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The accurate location of tectonic tremors helps improve understanding of their underlying physical processes. However, current location methods often do not statistically evaluate uncertainties to a satisfactory degree and do not account for potential biases due to subsurface structures not included in the model. To address these issues, we propose a novel three-step process for locating tectonic tremors. First, the measured time- and amplitude differences between station pairs are optimized to obtain station-specific relative time and amplitude measurements with uncertainty estimates. Secondly, the time– and amplitude–distance relationships in the optimized data are used to roughly estimate the propagation speed (i.e. shear wave velocity) and attenuation strength. Linear regression is applied to each event, and the resulting velocity and attenuation strength are used for quality control. Finally, the tremor location problem is formulated within a Bayesian framework where the model parameters include the source locations, local site delay/amplification factors, shear wave velocity and attenuation strength. The Markov chain Monte Carlo algorithm is used to sample the posterior probability and is augmented by a parallel tempering scheme for an efficient global search. We tested the proposed method on ocean-bottom data indicating an intense episode of tectonic tremors in Kumano-nada within the Nankai Trough subduction zone. The results show that the range of the 95 per cent confidence interval is typically <7 km horizontally and <10 km vertically. A series of experiments with different inversion settings reveals that adopting amplitude data and site correction factors help reduce random error and systematic bias, respectively. Probabilistic sampling allows us to spatially map the probability of a tremor occurring at a given location. The probability map is used to identify lineaments of tremor sources, which provides insights into structural factors that favour tremor activity. [ABSTRACT FROM AUTHOR]

Active‐source seismic surveys have resolved the fine‐scale P‐wave velocity (Vp) of the subsurface structure in subduction forearcs. In contrast, the S‐wave velocity (Vs) structure is poorly resolved despite its usefulness in understanding rock properties. This study estimates Vp and Vs structures of the Nankai Trough forearc, by applying transdimensional inversion to high‐frequency teleseismic waveforms. As a result, a thin (∼1 km) low‐velocity zone (LVZ) is evident at ∼6 km depth beneath the sea level, which is located ~3 km seaward from the outer ridge. Based on its high Vp/Vs ratio (∼2.5) and comparison to an existing seismic reflection profile, we conclude that this LVZ reflects a high pore pressure zone at the upper portion of the underthrust sediment. We infer that this overpressured underthrust sediment hosts slow earthquake activities and that accompanied strain release helps impede coseismic rupture propagation further updip. Plain Language Summary: Many geophysical surveys have investigated the subsurface structures of shallow subduction zones by estimating the propagation speed of compressional waves emitted from artificial explosive sources. Although shear and compressional wave speeds are necessary to understand rock properties (e.g., water pressure) of the subsurface, it has been difficult to constrain the shear wave speed with a high‐resolution. In this study, we estimate both compressional and shear wave speeds by applying an advanced technique to earthquake waveforms recorded at ocean‐bottom seismometers deployed at the Nankai subduction zone, Japan. The results show a sufficiently high spatial resolution to detect a thin (~1 km) layer, which we interpret as water‐rich subducted sediment. This water‐rich zone may promote slow slips on the megathrust fault and work as a barrier against rupture propagations during large earthquakes. Key Points: We applied transdimensional inversion of teleseismic waveforms to seafloor cabled stations at the Nankai subduction zoneThe resultant high‐resolution velocity structures allow a direct comparison to active source surveysA low‐velocity zone beneath the outer ridge is evident, which is interpreted as an overpressured portion of underthrust sediment [ABSTRACT FROM AUTHOR]