Mantle earthquakes beneath the South Iberia continental margin and Gulf of Cadiz – constraints from an onshore-offshore seismological network

Abstract

The Gulf of Cadiz and the passive continental margin of southern Iberia to the west of the Strait of Gibraltar locally accommodate the presently ongoing convergence between Africa and Eurasia by widespread, rather diffusive, seismic activity. Seismicity of the northern Gulf of Cadiz was derived from an amphibious seismological network, including 24 temporary marine offshore stations, besides the permanent networks in Portugal, Spain, and Morocco. During the 6 month of the offshore network operation, in total 86 local earthquakes were located at six or more offshore stations with the majority of earthquakes occurring to the southwest of Iberia and along the Algarve continental margin off southern Iberia. The distribution of events along the Algarve margin mimics features reported for the Atlantic passive continental margins of both South and North America. Focal mechanisms at the Portimão Bank support that seismically active areas are associated with compression. Similar stress patterns are reported for the east coast of South and North America.However, while earthquakes along the American east coast occur at crustal levels, earthquakes in the northern Gulf of Cadiz occur both in the lower crust and upper mantle, with the majority of events rupturing within the mantle, including a number of well-located earthquakes beneath crust forming the continent-ocean transition zone. The large number of earthquakes in the mantle might be caused by the unique geological setting, where deformation occurs in cool lithosphere of Mesozoic age. We suggest that seismicity along the Algarve margin is caused by re-activation of pre-existing margin-2 parallel faults rather than corresponding to newly formed structures related to a new developing plate boundary.

Seismogenic Necking During Slab Detachment: Evidence From Relocation of Intermediate‐Depth Seismicity in the Alboran Slab

“…This seems at odds with the necking model we propose in this paper. However, considering these earthquakes are all at shallower depths relative to the rest of the events and that intermediate-depth seismicity has been reported even farther west (Domingues et al, 2013;Geissler et al, 2010;Grevemeyer et al, 2016;Gutscher et al, 2009;Monna et al, 2013), these events likely lie in the mantle lithosphere of Atlantic plate.…”

A sanity check for earthquake recurrence models used in PSHA of slow deforming regions: the case of SW Iberia

“…In offshore SW Iberia, the source area for the large 1755 and 1969 magnitude earthquakes ( Fig. 1a), seismicity relocated by Ocean Bottom Seismometers (OBS) has shown that most of the small magnitude events are occurring at mantle depths, down to 50 km focal depths, again without a clear relationship with the known active tectonic features identified by geological and geophysical surveys (Geissler et al, 2010, Grevemeyer et al, 2016, Silva et al, 2017.…”

Lithospheric deformation in the Africa‐Iberia plate boundary: Improved neotectonic modeling testing a basal‐driven Alboran plate

“…The SW Iberia‐Gulf of Cadiz segment of the plate boundary is characterized by a WNW‐ESE oblique dextral convergence between Africa (Nubia plate) and Eurasia (Iberia block) of 4–5 mm/yr [ Fernandes et al ., ; Stich et al ., ; Nocquet , ]. The strain is accommodated by moderate‐ to large‐magnitude earthquakes at shallow to intermediate depths, up to 60 km, most of them clustering along lineations with preferred orientation of NNE‐SSW and WNW‐ESE [ Buforn et al ., ; Stich et al ., ; Geissler et al ., ; Custódio et al ., ; Grevemeyer et al , ], which mostly coincide with segments of thrust and strike‐slip faults and sometimes localize at fault intersections [ Rosas et al ., ]. An important cluster in south Portugal trends ENE‐WSW, along the Cadiz fault, possibly accommodating perpendicular shortening by thrusting and right‐lateral tectonics [ Duarte et al ., , ].…”