On September 6, 1993, at 0930 hours Pacific daylight time, a Cessna 182A, N4847D, encountered a surface irregularity during the landing roll on a dirt airstrip near Potrero, California. After encountering the irregularity, the nose gear collapsed and the aircraft nosed over onto it's back. The aircraft was owned and operated by the pilot and was on a local area personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed for the operation. The aircraft incurred substantial damage. The certificated commercial pilot and his one passenger were not injured. The flight originated at San Diego, California, on the day of the mishap at 0900 hours. In his statement, the pilot reported that the touchdown and initial ground roll were normal until the aircraft encountered a raised surface irregularity in the dirt airstrip. The nose wheel lifted after contact with the irregularity, and, when the nose wheel contacted the ground again, the landing gear appeared to collapse. The aircraft subsequently nosed over. The pilot stated that the nose wheel and fork assembly separated from the nose strut as the nose wheel was lifted off the airstrip by the irregularity. The pilot observed that the upper torque link had failed at a point about one inch below the upper attach point. The aircraft was examined by FAA airworthiness inspectors from the San Diego Flight Standards District Office with assistance from a mechanic who also holds an Inspection Authorization. The inspectors reported that the upper torque link fracture face exhibited overload characteristics with a clean appearance, and no discoloration or beach markings evident.