Spatial clustering of deep earthquakes controlled by water carriers

Deep earthquakes within subducting slab into the mantle transition zone (MTZ) often exhibit spatial variations along the strike of the slab. Existing mechanisms, including dehydration embrittlement, transformational faulting, and thermal shear instability, have been proposed to explain the cause of deep earthquakes; however, these hypotheses fail to account for the deep earthquake cluster within stagnant slab. Given that variable water input plays a crucial role in the distribution of seismicity within the arc system, spatial variations in the transport of subducted water could potentially control the clustering of deep-focus earthquakes in the MTZ. Northeast (NE) Asia is an ideal region to investigate this problem, where the Pacific slab stagnates continuously from north to south and extends westward for <1000 km in the MTZ, with deep seismicity occurring in clusters in the MTZ. Meanwhile, previous studies have shown that surficial water can be transported to the MTZ in this region (Xing et al., 2024), and the thermal state of subducting slab beneath NE Japan exhibits along-strike variability, with slab temperature decreasing gradually from north to south (Wada et al., 2015), implying the potential spatial variations in deep water cycling.

Here, we report major and trace element compositions, together with Sr-Nd-B isotopic data of basalts in NE Asia to trace deep water cycling and investigate the spatial co-variations between water carriers and deep earthquakes in Northeast Asia. Our results reveal prominent along-strike differences in B isotopic compositions. Northern arc basalts from Hokkaido show heavy and variable δ¹¹B values (−14.55‰ to +6.47‰), whereas associated intraplate basalts have light δ¹¹B values (−10.44‰ to −5.15‰). In contrast, southern arc basalts from Honshu display homogeneous and light δ¹¹B values (−4.7‰ to −3.1‰; Moriguti et al., 2004), against variable intraplate region (−8.42‰ to +7.71‰). These contrasts reflect distinct carriers transporting water into the MTZ. In the north, dehydration of hydrous minerals leaves minimal water carried by nominally anhydrous minerals, which corresponds to the absence of deep-focus earthquakes in the MTZ. Conversely, dense hydrous magnesium silicates transport large amounts of water into the MTZ in the south, consistent with a notable cluster of deep-focus earthquakes. Therefore, we conclude that water carriers into the MTZ critically control along-strike earthquake clustering.

This work was financially supported by the National Key R&D Program of China (Grant 2022YFF0801002) and the National Natural Science Foundation of China (Grant 42372065).

 

References:

Wada et al., 2015, Earth and Planetary Science Letters, v. 426, p. 76-88.

Moriguti et al., 2004, Chemical Geology, v. 212, p. 81-100.

Xing et al., 2024, Nature Geoscience, v. 17, p. 579-585.