SUMMARY The SW Iberian margin is well known for its complex tectonic setting and crustal structure and by the occurrence of moderate magnitude earthquakes and some great tsunamigenic earthquakes. Fortunately, many seismic reflection and refraction profiles have been carried out, providing detailed information about the crustal structure of the main geologic domains in this region. These studies show a first-order variation due to the transition from oceanic to continental domain, large-scale heterogeneities within the crust and an irregular Moho topography. Routine earthquake locations in this area have been usually computed using a general 1-D velocity model which is clear that cannot account for such a heterogeneous structure. In addition, regional seismic stations used to locate the Gulf of Cadiz seismicity are on land and far away to the east, implying large azimuthal gaps and distances. In this context, a 3-D approach seems necessary to properly solve the crustal velocity field and improve earthquake location in this area. With this purpose, we present a new digital 3-D P-wave velocity distribution for the crust and uppermost mantle derived from previously published controlled-source seismic experiments carried out in SW Iberia and the Gulf of Cadiz over the last 40 yr. We have reviewed more than 50 wide-angle and multichannel seismic reflection and refraction profiles and digitized the most significant published 2-D seismic velocity models, performing an updated compilation of crustal parameters (P-wave velocities and geometry and depth of the main crustal interfaces). These velocities as a function of position and depth have been interpolated using ordinary kriging algorithm to obtain, in the form of a regular georeferenced 20 × 20 × 1 km grid spacing, a high-resolution 3-D P-wave velocity distribution for the crust and uppermost mantle and a continuous Moho depth map of the whole area of this study (33°N–41°N latitude and 15°W–5°W longitude). Since current seismic location tools allow the implementation of 3-D grid structures, we have applied our 3-D model to relocate a selection of moderate earthquakes occurred in the studied region using a probabilistic nonlinear method. In the Gulf of Cadiz area the probabilistic approximation provides maximum likelihood hypocentres located within the uppermost mantle with the majority of depths ranging between 20 and 45 km. This model would subsequently be implemented at the Spanish Seismic Network for the routine relocation of the seismicity of the area.

A Spanish ALERT-ES project was set up to study the feasibility of setting up an earthquake early warning system to warn of potentially damaging earthquakes that can occur in the Cape of San Vicente (SV)-Gulf of Cadiz (GC) area, located in the south west of the Iberian Peninsula, such as the 1755 Lisbon earthquake. Four events, located close to the epicenters of the largest earthquakes in the area, were simulated using different seismic software packages (Earthworm, SeisComP3, and PRobabilistic and Evolutionary early warning SysTem [PRESTo]) and the errors were analyzed. In addition, a study about the blind zone and the lead time at six selected targets was carried out. The results show a blind zone in the southwest corner of Portugal for SV earthquakes and also a blind zone in the coastal area, from Portimao to Cadiz, for the GC earthquakes.

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The 2020–2021 Santa Fe seismic sequence is the first well-instrumented and widely felt seismic series occurred in the Granada basin during the Spanish instrumental period since the 1979 Fuente Vaqueros seismic series and the 1955–1956 Armilla and Purchil destructive earthquakes. It began in December 2020 with an Mw 3.7 earthquake followed by five Mw>4 earthquakes between 23 and 28 January 2021 and a long sequence of aftershocks during the next four months. Over six months later, on 12 August, another large earthquake of magnitude 4.5 stroke the area to the southwest of Santa Fe city where the previous shocks occurred. This seismic sequence offers a unique opportunity to study the seismic activity in this region—one of the most seismically active zones and with the highest seismic hazard in the Iberian Peninsula. The wide station coverage and good-quality data available allows us to perform high-precision absolute and relative relocations and to obtain the seismic moment tensor (MT) and focal mechanism of the largest earthquakes. Relocation results reveal a clustered distribution of the seismicity between Santa Fe and Pinos Puente faults, focused on a depth range between 2 and 5 km. Hypocenters follow a near-vertical pattern, and Mw>4 events display a northeast–southwest-dipping trend that seems to fit Pinos Puente fault plane. However, regional MT solutions show predominant normal fault mechanisms, with minor oblique component but with strike and dip orientations more compatible to Santa Fe fault. To explain this apparent discrepancy with the known mapped active faults, we propose a change in dip of Santa Fe fault plane from near vertical at shallow depths, bended below the first kilometers, to a possible fault splay in depth compatible with the known geologic structure of the area.

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