Spatiotemporal evolution of a deep short-term slow slip event in the Nankai subduction zone in November 2022 derived from strain, tilt, and GNSS records

Abstract

This study investigated the spatiotemporal evolution of a deep short-term slow slip event (SSE) in southwest Japan using three geodetic data types: tilt, strain, and GNSS. Because tilt and strain data are more sensitive to crustal deformation than GNSS data, their joint use enables us to estimate the detailed spatiotemporal evolution of SSE slip. In addition, a higher sampling rate of tilt and strain data improves their temporal resolution down to sub-daily time scales. We developed a geodetic inversion method jointly using the three types of geodetic data, and obtained the spatiotemporal evolution of an SSE slip, which occurred beneath the western Shikoku region in November 2022. Comparing results from all possible combinations of data types, we found that the small early-stage SSE slip could be resolved only when strain data was included in the dataset, which is considered to have the highest sensitivity to crustal deformation among the sensors. With the improved spatiotemporal resolution, we compared the spatiotemporal evolution of the SSE slip with that of tremor activities. The initiation timing of the SSE was consistent with the timing when tremor activities were initiated. SSE slips were spatiotemporally collocated with tremor activities, with a moment evolution quantitatively consistent with tremor energy radiation. The scaling factor between the aseismic moment of SSE and the seismic energy of tremor was estimated to be 10⁻¹¹. However, it is unclear whether we can regard this value as the scaled energy of a slow earthquake because some tremor energy might be missed through the tremor detection process. Our results show that joint use of tilt and strain is especially useful in obtaining not only spatial, but also spatiotemporal evolution of small SSE slip in southwest Japan and that SSE and tremors are representations of a single broadband slip phenomenon in different frequency bands.