DISS 3.2.GreDaSS: Seismogenic Source GRCS620 - Cephalonia Transform Fault Zone

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COMMENTS The Cephalonia Transform Fault Zone (CTFZ) is one of the most hazardous sources in the Ionian Sea and broader Greece, separating two different geodynamic settings: the Adria-Aegean plate convergence to the north and the subduction zone along the West Hellenic Arc to the south. This structure is well expressed morphologically as it has formed a linear and steep NE-SW-to-NNE-SSW-trending slope, clearly imaged in bathymetric maps. It consists of four segments: the CTFZ segments A, B and C (GRIS620, GRIS621 and GRIS622, respectively), and the Lefkada Fault Segment (GRIS625). Concerning the latter one, the rupture hypothesis of Benetatos et al. (2005; 2007) and Zahradnik et al. (2005) is adopted, although there is strong argument from other researchers (see Open Questions and relative Summaries below and at the corresponding sources). In any case, all the segments are associated with strong earthquakes. The geometry of the fault zone is obtained from the numerous focal mechanisms and the sea-bottom morphology. The most probable and, at the same time, worst case scenarios are i) the total rupture of all three CTFZ segments A, B and C, and ii) the rupture of the Lefkada Fault. In the first case the maximum expected magnitude would be 7.2, while in the second case the maximum expected magnitude would be similar to the Lefkada Fault Segment, that is 6.9. The above magnitudes have been calculated by the empirical relations of Wells and Coppersmith (1994). OPEN QUESTIONS There is an open issue about the August 14, 2003 Lefkada earthquake, which poses some problems concerning segmentation. . Indeed, based on the aftershock spatial distribution and the mainshock modelling, some researchers (Benetatos et al., 2005; 2007; Zahradnik et al., 2005) support the occurrence of a double event with the sliding of (at least) two patches along a single major segment that occupies the north-eastmost 50 km of the CTFZ. In contrast, other researchers (Karakostas et al., 2004; Karakostas, 2008; Papadopoulos et al., 2003; Papadimitriou et al., 2006) suggest that the earthquake sequence is the result of stress transfer loading nearby fault planes. Thus, a segmentation problem appears: does the ca. 50-55 km-long northeastern part of the CTFZ represents a unique segment that was ruptured in patches during the 2003 event, or does it consist of more than one segment that were partially triggered by stress transfer loading? SUMMARIES Dziewonski et al. (1983) In the CMT Harvard catalogue, the best double couple solution of the January 17, 1983 (Mw = 6.8) earthquake suggests the following nodal planes: Nodal plane 1: strike, dip and rake = 034°, 14° and 153° respectively. Nodal plane 2: strike, dip and rake = 151°, 84° and 78° respectively. The scalar moment is 2.35 E19 Nm and the centroid depth is 10.1 km. Similarly, the best double couple solution of the March 23, 1983 (Mw = 6.2) earthquake suggests the following nodal planes: Nodal plane 1: strike, dip and rake = 027°, 59° and 175° respectively. Nodal plane 2: strike, dip and rake = 120°, 86° and 32° respectively. The scalar moment is 2.23 E18 Nm and the centroid depth is 32.7 km. It is noteworthy to mention that the suggested epicentre is located offshore, south from Cephalonia and northwest from Zakynthos Islands. Scordilis et al. (1985) In this seismological paper, the properties of the 1983 earthquake sequence are analysed. The authors calculated the focal mechanisms from the first motion polarities of the main (January 17, Ms = 7.0) and second strongest (March 23, Ms = 6.2) events: Main event (January 17, 1983): Strike, dip and rake = 141°, 82° and 47° (nodal plane 1), and 041°, 44° and 13° (nodal plane 2), respectively. Second strongest event (March 23, 1983): Strike, dip and rake = 142°, 76° and 19° (nodal plane 1), and 047°, 72° and 15° (nodal plane 2), respectively. The spatial distribution of the hypocentres is concentrated in the first 12-13 km of the crust. 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 focal mechanism of the January 17, 1983 earthquake shows an almost pure thrust dip-slip fault (first motion polarities) or thrust dip-slip with very little strike-slip component (centroid-moment tensor solution), which is in contrast with focal mechanisms of other researchers. The properties of both events (January 17 and March 23, 1983) are shown below: January 17, 1983 (Ms = 7.0): strike, dip and rake = 135°, 83° and 90° (nodal plane 1), and 315°, 7° and 90° (nodal plane 2), respectively. Depth = 14 km. March 23, 1983 (Ms = 6.2): strike, dip and rake = 027°, 59° and 175° (nodal plane 1), and 120°, 86° and 32° (nodal plane 2), respectively. Depth = 19 km. Similarly, the source properties of the September 17, 1972 (Ms = 6.3) earthquake are shown below: strike, dip and rake = 306°, 80° and -26° (nodal plane 1), and 042°, 65° and -168° (nodal plane 2), respectively. Depth = 33 km. Kiratzi and Langston (1991) The authors calculated the source parameters of the January 17, 1983 (Ms = 7.1) Cephalonia earthquake from body wave inversion. The proposed nodal plane has the following parameters: 60° strike, 47° dip and 174° rake. The source depth is calculated at 8 km depth and the seismic moment equal to 7.3 E18 N m. 