Holocene earthquake record offshore Portugal (SW Iberia): testing turbidite paleoseismology in a slow-convergence margin.

The SW margin of the Iberian Peninsula hosts the present-day boundary between the Eurasian and AfricanPlates. Convergence (4-5 mm/yr) is accommodated through a wide deformation zone characterized bymoderate magnitude seismic activity. This zone has also been the source of the most important seismicevents in Western Europe, such as the 1755 Lisbon Earthquake and Tsunami and 1969 Horseshoe Earthquake.Despite efforts to identify active seismogenic structures in the Gulf of Cadiz in the last ten years, littleis known about its paleoseismic history. The turbidite paleoseismology approach was applied for the firsttime in a low-rate convergent margin to determine the recurrence interval of large earthquake events thatoccurred in SW Iberia during the Holocene. Four sediment cores collected at strategically located sitesoffshore Portugal (i.e. Tagus Abyssal Plain, Infante Don Henrique Basin and Horseshoe Abyssal Plain) revealthat these deep-sea basins preserve a record of episodic deposition of turbidites. In the SW Iberian Marginexcluding special climatic events, earthquakes are the most likely triggering mechanism for synchronous,widely-spaced distributed turbidites during the Holocene, when the sea level was relatively stable. Agecorrelation together with textural, mineralogical, physical properties and geochemical signatures of thenew cores complemented by pre-existing multicores and gravity cores reveals a total of 7 widespreadturbidite events for the Holocene. Precise dating of the most recent turbidite event (E1) based on 210Pb and137Cs geochronology provides an age of AD 1971 3. This age corresponds to a high-magnitude instrumentalearthquake in the region: the 1969 Horseshoe Earthquake (Mw 8.0). Calibrated 14C ages of subsequentwidespread turbidite events (E3 and E5) correlate with the dates of important historical earthquakesand paleotsunami deposits in the Gulf of Cadiz area, such as AD 1755 and 218 BC, respectively. If oldersynchronous events (E6, E8, and E10) with ages ranging from 4960-5510 yr BP to 8715-9015 yr BP are alsotaken into account, a great earthquake recurrence interval of about 1800 years is obtained for the Holocene.Our correlations suggest that the turbidite record may be considered as a proxy for paleoseismic activity inlow-convergence rate margins, and a valuable complementary tool in earthquake and tsunami hazardassessment along the coasts of the Iberian Peninsula and North Africa.