Earthquakes on Compressional Inversion Structures - Problems in Mechanics and in Hazard Assessment

Richard H. Sibson

Published September 27, 2016, SCEC Contribution #6489, 2016 SCEC Annual Meeting Talk on 9/11 18:00 (PDF)

The dip-distribution of near-pure reverse-slip ruptures includes a dominant ‘Andersonian’ cluster (dips of 30±5°) flanked by groups of low-angle thrusts (dips of 10±5°), and moderate-steep reverse faults (dips of 50±5°). These last are attributable in part to ‘compressional inversion’ – reactivation during crustal contraction of normal faults inherited from earlier crustal extension – identified by a distinctive structural-stratigraphic signature and shown by seismic profiling to be widespread. It is easier, in terms of required stress levels, to impose brittle deformation on the crust during extension; compressional inversion is thus an inevitable consequence of the Wilson Cycle of oceanic opening and closure. Examples of seismically active inversion provinces include NE Honshu, Japan, and the NW South Island, NZ, with damaging inland earthquakes < M7.5.

Reactivation mechanics does much to explain the observed dip distribution for reverse-slip ruptures, suggesting first, that low-displacement faults are characterized by ‘Byerlee’ friction (μs ~ 0.6), and second, that high fluid overpressures are needed for continued reactivation of moderate-steep reverse slip faults. Support for the latter comes from the existence of hydrothermal vein systems formed by ‘fault-valve’ action around reverse faults exhumed from seismogenic depths, and from geophysical anomalies (bright-spot reflectors, anomalously high Vp/Vs, high electrical conductivity) in the mid-crust of areas undergoing inversion. Compressional inversion earthquakes are predominantly ‘fluid-driven’ (H2O, CO2?) because failure on such structures is likely induced by accumulating fluid overpressure rather than by increasing differential stress.

Evaluating hazard from inversion structures is problematic because their surface expression is often structurally complex with misleading dip-separations, and may be obscured near the margins of sedimentary basins. Complexity also arises from competition between inversion structures and younger, more optimally oriented thrusts. Inversion structures are likely widespread throughout North America within the west coast plate boundary zone (e.g. Ventura and Los Angeles Basins within the Transverse Ranges), within mid-continental rift zones, and along the Atlantic seaboard where inherited Mesozoic rift structures are occasionally reactivated in reverse-slip earthquakes (e.g. 1982 Miramachi, New Brunswick; 1983 Goodnow, NY).

Key Words
Compressional Inversion, Mechanics, Hazard, Fluid Overpressure

Citation
Sibson, R. H. (2016, 09). Earthquakes on Compressional Inversion Structures - Problems in Mechanics and in Hazard Assessment . Oral Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology