Fig. 1 Multi-spectral image of the Coalinga - Kettleman Hills - Lost Hills fold and thrust belt in California.

We investigate the nature of blind-thrust earthquake segmentation through the development of 3D geologic and geophysical models of the western San Joaquin basin fold-and-thrust belt of central California. 3 major earthquakes occurred on distinct segments of a blind-thrust system in the western basin, beneath the New Idria, Coalinga, and Kettleman Hills anticlines, which are part of an en echelon fold system that extends south to Lost Hills. Previous studies have demonstrated that these anticlines are fault-related folds that develop in response to motions on the blind-thrust system. The offsets between the folds are thought to overlie geometric segment boundaries in the underlying blind-thrust fault, but the precise nature of the subsurface fault geometry remains unresolved. As recent studies on blind-thrusts have demonstrated, the geometric relationships between individual segments of a blind-thrust system have an influence on seismic hazard assessment. Therefore, to better define these geometric segment boundaries, we construct comprehensive 3D models of the blind-thrust system that integrate more than 10,000 km of industry seismic reflection data, 200 well logs, surface geology, remote sensing data, and seismicity. In these structural models, kinematic indicators such as growth strata allow us to distinguish between competing models of fold growth and fault activity. From this analysis, we determine that the northern system of fault-related folds implies discrete fault segment boundaries that correspond to the patterns of seismicity; whereas, the southern system appears to be more continuous along the trend with less well defined segment boundaries and minimal seismic activity. The changes in fold segmentation style and earthquake activity also correspond to the position on the San Andreas fault, which lies 40 km to the west, where motion changes from locked to creeping. This may imply that variations in stress transfer from the San Andreas influence blind thrust segmentation and fold growth in the western San Joaquin basin.

This work is supported by: NSF, Chevron-Texaco, Aera Energy, Schlumberger-Doll.