PKPdif波-内核低速层存在的地震学证据
PKPdif Wave may Indicate Existence of Low-velocityLayer in Earth's Inner Core

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