不同坡角斜坡动力响应频谱特征研究
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摘要
斜坡频谱特征曲线与斜坡形态密切相关,若入射地震波主频接近频谱曲线中极大谱比值所对应的卓越频率,就会放大斜坡动力响应,甚至造成斜坡失稳。以汶川极震区青川狮子梁斜坡为典型实例,根据青川3次余震作用下斜坡不同高程实测地震记录及地脉动测试结果,通过单点谱比法,获得斜坡地震动和地脉动的频谱特征曲线,通过对比分析曲线发现:坡顶卓越频率最小;同时,实测水平地震加速度峰值放大系数随高程增加表现出先减小后增加的"凹型"特征,坡顶的放大系数甚至达到坡底的1.25。如果从斜坡形态因素进行分析,坡顶存在低频但较高放大系数的现象可能与斜坡高差与入射波波长之比密切相关,在比值为0.2时,坡顶放大效应达到最大。为进一步确定斜坡形态对频谱特征影响,利用实测汶川地震卧龙波拟合成多频率地脉动作为动力输入,在FLAC有限差分软件中建立3种不同坡角均质斜坡模型,经动力计算后发现,3种模型坡顶、坡中及坡脚的卓越频率基本一致,该结果与输入单一频率Ricker子波时斜坡动力响应一致,说明斜坡角度改变不会影响斜坡的卓越频率。研究结论有助于增强斜坡自身频谱与动力响应之间关系的认识。
Morphological characteristics of slopes determine the predominant frequency,which may amplify the incident seismic wave of the same or similar frequency range,thereby increase the slope dynamic response,and even trigger landslides. The microtremors and seismic observations were carried out in the adits along the Shiziliang slope surface in the meizoseismal area of Wenchuan earthquake(Ms 8.0). The spectral ratios of horizontal to vertical component from the microtremor and the seismic records of 3 aftershocks were analyzed and the spectral characteristic curves were obtained. It was found that the minimum predominant frequency occurred at the slope crest. The PGA amplification decreased initially and then increased with the elevation,exhibiting a concave shape. The biggest PGA amplification reached 1.25 at the slope crest. The phenomenon of the low predominant frequency corresponding to the high PGA amplification at the top of slope may be related to the ratio of slope height to the wavelength of incident wave. The ratio was 0.2 when the PGA amplification reached themaximum value. To further investigate the morphological amplification effect on spectral characteristics,dynamic analysis was performed using the finite difference software FLAC by inputting the waves of multiple frequencies from the seismic acceleration records at Wolong station during Wenchuan earthquake(Ms8.0). 3 different angles of inclination were assumed for the slope being simulated. The calculated predominant frequencies of the slopes with different angles of inclination were found to be the same,which were verified by the results from inputting Ricker wavelets of different mono-frequencies. This indicated that the slope angle had no effect on the predominant frequency.
Morphological characteristics of slopes determine the predominant frequency,which may amplify the incident seismic wave of the same or similar frequency range,thereby increase the slope dynamic response,and even trigger landslides. The microtremors and seismic observations were carried out in the adits along the Shiziliang slope surface in the meizoseismal area of Wenchuan earthquake(Ms 8.0). The spectral ratios of horizontal to vertical component from the microtremor and the seismic records of 3 aftershocks were analyzed and the spectral characteristic curves were obtained. It was found that the minimum predominant frequency occurred at the slope crest. The PGA amplification decreased initially and then increased with the elevation,exhibiting a concave shape. The biggest PGA amplification reached 1.25 at the slope crest. The phenomenon of the low predominant frequency corresponding to the high PGA amplification at the top of slope may be related to the ratio of slope height to the wavelength of incident wave. The ratio was 0.2 when the PGA amplification reached themaximum value. To further investigate the morphological amplification effect on spectral characteristics,dynamic analysis was performed using the finite difference software FLAC by inputting the waves of multiple frequencies from the seismic acceleration records at Wolong station during Wenchuan earthquake(Ms8.0). 3 different angles of inclination were assumed for the slope being simulated. The calculated predominant frequencies of the slopes with different angles of inclination were found to be the same,which were verified by the results from inputting Ricker wavelets of different mono-frequencies. This indicated that the slope angle had no effect on the predominant frequency.
