Toward a Homogeneous Moment-Magnitude Determination for Earthquakes in Greece and the Surrounding Area

Transcription

1 Bulletin of the Seismological Society of America, Vol. 87, No. 2, pp , April 1997 Toward a Homogeneous Moment-Magnitude Determination for Earthquakes in Greece and the Surrounding Area by B. C. Papazachos, A. A. Kiratzi, and B. G. Karacostas Abstract A strategy is presented in order to produce a statistically homogeneous catalog of historical and present-century earthquakes of the Aegean area by expressing the size of all these earthquakes in the moment-magnitude scale. Records of the Mainka and Wiechert intermediate period (T o -- 4 sec) seismographs in Athens have been used to calculate the magnitudes of earthquakes that occurred in Greece and its surrounding area (Albania, S. Yugoslavia, S. Bulgaria, and W. Turkey) during the whole instrumental period (since 1911). This magnitude, M, has also been used to derive empirical relations with macroseismic data. It is shown here that M is equivalent to moment magnitude, Mw, for a wide range of earthquake sizes (5.0 _-< M <_-- 8.0), and to the surface-wave magnitude, M s, for large earthquake sizes (6 <_- M -<_ 8.0). It is also shown that the local magnitude, ML, for earthquakes in Greece is half a unit smaller than the moment magnitude in the low-magnitude range (4.5 _-< M/. =< 6.0). This could be attributed to a magnification smaller than the nominal for the Wood-Anderson seismographs in Greece (V ). Relations are also proposed to calculate moment magnitude when other magnitudes (M, AlL, Ms, and m9) are known for earthquakes in Greece. Introduction The most commonly used measure of the "size" of an earthquake is the earthquake magnitude. Because of the numerous combinations of wave types and recording instruments, any one earthquake can have a number of magnitudes assigned to it. All the magnitudes, however, stem from the work of Richter (1935), who defined the local magnitude scale, AlL, on the basis of the trace amplitudes of earthquakes recorded by Wood-Anderson seismographs. Since that time, other magnitudes have been defined using records from other types of seismographs and other wave types. Briefly, the surface-wave magnitude scale, Ms (Gutenberg, 1945), is based on the ground amplitudes of surface waves with period of about 20 sec (17-23 sec) at distances between 15 and 130, the body-wave magnitude scale, mb, is based on the ground amplitudes of body waves with period of about 1 sec recorded at long distances, and the moment-magnitude scale, M w (Hanks and Kanamori, 1979), is based on the scalar seismic moment, M 0. For the Aegean Sea and its surrounding area, relations have been derived to calculate the surface-wave magnitude of earthquakes using records from intermediate-period seismographs (Papazachos and Vasilicou, 1967; Papazachos and Comninakis, 1971). Empirical relations have also been derived for the calculation of local magnitude (Kiratzi, 1984; Kiratzi and Papazachos, 1984, 1986) and of duration magnitude, M D (Kiratzi and Papazachos, 1985; Papanastasiou, 1989). Relations between magnitudes in different scales (M s, ML, rob, and Mw) have also been proposed (Kiratzi, 1984; Kiratzi and Papazachos, 1984, 1986). Formulas are also available to calculate the magnitude of strong shocks using macroseismic data (Galanopoulos, 1961; Drakopoulos, 1978; Tassos, 1984; Papaioannou, 1984; Papazachos, 1992). Seismicity studies require homogeneous earthquake catalogs, that is, catalogs in which earthquake magnitudes are in the same scale and preferably in the seismic momentmagnitude scale, M w. Unfortunately, seismic moment magnitudes are available for only a limited number of strong recent earthquakes. For this reason, empirical relations are proposed in this article to easily estimate moment magnitudes for earthquakes in Greece that are recorded by the national