Abstract
Because they provide information about the spatial distribution of brittle deformation, both seismologists and experimentalists use b-values to study earthquake populations. Here, we present the b-values for intermediate-depth intraslab earthquakes in the Pacific slab beneath the Tohoku and Hokkaido regions, northeastern Japan and find a difference in the lower-plane event b-values in the double seismic zone. Lower-plane events reveal significantly larger b-values beneath Tohoku (0.96) than Hokkaido (0.86), implying that the brittle deformation beneath Hokkaido is more localized and leads to higher ratio of relatively large lower-plane events than occur beneath Tohoku. We also estimated the b-values for experimental earthquakes, and found they increase with increasing antigorite content in serpentinized peridotite. These experimental earthquakes already led to the “dehydration driven stress transfer” (DDST) model, which suggests that a highly hydrated peridotite is not required when oceanic mantle events occur. A comparison of experimental and natural earthquake b-values implies that lower-plane peridotite is more hydrated beneath the Tohoku region, which could also explain the difference in oceanic-plate velocity structures near the trench identified in Ocean Bottom Seismometer studies off Tohoku and Hokkaido. These results suggest that lower-plane events occur in fresh peridotite near serpentinized faults.
Highlights
In seismology, b-value analyses provide an important tool for understanding the nature of earthquakes
The preexisting spreading fabric drive the final hydration level[10] and may control the earthquake distribution for the entire subducting slab. This difference in trench orientation between the Hokkaido and Tohoku regions makes northeastern Japan a very interesting region for studies to understand the influence of the spreading faults, which could be a key to unraveling earthquake nucleation processes
The upper limit of the depth interval is fixed at 60 km because the lower limit of low angle thrust type events is ≈55 km beneath the Tohoku region and ≈60 km beneath the eastern Hokkaido region[31,32,33]
Summary
B-value analyses provide an important tool for understanding the nature of earthquakes. The preexisting spreading fabric drive the final hydration level[10] and may control the earthquake distribution for the entire subducting slab This difference in trench orientation between the Hokkaido and Tohoku regions makes northeastern Japan a very interesting region for studies to understand the influence of the spreading faults, which could be a key to unraveling earthquake nucleation processes. Synthetic serpentinized peridotites, i.e. olivine-antigorite aggregates, were deformed at high pressure using D-DIA19, and examined by combining synchrotron and acoustic-emission technologies in order to gain a better understanding of the intermediate-depth earthquake generation process in the mantle[18] In such synthetic serpentinized peridotite, a dehydration-driven stress transfer (DDST)[18] has been proposed as a possible mechanism for triggering intermediate-depth seismicity. The acceleration stage of strain localization is not included in the “dehydration-embrittlement” hypothesis[27]
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