The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes

Abstract

We expose the results of the study of the south-western Alboran seismic sequence of January-March 2016 and the stress perturbations it caused by means of Coulomb modeling. The use of data from numerous stations allowed us to relocate the largest events (Mw ≥ 3.8) south of those determined by IGN (Spain). The main shock of 25 January 2016 (Mw = 6.3) was relocated at 35.6133˚N × 3.6888˚W, at a hypocentral depth of 15.5 ± 6.0 km. The epicenters are aligned along two distinct clusters: The first runs N-S towards the Moroccan coast along Al-Idrissi fracture zone, while the second is centered on the Alboran ridge northern thrust fault. The focal mechanisms determined by different agencies correspond to a strike-slip/normal/reverse motion in the western cluster and thrusting in the eastern one. Coulomb stress change modeling shows that the main earthquake: (1) may have been triggered by stress accumulation by the 1994 and 2004 earthquakes; (2) has induced stress perturbations that provide a suitable explanation for the distribution of the aftershock clusters, including the eastern cluster. The prediction of the next event shows that it is likely to occur near the Moroccan coast.

Imaging the Growth of Recent Faults: The Case of 2016–2017 Seismic Sequence Sea Bottom Deformation in the Alboran Sea (Western Mediterranean)

“…The 2016–2017 seismic sequence (Figures and ) was initiated by a moderate earthquake (21 January 2016, M w = 5.1) followed closely by a stronger event (25 January, M w = 6.3) and by a long seismic series of decreasing activity during 2016–2017 (Buforn et al, ; Kariche et al, ; Medina & Cherkaoui, ). The relatively long distances from hypocenters to seismic stations, generally greater than 50 km and in some cases reaching more than 80 km, decrease the quality of earthquake locations.…”

“…Anyway, aftershock sequences can affect a wide zone (e.g., >10‐km wide with respect to the main fault during the Landers 1992 seismic sequence; Hauksson et al, ) favored by the structural complexity and preexisting structures. Despite that Buforn et al () establish horizontal and vertical errors for the 2016–2017 seismic sequence location, and Buforn et al () and Medina and Cherkaoui () relate the seismicity with the Al Idrisi Fault, the careful analyses of epicenter locations for most of the NNE‐SSW seismicity alignment (IGN, http://www.ign.es; Buforn et al, ; Kariche et al, ; Medina & Cherkaoui, ) point to that they are displaced westward with respect to the seafloor trace of the Al Idrisi Fault determined by marine geophysical data (Estrada et al, ; Martínez‐García et al, , ; Figures , , and ). This displacement of the earthquakes is higher when the standard velocity model (Carreño‐Herrero & Valero‐Zornoza, ) is considered (IGN, http://www.ign.es; Kariche et al, ; Medina & Cherkaoui, ): roughly 10 km for the first stage earthquakes (Figures a and a), as opposed to the roughly 5 km (Buforn et al, ) with the model by El Moudnib et al (; Figure b).…”

“…The 2004 seismic sequence was carefully relocated by Van der Woerd et al (). As the precise location of seismicity is sensitive to velocity models and to the distance of the seismic stations (Michelini & Lomax, ), literature (Buforn et al, ; Kariche et al, ; Medina & Cherkaoui, ) shows a noncoincident location for the 2016–2017 seismic sequence' epicenters. The 1993–1994 and 2016–2017 seismic sequences' epicenter and hypocenter locations were calculated through a standard procedure considering the first arrivals of P and S waves and a standard velocity model (Carreño‐Herrero & Valero‐Zornoza, ).…”

“…Seismicity generally occurs at shallow crustal levels (Buforn et al, ). Intermediate seismicity (40 to 120‐km deep) is mainly located along a N‐S elongated band in the western Alboran Basin that becomes NE‐SW northward (Buforn et al, ; López‐Casado et al, ; Medina & Cherkaoui, ; Morales et al, ). Deep seismicity (600 to 640‐km deep) is scarce but also occurs beneath the central Betic Cordilleras (Buforn et al, , ).…”

“…The main earthquake was felt in several coastal cities of northern Morocco and southern Spain, causing economic losses in both countries (http://www.ign.es/resources/noticias/Terremoto_Alboran.pdf). The earthquakes of this seismic sequence have been analyzed in detail by the IGN (http://www.ign.es), Buforn et al (), Medina and Cherkaoui (), and Kariche et al (). They consider different velocity models for epicenter locations, suggesting that activity occurred in the area nearby Al Idrisi Fault, yet they do not compare their results with the more accurate position of this fault obtained by marine geophysical research (Estrada et al, ; Lafosse et al, ; Martínez‐García et al, , ).…”