Seismological Society of America

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, 5610 }650 cal yr 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 (PCI, 1240 } 50 cal yr 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 PCI 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, whi