Seismic Swarm S20100718.3: Analysis of Activity Near Nikolski, Alaska

The seismic swarm designated S20100718.3 occurred approximately 56 km west-southwest of Nikolski, Alaska, in the central Aleutian Islands. It began at 14:44 on 17 July 2010 and concluded at 17:31 on 7 August 2010, spanning 506 hours and 47 minutes. During this period, 625 earthquakes were recorded, highlighting a concentrated episode of seismic unrest in a region known for its intense tectonic activity. Nikolski lies along the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent earthquakes and volcanic activity throughout the Aleutian arc. The swarm’s location aligns with the broader pattern of seismicity along the arc, where shallow crustal events often accompany stress accumulation at the plate interface or within the overriding plate. Analysis of the first 100 recorded events reveals predominantly shallow focal depths between 0 and 14 km, with the majority clustered between 3 and 10 km. Magnitudes ranged from 2.0 to 6.0, including multiple events of magnitude 4.0 or greater. Notable shocks included a magnitude 6.0 earthquake at 19:48 on 18 July at 10 km depth and several magnitude 4.1–4.7 events occurring within the first 48 hours. The temporal distribution showed an initial energetic phase on 17–18 July, followed by a gradual decline, consistent with swarm behavior rather than a classic mainshock-aftershock sequence. Such swarms are uncommon in the Nikolski area. Historical records since 2000 indicate only two prior swarms in the region: one in 2007 and the current 2010 episode. This low frequency underscores the episodic nature of clustered seismicity amid the steady background of subduction-related events. The shallow depths and rapid succession of events suggest possible involvement of fluid migration or localized stress release within the volcanic arc crust, although no immediate surface deformation or eruptive activity was associated with this swarm. Continued monitoring of the Aleutian subduction zone remains essential for understanding precursory patterns that may precede larger tectonic or volcanic events. References SeismoSight internal swarm classification S20100718.3 USGS Earthquake Hazards Program – Aleutian subduction zone overview Alaska Volcano Observatory – Regional tectonic setting reports