We develop a numerical model to simulate the occurrence of multiscale characteristic earthquakes in the off-Kamaishi area of the northeast Japan subduction zone. A notable feature of these earthquake sequences is that microearthquakes occur during the later half of interseismic period between two successive M ∼ 4.9 characteristic earthquakes, which we call moderate earthquakes. We introduce a microasperity, the rupture area of the microearthquakes, with a significantly smaller nucleation size than the size of a hyper-asperity, the rupture area for moderate earthquakes. In contrast, the nucleation size in the rest of the hyper-asperity, which we call a conditional asperity, is set to be comparable to the spatial size of the hyper-asperity. Such a hierarchical asperity model successfully reproduces the observed earthquake sequences. The simulation results show that both microearthquakes and moderate earthquakes nucleate at the microasperity. The stress level in the conditional asperity determines whether a rupture terminates just outside the microasperity or propagates out into the conditional asperity.