Detectability analysis of very low frequency earthquakes: Methods and application in Nankai using F-net and DONET broadband seismometers

Takemura, Shunsuke; Baba, Satoru; Yabe, Suguru; Yamashita, Yusuke; Shiomi, Katsuhiko; Matsuzawa, Takanori

doi:10.21203/rs.3.rs-2351814/v1

Cited by 3 publications

(2 citation statements)

References 34 publications

(44 reference statements)

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“…The detectability of VLFEs along the Nankai Trough was evaluated in Takemura et al. (2024). Shallow VLFEs lie between scaling laws of ordinary ( Mo ∝ T 3 ) and deep slow ( Mo ∝ T ) earthquakes.…”

Section: Discussionmentioning

confidence: 99%

Revisiting Seismic Energy of Shallow Tremors: Amplifications Due To Site and Propagation Path Effects Near the Nankai Trough

Takemura,

Emoto,

Yabe

2024

JGR Solid Earth

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We investigated the effects of the propagation path and site amplification of shallow tremors along the Nankai Trough. Using far‐field S‐wave propagation from intraslab earthquake data, the amplification factors at the DONET stations were 5–40 times against an inland outcrop rock site. Thick (∼5 km) sedimentary layers with VS of 0.6–2 km/s beneath DONET stations have been confirmed by seismological studies. To investigate the effects of thick sedimentary layers, we synthesized seismograms of shallow tremors and intraslab earthquakes at seafloor stations. The ratios of the maximum amplitudes from the synthetic intraslab seismograms between models with and without thick sedimentary layers were 1–2. This means that thin lower‐velocity (<0.6 km/s) sediments just below the stations primarily control the estimated large amplifications. Conversely, at near‐source (≤20 km) distances, 1‐order amplifications of seismic energies for a shallow tremor source can occur due to thick sedimentary layers. Multiple S‐wave reflections between the seafloor and plate interface are contaminated in tremor envelopes; consequently, seismic energy and duration are overestimated. If a shallow tremor occurs within underthrust sediments, the overestimation becomes stronger because of the invalid rigidity assumptions around the source region. After 1‐order corrections of seismic energies of shallow tremors along the Nankai Trough, the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. Hence, the physical mechanisms governing seismic slow earthquakes can be the same, irrespective of the region and source depth.

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Section: Discussionmentioning

confidence: 99%

Revisiting Seismic Energy of Shallow Tremors: Amplifications Due To Site and Propagation Path Effects Near the Nankai Trough

Takemura,

Emoto,

Yabe

2024

JGR Solid Earth

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“…Shallow VLFEs lie between scaling laws of ordinary (Mo ∝ T 3 ) and deep slow (Mo ∝ T) earthquakes. The detectability of VLFEs along the Nankai Trough was evaluated in Takemura, Baba, Yabe, Yamashita, et al (2022). Duration ranges were not different at different depths, but the seismic moments of shallow VLFEs were 1-2 orders larger than those at deeper depths (Ide et al, 2008).…”

Section: Scaled Energy Of Seismic Slow Earthquakes At Deep and Shallo...mentioning

confidence: 99%

Revisiting Seismic Energy of Shallow Tremors: Amplifications due to Site and Propagation Path Effects Near the Nankai Trough

Takemura,

Emoto,

Yabe

2024

Preprint

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We investigated the effects of the propagation path and site amplification of shallow tremors along the Nankai Trough. Using far-field S-wave propagation from intraslab earthquake data, the amplification factors at the DONET stations were 5–40 times against an inland outcrop rock site. Thick (~5 km) sedimentary layers with VS of 0.6–2 km/s beneath DONET stations have been confirmed by seismological studies. To investigate the effects of thick sedimentary layers, we synthesized seismograms of shallow tremors and intraslab earthquakes at seafloor stations. The ratios of the maximum amplitudes from the synthetic intraslab seismograms between models with and without thick sedimentary layers were 1–2. This means that the estimated large amplifications are primarily controlled by thin lower-velocity (< 0.6 km/s) sediments just below the stations. Conversely, at near-source (≤ 20 km) distances, 1-order amplifications of seismic energies for a shallow tremor source can occur due to thick sedimentary layers. Multiple S-wave reflections between the seafloor and plate interface are contaminated in tremor envelopes; consequently, seismic energy and duration are overestimated. If a shallow tremor occurs within underthrust sediments, the overestimation becomes stronger because of the invalid rigidity assumptions around the source region. After 1-order corrections of seismic energies of shallow tremors along the Nankai Trough, the scaled energies of seismic slow earthquakes were 10-10–10-9 irrespective of the region and source depth. Hence, the physical mechanisms governing seismic slow earthquakes can be the same, irrespective of the region and source depth.

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A review of shallow slow earthquakes along the Nankai Trough

Takemura,

Hamada,

Okuda

et al. 2023

Earth Planets Space

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Slow earthquakes occur at deep and shallow plate boundaries along the Nankai Trough. Deep slow earthquakes are continuously distributed along the 30–40 km depth contours of the upper surface of the subducted Philippine Sea Plate. In contrast, shallow slow earthquakes occur in limited regions: Hyuga-nada, off Cape Muroto, and southeast off the Kii Peninsula. This review provides an overview of the up-to-date seismological, geodetic, geological, and experimental results in the shallow Nankai area for a unified understanding of the spot-like occurrence of shallow slow earthquakes. Shallow slow earthquakes tend to be distributed in transitional regions between the frictionally locked and stably sliding zones on the plate boundary. Based on geological and experimental studies, the lithology of incoming sediments and their friction coefficients can be variable along the Nankai Trough. Laboratory friction experiments revealed that sediments under shallow plate boundary conditions often exhibit positive (a − b) values, while negative (a − b) is possible via several processes. Subducted seamounts create complex fracture networks and stress shadows in their surrounding areas; however, not all subducted seamounts are related to shallow slow earthquake activities. This incomplete correlation suggests that alternative factors are required to explain the spot-like distribution of shallow slow earthquakes in the Nankai subduction zone. High pore fluid pressure conditions around shallow slow earthquake zones were interpreted based on seismological structural studies. In addition, ambient noise monitoring revealed temporal changes in seismic velocity structures associated with shallow slow earthquake migrations. This result suggests a close link between pore fluid migration and shallow slow earthquake episodes. Because transient changes in pore fluid pressure can lead to various slip behaviors, the episodic migration of pore fluid around the plate boundary could promote shallow slow earthquake activity along the Nankai Trough. Graphical Abstract

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Detectability analysis of very low frequency earthquakes: Methods and application in Nankai using F-net and DONET broadband seismometers

Cited by 3 publications

References 34 publications

Revisiting Seismic Energy of Shallow Tremors: Amplifications Due To Site and Propagation Path Effects Near the Nankai Trough

Revisiting Seismic Energy of Shallow Tremors: Amplifications Due To Site and Propagation Path Effects Near the Nankai Trough

Revisiting Seismic Energy of Shallow Tremors: Amplifications due to Site and Propagation Path Effects Near the Nankai Trough

A review of shallow slow earthquakes along the Nankai Trough

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