Looking inside the Continents from Space: Insights into Earthquake Hazard and Crustal Deformation

当两个构造板块移动到一起或分开时，任何被夹在它们之间的大陆都会变形，导致主要的地质特征，如山脉带或沉积盆地的发展。当脆弱的近地表地壳试图适应这种变形时，地震就发生在地球内部的断层上。了解大陆如何变形以及地震将在哪里发生的需求是迫切的-自1900年以来，大陆内部有140万至170万人死于地震。全球定位系统可以提供非常精确的测量地面上的各个点如何移动，但这些点通常分布稀疏。在过去的二十年里，欧洲航天局（欧空局）设计的卫星已经证明了星载雷达测量地球表面位移的能力。雷达反复发出微波信号，这些信号从表面散射回来，并在返回航天器时进行测量。我们使用卫星在两个不同时间获得的雷达回波的差异来测量该点在干预时间间隔内的位移。几毫米或更小的位移可以用这种方法测量。当大陆地壳变形时，岩石继续弯曲，积累的应变将在未来的地震中释放出来。在评估地震危险性时，除了知道地震将发生的断层在哪里之外，还必须知道这种应变增长的速度。然而，这些速率很小，并且在存在由雷达散射的地面上的变化和雷达信号通过的大气的性质的变化引起的噪声的情况下，不容易使用雷达测量。此外，我们对卫星位置的认识误差也会影响我们的测量。可以设计出克服这些困难的方法，但由于在地震带的许多地方不定期和不频繁地获取雷达图像，目前的卫星应用这些方法的机会有限，我们有动力利用一个调查小组的努力来解决这些问题，因为欧空局计划在2013年中至晚些时候发射一颗新的雷达卫星Sentinel-1A。一个完全相同的伙伴，哨兵-1B将在18个月后发射。每一个航天器将通过地球表面上的一个给定的点每6天;一旦两者都在轨道上，任何一点将重新访问每3天。这一短时间间隔，加上将对航天器的每一次通过进行观测，以及它的位置将受到仔细控制和众所周知的事实，将意味着我们测量运动和应变速率的能力得到根本性的提高。通过将所有可用卫星轨迹的测量结果与任何可用的GPS数据结合起来，我们将能够详细绘制大面积的应变率。我们计划通过使用这样的观测来测试和约束物理模型，看看大陆内部在变形时会发生什么。因此，在地震中发生的位移由雷达测量可以用来推断已经发生在断层上的运动在深度。地震发生后，地球表面继续移动的方式告诉我们周围地区的力学性质，这是理解断层周围的力如何随时间变化的基本知识。在更大的范围内，大陆应变的空间分布告诉我们地壳强度的变化。有了这些约束条件，我们就可以检验关于大陆如何变形以及控制变形发生地点的主要因素是什么的相互竞争的假设。

As two tectonic plates move together or apart, any continent trapped between them deforms, causing major geological features such as mountain belts or sedimentary basins to develop. As the brittle, near-surface crust tries to accommodate the deformation, earthquakes occur on faults inside the earth. The need to understand how the continents deform, and where earthquakes will occur, is compelling - between 1.4 and 1.7 million people have died in earthquakes in the continental interiors since 1900.We can measure the way the continents are actively deforming using satellites. GPS can provide very precise measurements of how individual points on the ground move, but such points are often sparsely distributed. Over the past two decades, satellites designed by the European Space Agency (ESA) have demonstrated the ability of satellite-borne radar to measure displacements of the earth's surface. The radar repeatedly sends out bursts of a microwave signal that scatters back from the surface and is measured when it returns to the spacecraft. We use differences in the radar returns acquired by the satellite at two different times to measure the displacement of that point over the intervening time interval. Displacements of a few millimeters or less can be measured in this way.As the continental crust deforms, the rocks continue to bend, building up strain that will be released in future earthquakes. When assessing earthquake hazard, in addition to knowing where the faults are on which the earthquakes will occur, it is essential to know the rate at which this strain is growing. These rates are small, however, and not easy to measure using radar in the presence of noise caused by changes on the ground from which the radar scatters and in the properties of the atmosphere through which the radar signal passes. In addition, errors in our knowledge of the position of the satellites affect our measurements. Methods can be devised to counter these difficulties, but the opportunities to apply them has been limited with the current satellites by the irregular and infrequent acquisition of radar images over many parts of the seismic belts.We are motivated to bring the efforts of a team of investigators to bear on these questions because of the planned launch by ESA in mid-to-late 2013 of Sentinel-1A, a new radar satellite. An identical partner, Sentinel-1B will be launched 18 months later. Each spacecraft will pass over a given point on the earth's surface every 6 days; once both are in orbit any point will be revisited every 3 days. This short time interval, plus the fact that observations will be made for every pass of the spacecraft and its position will be carefully controlled and well known, will mean a radical improvement in our ability to measure rates of motion and strain. By combining the measurements from all available satellite tracks, together with any GPS data available, we will be able to map in detail over large areas the rates at which strain is building up. We plan to look at what happens inside the continents as they deform by using such observations to test and constrain physical models. Thus the displacements occurring in an earthquake measured by radar can be used to infer the movements that have taken place on the fault at depth. The way the earth's surface in the vicinity of an earthquake continues to move immediately after it tells us about the mechanical properties of the surrounding region, knowledge essential to understanding how the forces around a fault vary with time. On a larger scale, the spatial distribution of strain in the continents tells us about changes in the strength of the crust. With these constraints we can test competing hypotheses about how the continents deform and what are the major factors controlling where the deformation occurs.