SLIP DISTRIBUTION FOR THE 1928 CHIRPAN AND PLOVDIV MAIN SHOCKS AND EARTHQUAKE TRIGGERING

) is shown by thick blue line. The epicenters of the two strong events are depicted by red stars. Fault plane solutions are shown as lower – hemisphere equal –area projections. Locations of the nearby cities are shown by white squares. SGEM 200 6 - Section III 121 MODELING SURFACE DISPLACEMENTS We compare our modeled displacements with those obtained from geodetic measurements by Yankov (1945). Mapped surface ruptures (S. Bonchev and Bakalov, 1928; G. Bonchev, 1931) are shown in Figure 1, while the used fault plane solutions are based on previously published ones and information on surface fault expressions and the regional stress field. Vanneste et al. (2006) suggest that only the north surface break is the direct expression at the surface of the fault that generated the 14 April earthquake, which is a long regional fault striking E–W and dipping to the south. Dimitrov et al. (2006) failed in determining the fault plane solution from the collected available data of P first arrivals while Jackson and McKenzie (1988) suggested a focal mechanism from surface ruptures (strike = 105 o, dip = 45 o, rake = –90o). This latter solution is in agreement with a recently determined of a smaller magnitude event (09/09/1991, strike = 108 o, dip = 37 o, rake = –99o) by Alexiev and Georgiev (2002). In our paper, we adopted a solution in good agreement with the latter two (strike = 95 o, dip = 45 o, rake = –90o) and in accordance with the surface ruptures mapped by Bonchev and Bakalov (1928). Fault plane solutions published for the Plovdiv event from Glavcheva (1984), VanEck and Stoyanov (1996), Dimitrov and Ruegg (1994) indicated a WNW –ESE oriented normal fault with a significant dextral component. The data used for the solution were only from remote stations, since there were no data from proximate stations, and most probably, this fact resulted in the solution of normal/strike –slip character (Alexiev and Georgiev, 1996). The more recent determination, based on newly collected data by Dimitrov et al. (2006), give a north –dipping fault plane in good agreement with the main surface rupture (strike = 300 o, dip = 67 o, rake = –124 o). The most suitable orientation of the fault plane related to Plovdiv event is that being in agreement with surfa ce ruptures (strike = 300 o, dip = 62 o, rake = –65o) and the orientation of the axis of maximum extension in the area (Kotzev et al. 2006), resulting to a left–lateral strike slip component added to the normal faulting (Fig. 2). The parameters of these fault planes are summarized in Table 1. The fault considered as planar surfaces are shown in black in Figure 2 along with the simplified surface ruptures that are shown in red. We assumed that surface deformation was caused by variable slip on several fault segments of the two causative faults embedded in an elastic half space (Okada, 1992). For the Chirpan fault, we considered three fault segments with different slip values on them, the maximum being on the western and upper fault patch. Considering a fault length of 38 km and width of 14 km, and taking a rigidity of 33 GPa and dislocations of

00 m, 0.30 m and 0.11 m for the three segments, respectively, we obtain a scalar moment (Mo=μSu) of 0.623 1026 dyncm (Table 1). This corresponds to a moment magnitude of Mw6.5, in satisfactory agreement with the surface wave magnitude reported for this event (Ms6.8). For the Plovdiv fault, we considered sixteen fault patches with dislocation varying between some centimeters and 2.48 meters. If we consider a planer fault, its length equals to 47 km (53 km if we consider bends), which is used along with a fault width of 19 km to compute a scalar moment of 2.345 1026 dyncm. This gives a moment magn itude of Mw6.9, which is much closer to the value reported for this event (Ms7.0). 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 122 Table 1. Summary of fault parameters Date Fault segment Strike (o) Dip (o) Rake (o) Length (km) Width (km) (dyncm)

