Shallow very low frequency earthquakes accompanying low frequency tremors near the trench axis in the Costa Rica subduction zone

Slow earthquakes, such as low frequency tremors (LFTs), very low frequency earthquakes (VLFEs), and slow slip events (SSEs), are fault slip events with slower slip speed than regular earthquakes and occur mainly in subduction zones. LFTs and VLFEs are observed in the frequency range of 2-8 Hz and 0.02-0.05 Hz, respectively. In the Costa Rica subduction zone, thrust type large earthquakes with Mw of 7-8 occur repeatedly. Jiang et al. (2012; 2017) pointed out that large slip areas of SSEs were distributed in both downdip and updip areas of the coseismic slip of large earthquakes. Outerbridge et al. (2010) located LFTs in downdip of the coseismic slip area of large earthquakes, whereas Walter et al. (2011; 2013) found LFTs and VLFEs that occur in updip.

To clarify the relationship between slow and large regular earthquake distribution, we detected VLFEs in the Costa Rica subduction zone by the matched-filter technique. We used waveforms of a temporary seismic network, Tomography Under Costa Rica and Nicaragua (Abers & Fischer, 2003), recorded from August 2004 to January 2006. We computed synthetic waveforms at stations from each virtual source grid using OpenSWPC (Maeda et al., 2017) and a 3D velocity model. The grids were widely distributed on the plate boundary beneath both onshore and offshore areas. The focal mechanisms were assumed to be consistent with the geometry of the plate boundary and with plate motion. After applying a band-pass filter of 0.02-0.05 Hz, we calculated cross-correlation coefficients (cc) between the synthetic and observed seismograms every 1 s, and selected events whose cc exceeded the threshold defined as 9.5 times the median absolute deviation of the distributions.

We detected 76 VLFEs and many VLFEs occurred in September 2004 and August 2005. Most of VLFEs are located near the trench axis, in the depth of 5-10 km, updip of the coseismic slip areas of large earthquakes and overlaps with the shallower part of the large slip area of the August 2005 SSE. Furthermore, we found LFT signals in a frequency range of 2-8 Hz within the same time windows of VLFEs. Rough estimation of LFT locations tends to be consistent with VLFE regions. Although VLFEs or LFEs were detected in both updip and downdip areas in previous studies, we verified that they occurred in updip area by visual checking of waveforms and the cc distribution.