Seismic Swarm PS20170912.1: Analysis of September 2017 Events Southwest of Manuel Ávila Camacho, Mexico
A seismic swarm designated PS20170912.1 occurred in southern Mexico, centered 102 km southwest of Manuel Ávila Camacho (Ponte Duro). The sequence began at 14:55 on 11 September 2017 and concluded at 02:01 on 13 September 2017, spanning 35 hours and 5 minutes. Eight earthquakes were recorded during this interval, with magnitudes ranging from 4.6 to 5.5 and focal depths between 14 km and 67 km.
The events unfolded as follows: a magnitude 5.0 quake at 14:55:58 on 11 September at 32 km depth; a magnitude 5.5 event at 21:09:10 the same day at 27 km; another magnitude 5.0 at 22:20:53 at 48 km; a magnitude 4.6 at 01:00:19 on 12 September at 67 km; a magnitude 5.1 at 01:12:29 at 17 km; a magnitude 5.4 at 05:08:45 at 42 km; a magnitude 5.0 at 15:28:21 at 37 km; and a final magnitude 5.0 at 02:01:38 on 13 September at 14 km. These clustered occurrences reflect typical swarm behavior, where multiple events of moderate size occur without a single dominant mainshock.
The swarm location lies within the tectonically active southern Mexico region, part of the Middle America subduction zone. Here, the Cocos Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year along the Middle America Trench. This convergence generates frequent intermediate-depth seismicity and occasional shallow crustal events. Updated geological assessments confirm the area's position near the Tehuantepec Ridge, where variations in slab geometry influence rupture patterns and aftershock distributions.
The swarm followed closely after the magnitude 8.2 Tehuantepec earthquake of 8 September 2017, whose epicenter lay about 44 km from the swarm centroid. That event, one of the largest instrumentally recorded in Mexico, ruptured a substantial portion of the subducting slab at depths exceeding 40 km. Post-event analyses indicate that static stress changes from the mainshock likely contributed to triggering the subsequent swarm activity through Coulomb stress transfer on nearby fault segments.
Historical records since 1 January 2000 document only this single swarm in the immediate vicinity. No comparable sequences have been identified in the region prior to 2017, underscoring the relative rarity of swarm-type activity compared with typical aftershock sequences or isolated mainshocks in this subduction setting. The 2017 Tehuantepec mainshock remains the dominant strong event in the modern catalog for the area.
Seismic swarms in subduction environments often signal fluid migration or aseismic slip processes along the plate interface. In this instance, the depth range of the recorded events spans both crustal and upper-slab levels, consistent with stress perturbations following a major intraslab rupture. Continued monitoring by regional networks provides essential data for refining hazard models in southern Mexico, where population centers remain exposed to both large subduction earthquakes and associated secondary sequences.