Seismic Swarm S20041203.1: Analysis of Activity Near Al Aaroui, Morocco

Seismic swarm S20041203.1 began at 13:53 UTC on 2 December 2004 and concluded at 05:35 UTC on 8 December 2004, centered 7 km west-southwest of Al Aaroui in northeastern Morocco. Over 135 hours and 42 minutes, the sequence produced 112 earthquakes. Analysis of the first 100 events reveals predominantly shallow foci, with the majority occurring at depths between 0 and 10 km and only isolated events reaching 17–20 km. Magnitudes ranged from 2.6 to a peak of 5.1, recorded at 10:30 UTC on 4 December. The sequence featured frequent events in the 3.0–3.9 range, punctuated by several shocks exceeding 4.0, including a 4.9 event early on 2 December and a 4.6 event late on 4 December.

Temporal distribution shows intense clustering on 2–4 December, with more than 60 events in the initial 48 hours, followed by a gradual decline through 5–6 December. Depths remained consistently shallow for most events, suggesting rupture within the upper crust. No clear migration pattern is evident from the catalog, though repeated activity near the same epicentral area indicates a localized source volume.

Northeastern Morocco lies within the Rif orogenic belt, formed by the ongoing convergence between the African and Eurasian plates at rates of approximately 4–6 mm per year. This tectonic regime produces a complex network of thrust and strike-slip faults that accommodate northwest-southeast shortening. The Al Aaroui region sits near the southern margin of the Rif, where inherited structures from Miocene compression remain active. Historical seismicity in the broader area includes moderate events associated with the Nekor and Bokkoya fault systems, with notable activity documented in the late 20th and early 21st centuries.

The 2004 swarm occurred roughly nine months after the destructive 6.3 Al Hoceima earthquake of 24 February 2004, located about 80 km to the west. While the two sequences are distinct, both reflect the same regional stress field. Updated instrumental records from Moroccan and international networks confirm that northeastern Morocco experiences recurrent low-to-moderate magnitude swarms, typically lasting days to weeks and rarely exceeding magnitude 5.5.

Such swarms are characteristic of the Rif, where fluid migration and aseismic slip on shallow faults can trigger clusters of small events. The shallow depths observed in S20041203.1 align with the brittle upper crust typical of this tectonic setting. No surface rupture was reported, consistent with the modest magnitudes involved.

Continued monitoring by the Moroccan National Seismic Network and international agencies remains essential for understanding strain accumulation along the plate boundary. The 2004 sequence underscores the persistent seismic hazard in the Al Aaroui–Nador corridor despite the absence of great earthquakes in the instrumental era.

References

USGS Earthquake Catalog (2004–2024 updates)
Moroccan National Seismic Network annual reports
Fadil et al., Tectonics of the Rif Belt, Journal of Geophysical Research (updated syntheses)
IGN (Spain) and EMSC Mediterranean Seismicity Bulletins