人們知道地球的自轉周期并非是标準的24小時整，而是每天都有着微弱的變化。更為奇妙的是，處于地球最中心的固态内核，有着和外部圈層不一樣的自轉周期，也就是地球内核的差速旋轉現象。地球中心的地核的主要成分是鐵合金，分為液态的外核和固态的内核兩個主要圈層。外核的對流是地球磁場的來源，其施加在内核上的電磁力矩驅動了内核的差速旋轉。同時在萬有引力的作用下，固态且不均一的内核和地幔之間也有重力耦合效應，這将影響内核差速旋轉的狀态。然而無論是通過地球動力學的模拟還是地震學的觀測，地球内核差速旋轉的速度和模式的确定，都極具挑戰性。這一問題的解決将對地球深部的動力學過程和地核地幔的耦合機制提供全新的約束，有助于我們理解地球深部圈層的運行機制。該研究成為深地科學的重點探索任務。

beat365官方网站宋曉東教授和特聘副研究員楊翼博士最新的研究結果揭示了地球内核近七十年來的差速旋轉模式的變化。該研究利用在同一震源重複發生的天然地震追蹤地球内核的旋轉模式。重複地震的地震波在同一個台站接收到的記錄通常具有相同的波形，然而當地震波采樣到地球内核時，卻會顯示出不一樣的波形和到時（例如圖1），這是由于内核的旋轉導緻其内部的不均勻結構發生了橫向移動，兩次地震記錄有着空間中的同樣的路徑卻采樣到了不同的内核結構。通過分析長期的波形的變化模式，就可以推斷出内核旋轉的模式。令人驚訝的是，近十餘年來，這種地震信号的時變現象在全球的地震波路徑上統一消失了。經過更精确的分析得出了地球内核的差速旋轉在2009年就接近停止并開始緩慢地反向。類似的反向旋轉現象在上世紀70年代初期也有出現，意味着内核的旋轉很可能存在周期為六七十年左右的震蕩模式。

同時，在地磁場強度和地球日長（自轉周期）變化的信号中，甚至在全球平均海平面和氣溫變化當中，也存在着同樣的六七十年周期的信号，并且它們在相位上也存在着一定的對應關系。地磁場的變化意味着外核流體運動模式的變化，日長的變化代表着地幔和地殼旋轉的角速度變化，而全球海平面和氣溫變化源自地表（和大氣圈）的變化。由此可見地球的内核、外核、地幔和地表之間，由于電磁和重力等耦合效應，形成了一個周期為六七十年、從内核到淺表的耦合共振系統，這對于揭示地球作為一個系統的運行機制有重要意義（圖2）。

該成果于2023年1月23日正式發表在《自然.地球科學》(Nature Geoscience)上，研究論文題為“Multidecadal variation of the Earth’s inner-core rotation”，同刊發表的研究簡文題為“Rotation of the Earth's inner core changes over decades and has come to near-halt” （論文鍊接見文末）。楊翼博士為兩篇文章的第一作者，宋曉東是北大講席教授為兩篇文章的通訊作者。

該研究是中國地質科學院和beat365地空學院共建深地科學與探測技術實驗室（SinoProbe Laboratory）的成果，受到科技部“地球系統與全球變化”重點專項、自然科學基金地震聯合基金和青年基金、以及博士後面上基金的資助。

beat365官方网站設有5個本科生專業（地質、地球化學、固體地球物理學、空間科學與技術、地理信息系統）。地球科學與空間科學既是一門基礎科學，又是一門在現代國防和國民經濟建設中有着廣泛應用的科學。北大地質學及地球物理學均為我國同類專業中曆史最悠久的學科，地空學院是我國地球科學科研和人才培養的重要基地，承擔着為國家現代化建設輸送高級專門人才的重任，對我國現代科學發展和國民經濟建設發揮了重要作用。

深地科學與探測技術實驗室建于2020年，依托中國地質科學院，主要目标是聯合我國有關部門研究力量，組織實施國家深地領域研究任務，推進國際合作，打造國家級深地領域研究平台。實驗室聚焦深部結構探測、深部物質探測、深部資源探測和深部過程等前沿，搭建深地科學數據、探測技術與裝備和實驗測試支持等平台。實驗室面向國際開放與合作，向全球招聘科學家，與國際組織和機構、大學合作研究。前期深部探測實驗專項（SinoProbe-1，2008-2014)已奠定了紮實的基礎，儲備了技術裝備和人才隊伍，在國際上産生良好的反響。

聯系人：

作者：宋曉東教授, xiao.d.song@pku.edu.cn; 楊翼博士, yiy@pku.edu.cn

(北大地空學院);

深地實驗室: 董樹文教授，swdong@cags.ac.cn

鍊接：

圖1. 重複地震的内核信号随時間變化示例圖。在1998年到2004年間，地震波的波形存在明顯的變化，而在2009到2017年間，并未觀測到波形的變化。不經過内核的波形一直保持不變（圖b中的BC震相）。

Figure 1. Example of time-varying inner-core seismic waves. There is significant change between 1998 and 2004 (a), but not change between 2009 and 2017 (b) in the inner-core phases (DF, CD, and PKP coda). The outer-core phase (BC) always remains unchanged.

