Estimation of seismic velocity changes at different depths associated with the 2014 Northern Nagano Prefecture earthquake, Japan (M W 6.2) by joint interferometric analysis of NIED Hi-net and KiK-net records

Kaoru Sawazaki,Tatsuhiko Saito,Tomotake Ueno,Katsuhiko Shiomi

doi:10.1186/s40645-016-0112-7

Kaoru Sawazaki, Tatsuhiko Saito + Show 2 more

Open Access

https://doi.org/10.1186/s40645-016-0112-7

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Abstract

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 (M W 6.2). 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) 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.

Highlights

Even though seismic velocity changes associated with large earthquakes have been widely studied for decades (e.g., Poupinet et al 1984), the cause of the observed velocity changes is still debated
To extract the velocity change in the deep zone from the values of ΔV/VKiK and ΔV/VHi, we evaluate the sensitivity of the autocorrelation function (ACF) to partial velocity changes in the deep zone through a twodimensional wave propagation simulation using the finite difference method (FDM)
The evaluated sensitivity α is 0.78 under the reference velocity structure. This value indicates that almost 80% of the ACF energy senses changes in the medium below the High-sensitivity seismograph network of Japan (Hi-net) sensor installation depth of 150 m. Using this α value and Eq (3), we plot ΔV/Vdeep in Fig. 7, as well as the relative velocity change detected from the deconvolution waveform (DCW) of the Kiban Kyoshin strong-motion seismograph network of Japan (KiK-net) records (ΔV/Vshallow, red circles) and from the ACF of the Hi-net records (ΔV/ VHi, black circles)

Summary

Introduction

Even though seismic velocity changes associated with large earthquakes have been widely studied for decades (e.g., Poupinet et al 1984), the cause of the observed velocity changes is still debated. From the velocity changes and recoveries observed at different depths, and susceptibility values previously estimated in rock experiments, we discuss the contributions of dynamic and static strain changes to the damage at the subsurface. Applying the stretching technique to the reference and perturbed waveforms using lag times of 4–12 s, we compute the travel time shifts τshallow and τdeep due to the 3% velocity reductions in the shallow and deep zones, respectively.

Results

Conclusion