The great tsunami of March 11, 2011 in Japan - analysis of source mechanism and tsunamigenic efficiency

摘要

The great earthquake of March 11, 2011 that occurred off shore the Tohoku region of Japan generated a very destructive tsunami that was anomalously higher than expected from its seismic waves. To understand the cause of the anomaly, an examination was undertaken of the seismotectonics of the region, of the earthquake' focal mechanism, of rupture patterns and of the spatial and temporal sequencing and clustering of major aftershocks - the latter defining the limits of crustal displacements, the amount of energy release and the tsunami generating area. Based on this analysis we conclude that the great tsunami resulted from a combination of up-thrust tectonic motions and crustal deformations of the ocean floor and from additional uplift due to a large coseismic lateral movement, which compressed and deformed compacted sediments along the accretionary prism on the overriding tectonic plate. Although a uniform elastic deformation of the sedimentary layer can be estimated and provide a seismic moment, the deformation was not elastic and occurred randomly and non-uniformly along faults that were either oblique or parallel to the earthquake's overall rupture direction, some failing in a sequential bookshelf manner. Both the 1992 Nicaragua and the 2004 Sumatra earthquakes demonstrated that bookshelf failures of sedimentary layers can generate anomalously high tsunamis. The same mechanism was responsible for the high tsunami generated by both the 2011 and the 1896 Sanriku earthquakes in Japan. The efficiency of the 2011 tsunami generation was greater along the shallow eastern segment of the fault off the Miyagi Prefecture where most of the energy release of the earthquake and deformations occurred, while the segment off the Ibaraki Prefecture - where the rupture process was rapid -released lesser seismic energy, resulted in lesser compaction and deformations of sedimentary layers and thus to a tsunami of smaller height. Because of the complexity of the rupturing process, th--e extent of additional uplift due to the buckling of the sediments in the tsunami generation area of the fault is difficult to estimate. However, based on the present evaluation, it is concluded that the greater tsunamigenic efficiency and destructiveness of the 2011 tsunami along Honshu's coastlines was caused by vertical crustal displacements of more than 10 meters due to up-thrust faulting, and that lateral compression and folding of sediments contributed additional uplift of about 7 meters along the leading edge of the accretionary prism of the overriding tectonic plate - as it thrusted in an east-southeast direction.