Seismic Sequence Analysis of the Arraiolos Zone, South Portugal, and Its Seismotectonic Implications: A Detailed Analysis of the Period 15 January–30 June 2018

Abstract

The Arraiolos Zone has been affected by the persistent superficial seismicity (focal depth < 20 km) of a weak magnitude (M < 4) and some events of a higher magnitude (M > 4), and is mainly located around the Aldeia da Serra village. On 15 January 2018, at 11:51 UTC, the largest instrumental earthquake recorded in that area occurred, with a magnitude (ML 4.9) located northeast of Arraiolos, near the Aldeia da Serra village. This event was followed by a sequence of aftershocks with a magnitude (ML) ≤ 3.5. This seismic sequence was monitored by the designated temporary seismic network of Arraiolos, comprising 12 broadband seismic stations (CMG 6TD, 30 s) from the ICT (Institute of Earth Sciences, Évora) and 21 short-period stations (CDJ 2.0 Hz) from the IDL (Instituto Dom Luiz), distributed around the epicenter, within a radius of approximately 25 km. To infer the structure and kinematics of faults at depth and to constrain the crustal stress field in which the earthquakes occur, we use the polarities of the first P-wave arrivals and the S/P amplitude ratios to better constrain the focal mechanisms of 54 events selected, and apply the HASH algorithm. Overall, the good-quality (defined by the HASH parameters) focal solutions are characterized by a mixture of reverse and strike-slip mechanisms in our study area (AZS). Our seismicity and focal mechanism results suggest that the horizontal stress is more dominant than the vertical one and oriented in the NW–SE direction, parallel with the strike of the main faults. This analysis leads us to affirm that the ASZ is an active right-lateral shear zone.