2013年4月20日芦山地震最大加速度记录分析

2013年4月20日芦山地震是汶川地震后龙门山断裂带发生的又一次破坏性地震,中国数字强震动观测网络有100余台仪器触发并获得加速度记录.设置在四川省宝兴县防震减灾局的强震台(编号51BXD)在东西向获得了1.026g(-1 005.3gal)的峰值加速度记录,这是我国首次在自由地表获得峰值超过1g的记录,是我国强震动观测历史上的重要事件.经震后灾害调查,发现观测台站附近建筑物破坏并不严重,确定地震宏观烈度为Ⅶ度,这引起了社会的广泛关注与争议.通过现场考察,初步认定台站的特殊选址带来了局部地形影响,陡峭山坡放大了高频段地震动.还分析了宝兴县另外3个强震台站的观测环境,并采用全球新一代地震动预测方程进行对比分析,结果显示,该台在小于0.2 s范围内的反应谱放大明显.为了进一步说明台址受场地地形影响,对台站附近进行了背景噪声测试,采用水平/垂直谱比法计算了场地背景噪声的卓越周期、水平向放大系数,得出了随着地势由山脚向山坡上增高,场地卓越频率逐渐增大,且放大系数也随之增大的结论,这与51BXD强震动记录反应谱特征一致.最后,与汶川地震典型记录反应谱、抗震规范谱等进行对比,分析了51BXD强震动记录不同频段的特点;结合附近建筑物现场震后调查结果,初步解释了造成建筑物破坏较轻的原因,验证了局部地形影响是造成峰值加速度记录较大的主要原因. 

Maximum Acceleration Recording from Lushan Earthquake on April 20, 2013

The Lushan earthquake on April 20, 2013 is another destructive event after the Wenchuan earthquake in 2008. More than 100 strong motion stations were triggered in the main shock. The 51BXD station located in Baoxing Country recorded a peak ground acceleration (PGA) of more than 1g, that is 1.026g (-1 005.3 gal) in the EW direction. It has been the first time that more than 1g is recorded at free field site in China and it should be a milestone event in Chinese strong motion observation history. However, in the released official Lushan intensity map, the around area was classified to intensity Ⅶ, which raises an interesting question why such a great PGA leads to a low intensity. During the post-earthquake field survey, it was found that this station was located on a steep slope, not a real "free-field" station, so the recording could be affected by the topography. By comparing with 3 other nearby stations with the next generation attenuation models, it is shown that the earthquake response spectrum less than 0.2 s was amplified significantly. To support this evidence, a background noise test was performed around this area, and predominant period and horizontal acceleration amplification factor were calculated with the traditional spectral ratio method. The calculation results clearly show that from the bottom of the valley to the top, the predominant frequency increases and the amplification factors also raise with the predominant frequency increasing. In addition, the response spectrum of 51BXD recording was calculated and compared with some others from the Wenchuan earthquake to interpret why there were so slight damages to its around buildings. The preliminary conclusion is that the topography is the root cause for 1g peak ground acceleration recording in the Lushan earthquake.