Supershear earthquakes are rare but extremely powerful. They occur when the rupture along a fault moves faster than the speed at which shear waves travel through the surrounding rock. Much like supersonic jets produce a sonic boom when they exceed the speed of sound, supershear earthquakes generate Mach cones—shear shock fronts that carry concentrated energy and radiate intense shaking. These waves can travel much farther than those produced by traditional (subshear) earthquakes, causing widespread destruction. A recent example is the 2023 Kahramanmaraş earthquake doublet, which caused severe damage across southern Turkey and northern Syria. Despite their devastating impact, the physical conditions that allow such fast ruptures to occur remain poorly understood. This project addresses that gap by investigating the mechanics that govern the onset and recurrence of supershear earthquakes, particularly on long, mature faults that often appear stable. The research will improve earthquake hazard models and support safer infrastructure design in earthquake-prone regions. It will also help scientists better understand how faults accumulate and release stress across multiple earthquake cycles. Broader impacts include the training of graduate and undergraduate students through mentoring, hands-on research, and collaborative group activities. A new graduate course on dynamic earthquake mechanics will bring together students from civil engineering and geophysics. Project results will be