Transient creep in olivine controls post-seismic deformation

・In situ deformation experiments on olivine were performed using a high-flux synchrotron X-ray and a “mobile” multianvil apparatus at BL05XU, SPring-8. ・Transient creep of olivine aggregates follows the Burgers creep function at upper mantle pressures and temperatures. ・Time-dependent rheology of the shallow mantle observed in the late post-seismic deformation is explained by the Burgers model.

【Post-seismic deformation】
Upper: A cross-section view of a subduction zone just after an earthquake on the plate boundary. Tensile stress is coseismically induced in the plates.
Lower: A few years after the earthquake. Post-seismic displacements of land are significant due to the upwelling flow of the uppermost mantle induced by the earthquake. After Sun et al. (2014 Nature).

【Transient creep of olivine】
Differential stress (color symbols), pressure and strain (gray circles), and temperature (orange line) plotted against time in a deformation run for an olivine sample. A sudden increase in temperature (from 570 to 870 K) was followed by a synchronized pressure jump (due to thermal expansion of olivine). The decrease in stress and the increase in strain (due to transient creep of olivine) was delayed by 25 seconds. Note that stress values are obtained from diffraction peaks of olivine (blue: 021; pink: 101; orange: 130).

Geodetic observations reveal that the deformation of the crust and upper mantle after a great earthquake continues for decades (Fig. 1). Viscosities of the upper mantle estimated from early post-seismic deformation are often significantly low (1017−1018 Pa·s) just after the earthquake then continuously increase to a typical value of ~1020 Pa·s with the passage of time. This characteristic of post-seismic mantle flow cannot be explained by partial melting (nor water weakening) of upper mantle rocks. We performed small-strain deformation experiments on natural olivine, which is the major mineral in the upper mantle, via a state-of-the-art large-volume deformation apparatus (multianvil apparatus) combined with high-flux synchrotron X-ray observations. We have successfully shown that the reported time-dependent crustal deformation, which continues for decades after a great earthquake, is explained by the transient creep of olivine (Fig. 2).

Published: 19 Jun 2024


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Japan Society for the Promotion of Science (JSPS) KAKENHI 19H00722, 23H00147, Mitsubishi Foundation 202310008