A historical earthquake-related co-seismic deformation observed on beachrock beds along the southern shoreline of Lake Iznik is discussed as a new paleoseismic record for an 8th century AD earthquake in the Middle Strand of the North Anatolian Fault, NW Turkey. Electrical Resistivity Tomography (ERT) images beneath the beachrock surface allowed monitoring of the subsurface trace of a normal fault dipping north along a 100 m surface rupture. No strike-slip deformation exists along the rupture, suggesting that the deformation in the beachrock is connected with a secondary structure, and that the main surface rupture was under the lake waters. The deformed beds of the beachrock, dated using Optically Stimulated Luminescence (OSL) to 1.3 ± 0.15 ka, are overlain by an undeformed secondary deposition of beds dated to 1.2 ± 0.09 ka. This allows us to narrow down the time of the faulting and implying that it was most likely a result of the AD 715 earthquake.