Nonlinear earthquake response of marine sediments with distributed
Seismic waves can be significantly amplified by soft sediment layers.
Large dynamic strains can trigger a nonlinear response of shallow soils
having low strength, which is characterized by a shift of the resonance
frequencies, ground motion deamplification, and in some cases, soil
liquefaction. We investigate the response of marine sediments during
earthquake ground motions recorded along a fiber-optic cable offshore
the Tohoku region, Japan, with Distributed Acoustic Sensing (DAS). We
compute AutoCorrelation Functions (ACFs) of the ground motions from 103
earthquakes in different frequency bands. We detect time delays in the
ACF waveforms that are converted to relative velocity changes
(dv/v). dv/v drops, which are characteristic of soil
nonlinearity, are observed during the strongest ground motions.
Moreover, the dv/v values show a strong variability along the
cable. This study demonstrates that DAS can be used to infer the dynamic
properties of the shallow Earth with an unprecedented spatial