GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.3 GNGTS 2023 b -value analysis elucidates the control of subduction age and stress state on intraslab seismicity of the NW Pacific subducting plate N. A. Pino, C. Piromallo Since their first observations, the occurrence of intermediate depth (70-300 km) and deep (>300 km) earthquakes have induced Earth scientists to conceive the existence of a source mechanism different from brittle failure. Like shallow events, deeper earthquakes are associated with sliding along shear planes without significant volumetric component and respect the general frequency-magnitude distribution, but the increasing temperature and pressure with depth would hinder frictional sliding. Thus, alternative physical processes must be invoked. Several mechanisms have been proposed by various researchers in the last three decades, mostly depending on the subducting plate age and stress state. However, these properties of the descending slabs are commonly synthesized with single parameters, which do not reflect adequately the lateral and vertical heterogeneities within each slab and the marked differences among distinct subduction zones. In order to study the details of the spatial distribution of the seismicity with respect to various slab characteristics, we analyze the intraslab earthquakes in the NW Pacific subducting plate (Fig. 1), where abundant, precisely located (Japan Meteorological Agency, https://www.data.jma.go.jp/svd/eqev/data/bulletin/hypo_e.html ) seismicity occurs and the slab properties are sufficiently well documented as is the persistence below the 410 km phase transition of a Metastable Olivine Wedge (Jiang et al., 2014; Kawakatsu et al., 2011). In particular, we separately investigate the relations between the distribution of the seismicity and the age of the subducting lithosphere A L , the slab sink rate V Z , and the thermal parameter φ , defined as the product φ = A L ∙V Z . Exhaustive seismological analyses (Valleé, 2013) indicate that the intermediate and deep earthquakes share the same macroscopic properties of the crustal ones, in spite of the different source physical mechanisms; therefore we analyze the details of the distribution of the Gutenberg–Richter b -value along the Kurile and Izu–Bonin sectors of the subducting plate and interpret its variation in terms of stress state, analogously to what commonly done for frictional earthquakes. Our results (Pino et al., 2022) demonstrate that: i) the depth distribution of intermediate and deep earthquakes is primarily controlled by the age from subduction initiation (Fig. 2);