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    Solid earth sciences

    Session convener-recommended article JpGU Meeting 2015


    Estimation of seismic velocity change at different depths associated with the 2014 Northern Nagano Prefecture Earthquake, Japan (MW6.2), by joint interferometric analysis of NIED Hi-net and KiK-net records

    Sawazaki K, Saito T, Ueno T, Shiomi K

    Time lapse monitoring, Velocity change and recovery, Passive image interferometry, Wave propagation simulation, Dynamic and static strain changes, Slow dynamics

    Relative velocity changes estimated for the shallow (red circles) and the deep (blue circles) zones, where sensitivity α = 0.78 is used for the latter. Relative velocity changes detected from the Hi-net ACF are shown by black circles. The abscissa indicates the lapse time after the N. Nagano earthquake given by logarithmic scale. The m-values shown in the figure represent the speed of log(t)-recovery as defined by eq. (5) in the main text.

    To estimate the seismic velocity changes at different depths associated with a large earthquake, we apply passive image interferometry to two types of seismograms: KiK-net vertical pairs of earthquake records and Hi-net continuous borehole data. We compute the surface/borehole deconvolution waveform (DCW) of seismograms recorded by a KiK-net station and the autocorrelation function (ACF) of ambient noise recorded by a collocated Hi-net station, 26 km from the epicenter of the 2014 Northern Nagano Prefecture earthquake, Japan (MW6.2). Because the deeper KiK-net sensor and the Hi-net sensor are collocated at 150 m depth, and another KiK-net sensor is located at the surface directly above the borehole sensors, we can measure shallow (<150 m depth) and deep (>150 m depth) velocity changes separately. The sensitivity of the ACF to the velocity changes in the deeper zone is evaluated by a numerical wave propagation simulation. We detect relative velocity changes of −3.1 and −1.4% in the shallow and deep zones, respectively, within 1 week of the mainshock. The relative velocity changes recover to −1.9 and −1.1%, respectively, during the period between 1 week and 4 months after the mainshock. The observed relative velocity reductions can be attributed to dynamic strain changes due to the strong ground motion, rather than static strain changes due to coseismic deformation by the mainshock. The speed of velocity recovery may be faster in the shallow zone than in the deep zone because the recovery speed is controlled by initial damage in the medium. This recovery feature is analogous to the behavior of slow dynamics observed in rock experiments.