Electronic Supplement to
Holocene Behavior of the Brigham City Segment: Implications for Forecasting the Next Large-Magnitude Earthquake on the Wasatch Fault Zone, Utah

by Stephen F. Personius, Christopher B. DuRoss, and Anthony J. Crone

Section 7. Description of Calls Fort gravel pit fault exposures.

Several fortuitous temporary fault exposures were observed by the lead author in a then-active gravel pit near the settlement of Calls Fort during mapping of the BCS (Personius, 1988; Personius, 1990). This site lies about 12 km north of Brigham City and 4.5 km south of the Brigham City/Collinston segment boundary (see Fig. 1). Two exposures were examined in the north walls of the pit, one of the western (primary) fault trace and one of the eastern splay (Fig. S4a). No age data were obtained from these exposures. The western strand was better exposed in the pit wall and was comprised of more than a dozen splays in a 20-m-wide zone, most of which were inaccessible due to gravel pit operations (Fig. S4b). A slightly oblique exposure of the central, most-active part of the western strand (Fig. S4c) revealed evidence for at least three earthquake ruptures that post-date final withdrawal of Lake Bonneville from the elevation of the site (~1,350-1,360 m, ~100-110 m below the Provo shoreline) <15 ka. About 2.5 m of fault-scarp colluvium fills a graben complex adjacent to this part of the fault, but projections of faulted lacustrine contacts indicate only about 0.5 m of vertical separation across the graben. Total vertical separation across the exposed extent of the western strand is estimated to be about 2.2 m. The eastern strand at the Calls Fort gravel pit was not as well exposed, but two fault splays offset a carbonate-cemented lacustrine gravel overlying Paleozoic bedrock a total of 2 m (Fig. S4d). The faulted gravel is in turn overlain by 0.5-1.4 m of undifferentiated fault-scarp colluvium deposited during one or more earthquakes.

We draw several conclusions from our analysis of the Calls Fort pit exposures: 1) surface faulting from three or more post-Provo earthquakes extended at least 12 km north of Brigham City; 2) vertical separation estimates from the western and eastern strands indicate at least 4.2 m of cumulative offset during these events, and 3) both the number and size of the Calls Fort ruptures are minimum estimates due to incomplete exposure and lack of timing information, but most importantly, the exposures do suggest fewer and probably smaller earthquake ruptures than recorded in similar-aged deposits to the south near Brigham City.

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