Pdif波在中国CDSN台站的运动学特征
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摘要
利用中国数字地震台网(简称CDSN,下同)11个台的宽频带长周期(VLP)数字记录对1990-2012年间的124个极远震记录进行了分析处理,在震中距100.0°～179.0°之间,获取了CDSN11个台站的Pdif波的震相数据,得到了Pdif波在地球核幔界面上衍射的运动学特征和《Pdif波走时表》。所得结果大大拓展了Pdif的记录范围,在144.0°～179.0°之间弥补了《IASPEI1991地震波走时表》的缺失,为更大范围内的Pdif震相分析提供了参考标准,有助于提高中国地震台网的地震分析水平和积累震相分析经验,以及对地球核幔边界物理性质的认识。
The traveling-time curve of the P-wave ends at an epicentral distance(Δ)of about 100°. At epicentral distances larger than 100°(Δ>100°),P-waves pass through the core-mantle boundary(CMB)and start to be diffracted around the core with speeds of～13.7km/s.The diffracted wave is termed Pdif. Due to the sharp decrease of the P-wave velocity at the core-mantle boundary from 13.7km/ s to～8.0km/s,the short period P-wave is strongly refracted into the core with speeds of～8.0 to 11.2km/s.This refracted wave is termed PKP(PKPab,PKPbc and PKPdf). The Pdif wave was first recognized on seismogram by Denmark seismologist Lehmann in 1936.The traveling-time curve of the Pdif wave was well defined at distances of 105°to 130°by seismologists Jeffres and Bullen in 1940,using the seismic data from 1930to 1940,for earthquakes with depths from 33to 794km.In 1991,the traveling-time curve of the Pdif wave was updated by Kennett[6]for distances of 100°to 144°and depths of 0,100,300,and 600km using seismic data from 1964 to 1987. In the China Digital Seismograph Network(CDSN),the Pdif wave of strong earthquakes, whose distances range from about 100.2°to 179.0°,may be observed so that the Pdif phase can be analyzed perfectly.We collected and analyzed 124broad-band seismic recordings at the 11CDSN stations during aperiod of 1990to 2012,from very-far-field events with magnitudes of～6.0 to 8.8and depths～4to 631.3km.The kinematic characteristics of the Pdif phase are summarized as the following:(1)There is a good linear relationship between traveling-times and distances,and a systematic bias exists between our results and those of the IASPEI 1991table;for example,the difference reaches 11.2satΔ=100°,and 17.0satΔ=144°.(2)ForΔ>100°,the Pdif arrival time is about 4min and 10.7sto 29.8searlier than the PKP wave,and the time difference between the Pdif and PKP waves with distance could be formulated as: TPKP-Pdif =-2.835Δ+550.7(h ≤100km)(100.0°to 179.0°).(3)The Pdif wave is about 4min and 5.4sto 6min and 42.0searlier than the PP wave, and the time difference between the Pdif and PP waves with distance could be formulated as: TPP-Pdif =0.014Δ2+5.562Δ-163.7(h ≤100km)(100.0°to 179.0°).(4)For deep-focus earthquakes,the diffracted depth phases,pPdif and sPdif,may be observed.The time difference between pPdif-Pdif and sPdif-Pdif is similar to pP-P and sP-P. A time table,called the Traveling-time Table of the Pdif Wave at CDSN Stations,is proposed.The relationship between traveling-time and epicenter distance for Pdif is suggested to be: TPdif =4.573Δ+367.4(h ≤100km)(100.0°to 179.0°). The time table proposed in this study is a useful complement to the IASPEI1991table,and it could be used as a standard of Pdif wave analysis.It will be very useful for enhancing the earthquake analysis ability and accumulating analysis experience and for understanding physical characteristics of the mantle-core boundary.
The traveling-time curve of the P-wave ends at an epicentral distance(Δ)of about 100°. At epicentral distances larger than 100°(Δ>100°),P-waves pass through the core-mantle boundary(CMB)and start to be diffracted around the core with speeds of～13.7km/s.The diffracted wave is termed Pdif. Due to the sharp decrease of the P-wave velocity at the core-mantle boundary from 13.7km/ s to～8.0km/s,the short period P-wave is strongly refracted into the core with speeds of～8.0 to 11.2km/s.This refracted wave is termed PKP(PKPab,PKPbc and PKPdf). The Pdif wave was first recognized on seismogram by Denmark seismologist Lehmann in 1936.The traveling-time curve of the Pdif wave was well defined at distances of 105°to 130°by seismologists Jeffres and Bullen in 1940,using the seismic data from 1930to 1940,for earthquakes with depths from 33to 794km.In 1991,the traveling-time curve of the Pdif wave was updated by Kennett[6]for distances of 100°to 144°and depths of 0,100,300,and 600km using seismic data from 1964 to 1987. In the China Digital Seismograph Network(CDSN),the Pdif wave of strong earthquakes, whose distances range from about 100.2°to 179.0°,may be observed so that the Pdif phase can be analyzed perfectly.We collected and analyzed 124broad-band seismic recordings at the 11CDSN stations during aperiod of 1990to 2012,from very-far-field events with magnitudes of～6.0 to 8.8and depths～4to 631.3km.The kinematic characteristics of the Pdif phase are summarized as the following:(1)There is a good linear relationship between traveling-times and distances,and a systematic bias exists between our results and those of the IASPEI 1991table;for example,the difference reaches 11.2satΔ=100°,and 17.0satΔ=144°.(2)ForΔ>100°,the Pdif arrival time is about 4min and 10.7sto 29.8searlier than the PKP wave,and the time difference between the Pdif and PKP waves with distance could be formulated as: TPKP-Pdif =-2.835Δ+550.7(h ≤100km)(100.0°to 179.0°).(3)The Pdif wave is about 4min and 5.4sto 6min and 42.0searlier than the PP wave, and the time difference between the Pdif and PP waves with distance could be formulated as: TPP-Pdif =0.014Δ2+5.562Δ-163.7(h ≤100km)(100.0°to 179.0°).(4)For deep-focus earthquakes,the diffracted depth phases,pPdif and sPdif,may be observed.The time difference between pPdif-Pdif and sPdif-Pdif is similar to pP-P and sP-P. A time table,called the Traveling-time Table of the Pdif Wave at CDSN Stations,is proposed.The relationship between traveling-time and epicenter distance for Pdif is suggested to be: TPdif =4.573Δ+367.4(h ≤100km)(100.0°to 179.0°). The time table proposed in this study is a useful complement to the IASPEI1991table,and it could be used as a standard of Pdif wave analysis.It will be very useful for enhancing the earthquake analysis ability and accumulating analysis experience and for understanding physical characteristics of the mantle-core boundary.
