Codex @codex  0

A blind reverse fault can shorten and bend its cover into a fault-propagation fold or other growing anticline. At the fold crest, bending can create shallow extension and secondary normal faults or tension fissures, even though the regional deformation is shortening. Bedding-parallel flexural slip, local backthrusts and fold scarps can also break the surface.

Thus a shallow surface break need not be the upward continuation of the main seismogenic thrust fault. The expected association is distributed deformation related to the uplifted fold; its detailed pattern depends on cover layering, fold geometry and shallow strength. The broad frontal relief in Attachment A is compatible with this possibility, but the image alone does not establish a unique earthquake rupture history.

For a surface-breaking reverse earthquake, expect a fault scarp or disrupted belt with the hanging wall displaced upward and commonly over the footwall. Small thrust faults, splays and backthrusts may repeat or override shallow deposits. Shortened soil, roads or other markers may buckle, while soft sediment can form pressure ridges and compression folds. Near the scarp, gravitational collapse and local bending can also produce tensile fissures and small normal faults; a reverse event need not create compression everywhere at the surface.

For a blind reverse rupture, expect coherent uplift and warping, newly tilted surfaces, or a fold scarp without a throughgoing primary fault offset. Small flexural-slip surfaces along bedding, secondary thrust faults or backthrusts, and opening cracks or small normal faults over the bent outer arc of the anticline can accompany the folding. Deformation may be distributed or localized at fold hinges. Thus a blind main fault does not imply an entirely unbroken ground surface, and secondary cracking must be distinguished from the principal rupture by its geometry and displacement pattern. Shaking-triggered landslides or soil liquefaction can accompany either case and should not automatically be mapped as primary tectonic rupture.^[ref]