Slow earthquakes are mainly distributed in regions surrounding seismogenic zones along the plate boundaries of subduction zones. In the Central American subduction zone, large regular interplate earthquakes with magnitudes of 7–8 occur repeatedly around the Nicoya Peninsula, in Costa Rica, and a tsunami earthquake occurred off Nicaragua, just north of Costa Rica, in 1992. To clarify the spatial distribution of various slip behaviors at the plate boundary, we detected and located very low frequency earthquakes (VLFEs) around the Nicoya Peninsula using a grid-search matched-filter technique with synthetic templates based on a regional three-dimensional model. VLFEs were active in September 2004 and August 2005, mainly near the trench axis, updip of the seismogenic zone. The distribution of VLFEs overlapped with large slip areas of slow slip events. Low frequency tremor signals were also found in high-frequency seismogram envelopes within the same time windows as detected VLFEs; thus, we also investigated the energy rates of tremors accompanied by VLFEs. The range of scaled energy, which is the ratio of the seismic energy rate of a tremor to the seismic moment rate of accompanying VLFE and related to the rupture process of seismic phenomena, was 10-9–10-8. The along-dip separation of shallow slow and large earthquakes and the range of the scaled energy off Costa Rica are similar to those in shallow slow earthquakes in Nankai, which shares a similar thermal structure along the shallow plate boundary.

Slow earthquakes are mainly distributed in the vicinity of seismogenic zones of megathrust earthquakes and relationships between both types of earthquakes are expected. We examined the activity of very low frequency earthquakes (VLFEs), classified as one type of slow earthquake, around Japan because they have the potential to clarify detailed spatiotemporal slip behaviors at the plate boundaries. The distribution of the shallow VLFE activity rate is heterogeneous along trench axes and exhibits an anticorrelation relationship with the spatial distribution of the interplate coupling ratio, whereas deep VLFEs are distributed only in weakly coupled areas and the spatial variation of the activity rate is small. Furthermore, VLFEs are mainly hosted by low seismic velocity anomalies. Thus, slow earthquakes can be triggered by decreased effective stress due to the high pore fluid pressure within regions with weak interplate coupling and their activity can be an indicator of interplate slip behavior.

We detected shallow very low frequency earthquakes (VLFEs) off the Cape Muroto and Kii Channel in the Nankai subduction zone and estimated their moment rate functions. Combining the new and previously estimated catalogs, we obtained the comprehensive catalog of shallow VLFE moment rate functions along the Nankai Trough. We defined the shallow VLFE swarms and investigated the scaling relationships of their cumulative moments, activity area, and durations in each region. Detected swarms were considered candidates for shallow slow slip events. A similar scaling relationship was observed between the cumulative moments and activity areas, irrespective of regions. It indicates similar stress drops in each region. However, the relationship between the cumulative moments and durations varied. This difference was explained by the along-strike variations in the faulting conditions of shallow slow earthquakes, such as material or hydrological properties.

Cross-correlation analysis was applied to long-term onshore broadband records from April 2004 to March 2021 to detect and relocate shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula, along the Nankai Trough, Japan. We then determined the moment rate functions of detected shallow VLFEs using the Monte Carlo-based simulated annealing method. According to this new comprehensive catalog, shallow VLFEs are widespread beneath the accretionary prism toe, but shallow VLFEs with large cumulative moments are localized around the western edge of the paleo-Zenisu ridge, which is subducted beneath southeast off the Kii Peninsula. Our results from long-term shallow VLFE catalog are well consistent with previous studies in this region, suggesting that heterogeneous structures and stress conditions due to the subducted paleo-Zenisu ridge promote the occurrence of shallow slow earthquakes. The relocated shallow VLFE epicenters illustrated three major episodes characterized by a similar activity area and five minor episodes characterized by different areas. The three major episodes exhibited slow frontal migration with different initiation locations, directions, and speeds, as well as several rapid reverse migrations. Episodes of minor activity were distributed in different locations within part of the area of major activity. Different patterns of shallow VLFE migration could reflect temporal changes in the pore-fluid distribution or stress conditions of the plate boundary.

Slow earthquakes are generally distributed in regions surrounding seismogenic zones along the plate boundaries of subduction zones. In the Costa Rica subduction zone, large regular interplate earthquakes with a magnitude of 7–8 occur repeatedly, and a tsunami earthquake occurred in the northern part in 1992. To clarify the spatial distribution of various slip behaviors at the plate boundary in the Costa Rica subduction zone, we detected and located very low frequency earthquakes (VLFEs) using a grid-search matched-filter technique with synthetic templates based on a regional three-dimensional model. VLFEs were activated in September 2004 and August 2005, and most of the VLFEs were located near the trench axis at a depth range of 5–10 km, the updip of the seismogenic zone. The spatial distribution of VLFEs complements the slip areas of large earthquakes and the tsunami earthquake. Low frequency tremor signals were also found in high-frequency seismogram envelopes within the same time windows of detected VLFEs; thus, we also investigated the energy rates of tremors accompanied by VLFEs. The range of scaled energy, which is the ratio of the seismic energy rate of a tremor to the seismic moment rate of accompanying VLFE, was 10-9–10-8. This value is similar to that in shallow slow earthquakes in the Nankai subduction zone. The similarity of characteristics and distribution of shallow slow earthquakes in the Costa Rica and Nankai subduction zones may be due to common tectonic features, such as age, temperature, or the presence of accretionary prisms.