Faulting caused by earthquakes beneath the outer slope of the Japan trench.

Abstract

We determined fault plane solutions and focal depths for seven events which occurred in the Japan trench outer slope, using comparison between synthetic and observed long-and short-period teleseismic seismograms. The five normal fault solutions obtained in this study have nearly vertical nodal planes parallel to the trench and their depth range, 2-16 km below the seafloor, is in accord with bending of the oceanic lithosphere prior to subduction. The occurrence of both types of normal faulting, downthrow of both the continental and oceanic sides, is likely to be associated with the formation of horst-graben structures in the outer trench slope near the trench axis.One normal fault solution (31 km below the seafloor) that is significantly deeper than the other normal fault solutions has a nearly vertical nodal plane striking parallel to the ENE magnetic anomaly lineations in the region. This event is also characterized by its lower stress drop or slower rupture than the other events. This event would represent a reactivation of the initial structural fabric inherited from the accretion at the mid-oceanic ridge and indicates that the bending stress is small at this depth. One reverse fault solution, 41 km below the seafloor, is in accord with the compressional bending stress in the deeper portion of the lithosphere. These events indicate that a neutral surface of bending stress is around 30 km depth in this region at least for the past few decades, consistent with the elastic thickness of lithosphere in adjacent regions. This implies that the lithosphere is not entirely in deviatoric tension in this region, suggesting that the 1933 Sanriku earthquake may be a large bending event.

Insight into complex rupturing of the immature bending normal fault in the outer slope of the Japan Trench from aftershocks of the 2005 Sanriku earthquake (Mw = 7.0) located by ocean bottom seismometry

“…Although we could not determine focal mechanism solutions for the deep earthquakes, several earthquakes had P wave motions consistent with reverse faulting but not normal faulting, similar to the main shock. Existence of a thrust type earthquake with focal depth of about 40 km has been reported by Seno and Gonzalez [1987]. Although the epicenter of the earthquake was located at the axis of the Japan trench and about 100 km apart from the focal area of the 2005 earthquake, its focal depth is in good agreement with the deeper seismicity which we found by the OBS observation.…”

“…It is essential for understanding how deep water infiltrates into the oceanic lithosphere to clarify focal depth distribution of the bending earthquakes. Most of the previous studies were based on waveform analyses of teleseismic records and precise focal depths have been given to large events [e.g., Seno and Gonzalez , 1987] but few studies on microseismicity representing depth extent of seismogenic layer have been made [e.g., Hirata et al , 1985; Lefeldt et al , 2009].…”

Normal‐faulting earthquakes beneath the outer slope of the Japan Trench after the 2011 Tohoku earthquake: Implications for the stress regime in the incoming Pacific plate

“…This stress regime can be explained by bending of the incoming/subducting lithosphere. Similar regimes of shallow tensional stress and deep compressional stress in the Pacific plate along the Japan Trench have been reported on the basis of data before the 2011 Tohoku earthquake [ Seno and Gonzalez , 1987; Gamage et al , 2009]. However, focal mechanisms obtained from OBS data after the 2011 Tohoku earthquake show tensional stresses extending to depths of about 40 km in the Pacific plate without a change to compressional stress (Figure 3).…”

“…Dehydration embrittlement of the oceanic mantle, which may be hydrated in the trench–outer rise region, could have triggered the lower plane earthquakes [ Peacock , 2001]. Before the 2011 Tohoku earthquake, stresses in the incoming Pacific plate were tensional in the shallow part of the plate and compressional at greater depth, a response to bending of the lithosphere [ Seno and Gonzalez , 1987; Gamage et al , 2009]. Under these conditions, fluids would penetrate downwards only through the shallow tensional faults and hydration of the incoming plate would be limited to depths shallower than the zone of transition from tensional to compressional stress [ Lefeldt et al , 2009].…”

Outline of the 2011 off the Pacific coast of Tohoku Earthquake (M w 9.0) —Seismicity: foreshocks, mainshock, aftershocks, and induced activity—

“…1(a), 3(d)). It is thought that normal fault type events occur under the influence of the slab bending (Seno and Gonzalez, 1987). In the present case, in addition, some aftershocks beneath the outer rise were induced by the east-west extension field made by the mainshock which released the compressional stress caused by the subduction of the Pacific plate.…”