Comprehensive Detection of Very Low Frequency Earthquakes Off the Hokkaido and Tohoku Pacific Coasts, Northeastern Japan

Abstract

Very low frequency earthquakes (VLFEs), classified as one type of slow earthquake, occur near seismogenic zones and can be a proxy for interplate slow slip. We investigated VLFE activity off the Hokkaido and Tohoku Pacific coasts, where the Pacific plate subducts under the North American plate.

Slow Earthquakes Illuminating Interplate Coupling Heterogeneities in Subduction Zones

“…Generally, the detection procedure used for VLFEs was the same as that reported in Baba et al (2020). We placed 196 virtual epicentral grids on the Philippine Sea Plate boundary in Southwest Japan (Figure S1 in the supporting information) in intervals of 0.3° and computed synthetic waveforms for the 10 stations closest to each virtual source grid using the open‐source finite difference method code (OpenSWPC; Maeda et al, 2017) and by using a three‐dimensional velocity structure model of the Japan Integrated Velocity Structure Model (JIVSM; Koketsu et al, 2012).…”

Spatio-temporal Characteristics of Frictional Properties on the Subducting Pacific Plate Off the East of Tohoku District, Japan Estimated from Stress Drops of Small Earthquakes

“…They reported that a segmentation likely caused to cease the coseismic rupture of the 2011 Tohoku earthquake, which provides important information for a risk assessment from future major earthquakes. Baba et al (2020) detected very low frequency earthquakes (VLFEs) off the Hokkaido and Tohoku Pacific coasts by a matched-filter technique. They pointed out that their spatial distribution is consistent with the afterslip of the 2003 Tokachi-Oki earthquake (M W 8.0).…”

Seismic energy radiation and along-strike heterogeneities of shallow tectonic tremors at the Nankai Trough and Japan Trench

“…Seismic moment rates of shallow VLFEs that accompany shallow tremors were estimated by comparing the synthetic VLFE waveforms with the observed waveforms in a frequency band of 0.02-0.05 Hz using three-component seismograms from 10 F-net stations located close to the hypocenter of the VLFEs (Baba et al, 2020a). Synthetic VLFEs were calculated with OpenSWPC (Maeda et al, 2017) using the three-dimensional seismic velocity structure from the Japan Integrated Velocity Structure Model (Koketsu et al, 2012), the seismic moment tensor from the subducting plate model (Nakajima & Hasegawa, 2006;Nakajima et al, 2009;Kita et al, 2010), and the NUVEL-1A plate motion model (DeMets et al, 1994).…”

“…To discard bad estimations due to low signal to noise ratios, we excluded results with average cross correlation coefficients less than 0.25. This threshold is comparable to the threshold of VLFE detection using the matched filter technique of Baba et al (2020a). As the VLFE catalog of Nakano et al (2016Nakano et al ( , 2018 already includes information on the seismic moment rates, we applied this method only to the Japan Trench.…”

“…Observational networks have been deployed in offshore regions around Japan (Aoi et al, 2020), following these initial observations. Onshore observational data are still important for understanding the long-period seismicity of shallow slow earthquakes (Takemura et al, 2019;Baba et al, 2020a) because the observational period of the offshore data is still short. However, offshore observational instruments reveal detailed characteristics of the shallow slow earthquakes.…”