Shock Waves as a Possible Mechanism of Generation of Abnormally High Accelerations during the М 9.0 Tohoku Earthquake on March 11, 2011

Resumo

A mechanism of generation of abnormally high accelerations (>1g, with the maximum of ~3g), recorded during the 2011 Tohoku earthquake (M_w = 9.0), is proposed. Based on the records of vertical groups, soil behavior in near-fault zones is studied to reveal an atypical model: shear moduli increased under string motions, demonstrating strengthening of the soil, then decreased. This phenomenon can be explained under the supposition that soils suffered some additional effect. The shapes of accelerograms show a decrease in the duration and an increase in the intensity of strong motions with distance from the source, possibly indicating superposition of seismic waves and formation of a shock wave by a rapidly moving source (a tip of the crack propagating in the source).