Parkfield earthquakes occurred with exceptionally regular intervals (between 20 and 30 years) between 1857 and 1966. Sieh studied four of these events (1901, 1922, 1934, and 1966) and found that they helped to determine the southeast boundary for the origination of the dawn foreshock. The coverage and intensities of felt reports for that earthquake show a solid resemblance to the Parkfield events. Only one of the four Parkfield events was not felt further southeast than the dawn shock, so Sieh concluded that if the San Andreas was the source of the event, then it was reasonable to assume that the Parkfield to Cholame stretch of the fault was responsible for producing the dawn felt intensities, though the San Andreas is not the only possible source for the dawn event. For example, the November 22, 1952, magnitude six Bryson earthquake "nearly duplicates" the felt reports. That event may have occurred west of the San Andreas on the Nacimiento fault, though the highest felt reports were along the Rinconada fault around southwest of the San Andreas.