Spatiotemporal Distribution of Shallow Tremors Along the Nankai Trough, Southwest Japan, as Determined From Waveform Amplitudes and Cross‐Correlations

Abstract

AbstractShallow slow earthquakes in subduction zones are related to spatiotemporal variations of stress on the plate boundary adjacent to a megathrust earthquake. A dense network (DONET) of cabled ocean‐bottom seismometers (OBSs) deployed in the Nankai Trough seismogenic zone, southwest Japan, has enabled real‐time monitoring of tremors. Because the amplitudes of tremor signals are very low, even when recorded by OBSs, we developed a hybrid method that simultaneously uses traveltimes based on cross‐correlations and amplitudes of waveforms to determine tremor hypocenters. We applied our method to waveforms observed during five years of continuous DONET recording from April 2016 to September 2021 to detect more than 6,500 tremors, including three major episodes in 2016, 2018, and 2020–2021. The tremors of each episode were distributed over distances of >100 km along the trench axis in two regions, southeast of the Kii Peninsula and off Cape Muroto. We also estimated seismic energy rates and found that for the three major tremor episodes, energy rates increased gradually with time. The 2018 and 2020–2021 episodes began near a subducted seamount and a subducted ridge, respectively, and were likely related to fluid movements controlled by the topography of the downgoing plate. We showed that the tremors were spatiotemporally synchronized with slow slip events, and their spatial variations of energy rate corresponded to stress heterogeneity on the shallow plate boundary. Future real‐time monitoring of shallow tremors will contribute to our understanding of changes of stress state due to slow earthquakes at the shallow plate boundary.