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 two neighbouring events of January 17 and March 23, 1983 are shown below: January 17 (Ms = 7.0): strike, dip and rake = 48° ± 20, 56° ± 5 and 167° ± 25, respectively. Scalar moment = 2.08 E18 Nm and centroid depth = 11 ± 5 km. March 23 (Ms = 6.2): strike, dip and rake = 30° ± 25, 70° ± 10 and 176° ± 25, respectively. Scalar moment = 6.5 E17 Nm and centroid depth = 8 ± 4 km. Similarly, 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 Nm and centroid depth = 8 ± 4 km. 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°. Sachpazi et al. (2000) The authors carried out a seismological investigation on the CTFZ and the Western Hellenic Arc. They observed that west from the Cephalonia Island, the CTFZ is the boundary of the seismic deformation that occurs to the east in contrast with the western part where no deformation occurs. The focal mechanisms of the microearthquakes that occurred along the western coast of the island and its extension 20 km SSWwards document normal faulting in contrast with the strong events that show a strike-slip kinematics. 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 following events the authors used the parameters determined by Papadimitriou (1993) which are based on body-wave modelling. These are as follows: January 17, 1983 (Mw = 6.8): Nodal plane 1: Strike, dip and rake = 039°, 45° and 175°, nodal plane 2: Strike, dip and rake = 133°, 86° and 45°, depth = 11 km and Mo = 20.8 E18 Nm. March 23, 1983 (Mw = 6.1): Nodal plane 1: Strike, dip and rake = 031°, 69° and 174°, nodal plane 2: Strike, dip and rake = 123°, 84° and 21°, depth = 7 km and Mo = 1.92 E18 Nm. September 17, 1972 (Mw = 6.2): 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 Nm. Vannucci and Gasperini (2003; 2004) The focal mechanism solution of the EMMA catalogue suggests the source parameters for the following earthquakes: January 17, 1983 (Mw = 6.5): Strike, dip and rake = 135°, 83° and 90° (nodal plane 1), and 315°, 07° and 90° (nodal plane 2), respectively. The scalar moment is 6.918 E18 N m and the hypocentral depth is 14 km. March 23, 1983 (Mw = 6.0): Strike, dip and rake = 027°, 59° and 175° (nodal plane 1), and 120°, 86° and 32° (nodal plane 2), respectively. The scalar moment is 1.230 E18 N m and the hypocentral depth is 19 km. September 17, 1972 (Mw = 5.9): 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. Focaefs and Papadopoulos (2004) The authors studied the historical seismicity and reevaluated the intensities of strong historical earthquakes in the region of the Lefkada Island. They also established a relation between magnitude and maximum intensity from twenty-nine instrumental events that hit the area in the past. Karakostas et al. (2004) This paper deals with a seismological investigation of the August 14, 2003 (Mw = 6.2) Lefkada earthquake, based on the installation of a dense temporary seismographic network one day after the main shock. According to the spatial distribution of the aftershocks, the main rupture occupied the northwestern part of the CTFZ and trended towards southwest. Moreover, intense aftershock activity took place up to 40 km beyond the southern end of the main rupture which was explained as static stress changes on another fault near Cephalonia Island. Papadopoulos et al. (2004) The authors try to explain the unusual “off-fault” aftershock distribution that was observed after the August 14, 2003 Lefkada earthquake. The two ca. 15-20 km-distant clusters are attributed to two different faults, a hypothesis that the authors support with Coulomb stress transfer modelling. Pavlides et al. (2004) The authors documented the co-seismic surface effects of the August 14, 2003 Lefkada earthquake and investigated the structure of neotectonic faults at the central-northwestern part of the island. Benetatos et al. (2005; 2007) The authors carried out a seismological investigation of the August 14, 2003 (Mw = 6.2) Lefkada earthquake, which was produced by one of the two major segments that comprise the CTFZ. Their slip model indicates that the earthquake occurred as two distinct subevents, separated in space by approximately 40 km and in time by 14 sec. Rupture initiated in the north and propagated to the south with an average speed of 2.4 km/sec. The moment was released in three distinct patches. The first patch is located beneath the western coast of Lefkada Island and is confined in a small area (ca. 25x10 km2) that extends in depth from 10 to 25 km, having the maximum slip of 34 cm. The second patch is located offshore further to the south, close to the northern coast of Cephalonia Island, and is confined in an even smaller area (ca. 15x10 km2). The third patch, for which the uncertainty is the highest, occurred at the intersection of the two fault segments, which is actually the intersection of the Lefkada and Cephalonia segments of the CTFZ. It is quite smaller (ca. 10x10 km2) and is located at depths greater than ca. 25 km2. The authors also