Search

Giant earthquakes have repeatedly ruptured the Cascadia subduction zone, and similar earthquakes will likely also occur there in the near future. We employ a 3-D time-dependent thermomechanical model that incorporates an up-to-date description of the slab geometry to study the Cascadia subduction thrust. Results show a distinct band of 3-D slab dehydration that extends from Vancouver Island to the Seattle Basin and farther southward to the Klamath Mountains in northern California, where episodic tremors cluster. This distribution appears to include a region of increased dehydration in northern Cascadia. The phenomenon of heterogeneous megathrust seismicity associated with oblique subduction suggests that the presence of fluid-rich interfaces generated by slab dehydration favors megathrust seismogenesis in the northern part of this zone. The thin, relatively weakly metamorphosed Explorer, Juan de Fuca, and Gorda Plates are associated with an anomalous lack of thrust earthquakes, and metamorphism that occurs at temperatures of 500–700°C near the Moho discontinuity may represent a key factor in explaining the presence of the associated episodic tremor and slip (ETS), which requires a young oceanic plate to subduct at a small dip angle, as is the case in Cascadia and southwestern Japan. The 3-D intraslab dehydration distribution suggests that the metamorphosed plate environment is more complex than had previously been believed, despite the existence of channeling vein networks. Slab amphibolization and eclogitization near the continental Moho depth is thus inferred to account for the resultant overpressurization at the interface, facilitating the generation of ETS and the occurrence of small to medium thrust earthquakes beneath Cascadia.

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 30 May 2015 Chichi-jima M8 earthquake is one of the largest deep focus earthquakes ever recorded and its depth of 682 km puts it near the base of the mantle transition zone. Before source mechanisms and slip models of this earthquake can be reliably assessed, a better understanding of the tectonic setting and structures of the region near the origin is required. Here we present evidence from receiver functions, a method of isolating subsurface material contrast with converted seismic waves, that the earthquake initiated within the upper mantle transition zone, above a significantly depressed 660 km phase boundary. Additionally, we observe multiple conversions within and below the transition zone, which we associate with seismic waves passing into and out of segments of the subducting Pacific plate. From this, we infer slab material is piling up at the base of the transition zone and segments are penetrating into the lower mantle.

Three-dimensional numerical modeling has been performed to account for possible earthquake migration phenomena along a northwestern Pacific subduction slab, on the assumption that the lithosphere is a linear visco-elastic body, but the asthenosphere and the mesosphere respond to the applied stress as non-linear Newtonian materials obeying the power law of creep. On the basis of a three-dimensional thermal structure of the northwestern Pacific region, the preliminarily computed stress regimes show that: 1) to induce a stick-slip type trench event, prefailures at a depth of about 40 km in the front of the locked zone between two lithospheric plates are required; 2) the compositional density contrast due to phase change at typical depths is prerequisite of stress concentration to cause a dislocation type deep-focus earthquake; and 3) the co-seismic change in gravitational potential is a controlling factor for earthquake migration downward along the subduction slab.