Paper No. 62-5
Presentation Time: 5:05 PM
PENROSE MEDAL: ROCKS AND SHOCKS: CONTINENTAL MARGIN TECTONICS, GIANT ARC MEGATHRUST EVENTS, TSUNAMI GENERATION, AND PALEOSEISMOLOGY
Copper River Delta (CRD) on the Gulf of Alaska coast in the eastern Aleutian arc is 135 km north of the PAC/NA plate boundary at the Aleutian Trench and ~12 km above the gently north-dipping Aleutian megathrust (orthogonal convergence rate ~5 cm/year). The Mw 9.2 1964 Alaska earthquake was generated by >25 m slip displacement on the megathrust that resulted in 2.2 ± 0.2 m coseismic uplift in the Alaganic Slough estuary of the CRD. The uplift abruptly brought a 12-km wide zone of intertidal mud flats above the highest tide level resulting in rapid conversion to subaerial fresh-water peat marsh, ponds, and patchy forest. Slough bank exposures and 5 cored drill holes to ~14 m below the uplifted marsh surface record 8 pre-1964 peat layers of dominantly fresh-water flora 2–45 cm thick. Peat layers commonly have sharp basal contacts and are overlain gradationally mainly by laminated gray intertidal mud layers 50–250 cm thick that contain minor fine sand laminae, salt-tolerant plant fossils, and marine microfossils. Each peat/mud “couplet” defines a complete earthquake cycle. The base of the peat defines the "event horizon", the time when intertidal mud flats were elevated during great earthquakes into the supratidal environment where fresh water peat could form. Gradual interseismic regional subsidence ultimately submerges the peat and preserves it by burial in intertidal mud to complete the cycle. Multiple high-precision 14C samples were used to date and correlate the peat event horizons between core holes and to calculate return intervals (RIs). Our data indicate that the 8 pre-1964 mean event horizons range from ~780 (E-1) to 6560 (E-8) calendar years before present (cybp). The Mw 9.2 1964 Alaska earthquake (E-0), with coseismic uplift at the Alaganic Slough area of 2.2± 0.2 m, followed a recurrence interval (RI) of ~780 years. The 7 pre-1964 RIs range from ~487 to 1300 cybd. Assuming that uplift and magnitude scale roughly with RI, uplift per event is ~0.8–2.3 m (average ~1.5 m) and magnitudes of the 8 events for which RI’s are known range from ~Mw 9.0 to 9.3. We find no evidence of earthquake uplift events and peat in the CRD strata less than Mw 9.0. Our data indicate that: (1) Energy release in this segment of the Aleutian megathrust is primarily during very large Mw 9+ seismic events and (2) Minimum median RI is ~480 years and Mw is ~9.05; (3) Maximum RI is ~1125 and Mw is 9.3; (5) the Mw 9.2 1964 earthquake is an average event at this locality.