The 2016 M7.1 Pedro Bay Earthquake and Alaskan Subduction Zone Geology

The M7.1 earthquake of 24 January 2016 struck at 10:30 UTC, with its epicenter located 47 km east-southeast of Pedro Bay, Alaska, at a depth of 125.6 km. This intermediate-depth event occurred within the subducting Pacific plate beneath the North American plate in the Cook Inlet region. The quake produced moderate to strong shaking across parts of the Alaska Peninsula and was felt as far as Anchorage, though it caused limited structural damage due to its depth and the sparse population in the immediate epicentral area.

Pedro Bay lies on the western shore of Iliamna Lake, within a tectonically active corridor influenced by the Aleutian subduction zone. Here the Pacific plate converges with and descends beneath the North American plate at rates of approximately 6–7 cm per year. This ongoing subduction generates both shallow megathrust earthquakes and deeper intraslab events like the 2016 quake. The latter type typically occurs within the bending or dehydrating slab at depths between 70 and 200 km, where mineral phase changes can trigger brittle failure.

Geologically, the region features a complex assemblage of Mesozoic to Cenozoic volcanic arcs, accreted terranes, and sedimentary basins. The Bruin Bay Fault and related structures accommodate some of the regional strain, while active volcanism at nearby Redoubt and Iliamna volcanoes reflects the same subduction-driven magmatism. Historical seismicity in south-central Alaska includes the great 1964 M9.2 Prince William Sound earthquake, which ruptured a much shallower portion of the plate interface roughly 300 km to the east. Since 2000, the M7.1 event near Pedro Bay stands as the strongest recorded in the immediate vicinity.

Post-event analysis by seismic networks confirmed aftershock activity concentrated along a steeply dipping plane consistent with intraslab faulting. Ground-motion recordings indicated peak accelerations below levels expected to produce widespread landslides or liquefaction, aligning with the event’s depth. Long-term monitoring continues to refine slab geometry and stress transfer models for the Cook Inlet segment of the subduction zone.

References

USGS Earthquake Catalog
Alaska Earthquake Center annual reports
NOAA National Centers for Environmental Information