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

Abstract

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.

A “Geodetic Gap” in the Calabrian Arc: Evidence for a Locked Subduction Megathrust?

“…Over the past few years a similar behavior has been documented on different occasions: slip along the subduction interface triggered extensional earthquakes in the adjacent forearcs following both the 2010, Geophysical Research Letters 10.1002/2017GL076554 M w 8.8 Maule, Chile (Aron et al, 2013;Ryder et al, 2012) and the 2011, M w 9.0, Tohoku-Oki, Japan (Tsuji et al, 2013;Umeda, 2015) earthquakes. Similarly, slow slip events along the Calabrian subduction interface could trigger extensional earthquakes in its forearc.…”

Transient Electromagnetic Investigations in a Tectonic Domain of the Kachchh Intraplate Region, Western India: A Morphotectonic Study of the Kachchh Mainland Fault

“…The Jhurio and Habbo anticlinal topographies (seen as high BG in Figure ) are more domal compared to the other asymmetrical anticlines. Umeda () reported a localized extensional tectonics in an overall reverse‐faulting regime in Northeast Japan that represents the pre‐Tohoku‐Oki earthquake stress field in the area as a normal‐faulting stress regime. Further, a study by Monaldi et al () in the inverted Salta group rift basin of the northwestern Argentina identified an east‐trending extensional structures including half gardens, rollover anticlines, and extensional fault‐propagation folds at its eastern border, which they interpreted as the preserved extensional structures.…”

Near-Source Waveform Modeling to Estimate Shallow Crustal Attenuation and Radiated Energy of Mw 2.0-4.5 Earthquakes

“…Additionally, p revious studies have suggested that fluids exist in fault zone s in this region and affect the intense seismicity (Kato et al, 2013;Umeda et al, 2015;Yoshida et al, 2015;Zhao et al, 2015;Usuda et al, 2021). Yoshida (2021) examined the seismic waves in a fluid -driven earthquake swarm in Japan and suggested that the near-source (< a few kilometers) 𝑄 𝑠 -value was very small (< 50) during intense fluid migration.…”