point out that although the length of the ruptured zone, which is more than 55 km from the initiation of the rupture in the north to the end of the southern slip patch, is too large to be accounted for by a single Mw = 6.2 event according to empirical relationships (e.g. Wells and Coppersmith, 1994), the resolved moment in the two patches separately, provides dimensions that are in agreement with these relationships. Ekstrom et al. (2005) In the CMT Harvard catalogue, the best double couple solution of the August 14, 2003 (Mw = 6.2) earthquake suggests the following nodal planes: Nodal plane 1: strike, dip and rake = 018°, 59° and -174° respectively. Nodal plane 2: strike, dip and rake = 285°, 85° and -31° respectively. The scalar moment is 2.98 E18 N m and the centroid depth is 15 km. Zahradnik et al. (2005) The authors performed moment tensor inversion of the August 14, 2003 Lefkada earthquake for multiple point sources, which was extended to full waveform data at regional (or local) distances. According to their results, the rupture process was dominated by two main events one at the Lefkada Island and the other at the Cephalonia Island, with spatial and temporal separation of 40 km and 14 s, respectively. The focal mechanisms of the two main sub-events are predominantly right-lateral strike slip of south-southwest and north-northeast orientation. The Cephalonia sub-event occurred on a less steeply dipping fault with a small thrust component. The whole fault length, measured from the hypocenter up to the most distant sub-event 2 is 45 km which is too long for an Mw = 6 event according to empirical relationships. This difference is explained with the double-event hypothesis. The authors conclude that the inversion revealed a non-uniform rupture process, not only regarding its space-time development, but also regarding the focal mechanism. Papadimitriou et al. (2006) This seismological study deals with the seismological investigation of the August 14, 2003 (Mw = 6.3) Lefkada earthquake and the induced stress transfer changes. The source parameters of this event were determined using body-wave modelling. The focal depth was found equal to 9 km and the focal mechanism suggests a dextral strike-slip fault plane, with strike, dip and rake equal to 015°, 80°, and 170°, respectively. The seismic moment is calculated to 2.9 E18 N m, while based on the velocity rupture (3 km/s), the fault length is estimated at 24 km and the co-seismic slip at 40 cm. The horizontal aftershock distribution reveals two clusters, one under the Lefkada Island and one under the northwestern part of the Cephalonia Island. The authors also present i) the horizontal microearthquake distribution of the 1989 experiment, when a temporary seismological network consisting of 54 portable seismological stations was installed in northwestern Greece for a period of two months for the purpose of recording microseismicity (Hatzfeld et al., 1990), and ii) the spatial aftershock distribution of a moderate (Mw = 5.1) event that occurred on November 29, 1994 close to the western coast of Lefkada Island and which was recorded by a temporary seismological network installed by the University of Athens. The latter investigation shows a cluster of events under the central-western part of the island that reaches a depth of ca. 12 km. After applying the Coulomb stress analysis, the authors suggest that the southern cluster is triggered by the main event, as an effect of stress transfer loading. 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 Island. The southern cluster probably depicts 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 (2008) The author reassesses the August 14, 2003 earthquake sequence in order to clarify the rupture process and aftershock evolution after the argument of Zahradnik et al. (2005) and Benetatos et al. (2005; 2007). He relocated the aftershock sequence suggesting a maximum depth of around 13 km and re-estimated the parameters of the main rupture showing a northeast-to-southwest direction of the sequence development. 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. They observed that the best located events that occurred in the area of Cephalonia form two clusters supporting a NNE-SSW-striking thrust faulting. Rondoyanni et al. (2012) This paper deals with a morphotectonic investigation on the Lefkada Island and Coulomb stress transfer modelling. According to field observations, the authors consider the Athani Fault as the most important neotectonic structures among others. This strike-slip structure is running parallel to and near the west coast of the island and shows similarities with the northern branch of the CTFZ. In more detail, the authors observed a 17 km-long fault trace, well delineated on satellite imagery and aerial photographs, that shows few exposed polished surfaces bearing striations with 5°-10° pitch usually on dolomitic limestones. The most impressive fault surface was observed near the village Dragano, where it has a dip of 65° and documents ‘‘canellures’’ of 8° dip. The meso-structural analysis of the Athani Fault suggests the following stress axes: ó1 = 6°/N233°, ó2 = 83°/N59°, ó3 = 0.7°/N323°.