Magnitude Distribution During Transformational Faulting : Implication for Deep-Focus Earthquakes

Titelangaben

Sawa, S. ; Gasc, J. ; Miyajima, Nobuyoshi ; Schubnel, A. ; Baïsset, M. ; Muto, J.:
Magnitude Distribution During Transformational Faulting : Implication for Deep-Focus Earthquakes.
In: Journal of Geophysical Research: Solid Earth. Bd. 130 (2025) Heft 8 . - e2025JB031779.
ISSN 2169-9356
DOI: https://doi.org/10.1029/2025JB031779

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Abstract

Deep-focus earthquakes occur at 300–660 km depth. Geophysical observations and deformation experiments have proposed the olivine-spinel (wadsleyite/ringwoodite) phase transformation as the source of the mechanical instability. While geophysical observations indicate that fault geometry influences the b-values of the Gutenberg-Richter law for transformational faulting, deformation experiments reveal that b-values are also influenced by rock properties, including structural heterogeneity. Grain sizes play a crucial role in the rate of phase transformation, impacting the occurrence of faulting. Consequently, grain sizes may also influence b-values. We conducted deformation experiments on germanate olivine (Mg2GeO4), an analog to silicate olivine, with various grain sizes to reveal the effect of grain size on b-value during transformational faulting. We used a Griggs-type deformation apparatus and measured acoustic emissions (AE) with an AE transducer, which was calibrated by laser-doppler interferometry. This calibration enabled the acquisition of AE waveforms with a unit of velocity (m/s), facilitating comparison to natural earthquakes. b-values in the fine-grained aggregates (a few um) are smaller than those in the coarse-grained aggregates (hundreds of um) at the same deformation conditions. In coarse-grained aggregates, heterogeneous formation of spinel aggregates contributes to high b-values. Conversely, in fine-grained aggregates, homogeneous formation of spinel grains at the grain boundaries results in lower b-values. Therefore, the homogeneity (or heterogeneity) of spinel formation could be one of the factors controlling the b-values of deep-focus earthquakes.