Seismic swarm S20110828.1 occurred across Spain from 00:07 on 28 August 2011 to 14:02 on 29 August 2011. In 37 hours and 54 minutes, the sequence comprised 52 earthquakes. Magnitudes ranged from 0.6 to 2.7, with the largest event recorded at 06:40 on 28 August. Depths remained shallow, predominantly between 0 and 15 km, consistent with low-energy crustal activity.
The temporal distribution showed clustering in the first 12 hours, followed by a gradual decline. Early events included multiple tremors near magnitude 1.0–1.6 at depths of 5–12 km. Later activity featured isolated events up to magnitude 2.5 at 10 km depth. This pattern reflects typical swarm behavior, where numerous small events occur without a dominant mainshock-aftershock sequence.
Spain lies at the convergent boundary between the Eurasian and African tectonic plates. Moderate seismicity concentrates along the southern and southeastern margins, particularly in the Betic Cordillera and the Alboran Sea region. Historical records document destructive events such as the 1884 Andalusian earthquake and the 1954 Granada quake. Contemporary monitoring by the Instituto Geográfico Nacional confirms that swarms of low-magnitude events recur in these zones due to ongoing plate convergence and local fault interactions.
The 2011 swarm parameters align with background microseismicity levels observed in intraplate and plate-boundary settings across the Iberian Peninsula. Depths under 15 km indicate activity within the upper crust, where brittle failure predominates. No surface rupture or significant damage was associated with events of this scale.
Such sequences provide data for refining seismic hazard models. Continuous observation improves understanding of stress transfer along regional faults and supports probabilistic assessments used in building codes. Updated national seismic catalogs incorporate these micro-events to enhance resolution of active structures.