Tidal Loading and Asthenospheric Anelasticity

潮汐载荷和软流圈滞弹性

• 批准号: NE/R010234/1
• 负责人: Peter Clarke
• 金额: $ 51.5万
• 依托单位: Newcastle University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2018
• 资助国家: 英国
• 起止时间: 2018 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FR010234%2F1
• 关键词: Tidal; Loading; Asthenospheric; Anelasticity

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.

地球表面在水平和垂直方向上以几个小时为单位振荡，幅度可达几厘米，因为地球表面在海洋重量的作用下会变形，海洋潮汐会定期重新分布。这些“海洋潮汐加载”变形太小、缓慢且空间平滑，对我们人类来说是显而易见的，但它们可以通过精确的卫星定位技术（例如 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（近年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学（对力的变形行为） 。由于海洋潮汐在几十到数百公里的空间尺度上是相似的，因此海洋潮汐载荷告诉我们大多数有关地球在最接近地表（对应于地壳和最上地幔）的几百公里内的行为。一个重要的问题是，地球是否表现得完全有弹性（就像一个橡皮球，因为它在非常短的地震时间尺度上表现得如此），或者它是否表现得非弹性（即不完全有弹性；更像是一个湿海绵球或充气不足的足球，去除力后，在恢复到原始形式之前会出现一段时间延迟）。地球行为从弹性变为非弹性（或者在地质时间尺度上更像流体）的方式不仅本身具有科学意义，而且影响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。海洋潮汐是唯一一种在这些深度影响地球的规律的、众所周知的现象，它使我们能够对其行为进行建模，以便我们以后能够理解其他不太规律、因此难以处理的现象。因此，规律的海潮强迫地球变形，主要是在半日（大约 12 小时）和日间（大约 24 小时）周期，提供了一种在 GPS 出现之前我们无法理解地球行为的方法。现在，这对于我们使用大地测量学来研究地震复发、海平面上升和其他地质灾害的方式非常重要。精确的 GPS 大地测量学使我们能够测量迄今为止的海潮载荷变形。无与伦比的准确性和空间覆盖范围（正如我们最近针对西欧主要“M2”潮汐成分所证明的那样）。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下存在问题。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解决这一障碍的方法。我们将利用多GNSS数据观测地球表面的潮汐简谐运动，并推断固体地球在半日和日潮汐时间尺度的全范围内的滞弹性变形程度。我们的观测将使我们能够在这个研究不足的时间尺度上研究地球最上地幔的软“软流圈”层的行为，这将对（例如）慢滑事件和短期滑移事件的理解产生影响。俯冲带（发生最大地震的地方）的震后弛豫。除了这些更“蓝天”的方面之外，改进的地球对表面质量载荷的近瞬时响应的正演模型（来自我们的工作）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 测量验潮仪垂直陆地运动的数据，以纠正海平面变化观测结果。

项目成果

Asthenospheric anelasticity effects on ocean tide loading around the East China Sea observed with GPS

• DOI: 10.5194/se-11-185-2020
• 发表时间: 2020-02
• 期刊: Solid Earth
• 影响因子: 3.4
• 作者: Junjie Wang;N. Penna;P. Clarke;M. Bos

Benefits of combining GPS and GLONASS for measuring ocean tide loading displacement

• DOI: 10.1007/s00190-020-01393-5
• 发表时间: 2020-07
• 期刊: Journal of Geodesy
• 影响因子: 4.4
• 作者: Majid Abbaszadeh;P. Clarke;N. Penna