Seismic Analysis of the 2020–2021 Santa Fe Seismic Sequence in the Granada Basin, Spain: Relocations and Focal Mechanisms

Abstract

Abstract 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.