Why the Mw 6 parkfield earthquake expected in the 1985–1993 interval was postponed till 2004?

The earthquake prediction released by USGS in 1985 to the public stated that there was an Mw6.0 earthquake occurring in the Parkfield area by January 1993 with a 95% chance. This prediction has proved unsuccessful. Some researchers analyzed the reasons for the failure of the prediction on the basis of statistics. In this study, we explored this problem by means of b-value and the Earth's rotation-correlated seismicity. The results show that an increasing trend in b-value occurred prior to May 1992 and a decreasing trend with a relative decrease of more than 35% from June 1992 to early 1999. In several years before the 2004 Mw 6.0 Parkfield earthquake, the b-value remained stable at the lowest level. A significant correlation between seismic activity and Earth's rotation occurred only when the 2004 mainshock was approaching. It has been well- known that when a stressed focal medium is approaching failure due to action of a critical stress, the focal medium will reach a critical state. At this point, seismicity could be significantly correlated to Earth's rotation. So, significantly Earth's rotation-related seismic activity indicated that the focal medium in the Parkfield area was at a critical state. Since the decline in b-value corresponds to increasing stress and the significantly Earth's rotation-related seismic activity indicates the critical state of the focal medium, we suggest that two precursors are the decline (several years to more than a decade) in b-value and the significantly Earth's rotation-related seismic activity. These two precursors did not emerge during or before the 1985 Parkfield earthquake prediction period. Instead, they appeared preceding the 2004 Mw 6.0 Parkfield mainshock. This may be the reason that the Mw 6 Parkfield earthquake did not occurred till 2004.