network or for which macroseismic data are available. The records used are from the Wiechert and Mainka seismographs that have been in operation in Athens since 1911, as well as from Wood-Anderson seismographs and other short-period seismographs installed during the last three decades in Greece. In this article, the symbols ML and Mw are used to denote original local magnitude (calculated from the records of Wood-Anderson seismographs) and original moment magnitude (calculated from the scalar seismic moment), respectively. When such magnitudes are calculated through empirical relations, from other kinds of records (from 474

2 Toward a Homogeneous Moment-Magnitude Determination for Earthquakes in Greece and the Surrounding Area 475 Wiechert and Mainka intermediate-period seismographs, from short-period seismographs, or from macroseismic data), the symbols M*, and M* are used. Magnitude Based on Records from the Wiechert and Mainka Seismographs The magnitude M of a shallow earthquake (h -< 40 km) that occurs in Greece and its surrounding area and is recorded by the Wiechert (T O = 5 sec, critical damping e = 5, V = 180) or Mainka (To = 3 sec, critical damping e = 4, V = 60) seismographs at the seismological station of Athens is calculated using the following relation: M = log a log A (1) (Papazachos and Vasilicou, 1967), where a is the average ground amplitude (in/zm), as inferred from the maximum recorded amplitude at the two horizontal components (N-S, E-W), and A (<600 km) is the epicentral distance from Athens. Furthermore, Papazachos and Comninakis (1971) derived the following relation to calculate the magnitude M of intermediate-depth Greek earthquakes (40 km < h =< 180 krn) (Benioff zone of southern Aegean Sea): M = log a R/ , (2) where a (in/zm) is the same as in relation (1) and R (in km) is the hypocentral distance from Athens. The coefficients in relations (1) and (2) were determined from a regression analysis with surface-wave magnitudes, Ms, calculated from surface waves recorded at distant stations (Uppsala, Pasadena, Berkeley). New data collected afterward have shown the effectiveness of relations (1) and (2) to calculate surface-wave magnitudes of strong earthquakes with magnitudes between 6 and 8 (Kiratzi, 1984; Kiratzi and Papazachos, 1984; Kiratzi, 1989). Therefore, for this magnitude range, it is M= M s, 6.0 <M s <- 8.O. (3) It has been further shown that for earthquakes in Greece (Papazachos, 1989), the magnitude M is equivalent to moment magnitude, Mw, for a wide range of magnitudes; that is M= Mw, 5.0--<M_--<8.0. (4) For this reason, although relations (1) and (2) have been derived using data for strong earthquakes (M s >- 6.0), they have been routinely used to calculate magnitudes M for smaller events also. In this article, we present additional data that strongly support relation (4). Macroseismic Magnitude Using macroseismic information for the shallow strong earthquakes that occurred in Greece since 1950, the following relation has been determined: M = log R R (5) (Papazachos, 1992), where I is the macroseismic intensity and R is the hypocentral distance in kilometers. Macroseismic intensities, which correspond to the Modified Mercalli scale, have been estimated mainly using the extensive information collected by the National Observatory in Athens. Relation (5) has been used to calculate the magnitudes of the strong shallow historical earthquakes in Greece (550 B.C. to 1910). Relations have been also proposed for the determination of the magnitudes of the intermediate-depth historical strong earthquakes of the southern Aegean Sea (Tassos, 1984; Papaioannou, 1984). Local Magnitude, M L Local magnitude is based on records of Wood-Anderson seismographs [To = 0.8, static magnification V = 2080 _+ 60, (Uhrhammer and Collins, 1990)]. A Wood-Anderson seismograph is in operation in Athens since 1964, and