Scaled Energy Estimation for Shallow Slow Earthquakes

Abstract

Deep and shallow slow earthquakes occur at the edge of the seismogenic zone in the Nankai subduction zone. Deep slow earthquakes occur under high‐temperature and high‐pressure conditions, whereas shallow slow earthquakes occur at lower temperatures and pressures. Although these two types of slow earthquake show qualitatively similar behaviors, such as slow deformation and depleted high‐frequency seismic signals, their similarities have not yet been evaluated in quantitative ways. In this work, we analyze shallow tectonic tremors accompanied by very low frequency earthquakes off Kii Peninsula, Japan. We estimate the seismic energy of the shallow tectonic tremors by correcting for the site amplifications of ocean bottom seismic stations and seismic attenuation in the shallow accretionary prism. The estimated seismic energy rates of these tectonic tremors are compared with seismic moment rates of accompanying very low frequency earthquakes, revealing that the scaled energy of shallow slow earthquakes is roughly constant, ~10−9 to 10−8, which is 0 to 1 order of magnitude larger than the typical scaled energy of deep slow earthquakes.

Event Size Distribution of Shallow Tectonic Tremor in the Nankai Trough

“… β controls the slope of the distribution; β = 2/3 b in the ordinary Gutenberg‐Richter law. Because the scaled energy of slow earthquakes is constant (Ide & Maury, ; Ide & Yabe, ; Yabe et al, ), we fit the TGR model to the tremor size distributions by replacing the seismic moment with the seismic energy according to the method described by Kagan (). By assuming M t = 3.0E4 J, we obtained corner energy values of 1.3E6, 9.0E6, 1.0E7, and 1.7E5 J for tremor activity in 2009, 2015, 2016, and 2018, respectively, and respective β values of 0.72, 1.0, 0.70, and 0.38.…”

“…We next measured event size by computing the radiated seismic energy from the tremor source, basically following the method of Yabe and Ide () and Yabe et al (). The seismic energy E i of tremor event i estimated at the j th station can be calculated as follows: where is the ground velocity at time t , R ij is the distance between the source of event i and station j , and A j is the site amplification factor.…”

“… ρ , V s , and C are the density of the medium, S wave velocity, and path attenuation factor, respectively. We assumed ρ = 2,700 kg/m 3 and V s = 3,500 m/s, following Yabe et al (). For tremor energy estimations, we band‐pass filtered the three‐component velocity seismograms between 2 and 8 Hz, squared the result, and then low‐pass filtered the resultant seismograms at 0.1 Hz to obtain envelope waveforms.…”

“…We note that activities east and west of DONET1 stations in 2016, as detected by Annoura et al (2017), were discarded in the robust estimations of the daily average of tremor source locations because they were minor compared with activities beneath DONET1 stations. We next measured event size by computing the radiated seismic energy from the tremor source, basically following the method of Yabe and Ide (2014) and Yabe et al (2019). The seismic energy E i of tremor event i estimated at the jth station can be calculated as follows:…”

“…Observed seismograms of shallow slow earthquakes are very similar to those of deeper earthquakes despite the very different physical conditions (e.g., temperature and pressure) in the source region (Saffer & Wallace, ; Syracuse et al, ). The scaled energy values, defined as the proportionality constant of the seismic energy to seismic moment, of shallow slow earthquakes are similar to, but slightly larger than, those of their deeper counterparts (Yabe et al, ). These shallow slow earthquakes can also be considered broadband slow earthquakes (Kaneko et al, ; Nakano et al, ).…”