Prevalence of Shallow Low‐Frequency Earthquakes in the Continental Crust

Abstract

Low‐frequency earthquakes (LFEs) that predominantly occur at depths of 20–30 km are categorized as a particular class of earthquakes whose spectral power is concentrated at 1–4 Hz. While the tectonic LFEs along megathrust boundaries occur as shear failure, the genesis of LFEs in the continental plate is poorly understood due to the diversity of focal mechanism solutions. Here we conduct a systematic survey of LFEs using two metrics (frequency index and peak frequency) that characterize the frequency content of the waveforms, and show that LFEs are prevalent in the upper crust beneath the Japanese Islands, even in non‐volcanic regions. Shallow LFEs are most common near tectonic boundaries and are temporarily activated in the aftershock sequences of large (M ≥ 6.5) crustal earthquakes. The widespread distribution of shallow LFEs suggests that a lower crustal rheology is not necessary for their genesis. We infer that failure along frictionally weakened faults due to high pore‐fluid pressures is a primary control for the enrichment of low‐frequency energy. The observed differences in the frequency content are probably due to differences in the pore‐fluid pressure along each fault, which influences the rupture velocity and magnitude of the tensile component during shear failure. Our observations may lead to a more unified model of earthquake generation, thereby providing a better understanding of how earthquakes release the stress accumulated in the Earth.

Crustal structure beneath earthquake swarm in the Noto peninsula, Japan

“…However, the relocated hypocenters with the 3D velocity model do not have enough resolution to discuss the detailed temporal and spatial evolution of seismicity. Furthermore, this study has no observations to suggest what broke the low-permeability seal, even though the candidates would probably include either episodic aseismic deformation (Nakajima and Uchida 2018), or stress loading by external forces such as strong motion (Nakajima and Hasegawa 2021), or fast increases in porefluid pressure itself to some threshold value (Shapiro et al 2018).…”

Prevalence of Repeating Earthquakes in the Continental Crust and Subducting Slabs: Triggering of Earthquakes by Aseismic Slip

“…The temporal variations in the stress drop can account for the magnitude variability observed for repeater sequences, from small (<0.2–0.3) (e.g., Abercrombie et al., 2020; Matsuzawa et al., 2002; Naoi et al., 2015; Uchida et al., 2019) to large (≥1.0) magnitude differences (e.g., Cauchie et al., 2020; Chen et al., 2008, 2009; Lengliné et al., 2014; Lin et al., 2016; Yamaguchi et al., 2018). Very large variations in the stress drop by two orders of magnitude or more are often observed for non‐repeating earthquakes in various tectonic regimes, for example, in Parkfield, California (e.g., Allmann & Shearer, 2007), in the continental crust beneath the Japanese Islands (e.g., Nakajima & Hasegawa, 2021), and in the subducting Pacific slab (e.g., Kita & Katsumata, 2015; Tsuchiyama & Nakajima, 2021). We interpret that these large variations in the stress drop are primarily caused by the temporal and spatial variations in pore‐fluid pressures at asperity patches, even though the heterogeneous strength within an asperity patch could generate variable stress drops (Lin & Lapusta, 2018).…”

“…We defined afterslip-type repeaters in the continental crust that occurred in aftershock areas of 10 M ≥ 6.5 earthquakes (mainshocks) in the analyzed period of 2003-2017 (areas outlined in blue in Figure 6a) because the repeaters in these areas are likely to be affected by afterslip of the mainshock (see Table 1 in Nakajima & Hasegawa, 2021, for details of the 10 mainshocks). We classified all other repeaters outside the 10 aftershock areas as burst-type repeaters, where no marked aseismic slip was geodetically observed.…”

“…(h) Histograms of frequency index (FI) for (red) repeaters and (gray) non‐repeaters for the magnitude range of 1.5 ≤ M ≤ 2.5. FI values are taken from Nakajima and Hasegawa (2021).…”

Strongly Scattering Medium Along Slow Earthquake Fault Zones Based on New Observations of Short‐Duration Tremors

“…They are often considered as building blocks of tremors (Shelly et al, 2007;Figure 1d) and of the broadband slow earthquake phenomena (i.e., Brownian walk model) (Ide, 2008;Ide & Yabe, 2018). There are also reports on LFEs that are not associated with tremor signals (Arai et al, 2016;Aso et al, 2013;Nakajima & Hasegawa, 2021). Meanwhile, to date, LFEs have not been reported in association with shallow (<15 km) tremor signals.…”