another one was also in operation for some time at the station of Valsamata (western Greece). Records from these seismographs have been used to determine the attenuation function applicable to Greece (Kiratzi, 1984; Kiratzi and Papazachos, 1984). Since January 1981, the Geophysical Laboratory of the University of Thessaloniki operates in northern and central Greece a regional network (THENET) of 12 seismological stations with well-calibrated instruments. Each station is equipped with three-component (N-S, E-W, Z) short-period seismographs (Teledyne-Geotech type). Ground amplitudes recorded by these seismographs are used to determine the local magnitude, M*, by the following relation: M* = log a log R Cs, 3.0 =< My <-- 6.0, (6) where a (in/zm) is the ground amplitude as inferred from the maximum recorded amplitude, R is the hypocentral distance, and C s is a station correction (Kiratzi, 1984; Kiratzi and Papazachos, 1986). The coefficients in relation (6) were derived from a regression analysis with ML determined from the records of the Wood-Anderson seismographs at the stations of Athens and Valsamata, and the standard deviation is A value of M* is calculated for each THENET station, and the average of these values and of the value calculated from the Wood-Anderson seismograph in Athens is adopted as the ML* magnitude for each earthquake. Signal duration, D (in sec), is also used to calculate local magnitude from THENET records by the following relation:

3 476 B.C. Papazachos, A. A. Kiratzi, and B. G. Karacostas M~ = MD = Co + C1 log D A, (7) 3.0 _-< ML <= 6.0 (Kiratzi, 1984; Kiratzi and Papazachos, 1985), where Co is the station correction, and A is the epicentral distance in kilometers. C1 is equal to 1.97 when the duration is measured from the P-wave onset up to the time when the peak-to-peak amplitude becomes 2 mm, while C1 is equal to 2.31 when the total duration is measured from the P-wave onset up to the noise level. Empirical relations have also been derived to calculate local magnitude for earthquakes in Greece from accelerograph records (Hatzidimitriou et al., 1993). -J Data from ML = 0.58Ms+2.14, N=395, rms=0.19 ~lqw mo q,l', ~ql-- qll o* Relations between Different Magnitudes Here we consider several earthquake data sets used to establish the relation between several types of magnitudes. Data from 395 earthquakes that occurred in Greece and its surrounding area between 1981 and 1993 for which both original Ms magnitudes (from ISC or NEIS) and original ML magnitudes (from the Wood-Anderson seismograph in Athens) are available. All the data are shown in Figure 1, which also shows the data after a moving average analysis was applied. The relation that fits the data in a least-squares sense is 3.5 ta 3.0 6,0 -- 5,5 ~5.0 _a Ms (ISC,NEIS) AlL = (0.58 _+ 0.02)M s _+ 0.07, (8) 3.0 _-< M s <--_ 6.0, with a standard deviation equal to This relation is similar to the one suggested by Ambraseys and Bommer (1990), which holds for 3.0 -<_ ML < For 245 earthquakes that occurred in Greece during the period 1969 to 1987, we calculated M from the records of the Mainka or Wiechert seismographs (relations 1 and 2), and ML from the records of the Wood-Anderson seismographs following the relations of Kiratzi and Papazachos (1984). Figure 2 shows the data and the relation that fit the data in a least-squares sense: M = ( )ML , (9) 3.0 =< M L =< 6.0. The standard deviation is equal to Relation (9) gives results similar to those obtained using a simple relation (M = ML + 0.5) that is routinely applied in Greece to calculate M when ML is available (Kiratzi, 1984; Latoussakis, 1984). Using data from 1412 earthquakes that occurred in Greece and its surrounding area during the period 198t to 1993, the following relation between the body-wave magnitude, mb, reported by NEIS or ISC, and the ML magnitude (from the Wood-Anderson seismographs) was