April Chirpan 95 45 –90o 38 14 0.623 1026

April Plovdiv 300 62 –65o 47 17 2.345 1026 Fig. 2. Observed coseismic displacements produced by 14 and 18 April 1928 earthquakes (upper part of the figure, Yankov, 1945; figure taken from http://www.geology.bas.bg/paleo). Horizontal displacement field on the surface of a homogeneous half–space due to Chirpan and Plovdiv earthquakes. Contours denote the amplitude of displacement in meters (lower part of the figure) . SGEM 200 6 - Section III 123 Our synthetic displacements (Fig. 2, lower part) fit quite well those observed by Yankov (1945) as far as the shape and the magnitude concerns (Fig. 2, upper part). The abrupt alteration between uplift and subsidence around the middle of the Plovdiv fault cannot be predicted by the elastic model, which requires continuous displacements. This twisting of the isocontours is probably due to local landslides. Our slip model gives better fit that the model of Dimitrov et al. (2006), who reproduced the displacement field using a homogeneous slip equal to 0.7 m on the Chirpan fault and ten fault patche s for the Plovdiv fault with maximum slip equal to 2.6 m. COSEISMIC DEFORMATIO N INDUCED BY THE CHI RPAN EARTHQUAKE ON THE PLOVDIV FAULT To assess the nature of any interaction between the two large events, we calculated static Coulomb failure stress change ( CFF ) using the recovered slip distributions. Although the coseismic strain concentration on the Plovdiv fault adjacent to Chirpan rupture has been documented in a previous study (Papadimitriou et al., 2006), we examine the stres s pattern here, since the geometry and slip distribution on the Chirpan is redefined in the present study in more detail. We used the formula of Okada (1992), assuming the fault system to be buried in a homogeneous half space with an apparent frictional coefficient of 0.6 since the calculations are performed close in time with the main shocks occurrence. The shear modulus and Poisson's ratio are fixed as 33 GPa and

25, respectively. Using the information outlined in the previous section and Table 1, we modeled the CFF in our study area. The stress field is calculated assuming an oblique normal faulting for the Plovdiv earthquake. Figure 3 shows the distribution of CFF immediately after the Chirpan earthquake on a horizontal plane at a depth of 10 km. The CFF is positive where the Plovdiv fault is located, with positive static stress changes reaching up to 10 bars, as well as where the aftershock activity was concentrated. This elastic stress change may lead to earlier failure at the second rupture zone, a phenomenon known as clock advance . Reasenberg and Simpson (1992) and King et al. (1994) have suggested that static stress changes as low as 0.1 bar can induce aftershocks. The numerous aftershocks reported, as mentioned in a previous section, may well be explained by induced stress. The fact that strong earthquake occurrence on adjacent faults and fault segments as well as aftershocks may be a product of static stress transfer together with other examples (e.g. Landers –Big Bear sequence) suggests that the seismic hazard posed by seismic activity off the main fault can be assessed by stress –transfer calculations. Thus, shortly after a strong earthquake occurrence a short –term hazard assessment is feasible regarding triggered seismicity that in some cases may occur closer to urban areas than the main shock. 6th International Multidisciplinary Scientific GeoConference SGEM2006 www.sgem.org Int er nat ional Confer ence SGEM 200 6 124 Fig. 3. Coulomb static stress calculation at a depth of 10 km for the slip of the Chirpan earthquake computed for the faulting geometry of the Plovdiv earthquake, assuming normal faulting with a sinistral strike –slip component. Changes are denoted by the color scale at bottom (in bars). Red areas denote increase of the likelihood of slip on faults with the same geometry. DISCUSSION AND CONCL USION S This study represents a step forward toward defining the slip distribution on faults of past earthquakes for which the relevant geodetic data are available. We tried to constrain the distribution of slip over the several fault patches in which the two main faults were divided. The compliant fault zone model that better explains the surface deformation successfully reveals that dip–slip is the dominant coseismic motion for the Chirpan earthquake, while the Plovdiv faulting encompasses strike –slip motion in addition to the dominant regional extension. With better knowledge of the fault geometry and coseimic slip distribution for the Chirpan earthquake, stress changes were estimated in the study area. The fault zone near the hypocenter of the Plovdiv earthqua ke is loaded by slip associated with the Chirpan event. The Plovdiv earthquake did not occur immediately but the loading was sufficient to initiate the inevitable failure. Strong earthquakes are sometimes clustered in time, suggesting that fault failure on one fault may affect earthquake occurrence on another fault. The phenomenon of double strong event occurrence is quite common in the area of Bulgaria and northern Greece (see 1904, 1930, 1978 seismic sequences, more details in Papadimitriou et al., 2006). It may well be, then, that a recognizable pattern of stress and fault zone behavior will be the basis for anticipating the characteristics of future seismic excitations, this contributing to the assessment of the seismic hazard in our study area. SGEM 200 6 - Section III 125 Acknowl edgements. The stress tensors were calculated using the DIS3D code of S. Dunbar, which later improved by Erikson (1986) and the expressions of G. Converse. The GMT system (Wessel and Smith, 1998) was used to plot the figures. This study was supported by the bilateral research project between Greece and Bulgaria EPAN – M.4.3.6.1 and NZ–BG–9/05. Geophysics Department contribution 669. REFERENCES

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