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.