Seismic distribution and dehydration of MORB associated with subduction of the Pacific and the Philippine Sea plates beneath the Tohoku and Kanto districts, Japan

Abstract:

We performed 3D parallelepiped numerical simulation for thermal regime and water content distribution associated with subduction of the Pacific (PAC) and the Philippine Sea (PHS) plates beneath the Tohoku and the Kanto districts from northeast to central Japan. The two plates are overlapped beneath the Kanto district. From our results, a remarkable cold interplate zone was recognized extending westward off the Tohoku district where the 2011 Tohoku-oki earthquake (M9.0) and a large number of earthquakes occurred. However, in the cold zone, a north-south trending remarkable aseismic belt was found approximately 200~300 km to the east offshore (Fig. 1). In this region, dehydration of hydrous minerals would not take place due to apparent increase in water content associated with the phase transformation from pumpellyite-actinolite (4.4 wt%) to lawsonite blueschist (5.4 wt%) adjacent to the Japan Trench. Contrastingly, at offshore of the Tohoku district, clustered earthquakes with magnitude greater than 4.0 lie in the belt of the transition zone from lawsonite blueschist (5.4 wt%), epidote blueschist (3.0 wt%), and lawsonite amphibole eclogite (3.0 wt%) to amphibole eclogite (< 1.0 wt%) on upper surface of the PAC plate. Over 80% of the earthquakes with magnitude greater than 4.0 were found right on this dehydration belt of MORB. Similarly, most of the earthquakes with magnitude less than 4.0 are distributed there, or down along the Wadachi-Benioff zone of the PAC plate where magnitude of earthquakes are getting smaller. We conjectured that the severe dehydration occurs to release nearly 80% of the water which was originally included in the hydrous minerals of MORB, resulting in a more hydrous mantle wedge and plate interface, and subsequently triggered the clustering earthquakes. From the phase diagram of MORB (0~7 km), we found an apparent seismic gap in a depth range of 50~100 km (pressure of 2 ~2.5 GPa) under approximately ~600°C, a transition temperature to eclogite, where occurrences of any earthquakes are inhibited (Fig. 2). The profile passing through the coldest part in the slab-slab contact zone of the PAC and PHS plates was found to have undergone the most earthquakes. Therefore, dehydration is considered to control the distribution of seismicity in the subducted PAC plate beneath the Tohoku and the Kanto districts.