15:30 〜 15:45
[SSS10-23] Magnitude distribution during phase transformational faulting: Implication for deep-focus earthquakes
★招待講演
キーワード：深発地震、アコースティックエミッション、応力降下量、相転移
Deep-focus earthquakes occur at depths of 300-660 km in subducted slabs. Geophysical observations (e.g., Zhan et al., 2014) and deformation experiments (e.g., Green and Burnley, 1989) suggest that phase transformations from olivine to wadsleyite and ringwoodite are involved in the faulting process. Geophysical observations further indicate that fault geometry influences the b values in the Gutenberg-Richter law for phase transformational faulting. Meanwhile, deformation experiments reveal that b values are also influenced by rock properties, including structural heterogeneity. Rock grain sizes play a crucial role in the transformation rate, impacting the occurrence of faulting. Consequently, grain sizes may also indirectly influence b values.
We conducted deformation experiments on germanate olivine, an analog of silicate olivine, with various grain sizes (almost 10 - 200 μm) to assess the effect of grain size on b values during phase transformational faulting. We used a Griggs-type deformation apparatus and measured acoustic emissions (AE) with a transducer calibrated by laser-doppler interferometry. This calibration enabled the acquisition of AE waveforms in units of velocity (m/s), enabling a comparison to natural earthquakes. Confining pressure, temperature, and strain rate were 1.2-1.5 GPa, 973-1265 K, and 4.2×10-5-1.7×10-4 s-1, respectively. The AEs obtained upon phase-transformational faulting follow the scaling law between stress drop and corner frequencies observed in natural earthquakes. The b values of these AEs also are similar to those of natural deep-focus earthquakes and range from 0.6 to 1.1. In fine-grained aggregates, b-values are smaller than those in coarse-grained aggregates under the same deformation conditions. In fine-grained aggregates, the homogeneous formation of spinel grains at olivine grain boundaries results in lower b values. Conversely, in coarse-grained aggregates, the heterogeneous formation of spinel aggregates inside olivine grains contributes to high b values. Therefore, the heterogeneity of spinel formation appears to be a controlling factor for b values in phase transformational faulting associated with deep-focus earthquakes.
References
Burnley, P., H. W. Green and D. J. Prior (1991), JGR, vol 96, 425-443.
Zhan, Z., Kanamori, H., Tsai, V. C., Helmberger, D. V., & Wei, S. (2014), EPSL, vol 385, 89-96.
We conducted deformation experiments on germanate olivine, an analog of silicate olivine, with various grain sizes (almost 10 - 200 μm) to assess the effect of grain size on b values during phase transformational faulting. We used a Griggs-type deformation apparatus and measured acoustic emissions (AE) with a transducer calibrated by laser-doppler interferometry. This calibration enabled the acquisition of AE waveforms in units of velocity (m/s), enabling a comparison to natural earthquakes. Confining pressure, temperature, and strain rate were 1.2-1.5 GPa, 973-1265 K, and 4.2×10-5-1.7×10-4 s-1, respectively. The AEs obtained upon phase-transformational faulting follow the scaling law between stress drop and corner frequencies observed in natural earthquakes. The b values of these AEs also are similar to those of natural deep-focus earthquakes and range from 0.6 to 1.1. In fine-grained aggregates, b-values are smaller than those in coarse-grained aggregates under the same deformation conditions. In fine-grained aggregates, the homogeneous formation of spinel grains at olivine grain boundaries results in lower b values. Conversely, in coarse-grained aggregates, the heterogeneous formation of spinel aggregates inside olivine grains contributes to high b values. Therefore, the heterogeneity of spinel formation appears to be a controlling factor for b values in phase transformational faulting associated with deep-focus earthquakes.
References
Burnley, P., H. W. Green and D. J. Prior (1991), JGR, vol 96, 425-443.
Zhan, Z., Kanamori, H., Tsai, V. C., Helmberger, D. V., & Wei, S. (2014), EPSL, vol 385, 89-96.