EVIDENCE FROM THE MAPLETON MEGATRENCH FOR MORE FREQUENT HOLOCENE EARTHQUAKES ON THE PROVO SEGMENT OF THE WASATCH FAULT ZONE, UTAH

We excavated a >9-m-deep and ~84-m-long “megatrench” across a 19-m-high scarp on post-Bonneville fan alluvium along the Provo segment of the Wasatch fault zone at the eastern margin of the Basin and Range Province. The purpose of our study was to extend the paleoseismic record back through the Holocene to investigate possible large variations in recurrence rates observed on adjacent segments. The trench exposed a complex, 50-m-wide deformation zone with evidence of multiple surface-faulting events that occurred throughout the Holocene on 4 footwall and 6 antithetic faults. The evidence included 18 colluvial-wedge and fissure-fill deposits, 7 buried scarp free-faces, stratigraphic 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 ~13.5 ka. A big surprise was over 4 m of mid to late Holocene fan alluvium deposited on and above a 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 slope. This highlights the importance of understanding paleotopography when determining slip rates from large scarps, and may explain anomalously high rates for some large scarps elsewhere on the Provo segment. Radiocarbon analyses of 45 charcoal samples provide age constraints, and our preliminary evaluation suggests that at least 8, possibly 11, large surface-faulting earthquakes occurred at the Mapleton site between 600 yrs ago and 10,085 +/- 35 C14 yr B.P. At least 4, possibly 5, of these events occurred since 5,305 +/- 50 C14 yr B.P. Applying calendar calibrations yields average recurrence intervals of 1,100 to 1,600 yrs. These intervals are much shorter than determined by previous studies, and the preferred value of 2,400 yrs assigned by the Utah Quaternary Fault Parameters Working Group. However, they are similar to average mid to late Holocene intervals of the adjacent Salt Lake City, Weber, and Brigham City segments of 1,300 to 1,400 yrs. They are also more consistent with the prominent Holocene geomorphic expression of this more-active central segment, and suggest that earthquake recurrence has not varied significantly from the early to late Holocene.