Large earthquakes often occur on faults that were known to have produced destructive events in the past. However, anticipating the characteristics of these earthquakes and their impacts remains a great challenge. The 2025 M_w 7.7 Mandalay earthquake was produced by rupture of an unusually long stretch of the Sagaing Fault in Myanmar, primarily a section that had not broken since 1839 and was considered a zone of high hazard as well as sections that experienced more recent earthquakes. These observations challenge the usual approaches used in seismic hazard studies to evaluate how a fault is spatially divided and strain is built up and released over time. We show that physics-based simulations of earthquake sequences can provide an alternative approach.