3-D thermal regime and dehydration processes around the regions of slow earthquakes along the Ryukyu Trench

Nobuaki Suenaga,Shoichi Yoshioka,Yingfeng Ji

doi:10.1038/s41598-021-90199-2

Abstract

Several interplate seismic events, such as short-term slow slip events (S-SSEs) and low-frequency earthquakes (LFEs), have been identified in the Ryukyu Trench, southwestern Japan. As one of the specific characteristics of this seismicity, the depths at which S-SSEs occur at the plate interface beneath Okinawa Island are approximately 5–10 km shallower than those beneath the Yaeyama Islands. To elucidate the cause of this difference in depth, we constructed a three-dimensional, Cartesian thermomechanical subduction model and applied the subduction history of the Philippine Sea (PHS) plate in the model region. As a result, the interplate temperatures at which S-SSEs take place were estimated to range from 350 to 450 °C beneath Okinawa Island and from 500 to 600 °C beneath the Yaeyama Islands. The former temperature range is consistent with previous thermal modelling studies for the occurrence of slow earthquakes, but the latter temperature range is by approximately 150 °C higher than the former. Therefore, explaining how the depth difference in S-SSEs could be caused from the aspect of only the thermal regime is difficult. Using phase diagrams for hydrous minerals in the oceanic crust and mantle wedge, we also estimated the water content distribution on and above the plate interface of the PHS plate. Near the S-SSE fault planes, almost the same amount of dehydration associated with phase transformations of hydrous minerals from blueschist to amphibolite and from amphibolite to amphibole eclogite within the oceanic crust were inferred along Okinawa Island and the Yaeyama Islands, respectively. On the other hand, the phase transformations within the mantle wedge were inferred only beneath the Yaeyama Islands, whereas no specific phase transformation was inferred beneath Okinawa Island around the S-SSE occurrence region. Therefore, we conclude that dehydrated fluid derived from the oceanic crust at the plate interface would play a key role in the occurrence of S-SSEs.

Highlights

In southwestern Japan, the Philippine Sea (PHS) plate is subducting beneath the Amurian plate along the Ryukyu Trench[1] (Fig. 1)
The interplate temperature and the mantle wedge temperature, which is taken 5 km above the plate interface, decrease by at most 200 °C when incorporating the effect of trench retreat and the recent spreading rate of the Okinawa Trough into the model; these changes are due to the development of lateral flow in the mantle wedge
The results show that the shallower region where short-term slow slip events (S-SSEs) and low-frequency earthquakes (LFEs) occur beneath Okinawa Island is adjacent to the deeper region where the phase transformation from blueschist to amphibolite within the oceanic crust takes place (Figs. 3b and 4a)

Summary

Introduction

In southwestern Japan, the Philippine Sea (PHS) plate is subducting beneath the Amurian plate along the Ryukyu Trench[1] (Fig. 1). LFE distributions, southward migrations have been identified beneath both Okinawa Island and the Yaeyama Islands[3] This mobility of the source of LFEs would indicate the existence of fluid near the plate b oundary[6]. To investigate the similarity or difference between the Okinawa Island and Yaeyama Islands regions with respect to the thermal regimes and the dehydration processes near the S-SSEs and LFEs, we performed several numerical simulations of temperature fields associated with the subduction of the PHS plate using a 3-D Cartesian thermomechanical subduction model and estimated the water content distributions on and above the subducting PHS plate using phase diagrams for hydrous mid-ocean ridge basalt (MORB) of the oceanic crust and ultramafic rocks of the mantle wedge. The domains of the upper crust, lower crust, and accretionary prism were rigid; the other domains were composed of convective regions

Methods

Results

Conclusion