DISS 3.2.GreDaSS: Seismogenic Source GRIS015 - Righeo Fault

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References

COMMENTS The first who mapped and correlated the fault with the March 8, 1957 Velestino earthquake was Caputo (1990; 1995). It is partly under-lapped by the Dasolofos Fault (GRGG016) with which it forms a left stepping geometry structure. Most of the geometrical-kinematic parameters derive from the detailed mapping of Caputo (1990; 1995), while maximum depth and width are obtained from the microseismic spatial distribution suggested by Kementzetzidou (1996) and Hatzfeld et al. (1999). Slip per event and Mw are based on analytical and empirical relationships (Kanamori and Anderson, 1975; Hanks and Kanamori, 1979, Wells and Coppersmith, 1994). Righeo Fault can be related with many historical and instrumentally recorded events. According to the earthquake catalogue of Papazachos and Papazachou (1997), four large events with M > 6.0 occurred in the last 260 years in the surrounding area of the fault: in 1743 (M = 6.6), 1773 (M = 6.4) and the 1957 sequence mainly consisting of a large foreshock (M = 6.5) followed by the mainshock (M = 6.8) 7 minutes later. For the historical events the locations’ uncertainties are large. In the earthquake catalogue of the Institute of Geodynamics (National Observatory of Athens) there is a good match for the 1957 sequence. The microseismic survey carried out by Hatzfeld et al. (1999) shows a concentration of seismic activity near the region of Velestino suggesting Righeo Fault as a possible candidate, although the authors prefer the antithetic structure of Nea Pagasae Fault due to the dip angle that better fits the spatial distribution of the earthquakes. OPEN QUESTIONS It is still not very clear if Righeo Fault can be related with most of the historical earthquakes. Even for the 1957, Velestino earthquake the evidences are not so strong. SUMMARIES Ambraseys and Jackson (1990) The authors, studying the effects of strong earthquakes in Central Greece from 1890 to 1988, provide information about the two earthquakes that occurred on March 8, 1957 in Velestino. The shocks occurred 7 minutes apart, in a region which had already suffered the 1954-5 events. The authors report that although rockfalls, landslides and ground failures were produced, clear evidence of surface faulting does not exist. Only a series of breaks about 1 km-long in limestone, showing normal faulting north of Aerino and Kokina, and another break, about 500 m long in alluvium between M. Perivolaki and Velestino are attributed by local people to these particular earthquakes. The suggested magnitudes of the foreshock and mainshock are Ms = 6.5 and Ms = 6.6, respectively, while strong aftershocks were continued until late November. The authors propose normal faulting with 110° strike, 45° dip, -090° rake in proximity to the 1980 earthquakes, and 11 km depth assessed from macroseismic data. Strike is taken from the trend of prominent topographic escarpments in the epicentral region. Caputo (1990; 1995) He is the first that mapped the fault on the northern border of the Vasilika Basin. According to his geological and morphotectonic mapping, it is an at least 20 km-long, south-dipping normal fault, trending ESE-WNW. He also observed a straight morphological sharp scarp along strike affecting Pliocene fluvio-lacustrine deposits and the Pleistocene Red Beds. The author connects the fault with the March 8, 1957 Velestino earthquake based on the macroseismic epicentres of the main shock and aftershocks given by Ambraseys and Jackson (1990). However, he considers that the isoseismal map of the latter is rather doubtful due to the “cumulative” effect of the two, seven minutes apart, events. Caputo and Pavlides (1991, 1993) The authors provide some meso-structural information and a large scale neotectonic map during their investigation about the geodynamic regime of Thessaly and surrounding area. Pavlides (1993) The fault is included in a small scale neotectonic map of the large fault zone of Southern Thessaly. Papazachos and Papazachou (1997, 2003) According to the earthquake catalogue, the Velestino earthquake in March 8, 1957 (Mw=6.8) was preceded by a large foreshock seven minutes before (12:14 local time, Mw = 6.5). The largest aftershock (Mw = 6.0) occurred at 23:35 (local time) of the same day. Kementzetzidou (1996) and Hatzfeld et al. (1999) The authors installed a temporary network of 68 seismological stations during the summer of 1992 in the region of Thessaly and Evia in order to determine the western extension of the North Anatolian Fault system. They recorded 510 earthquakes and computed 80 focal mechanisms. In the northern part of Pagasitikos Gulf the seismic activity is concentrated in two clusters that align approximately E–W. One of these is located near the region of Velestino, where according to Caputo (1990), four faults with the same strike exist. However, the authors could not relate it with any of the structures, even though they prefer the Nea Pagasae Fault, an antithetic structure, because the dip fits the spatial distribution of the earthquakes in the depth of about 5 km. Goldsworthy et al. (2002) After revising the historic seismicity and other neotectonic investigations in the southern zone of Thessaly, the authors present these features on a regional map of Thessaly. Vannucci and Gasperini (2003) The fault plane solutions and the seismic parameters obtained from the database of Earthquake Mechanisms for European Area (EMMA), suggest the following attributes for the foreshock (12:14 local time) of the March 8, 1957 and the respective nodal plane: Mw = 6.4, Mo = 4.898e+18, strike = 58, dip = 41 and rake = -128. Ambraseys (2009) The author refers to the February 21, 1743 Thessaly and March 16, 1773 Almyros earthquakes with no information about the location and magnitude of the events.