LESSONS LEARNED FROM THE MAPLETON MEGATRENCH ABOUT THE RUPTURE BEHAVIOR OF THE WASATCH FAULT ZONE, UTAH

Excavation of a megatrench across a large scarp on post-Bonneville fan alluvium along the Provo segment of the Wasatch fault zone revealed evidence for surface-faulting events as old as 13 ka and provided multiple lessons about obtaining long paleoseismic records. The trench exposed 33 m of vertical relief across a complex, 50-m-wide deformation zone with evidence of multiple events that occurred throughout the Holocene on 4 footwall and 6 antithetic faults. Paleoseismic evidence included 18 colluvial-wedge and fissure-fill deposits, 7 buried scarp free-faces, stratigraphic truncations and differential offsets of fan sediments, and fault terminations. Debris flows and stream alluvium comprise the exposed fan deposits and ages range from historic to ~15,100 cal BP based on radiocarbon analyses of 46 charcoal samples from the trench and soil pits. Surprisingly, over 7.5 m of mid to late Holocene fan alluvium draped a large, steep (>30 degrees) paleo-scarp created by older (>7.3 ka) events on footwall faults over 16 m upslope from the main fault at the base of the scarp. This highlights the importance of adequately exposing the entire deformation zone to obtain complete faulting histories. Our preliminary evaluation indicates that at least 7, possibly 11 or more, large surface-faulting earthquakes occurred since 13,040 cal BP., and recurrence does not appear to have varied significantly during the Holocene, however, the pre-6 ka record remains incomplete. Most of the uncertainty lies in possible correlations between older events and additional analyses are ongoing. Correlations between younger events are more straightforward. At least 4, possibly 5 earthquakes occurred since 6.1 ka, including a previously unrecognized event that occurred about 1600 (1000 to 1800) cal BP. Another surprise included unusually large displacement (est. 4.7 ± 0.5 m) for the youngest event, and the possibility of coseismic rupture with the adjacent Santaquin section of the Nephi segment to the south. The four youngest events on the Provo segment occurred between 500 (350 to 650) cal BP and 4850 (4450 to 5050) cal BP, yielding a preferred average recurrence interval of 1450 years. This is much shorter than determined by previous studies, and the preferred consensus value of 2400 years assigned by the Utah Quaternary Fault Parameters Working Group.