Abstract

Traditional paradigms of continental seismicity assert the stationarity of the earthquake process and a causal association of earthquakes with active faults, increasing levels of stress, and crustal structures, in a framework of Plate Tectonics. I propose, instead, that the seismicity associated with a magnitude six or greater intraplate continental earthquake is a transient phenomenon responding to a perturbation in crustal strength independent of existing faults and crustal structures. Regional plate stress may still provide the driving energy, but the causative stress is released by a perturbation in crustal strength in the vicinity of a major earthquake. The timing of a major earthquake and the characteristics of the associated seismicity may then be described by a sequence of five phases which are as follows: (1) Initiation. A major intraplate continental earthquake is initiated with a disturbance in the hydraulic or thermal properties of the crust below the epicenter. Such disturbances could be induced by the intrusion of a sill or by partial melting. (2) Strength corrosion. A corrosion in crustal strength follows the upward migration of fluids or heat from the area of recent disturbance. (3) Stress concentration. As a weakened central zone deforms in response to tectonic plate stress, stresses are concentrated in the surrounding rigid crust. (4) Failure. A major earthquake occurs when the stress surrounding the weakened core exceeds the crustal strength, either because the concentrated stresses are anomalously high or because the dispersing fluids have spread beyond the core. (5) Crustal healing. The final phase in the occurrence of a major intraplate continental earthquake is an extended aftershock sequence which is concentrated along the rupture zone of the main event. The occurrence of a major intraplate earthquake as described above releases the strain energy in a perturbed area. Additional major events would be unlikely until the strength has recovered sufficiently to equalize intraplate stress and permit a repeat of the cycle.

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