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[1] We compute apparent stress for 114 aftershocks (0.9 ≤ ML ≤ 3.7) of the 1999 Mw = 6.9 Quepos, Costa Rica, thrust-faulting earthquake to examine the influence of subducting plate topographic complexity near the Osa Peninsula on earthquake rupture. Using seismic coda techniques, we find a heterogeneous distribution in apparent stress of 0.1–2.5 MPa (mean 0.6 MPa) for these aftershocks. Mean aftershock apparent stress is more than twice the global mean for thrust-faulting earthquakes at oceanic subduction zones and 1.5 times the mean for events just northward along the margin near the Nicoya Peninsula where the subducting plate has lower relief. We also find constant source scaling for the Osa aftershocks. The variation in apparent stress found near the Osa Peninsula, and high mean as compared to global and regional values, suggest areas of stress concentration in the region of bathymetric complexity in the subduction zone.