DISS 3.2.GreDaSS: Seismogenic Source GRCS002 - North Tyrnavos Basin

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COMMENTS The North Tyrnavos Basin CS bounds the northern margin of the Tyrnavos Basin containing two major underlapping and right-stepping ISs, the Rodia (GRIS002) and Gyrtoni (GRIS003) Faults. Although the fault linkage along this fault zone is not clear, the morphological evidences suggest an overstepping structure with the recent WNW-ESE trending faults affecting and cutting the older NW-SE structures. Maximum depth and width are based on microseismicity and aftershock distribution from both Volos broader area, Southern Thessaly (Kementzetzidou, 1996; Hatzfeld et al., 1999) and Kozani 1995 seismogenic volume, Western Macedonia (Hatzfeld et al., 1997). The estimated long-term (Late Pleistocene-Present period) slip-rate of 0.1-1.0 mm/a bolongs to the Rodia IS after Caputo and Helly (2005). Since the large over-stepping distance between the fault segments implies difficulties of rupture propagation between them, the maximum expected magnitude of the largest fault segment can be estimated with the use of empirical relationships independently. Therefore, the highest value of 6.1 from both Rodia and Gerakarou segments is selected as the maximum expected magnitude for the whole zone. OPEN QUESTIONS The North Tyrnavos Basin CS shows a rather complex geometry. The interaction of the fault segments is unknown. SUMMARIES Demitrack (1986) The author performed a detailed stratigraphic study of the Late Pleistocene and Holocene deposits and soils of the Larissa Plain among which those close to the Rodia Fault. She clearly recognises and maps abrupt changes in the deposits distribution and mentions the existence of faults cutting soils of ages between 100.000 and 10.000 years (Late Pleistocene-Holocene). At this regard, particularly interesting is the straight contact between the 'Pinios alluvium' (latest Holocene) and the 'Rodia alluvium' (Early Holocene), extending South and North of the fault, respectively. Moreover, according to a morphological analysis made on the streams' profiles flowing southwards toward the plain, Demitrack states the existence of some steps and suggests the presence of as many faults. Caputo (1993) The author presents a more detailed description of the fault geometry, kinematics and age showing that the structure consists of several segments with slightly different orientations ranging from NW-SE to E-W due to the oblique reactivation of Pliocene normal fault planes (NW-trending) connected by newly formed E-W segments. Several morphotectonic features such as triangular facets developed in the basement of the footwall block and numerous straight morphological scarps aligned all along the fault within the late Quaternary deposits of the hanging-wall block have been mapped and described. Along the different segments, activity has been documented during Late Pleistocene and Holocene. Faulting seems to have a typical basin-ward migration. An average slip-rate in the order of few mm/a is also suggested. Caputo and Pavlides (1993) The authors discuss about the Cainozoic geodynamic evolution of Thessaly. They conclude that the Pliocene-Early Pleistocene extensional phase, which generated the major NW-SE boundary faults and basins, is followed by a N-S extension which is still active forming a new ESE-WNW trending fault system among which is the Rodia Fault. It is also suggested that there is no evidence of continuation from the North Aegean Trough westwards to Thessaly and its surroundings because they did not detect any structures directly or indirectly associated with major strike-slip faulting. Caputo et al. (1994) The authors document the Holocene activity of the major faults bordering the Tyrnavos Basin reconstructing their role in the palaeogeographic, sedimentary and tectonic evolution of the area. In particular, they consider the repeated damming effects of the Rodia Fault during its Holocene activity as the responsible of recurrent flooding periods affecting the northern Larissa Plain crossed by the Pinios River, the longest river of Greece. Caputo (1995) In this paper the author includes a short description of major faults affecting the broader Thessaly in order to compare and explain a time difference observed between the northern and southern part of Thessaly regarding the earthquakes occurrence. A large seismic gap is proposed for the northern sector of Thessaly according to historical seismicity and active fault distribution. A possible long-term slip-rate of 1-4 mm/a is tentatively suggested. Caputo and Helly (2000) They performed seismic reflection, GPRs and micro-topographic profiles across the morphotectonic escarpment separating the 'Pinios alluvium' and the 'Rodia alluvium' (Demitrack, 1986) and the morphotectonic escarpment of the Gyrtoni Fault. Eventually they excavated a palaeoseismological trench documenting at least two major dislocation events in post-Neolithic times. Goldsworthy and Jackson (2000) The authors after a morphotectonic study suggest that much of the topography in the footwall was produced by an earlier phase of deformation unconnected with motion on the Rodia Fault and perhaps even prior to the late Tertiary extension in this part of Greece. They observed that streams cross the fault at positions unrelated to its continuity or segmentation, but incision changes abruptly to deposition as the drainage crosses the line of the fault itself. According to them this is a typical behaviour of faulting in schists such as in the Rodia Fault case. On the other hand they are reluctant to attribute stream valleys’ semi-regular spacing, position or catchments to any feature of the active Rodia Fault because they support that these valleys may well have been influenced by earlier structural trends, regional tilting or anisotropic fabrics within the schist. Caputo et al. (2003) The authors performed several ERTs across the major active faults of the Tyrnavos Basin among which the Rodia and Gyrtoni Faults. The authors applied the method with 10 m electrode spacing showing a strong relative resistivity contrast and clearly indicating the locations of the fault planes. Oliveto et al. (2004) The authors applied the horizontal-to-vertical spectral ratios method (HVSR) in the Tyrnavos Basin in order to measure the differential sedimentary thickness across the major boundary faults among which the Rodia and Gyrtoni Faults. Caputo and Helly (2005) The authors revise the instrumental and historical seismicity of Larissa’s broader area and archaeological evidences to suggest that since 15th century there was no significant seismic activity until the 1941 Larissa earthquake. On the other hand, they document two strong earthquakes affecting Larissa before the 15th century whose epicentres are probably located within the Tyrnavos Basin. The estimated magnitudes are as high as about 6.5. It is suggested that after 4 centuries of quiescence the 1941 Larissa event is not enough to fill the seismic gap for Northern Thessaly as it was proposed by Caputo (1995). They propose that the recurrence interval for local earthquakes affecting the investigated area is 0.5-1.5 ka which implies at least two strong events during the last 2200 years. As a final conclusion, they suggest that the seismic hazard of Northern Thessaly is likely high. Caputo and Helly (2005) The authors performed a morphotectonic, structural and palaeoseismological research for the Rodia Fault. The fault shows two strong morphological evidences of Late Quaternary activity. The first is the development of triangular and trapezoidal facets particularly in the central part of the fault and the second is represented by the morphological scarps formed in the Late Pleistocene and Holocene deposits. The structural evidences are also essential and include many slickensides formed on the bedrock and the Pliocene deposits indicating a dip-slip normal displacement, northward tilting of the Late Pleistocene(-Holocene?) alluvial deposits antithetic to the fault plane and some up-to-one metre-thick fault gouges separating the latest Pleistocene-Early Holocene poorly consolidated sediments and the metamorphic bedrock. The latter is locally characterised by an intense fabric where a complete re-orientation of the grains has occurred and the original layering has completely disappeared. The palaeoseismological investigation (three trenches) was carried out parallel to GPR and seismic reflection profiles (Caputo and Helly, 2000) as well as ERTs (Caputo et al., 2003). They document the occurrence of a Holocene morphogenic earthquake dated 2-3 ka BP with a vertical displacement 25-30 cm and a magnitude of 6.4 ±0.2. Based on all new evidences and data, the fault is assigned to class B (slip-rate 0.1-1.0 mm/a) according to RGAFJ (1992) classification.