17:15 〜 18:45
[SSS03-P01] Estimations for seismic energy and source duration of shallow tremors along the Nankai Trough incorporating the effects of thick sedimentary layer
キーワード：スロー地震、南海トラフ、浅部プレート境界
Slow earthquakes have been observed along the Nankai Trough at shallow (<10 km) and deep (30-40 km) depths. Estimations of the source parameters of slow earthquakes at both depths have been conducted to understand their physical properties. However, because of propagation effects within the thick sedimentary layer (Takemura et al., 2020, 2023ab) and large site amplifications (e.g., Kubo et al., 2018; Yabe et al., 2019), precise estimations of source parameters for shallow tremors are still challenging issues even for using near-source DONET observations. Here, to estimate the source parameters of shallow tremors precisely, we propose the nearest single-station method for source radiated energy and durations of shallow tremors.
Based on the shallow tremor catalog by Tamaribuchi et al. (2022) and the DONET record (https://doi.org/10.17598/NIED.0008), we estimated source radiated energies and durations of shallow tremors that occurred southeast off the Kii Peninsula. After correcting site amplifications based on far-filed S-wave propagation, we synthesized three-component squared velocity envelopes (hereafter “squared envelopes”) at each DONET station. The 10-s moving average was applied for stable estimations. We measured half-value widths (t) of the nearest squared envelopes as source durations of shallow tremors. Because the envelope broadening due to the thick sedimentary layer becoming strong with increasing distance, we only used the nearest station. Then, we integrated squared envelopes within time windows of half-value widths (t0). First, we obtained a source radiated energy E0 via the conventional method, assuming simple far-field S-wave propagation, a density of 2,700 kg/m3, S-wave velocity VS of 3,500 m/s, and QS = 853 for the 2-8 Hz band. However, distant-dependent propagation amplification due to a thick sedimentary layer (Takemura et al., 2023b) was still included in source radiated energy E0. By correcting the effects of propagation amplification, we precisely obtained source radiated energies E of shallow tremors as possible.
The energy rates (E0/t0) via the conventional method are 10-0.8-105.9 J/s, which are similar to the estimated values of Yabe et al. (2019). After correcting the effects of propagation amplification, the energy rates (E/t0) and half-value widths (t0) of shallow tremors are 10-1.1-105.7 J/s and 5-300 s, respectively. The energy rates of shallow tremors are wider values than those of deep tremors (Yabe et al. 2014). Assuming the scaled energy of 10-9 (Yabe et al., 2019; Takemura et al., 2023b), we evaluated cumulative moments of shallow tremors from April 2016 to March 2021. The cumulative moment of shallow tremors is 6.4*1017 Nm, which is similar order to a cumulative moment from shallow VLFEs in this region (Takemura et al., 2022). This result indicates that large events of shallow tremors/VLFEs tend to be dominant southeast off the Kii Peninsula.
Based on the shallow tremor catalog by Tamaribuchi et al. (2022) and the DONET record (https://doi.org/10.17598/NIED.0008), we estimated source radiated energies and durations of shallow tremors that occurred southeast off the Kii Peninsula. After correcting site amplifications based on far-filed S-wave propagation, we synthesized three-component squared velocity envelopes (hereafter “squared envelopes”) at each DONET station. The 10-s moving average was applied for stable estimations. We measured half-value widths (t) of the nearest squared envelopes as source durations of shallow tremors. Because the envelope broadening due to the thick sedimentary layer becoming strong with increasing distance, we only used the nearest station. Then, we integrated squared envelopes within time windows of half-value widths (t0). First, we obtained a source radiated energy E0 via the conventional method, assuming simple far-field S-wave propagation, a density of 2,700 kg/m3, S-wave velocity VS of 3,500 m/s, and QS = 853 for the 2-8 Hz band. However, distant-dependent propagation amplification due to a thick sedimentary layer (Takemura et al., 2023b) was still included in source radiated energy E0. By correcting the effects of propagation amplification, we precisely obtained source radiated energies E of shallow tremors as possible.
The energy rates (E0/t0) via the conventional method are 10-0.8-105.9 J/s, which are similar to the estimated values of Yabe et al. (2019). After correcting the effects of propagation amplification, the energy rates (E/t0) and half-value widths (t0) of shallow tremors are 10-1.1-105.7 J/s and 5-300 s, respectively. The energy rates of shallow tremors are wider values than those of deep tremors (Yabe et al. 2014). Assuming the scaled energy of 10-9 (Yabe et al., 2019; Takemura et al., 2023b), we evaluated cumulative moments of shallow tremors from April 2016 to March 2021. The cumulative moment of shallow tremors is 6.4*1017 Nm, which is similar order to a cumulative moment from shallow VLFEs in this region (Takemura et al., 2022). This result indicates that large events of shallow tremors/VLFEs tend to be dominant southeast off the Kii Peninsula.