Earthquake Swarm S20021227.1 Near Cantwell, Alaska

SeismoSight recorded Swarm S20021227.1 beginning at 17:09 on 26 December 2002 and concluding at 12:17 on 3 January 2003. The sequence occurred 46 km ENE of Cantwell in the central Alaska Range and comprised 98 earthquakes over 187 hours and 7 minutes.

The swarm initiated with low-magnitude events near 3–4 km depth. Activity intensified on 27 December, including a magnitude 4.3 earthquake at 6 km depth followed by numerous events between 1 and 13 km. Subsequent days featured a broad depth range extending to 19 km, with the largest subsequent shock reaching magnitude 3.5 on 2 January 2003 at 5 km depth. Magnitudes remained predominantly below 2.0, producing a classic swarm pattern of clustered, non-mainshock-aftershock seismicity.

Cantwell lies within the tectonically active Alaska Range, where ongoing convergence between the Pacific and North American plates drives right-lateral strike-slip motion along the Denali Fault system. The fault accommodates approximately 8–12 mm per year of slip and has generated large historical earthquakes, including the magnitude 7.9 Denali event of 2002. Crustal seismicity in this region commonly occurs at depths of 5–15 km within Paleozoic to Mesozoic metamorphic and igneous rocks intruded by younger granitic bodies. Shallow seismicity such as that observed in the swarm reflects brittle failure above the brittle-ductile transition.

Since 1 January 2000, nine earthquake swarms have been documented in the broader area, with S20021227.1 representing the earliest recorded under the current classification scheme. These swarms provide valuable data on fluid migration and stress transfer along subsidiary structures adjacent to the main Denali Fault trace.

The 2002–2003 swarm offers insight into episodic, low-magnitude energy release that may precede or accompany larger tectonic adjustments in south-central Alaska. Continued monitoring by regional networks remains essential for understanding long-term seismic hazard in this rapidly deforming mountain belt.

References

Alaska Earthquake Center, University of Alaska Fairbanks.
U.S. Geological Survey Earthquake Hazards Program.
SeismoSight internal swarm classification records.