用Nakamura技术评估天津地区场地效应
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摘要
用传统的经验转换函数方法评估场地效应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。这种方法在天津市这样的大面积平原多震地区无法实现。天津市辖区面积11000km2,只有北部的蓟县是山区,市区是平原,南部的塘沽区、汉沽区、大港区是滨海第四纪巨厚沉积区。天津南部滨海区在1976年唐山MS7·8地震时受灾最重,死亡人员超过20000。天津是1949年以来我国唯一遭遇麦卡利地震烈度达到Ⅷ震灾的直辖市,也是此间我国遭遇地震最大(1976年宁河MS6·9)的城市。十五期间投巨资建立了数字地震观测台网,位于不同场地上的地震观测点清晰、完整地记录了若干有感地震。本文利用近场地面自由场地震三分向记录和Nakamura技术研究了天津地区的场地效应。并与唐山地震时天津地区实际遭受的宏观地震烈度相对比,结合天津地区的地下结构进行了分析。结果表明,Nakamura技术所获得的反映震害的指标(Nakamura技术的放大倍数)与实际震害基本一致。说明Nakamura技术在与天津地区地质构造类似的城市(平原、第四纪沉积层巨厚)如北京、上海、成都具有推广应用价值。我国正在建设大量自由场、数字化、宽频带、大动态地震观测点,该方法具有一定的应用前景。
Site response is examined with empirical transfer function, which demands an array with small calibration in history. Among all the seismographs deployed, one seismograph should be located on rock to serve as reference site, and that is not far away from the others. Empirical transfer function method is not applicable to Tianjin, an extensive plain area where no rock is exposed at all. Tianjin boasts an area of 11000km2,rock sites are located in Jixian, northern part of Tianjin only. The whole urban area is plain, the southern districts of Tanggu,Hangu,and Dagang are coast zones which are adjacent to the Bohai Sea, where thick Quartery sediment is deposited. When the Tangshan M_S7.8 earthquake hit in 1976, over 20000 residents were killed. Tianjin is the sole mega city which suffered an seismic intensity of Ⅷ(Mercalli scale) in China after 1949, it is the city where the maximum earthquake burst (Ninghe M_S6.9,1976) as well. During the 10 th national five-year plan, permanent free surface digital seismic network was deployed in Tianjin, fine accelerograms from different observatories were obtained for a couple of felt earthquakes. Site response in the whole Tianjin area is studied with near field, free surface, 3 component data through Nakamura technique. Comparisons are made with the actual seismic intensity during the Tangshan M_S7.8 earthquake and the genuine tectonic underlying Tianjin. The results demonstrate that the amplitude of Nakamura technique is basically in agreement with the real seismic hazard and underground tectonic. A conclusion can be approached that Nakamura technique is of great value to the cities which are similar in geological context with Tianjin (plain, covered by thick Quartery sediment) such as Beijing, Shanghai, and Chengdu. A great deal of free field, high dynamic, digital observatories are being deployed in China,Nakamura technique is promising.
Site response is examined with empirical transfer function, which demands an array with small calibration in history. Among all the seismographs deployed, one seismograph should be located on rock to serve as reference site, and that is not far away from the others. Empirical transfer function method is not applicable to Tianjin, an extensive plain area where no rock is exposed at all. Tianjin boasts an area of 11000km2,rock sites are located in Jixian, northern part of Tianjin only. The whole urban area is plain, the southern districts of Tanggu,Hangu,and Dagang are coast zones which are adjacent to the Bohai Sea, where thick Quartery sediment is deposited. When the Tangshan M_S7.8 earthquake hit in 1976, over 20000 residents were killed. Tianjin is the sole mega city which suffered an seismic intensity of Ⅷ(Mercalli scale) in China after 1949, it is the city where the maximum earthquake burst (Ninghe M_S6.9,1976) as well. During the 10 th national five-year plan, permanent free surface digital seismic network was deployed in Tianjin, fine accelerograms from different observatories were obtained for a couple of felt earthquakes. Site response in the whole Tianjin area is studied with near field, free surface, 3 component data through Nakamura technique. Comparisons are made with the actual seismic intensity during the Tangshan M_S7.8 earthquake and the genuine tectonic underlying Tianjin. The results demonstrate that the amplitude of Nakamura technique is basically in agreement with the real seismic hazard and underground tectonic. A conclusion can be approached that Nakamura technique is of great value to the cities which are similar in geological context with Tianjin (plain, covered by thick Quartery sediment) such as Beijing, Shanghai, and Chengdu. A great deal of free field, high dynamic, digital observatories are being deployed in China,Nakamura technique is promising.
引文
马淑芹、龚绍京、栗连弟、戚成柱,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。
谢礼立,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。
Borcherdt R D.1970.Effects of local geology on ground motion near San Francisco Bay.Bull SeismSoc Amer,60,29~61.
Lermo J.1993.Site Effect Evaluation Using Spectral Ratios with only One Station.BSSA,83(5):1574~1594.
Ma S.2004.Calibration of Wellington3D Shaking Hazard Model.MSC thesis,School of Earth Sciences,Victoria University ofWellington,NewZealand,163.
Ma S,Savage M,Benites R,YuJ.2006.Development of the hybrid1D+3Dseismic shaking hazard modellingtechnique.GeophysicalJournal International,Great Britain,accepted.
Nakamura Y.1989.A methodfor dynamic characteristics estimation of subsurface using microtremor on the ground surface,QR RTR1,30(1).
Taber J.1992.Frequency dependent amplification of weak ground motions in Porirua and Lower Hutt,NewZealand.Bull.NZ Natl.Soc.Earthquake Engineering,25,303~331.
谢礼立,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。
Borcherdt R D.1970.Effects of local geology on ground motion near San Francisco Bay.Bull SeismSoc Amer,60,29~61.
Lermo J.1993.Site Effect Evaluation Using Spectral Ratios with only One Station.BSSA,83(5):1574~1594.
Ma S.2004.Calibration of Wellington3D Shaking Hazard Model.MSC thesis,School of Earth Sciences,Victoria University ofWellington,NewZealand,163.
Ma S,Savage M,Benites R,YuJ.2006.Development of the hybrid1D+3Dseismic shaking hazard modellingtechnique.GeophysicalJournal International,Great Britain,accepted.
Nakamura Y.1989.A methodfor dynamic characteristics estimation of subsurface using microtremor on the ground surface,QR RTR1,30(1).
Taber J.1992.Frequency dependent amplification of weak ground motions in Porirua and Lower Hutt,NewZealand.Bull.NZ Natl.Soc.Earthquake Engineering,25,303~331.
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