Abstract

The Brigham City segment (BCS), the northernmost Holocene‐active segment of the Wasatch fault zone (WFZ), is considered a likely location for the next big earthquake in northern Utah. We refine the timing of the last four surface‐rupturing (∼Mw 7) earthquakes at several sites near Brigham City (BE1, 2430±250; BE2, 3490±180; BE3, 4510±530; and BE4, forumla B.P.) and calculate mean recurrence intervals (1060–1500  yr) that are greatly exceeded by the elapsed time (∼2500  yr) since the most recent surface‐rupturing earthquake (MRE). An additional rupture observed at the Pearsons Canyon site (PC1, forumla B.P.) near the southern segment boundary is probably spillover rupture from a large earthquake on the adjacent Weber segment. Our seismic moment calculations show that the PC1 rupture reduced accumulated moment on the BCS about 22%, a value that may have been enough to postpone the next large earthquake. However, our calculations suggest that the segment currently has accumulated more than twice the moment accumulated in the three previous earthquake cycles, so we suspect that additional interactions with the adjacent Weber segment contributed to the long elapse time since the MRE on the BCS. Our moment calculations indicate that the next earthquake is not only overdue, but could be larger than the previous four earthquakes. Displacement data show higher rates of latest Quaternary slip (∼1.3  mm/yr) along the southern two‐thirds of the segment. The northern third likely has experienced fewer or smaller ruptures, which suggests to us that most earthquakes initiate at the southern segment boundary.

Online Material: Cross sections, age estimates, evaluation of Box Elder delta earthquake chronology, code for OxCal models, trench logs, sources of surface offset data, and description of Call Fort gravel pit fault exposures.

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