Late Quaternary faulting and thermoluminescence dating of the East Cache fault zone, north-central Utah

Abstract

The East Cache fault zone (ECFZ), the southernmost of five en-echelon Quaternary normal faults between the Wasatch and Teton faults, displays evidence of two large paleoearthquakes in the last 15,000 years (15 ka). Stratigraphic data from a trench on the central segment of the ECFZ are augmented with one radiocarbon date, the regional radiocarbon chronology, and 14 thermoluminescence (TL) age estimates. Of the 14 TL analyses performed by Alpha Analytic Inc., only 8 satisfied our criteria for statistical acceptability (< 20 per cent coefficient of variation, < 50 per cent extrapolation to residual TL level). The TL age estimates broadly bracket the earlier faulting event at between 8.7_ 1.0 and 17.4 + 3.0 ka, and the later event at between 2.5 _+ 0.5 and 8.7 _+ 1.0 ka. One radiocarbon age further constrains the earlier event to be younger than 15,540 _ 130 yr BP. Correlation of local Quaternary deposits to the regional radiocarbon chronology provides indirect, but more tightly limiting, age constraints of ca. 4 to ca. 7 ka for the later event, and ca. 13 ka to 15,540 +_ 130 yr BP for the earlier event. Both events had small vertical displacements at the trench site (0.8 to 1.8 m), but the cumulative net vertical tectonic displacement elsewhere is up to 4.2 m, suggesrive of M s 6.9 to 7.1 paleoearthquakes. Single-event displacements and the extent of fault scarps suggest that these two paleoearthquake ruptures were probably limited to the 20-kin-long central segment of the East Cache fault zone near Logan, Utah. The elapsed time since the most recent event on the central segment (ca. 4 to ca. 7 ka) is somewhat less than the estimated recurrence interval between the latest two paleoearthquake (8.6 _ 3.0 ka). However, comparison of only a single poorly constrained recurrence interval with the elapsed time does not result in a confident assessment of future earthquake potential.