VARIATIONS IN THE TIMING AND PATTERN OF RUPTURING ALONG THE NEPHI SEGMENT OF THE WASATCH FAULT ZONE, CENTRAL UTAH

The Wasatch Fault (WF) is a 370 km long segmented normal fault related to eastern Basin and Range extension. The fault is significant as several segments may exhibit an accelerated short-term, Holocene fault slip rate compared to the long-term (Late Pleistocene) rate. The long-term rate is not very well known for the individual segments, but is important in assessing the potential for temporally clustered earthquakes. Near the southern extent of the WF, the Nephi segment is composed of two strands with multiple Holocene surface ruptures: a 17 km long eastern strand and a 25 km long western strand. This study investigates the complex nature of the fault strands, documents changes in the fault displacement along strike, and constrains the short- and long-term slip rates. The examination of the spatial and temporal complexities of the Nephi segment, particularly earthquake clustering and fault strand migration, is significant as it will allow for a more accurate delineation of the strain release and earthquake hazards along a major Basin and Range normal fault.

Scarp diffusion modeling and vertical displacement data yield the rate and pattern of rupturing along the Nephi segment. Surface ruptures vertically displace Late Holocene alluvium ~1-4 m, Mid Holocene alluvium 3.5-7 m, latest Pleistocene alluvium 6-10 m and Late Pleistocene alluvium 12.5-19 m. The short-term vertical displacement rate, found for ruptures between <1 and 15 ka, ranges from .6-1.5 mm/yr, with a mean of .9 mm/yr, in contrast to the long-term rate of ~.3 mm/yr since 43-60 ka. The western and eastern strands of the segment may be rupturing independently based on: 1) a lack of Quaternary scarps on a relay fault connecting the two strands; 2) an increase in rupture complexity, and asymmetric decrease in displacement toward the area of strand overlap; and 3) a disparity in the vertical offsets related to the most recent events along the eastern (~1 m) and western (~2 m) strands. The long-term fault slip rate is on average 3 times less than the short-term rate, suggesting earthquake clustering during the Holocene, following a period of quiescence in the Late Pleistocene. The results of this study are consistent with previous analyses and indicate that resolving the long-term fault slip rate is important in clarifying the earthquake behavior and recurrence trends of the WF.