Source parameters of the moderate-sized (ML 4.9) 2018 Arraiolos, Portugal earthquake and its sequence

Abstract

The Arraiolos zone is one of the most seismically active intraplate regions in mainland Portugal and is characterized by low- to moderate-magnitude events. However, there is a need to analyse the characteristics of the seismic source parameters in this zone, which is crucial for understanding the origin of the zone’s seismicity and gaining insights into the stresses involved in the rupture processes of seismic events. On January 15, 2018, an earthquake of magnitude 4.9 M_L occurred northeast of Arraiolos, near Aldeia da Serra. This event was followed by aftershocks with magnitudes of up to 3.5 (M_L). In the present study, we estimated the source parameters and respective scaling relationships of 82 earthquakes (0.6 ≤ M_L ≤ 4.9) that occurred in the study area following the 2018 Arraiolos earthquake. The source parameters were estimated from the displacement spectra of the P-waves by automatic fitting of the Brune spectral model (ω⁻²). Consequently, the parameters characterizing the seismic source, such as the scalar seismic moment (M₀), moment magnitude (M_w), corner frequency (\({f}_{c}\)), source radius (r_o) and stress drop (Δσ), were determined. The results show that the moment magnitude (M_w) varies between 0.9 and 4.3. The source radii range from 31.5 m to 775.9 m, and the stress drop values (Δσ) range from 0.4 bars to 97.0 bars, with an average of 7.3 bars. Furthermore, most earthquakes occur mainly between 12 and 13 km depth, with a range of stress drop values (including low and high values). The linear relationships between the local and moment magnitudes are consistent. An increasing trend was observed between the source radii and the seismic moments, as expected. It was found that there is no linear correlation between stress drops and seismic moments. This suggests that the dynamic parameters controlling the rupture of larger magnitude earthquakes may be different from those of weaker earthquakes. Studying seismic source parameters in the Arraiolos area is crucial for understanding the seismogenic dynamics, as well as to improve regional hazard assessment.