Characteristics Of Near-field Ground-motion Observed From KiK-net And K-NET Stations During Mw 7.0 Kumamoto Earthquake

This study selects the three-component records of 255 near-field strong motion stations of the Japan Kumamoto MW 7.0,to study the attenuation,vertical vibration characteristics.How the shallow-site amplification and rupture directivity effects would affect the near fault ground motion is also studied.The near-field ground motion attenuation and distribution are studied at the very beginning.The near-field horizontal ground motion PGA,PGV and spectra acceleration in period 0.2s,1.0s,2.0s,3.0s,5.0s and 10.0s are compared with the NGA-West2 ground motion model predictions to study the Kumamoto surface and borehole PGA/PGV and spectra acceleration attenuations.Ground Motion recordings of 12 stations on the rupture propagation direction and 9 stations opposite the rupture propagation direction are also compared with the NGA-West2 model predictions to study the horizontal spectra acceleration differences.From the results,the following conclusion could be made:(1)for borehole observations,the ground motion predictions of NGA-West2 model are higher than observation in PGA and 0.2s spectra acceleration,while consistent with observation in PGV and long period ground motion;(2)for surface observations,the NGA-West2 ground motion model predicts consistent PGA,PGV and 0.2s spectra acceleration observation,but higher in long period ground motion.(3)rupture directivity effects are observed from both the acceleration and velocity time history records.The effects decrease with the increase of distance to fault.(4)seen from the spectra acceleration,the rupture directivity effects amplify the near-field ground motion on the direction of the rupture propagation obviously,especially in the long periods.Then,the vertical and horizontal ground motions are compared.The 255 near-field stations are grouped according to their locations(surface/borehole),fault distanceand site conditionto study the vertical/horizontal ground motion ratio of Kumamoto earthquake.The following conclusion could be made:(1)the mean ground motion V/H ratio of the borehole station spectra acceleration is higher than 2/3.The mean ground motion V/H ratios of the spectra acceleration of 0.4s-1s and 3s-10 s are higher than all other periods.The mean ground motion V/H ratio of short period(less than 0.1s)for D type sites,who has low shear wave velocities,is higher than type B and C sites.(2)the mean ground motion V/H ratio of the surface station spectra acceleration is not overall larger than 2/3,with few examples showing their ground motion V/H larger than 1.Short period(less than 0.1s)and 3-10 s spectra acceleration V/H ratios are obviously higher than middle-period ones.For periods above 1s,the mean of V/H ratio increase with the period.The type B sites with higher shear-wave velocities have higher mean V/H ratio than C/D/E type sites,especially in short periods.The stations whose fault distance is less than 30 km record higher mean V/H ration than other stations,which could be attributed to the near-field effect of the vertical ground motion.(3)in all,the variations with period of surface and borehole mean V/H ratio are similar,but borehole stations have higher mean V/H ration.Finally,the shallow-site amplification research is conducted.By comparing the surface and borehole records of the KiK-net stations,this study discuss the effect of the shallow-site amplification.It shows:(1)the PGA,PGV and the spectra acceleration of 0.2s,1.0s and 2.0s show obvious amplifications for the surface stations,which decreases when the site shallow layer shear wave velocities increase.The amplification is small for the periods of 3s,5s and 10 s.(2)obvious non-linear reaction could be observed from the near-field records.The predominant period of the amplification curve moves to the long period,when the strong motion of PGA > 100 gal is applied.And the surface/borehole amplification factor decreases considerably.(3)the surface/borehole spectra acceleration ratio varies with the periods,and reaches the strongest between the period of 0.1-0.5s.With the decrease of the site shear wave velocity VS30,the effect of shallow-site amplification moves to the long period,showing a strong non-linear effect.