Paleoseismic results from the Alpine site, Wasatch fault zone: Timing and displacement data for six holocene earthquakes at the Salt Lake City–Provo segment boundary

Abstract

To improve the characterization of Holocene earthquakes on the Wasatch fault zone (WFZ), we conducted light detection and ranging (lidar)‐based neotectonic mapping and excavated a paleoseismic trench across an 8‐m‐high fault scarp near Alpine, Utah, located &lt; 1 &#x2009;&#x2009; km "> < 1 km <1 km south of the boundary between the Salt Lake City and Provo segments (SLCS and PS). We document evidence for six paleoearthquakes (AL6–AL1) from scarp‐derived colluvial wedges and crosscutting relations. A ground‐penetrating radar survey across the scarp resolved fault‐zone width, but not paleoearthquake stratigraphy. Bayesian (OxCal) modeling of 13 radiocarbon and 13 optically stimulated luminescence ages indicates that six earthquakes occurred &#x223C; 6.2 &#x2013; 0.4 &#x2009;&#x2009; ka "> ∼ 6.2 – 0.4 ka ∼6.2–0.4 ka . Interseismic recurrence ranges from 0.2 to 1.8 ky (mean 1.2 ky). We estimate 6.5 &#xB1; 0.7 &#x2009;&#x2009; m "> 6.5 ± 0.7 m 6.5±0.7 m of cumulative vertical tectonic displacement across the &#x2265; 14 &#x2010; m &#x2010; wide "> ≥ 14 ‐ m ‐ wide ≥14‐m‐wide fault zone used near‐field observations of scarp‐derived colluvial‐wedge thicknesses, antithetic faulting, and graben backtilting. This is similar to our independent estimate of 6.5 &#xB1; 0.5 &#x2009;&#x2009; m "> 6.5 ± 0.5 m 6.5±0.5 m using far‐field observations of the offset ground surface and correlation of alluvial‐fan stratigraphy across the WFZ. These results suggest that colluvial‐wedge thickness at the Alpine site approximates one‐half the original west‐facing fault scarp height. Per‐event vertical displacements range from 0.8 to 1.2 m (mean 1.1 m), which we use to estimate surface rupture lengths that may exceed 50 km from earthquakes as large as moment magnitude ( M w "> M w Mw ) &#x223C; 7.0 "> ∼ 7.0 ∼7.0 . The late Holocene average vertical slip rate is 0.9 &#x2009;&#x2009; mm / yr "> 0.9 mm / yr 0.9 mm/yr ( 0.7 &#x2013; 1.2 &#x2009;&#x2009; mm / yr "> 0.7 – 1.2 mm / yr 0.7–1.2 mm/yr range). Earthquake frequency has increased in the past &#x223C; 1 &#x2009;&#x2009; ky "> ∼ 1 ky ∼1 ky , whereas displacement per event has been similar for the past &#x223C; 6 &#x2009;&#x2009; ky "> ∼ 6 ky ∼6 ky , suggesting that strain accumulation is not the sole factor that controls the frequency and size of earthquakes at the SLCS–PS segment boundary. These findings can be used for a more nuanced characterization of earthquakes at the SLCS–PS boundary and improve earthquake hazard assessments along the Wasatch Front.