Studies on the physical properties of the entire Izu–Bonin–Mariana (IBM) subduction zone contribute to comprehensive seismotectonic understanding and earthquake potential assessment, especially given previous controversial conclusions. Determining seismic b‐value is a method that has been used for other regions and is adopted here to study the spatiotemporal variations along the IBM system. Based on the frequency–magnitude distribution relation log10(N)=abM, b‐values are mapped within the subduction zone using earthquakes with Mw2 after 2005. The b‐value anomalies in cross sections indicate detailed seismotectonic characteristics against the regional geological background. The common characteristics from north to south: (1) regional high b‐values at shallow depths in the overriding are associated with relatively low temperatures in thermal model, the bottom half of which correspond with highly serpentinized mantle wedge; and (2) low b‐values at intermediate depths are associated with high temperatures along the primarily heated hydrated slab. In the Izu–Bonin segment, low b‐values around the slab deflection at deep depths respond to stress buildup and shearing instability of metastable olivine in primarily heated hydrated slabs. In the Mariana segment, high b‐values beneath the volcanic region at depths from the surface to 50 km and between 50 and 100 km are associated with extension and volcanism and the melting region, respectively. Temporal b‐value variations indicate regional changes before and after large events for further seismic risk analysis. Stress drops of large intermediate and deep earthquakes are negligible to local stress state in strong flexure of the incoming slab. The rupture zone around the Pagan region at an approximate depth of 200 km and the region around the rifting–spreading transition in the northern Mariana trough at depths between 180 and 350 km are areas for potential large earthquakes.

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