Seismic Swarm S20021104.2 in the Denali Fault Region

Seismic swarm S20021104.2 was recorded from 18:47 on 3 November 2002 until 22:06 on 22 November 2002, approximately 84 km east-southeast of McKinley Park, Alaska. Over 459 hours and 19 minutes, the swarm comprised 621 earthquakes. This activity occurred along the Denali Fault system, a major right-lateral strike-slip structure marking the boundary between the Pacific and North American plates in south-central Alaska.

The Denali Fault extends roughly 2,000 km across Alaska and has produced repeated large earthquakes throughout the Holocene. Regional tectonics reflect oblique convergence, with the Yakutat terrane collision driving rapid slip rates of 8–12 mm per year along the central fault segment. Shallow crustal earthquakes dominate the seismicity, consistent with the predominantly strike-slip regime.

The swarm initiated shortly before the Mw 7.9 Denali Fault earthquake of 3 November 2002, whose epicenter lay only 5 km from the swarm centroid. That mainshock ruptured approximately 340 km of the Denali and Totschunda faults, producing surface offsets up to 8.8 m. Post-mainshock aftershocks and triggered sequences were widespread, yet S20021104.2 exhibited distinct swarm-like characteristics with numerous events clustered in time and space.

Analysis of the first 100 events reveals predominantly shallow focal depths between 0 and 18 km, with the majority occurring at 1–8 km. Magnitudes ranged from 1.0 to 4.7, including twelve events of M 4.0 or greater within the initial 24 hours. Early activity featured rapid succession of moderate shocks (M 4.2–4.7) between 22:20 and 23:58 on 3 November, followed by a gradual decline into smaller events. Depths remained consistently crustal, showing no evidence of deeper migration during this interval.

Subsequent events through 5 November continued at a high rate, with magnitudes mostly between 1.8 and 3.6. The temporal pattern indicates classic swarm behavior driven by fluid migration or aseismic slip rather than a single large aftershock sequence. Such swarms have occurred previously in the Denali region; historical records since 2000 document only one prior swarm in 2002, underscoring the relative rarity of this mode of seismicity.

The 2002 Denali mainshock remains the largest earthquake in the United States since 1964. Its rupture highlighted the seismic hazard posed by the Denali Fault to nearby infrastructure, including the Trans-Alaska Pipeline. Ongoing geodetic monitoring shows continued strain accumulation west of the 2002 rupture segment, indicating that the fault retains potential for future large events.

Continued study of swarms such as S20021104.2 improves understanding of fault interaction and precursory processes along mature strike-slip systems. Real-time seismic networks now provide higher-resolution catalogs that help distinguish swarm activity from classic aftershock sequences, aiding hazard assessment in tectonically active regions like central Alaska.

References

USGS Earthquake Hazards Program – Denali Fault Earthquake 2002
Alaska Earthquake Center – Regional Seismicity Catalog
Global Centroid Moment Tensor Project – Mw 7.9 Denali Event
USGS National Earthquake Information Center – Historical Earthquake Database