利用日本K-NET和KiK-net强震动台网获取的距离发震断层100 km以内136个强震动台站的三分量加速度记录,研究熊本Mw7.0地震地震动的长周期特性.基于残差分析研究不同周期地震动的空间分布差异,将观测分析结果与美国NGA经验模型、汶川和芦山地震观测结果进行对比,揭示此次熊本地震近场强震动的长周期特点及其形成机理.研究结果表明:(1)虽然总体上此次地震的近场地震动水平与美国NGA-West2经验模型的预测结果接近,但周期2 s以上地震动的分布在断层不同方位有系统性差异,在断层的北东方位,周期2.0～10.0 s的反应谱高于NGA-West2经验模型的预测结果,在西南方位,谱值低于经验预测模型.(2)我们认为此次地震2.0～10.0 s的长周期地震动的空间分布差异主要受破裂方向性的影响,在破裂传播的正前方,周期T=2.0 s,3.0 s,5.0s,7.5s和10.0s的加速度谱被放大到整体观测平均水平的1.4～2.0倍.从周期T=2.0 s到10.0 s,破裂向前方向的放大作用和破裂反方向的减弱作用均有所增强.此次地震观测到的速度大脉冲记录均位于断层的东北方位,这与方向性脉冲的产生机理相吻合,速度大脉冲对加速度反应谱有显著的长周期放大作用,放大倍数值可以超过4.0,放大作用的影响主要位于脉冲的特征周期Tp附近.(3)近断层记录在建筑结构敏感的周期(0.5～2.0 s)的反应谱达到芦山地震的3～6倍,虽然与芦山地震震级接近,此次地震近断层地震动破坏力大大超过了芦山Mw6.8地震,甚至超过了汶川Mw7.9地震,这种长周期特点应该引起工程抗震设计和相关研究人员的重视.%Using ground acceleration time series recorded by 136 near-field strong motion stations of K-NET and KiK-net within 100 km from ruptured fault,we investigate long period strong motion during the Mw7.0 Kumamoto earthquake.In order to reveal the characteristics and mechanism of long-period motion during this event,we investigate spatial variability of observed ground motions based on residual analysis,and compare the observation with NGA-West2 prediction models and with Wenchuan and Lushan earthquake in China.Following conclusion can be made based on our study:(1) although overall observation shows good coincidence with the prediction from the NGA-West2 models,long-period motion over 2.0 s are systematically biased by the azimuth.Ground motions on the northeast side of the fault are much higher than the prediction from NGA-West2 models,while lower than the prediction on the southwest side of the fault.(2) The spatial distribution of long-period motion of 2.0 s～10 s are dependent on the rupture directivity.Ground motions on the northeast side of the fault in the forward direction are much higher than average,right ahead of the rupture forward,spectral accelerations (SAs) at 2.0,3.0,5.0,7.5 and 10.0 s are strongly amplified and are about 1.4～2.0 times of the average level.Both the amplification effects in the forward and de-amplification effects in the backward increase with period from 2.0 to 10.0 s.All four ground velocity pulses are observed in the northeast side of the fault,which coincide well with the effects of rupture directivity.Acceleration spectra at long period are highly amplified due to the present of large velocity pulses.Amplification effects are significant near pulse period Tp,with largest amplification factor exceeding 4.0.(3) Spectra of near-fault strong motion at period 0.5～2.0 s reach 3～6 times of the observation during Lushan earthquake.With a similar magnitude as Lushan earthquake,the near-fault motions during this event are much more destructive,especially for building structures which are vulnerable to long-period motion.Such long-period motion should be well understood by researchers and seismic resistant designers.
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