derived: Dg Ms ( ISC, NEIS) Figure 1. Local magnitude, ML (from Wood-Anderson records, Athens) versus surface-wave magnitude, Ms, (from ISC or NEIS) for 395 earthquakes that occurred in the Aegean area during the period 1981 to 1993 (upper part: original data; lower part: the same data after a moving average analysis is applied). mb = (0.76 _ 0.04)M L , (10) < ML < The standard deviation is equal to The data are shown in Figure 3, which also shows the data after a moving average analysis was applied. Determination of Moment Magnitude for Earthquakes in the Aegean Area Using Local Data Moment magnitudes, Mw, calculated using original data (seismic moment values) are available for only about 200 earthquakes in Greece. For all other known earthquakes in this area, values of M (relations 1, 2, and 5) and ML (from Wood-Anderson records or through relations 6 and 7) are available. However, for the compilation of homogeneous catalogs and other purposes, it is necessary to have all mag-

4 Toward a Homogeneous Moment-Magnitude Determination for Earthquakes in Greece and the Surrounding Area [ 6.5L- '"[ E,'-..F_ e,,,i 4.5 ~- ~"*el Data from L o _ l ~ M = 0.97ML+ 0.58, 4.0 ~,dt~e"" N=245, rms= ~ "re 3. o ~ ML _ [ mb ML Figure 2. Magnitude M (from Wiechert or Mainka seismographs) versus local magnitude, M r (from Wood-Anderson seismographs) for 245 earthquakes that occurred in the Aegean area during the period 1969 to m b nitudes in the same scale and preferably in the momentmagnitude scale. The reported magnitudes of all earthquakes in Greece recorded by the Wiechert or Mainka seismographs (since 1911) are in the M scale (relations 1 and 2), and this is also the case for all historical earthquakes (relation 5). This magnitude, however, is equivalent to moment magnitude, Mw (relation 4). Therefore, moment magnitude, M*, for earthquakes in Greece can be estimated by M~v=M, < M < , (11) where M is calculated by relations (1), (2), and (5). The magnitudes of all earthquakes in Greece recorded by the Wood-Anderson seismographs (since 1964) or by other short-period seismographs (relations 6 and 7) are in the M L scale. Since this magnitude is related to M (relation 9) and since M is equivalent to M w (relation 11), the following relation can be used to estimate the moment magnitude: M*= 0.97M L , 4.5_--<M L_-<6.0 (12) when ML (or M~) is known. Relations (11) and (12) are very convenient because they can be used to express all magnitudes in the seismic moment scale and to obtain homogeneous catalogs. It is necessary, though, to further check the validity of these relations and estimate their uncertainties using originally determined moment magnitudes. Table 1 lists all earthquakes known to us (169 events) that occurred in Greece and its surrounding area since 1963, ( H L Figure 3. Magnitude m b (reported by NEIS or ISC) versus local magnitude, ML (from Wood-Anderson seismographs), for 1412 earthquakes that occurred in the Aegean area during the period 1981 to 1993 (upper part: original data; lower part: the same data after a moving average analysis is applied). for which the seismic moment, M o, has been calculated. All listed Mo values (in dyne-cm) are from the Harvard catalog for the period 1981 to 1995, while for the period 1963 to 1980, M o values are the average of several published values (referenced in Table 1). The Mw, listed in the same table, is the moment magnitude calculated by the known formula of Hanks and Kanamori (1979). The M* magnitudes, listed in the same table, have been determined by relations (1) or (2) and (11) for the period 1963 to 1980 and by relation (12) for the period 1981 to Figure 4 shows the distribution of the epicenters for this data set, and Figure 5 shows a plot of M*, calculated from local data, versus the original moment magnitude, M w. The mean of the differences M* - M w is almost equal to zero with a standard deviation tr equal to 0.22, for both data sets and for earthquakes with moment magnitude down to 5.0. This uncertainty is not very large if we take into

