Late Holocene partitioning between regional and co-seismic uplift on the Scilla fault, Messina Straits: Seismotectonic implications and long-term displacement clustering

Detailed mapping and extensive radiometric dating of raised Late Holocene shorelines along the Messina Straits in the southern Calabrian arc, the fastest uplifting sector of the central Mediterranean region (WESTAWAY, 1993), indicate that vertical displacement occurred as an alternation of steady and episodic motions. Abrupt displacements are attributed to co- and post-seismic footwall uplift along the Scilla normal fault, which runs immediately offshore the studied coastal outcrops along the eastern side of the Straits (fig, 1). Two co-seismic displacement events are constrained in different coastal sites located 20 km apart at ∼1.8 and ∼3.5 Ka. Co-seismic displacements show a consistent site value and pattern of along-strike variation, suggestive of characteristic-type behaviour (SCWHARZ & COPPERSMITH, 1984) for the Scilla fault, which, based on coastal findings and bathimetry considerations, is traced for a length of ∼30 km (fig. 1). The rather large (typically l m and up to 2 m) vertical slip per event together with the inferred fault length indicate that a substantial fraction of co-seismic displacement permitted by scaling relations is accommodated by footwall uplift. Fault length and displacement per event also point to past M-7 earthquakes which have a ∼1.6-1.7 recurrence time. Our paleoseismological record suggests that the last rupture on the Scilla fault inferred (MONACO & TORTORICI, 2000; JACQUES et alii, 2001) during the February 6, 1783 M∼6.3 earthquake was at the expected time but did not release the entire loaded stress. Precise compensation for eustatic changes and glacio-hydroiso-static adjustments (LAMBECK et alii, 2002) specifically reconstructed for the Scilla coast, constrains Late Holocene steady uplift during the interseismic intervals at 1.0 mm/yr, a value consistent with longterm (0.1-1 Ma) estimates. Thus, Late Holocene total uplift of the Scilla coastline at -1.6-2.1 mm/yr (fig. 1) is almost equally balanced between the steady and the stick-slip components. Appraisal of the present elevation attained by a suite of 125 Ka and younger marine terraces in the footwall of the fault (fig. 1) indicates that Late Holocene uplift occurred ∼100% faster than tha value averaged since the Late Pleistocene. Given the ostensible constancy in regional uplift rate, the Holocene uplift rate increase is attributable to a period of enhanced slip rate on the Scilla fault. Displacement of the Late Pleistocene marine terraces across the western, onland sector of the Scilla fault, however, also points to higher slip rates during the 125-100 Ka time span. Intervals of enhanced slip rate on the Scilla fault were, clustered during 125-100 Ka BP and the last 10 Ka, and intercalated with a ∼90 Ka long period of tectonic quiescence, when the region only experienced steady regional uplift.