Migration of Deep Low-Frequency Tremor Triggered by Teleseismic Earthquakes in the Southwest Japan Subduction Zone

Ryo Kurihara, Kazushige Obara, Akiko Takeo, Takuto Maeda

Geophysical research letters, 45 (2018)

Deep low-frequency tremor triggered by teleseismic surface waves is concentrated in several locations along the ambient tremor source region in the Nankai subduction zone, southwest Japan. We systematically investigated triggered tremor in northern Kii and western Shikoku, at 71 teleseismic earthquakes by applying a matched filter technique. We detected 9 and 17 events of triggered tremor in northern Kii and western Shikoku, respectively. In addition, we observed tremor migration episodes, whose directions were always along-dip. In northern Kii, seven episodes showed downward migration at a speed of 5–20 km/hr. In western Shikoku, six episodes migrated at a speed of 10–100 km/hr, in which five episodes showed upward migration. Migration of the triggered tremor suggests that dynamic stress perturbation from distant earthquakes may trigger propagating creep events caused by fluid.

Waveforms and spatiotemporal distribution of detected triggered tremor in northern Kii for the 2007 Kuril earthquake (Mw 8.1). (a, top) Transverse, radial, and vertical components of normalized displacement waveforms at a period of 10–40 s (0.025–0.1 Hz) and vertical component velocity waveforms at 2–8 Hz recorded at station KWBH. The amplitude of vertical component is adjusted in order to clearly show the waveform of triggered tremor. (bottom) Along-dip locations of triggered tremor as a function of time. The colored circle shows the location of triggered tremor in the rectangle in (b). The slope of the broken line shows the velocity of the detected tremor in the 1,200-s time window. (b) Locations of detected triggered tremor (colored circles). The color bar indicates the time from the origin of the teleseismic event. The gray dots show the locations of used templates.