PKPdif波-内核低速层存在的地震学证据
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摘要
通过对中国数字地震仪台网(CDSN)11个台1993—2012年间30个极远震记录的分析,识别出了PKP波入射到内外核界面(ICB)上的衍射波PKPdif波。PKPdif波在震中距120°左右时出现在PKIKP波之前,在震中距150°以后出现在PKIKP波之后,是一个长周期波。为了解释PKPdif波超前于PKIKP波的原因,设想在距地面约5 156.1～5 372.2 km的深度,即内外核界面下有可能存在一个PKIKP波的低速层(厚度约216.1 km)。该设想符合Jeffreys速度模型,给出的PKPdif波在震中距119.4°～176.1°间的走时表填补了目前使用的《IASPEI1991地震波走时表》的空白。讨论PKPdif波的运动学特征有助于对内外核界面物理性质的认识,有助于提高分析震相的水平和积累震相分析经验。
In this study, we analyzed the records of 30 earthquakes recorded from 11 stations in the China Digital Seismographic Network(CDSN) during 1993-2012. We could observe a PKPdif phase at the epicentral distance from about 120°. The travel-time of a PKPdif wave was shorter than that of a PKIKP wave in the epicentral distance range of 120° to 150°, but longer for epicentral distances of more than 150°. We regarded PKPdif waves as diffracted PKP waves with a long period at the inner core boundary. We discussed the cause and propagation mechanism of PKPdif waves and proposed an assumption, in agreement with the Jeffreys model that a low-velocity layer of a PKIKP wave exists at a depth range of 5 156.1-5 372.2 km under the Earth's surface. We suggested the kinematic characteristics of the PKPdif waves recorded by the CDSN stations. We defined the relationship between the PKPdif travel-time and the epicentral distance, which filled the gap in the current "IASPEI1991 Seismological Tables". We constructed a travel-time table of PKPdif waves within an epicentral range of 119.4° to 176.1°. We also discussed the kinematic characteristics of PKPdif waves to better understand the physical properties of the ICB, improve phase identification, and accumulate analytical experience regarding the seismic phase.
In this study, we analyzed the records of 30 earthquakes recorded from 11 stations in the China Digital Seismographic Network(CDSN) during 1993-2012. We could observe a PKPdif phase at the epicentral distance from about 120°. The travel-time of a PKPdif wave was shorter than that of a PKIKP wave in the epicentral distance range of 120° to 150°, but longer for epicentral distances of more than 150°. We regarded PKPdif waves as diffracted PKP waves with a long period at the inner core boundary. We discussed the cause and propagation mechanism of PKPdif waves and proposed an assumption, in agreement with the Jeffreys model that a low-velocity layer of a PKIKP wave exists at a depth range of 5 156.1-5 372.2 km under the Earth's surface. We suggested the kinematic characteristics of the PKPdif waves recorded by the CDSN stations. We defined the relationship between the PKPdif travel-time and the epicentral distance, which filled the gap in the current "IASPEI1991 Seismological Tables". We constructed a travel-time table of PKPdif waves within an epicentral range of 119.4° to 176.1°. We also discussed the kinematic characteristics of PKPdif waves to better understand the physical properties of the ICB, improve phase identification, and accumulate analytical experience regarding the seismic phase.
引文
[1] GUTENBURG B,RICHTER C F.On Seismic Waves:Ⅰ[J].Gerlands Beitr Geophysik,1934,43:56-133.
[2] BULLEN K E,BURKE-GAFFNEY T N.Diffracted Seismic Waves near the PKP Custic[J].Geophys J Int,1958:9-17.
[3] BOLT B A.The Velocity of Seismic Waves near the Earth's Center[J].BSSA,1964,54(1):191-208.
[4] PETER Borman.New Manual of Seismological Observatory Practice (Chinese)[M].Beijing: Seismological Press,2006.
[5] 沈旭章,周蕙兰.用PKIKP的前驱震相探测青藏高原东部地幔底部的散射体[J].科学通报,2009,54(24):3844-3851.SHEN Xuzhang,ZHOU Huilan.Locating Seismic Scatters at the Mantle Beneath Eastern Tibet with PKIKP Precursors[J].Chinese Sci Bull,2009,54(24):3844-3851.
[6] 许健生,隗永刚,张淑珍,等.Pdif波在中国CDSN台站的运动学特征[J].地震工程学报,2013.35(3):610-617.XU Jiansheng,WEI Yong-gang,ZHANG Shuzhen.The Kinematical Characteristics of Pdif-waves at CDSN Stations[J].China Earthquake Engineering Journal,2013.35(3):610-617.
[7] SONG Xiaodong,HELMBERGER Don V.Velocity Structure near the Inner Core Boundary from Waveform Modeling[J].J G R,1992,97(B5):6573-6586.
[8] JEFFREYS.The Times of the Core Waves[J].Mon Not R Astr Soc Geophys,1939(Suppl4):594-615.
[9] JEFFREYS H,BULLEN K E.Seismological Tables[Z].Brit Assoc Adc Sci,1940.
[10] KENNETT B L N.IASPEI 1991 Seismological Tables (Chinese)[M].Beijing: Seismological Press,1993:108-130.
[11] PIETRO Caloi.Seismic Waves from the Outer and the Inner Core[J].Geophs J,1961,4:139-150.
[12] GEORGE L Choy.The Structure of the Inner Core Inferred from Short-period and Broadband GDSN Data[J].G G R Astr Soc,1983,72:1-21.
[2] BULLEN K E,BURKE-GAFFNEY T N.Diffracted Seismic Waves near the PKP Custic[J].Geophys J Int,1958:9-17.
[3] BOLT B A.The Velocity of Seismic Waves near the Earth's Center[J].BSSA,1964,54(1):191-208.
[4] PETER Borman.New Manual of Seismological Observatory Practice (Chinese)[M].Beijing: Seismological Press,2006.
[5] 沈旭章,周蕙兰.用PKIKP的前驱震相探测青藏高原东部地幔底部的散射体[J].科学通报,2009,54(24):3844-3851.SHEN Xuzhang,ZHOU Huilan.Locating Seismic Scatters at the Mantle Beneath Eastern Tibet with PKIKP Precursors[J].Chinese Sci Bull,2009,54(24):3844-3851.
[6] 许健生,隗永刚,张淑珍,等.Pdif波在中国CDSN台站的运动学特征[J].地震工程学报,2013.35(3):610-617.XU Jiansheng,WEI Yong-gang,ZHANG Shuzhen.The Kinematical Characteristics of Pdif-waves at CDSN Stations[J].China Earthquake Engineering Journal,2013.35(3):610-617.
[7] SONG Xiaodong,HELMBERGER Don V.Velocity Structure near the Inner Core Boundary from Waveform Modeling[J].J G R,1992,97(B5):6573-6586.
[8] JEFFREYS.The Times of the Core Waves[J].Mon Not R Astr Soc Geophys,1939(Suppl4):594-615.
[9] JEFFREYS H,BULLEN K E.Seismological Tables[Z].Brit Assoc Adc Sci,1940.
[10] KENNETT B L N.IASPEI 1991 Seismological Tables (Chinese)[M].Beijing: Seismological Press,1993:108-130.
[11] PIETRO Caloi.Seismic Waves from the Outer and the Inner Core[J].Geophs J,1961,4:139-150.
[12] GEORGE L Choy.The Structure of the Inner Core Inferred from Short-period and Broadband GDSN Data[J].G G R Astr Soc,1983,72:1-21.