Slow Earthquakes in the Microseism Frequency Band (0.1–1.0 Hz) off Kii Peninsula, Japan

• Published in: Geophysical research letters Vol. 45; no. 6; pp. 2618 - 2624
• Main Authors: Kaneko, Lisa, Ide, Satoshi, Nakano, Masaru
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.03.2018
• Subjects: Broadband; Deformation; Detection; DONET1; Earth; Earthquakes; Locations (working); Low frequencies; Low frequency; microseism frequency band; Microseisms; Nankai; Noise levels; Ocean bottom; Ocean bottom seismometers; Ocean floor; Oceans; Seismic activity; Seismometers; Slip; slow earthquake; tremor; Tremors; VLF; Japan
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL076773

It is difficult to detect the signal of slow deformation in the 0.1–1.0 Hz frequency band between tectonic tremors and very low frequency events, where microseism noise is dominant. Here we provide the first evidence of slow earthquakes in this microseism band, observed by the DONET1 ocean bottom seismometer network, after an Mw 5.8 earthquake off Kii Peninsula, Japan, on 1 April 2016. The signals in the microseism band were accompanied by signals from active tremors, very low frequency events, and slow slip events that radiated from the shallow plate interface. We report the detection and locations of events across five frequency bands, including the microseism band. The locations and timing of the events estimated in the different frequency bands are similar, suggesting that these signals radiated from a common source. The observed variations in detectability for each band highlight the complexity of the slow earthquake process. Plain Language Summary Slow earthquakes, which are thought to have a relation to disastrous earthquakes, are divided into deep tectonic tremors (2–8 Hz), very low frequency events (VLFEs; 0.01–0.05 Hz), and slow slip events, each of which is observed in a different frequency band. Between tectonic tremors and VLFEs, “microseism frequency band (0.1–1.0 Hz),” the existence of signals is largely unknown, and if present, they are difficult to find because of the large noise levels of the Earth. Therefore, it remains unclear whether tremor, LFE, and very low frequency event each represent an independent phenomenon or if they are different manifestations of a unified broadband phenomenon. Here we provide the first evidence of slow earthquakes in this microseism band, observed by the DONET1 ocean bottom high‐sensitivity seismometer network off Kii Peninsula, Japan. We found the timing and place of slow earthquakes for each frequency band. The locations and timing of the events estimated in the different frequency bands are similar, suggesting that these signals radiated from a common source. These results strongly suggest that this series of slow earthquakes is a united broadband phenomenon, which can change our perspective of slow earthquakes. Key Points We identified slow earthquakes across a wide frequency range, including the microseism frequency band, at the Nankai subduction zone Hypocenters of slow earthquakes determined in different frequency bands are located close to each other Signal amplitude can be variable across a broad frequency range, highlighting the complex nature of slow earthquakes