The Parkfield–Cholame region, 40 to 60 km southeast of Priest Valley, lies near the junction of two contrasting segments of the San Andreas fault. To the southeast is the ∼350-km locked segment that had up to 9-m displacements in the 1857 earthquake of M 7.9. To the northwest is the ∼120-km creeping segment that since 1922 has generated four earthquakes of M ∼6, in 1922, 1934, 1966, and 2004. The 2004 earthquake was centered 10 km southeast of Parkfield, and the other three events were centered 7 to 27 km northwest of Parkfield.
From 1857 to 1922, earthquakes of M ∼6 or larger have occurred eight times, on the San Andreas fault zone within ∼15 to 75 km northwest of Parkfield, in 1857, 1860, 1877, 1881, 1882, 1885, 1901, and 1908 (Toppozada et al., 2002). Only three of these earthquakes, 1857, 1881, and 1901, were considered in Bakun and Lindh’s (1985) prediction, which focused exclusively on the ∼35-km segment centered near Parkfield. This gave the impression of quasi-regular recurrence of Parkfield region events (Fig. 1). When we include the five recently identified pre-1922 events within ∼15 to 75 km northwest of Parkfield, the picture changes to San Andreas fault seismicity decreasing with time since 1857.
The 2004 earthquake came 16 years later than predicted, and at the end of the longest interevent interval of 38 years. This reinforces the decreasing seismicity observed after the major 1857 earthquake (Toppozada et al., 2002; their figure 10). The 1857 earthquake had immediate foreshocks and an apparent rupture end ∼20 to 60 km northwest of Parkfield, near Priest Valley (Sieh, 1978a,b).
The 2004 earthquake had the most southeasterly of the Parkfield earthquake epicenters, but its intensity center or moment centroid was near Parkfield (Bakun et al., 2005). Pre-1934 earthquakes were located at their centers of maximum intensities, which approximate the moment centroids. The 12 Parkfield–Lonoak region earthquakes extended to the northwest beyond the 1857 rupture end in 1882 and 1885. Since ∼1901 they have migrated generally to the southeast. This suggests that the reach into the creeping zone of the stress loading from the 1857 fault displacements of 9 m, ∼80 km southeast of Parkfield, has decreased with time due to dissipation of stress by creep in the fault zone northwest of Parkfield. If the southeasterly migration continues, the next Parkfield earthquake could occur near or southeasterly of the 2004 Cholame epicenter. The 1857 displacement was ∼3.5 m within 10 km of Cholame (Sieh, 1978a) and has been more than recovered by the strain accumulation rate of 35 mm/year (Sieh and Jahns, 1984) from 1857 to 2005. Thus a M ∼7 or greater earthquake is now possible near Cholame and the locked zone to the southeast, which has had no known earthquakes since 1857.