引文
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[14]ASHFORD S A,SITAR N,LYSMER J,et al.Topographic effects on the seismic response of steep slopes[J].Bulletin of the Seismological Society of America,1997,87:701–709.
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[16]YIN Y P.Catastrophic landslides associated with the Wenchuan Ms8.0 Earthquake[J].Bulletin of Engineering Geology and the Environment,2010,70(1):15–32.
[2]殷跃平,王猛,李滨,等.汶川地震大光包滑坡动力响应特征研究[J].岩石力学与工程学报,2012,31(10):1 969–1 982.(YIN Yueping,WANG Men,LI Bin,et al.Dynamic response characteristics of Daguangbao landslide triggered by Wenchuan earthquake[J].Chinese Journal of Rock Mechanics and Engineering,2012,31(10):1 969–1 982.(in Chinese))
[3]YIN Y P,WANG F W,SUN P.Landslide hazards triggered by the2008 Wenchuan Ms 8.0 Earthquake,Sichuan,China[J].Landslides,2009,6(2):139–152.
[4]HUANG R Q,PEI X J,FAN X M,et al.The characteristics and failure mechanism of the largest landslide triggered by the Wenchuan earthquake,May 12,2008,China[J].Landslides,2012,9(1):131–142.
[5]崔芳鹏,胡瑞林,殷跃平,等.纵横波时差耦合作用的斜坡崩滑效应离散元分析——以北川唐家山滑坡为例[J].岩石力学与工程学报,2010,29(2):319–327.(CUI Fangpeng,HU Ruilin,YIN Yueping,et al.Discrete element analysis of collapsing and sliding response of slope triggered by time difference coupling effects of P and S seismic waves—taking Tangjiashan landslide in Beichuan County for example[J].Chinese Journal of Rock Mechanics and Engineering,2010,29(2):319–327.(in Chinese))
[6]SUN P,YIN Y P,WU S R,et al.Does vertical seismic force play an important role for the failure mechanism of rock avalanches—a case study of rock avalanches triggered by the Wenchuan Ms 8.0Earthquake of May 12,2008,Sichuan,China[J].Environmental Earth Sciences,2012,66(5):1 285–1 293.
[7]COLLIER C J,ELNASHAI A S.A procedure for combining vertical and horizontal seismic action effects[J].Journal of Earthquake Engineering,2001,5(4):521–539.
[8]ATHANASOPOULOS G A,PELEKIS P C,LEONIDOU E A.Effects of surface topography on seismic ground response in the Egion(Greece)15 June 1995 earthquake[J].Soil Dynamics and Earthquake Engineering,1999,18:135–149.
[9]王海云,谢礼立.自贡市西山公园地形对地震动的影响[J].地球物理学报,2010,53(7):1 631–1 638.(WANG Haiyun,XIE Lili.Effrcts of topography on ground motion in the Xishan park,Zigong city[J].Chinese Journal of Geopgys,2010,53(7):1 631–1 638.(in Chinese))
[10]CELEBI M.Topographical and geological amplifications determined from strong-motion and aftershock records of the 3March 1985 Chile earthquake[J].Bulletin of the Seismological Society of America,1987,77(4):1 147–1 167.
[11]LENTI L,MARTINO S.New procedure for deriving multifrequential dynamic equivalent signals(LEMA_DES):a test-study based on Italian accelerometric records[J].Bulletin of Earthquake Engineering,2012,8:813–846.
[12]罗永红.地震作用下复杂斜坡响应规律演技[博士学位论文][D].成都:成都理工大学,2011.(LUO Yonghong.Study on complex slopes response law under earthquake action[Ph.D.Thesis][D].Chengdu:Chengdu University of Tecnology,2011.(in Chinese))
[13]NAKAMURA Y.A method for dynamic characters estimation of subsurface using microtremors on the ground surface[J].QR of RTRI,1989,30(1):25–33.
[14]ASHFORD S A,SITAR N,LYSMER J,et al.Topographic effects on the seismic response of steep slopes[J].Bulletin of the Seismological Society of America,1997,87:701–709.
[15]LYSMER J,KUHLEMEYER R L.Finite dynamic model for infinite media[J].Journal of Engineering Mechanics,1969,95(EM4):859–877.
[16]YIN Y P.Catastrophic landslides associated with the Wenchuan Ms8.0 Earthquake[J].Bulletin of Engineering Geology and the Environment,2010,70(1):15–32.
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