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

GSTDTAP

项目编号	1519035
	RAPID: Capturing the physics of mountain uplift near the South Alpine Fault New Zealand
	Roger Bilham
主持机构	University of Colorado at Boulder
项目开始年	2015
	2015-01-15
项目结束日期	2015-12-31
资助机构	US-NSF
项目类别	Standard Grant
项目经费	31500(USD)
国家	美国
语种	英语
英文摘要	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 Mw>8 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.
来源学科分类	Geosciences - Earth Sciences
文献类型	项目
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/67483
专题	环境与发展全球科技态势
推荐引用方式 GB/T 7714	Roger Bilham.RAPID: Capturing the physics of mountain uplift near the South Alpine Fault New Zealand.2015.