We analyze and interpret two marine multichannel seismic reflection/onshore-offshore/ocean bottom seismometer (OBS) seismic profiles that cross in the region of the Mendocino Triple Junction over the fault plane of the 1992 Petrolia, California, Ms = 7.1 earthquake. From the offshore reflection data and the wide-angle constraints provided by the onshore-offshore and OBS data we have determined a rough three-dimensional view of crustal and uppermost mantle structure beneath the triple junction region. This structure is also consistent with gravity data. The upper and mid crust has seismic velocities and densities consistent with accretionary complex rocks of the Franciscan terrane; a ubiquitous lower crustal layer with velocities of 6.5--7.1 km s-1 is 5--7 km thick throughout most of the region. Over an area extending at least 25 km to the south and 60 km to the west of the triple junction the lower crustal layer is up to 12 km thick; this area is overlain by undisturbed pre-11 Ma sediments, indicating little deformation since that date. The thick lower crust extends beneath the coast, suggesting that the deep boundary between Pacific and North American terranes lies onshore. The northeastern edge of the thick lower crustal layer is marked by concentrated seismicity, with the hypocenter of the 1992 Petrolia earthquake lying close to the NE corner. We speculate that at depth the Pacific lower crust acts an asperity acting to concentrate stress, from which the Petrolia earthquake ruptured.