5 478 B. C. Papazachos, A. A. Kiratzi, and B. G. Karacostas Table 1 Parameters of the Earthquakes Used to Check the Validity of the Magnitude Scales Applied in Greece Date Mo (m/d/y) Origin Time Lat. Long. h (kin) M~v (* 1024 dyne.cm) Mw Reference 07/26/63 04:17: /18/63 16:58: /16/63 13:47: /11/64 16:00: /29/64 4:21: /17/64 2:34: /06/64 14:31: /09/65 17:57: /31/65 9:47: /05/65 3:12: :57: /27/65 14:09: /13/65 20:01: /06/65 3:18: ,6 11/28/65 5:26: /20/65 00:08: /05/66 2:01: /09/66 00:42: /29/66 2:39: t /04/67 17:58: /01/67 7:09: /30/67 7:23: /19/68 22:45: /30/68 17:40: /05/68 7:52: /14/69 23:12: /03/69 00:59: /23/69 21:08: /25/69 13:21: ,10 03/28/69 1:48: /12/69 15:13: ,6 07/08/69 8:09: /28/70 21:02: ,10 04/08/70 13:50: ,6 04/16/70 10:42: /19/70 13:29: t /23/70 9:0 t : /25/71 5:43: ,11 05/04/72 21:39: ,12 09/13/72 4:13: ,6 09/17/72 14:07: ,6 11/29/73 10:57: ,6 01/08/75 19:32: /27/75 5:15: /12/75 13:10: /22/75 00:44: /11/76 16:59: /18/77 9:27: /11/77 23:19: ,6 11/28/77 2:59: /23/78 23:34: /20/78 20:03: ,14,15,16 04/15/79 6:19: ,17,18 04/15/79 14:43: /15/79 6:59: /24/79 17:23: /14/79 11:44: /15/79 11:34: /16/79 18:42: (continued)

6 Toward a Homogeneous Moment-Magnitude Determination for Earthquakes in Greece and the Surrounding Area 479 Table 1 Continued Date Mo (m/d/y) Origin Time Lat. Long. h (km) M~v (* 10 z4 dyne.cm) Mw Reference 07/18/79 13:12: /23/79 1 t:41: /09/80 2:11: /80 19:39: /11/80 09:15: /24/81 20:53: /25/81 02:35: /04/81 21:58: /05/81 06:59: /07/81 11:34: /10/81 15:16: / :41: /28/81 17:20: /13/81 23:25: /t9/81 14:10: /27/81 17:39: /29/81 08:00: /18/82 19:27: t /22/82 03:04: t /16/82 23:41: /03/83 00:12: /17/83 12:41: /19/83 00:02: /31/83 15:27: /21/83 00:13: /19/83 21:41: /23/83 23:51: /24/83 04:17: /05/83 12:01: /14/83 02:54: /06/83 15:43: /26/83 12:52: / :59: /10/83 10:16: /21/83 20:34: t /11/84 08:02: /06/84 09:12: / :57: /17/84 07:48: /21/84 10:43: / : 57: /21/85 08:49: /30/85 18:14: /23/85 16:02: /22/85 21:32: /07/85 10:20: /27/85 16:39: / :50: /09/85 23:30: /21/85 21:57: /03/86 01:24: /25/86 01:41: /22/86 19:52: /08/86 04:55: /13/86 17:24: /02/86 10:12: /11/86 09:00: (continued)

7 480 B.C. Papazachos, A. A. Kiratzi, and B. G. Karacostas Table 1 Continued Date M o (m/d/y) Origin Time Lat. Long. h (kin) M~v (*1024 dyne.em) M w Reference 02/27/87 23:34: /12/87 02:47: /29/87 18:40: /10/87 14:50: /19/87 18:45: /09/88 01:02: /24/88 20:49: /18/88 05:17: /05/88 20:03: /16/88 12:34: /20/88 21:01: /19/89 14:28: /24/89 00:40: /17/89 05:42: /19/89 05:36: /28/89 13:29: /27/89 23:06: /28/89 13:30: /07/89 19:45: /14/89 18:06: /20/89 18:32: /24/89 02:13: /27/89 01:21: /05/89 06:52: /16/90 02:16: /09/90 11:22: /18/90 11:29: /21/90 06:57: /19/91 12:09: /26/91 11:43: /18/91 14:04: /23/92 04:24: /20/92 05:37: /30/92 11:44: /23/92 20:12: /06/92 19:08: /18/92 21:10: /21/92 05:07: /05/93 06:55: /18/93 15:47: /26/93 11:58: /13/93 23:26: /I 4/93 12:31: /04/93 05:18: /11/94 07:22: /28/94 15:45: /25/94 02:30: /16/94 23:09: /23/94 06:46: /24/94 02:05: /13/95 08:47: /15/95 00:15: /01/95 15:57: M~v: Moment magnitude calculated using appropriate relations discussed in the present article. Mw: Moment magnitude calculated from the scalar seismic moment. 1. North (1977), 2. Papadimitriou (1993), 3. Bezzeghoud (1987), 4. Taymaz et al. (1990), 5. Papadimitriou (1988), 6. Ioannidou (1989), 7. Anderson and Jackson (1987), 8. Kiratzi et al. (1990), 9. Eyidogan and Jackson (1985), 10. Jackson and McKenzie (1984), 11. Kiratzi (1991), 12. Kiratzi and Langston (1989), 13. Prochazkova (1979), 14. Ekstr6m and England (1989), 15. Kulhanek and Meyer (1983), 16. Barker and Langston (1981), 17. Kanamori (1981), 18. Kanamori and Given (1981), 19. Boore et al. (1981), 20. CMT Harvard determinations.

