This workshop was the third in our series of four SCEC5 workshops designed to evaluate the importance of each of the four ingredients required for dynamic earthquake rupture simulations. The four ingredients are: fault geometry, fault friction, rock properties, and initial stress conditions (Figure 1). The previous two workshops in the ‘ingredients’ series were the November 2018 SCEC workshop that focused on ingredient #1, fault geometry, and the January 2020 SCEC workshop that focused on ingredient #2, fault friction. This October 2020 workshop focused on ingredient #3, rock properties. This workshop included presentations about how our choices of velocity structures and off-fault yielding affect computationally simulated earthquake rupture behavior and ground motion. We also had one presentation that introduced everyone to a new 3D rupture dynamics benchmark that simulates thermal pressurization, and we were shown the results from the 5 codes that had simulated this benchmark by the time of the workshop.