用Nakamura方法评估北京地区场地效应
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摘要
传统的评估场地效应的经验转换函数方法需要有一个小孔径台阵,布设若干地震计,其中须有离软土台不远的基岩台,但因北京市位于很大部分无基岩出露的大面积平原地区,所以无法实现。我们用Nakamura方法对北京地区进行场地效应评估。北京市"十五"期间建立了数字地震观测台网,位于不同场地上的强震观测点清晰完整地记录了若干有感地震。本文利用近场地面自由场强震三分向地震记录,通过Nakamura方法研究了北京地区的场地效应。并与1679年三河-平谷MS8.1地震时北京地区实际遭受的宏观地震烈度相对比,结合北京地区的地下结构进行了分析。结果表明,用Nakamura方法所获得的反映震害的指标(Nakamura谱比率)有3个高值异常区,分别在北京东北部的怀柔县(值11.1)、北京昌平区西部(值10.1)和市中心区(值8.1),这与三河-平谷MS8.1地震时实际震害和观测点的地下局部构造基本一致。说明Nakamura方法在与北京地区地质构造类似的城市(平原、第四纪沉积层覆盖)如上海、成都、天津具有推广应用价值。我国正在建设大量自由场、数字化、宽频带、大动态强震观测点,该方法对于工程安全性评价具有一定的应用前景。
Site response is examined with empirical transfer function in history, which demands an array with small calibration. Among all the seismographs deployed, one seismograph must be located on rock to serve as reference site, and is not far away from the others. Empirical transfer function method is not applicable to Beijing, where no rock is exposed in the urban area. Beijing boasts an area of 16000 km2, its southeastern part is plain. The Nakamura technique is adopted in the Beijing area for site response. During the 10th "Five-year Plan", permanent digital seismic strong motion network was deployed in Beijing, fine accelerograms from many observatories were obtained for a couple of felt earthquakes. Site response in the Beijing area is studied with near field, free surface, strong motion, 3 component data through the Nakamura technique. Comparisons are made with the actual seismic intensity during the Pinggu M_S8.1 earthquake, 1679 and the genuine tectonic underlying Beijing. It shows that peak Nakamura spectral ratio excels in 3 areas: Huairou county in northeast Beijing (11.1), western part in Changping county in Beijing (10.1) and central part in the urban area. The results demonstrate that the amplitude of Nakamura technique is basically in agreement with the real seismic hazard and underground tectonics. A conclusion can be approached that the Nakamura technique is of great value to the cities which are similar in geological context with Beijing (plain, covered by thick Quartery sediment) such as Tianjin, Shanghai and Chengdu. A great deal of free field, high dynamic, digital strong motion observatories are being deployed in China, the Nakamura technique is promising and useful.
Site response is examined with empirical transfer function in history, which demands an array with small calibration. Among all the seismographs deployed, one seismograph must be located on rock to serve as reference site, and is not far away from the others. Empirical transfer function method is not applicable to Beijing, where no rock is exposed in the urban area. Beijing boasts an area of 16000 km2, its southeastern part is plain. The Nakamura technique is adopted in the Beijing area for site response. During the 10th "Five-year Plan", permanent digital seismic strong motion network was deployed in Beijing, fine accelerograms from many observatories were obtained for a couple of felt earthquakes. Site response in the Beijing area is studied with near field, free surface, strong motion, 3 component data through the Nakamura technique. Comparisons are made with the actual seismic intensity during the Pinggu M_S8.1 earthquake, 1679 and the genuine tectonic underlying Beijing. It shows that peak Nakamura spectral ratio excels in 3 areas: Huairou county in northeast Beijing (11.1), western part in Changping county in Beijing (10.1) and central part in the urban area. The results demonstrate that the amplitude of Nakamura technique is basically in agreement with the real seismic hazard and underground tectonics. A conclusion can be approached that the Nakamura technique is of great value to the cities which are similar in geological context with Beijing (plain, covered by thick Quartery sediment) such as Tianjin, Shanghai and Chengdu. A great deal of free field, high dynamic, digital strong motion observatories are being deployed in China, the Nakamura technique is promising and useful.
引文
胡平,2003,强震台网建设,筑减灾基业,146～151,北京:地震出版社。
马淑芹、龚绍京、栗连弟等,2001,使用地面单台三分向数字地震资料评估场地效应方法的初步研究,西北地震学报,23(3),269～273。
刘恢先,1986,唐山大地震震害(四),1～702,北京:地震出版社。
郭增建、陈鑫连,1991,城市地震对策,1～502,北京:地震出版社。
谢礼立、李沙白、章文波,1999,唐山响堂三维场地影响观测台阵,地震工程与工程振动,19(2):1～8。
谢毓寿、蔡美彪,1987,中国地震历史资料汇编(第三卷(上),1～540,北京:科学出版社。
修济刚、高旭、郭大庆、宋永增、罗咏生、刘松青,1999,北京市防震减灾工作的进展,国际地震动态,3。
Borcherdt R D,1970,Effects of local geology on ground motion near San Francisco Bay,Bull SeismSoc Amer,60,29～61.
Nakamura,Y.,1989,A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface,QRRTR1,30(1).
Lermo,J.,1993,Site effect evaluation using spectral ratios with only one station,BSSA,83(5):1574～1594.
Ma,S.,2004,Calibration of Wellington3Dshaking hazard model,MSc thesis,1～163,School of Earth Sciences,Victoria Universityof Wellington,,Wellington,NewZealand.
Ma,S.,Savage,M.,and Yu,J.,2007,Modelling seismic ground motion in hutt valley,New Zealand by discrete wavenumbermethod,Bulletin of the NewZealand Societyfor Earthquake Engineering,NewZealand,accepted.
马淑芹、龚绍京、栗连弟等,2001,使用地面单台三分向数字地震资料评估场地效应方法的初步研究,西北地震学报,23(3),269～273。
刘恢先,1986,唐山大地震震害(四),1～702,北京:地震出版社。
郭增建、陈鑫连,1991,城市地震对策,1～502,北京:地震出版社。
谢礼立、李沙白、章文波,1999,唐山响堂三维场地影响观测台阵,地震工程与工程振动,19(2):1～8。
谢毓寿、蔡美彪,1987,中国地震历史资料汇编(第三卷(上),1～540,北京:科学出版社。
修济刚、高旭、郭大庆、宋永增、罗咏生、刘松青,1999,北京市防震减灾工作的进展,国际地震动态,3。
Borcherdt R D,1970,Effects of local geology on ground motion near San Francisco Bay,Bull SeismSoc Amer,60,29～61.
Nakamura,Y.,1989,A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface,QRRTR1,30(1).
Lermo,J.,1993,Site effect evaluation using spectral ratios with only one station,BSSA,83(5):1574～1594.
Ma,S.,2004,Calibration of Wellington3Dshaking hazard model,MSc thesis,1～163,School of Earth Sciences,Victoria Universityof Wellington,,Wellington,NewZealand.
Ma,S.,Savage,M.,and Yu,J.,2007,Modelling seismic ground motion in hutt valley,New Zealand by discrete wavenumbermethod,Bulletin of the NewZealand Societyfor Earthquake Engineering,NewZealand,accepted.
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