Lamb Simon
Institute of Geophysics, Victoria University of Wellington, Wellington, New Zealand.
Philos Trans A Math Phys Eng Sci. 2021 Mar 22;379(2193):20190414. doi: 10.1098/rsta.2019.0414. Epub 2021 Feb 1.
Satellite-based measuring systems are making it possible to monitor deformation of the Earth's surface at a high spatial resolution over periods of several decades and a significant fraction of the seismic cycle. It is widely assumed that this short-term deformation directly reflects the long-term pattern of crustal deformation, although modified in detail by local elastic effects related to locking on individual faults. This way, short-term deformation is often jointly inverted with long-term estimates of fault slip rates, or even stress, over periods of 10 s to 100 s kyrs. Here, I examine the relation between these two timescales of deformation for subduction, continental shortening and rifting tectonic settings, with examples from the active New Zealand and Central Andean plate boundary zone. I show that the relation is inherently non-unique, and simple models of locking on a deep-seated megathrust or decollement, or mantle flow, provide excellent fits to the short-term observations without requiring any information about the geometry and rate of surface faulting. The short-term deformation, in these settings at least, cannot be used to determine the behaviour of individual faults, but instead places constraints on the forces that drive deformation. Thus, there is a fundamental difference between the stress loading and stress relief parts of the earthquake cycle, with failure determined by dynamical rather than kinematic constraints; the same stress loading can give rise to widely different modes of long-term deformation, depending on the strength and rheology of the deforming zone, and the role of gravitational stresses. The process of slip on networks of active faults may have an intermediate timescale of kyrs to 10 s kyrs, where individual faults fail piecemeal without any characteristic behaviour. Physics-based dynamical models of short-term deformation may be the best way to make full use of the increasing quality of this type of data in the future. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.
基于卫星的测量系统使得在几十年的时间跨度以及地震周期的很大一部分时间内，以高空间分辨率监测地球表面变形成为可能。人们普遍认为，这种短期变形直接反映了地壳变形的长期模式，尽管会因与单个断层锁定相关的局部弹性效应而在细节上有所改变。通过这种方式，短期变形常常与长达10到100千年期间的断层滑动速率甚至应力的长期估计值一起进行联合反演。在此，我以活跃的新西兰和安第斯中部板块边界带为例，研究俯冲、大陆缩短和裂谷构造环境中这两种变形时间尺度之间的关系。我表明这种关系本质上是非唯一的，并且深部大逆冲断层或滑脱面的锁定或地幔流的简单模型，无需任何关于地表断层几何形状和速率的信息，就能很好地拟合短期观测结果。至少在这些环境中，短期变形不能用于确定单个断层的行为，而是对驱动变形的力施加限制。因此，地震周期的应力加载和应力释放部分存在根本差异，破坏由动力学而非运动学约束决定；相同的应力加载可能会导致截然不同的长期变形模式，这取决于变形带的强度和流变学以及重力应力的作用。活动断层网络上的滑动过程可能具有从千年到10万年的中间时间尺度，其中单个断层逐个破裂，没有任何特征性行为。基于物理的短期变形动力学模型可能是未来充分利用这类质量不断提高的数据的最佳方式。本文是“利用地质记录理解地震”讨论会议题的一部分。