引文
[1]赵荣国.中国763长周期台网观测Δ=175.6°的幔-核界面衍射波[J].国际地震动态,1987,(3):14-16.ZHAO Rong-guo.Waves Diffracted at the Mantle-core Boundary Were Observed by Type-763 Long-period Seismograph Network of China[J].Recent Developments in World Seismology,1987,(3):14-16.(in Chinese)
[2]郭瑛.关于Δ≥140°的地震衍射波Pdif[J].地震地磁观测与研究,1989.10(4):27-30.GUO Ying.On Diffracted Wave Pdif forΔ≥140°[J].Seismological and Geomagnetic Observation and Research,1989.10(4):27-30.(in Chinese)
[3]唐燕娟.对P波衍射波Pdif震相记录范围的讨论[J].地震地磁观测与研究,1997.18(2):32-36.TANG Yan-juan.A Discussion on Recorded Distance for Diffraction Wave Pdif of Wave P[J].Seismological and Geomagnetic Observation and Research,1997.18(2):32-36.(in Chinese)
[4]许健生,隗永刚,张淑珍.Pdif震相的记录特征[J].地震地磁观测与研究,2011.32(4):115-121.XU Jian-sheng,WEI Yong-gang,ZHANG Shu-zhen.The Recording Character of Pdif Phase[J].Seismological and Geomagnetic Observation and Research,2011.32(4):115-121.(in Chinese)
[5]许健生,隗永刚,张淑珍.Pdif震相在CDSN兰州地震台的记录特征[J].西北地震学报,2012.34(3):239-244.XU Jian-sheng,WEI Yong-gang,ZHANG Shu-zhen.The Recording Characters of Pdif Phase in Seismic Wave at CDSN Lanzhou Seismic Station[J].Northwester Seismological Journal,2012.34(3):239-244.(in Chinese)
[6]B L N Kennett.IASPEI 1991地震走时表[M].北京:地震出版社,1993:108-130.B L N Kennett.IASPEI 1991Seismological Tables[M].Beijing;Seismological Press,1993:108-130.(in Chinese)
[2]郭瑛.关于Δ≥140°的地震衍射波Pdif[J].地震地磁观测与研究,1989.10(4):27-30.GUO Ying.On Diffracted Wave Pdif forΔ≥140°[J].Seismological and Geomagnetic Observation and Research,1989.10(4):27-30.(in Chinese)
[3]唐燕娟.对P波衍射波Pdif震相记录范围的讨论[J].地震地磁观测与研究,1997.18(2):32-36.TANG Yan-juan.A Discussion on Recorded Distance for Diffraction Wave Pdif of Wave P[J].Seismological and Geomagnetic Observation and Research,1997.18(2):32-36.(in Chinese)
[4]许健生,隗永刚,张淑珍.Pdif震相的记录特征[J].地震地磁观测与研究,2011.32(4):115-121.XU Jian-sheng,WEI Yong-gang,ZHANG Shu-zhen.The Recording Character of Pdif Phase[J].Seismological and Geomagnetic Observation and Research,2011.32(4):115-121.(in Chinese)
[5]许健生,隗永刚,张淑珍.Pdif震相在CDSN兰州地震台的记录特征[J].西北地震学报,2012.34(3):239-244.XU Jian-sheng,WEI Yong-gang,ZHANG Shu-zhen.The Recording Characters of Pdif Phase in Seismic Wave at CDSN Lanzhou Seismic Station[J].Northwester Seismological Journal,2012.34(3):239-244.(in Chinese)
[6]B L N Kennett.IASPEI 1991地震走时表[M].北京:地震出版社,1993:108-130.B L N Kennett.IASPEI 1991Seismological Tables[M].Beijing;Seismological Press,1993:108-130.(in Chinese)
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