Seismic Swarm Analysis: January 2016 Events Near Al Hoceïma, Morocco

A seismic swarm designated PS20160125.1 was recorded 50 km north-northeast of Al Hoceïma in northern Morocco on 25 January 2016. The sequence began at 04:22 and concluded at 06:10, encompassing six earthquakes within a span of one hour and forty-eight minutes. The events exhibited magnitudes ranging from 4.7 to 6.3, with focal depths primarily clustered around 10–12 km.

The initial event registered a magnitude of 6.3 at 12 km depth. Subsequent shocks included a magnitude 5.0 at 10 km depth, followed by a magnitude 5.3 also at 10 km. Additional events comprised magnitudes of 4.7, 5.1, and 5.2, each at 10 km depth. This rapid succession of moderate-to-strong tremors without a prolonged aftershock tail characterizes the swarm-type behavior observed.

The region lies within the Rif Mountains, part of the broader Alpine-Mediterranean orogenic belt. Tectonic deformation arises from the oblique convergence between the Nubian and Eurasian plates at rates of approximately 4–6 mm per year. This setting produces a combination of thrust and strike-slip faulting, with the Nekor Fault and associated structures contributing to elevated seismicity. Northern Morocco has experienced recurrent moderate earthquakes, consistent with the compressional regime along the plate boundary.

Historical records indicate limited swarm activity since 2000, with only three documented sequences prior to 2016. These occurred in 2004 (two events) and 2007 (one event), underscoring the infrequent nature of clustered seismicity in the immediate vicinity. The 2016 swarm aligns with this pattern of episodic, short-duration activity rather than prolonged mainshock-aftershock sequences.

Such swarms provide insight into fluid migration or stress triggering along pre-existing faults within the Rif domain. Depths around 10–12 km correspond to the brittle-ductile transition zone, where small stress perturbations can initiate multiple ruptures in quick succession. The absence of a dominant aftershock decay further supports classification as a swarm driven by localized aseismic processes.

References

USGS Earthquake Catalog EMSC Seismic Database Moroccan National Institute of Geophysics Scientific literature on Rif tectonics (e.g., journal articles on Nubian-Eurasian convergence)