DISS 3.2.GreDaSS: Seismogenic Source GRIS622 - CTFZ segment C

Source Info Summary

Commentary

Pictures

References

COMMENTS This seismogenic source is responsible for the September 17, 1972 (Mw = 6.2 after Kiratzi and Louvari, 2003) earthquake and belongs to the major dextral transform fault zone of Cephalonia (or else CTFZ, corresponding to the GRCS620 of this database), that has produced a prominent linear submarine escarpment affecting the western coasts of Cephalonia and Lefkada Islands. In fact, this is the third recognised fault segment from the southwestern tip of the fault zone, as it is inferred from various epicentral locations (e.g. Papazachos and Papazachou, 1997; 2003; Papazachos et al., 2000; Vannucci and Gasperini, 2003; 2004). The hypocentral distributions of the 1983 aftershock sequence (Scordilis et al., 1985) and other microearthquake investigations (Karakostas et al., 2010), suggest a local seismogenic layer thickness of ca. 12 km (from the sea-floor), that is greater than the maximum fault depth of this source which is restrained to 7.5 km according to the hypocentral depth of the 1972 event (Baker et al., 1997; Kiratzi and Louvari, 2003) and the magnitude-scaling relationships (Wells and Coppersmith, 1994). Strike, dip and rake are obtained from various focal mechanisms, although sea-bottom morphology is also taken into account for the former. Based on all the aforementioned constraints (minimum and maximum depth, geometry and magnitude scaling relationships) the source dimensions are limited to ca. 18 x 8 km (length x width respectively). If we consider the smaller dimensions of this source compared to the broader fault zone, then the worst case scenario, as a stand-alone structure, would involve a fault plane that would be wider down to the bottom of the seismogenic layer and approximately 10% longer according to the available spacing between the segments. In this case, the maximum expected magnitude would be 6.6 (based on Wells and Coppersmith, 1994), which is greater than the magnitude of 6.2 of the largest associated event. Further discussion about the earthquake potential can be found in the comments of the CTFZ (GRCS620). Slip per event derives directly from Mo (Aki, 1966). OPEN QUESTIONS There are some uncertainties regarding the exact position of this segment. Moreover, the 33 km hypocentral depth of the 1972 event suggested by Anderson and Jackson (1987) is probably overestimated. SUMMARIES Anderson and Jackson (1987) The authors computed several focal mechanisms from the Adriatic and Ionian regions based on first motion polarities and centroid-moment tensor solutions. The source properties are shown below: September 17, 1972 (Ms = 6.3): strike, dip and rake = 306°, 80° and -26° (nodal plane 1), and 042°, 65° and -168° (nodal plane 2), respectively. Depth = 33 km. Hatzfeld et al. (1995) Based on a 7-week monitoring carried out by a network of 51 seismograph stations in northwestern Greece, the recorded microseismic activity shows a prevailing right-lateral strike-slip faulting along the Islands of Cephalonia and Lefkada, with only few exceptions of reverse faulting. The proposed cross section shows a cluster of earthquakes under the Cephalonia Island, the majority of which reaches a maximum depth of ca. 22 km. The seismic activity is characterised as significant and it is not extended under the submarine valley west of the islands. Furthermore, seismic activity is higher around Cephalonia than around Lefkada, but the authors did not observe a clear relationship with surface faulting. Baker et al. (1997) In this paper several focal mechanisms are computed by body-wave modelling for the broader area of the Adriatic and Ionian Sea. The parameters for the September 17, 1972 (Ms = 6.3) earthquake, are shown below: Nodal plane 1: strike, dip and rake = 39° ± 15, 61° ± 8 and -173° ± 30, respectively. Nodal plane 2: strike, dip and rake = 306°, 84° and -29°, respectively. Scalar moment = 2.7 E17 N m and centroid depth = 8 ± 4 km. Papazachos and Papazachou (1997; 2003) In this earthquake catalogue, the authors refer to the damages caused by the September 17, 1972 (M 6.3) Cephalonia event. Louvari et al. (1999) This paper studies the CTFZ from a seismological point of view. The authors revised the focal mechanisms proposed by i) Papadimitriou (1993) and Baker et al. (1997) for at least moderate events, and ii) Hatzfeld et al. (1995) for microearthquakes. The authors also determined new focal mechanisms by waveform modelling for other large events in the study area. Based on their collected and original data, they suggest that the CTFZ consists of two major segments forming an angular boundary, the Cephalonia and Lefkada fault segments. Regarding the Cephalonia segment, a representative nodal plane is given after the summation of the available data with a strike of 38°, a dip of 63° and a rake of 172°. Kiratzi and Louvari (2003) This paper presents a database of source parameters in the Aegean for the time period between 1953 and 1999. Besides the parameters computed by the authors, the database also contains parameters determined by other researchers. For the September 17, 1972 (Mw = 6.2) earthquake, the authors used the parameters determined by Papadimitriou (1993) which are based on body-wave modelling. These are as follows: Nodal plane 1: Strike, dip and rake = 045°, 68° and -174°. Nodal plane 2: Strike, dip and rake = 313°, 84° and -22°. Depth = 8 km and Mo = 2.16 E18 N m. Vannucci and Gasperini (2003; 2004) The focal mechanism solution of the EMMA catalogue suggests the following nodal planes for the September 17, 1972 (Mw = 5.9) earthquake: Strike, dip and rake = 306°, 80° and -26° (nodal plane 1), and 042°, 65° and -168° (nodal plane 2), respectively. The scalar moment is 8.710 E17 N m and the hypocentral depth is 33 km. Poscolieri et al. (2006) The authors studied the crustal stress and seismic activity in the Ionian Sea. Regarding the August 14, 2003 (Mw = 6.2) Lefkada earthquake sequence, two clusters were observed: one situated along the western coast of Lefkada Island and the other along the northwestern coast of the Cephalonia Islandvery near to the CTFZ segment “C” of this database. The spatial distribution of this cluster has a high concentration down to the first 20 km of the crust, while few sparse hypocentres reach the depth of ca. 35 km. Karakostas et al. (2010) This paper deals with the microearthquake investigation of the CTFZ. According to the authors the focal depths of all the events recorded by the local network are between 0 and 24 km, with the vast majority of them between 4 and 16 km, while the depth distribution of the best located events reveals that the seismogenic layer extends between 4 and 15 km, with a maximum of activity at 10-11 km depth.