Tidal Loading and Asthenospheric Anelasticity

地球表面在水平和垂直方向上每隔几小时就会振荡几厘米，因为它在海洋的重量下会变形，而海洋的重量又会被潮汐规律地重新分配。这些“海潮载荷”的变形对我们人类来说太小、太慢、空间平滑，无法察觉，但它们可以通过精确的卫星定位技术（如GPS）检测到。这使我们能够在地震后的地震振动频率（秒到分钟）和钱德勒摆动（在近一年的时间尺度上几乎有规律的旋转运动）之间的时间尺度上研究地球的流变性（对力的响应的变形行为）。因为在几十到几百公里的空间尺度上，海潮是相似的，所以海潮负荷告诉我们地球在离地表最近的几百公里范围内（对应于地壳和上地幔）的行为。一个重要的问题是，地球是否表现出完全的弹性（就像一个橡胶球，因为它在非常短的地震时间尺度上表现得很好），或者它是否表现出非弹性（即不是完全弹性，更像一个湿海绵球或充气不足的足球，在去除力之后表现出时间延迟，然后恢复到原来的形式）。地球的行为从弹性到非弹性（或者在地质时间尺度上更像流体）的变化方式不仅在科学上很有趣，而且它影响了我们如何从地球形状的大地测量中推断其行为的其他方面。海洋潮汐是唯一有规律的、众所周知的、影响地球在这些深度的现象，并使我们能够模拟其行为，以便我们以后能够了解其他不太规律的、因此不太容易处理的现象。因此，主要在半日（大约12小时）和日（大约24小时）期间，有规律的海洋潮汐强迫地球变形，提供了一种理解地球行为的方法，这种方法在GPS出现之前是无法实现的，现在对我们使用大地测量学研究地震复发、海平面上升和其他地质灾害的方法很重要。精确的GPS大地测量使我们能够以迄今为止无与伦比的精度和空间覆盖范围测量海洋潮汐载荷变形（正如我们最近为西欧占主导地位的“M2”潮汐成分所展示的那样）。然而，GPS在与GPS卫星的轨道和几何重复周期相对应的某些潮汐和近年频率时存在问题。多gnss（全球导航卫星系统：GPS、GLONASS、北斗和伽利略）定位的新发展提供了一种绕过这一障碍的方法。我们将利用多gnss数据观测地球表面的潮汐调和运动，并在半日和日潮汐时间尺度的全范围内推断固体地球的非弹性变形程度。我们的观测将使我们能够在这个研究很少的时间尺度上研究地球上最上层地幔的软“软流层”层的行为，这将对（例如）理解俯冲带（最大地震发生的地方）的慢滑事件和短期地震后松弛有影响。除了这些更多的“蓝天”方面，改进的地球对表面质量负荷的近瞬时响应的正推模型（由我们的工作产生）将对测量关键气候变化变量的用户产生直接的实际影响，例如GRACE卫星测量水和冰的质量传递，以及GNSS测量潮汐计垂直陆地运动以校正海平面变化观测。

The Earth's surface oscillates on timescales of a few hours, both horizontally and vertically, by up to several centimetres because it deforms under the weight of the oceans which is regularly redistributed by the ocean tides. These 'ocean tide loading' deformations are too small, slow, and spatially smooth to be apparent to us humans, but they can be detected by precise satellite positioning techniques such as GPS. This allows us to investigate the Earth's rheology (deformational behaviour in response to forces) at time scales intermediate between the frequencies of seismic vibrations following earthquakes (seconds to minutes) and the Chandler wobble (an almost-regular rotational movement at near-yearly timescales). Because the ocean tides are similar over spatial scales of a few tens to hundreds of kilometres, ocean tide loading tells us most about the Earth's behaviour within the few hundred kilometres nearest the surface (corresponding to its crust and uppermost mantle). An important question is whether the Earth behaves perfectly elastically (like a rubber ball, as it does over very short seismic timescales), or if it behaves anelastically (i.e. not perfectly elastically; more like a wet sponge ball or an under-inflated football, that exhibits a time delay after the removal of the force before it returns to its original form). The way in which the Earth's behaviour changes from elastic to anelastic (or even more fluid-like over geological timescales) is not just scientifically interesting in itself, but it affects how we can infer other aspects of its behaviour from geodetic measurements of Earth's shape. The ocean tides are the only regular, well-known, phenomena that affect the Earth at these depths, and allow us to model its behaviour so we can later understand other less-regular and therefore less-tractable phenomena. Thus, the regular ocean tide forcing of the Earth's deformation, dominantly at semi-diurnal (roughly 12-hour) and diurnal (roughly 24-hour) periods, provides a way to understand Earth's behaviour in ways we could not before the advent of GPS and which are now important to the way we use geodesy to study earthquake recurrence, sea level rise, and other geohazards.Precise GPS geodesy allows us to measure ocean tide loading deformations with hitherto unsurpassed accuracy and spatial coverage (as we recently demonstrated for the dominant 'M2' tidal constituent in western Europe). However, GPS is problematic at certain tidal and near-annual frequencies corresponding to the GPS satellites' orbital and geometry repeat periods. New developments in multi-GNSS (Global Navigation Satellite Systems: GPS, GLONASS, Beidou, and Galileo) positioning offer a way around this obstacle. We will use multi-GNSS data to observe the tidal harmonic motions of the Earth's surface and infer the degree of anelastic deformation of the solid Earth over the full range of semi-diurnal and diurnal tidal timescales. Our observations will allow us to investigate the behaviour of the soft 'asthenosphere' layer of the Earth, in the uppermost mantle, at this poorly-studied timescale, which will have implications for (e.g.) the understanding of slow slip events and short-term postseismic relaxation in subduction zones (where the largest earthquakes occur). In addition to these more "blue-sky" aspects, improved forward models (resulting from our work) of the Earth's near-instantaneous response to surface mass loads will have immediate practical consequences for users measuring key climate change variables, e.g. GRACE satellite measurements of water and ice mass transfer, and GNSS measurements of tide gauge vertical land motion to correct sea level change observations.