強震観測記録に基づく相模トラフ沿いの地震の長周期・長時間地震動の予測式

抄録

 We develop long-period and long-duration ground motion prediction equations for earthquakes along the Sagami Trough based on strong motion records to utilize the equations to design input motions for high-rise buildings. Equations of 5%-damped acceleration response spectra SA and the average and variance of group delay time in the period range of 0.1 to 10 s are developed. Mw range of dataset is 3.7 to 8.2. The fault distance range is 1.2 to 250 km. The main features of the equations are as follows.

 1) Since the strong motion records of big Sagami Trough earthquakes are rare, many records of earthquakes with M_J≧ 4.0 along the Sagami Trough are added to records of big subduction-zone earthquakes (M_J≧6.5) in whole Japan. The amplification factors for SA tend to be smaller than those for earthquakes along the Nankai Trough obtained by our previous paper. This result is the same result showed by 3-D FDM by Fujiwara et al..

 2) Records of crustal earthquakes are used only to estimate a coefficient which represents the saturation effect to S_A near fault regions because the upper depth of the Philippine Sea plate in south western part of Kanagawa prefecture is less than 10 km. The saturation effect is stronger than that for earthquakes along the Nankai Trough obtained by our previous paper.

 3) Magnitude coefficients for wide Mw range data are modeling well by introducing hinge magnitudes used in S_A equations of NGA-WEST2.

 4) Differences between interplate and intraplate earthquakes are modeling by using dummy valuables as fault depth dependency coefficients for S_A. The S_A is larger as the depth is shallower in the long period range. This tendency reflects feature of surface waves.

 5) Site coefficients of S_A and group delay time obtained at the surface are converted to site coefficients at the engineering bedrock with an S-wave velocity of 400 m/s based on 1-D wave propagation theory using shallow structures modeled by National Research Institute for Earth Science and Disaster Resilience (NIED). Even at a period of 1 s amplification factors for S_A at the surface were affected by shallow structures in the wide region around the center of the Kanto plane but the effects are removed in amplification factors at the engineering bedrock.

 6) Site coefficients of group delay time at the engineering bedrock become longer by shallow structures even in the periods longer than the predominant periods of shallow structures. This result means that 3-D FDM using the subsurface structure under the engineering bedrock cannot simulate these effects on duration by shallow structures.

 The developed equations are applied to the 1923 Taisho Kanto (Mw7.9) type earthquake using fault models by the Headquarters for Earthquake Research Promotion (HERP). The long-period ground motion predicted near Hongo is consistent to the observed and restored record there. Predicted S_A are comparable to those predicted by 3-D FDM by HERP. This result is reasonable because the scaling relation of spectral level of the 1923 Taisho Kanto earthquake and the source models by HERP almost agrees with the previous empirical relation for earthquakes along the Pacific plate which are major dataset of our equations. We also applied the developed equations to the hypothetical south Tokyo metropolitan earthquake (Mw7.3) using fault models by Cabinet Office. The predicted S_A is a little larger and duration is a little longer than those predicted by stochastic Green’s function method by Cabinet Office. This result is reasonable because surface waves are not considered in the stochastic Green’s function method.