Earthquakes with non-double-couple (NDC) components are indicators of geometric fault complexity, anisotropy, tensile failure, and fluid flow effects. NDC earthquakes have been observed in volcanic zones and interpreted as faulting related to fluid effects in the hypocentral region. In this study, we provide evidence of the occurrence of microearthquakes with NDC components in a tectonic zone. Aftershocks of the 2000 Western Tottori earthquake revealed definite NDC components based on a polarity analysis of the first P wave motion data from a hyperdense seismic observation. We modeled these events using both shear faulting with tensile failure and multiple shear ruptures. Six of the eight NDC events were well modeled by shear faults accompanied with an opening tensile crack. These results suggest that pressurized fluid or a weak tensile crack may exist along the rupture zone of the Tottori earthquake.Plain Language Summary An earthquake is caused by a relative slip between two rock faces that breaks rocks around them. The characteristics of the source process can inform us about how the rock was deformed, which is important to deepen our understanding of earthquake generation. We studied small earthquakes in the area where a large earthquake, the 2000 Western Tottori Earthquake in Japan, occurred, using hyperdense seismic observation data. In the observations, 1,000 sensors were placed in the area, 1 km from each other. We found that small earthquakes did not only slip along the fault surfaces, but also induced an opening of tensile cracks. The opening can be explained by a local high-pressure fluid or/and an ease of crack opening in the area. According to previous studies, both possibilities could be related to the large earthquake occurrence. Therefore, we conclude that the rock was damaged by fluid infiltration and dynamic shaking.