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[SCG45-P06] Experimental study of lawsonite blueschist for the mechanism of intermediate-depth earthquakes: insights from acoustic emission
Keywords:lawsonite blueschist, acoustic emission, deformation experiment
In cold subduction zones, such as those beneath northeastern (NE) Japan, intermediate-depth earthquakes occur at 60-100 km depth within the oceanic crust. The mechanism is considered to be associated with phase transitions and brittle failure of lawsonite and glaucophane in lawsonite blueschist, based on experimental studies and seismological observations (Incel et al., 2017; Kita et al., 2006; Okazaki and Hirth, 2016; Shiraishi et al., 2022). Most previous experimental studies have used powdered samples, and deformation experiments using lawsonite blueschist itself are limited. The foliation and the presence of minerals other than lawsonite and glaucophane are expected to influence the process of microfracture in blueschist.
In this study, we conducted shear deformation experiments on lawsonite blueschist using a Green-type solid-medium triaxial deformation apparatus under pressure-temperature (P-T) conditions (1 GPa, 200℃, and 2 GPa, 400℃) corresponding to those at the upper Wadati–Benioff plane to investigate the mechanical behavior in lawsonite blueschist. Two types of experiments were performed: deformation with temperature ramping and steady-temperature deformation. In the temperature ramping experiments, the temperature was systematically increased during deformation to cross the phase transition boundaries of lawsonite and glaucophane. The shear direction was set parallel to the foliation. This setup enabled us to examine the relationship between mineral reactions and fault development in lawsonite blueschist. The acoustic emission (AE) system was used to simultaneously monitor seismic characteristics during brittle failure.
At the same temperature ramping rate and equivalent strain rate as in previous studies that supported lawsonite dehydration in powdered samples, lawsonite blueschist exhibited slow stress drop accompanied by AE occurrence. In experiments at both confining pressures of 1 GPa and 2 GPa, lawsonite blueschist consistently showed substantial weakening immediately after the temperature increase. This was accompanied by AE with a large root-mean-square (RMS) signal, indicative of brittle failure during stress drop, which led to fault slip destabilization. Recovered samples from temperature ramping experiments exhibited intense shear localization including submicron-sized gouge and fractured minerals derived from starting material, such as lawsonite, glaucophane, and chlorite. In contrast, during the steady-temperature deformation experiment, no substantial weakening was observed for the same deformation duration within the stability field of lawsonite and glaucophane.
The absence of phase transition products along the shear planes indicates that intact samples underwent stable slip during temperature ramping deformation without mineral reactions, in contrast to previous studies using powdered samples at the same temperature ramping rate. The substantial weakening was observed immediately after the temperature increased, while stress continued to accumulate during steady-temperature deformation. This suggests that the weakening of lawsonite blueschist is primarily driven by ductile deformation induced by temperature ramping. The AE generated by brittle failure during stable slip indicates that lawsonite blueschist exhibits semi-brittle behavior in the absence of mineral reactions.
In cold subduction zones, we propose that the semi-brittle behavior of lawsonite blueschist may facilitate fault development and small earthquakes in the oceanic crust at depths up to 70 km, where P-T conditions are comparable to those in our study.
References
Incel, S., Hilairet, N., Labrousse, L., John, T., Deldicque, D., Ferrand, T., et al. (2017), EPSL, vol 459, 320–331.
Kita, S., Okada, T., Nakajima, J., Matsuzawa, T., and Hasegawa, A. (2006), GRL, vol 33, L24310.
Okazaki, K., and Hirth, G. (2016), Nature, vol 530(7588), 81–84.
Shiraishi, R., Muto, J., Tsunoda, A., Sawa, S., and Suzuki, A. (2022), JGR, vol 127, e2021JB022134.
In this study, we conducted shear deformation experiments on lawsonite blueschist using a Green-type solid-medium triaxial deformation apparatus under pressure-temperature (P-T) conditions (1 GPa, 200℃, and 2 GPa, 400℃) corresponding to those at the upper Wadati–Benioff plane to investigate the mechanical behavior in lawsonite blueschist. Two types of experiments were performed: deformation with temperature ramping and steady-temperature deformation. In the temperature ramping experiments, the temperature was systematically increased during deformation to cross the phase transition boundaries of lawsonite and glaucophane. The shear direction was set parallel to the foliation. This setup enabled us to examine the relationship between mineral reactions and fault development in lawsonite blueschist. The acoustic emission (AE) system was used to simultaneously monitor seismic characteristics during brittle failure.
At the same temperature ramping rate and equivalent strain rate as in previous studies that supported lawsonite dehydration in powdered samples, lawsonite blueschist exhibited slow stress drop accompanied by AE occurrence. In experiments at both confining pressures of 1 GPa and 2 GPa, lawsonite blueschist consistently showed substantial weakening immediately after the temperature increase. This was accompanied by AE with a large root-mean-square (RMS) signal, indicative of brittle failure during stress drop, which led to fault slip destabilization. Recovered samples from temperature ramping experiments exhibited intense shear localization including submicron-sized gouge and fractured minerals derived from starting material, such as lawsonite, glaucophane, and chlorite. In contrast, during the steady-temperature deformation experiment, no substantial weakening was observed for the same deformation duration within the stability field of lawsonite and glaucophane.
The absence of phase transition products along the shear planes indicates that intact samples underwent stable slip during temperature ramping deformation without mineral reactions, in contrast to previous studies using powdered samples at the same temperature ramping rate. The substantial weakening was observed immediately after the temperature increased, while stress continued to accumulate during steady-temperature deformation. This suggests that the weakening of lawsonite blueschist is primarily driven by ductile deformation induced by temperature ramping. The AE generated by brittle failure during stable slip indicates that lawsonite blueschist exhibits semi-brittle behavior in the absence of mineral reactions.
In cold subduction zones, we propose that the semi-brittle behavior of lawsonite blueschist may facilitate fault development and small earthquakes in the oceanic crust at depths up to 70 km, where P-T conditions are comparable to those in our study.
References
Incel, S., Hilairet, N., Labrousse, L., John, T., Deldicque, D., Ferrand, T., et al. (2017), EPSL, vol 459, 320–331.
Kita, S., Okada, T., Nakajima, J., Matsuzawa, T., and Hasegawa, A. (2006), GRL, vol 33, L24310.
Okazaki, K., and Hirth, G. (2016), Nature, vol 530(7588), 81–84.
Shiraishi, R., Muto, J., Tsunoda, A., Sawa, S., and Suzuki, A. (2022), JGR, vol 127, e2021JB022134.