The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region

Abstract

The 2016 January 25 earthquake (M w 6.3) follows in sequence from the1994 May 26 earthquake (M w 6.0) and the 2004 February 24 earthquake (M w 6.4) in the Rif Mountains and Alboran Sea. The earlier two seismic events which were destructive took place on inland conjugate faults, and the third event occurred on an offshore fault. These earthquake sequences occurred within a period of 22 yr at ∼25 km distance and 11-16-km depth. The three events have similar strike-slip focal mechanism solutions with NNE-SSW trending left-lateral faulting for the 1994 and 2016 events and NW-SE trending right-lateral faulting for the 2004 event. This shallow seismic sequence offers the possibility (i) to model the change in Coulomb Failure Function (CFF with low μ including the pore pressure change) and understand fault-rupture interaction, and (ii) to analyse the effect of pore fluid on the rupture mechanism, and infer the clock-time advance. The variation of static stress change has a direct impact on the main shock, aftershocks and related positive lobes of the 2004 earthquake rupture with a stress change increase of 0.7-1.1 bar. Similarly, the 2004 main shock and aftershocks indicate loading zones with a stress change (>0.25 bar) that includes the 2016 earthquake rupture. The tectonic loading of 19-24 nanostrain yr −1 obtained from the seismicity catalogue of Morocco is comparable to the 5.0 × 10 17 N•m yr −1 seismic strain release in the Rif Mountains. The seismic sequence is apparently controlled by the poroelastic properties of the seismogenic layer that depend on the undrained and drained fluid conditions. The short interseismic period between main shocks and higher rate of aftershocks with relatively large magnitudes (4 < M w < 5.5) implies the pore-fluid physical effect in undrained and drained conditions. The stress-rate ranges between 461 and 582 Pa yr −1 with a CFF of 0.2-1.1 bar. The computed clock-time advance reaches 239 ± 22 yr in agreement with the ∼10 yr delay between main shocks. The calculated static stress change of 0.9-1.3 bar, under pore-fluid stimulus added with well-constrained geodetic and seismic strain rates are critical for any seismic hazard assessment.

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, ).…”

“…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, , ).…”

Active Faulting Geometry and Stress Pattern Near Complex Strike‐Slip Systems Along the Maghreb Region: Constraints on Active Convergence in the Western Mediterranean

“…This NE-SW 10.1029/2018TC004983 Tectonics trending fault system was active throughout the Neogene and Quaternary period, by contributing/reacting to the westward motion of the Gibraltar Arc (e.g., Bousquet, 1979;Masana et al, 2004). In the Al-Hoceima/ Nekor region (southern limit of the CF shear zone), a band of strike-slip faults oriented roughly NE-SW, and associated with Neogene volcanism (Bousquet, 1979), is the site of shallow seismicity where the most important events are the 1994 and 2004 (M w 6.4) Al Hoceima earthquakes (e.g., Kariche et al, 2018;Medina, 1995;Ousadou et al, 2014;Stich et al, 2003). Oblique to the CF shear fracture, the current reactivation of the EW to SE trending conjugate Yusuf fault is marked by a right-lateral with normal component type of focal mechanism ( Figure 5) in the vicinity of its pull-apart basin .…”