RAPID: Capturing the physics of mountain uplift near the South Alpine Fault New Zealand

虽然山脉通常被认为是景观中的固定物，但在地质时间尺度上，它们是短暂的现象，只存在于岩石隆起和侵蚀的竞争过程几乎完全平衡的地方。在隆起超过侵蚀的地方，山的高度增加。没有隆起，山脉最终会被侵蚀成平缓的丘陵，最终变成平坦的平原。三种类型的过程可以解释山脉内岩石的上升。第一种是由挤压岩石引起的，类似于水平挤压橡皮擦，其表面将与水平收缩量成正比。第二种是由向地下斜坡的水平滑动引起的，这是一个逆冲断层，类似于由板块构造驱动的电梯上的岩石上升。第三个过程导致岩石因山谷的切割和河流的冲刷而上升。这种上升（被称为均衡）类似于冰山从海上升起，因为它暴露的表面被温暖的空气融化。这些过程中的每一种都会导致岩石的上升，但每种情况下损失的质量是不同的。通过测量山脉中岩石的上升和相关的质量变化，人们可以区分这三种机制。用GPS的方法可以很容易地测量出山顶的高度。山脉中质量的减少可以用重力仪来测量。2000年1月，PI在新西兰的南阿尔卑斯山测量了这两个数据，但为了确定答案，还需要进行第二次测量。全球定位系统的测量显示，在过去的15年里，高度增加了约80毫米。2015年1月，PI将首次使用NSF绝对重力仪测量重力减小。除非发生地震，否则导致新西兰南阿尔卑斯山脉高海拔的机制将被揭示出来。然而，如果在进行测量之前发生地震，那么由于剧烈震动引发的山体滑坡引发的群众运动，答案可能会变得模糊。因此，迫切需要获得NSF RAPID奖，并在新西兰南阿尔卑斯山进行及时的测量。2000年，国家科学基金会资助了在新西兰进行的16次重力绝对值测量，主要是在南阿尔卑斯山附近，利用重力（对当地密度变化敏感）和GPS（不敏感）的组合来研究地震间山脉隆起的物理现象。FG5重力仪具有独特的无漂移特性，因为它使用原子钟和稳定激光器测量真空中自由下落镜的加速度，精度达到10^-11。提出的造山机制（泊松比隆升、盘状断层滑动或侵蚀引起的均衡隆升）与不同的隆升与重力减少的比率有关。现在，15年过去了，8厘米的隆起已经发生，PI计划将同样的重力仪带到同样的顶点，以确定相关的重力下降。重力仪的精度为1µGal，重力的下降应该在27µGal左右，这是一个显著的信噪比。紧迫性与测量有关，因为如果当地发生地震，就像在克赖斯特彻奇测量附近发生的那样，山体滑坡可能会使重力信号模糊不清。事实上，阿尔卑斯断层已经过了它的中位复发间隔几十年，而且在Mw8级地震中破裂的可能性足够高，因此在未来几年内使用的捕捉其滑动速度的设备有很大的机会做到这一点。虽然破裂的传播速度可以很容易地通过地震仪（2.5-3.5公里/秒）来量化，但地表断层的滑动速度（0.1-10m/秒）迄今为止从未被测量过。在重力测量期间，将确定安装两个破裂仪的合适位置。每一根石墨棒都斜插在断层上，在断层滑动时从旋转的滚筒上拉出一根电线。其测量范围为13 m，分辨率为0.1 mm，采样率为1s，系统可以在太阳能电池板上无限期运行。

Although mountains are commonly considered fixtures in the landscape, at geological time-scales they are transient phenomena that exist only where the competing processes of rock uplift and erosion are almost exactly balanced. Where uplift exceeds erosion the height of mountains increase. Without uplift a mountain range will eventually be eroded to gentle hills, and eventually to a flat plain. Three types of processes can account for the rise of rocks within a mountain range. The first is caused by squeezing a rock, akin to squeezing a rubber eraser horizontally whose surface will rise in proportion to the amount of horizontal contraction. The second is caused by horizontal slip toward a subsurface ramp, a thrust fault, which is similar to rock rising on an elevator powered by plate tectonics. The third process causes rocks to rise in response to the incision of valleys and the removal of rock by rivers. This rise (known as isostasy) is similar to the rise of an iceberg from the sea as its exposed surface is melted by warm air. Each of these processes results in the rise of rocks, but the mass lost in each case is different. By measuring the rise of the rocks and the associated mass change in a mountain range one can distinguish between these three mechanisms. The rise of a mountain summit can easily be measured using GPS methods. The reduction of mass in a mountain range can be measured with a gravity meter. The PI measured both in the southern Alps of New Zealand in January 2000 but to determine the answer a second measurement is need. GPS measurements indicate the height has increased by about 80 mm in the past 15 years. In January 2015 the PI shall for the first time measure the reduction of gravity using the NSF absolute gravimeter. Unless an earthquake intervenes, the mechanisms that cause the high elevations of mountains in the southern Alps of New Zealand will be revealed. Should an earthquake occur before the measurements are undertaken, however, the answer may be rendered ambiguous by mass movements accompanying the landslides triggered by severe shaking. For this reason it is urgent to have this NSF RAPID award and make a timely measurement in the southern Alps of New Zealand. In 2000, NSF funded 16 measurements of absolute value of gravity in New Zealand, mostly near the Southern Alps to investigate the physics of interseismic mountain uplift using a combination of gravity (which is sensitive to local density variations) and GPS (which is not). The FG5 gravimeter is uniquely drift-free since it measures the acceleration of a freely falling mirror in a vacuum to an accuracy of 10^-11 using an atomic clock and a stabilized laser. Proposed mountain building mechanisms (Poissons-ratio uplift, listric fault slip, or erosion-induced isostatic uplift) are associated with different ratios of uplift to gravity decrease. Now, 15 years later, 8 cm of uplift has occurred and the PI plans to take the same gravimeter to the same summit points to determine the associated lowering in gravity. The accuracy of the gravimeter is 1 µGal, and the decrease in gravity should be about 27 µGal- a significant signal-to-noise ratio. Urgency is associated with the measurements in that should a local earthquake occur, as it did near the Christchurch measurements, landslides may render the gravity signal ambiguous. The Alpine fault is in fact many decades past its median recurrence interval and the probability of rupture in a Mw8 earthquake is sufficiently high that a device to capture its slip velocity operated in the next few years has a significant chance of doing so. Although the propagation velocity of a rupture can easily be quantified from seismometers (2.5-3.5 km/s) the sliding velocity of the surface fault (0.1-10m/s) has hitherto never been measured. During the gravity measurements suitable locations to install two rupture meters will be identified. Each consists of a graphite rod anchored obliquely across the fault, that pulls a wire from a rotating drum during fault slip. Its measurement range is 13 m, its resolution is 0.1 mm, its sampling rate is 1 s and the systems can run indefinitely on solar panels.