8 Toward a Homogeneous Moment-Magnitude Determination for Earthquakes in Greece and the Surrounding Area O qtm IONIAN SEA V ~lo?~.~ *, z,o~5. - ',_~-----~. I I AEG~EAN " SEA Q "0 ~ 0_~. s~ o ~ t * 3O I " account that standard deviations of moment magnitude values calculated for the same earthquake by different institutes have an average value of 0.14 for earthquakes in Greece. It is not known if relations (11) and (12) hold for M w < 5.0. Further research work is needed to determine such relations for smaller magnitudes. It is now clear that moment magnitudes in Greece can be calculated from seismic moment values, when such values are available, or can be estimated from local records (relations 11 and 12). Linear regression analysis (assuming the slope of the line equal to 3/2, Kanamori and Anderson, 1975), applied to the data from the shallow earthquakes in Greece, listed in Table 1, leads to the following relation: log M 0 = 1.5 M~v , (13) MEDITERRANEAN SEA while the same analysis applied to the data for the intermediate-depth earthquakes gives the relation Figure 4. Epicenters of earthquakes that occurred in the Aegean sea and its surrounding area between 1963 and 1995 and for which moment magnitude is available (see Table 1). 3O logm0 = 1.5 M~v (14) These relations are in good agreement with previous ones that others determined for the same area (Main and Burton, 1990; Papazachos and Kiratzi, 1992; Kiratzi and Papazachos, 1994). Summary and Discussion ,6o I Data from //'MM~=Mw o Data from o o/o & %" *o oe g/ O~Q The present study shows that the moment magnitude, M w, can be estimated for earthquakes in the Aegean Sea and its surrounding area using scaling relations between standard magnitude scales (M, ML, Ms, and mb). This, however, must be the approach only if the scalar seismic moment of the earthquake is not available. The magnitude M calculated by relations (1), (2), and (5) is equivalent to moment magnitude for a wide range of magnitudes. Therefore, moment magnitude can be calculated by the relation M~v = M, 5.0 ~ M ~ 8.0. (15) When the local magnitude M c (or M~) is available, the moment magnitude can be calculated by the relation 4, MW Figure 5. Moment magnitude, M~v, calculated using relations (11) and (12) versus moment magnitude, Mw, based on original scalar seismic moment values for 169 earthquakes that occurred in the Aegean area. Solid circles denote data from the period 1963 to 1980, and open circles denote data from the period 1981 to 1995, while the straight line is form* = M w. M*= 0.97M L , 4.5--<M L=<6.0. (16) From relation (3) and then (8) and (12), it comes out that the moment magnitude can be calculated when M s is available by the following relations: M~v = Ms, 6.0 _--< M s <= 8.0, (17) M* = 0.56Ms , 4.2 _--< M s <= 6.0. (18)

9 482 B.C. Papazachos, A. A. Kiratzi, and B. G. Karacostas Relations (10) and (12) give the following formula that can be used to calculate moment magnitude when mb is known: M* = 1.28 m b , 4.8 _--< m b =< 6.0. (19) Figure 6 graphically shows the relations between the standard magnitudes (M, M s, M L, and mb) and the moment magnitude, Mw, for earthquakes in Greece. The magnitude M, calculated from the records of the Wiechert or Malnka seismographs in Athens, is equivalent to Mw for a wide range of magnitudes (5.0 _-< M w <-<_ 8.0). For earthquakes with M w _--> 6.0, M s = Mw, while for earthquakes with M w < 6.0, M s < M w. The body-wave magnitude, mb, is smaller than M w for 5.0 < Mw <= 6.6. It is interesting to note that for Greek earthquakes M z is smaller than Mw by half a magnitude unit, while in other areas M L = Mw for M L < 6.0 (Heaton et al., 1986). This difference is probably attributed to a miscalibration of the Wood-Anderson seismographs in Greece. It is well known that Wood-Anderson seismographs are notoriously difficult to calibrate. The static