Simulation of Long-Period Ground Motions for the 1923 Kanto Earthquake

1923年关东大地震是在东京用尤因地震仪和今村地震仪记录的。早期的研究者从这两个地震图中估计的地面运动有很大的不同。如果我们假设尤因地震仪在震动时的固体摩擦力很大，这个矛盾就可以得到调和。固体摩擦可以降低仪器的共振，并且可以正确地记录具有大振幅的长周期地面运动。结果表明，关东地震的地震动具有较大的长周期分量，其速度反应谱为120 cm/s，周期为13 s。这种谱峰位置比通常在东京观测到的微震动的谱峰位置长得多。本文以1923年日本关东大地震和洛杉矶盆地下的一个假定地震为例，通过数值模拟，得到了比东京和洛杉矶的微震周期长得多的长周期地震动。然而，在长周期范围内，即使子断层的特征破裂持续时间长于反应谱的峰值周期，断层上的子断层的大小也不影响反应谱的峰值位置。结果表明，小震资料与大震模型谱峰位置的差异是由几公里至十公里深震源激发的长周期面波和破裂传播的方向性引起的。

The 1923 Kanto earthquake was recorded in Tokyo with a Ewing seismograph and an Imamura seismograph. The ground motions estimated by earlier investigators from these two seismograms differed significantly. This conflict can be reconciled if we assume that the solid friction of the Ewing seismograph was very high during shaking. Solid friction can reduce the resonance of the instrument, and long-period ground motions with large amplitudes can be recorded correctly. We conclude that the ground motion of the Kanto earthquake had a vary large long-period component with a velocity response spectrum of 120 cm/sec at a period of 13 sec. This spectral peak position are considerably longer than that for micro-tremors usually observed in Tokyo. By way of examples to investigate the difference in the spectral peak positions inferred from small earthquake data and from large earthquake model, we take the 1923 Kanto earthquake and a hypothetical earthquake beneath the Los Angeles basin.Several numerical simulations produce large long-period ground-motions whose periods are considerably longer than that for micro-tremors usually observed in Tokyo and Los Angeles. Size of subfault on the fault, however, does not affect the peak position of response spectra in long-period range, even if the characteristic rupture duration of the subfault is longer than the peak period of response spectra. Our result shows that the difference in the spectral peak positions between small earthquake data and a large earthquake model is caused by the excitation of long-period surface waves due to a source of several to ten kilometers deep and by the directivity of rupture propagation.