圖2. 從地表到内核的耦合共振系統示意圖。其中包括内核（差速旋轉）、外核（磁場變化）、地幔地殼（日長變化）、地表和大氣（全球平均海平面和氣溫變化）等多個圈層（和對應的地球物理觀測）。

Fig 2. A possible resonating Earth system from the surface to the inner core. The system includes the inner core (from its differential rotation), the outer core (from the magnetic field changes), mantle and crust (from the LOD variations), and the surface (from the global mean sea level rise and temperature).

News Release

Chinese scientists found rotation of the inner core changes over decades

The Earth’s rotation is not exactly 24 hours per cycle, but slightly changes every day. What is more surprising is that the Earth’s inner core has a different rotation period from the outer Earth layers. The phenomenon, known as the inner-core rotation, was discovered in 1996. The Earth's core is made of iron alloy with some light elements, which is divided into two major layers: the liquid outer core and the solid inner core. The convection in the outer core generates the geomagnetic field, and the electromagnetic torque exerted on the inner core drives the inner-core rotation. Meanwhile, under the influence of gravity, the solid and heterogeneous inner core and mantle are coupled gravitationally, which affects the inner-core rotation. However, determination of the speed and mode of the inner-core rotation have been quite challenging in the geophysical community, from geodynamic simulations and seismological observations. Solving the problem will provide new constraints on the dynamical processes in the deep Earth dynamics and the core-mantle coupling mechanism, and will help us better understand the inner working of the Earth.

Prof. Xiaodong Song at the School of Earth and Space Sciences (SESS), Peking University, and Dr. Yi Yang, an Associate Research Scientist in Song’s group, recently revealed the pattern changes of the Earth’s inner-core rotation over the past six decades. Their study traces the pattern of the Earth’s inner-core rotation using natural repeating earthquakes that occur at the same source location and with the same focal mechanism. Seismic waves from the repeating earthquakes usually have the same waveform at the same station, but when the waves interact with the Earth’s inner core, they show different waveforms and arrival times (e.g., Fig. 1), because the rotating inner core shifts its lateral heterogeneous structures, and the two earthquakes have the same raypath but sample different inner-core structures. By analyzing the pattern of the waveform changes, we can infer the mode of the inner-core rotation. Most surprisingly, the temporal change in seismic waves simultaneously disappeared along global paths in the last decade or so. More precise inversions using the travel-time temporal changes have shown that the inner-core rotation came to a halt in ~2009 and then a reverse. A similar reversed rotation was found in the early 1970s, implying a likely oscillating pattern with a period of six to seven decades.

Such a multilectal oscillation (six to seven decades) also exists in several other important geophysical observations, in the geomagnetic dipole changes, the Earth’s length of day (LOD, i.e., rotation period) variations, and even the global mean sea level and temperature changes. The geomagnetic dipole changes imply the changes of outer-core fluid motions; the change of day length represents the change of angular velocity of mantle and crust rotation, and the change of global sea level and temperature originates from the change of surface and atmosphere. Thus, the result suggests a coupling and resonating system with a multidecadal period, from the inner core to the shallow surface. The dynamics of the whole earth system involves the inner core, outer core, mantle, crust, and surface of the Earth, with coupling effects across those layers, such as electromagnetic and gravitation coupling (Figure 2).

The related papers were newly published on January 23, 2023 in Nature Geoscience. The research paper is entitled “Multidecadal variation of the Earth’s inner core rotation” and the research briefing is entitled “Rotation of the Earth’s inner core changes over decades and has come to near-halt” (see the links at the end of the report). Dr. Yi Yang is the first author of both papers, and Prof. Xiaodong Song is the corresponding author of the papers. This research is affiliated with the Sinoprobe Laboratory, which is jointly set up by Chinese Academy of Geological Sciences and Peking University. The research was supported by the National Key R&D Program of China on “Earth System and Global Change”, the National Natural Science Foundation of China, and China Postdoctoral Science Foundation.

The School of Earth and Space Sciences (SESS) at Peking University offers 5 undergraduate majors (Geology, Geochemistry, Solid Geophysics, Space Science and Technology, and Geographic Information System). Earth and space sciences are not only a basic science, but also a science widely used in modern societies and economic development. The geology and geophysics programs of Peking University are the oldest among similar disciplines in China. The SESS is an important base for earth science research and talent training in China. It has played an important role in the scientific and economic developments in the country.

The Deep Earth Science and Detection Technology Laboratory was established in 2020, based on the Chinese Academy of Geological Sciences. The main goal is to unite the research forces of relevant components in the country, organize and implement national deep earth research tasks, promote international cooperation, and build a national deep earth research platform. The laboratory focuses on the frontiers of deep structure, deep material, deep resource and deep processes, and builds platforms for deep earth data, detection technology and equipment, and experiment support. The laboratory is open to researchers in the world, through collaborations and recruitments, and cooperates with international organizations and research institutions and universities. The previous deep exploration project (SinoProbe-1, 2008-2014) has laid a solid foundation, built technical equipment and a talent team, and has generated good international response.

Contacts:

Authors: Prof. Xiaodong Song, xiao.d.song@gmail.com; Dr. Yi Yang, yiy@pku.edu.cn

SinoProbe Laboratory: Prof. Shuwen Dong, swdong@cags.ac.cn

Links: