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We have determined the three‐dimensional P wave velocity structure within the area of the Hengill‐Grensdalur central volcano complex, southwest Iceland, from the tomographic inversion of 2409 P wave arrival times recorded by a local earthquake experiment. The aperture of the 20‐element seismic network utilized in the inversion permitted imaging of a 5‐km‐thick crustal volume underlying a 15×14 km² area. Within this localized volume are located the underpinnings of the active Hengill volcano and fissure swarm, the extinct Grensdalur volcano, and an active high‐temperature geothermal field. It was thus expected that the characteristic length scale of heterogeneity would be of the order of a kilometer. In order to image heterogeneous seismic velocity structure at this scale we paid particular attention to the fidelity of the assumed model parameterization, defined as the degree to which the parameterization can …