Depth estimation of shallow low-frequency tremor by inversion technique using differential travel times between ocean bottom seismometers

Summary Low-frequency tremors provide important constraints on the mechanical and hydrological conditions of subduction plate interfaces, yet their focal depth distributions remain challenging to be resolved compared to those of fast earthquakes. In this study, low-frequency tremors in the Hyuga-nada region are independently relocated using earthquake location techniques with travel-times by converting cross-correlation-derived arrival-time delays into pseudo-arrival times for each single station, without imposing a priori depth constraints. The resulting hypocenter distribution reveals systematic spatial patterns that are robust across multiple observation periods. Hypocenter locations are determined through a multi-stage relocation procedure, in which an initial maximum-likelihood location incorporating station correction terms is followed by a double-difference relocation using a three-dimensional velocity model. Only events meeting stringent quality thresholds on converted arrival counts, azimuthal gap, and initial hypocentral depth uncertainty are retained. Location uncertainties are quantified at each stage using a Bayesian framework and jackknife resampling, which respectively characterize the probabilistic confidence of the solution and its robustness against the influence of individual stations and potentially biased converted arrival times. These steps are iteratively applied to refine source parameters, thereby improving depth resolution and overall location stability. The relocated low-frequency tremors are predominantly concentrated at shallower depths than fast earthquakes and are distributed along the plate interface. Comparison with seismic reflection profiles indicates that low-frequency tremors are generally distributed in the vicinity of strong reflective interfaces along the plate boundary, supporting previous interpretations that associate low-frequency tremors with mechanically weak, fluid-rich layers. The relocated tremor locations further show a clear spatial correspondence with a subducted seamount inferred from magnetic anomaly data, consistent with mechanical and hydrological contrasts associated with seamount subduction, particularly along the downdip flank, which may reflect stronger compression and enhanced fluid drainage. These results demonstrate that low-frequency tremors in the Hyuga-nada region are strongly controlled by the plate interface and structural heterogeneity. The relocation framework presented here provides a unified basis for integrated analyses of fast and slow earthquake processes in subduction zones.