Abstract

Compressional inversion involves reverse‐slip reactivation (± strike slip) of normal faults inherited from earlier crustal extension during crustal shortening. Examples of seismically active inversion provinces with damaging earthquakes M<7.8 range from active island arc systems to formerly rifted cratonic crust. Inversion structures are characterized by a distinctive structural‐stratigraphic signature shown by seismic reflection profiling to be quite widespread. Compressional inversion is largely responsible for an anomalous group of seismically active reverse faults dipping at 45°–60°, distinct from dominant Andersonian thrust dips of 30°±5°. Time‐averaged slip rates on such structures are generally slow (about 1  mm/yr, or less) and total reverse displacement is often limited. Reactivation mechanics does much to explain the observed dip distribution for reverse‐slip ruptures, suggesting first that low‐displacement faults are characterized overall by Byerlee friction (μs0.6), and second that high fluid overpressures are needed for reactivation of moderate‐steep reverse faults. Support for the latter comes from hydrothermal veining apparently produced by fault‐valve action on reverse faults exhumed from seismogenic depths, and from distinctive geophysical anomalies in the midcrust of areas undergoing active inversion. Nucleation of earthquake ruptures on inversion structures appears to be fluid driven (H2O, CO2, etc.), with failure triggered by locally rising fluid overpressure rather than by increasing differential stress alone. Evaluation of seismic hazard from inversion structures is problematic because of their slow slip rate and long recurrence intervals, and also because their surface expression is structurally complex and often obscured. Structural‐stratigraphic complexity is compounded by competition between inversion structures and younger, more optimally oriented thrust faults, and by subsidiary strike‐slip accompanying inversion.

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