A seismic zone that has been a persistent source of seismicity for at least two centuries contains most of the known earthquakes in southeastern Pennsylvania and is named the Lancaster seismic zone (LSZ) from the county it approximately occupies. The LSZ is identified by new epicenters for known and previously unknown earthquakes obtained from a systematic search of newspapers covering one and a half centuries of the preinstrumental period (1750 to 1900). The new preinstrumental epicenters and reliable instrumental epicenters indicate that the LSZ is about 50 km long and spans the belt of allochthonous Appalachian crystallines between the Great Valley and the Martic Line, an early Paleozoic suture. The LSZ also bridges across the Newark-Gettysburg Triassic Basin. The overall level and character of seismicity on the LSZ has been uniform at least since 1800; nevertheless, three bursts can be identified. The latest burst started in the middle 1960's. The 23 April 1984 Martic earthquake (MbLg = 4.1) is the largest in this burst and resembles in size the largest events in the other bursts. The 1984 event is located in the southernmost portion of the LSZ. Hypocenter distribution and first-motion data from several locally recorded events in the 1984 sequence suggest a seismogenic fault in a narrow depth range centered at 4.5 km. The inferred rupture dips steeply east with reverse and right-lateral movement consistent with ENE horizontal maximum compression. The Rockhill Jurassic dike and associated faults strike NNE and may control the geometry of the 1984 rupture and the overall distribution of seismicity in the LSZ.