Triggering of earthquake swarms following the 2011 Tohoku megathrust earthquake

Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo; Hama, Yuki

doi:10.1002/2014jb011598

Cited by 13 publications

(7 citation statements)

References 61 publications

(89 reference statements)

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“…To explain the anomalous IFPB activity and the clustering of deep (> 10 km) earthquakes off-shore IFPB, Imanishi et al (2012) proposed that a seaward-dipping low-angel normal fault beneath the IFPB was activated due to stress imparted by the TOE. This structure also aligns with major discontinuities resolved in tomographic and magnetotelluric inversions (Shelly et al, 2006;Umeda et al, 2015;Zhao et al, 2015). The on-and off-shore post-TOE hypocentral location and moment distributions are presented in Figure 8.…”

Section: Inversion Resultssupporting

confidence: 68%

“…According to this scenario, off-megathrust fault creep plays a role similar to the one played by moderate aftershocks, promoting long-range seismic fault interactions through multiple stress transfers (Felzer et al, 2002;Inbal et al, 2017;Ziv, 2006), an idea that is in line with the current understanding of earthquake interaction. Indeed, several independent evidences from tomographic (Shelly et al, 2006), magnetotelluric and geochemical studies (Umeda et al, 2015) support the existence of a seaward dipping discontinuity situated between the subduction interface and the IFPB. Unlike portions of the near-coast forearc to the north and south of Ibaraki, following the TOE, the volume surrounding that discontinuity hosted intense aftershock activity.…”

Section: Introductionmentioning

confidence: 99%

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The elusive role of aseismic slip along a seaward dipping normal fault in the indirect triggering of a normal faulting earthquake sequence in northeast Japan following the 2011 Tohoku-Oki megathrust

Magen,

Inbal,

Ziv

et al. 2024

Preprint

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Although blind normal faults are common in subduction environments, their rheology, kinematics and interaction with the upper crust are poorly constrained. A month-long shallow normal faulting sequence in the Ibaraki-Fukushima prefectural border (IFPB), northeast Japan, which followed the M9.0 Tohoku-Oki earthquake (TOE) and culminated in the M6.7 Iwaki earthquake, provides a window into megathrust-to-normal fault interaction. Stress change calculations indicate that direct triggering by the TOE co- and post-seismic slip does not provide a plausible explanation for the IFPB earthquake sequence. In quest for an alternative triggering mechanism, we analyzed post-TOE GNSS data from eastern IFPB. A key step in this analysis is the removal of the large-scale post-TOE displacement field, after which a distinct highly-localized strain along the coastline becomes apparent. The accumulation of this strain was mostly aseismic, and migrated with time prior to the Iwaki earthquake in a manner that correlates well with post-TOE local seismicity. We attribute the pre-Iwaki earthquake strain accumulation to aseismic slip along low-angle seaward dipping blind normal fault, activated by the TOE. Stresses transferred by this slip episode accelerated the failure along the IFPB shallow normal faults. This indirect triggering of the Iwaki earthquake sequence by the TOE highlights the complexity of stress transfers in subduction environments.

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Section: Inversion Resultssupporting

confidence: 68%

Section: Introductionmentioning

confidence: 99%

The elusive role of aseismic slip along a seaward dipping normal fault in the indirect triggering of a normal faulting earthquake sequence in northeast Japan following the 2011 Tohoku-Oki megathrust

Magen,

Inbal,

Ziv

et al. 2024

Preprint

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“…For the 2-D inversion, the position of the MT stations was projected onto the appropriate profile line (red line in Fig 1(b)), and the response functions were rotated to the new 2-D coordinate system.From a geological point of view, such a 2-D strike angle seems reasonable in the southern Tohoku area, considering the azimuths of the coastlines, distribution of volcanoes, and direction of active faults. Previous studies around the area byAsamori et al (2011) andUmeda et al (2015) inferred an N30°E − S30°W strike direction, which is close to our estimation.…”

supporting

confidence: 92%

“…Apart from volcanoes, but still fluid-driven, a swarm of normal-frequency earthquakes has been observed around Iwaki City, the fore-arc side of southern Tohoku, since the 2011 Tohoku-Oki earthquake (Fig 1). The swarm is triggered by lower crustal metamorphic fluids that invade the brittle upper crust (Umeda et al, 2015). In addition, the back-arc of southern Tohoku also corresponds to the northern end of the Niigata-Kobe Tectonic Zone (NKTZ), a long deformation belt with a high strain rate and many active fault zones (e.g., Sagiya et al, 2000;Heki and Miyazaki, 2001;Fig 1).…”

