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Complex Earthquake Deformation Drives Relative Sea-Level Change Where Oblique Contraction Focuses Rock Uplift West of the Fairweather Fault, Southeast Alaska

Description: 

Along the Yakutat-(Pacific)-North America plate boundary in southeast Alaska, episodic earthquake deformation controls relative sea-level (RSL) change by driving rapid rock uplift along Earth’s fastest slipping (≥49 mm/yr) ocean-continent transform fault, the Fairweather fault. Between Icy Point and Lituya Bay, the near-vertical Fairweather fault focuses rock uplift and rapid right-lateral slip by accommodating both vertical and fault-parallel strain during ruptures with a substantial vertical-slip component and separate, predominantly strike-slip events. We use 1-m resolution digital elevation models and offshore seismic reflection profiles to map active faults and uplifted marine and fluvial terraces, and document past reverse fault earthquakes with a maximum of 3–5 m coseismic uplift per event. We differentiate the tectonic from the glacial isostatic contribution to RSL over the past 7 ka by comparing RSL curves at Icy Point and tectonically stable Icy Strait. Radiocarbon and luminescence dating provide timing to estimate 4.6–9.0 mm/yr Holocene rock uplift rates. These rates result from plate-boundary strain that is partitioned onto west-verging reverse faults that form, together with the steeply dipping Fairweather fault, a 10-km-wide, asymmetric, positive flower structure along a 20°, 30-km-long restraining double bend in the Fairweather fault. The principal reverse fault in the flower structure is the offshore, blind Icy Point-Lituya Bay fault, above which 9–12 uplifted marine shorelines imply Holocene ruptures every 460–1040 years. Evaluated over a range of dips, the uplift on this reverse fault implies a maximum of 3.1–10 m dip-slip per event and estimated earthquake magnitudes of M_w 7.0–7.5. Our findings suggest that, first, oblique slip on the Fairweather fault at seismogenic depths occurs with and without co-rupture on the reverse fault. Second, sudden vertical land movements during earthquakes can dominate RSL changes along restraining bends in strike-slip plate boundaries. Therefore, projections of global sea level rise also must include regional tectonic vertical deformation along plate boundaries.

Session: Seismic Cycle-Driven Sea-Level Change Over Decades to Centuries: Observations and Projections [Poster Session]

Type: Poster

Date: 5/3/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Robert

Student Presenter: No

Invited Presentation: 

Authors