Historically, the Parkfield segment has represented a transition in fault behavior along the San Andreas fault (saf). Despite a ∼33 mm/yr long-term slip rate along the saf, slip has not been observed on the Cholame segment since the great 1857 Fort Tejón earthquake. During that time, the Parkfield segment has experienced slip from at least six ∼M 6.0 earthquakes, aseismic fault creep, and from minor earthquakes. Data from aseismic slip-rate studies and historical earthquake studies allow us to estimate the total slip released along this portion of the saf since 1857. Assuming the saf should slip at the long-term slip rate, a slip deficit of ∼5 m exists along the Cholame segment since 1857. The slip deficit is approximately equivalent to 1857 offsets measured on the Cholame and the southeast portion of the Parkfield segment. Thus, the slip deficit in southeast Parkfield and Cholame may be as great as the slip accommodated along these segments in 1857. The slip deficit abruptly decreases to the northwest across the Parkfield segment. It is < 2 m near the town of Parkfield and ∼0 m northwest of Middle Mountain. An ∼M 7 event rupturing all or part of the Cholame segment and the southeastern Parkfield segment (slip decreasing to the northwest) would release the accumulated slip and is plausible. Importantly, this study also shows that the change in the pattern of strain release occurs in the middle of the Parkfield segment, rather than at its ends.

Online material: Tables showing aseismic and coseismic slip estimations and calculations.

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