Coulomb Stress Perturbations Related to the Al Hoceima (Morocco) Earthquakes of 1994 and 2004

Abstract

This paper exposes the results of the study of the stress perturbations caused by the Al Hoceima (Morocco) earthquakes of 1994 and 2004 by means of Coulomb modeling. Modeling was based on the compilation of all the studies carried out after both events, including seismological (location and depth of the main shocks and aftershocks), seismotectonic (source parameters, stress field), geodetic (GPS), tomographic and geological ones. It shows that the first earthquake is likely to have induced the second one when adopting appropriate epicenter locations, source and receiver fault planes. In detail, the model shows that motion along the N23E oriented Bousekkour-Aghbal fault in 1994 activated the eastern segment of a previously unknown NW-SE fault located at its southern end, which in turn originated the 2004 earthquake. The model also provides a suitable explanation for the distribution of the aftershock clusters. Finally, an attempt of prediction of the next event shows that it is likely to occur on NE-SW planes located to the NW (Bokkoya and offshore) and SE (reaching the Nekor fault) of the NW-SE fault, while Al Hoceima city should remain in a shadow zone.

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

“…For instance, the CSC during the seismic crisis of 2016 ( Figure 7) shows that there is an increase in CS to the north and to the south, spanning the Al-Idrissi FZ and the Algarrobo-Herradura-Djibouti banks off the Spanish margin ( Figure 1). An interesting feature is that the area NE of Al Hoceima, which was in a CS decrease zone [3], has undergone an increase of CS after 2016, and should be considered in the next years. The same can be said about the Alboran ridge, which has been activated in 2016.…”

“…CSC calculations were carried out taking into account the regional state of stress, derived from our previous R4DT determination for the Al Hoceima area [26]: σ 1 and σ 3 horizontal at 150˚E and N60˚E respectively. The stress magnitudes used by software Coulomb are 100 bar for σ 1 ; 30 bar for σ 2 ; and 0 bar for σ 3 . No vertical gradient was taken into account.…”

“…Two strips appear to be the most conspicuous: the eastern NNE-SSW alignment, with shallow hypocenters (<20 km) [21], belongs to the TASZ and runs from Almeria to The regional state of stress determined from available fault-plane solutions is a N140-N160 horizontal maximum compressional stress and an ENE-WSW to vertical minimum compressional stress [26]. Local states of stress may differ from the regional one because of local effects reflected by Coulomb stress changes at fault tips [3].…”

“…This lineament was later named "Trans-Alboran Shear Zone" (TASZ) [2], a term that was adopted by all following authors. Along the TASZ, In order to understand the causes of the January-March 2016 seismic crisis in the Alboran area north of Al Hoceima, the seismic sequence and the Coulomb stress changes (CSC) were studied with the objective to determine how the major events did jump in space (and when possible in time), taking into account the results of a previous study conducted for the large events of 1994 and 2004 [3] and the numerous recent geological and geophysical studies of the Alboran basin, which provide a new and more precise knowledge on the relations between seismicity and tectonics in the area. A similar study on the subject has been presented recently [4] based on data by Instituto Andaluz de Geofísica (IAG, Granada, Spain); however, our study, based on a long field experience since 1988, includes new relocations, new determinations of the stress tensor, the inclusion of the effects of previous events and more detailed geological and CSC analyses.…”

The Midelt earthquake of November 17th 2019, and its implications on the present-day tectonics of the junction of the High and Middle Atlas (Morocco)

“…(2) The recurrence of earthquakes based only on time has been seriously questioned by the seismotectonic studies, which have shown that, after an earthquake, stress can be transferred to other segments or to the tips of the fault (Cowie and Scholz 1992;King et al 1994). Moroccan examples are those of the Mw~5 Erfoud earthquakes of 23 and 30 October 1992 (Bensaid et al 2012), and the Mw~6 Al Hoceima area earthquakes of 1994, 2004 and 2016 (Medina 2015;Medina and Cherkaoui 2017).…”