Tidal Loading and Asthenospheric Anelasticity

地球表面在几个小时的时间尺度上水平和垂直地振荡，最多几厘米，因为它在海洋的重量下变形，海洋潮汐定期重新分配。这些“海潮负荷”变形太小，太慢，而且在空间上太平滑，对我们人类来说很明显，但它们可以通过精确的卫星定位技术（如GPS）检测到。这使我们能够调查地球的流变学（变形行为，以响应力）的时间尺度之间的地震振动的频率（秒到分钟）和钱德勒摆动（一个几乎有规律的旋转运动，在近每年的时间尺度）的中间。由于海洋潮汐在几十到几百公里的空间尺度上是相似的，海洋潮汐负荷告诉我们最接近地表的几百公里内（对应于地壳和最上地幔）的地球行为。一个重要的问题是，地球的行为是完全弹性的（像一个橡胶球，就像它在很短的地震时间尺度上所做的那样），还是非弹性的（即不是完全弹性的;更像是一个湿海绵球或充气不足的足球，在力移除后，在它恢复到原来的形式之前会有一段时间延迟）。地球的行为从弹性到滞弹性（甚至在地质时间尺度上更像流体）的变化方式不仅本身在科学上很有趣，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，并允许我们模拟其行为，以便我们以后可以了解其他不太规律，因此不太容易处理的现象。因此，地球变形的规律性海潮强迫，主要是在半日潮（约12小时）和昼夜（大约24小时）的周期，提供了一种方法来了解地球的行为方式，我们不能在GPS的出现，现在是重要的方式，我们使用大地测量学研究地震复发，海平面上升，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潮负荷变形（正如我们最近在西欧对占主导地位的“M2”潮汐成分所证明的那样）。然而，GPS在对应于GPS卫星的轨道和几何重复周期的某些潮汐和近年频率上是有问题的。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。我们将使用多重全球导航卫星系统数据观测地球表面的潮汐谐波运动，并推断固体地球在半日和昼夜潮汐时标的整个范围内的滞弹性变形程度。我们的观察将使我们能够调查地球软“软流圈”层的行为，在最上层的地幔，在这个研究不足的时间尺度，这将对（例如）对俯冲带（最大地震发生的地方）的缓慢滑动事件和短期震后松弛的理解。除了这些更加“蓝天”的方面之外，地球对表面质量负荷的近瞬时响应的改进的前向模型（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卫星对水和冰质量转移的测量，以及全球导航卫星系统对潮汐仪垂直陆地运动的测量，以纠正海平面变化观测。

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.