Outer trench-slope faulting and the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake

Abstract

The 11 March 2011 off the Pacific coast of Tohoku Earthquake (M w 9.0) produced megathrust displacements of at least 40 m. The resulting tsunami devastated the Honshu coast southwest of regions struck by earthquakegenerated tsunami in 1611, 1896 and 1933. The 1896 Meiji-Sanriku earthquake was also an underthrusting earthquake, but the 1933 Sanriku-oki earthquake was a trench-slope normal faulting event; both generated inundation heights of 10 to 25 m along the coast of Iwate prefecture. Possible occurrence of a great outer trenchslope earthquake seaward of the 2011 Tohoku Earthquake along a southwestward extension of the 1933 fault zone is a concern. The second largest 2011 aftershock, an outer rise M w 7.7 normal faulting earthquake occurred near the southern end of the 1933 rupture. Additional aftershock activity has been distributed along a trend below the trench and diffusely spread in the outer rise, seaward of the megathrust region where the largest slip occurred. Coulomb stress perturbations of at least 5-10 bars are calculated for outer rise normal fault geometries for mainshock slip models. Whether a future great trench slope event will occur is uncertain, but the potential tsunamigenic hazard can be gauged by the huge inundations accompanying the 1933 rupture.

Spatio-temporal distribution of seismic moment release near the source area of the 2011 Tohoku-Oki earthquake

“…On the basis of our observations and the results of previous studies, we extend a conceptual model originally from Lay et al (2011a), regarding the possible stress variation and failure processes along the Japan Trench (Fig. 7), to explain three distinct M 0 processes (Category 1 to 3) of the five rupture segments (Segments A to E) along the Japan Trench.…”

“…In summary, we integrate our results with the schematic cross-sections proposed by Lay et al (2011a) regarding the possible stress variation and failure processes along the Japan Trench (Fig. 7).…”

Possible large near-trench slip during the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake

“…1 show some correspondence with the predicted Coulomb stress variations, but many different fault geometries and depths would need to be evaluated. These calculations can be compared with those for the joint seismic and geodetic inversion model of Ammon et al (2011), which are presented in Lay et al (2011c).…”

“…The occurrence of tsunami earthquakes, which characteristically have long source process times and weak short-period radiation (e.g., Kanamori, 1972;Kanamori and Kikuchi, 1993;Polet and Kanamori, 2000;Bilek and Lay, 2002; al., 2006) must involve rather large ocean bottom deformations indicative of large slips on the megathrust or on splay faults (Fukao, 1979;Pelayo and Wiens, 1992). The up-dip limit of co-seismic slip in large earthquakes is also important for consideration of how stresses are communicated to the trench-slope and outer rise intraplate environments, and the potential for tsunamigenic normal faulting ruptures (Kanamori, 1971;Ammon et al, 2008;Lay et al, 2011c). Given the common presence of a sedimentary wedge and the relatively hydrated conditions likely to exist in a shallow megathrust, it is usually uncertain whether the frictional regime will favor stable sliding or large strain accumulation.…”

A rupture model of the 2011 off the Pacific coast of Tohoku Earthquake

“…Recent large earthquake sequences have illuminated the importance of such interactions during great earthquake sequences (Ammon et al, 2008;Lay et al, 2009Lay et al, , 2010a. We refer the reader to Lay et al (2011c) for a discussion of the implications such large slip may have on processes in adjacent Pacific plate. Finally, an additional intriguing aspect of the model is its difference from short-period teleseismic P-wave back projection results that suggest strong short-period P-wave radiation occurred along the megathrust, downdip of the hypocenter.…”