Abstract
We here show relocated hypocenters within a seismic sequence with normal faults in the northern part of Ibaraki Prefecture, triggered after the M 9.0 2011 off the Pacific coast of Tohoku Earthquake. Depth-sections of the hypocenters from the center of the northern Ibaraki region to the north show an earthquake alignment dipping westwards at 40° to 50° at depths shallower than 10 km. On the other hand, hypocenters from the center to the south show a cross-cutting geometry consisting of conjugate westward- and eastward-dipping planes at the same depths. The dip angles of the hypocenter alignments are roughly consistent with the nodal planes of focal mechanisms of large normal earthquakes, and exhibit optimal-orientations in terms of the frictional failure criterion. Furthermore, comparison of the focal mechanisms recorded before and after the 2011 Tohoku Earthquake suggests that the stress field abruptly changed from horizontal compression to extension in the study area. The most plausible explanation of the drastic stress changes is a significant reduction in trench-normal compressive stress compared with reduction in trench-parallel stress accompanying large horizontal extensional deformation within the overlying plate.
Highlights
The 2011 off the Pacific coast of Tohoku EarthquakeM 9.0 ruptured a fault area nearly 500 km along-strike length and 200 km down-dip width of the fault off-shore of the Tohoku region along the Japan Trench subduction zone as a huge thrust earthquake at 14:46 (JST = UT + 9 hours) on March 11, 2011 (Japan Meteorological Agency (JMA))
The focal mechanism of this earthquake is a reverse fault type with a low-dipping angle, suggesting that the fault is located along the interface between the subducting Pacific plate and the overlying plate
Ide et al (2011) imaged a spatio-temporal slip distribution of the 2011 Tohoku Earthquake applying finite-source modeling to global broadband seismograms
Summary
The rate of seismicity abruptly increased after the 2011 Tohoku Earthquake, and several large magnitude events as large as Mjma 6.1 occurred (JMA catalog) (Fig. 1). The most striking feature of the induced seismicity is that the focal mechanisms reveal normal faulting with a T -axis orientated in a roughly E-W direction (Fig. 1(b)).
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