Magnitude distribution during phase transformation faulting: Implication
for deep-focus earthquakes
Abstract
Deep-focus earthquakes occur at 300-660 km depth. Geophysical
observations and deformation experiments propose the olivine-spinel
(wadsleyite/ringwoodite) phase transformation as the faulting mechanism.
While geophysical observations indicate that fault geometry influences
the b values in the Gutenberg-Richter law for the phase transformation
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, an
analog silicate olivine material, with various grain sizes to reveal the
effect of grain size on the difference in b value during the phase
transformation 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 μm) are smaller than those in the coarse-grained
aggregates (hundreds μm) at the same deformation conditions. In the
coarse-grained aggregates, the heterogeneous formation of spinel
aggregates contributes to high b values. Conversely, in the fine-grained
aggregates, the homogeneous formation of spinel grains inside olivine at
the grain boundaries results in lower b values. Therefore, the
homogeneity (or heterogeneity) of spinel formation appears to be a
controlling factor for b values in phase transformation faulting
associated with deep-focus earthquakes.