Section: Introductionmentioning

confidence: 99%

Delineating fluid distribution beneath the southern Tohoku, Northeast Japan, with a joint inversion of magnetotelluric and geomagnetic transfer functions

Diba

Uyeshima

Ichiki

et al. 2022

Preprint

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The southern part of Tohoku, Northeast Japan, is an area with significant in-land activities owing to the ongoing subduction mechanism. Among these are active volcanoes distributed on the volcanic front and back-arc, active faults throughout the area, and a recently observed swarm of shallow earthquakes on the fore-arc side. As fluids play an essential role in arc magmatism and the associated seismicity, this study aims to understand the deep fluid distribution beneath southern Tohoku to clarify the origin of the activities. A magnetotelluric survey delineating the subsurface electrical resistivity structure was used as the bulk resistivity is sensitive to the composition and connectivity of fluids. Using a newly developed joint inversion code, we estimated the resistivity structure using the inter-station horizontal magnetic field transfer function (HMTF) in addition to the conventional magnetotelluric response functions. Joint inversion with HMTF improved the recovery of low-resistivity anomalies owing to the sensitivity of the HMTF to electrical current concentration, resulting in a model with smaller data misfits. The main feature of the resulting resistivity structure is that, instead of under the volcanic front, a deep conductive body is found under the back-arc side in a position closer to a back-arc volcano (Mt. Numazawa) and a swarm of low-frequency earthquakes. Petrological studies indicate that the deep source of fluids supplying to Mt. Numazawa may be the same as that of Mt. Azuma and Mt. Adatara on the volcanic front. Magmatic fluids ascend from the upper mantle to the upper crust via different branches, resulting in multiple eruption centers. Thus, we inferred that the conductor reflects the fluid path to Mt. Numazawa. The high conductivity, especially in the uppermost mantle, may be caused by flux melting, where water or other volatiles released from the subducting slab reduce the solidus of high-temperature basaltic rocks.

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“…Recently, MT data were acquired along a 90 km long profile oriented in a WNW-ESE direction crossing the seismic source region of the earthquake swarms, nearly perpendicular to the strike of the NE Japan arc (Fig. 4) (Umeda et al 2015). The inversion of the MT data shows that the swarm-like earthquake source region coincides with the high electrical resistivity region adjacent to a conductor with a width of 20 km at depths of ~15 km depths, and extending down to the base of the crust (Fig.…”

Section: Geophysical Observationsmentioning

confidence: 99%

Localized extensional tectonics in an overall reverse-faulting regime, Northeast Japan

Umeda

2015

Geosci. Lett.

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In Northeast Japan, it has been recognized that trench-normal compressional stresses, aligned in the approximate direction of plate convergence, tend to dominate stress fields over a broad region. However, a particularly notable event was the shallow, normal-faulting earthquake swarms with a T-axis oriented in the E-W or NW-SE directions that occurred immediately after the 2011 Tohoku-Oki earthquake near the Pacific coast in the Southeast Tohoku district. The stress tensor inversion represents the pre-Tohoku-Oki earthquake stress field in this area as a normal-faulting stress regime with the minimum principal horizontal stress oriented in a roughly NW-SE direction. Additionally, the stress regime varies with depth from normal faulting at shallow depths (<15 km) to thrust faulting at greater depths. Seismic tomography and magnetotelluric soundings defined a geophysical anomaly with low seismic velocity and low resistivity clearly visible beneath the swarm activity, strongly supporting the existence of an interconnected network with fluid-filled porosity. The upper boundary of the conductor is in good agreement with an extensionalcompressional stress transition zone. A plausible explanation for these drastic changes in the stress regime is upward flexure of the upper crust due to partly anelastic deformation in the weakened lower crust. Additionally, remarkable upwarping and localized extensional tectonics during the late Pleistocene reflect the long-term rheological heterogeneities in the crust beneath the seismic source region.

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Triggering of earthquake swarms following the 2011 Tohoku megathrust earthquake

Cited by 13 publications

References 61 publications

The elusive role of aseismic slip along a seaward dipping normal fault in the indirect triggering of a normal faulting earthquake sequence in northeast Japan following the 2011 Tohoku-Oki megathrust

The elusive role of aseismic slip along a seaward dipping normal fault in the indirect triggering of a normal faulting earthquake sequence in northeast Japan following the 2011 Tohoku-Oki megathrust

Delineating fluid distribution beneath the southern Tohoku, Northeast Japan, with a joint inversion of magnetotelluric and geomagnetic transfer functions

Localized extensional tectonics in an overall reverse-faulting regime, Northeast Japan

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