We determined the mainshock source parameters, relocated aftershocks, and modeled broadband waveforms from 14 stations within 300 km of the 11 May 2012 Mw 4.8 Sunan earthquake. The focal mechanism, determined by the cut‐and‐paste method, is predominantly strike slip with a component of thrust and occurred at a depth of 5.1 km; the shallow focal depth likely contributed to the strong shaking in the epicentral area. The strike and dip of nodal plane I are consistent with the strike and dip of the Huangcheng‐Shuangta fault, which may have hosted the devastating 1927 Ms 8.0 Gulang earthquake and the surface trace for which is <2 km northeast of the Sunan epicenter. We used the double‐difference location method to relocate aftershocks and found that they occurred at depths from 4 to 7 km on a well‐defined plane dipping to the northwest. This plane closely corresponds with nodal plane II of our focal mechanism. The dip of the relocated aftershocks is consistent with the dip of nodal plane II. Therefore, this earthquake likely occurred on a secondary fault, in a region undergoing transpression, which indicates that deformation is not restricted to the Huangcheng‐Shuangta fault.