3:45 PM - 4:00 PM
[MIS17-08] Mechanism for deep earthquakes: insights from high pressure and temperature experiments
Keywords:deep earthquakes, deformation experiments, phase transformation, semi brittle
Deep earthquakes occur at 60-660 km depth in subduction zones. In particular, earthquakes at 60-300 km depth and 410-660 km depth are called intermediate-depth earthquakes and deep-focus earthquakes, respectively. In the upper mantle, most intermediate-depth earthquakes are distributed at the upper and bottom planes of subducting slabs, forming what is called double seismic zone (Wadati-Benioff zones). In the mantle transition zone, most deep-focus earthquakes are also aligned with the upper and bottom planes of metastable olivine wedge (MOW).
With increasing depth, normal stress and temperature increase, and rock deformation mechanisms change from brittle to ductile deformation. This indicates that earthquakes caused by frictional sliding of faults and brittle fracture are inhibited. Therefore, previous studies have suggested alternative mechanism for deep earthquakes: dehydration embrittlement of hydrous minerals, thermal instability, phase transformational faulting, dehydration-driven stress transfer, and so on. I focused on dehydration embrittlement and phase transformational faulting, and have conducted rock deformation experiments under high pressure and temperature.
The upper plane of the double seismic zone in cold subducting slabs is considered to be composed of lawsonite blueschist. Shiraishi et al. (2022) performed deformation experiments on lawsonite powder and revealed that drastic stress drops with brittle fracture occurred without dehydration of lawsonite, and gradual stress drops were accompanied by the dehydration. This implies that semi-brittle behavior could be dominant even in deeper parts of the subducting slabs. At depths greater than 200-300km, dense hydrous magnesium silicates (DHMS), including Phase A, Phase D, and Phase E, become water carriers. To reveal the influence of Phase A on deep earthquakes, I performed deformation experiments on the Phase A + olivine aggregates with acoustic emission (AE) sensor which can detect crack formation. As a result, the number of AEs in the sample with a small amount of Phase A was larger than in 100 % Phase A sample. This implies that strong material such as olivine is required to generate large earthquakes during dehydration of hydrous minerals. At the mantle transition zone, a phase transformation of olivine to wadsleyite and ringwoodite is considered to be one of the mechanisms for deep-focus earthquakes. I performed the deformation experiments of germanate olivine, which is an analog material of silicate olivine, with several grain sizes to reveal the effect of microstructure of olivine on the mechanism for deep-focus earthquakes. Furthermore, calibration of the AE sensor enables us to calculate corner frequency and stress drop. I revealed that the corner frequency and stress drop measured in the laboratory follow the relationship in natural earthquakes, and the difference in the grain sizes affects b values in Gutenberg-Richter law.
In conclusion, even in the deeper parts of slabs, semi-brittle behavior can be observed. Furthermore, the calibration of AE sensors enables quantitative experimental seismology comparable to natural earthquakes.
Shiraishi, R., Muto, J., Tsunoda, A., Sawa, S., & Suzuki, A. (2022). Localized deformation of lawsonite during cold subduction. Journal of Geophysical Research: Solid Earth, 127, e2021JB022134.
With increasing depth, normal stress and temperature increase, and rock deformation mechanisms change from brittle to ductile deformation. This indicates that earthquakes caused by frictional sliding of faults and brittle fracture are inhibited. Therefore, previous studies have suggested alternative mechanism for deep earthquakes: dehydration embrittlement of hydrous minerals, thermal instability, phase transformational faulting, dehydration-driven stress transfer, and so on. I focused on dehydration embrittlement and phase transformational faulting, and have conducted rock deformation experiments under high pressure and temperature.
The upper plane of the double seismic zone in cold subducting slabs is considered to be composed of lawsonite blueschist. Shiraishi et al. (2022) performed deformation experiments on lawsonite powder and revealed that drastic stress drops with brittle fracture occurred without dehydration of lawsonite, and gradual stress drops were accompanied by the dehydration. This implies that semi-brittle behavior could be dominant even in deeper parts of the subducting slabs. At depths greater than 200-300km, dense hydrous magnesium silicates (DHMS), including Phase A, Phase D, and Phase E, become water carriers. To reveal the influence of Phase A on deep earthquakes, I performed deformation experiments on the Phase A + olivine aggregates with acoustic emission (AE) sensor which can detect crack formation. As a result, the number of AEs in the sample with a small amount of Phase A was larger than in 100 % Phase A sample. This implies that strong material such as olivine is required to generate large earthquakes during dehydration of hydrous minerals. At the mantle transition zone, a phase transformation of olivine to wadsleyite and ringwoodite is considered to be one of the mechanisms for deep-focus earthquakes. I performed the deformation experiments of germanate olivine, which is an analog material of silicate olivine, with several grain sizes to reveal the effect of microstructure of olivine on the mechanism for deep-focus earthquakes. Furthermore, calibration of the AE sensor enables us to calculate corner frequency and stress drop. I revealed that the corner frequency and stress drop measured in the laboratory follow the relationship in natural earthquakes, and the difference in the grain sizes affects b values in Gutenberg-Richter law.
In conclusion, even in the deeper parts of slabs, semi-brittle behavior can be observed. Furthermore, the calibration of AE sensors enables quantitative experimental seismology comparable to natural earthquakes.
Shiraishi, R., Muto, J., Tsunoda, A., Sawa, S., & Suzuki, A. (2022). Localized deformation of lawsonite during cold subduction. Journal of Geophysical Research: Solid Earth, 127, e2021JB022134.