Abstract
The study of slow earthquake activity, which occurs in the shallow and deep sides of seismogenic zone, is crucial for understanding subduction zones, including variations in frictional properties with depth and interplate coupling. Observations at the seafloor are necessary, particularly for shallow slow earthquakes occurring in offshore areas; however, few observations of such activity have been made. We conducted long-term seismic observations on the seafloor in the Hyuga-nada region, located at the western end of the Nankai Trough, to characterize shallow low-frequency tremor activity from 2014 to 2017. Although these observations lasted for only a few years, the occurrence frequency of shallow tremors in Hyuga-nada was lower than that of deep tremors in the Nankai Trough, and major activity involving migration occurred only once every two or more years. In contrast, minor activity with a duration of a few days occurred several times a year. Major activities in 2015 were accompanied by migration similar to those in 2013. The tremors in 2013 were characterized by south to north migration at a rate of 30–60 km/day. However, the tremors in 2015 were characterized by west to east migration, and the activity area extended further to the east. The migration rates were also much slower (several to 20 km/day) than in 2013. These different migration properties likely reflect the state of interplate coupling in the down-dip side of shallow slow earthquake area. Minor activity was identified, including tremors triggered by the 2015 Nepal and 2016 Kumamoto earthquakes. Activity occurred mainly in the focal regions of major activities. Very-low-frequency earthquakes (VLFEs) occurred concurrently with tremors, and their epicenters coincided within the margin of error. However, the VLFEs were mostly peripheral to the shallow tremor concentration zones. This indicates that minor heterogeneities in frictional properties are present along the shallow plate boundary.
Highlights
During the last two decades, different slow earthquakes, including tectonic low-frequency tremors, very-lowfrequency earthquakes (VLFEs), and episodic slow slipYamashita et al Earth, Planets and Space (2021) 73:196 and cable-based seafloor monitoring systems have been constructed, thereby enabling the observation of shallow slow earthquakes (Annoura et al 2017; Araki et al 2017; Ito et al 2015; Nishikawa et al 2019; Obana and Kodaira 2009; Ohta et al 2019; Plata-Martinez et al 2021; Tanaka et al 2019; Todd et al 2018; Tonegawa et al 2020; Wallace et al 2016; Yamashita et al 2015).Yamashita et al (2015) investigated the detailed properties of shallow low-frequency tremors in the Hyuga-nada region at the western end of the Nankai Trough, Japan, using ocean bottom observations of seismicity near the trench (Fig. 1)
We report on continuous seafloor observations, obtained using short-period and broadband ocean bottom seismometers since 2014, to clarify the nature of shallow slow earthquakes in the Hyuga-nada region and to understand their universality and regional characteristics
Results the monitoring conducted by Yamashita et al (2015) lasted for less than 4 years, the occurrence frequency of shallow tremors in the Hyuga-nada region appears to be lower than that of deep tremors in the Nankai Trough (e.g., Obara 2010)
Summary
During the last two decades, different slow earthquakes, including tectonic low-frequency tremors, very-lowfrequency earthquakes (VLFEs), and episodic slow slipYamashita et al Earth, Planets and Space (2021) 73:196 and cable-based seafloor monitoring systems have been constructed, thereby enabling the observation of shallow slow earthquakes (Annoura et al 2017; Araki et al 2017; Ito et al 2015; Nishikawa et al 2019; Obana and Kodaira 2009; Ohta et al 2019; Plata-Martinez et al 2021; Tanaka et al 2019; Todd et al 2018; Tonegawa et al 2020; Wallace et al 2016; Yamashita et al 2015).Yamashita et al (2015) investigated the detailed properties of shallow low-frequency tremors (shallow tremors) in the Hyuga-nada region at the western end of the Nankai Trough, Japan, using ocean bottom observations of seismicity near the trench (Fig. 1). Complete episodes of shallow tremors that lasted up to a month were detected that exhibited migration properties similar to those of deep tremors This activity was linked to shallow VLFE activity, as shown by data acquired using a land-based broadband seismic network. Tremors and VLFEs often coincide, and their migration patterns resemble deep tremors during episodic tremor and slip (ETS) events (e.g., Ito et al 2007) This similarity suggests that tremors at this shallow plate boundary are coupled with VLFEs and short-term SSEs (Yamashita et al 2015). Analyses of 2010 activity by Asano et al (2015) revealed a similar migration pattern for shallow VLFEs in the Hyuga-nada region According to both studies, the shallow and deep slow earthquake activity occurred repeatedly in the same area
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