concentric other 25% of the total slip appears at shallower depths (ϳ5– fringes, corresponding to 56 mm in slant range, were ob- 8 km, taking into account the fault dip) and indicates a strong served in the meizoseismal area. The two fringes present an directivity toward east-southeast. Although damage is usu- asymmetry at the E–SE of the affected area, where most of ally associated with areas of high ground velocity or accel- the damage was reported. As can be seen in Figure 6b, the eration, this observation might be of great significance, be- synthetic displacement field deduced from the estimated cause most of the severely affected areas were reported close slip-distribution model also shows an asymmetry toward this to the east edge of the fault. The remaining 25% of the total direction. This anomaly is connected to the shallower slip slip appears as areas of very low slip around the two main patch (Fig. 6c), because the deep slip patch results in a per- patches. fectly symmetric displacement field at the surface (Fig. 6d). The absolute values of the synthetic displacements are also Forward Modeling to Calculate in good agreement with the results of Kontoes et al. (2000), the Displacement Field although this greatly depends on the employed value of the scalar seismic moment, which is used to scale the subfault The estimated slip pattern was tested in terms of its slip. capability of explaining the distribution of damage during the Athens mainshock. This test was performed through for- Conclusions and Discussion ward calculations of the displacement field at the surface and comparison of the resulting pattern with the displacement We present results of the source process during the 1999 field estimated from interferometric data by Kontoes et al. Athens earthquake and the effect that this process had on the (2000). The forward calculations were performed using the distribution of damage. The source was investigated through code of Kaverina et al. (2002), which includes subroutines a source time function inversion technique, which revealed by Okada (1985). a concentration of slip in two patches. Most of the slip We performed three calculations of the displacement (ϳ50% of the total slip) occurred at a depth greater than the 780 Z. Roumelioti, D. Dreger, A. Kiratzi, and N. Theodoulidis

Distance along strike (Km) (Papazachos et al., 2001; Gazetas, 2001). In particular, our NW SE 0 results confirm the assumption that the Aegaleo Mountain Range at the southeast of the activated area may have acted as a geometric barrier to the downward propagation of the rupture at the eastern part of the seismogenic fault. This -5 might have caused a confinement of the slip at shallower depths, which acted catastrophically at the east-northeast part of the deformed area. 50 -10 On the other hand, most of the seismic energy was re- leased uniformly, within a much larger area, as a result of 50 the downward propagation of the rupture in the deep-slip -15 patch. The downward propagation of the rupture is of great 50 interest because other studies related to the Athens earthquake imply upward propagation (Papadopoulos et al., Distance along dip (Km) 2000; Papazachos et al., 2001). Their assumption is basically -20 related to the depth of the mainshock hypocenter, which appears to be below the zone of aftershock concentration. Our attempts to estimate the slip model on the basis of larger -25 depth values (compared with the finally employed value of 0 5 10 15 20 25 8 km) resulted in a dislocation of the entire model to greater depths, without significant changes in the slip–hypocenter relative position. In these models, most of the slip appeared 0102030405060708090 at unrealistically great depths (20–25 km), compared with Slip (cm) what is commonly believed for Greek earthquakes (Papa- zachos and Papazachou, 1997). Furthermore, the corre- Figure 5. Slip distribution during the Athens sponding synthetic surface-displacement fields were also earthquake obtained from the STF inversion using a rupture velocity of 2.7 km/sec and a rise time of 0.4 shifted toward the South, deviating from the interferometric sec. Contours are for 10 cm of slip. Star denotes the observations. These results comprised our basic restrictions hypocentral location, and dotted frames the areas of regarding the choice of the finally used value for the hypo- maximum concentration of slip. central depth. In conclusion, the 1999 Athens mainshock can be re- hypocentral depth, whereas a significant amount (25%) was garded as another demonstration of the impact that the concentrated at a shallower patch, which nevertheless did source process can have in the near field of a seismogenic not reach the surface. The activated area was confined to fault, even in cases of earthquakes of moderate magnitude. about 100 km2, and a maximum variance reduction between Such events impose the need for a deterministic inclusion of the synthetic and the observed STF was obtained for a rise the seismic source in the simulation of future strong ground time of 0.4 sec and a rupture velocity close to 2.7 km/sec. motions. However, we should not ignore our incapability of The credibility of the estimated slip model was tested predicting the behavior of a seismogenic fault in a future through forward calculations of the surface-displacement earthquake and therefore the difficulty in deterministically field and comparison of the results with interferometric data. modeling the slip distribution of a future event. Recent at- The asymmetry in the observed displacement field (Kontoes tempts to identify systematic features of slip models for use et al., 2000), which is connected to the irregular distribution in the prediction of strong ground motions (Somerville et of damage in the epicentral area, was successfully repro- al., 1999) can be considered as optimistic steps toward the duced in the synthetic field. Taking into account the inability particular problem. Nevertheless, the number of such studies of the methodology used to reveal small-scale characteristics is still relatively small and does not ensure the identification of the slip history, the reproduction of the anomaly in the of systematic characteristics in the slip patterns. Especially shape of the simulated displacement field is supportive of in Greece, studies regarding the earthquake source processes the intensity of the modeled directivity effects toward east- are extremely limited (to our knowledge, the only relevant southeast. work is that of Meyer et al., 1998) considering the plethora The investigation of the contribution of each one of the of earthquakes. This can, in part, be attributed to the fact two prevailing slip patches to the overall surface-displace- that broadband data are only recently available. We believe ment field revealed that the shallower slip patch is probably that the national broadband network, which is now in full responsible for the asymmetric deformation of the affected operation by National Observatory of Athens, will greatly area. The channeling of the energy toward the East can be improve our understanding of the role of the earthquake attributed to the morphological characteristics of the area source to the ground motions. Slip Distribution of the 7 September 1999 Athens Earthquake Inferred from an Empirical Green’s Function Study 781

Figure 6. (a) Coseismic interferogram estimated by Kontoes et al. (2000). (b) Syn- thetic surface-displacement field for the Athens earthquake obtained using the slip- distribution model of Figure 5. (c) Synthetic surface-displacement field for the Athens earthquake produced by the shallower slip patch depicted in Figure 5. (d) Synthetic surface displacement field for the Athens earthquake produced by the deep slip patch of Figure 5.

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