Seismic attenuation structure associated with episodic tremor and slip zone beneath Shikoku and the Kii peninsula, southwestern Japan, in the Nankai subduction zone

Kita, S.; Matsubara, Makoto

doi:10.1002/2015jb012493

Cited by 24 publications

(28 citation statements)

References 63 publications

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“…Evidence includes spatial correlations between recurrence intervals of SSEs and geologic terranes30, between locations of long-term SSEs and LFEs and heterogeneous fluid transport properties in the overlying plate31, and between segment boundaries of short-term SSEs and areas where seismic properties above the megathrust varies sharply32. Our observations add to the growing body of evidence that LFEs occur only under undrained conditions and that a highly metamorphosed, permeable overlying plate inhibits the generation of LFEs.…”

Section: Discussionmentioning

confidence: 52%

Tremor activity inhibited by well-drained conditions above a megathrust

Nakajima

Hasegawa

2016

Nat Commun

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Tremor occurs on megathrusts under conditions of near-lithostatic pore-fluid pressures and extremely weakened shear strengths. Although metamorphic reactions in the slab liberate large amounts of fluids, the mechanism for enhancing pore-fluid pressures along the megathrust to near-lithostatic values remains poorly understood. Here we show anti-correlation between low-frequency earthquake (LFE) activity and properties that are markers of the degree of metamorphism above the megathrust, whereby LFEs occur beneath the unmetamorphosed overlying plate but are rare or limited below portions that are metamorphosed. The extent of metamorphism in the overlying plate is likely controlled by along-strike contrasts in permeability. Undrained conditions are required for pore-fluid pressures to be enhanced to near-lithostatic values and for shear strength to reduce sufficiently for LFE generation, whereas well-drained conditions reduce pore-fluid pressures at the megathrust and LFEs no longer occur at the somewhat strengthened megathrust. Our observations suggest that undrained conditions are a key factor for the genesis of LFEs.

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Section: Discussionmentioning

confidence: 52%

Tremor activity inhibited by well-drained conditions above a megathrust

Nakajima

Hasegawa

2016

Nat Commun

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“…They suggested that high-V P /V S zones around the plate interface are generated due to the dehydration of fluid from the subducting plate and that LFEs occur due to the increased pore pressure along the plate interface. Kita and Matsubara (2016) estimated a seismic attenuation structure (Q P ) beneath Shikoku and Kii peninsula. The Q P in the central Shikoku seems to be relatively high in the oceanic crust of the subducting plate as well as in the overlying plate.…”

Section: Resultsmentioning

confidence: 99%

Stress state along the western Nankai Trough subduction zone inferred from b-values, long-term slow-slip events, and low-frequency earthquakes

Chiba

2020

Earth Planets Space

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The b-value of the Gutenberg-Richter law represents the ratio of earthquake magnitude to frequency of occurrence and is inversely proportional to differential stress. Repeating long-term slow-slip events (SSEs) and low-frequency earthquakes (LFEs) occur at subducting plate interfaces and have stress-dependent characteristics near the interface. In this study, a comprehensive regional b-value distribution is produced for the western Nankai Trough region, which highlights the relationship between b-values, SSEs, and LFEs. b-values vary along the strike direction of the subducting plate and are significantly lower (b ∼ 0.6) in central Shikoku district than elsewhere, where LFEs frequently occur. However, b-values in the source regions of other LFEs are moderate to high. These findings imply that b-values in the focal region are controlled by more than the LFE source process; indeed, if this source process were solely responsible, then high b-values would be expected. Meanwhile, the V P /V S and Q P around the plate interface in central Shikoku estimated from seismic velocity and attenuation structure are smaller and larger than those in other regions with LFEs, respectively. SSEs with the migration toward central Shikoku also occurred during the analysis period, suggesting significant accumulation of shear stresses in the focal region, which reduced the b-values. These findings suggest that the spatial distributions of b-values are influenced by complicated stress and shear strength perturbations caused by SSEs and LFEs. On the other hand, the b-values in the region that underwent the greatest slip during the 1946 Nankai earthquake are not necessarily low, although the area covered by the b-value distribution is small owing to the lack of events on the updip side. Whereas the asperity areas of huge earthquakes are characterized by low b-values, the b-value distribution in the Nankai megathrust area is more complicated. It is considered that slow earthquakes, including SSEs and LFEs, are related to megathrust earthquakes via stress transfer from slow earthquakes to adjacent megathrust source regions. A unified analysis of b-values in the source regions of slow and megathrust earthquakes may be required to make precise estimates of the seismic hazard produced by a megathrust event.

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“…A constant stress loading rate means that frequent SSEs will occur at fault patches with smaller stress drops. Previous studies have attributed SSE segmentation to spatial variations in fault strength and/or effective normal stress that may be related to stress drop (e.g., Audet & Bürgmann, ; Brudzinski & Allen, ; Kita & Matsubara, ; Nakajima & Hasegawa, ). Numerical simulations using the rate‐ and state‐dependent friction (RSF) law also reproduced SSE segmentation when along‐strike variations in the effective normal stress were applied (Li et al, ; Liu, ).…”

Section: Discussionmentioning

confidence: 99%

Along‐Strike Variation and Migration of Long‐Term Slow Slip Events in the Western Nankai Subduction Zone, Japan

2019

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We detected 24 long-term slow slip events (SSEs) in the western Nankai subduction zone, Japan, from 1996 to 2017 using a newly developed method: grid-based determination of slow slip events (GriD-SSEs). GriD-SSE systematically detects long-term SSEs and determines their fault parameters by fitting modeled displacement time series to observed Global Navigation Satellite System data. The detected SSEs included 11 previously undocumented events. The results suggest an along-strike segmentation of long-term SSEs. The along-strike variations in SSE recurrence intervals and total slip amounts appear to correlate with the variation of updip interplate locking. Long-term SSEs are less frequent in the downdip extensions of the Nankai megathrust (M W 8-9) and Hyuga-nada earthquakes (M W 7.5), with~6-year recurrence intervals and total slip amounts that account for only 10-20% of the plate convergence. However, long-term SSEs do occur frequently downdip of the Hyuga-nada stable sliding zone and of the gap between the Nankai and Hyuga-nada earthquakes, with 2-to 3-year recurrence intervals and total slip amounts that account for 50-60% of the plate convergence. This correlation suggests that the long-term SSE activity is controlled by the updip locked areas owing to the stress shadowing effect. We also observed that the long-term SSEs repeatedly migrated in the downdip regions from the southwestern creeping zone to the northeastern Nankai megathrust locked zone. This SSE migration may represent the current state of stress accumulation and release processes that are controlled by the along-strike variations in interplate coupling along the entire western Nankai subduction zone.

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Seismic attenuation structure associated with episodic tremor and slip zone beneath Shikoku and the Kii peninsula, southwestern Japan, in the Nankai subduction zone

Cited by 24 publications

References 63 publications

Tremor activity inhibited by well-drained conditions above a megathrust

Tremor activity inhibited by well-drained conditions above a megathrust

Stress state along the western Nankai Trough subduction zone inferred from b-values, long-term slow-slip events, and low-frequency earthquakes

Along‐Strike Variation and Migration of Long‐Term Slow Slip Events in the Western Nankai Subduction Zone, Japan

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