The role of paleoseismology in studying an emerging or blind Fault: the case of Nisi fault, NW – Peloponnese, Greece

Geologic data at epicentral areas regarding surface slip following large earthquakes are occasionally contradicting in the exact locations of fault traces. In addition, this task is more difficult when a blind fault or previously unknown fault ruptures as is the case for the M_w: 6.4 June 8^th 2008 Movri Mountain earthquake that struck NW – Peloponnesus, Greece is involved. This earthquake caused widespread deformation and damages in buildings, as well as extensive hazards especially in terms of ground surface ruptures. Three surface ruptures were triggered by the Movri Mountain earthquake showing the following geological characteristics. Vertical displacements, up to 30 cm have been identified along a major high angle NNW-striking, 6 km long segment of the co-seismic rupture around the Nisi village area (8 km SE of Varda). Along this rupture zone we also observed several secondary on fault coseismic features, such as landslides and liquefaction phenomena. NNE-trending ruptures in the Petrochori area (18 km ENE of Varda) were mainly observed along a ~500 m wide zone of diffused deformation, accompanied by many landslide phenomena. This rupture zone has a length of ~4km and aligns with the up to now aftershock distribution. The third set of WNW striking surface ruptures has been identified throughout the broader epicentral area. However, a major 3 km long WNW-trending zone was mapped west of Michoi village. This orientation displays significant left-lateral component of horizontal motion. The most promising rupture, lying near the epicenter of the event, attains a maximum offset of 30 cm. In this surface rupture a paleoseismological trench was excavated in order to investigate if it was of tectonic origin or just a gravitational effect. Based on seven ¹⁴C samples, we identify two surface – rupturing earthquakes in the last 1Ka prior the recent event. Thus, observations from paleoseismology suggest that the Nisi fault earthquake appear not to be random over the last centuries. Our ¹⁴C data support the view that this fault resembles other known faults lying along the south coast of the Gulf of Corinth, in terms of slip rate and recurrence interval such as Helike and Schinos faults. Therefore, we infer that the Nisi fault displays a slip rate on the order of 1.5 mm/yr.