magnification is typically 2080 (Boore, 1989; Uhrhammer and Collins, 1990) rather than the nominal 2800, so miscalibration is a candidate for this magnitude difference. Also, if the Wood-Anderson seismographs were set up in a manner that the light beam missed the doubling mirror, the magnification would be about Thus, it is possible to have a systematic error of -0.43, which is close to the difference reported here lv~ L /.J, 4.0' Moment magnitude, M w Figure 6. Graphical relations between several magnitudes (M, M r, M s, and mb) versus moment magnitude, Mw, for earthquakes in the Aegean area and its surroundings. The Geophysical Laboratory of the University of Thessaloniki has compiled a large catalog that includes historical earthquakes (since 550 B.C.) with M => 6.0, calculated by relation (5) and earthquakes from the period 1911 to 1996 with M = 5.0, calculated by relations (1) or (2) or (12). Part of this catalog has already been published (Comninakis and Papazachos, 1986). All the reported magnitudes in this catalog are in the seismic moment-magnitude scale and is homogeneous at least as far as magnitude determination is concerned. Acknowledgments The authors would like to express their sincere gratitude to Dr. Jose Pujol, associate editor of BSSA, and to the two anonymous reviewers, for their critical suggestions that greatly improved our original manuscript. This work was funded by EEC Contract ENV4-CT , Climatology and Natural Hazards. References Ambraseys, N. and J. Bommer (1990). Uniform magnitude re-evaluation for the strong-motion database of Europe and adjacent areas, European Earthquake Eng. 2, Anderson, H. and J. Jackson (1987). Active tectonics of the Adriatic region, Geophys. J. R. Astr. Soc. 91, Barker, G. and C. Langston (1981). Moment tensor inversion of complex earthquakes, Geophys. J. R. Astr. Soc. 68, Bezzeghoud, M. (1987). Inversion et analyse spectrale des ondes, P.Ph.D. Thesis, University Paris VII, 232 pp. Boore, D. (1989). The Richter scale: its development and use for determining earthquake source parameters, Tectonophysics 166, Boore, D., J. Sims, H. Kanamori, and S. Harding (1981). The Montenegro, Yugoslavia, earthquake of April 15, 1979: source orientation and strength, Phys. Earth Planet. Interiors 27, Comninakis, P. E. and B. C. Papazachos (1986). A catalog of earthquakes in Greece and the surrounding area for the period Publ. of the Geophys. Lab., Aristotle University of Thessaloniki, No. 1, 172 pp. Drakopoulos, J. (1978). Attenuation of intensities with distance for shallow earthquakes in the area of Greece, Boll. Geof Teor. Appl. 20, Ekstrrm, G. and P. England (1989). Seismic strain rates in regions of distributed continental deformation, J. Geophys. Res. 94, Eyidogan, H. and J. Jackson (1985). A seismological study of normal faulting in the Demirci, Alasehir and Gediz earthquakes of in western Turkey; implications for the nature and geometry of deformation in the continental crust, Geophys. J. R. Astr. Soc. 81, Galanopoulos, A. ( 1961 ). On magnitude determination by using macroseismic data, Ann. Geof 14, Gutenberg, B. (1945). Amplitudes of surface waves and magnitudes of shallow earthquakes, Bull. Seism. Soc. Am. 35, Hanks, T. and H. Kanamori (1979). A moment magnitude scale, J. Geophys. Res. 84, Hatzidimitriou, P., C. Papazachos, A. Kiratzi, and N. Theodulidis (1993). Estimation of attenuation structure and local magnitude based on acceleration records in Greece, Tectonophysics 217, Heaton, T., F. Tajima, and A. Mori (1986). Estimating ground motions using recorded accelerograms, Surv. Geophys. 8, Ioannidon, H. (1989). Seismic source parameters determined by the inversion of body waves: Greece and the surrounding area, Ph.D. Thesis, Athens University, 185 pp. Jackson, J. and D. P. McKenzie (1984). Active tectonics of the Alpine-

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