We investigated temperature and mantle flow distributions associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan, by constructing a three‐dimensional parallelepiped model incorporating a past clockwise rotation, the bathymetry of the Philippine Sea plate, and distribution of the subducting velocity within its slab. The geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3‐D simulation. Using the model, we estimated a realistic and high‐resolution temperature field on the subduction plate interface, which was constrained by a large number of heat flow data, and attempted to clarify its relationship with occurrences of megathrust earthquakes, long‐term slow slip events (L‐SSEs), and nonvolcanic low‐frequency earthquakes (LFEs). Results showed that the oblique subduction coupled with the 3‐D geometry of subducting PHS plate was a key factor affecting the interplate and intraplate temperature distributions, leading to a cold anomaly in the plate interface beneath western Shikoku, the Bungo Channel, and the Kii Peninsula. Temperatures in the slab core in these regions at a depth near the continental Moho were nearly 200°C lower than that in eastern Shikoku, indicating a high thermal lateral heterogeneity within the subducting plate. The geothermal control of the LFEs beneath western Shikoku was estimated to be within a range from 400 to 700°C, and the interplate temperature for the L‐SSEs with a slip larger than 15 cm beneath the Bungo Channel was estimated to be approximately 350–500°C. A large horizontal temperature gradient of 2.5 ~ °C/km was present where the LFEs occurred repeatedly. The steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid‐ocean ridge basalt of the subducting PHS plate.

We developed a 3‐D thermal convection model to estimate the thermal regime, water content distribution, and slab dehydration beneath southwestern and central Japan, where deep tectonic tremors and short‐term slow slip events (S‐SSEs) are frequently observed on the plate interface extending from the western Shikoku to the Tokai district. The results showed that the pressure‐temperature (P‐T) conditions for the S‐SSEs, deep tectonic tremors, and regular earthquakes indicate that they probably originated from slab dehydration in association with subduction, resulting in a phase transformation from prehnite‐actinolite/lawsonite blueschist to amphibolite with a large thermal gradient. Slab dehydration and the thermal gradient in the dip direction are considered key factors for controlling the seismogenesis of slow and regular earthquakes in the Philippine Sea plate beneath southwestern and central Japan, although the deep tectonic tremors are more complicated because they are likely favored by a comparatively low temperature gradient in southwestern Japan.

The downdip limit of seismogenic interfaces inferred from the subduction thermal regime by thermal models has been suggested to relate to the faulting instability caused by the brittle failure regime in various plate convergent systems. However, the featured three‐dimensional thermal state, especially along the horizontal (trench‐parallel) direction of a subducted oceanic plate, remains poorly constrained. To robustly investigate and further map the horizontal (trench‐parallel) distribution of the subduction regime and subsequently induced slab dewatering in a descending plate beneath a convergent margin, we construct a regional thermal model that incorporates an up‐to‐date three‐dimensional slab geometry and the MORVEL plate velocity to simulate the plate subduction history in Hikurangi. Our calculations suggest an identified thrust zone featuring remarkable slab dehydration near the Taupo volcanic arc in the North Island distributed in the Kapiti, Manawatu, and Raukumara region. The calculated average subduction‐associated slab dehydration of 0.09 to 0.12 wt%/km is greater than the dehydration in other portions of the descending slab and possibly contributes to an along‐arc variation in the interplate pore fluid pressure. A large‐scale slab dehydration (>0.05 wt%/km) and a high thermal gradient (>4 °C/km) are also identified in the Kapiti, Manawatu, and Raukumara region and are associated with frequent deep slow slip events. An intraslab dehydration that exceeds 0.2 wt%/km beneath Manawatu near the source region of tectonic tremors suggests an unknown relationship in the genesis of slow earthquakes.

Dual subduction represents an unusual case of subduction where one oceanic plate subducts on top of another, creating a highly complex tectonic setting. Because of the complex interaction between the two subducted plates, the origin of seismicity in such region is still not fully understood. Here we investigate the thermal structure of dual subduction beneath Kanto, central Japan formed as a consequence of a unique case of triple trench junction. Using high-resolution three-dimensional thermo-mechanical models tailored for the specific dual subduction settings beneath Kanto, we show that, compared with single-plate subduction systems, subduction of double slabs produces a strong variation of mantle flow, thermal and fluid release pattern that strongly controls the regional seismicity distribution. Here the deepening of seismicity in the Pacific slab located under the Philippine Sea slab is explained by delaying at greater depths (~150 km depth) of the eclogitization front in this region. On the other hand, the shallower seismicity observed in the Philippine Sea slab is related to a young and warm plate subduction and probably to the presence of a hot mantle flow traveling underneath the slab and then moving upward on top of the slab.