Abstract
We develop a folding boundary element model in a medium containing a fault and elastic layers to show that anticlines growing over slipping reverse faults can be significantly amplified by mechanical layering buckling under horizontal shortening. Previous studies suggested that folds over blind reverse faults grow primarily during deformation increments associated with slips on the fault during and immediately after earthquakes. Under this assumption, the potential for earthquakes on blind faults can be determined directly from fold geometry because the amount of slip on the fault can be estimated directly from the fold geometry using the solution for a dislocation in an elastic half-space. Studies that assume folds grown solely by slip on a fault may therefore significantly overestimate fault slip. Our boundary element technique demonstrates that the fold amplitude produced in a medium containing a fault and elastic layers with free slip and subjected to layer-parallel shortening can grow to more than twice the fold amplitude produced in homogeneous media without mechanical layering under the same amount of shortening. In addition, the fold wavelengths produced by the combined fault slip and buckling mechanisms may be narrower than folds produced by fault slip in an elastic half space by a factor of two. We also show that subsurface fold geometry of the Kettleman Hills Anticline in Central California inferred from seismic reflection image is consistent with a model that incorporates layer buckling over a dipping, blind reverse fault and the coseismic uplift pattern produced during a 1985 earthquake centered over the anticline forelimb is predicted by the model.
Highlights
Geodetic observations have been interpreted as evidence that anticlines over blind faults grow as a consequence of slip on faults during earthquakes
We develop a boundary element model of the growth of an anticline over a fault embedded in a medium with elastic layers that slip at the contacts
We will demonstrate that an alternative explanation is that the 1985 earthquake may very well be typical of earthquakes on the major fault underlying the anticline, but the coseismic deformation pattern does not match the fold geometry because the fold did not grow solely as a consequence of slip on the underlying fault
Summary
Geodetic observations have been interpreted as evidence that anticlines over blind faults grow as a consequence of slip on faults during earthquakes. King and Stein (1983), Stein and King (1984), and Stein and Ekström (1992) proposed that a string of Quaternary folds in Central California at Coalinga and Kettleman Hills are produced by sudden, incremental growth during repeated earthquakes on underlying blind reverse faults (Fig. 1) This idea is leveraged by observations at Coalinga Anticline that show a similarity in pattern between uplifted river terraces, current topography and coseismic vertical displacements determined from leveling measurements before and after the 1983 Coalinga earthquake. We demonstrate that fault-cored folds in a mechanically layered medium can be significantly amplified and localized by buckling under horizontal compression To demonstrate this point, we examine the subsurface geometry and surface deformation measurements from the active Kettleman Hills and Coalinga Anticlines in Central California
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