Paleoearthquake history of the Spili fault

Η παλαιοσεισμική δραστηριότητα στο ρήγμα του Σπηλίου μελετήθηκε χρησιμοποιώντας μία πρωτοποριακή μέθοδο που συνδιάζει μετρήσεις Σπανίων Γαιών (REE) και κοσμογενών ισοτόπων 36Cl πάνω στην σεισμικά αποκαλυμμένη επιφάνεια του ρήγματος. Η ανάλυση των δεδομένων δείχνει ότι το ρήγμα είναι ενεργό και έχει φιλοξενήσει τουλάχιστον 5 μεγάλου-μεγέθους σεισμούς τα τελευταία 16500 χρόνια. Οι δύο πιο πρόσφατοι σεισμοί έλαβαν χώρα κατά την περίοδο 100-900 ετών πρίν από σήμερα και άθροισαν συνολικά 3.5 μέτρα σεισμικής μετατόπισης. Η χρονολογία των παλαιότερων 3 σεισμών προσδιορίσθηκε στα 7300, 16300 και 16500 χρόνια πριν από σήμερα με σεισμικές ολισθήσεις 2.5, 1.2 και 1.8 μέτρα, αντίστοιχα. Από το μέγεθος των σεισμικών ολισθήσεων συμπεραίνουμε ότι το μέγεθος των σεισμών που προκλήθηκαν από το ρήγμα του Σπηλίου κυμάνθηκε από Μ 6.3-7.3 ενώ ο μέσος ρυθμός επανάληψης τους ήταν ~4200 χρόνια. Τα παραπάνω δεδομένα αποκαλύπτουν ότι το ρήγμα του Σπηλίου είναι ένα από τα πιο ενεργά ρήγματα στην Κρήτη και οι σεισμικές παράμετροί που σχετίζονται με την δραστηριότητά του πρέπει να συμπεριληφθούν στο μοντέλο σεισμικής επικινδυνότητας της Ελλάδας.The paleoearthquake activity on the Spili Fault is examined using a novel methodology that combines measurements of Rare Earth Elements (REE) and of in situ cosmogenic 36Cl on the exhumed fault scarp. Data show that the Spili Fault is active and has generated a minimum of five large-magnitude earthquakes over the last ~16500 years. The timing and, to a lesser degree, the slip-size of the identified paleoearthquakes was highly variable. Specifically, the two most recent events occurred between 100 and 900 years BP producing a cumulative displacement of 3.5 meters. The timing of the three older paleoearthquakes is constraint at 7300, 16300 and 16500 years BP with slip sizes of 2.5, 1.2 and 1.8 meters, respectively. The magnitude of the earthquakes that produced the measured co-seismic displacements, ranges from M 6.3-7.3 while the average earthquake recurrence interval on the Spili Fault is about 4200 years. The above data suggest that the Spili is among the most active faults on Crete and its earthquake parameters may be incorporated into the National Seismic Hazard Model

Paleoseismological investigations along the Kera fault zone, Western Crete: implications for seismic hazard assessment

The island of Crete is the principal landmass in the Aegean arc system. Collision of the Euroasian plate in the north and the African plate in the south gives rise to the subduction related deformation along the Hellenic arc. As a result of the complex deformation, the area is characterized by high seismic activity. Paleoseismic investigations performed along the Kera fault scarp, which is part of a N-S oriented fault system along the Spatha peninsula (NW-Crete), show clear evidence of repeated normal faulting events. Five distinct episodes of faulting are observed. The first two are probably of Middle-Miocene or younger age representing older tectonic episodes, whereas the last three indicate co-seismic displacements most likely during the Pleistocene and Holocene. This is in good agreement with the previous estimates of Holocene average slip rate and the recurrence time estimate of large earthquakes in the order of ca. lmm/yr and 3000yrs, respectively. The Kera fault represents a NE-SW oriented bend in a N-S fault system and therefore has a minor left-lateral strike-slip component. During the 1980's at least three earthquakes could be associated with the Kera fault. More recently, in 1999, there were three small (with magnitudes between 3.0-4.5) offshore events that are probably associated with the same fault system in the offshore extension (to the north) of the N-S oriented faults along the Spatha peninsula. The existence of these earthquakes as well as the recent paleoseismic results clearly demonstrates the need of revising the seismic hazard assessment of the area. The length of the N-S oriented fault system, where the Kera fault represents the middle segment, reaches to a total of 30 km., and is capable of generating an earthquake of magnitude in the range 6.0-6.7. Such a (shallow) earthquake occurring at a short distance to the densely populated north-western coast of Crete is likely to have significant consequences