人们知道地球的自转周期并非是标准的24小时整，而是每天都有着微弱的变化。更为奇妙的是，处于地球最中心的固态内核，有着和外部圈层不一样的自转周期，也就是地球内核的差速旋转现象。地球中心的地核的主要成分是铁合金，分为液态的外核和固态的内核两个主要圈层。外核的对流是地球磁场的来源，其施加在内核上的电磁力矩驱动了内核的差速旋转。同时在万有引力的作用下，固态且不均一的内核和地幔之间也有重力耦合效应，这将影响内核差速旋转的状态。然而无论是通过地球动力学的模拟还是地震学的观测，地球内核差速旋转的速度和模式的确定，都极具挑战性。这一问题的解决将对地球深部的动力学过程和地核地幔的耦合机制提供全新的约束，有助于我们理解地球深部圈层的运行机制。该研究成为深地科学的重点探索任务。

北京大学地球与空间科学学院宋晓东教授和特聘副研究员杨翼博士最新的研究结果揭示了地球内核近七十年来的差速旋转模式的变化。该研究利用在同一震源重复发生的天然地震追踪地球内核的旋转模式。重复地震的地震波在同一个台站接收到的记录通常具有相同的波形，然而当地震波采样到地球内核时，却会显示出不一样的波形和到时（例如图1），这是由于内核的旋转导致其内部的不均匀结构发生了横向移动，两次地震记录有着空间中的同样的路径却采样到了不同的内核结构。通过分析长期的波形的变化模式，就可以推断出内核旋转的模式。令人惊讶的是，近十余年来，这种地震信号的时变现象在全球的地震波路径上统一消失了。经过更精确的分析得出了地球内核的差速旋转在2009年就接近停止并开始缓慢地反向。类似的反向旋转现象在上世纪70年代初期也有出现，意味着内核的旋转很可能存在周期为六七十年左右的震荡模式。

同时，在地磁场强度和地球日长（自转周期）变化的信号中，甚至在全球平均海平面和气温变化当中，也存在着同样的六七十年周期的信号，并且它们在相位上也存在着一定的对应关系。地磁场的变化意味着外核流体运动模式的变化，日长的变化代表着地幔和地壳旋转的角速度变化，而全球海平面和气温变化源自地表（和大气圈）的变化。由此可见地球的内核、外核、地幔和地表之间，由于电磁和重力等耦合效应，形成了一个周期为六七十年、从内核到浅表的耦合共振系统，这对于揭示地球作为一个系统的运行机制有重要意义（图2）。

该成果于2023年1月23日正式发表在《自然.地球科学》(Nature Geoscience)上，研究论文题为“Multidecadal variation of the Earth’s inner-core rotation”，同刊发表的研究简文题为“Rotation of the Earth's inner core changes over decades and has come to near-halt” （论文链接见文末）。杨翼博士为两篇文章的第一作者，宋晓东是北大讲席教授为两篇文章的通讯作者。

该研究是中国地质科学院和北京大学地空学院共建深地科学与探测技术实验室（SinoProbe Laboratory）的成果，受到科技部“地球系统与全球变化”重点专项、自然科学基金地震联合基金和青年基金、以及博士后面上基金的资助。

北京大学地球与空间科学学院设有5个本科生专业（地质、地球化学、固体地球物理学、空间科学与技术、地理信息系统）。地球科学与空间科学既是一门基础科学，又是一门在现代国防和国民经济建设中有着广泛应用的科学。北大地质学及地球物理学均为我国同类专业中历史最悠久的学科，地空学院是我国地球科学科研和人才培养的重要基地，承担着为国家现代化建设输送高级专门人才的重任，对我国现代科学发展和国民经济建设发挥了重要作用。

深地科学与探测技术实验室建于2020年，依托中国地质科学院，主要目标是联合我国有关部门研究力量，组织实施国家深地领域研究任务，推进国际合作，打造国家级深地领域研究平台。实验室聚焦深部结构探测、深部物质探测、深部资源探测和深部过程等前沿，搭建深地科学数据、探测技术与装备和实验测试支持等平台。实验室面向国际开放与合作，向全球招聘科学家，与国际组织和机构、大学合作研究。前期深部探测实验专项（SinoProbe-1，2008-2014)已奠定了扎实的基础，储备了技术装备和人才队伍，在国际上产生良好的反响。

联系人：

作者：宋晓东教授, xiao.d.song@pku.edu.cn; 杨翼博士, yiy@pku.edu.cn

(北大地空学院);

深地实验室: 董树文教授，swdong@cags.ac.cn

链接：

图1. 重复地震的内核信号随时间变化示例图。在1998年到2004年间，地震波的波形存在明显的变化，而在2009到2017年间，并未观测到波形的变化。不经过内核的波形一直保持不变（图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.

图2. 从地表到内核的耦合共振系统示意图。其中包括内核（差速旋转）、外核（磁场变化）、地幔地壳（日长变化）、地表和大气（全球平均海平面和气温变化）等多个圈层（和对应的地球物理观测）。

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: