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

当两个构造板块一起或分开运动时，夹在它们之间的任何大陆都会发生变形，从而导致山带或沉积盆地等主要地质特征的形成。由于脆弱的近地表地壳试图适应这种变形，地震就发生在地球内部的断层上。人们迫切需要了解大陆是如何变形的，以及地震将发生在哪里——自1900年以来，有140万到170万人死于大陆内部的地震。我们可以用卫星来测量大陆活跃变形的方式。GPS可以非常精确地测量地面上各个点的移动情况，但这些点通常分布稀疏。在过去的二十年里，欧洲空间局（ESA）设计的卫星已经证明了卫星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。我们利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间隔时间内的位移。用这种方法可以测量几毫米或更小的位移。随着大陆地壳的变形，岩石继续弯曲，形成的张力将在未来的地震中释放出来。在评估地震危险时，除了知道地震发生的断层在哪里之外，还必须知道这种应变的增长速度。然而，这些速率很小，而且由于雷达散射的地面上的变化和雷达信号所经过的大气的特性所引起的噪声存在，用雷达测量这些速率并不容易。此外，我们对卫星位置的认识有误也会影响我们的测量。可以设计一些方法来克服这些困难，但是目前使用的卫星由于在地震带的许多地方不定期和不经常地获取雷达图像而限制了应用这些方法的机会。由于欧空局计划在2013年中后期发射一颗新的雷达卫星Sentinel-1A，我们有动力让一组调查人员努力解决这些问题。一个相同的伙伴，哨兵- 1b将在18个月后发射。每艘宇宙飞船将每6天经过地球表面的一个给定点；一旦两者都进入轨道，任何地点将每3天重新访问一次。这样短的时间间隔，再加上航天器每经过一次都要进行观测，而且它的位置将被仔细控制和清楚地知道，这意味着我们测量运动速率和应变的能力将得到根本的提高。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，我们将能够详细地绘制出大范围内压力增加的速率。我们计划通过这些观测来测试和约束物理模型，看看大陆变形时内部发生了什么。因此，用雷达测得的地震位移可以用来推断断层深处发生的运动。地震发生后，附近的地表继续移动的方式告诉我们周围地区的力学特性，这是理解断层周围的力如何随时间变化的关键知识。在更大的尺度上，大陆应变的空间分布告诉我们地壳强度的变化。有了这些限制条件，我们就可以检验关于大陆如何变形以及控制变形发生的主要因素是什么等相互矛盾的假设。

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.