The Nisi Fault as a key structure for understanding the active deformation of the NW Peloponnese, Greece

Abstract

The previously unknown Nisi Fault in NW Peloponnese was ruptured during the 2008 Movri Mountain earthquake attaining a maximum offset of 25cm. The fault is interpreted as a branch of a flower structure above a blind strike-slip fault. We investigate the Nisi Fault seismotectonic evolution using morphotectonic analysis in order to determine whether the landscape is affected by tectonic forcing and paleoseismology to determine earthquake recurrence interval and fault slip rates. We applied several geomorphic indices, such as the asymmetry factor (AF), the stream length–gradient index (SL), the valley floor width to valley height ratio (Vf), the mountain-front sinuosity (Smf), the drainage basin shape (Bs) and the hypsometric curve (Hc), in four large drainage basins of the study area. The results show that fault-related vertical motions and the associated tilting influenced the drainage geometry and the landscape development. Values of stream-gradient indices (SL) are relatively high close to the fault trace. Mountain-front sinuosity (Smf) mean values along the fault zones range from 1.12 to 1.23. Valley floor width to valley height ratios (Vf) mean values along the studied fault range between 0.21 and 2.50. Drainage basin shape (BS) mean values along the fault range from 1.04 to 3.72. Lateral fault growth was likely achieved by propagation primarily towards north-northwestward. The paleoseismic history of the fault, investigated by a trench and 14C dating of seven samples, indicates two morphogenic earthquakes in the last 1kyr. Therefore, we suggest that the Nisi Fault displays a slip rate on the order of 1mm/yr and a recurrence interval ranging between 300 and 600years. From a seismotectonic point of view, the fault is classified as high activity rate, with abundant but discontinuous geomorphic evidence of its activity. Other similar faults affecting the western Peloponnese can be envisaged with a similar procedure. Additionally, the seismic history and surface expression of the Nisi Fault, resembles other faults of similar length in Greece and Italy. Particularly, a crucial issue in terms of seismic risk management is that neotectonic analysis has to be envisaged carefully on short in length faults, since these faults can be possibly related to strong earthquakes.