2018年2月6日花莲M_W6.4地震近场地震动方向性效应

Directivity effects on strong ground motion from the February 6,2018 M_W6.4 Hualien earthquake

利用中国台湾省内222个强震动台站以及Palert地震预警系统520个台站所观测的三分量加速度记录,研究此次花莲M W6.4地震近场强地震动空间分布和衰减特征,将观测结果与美国NGA-West2地震动经验预测模型进行对比,揭示此次台湾花莲地震近场地震动的长周期特点,基于回归残差分析研究地震动峰值加速度(PGA)、峰值速度(PGV)和不同周期地震动的空间分布差异,定量考察近场地震动的方向性效应.研究结果表明:(1)整体上此次地震的近场PGV观测值和周期1.0 s以上的长周期加速度谱值与美国NGA-West2地震动预测模型结果接近,PGA观测值和周期小于1.0 s的加速度反应谱略低于预测模型结果.从空间分布来看,周期1.0 s以上的长周期地震动在断层的不同方位有系统性差异,在破裂传播前方(震中西南方位),周期大于1.0 s时的反应谱明显高于美国NGA-West2地震动经验预测模型,在破裂传播后方(震中东北方位),周期大于1.0 s时的反应谱低于经验预测模型,表明此次地震近场地震动具有显著的方向性效应.(2)破裂传播的方向性效应主要影响周期超过1.0 s的长周期,而对PGA以及周期小于1.0 s的短周期地震动影响较弱.在破裂传播前方,周期1.0~10.0 s的加速度反应谱值被增强到整体观测平均水平的1.16~1.52倍;在破裂传播后方,周期1.0~10.0 s的加速度反应谱值被减弱到整体观测平均水平的0.36~0.70倍.(3)此次地震破裂方向性效应的影响表现出明显的窄带效应,破裂方向性的影响(包括破裂传播前方的增强作用和破裂传播后方的减弱作用)在周期T=3.0 s时达到最大,在该周期破裂传播前方的增强系数为1.52,破裂传播后方的减弱系数为0.36.从周期T=3.0 s到10.0 s,破裂方向性效应的影响随周期增大总体上呈减弱趋势,这与2016年日本熊本M W7.0地震破裂方向性效应的影响特点显著不同.

Using three-component acceleration records from 222 strong motion stations of Taiwan province,China and 520 stations of Palert Earthquake Early Warning System,we investigate spatial variability and attenuation characteristics of observed near-field strong ground motion during the Hualien M W6.4 earthquake,and compare the observation with NGA-West2 prediction models.We reveal the characteristics of long-period motions during this event and quantitatively analyze the directional effects on strong motion based on residual analysis.Following conclusion can be made based on our study:(1)overall observations at PGV and long-period spectral accelerations above period 1.0 s show good coincidence with the NGA-West2 prediction models,while the PGA observations and acceleration response spectra below period 1.0 s are slightly lower than those of the prediction models.Long-period motions above period 1.0 s are systematically biased by the azimuth in terms of spatial distribution.In the rupture forward(South-west of epicenter),observed spectral accelerations above period 1.0 s are much higher than the NGA-West2 prediction models,while lower than the NGA-West2 prediction models in the backward(North-east of epicenter).(2)The rupture directivity mainly affects the long-period motions above period 1.0 s and the effects on PGA and short-period motions are relatively weak.The ground motions right ahead of the rupture forward at period 1.0~10.0 s are strongly amplified and are 1.16~1.52 times of the average level.Observed ground motions in the backward at period 1.0~10.0 s are reduced to 0.36~0.70 times of the average level.(3)The rupture directivity during the Hualien earthquake shows clear narrow-band effect.The influence of the rupture directivity including the amplification effects in the forward and de-amplification effects in the backward reaches its maximum values at the period T=3.0 s.The maximum amplification and de-amplification coefficients are 1.52 and 0.36 respectively.From the period 3.0 s to 10.0 s,the influence of rupture directivity gradually weakens with the increase of period on the whole,which is significantly different from the characteristics of the rupture directivity effects during the 2016 Kumamoto M W7.0 earthquake in Japan.