Deep crustal structure of the North Anatolian Fault Zone and the earthquake cycle

地球表面被分成无数的构造板块，这些板块不断地移动。板块之间的相对运动是地球上大多数地震活动的来源，这些地震活动通常集中在狭窄的断层带，在那里板块相互碰撞，拉开或滑动。在断裂带内，地壳上部10-15公里的变形局限于狭窄的断层面。当板块运动引起的断层面上的应力克服摩擦阻力时，地震就会发生，这对生活在断层带的社区来说是一个重大的危险-仅在21世纪的第一个十年，地震就造成70万人死亡。在走滑断层带中，板块相互滑动，地震通常只破坏上地壳。我们知道下地壳（深度超过10-15公里）一定在持续变形，因为我们可以测量地震之间地表的变形。但是由于岩石样本或其他直接测量不容易从这些深度获得，我们对下地壳如何表现以及如何影响上地壳的应力加载以引起大地震的了解很少，我们提出了一个跨学科的项目，旨在了解地震加载周期（应力如何通过板块运动建立并在地震中释放）沿着一个主要的欧洲断层，土耳其的北安纳托利亚断层带（NAFZ）。NAFZ是一个走滑断层，其长度和滑动速率与加州的圣安德烈亚斯断层相当。它穿过土耳其北方人口稠密的地区，构成了一个主要的地震危险-超过1000公里的断层在20世纪的12次大地震中破裂。1999年8月17日在伊兹米特和87天后在杜兹采发生的两次大地震中，西北非军事区西端破裂，造成3万多人死亡。伊斯坦布尔南部仍然存在地震空区，这是一个人口超过1000万的城市中心，在未来几十年内有60%的可能性发生重大地震（Parsons等人，2000）。我们的目标是测量下地壳断层的性质，以限制沿着NAFZ的地震荷载周期。该项目包括：㈠一项新的高分辨率地震实验，目的是解析断层带的深层结构; ㈡对代表断层下中下地壳的一个挖掘断层带进行地质分析; ㈢对地表位移的卫星测量进行分析。这些研究的结果将用于建立地震荷载循环的计算模型。在这个项目中，我们的目标是解释构造板块的运动如何与断裂带相互作用，以及这是如何受到下地壳结构的影响。这将最终有助于更好地评估与大断层带相关的地震危险性。地球物理和地质数据的合成以及地球动力学建模将指导未来对其他主要走滑断层带的调查。

The Earth's surface is broken into numerous tectonic plates, which are continually moving. The movement of the plates relative to each other is the source for most earthquake activity on Earth, which is typically focussed into narrow fault zones where the plates collide, pull apart, or slide past each other. Within the fault zones the deformation in the upper 10-15 km of the Earth's crust is localised onto narrow fault planes. Earthquakes occur when the stresses on the fault planes caused by plate motions overcome frictional resistance, and these represent significant hazard for communities living in fault zones - in the first decade of the 21st century alone, earthquakes killed 700,000 people. In strike-slip fault zones, where plates slide past each other, earthquakes typically only break the upper crust. We know that the lower crust (deeper than 10-15 km) must be deforming continuously, because we can measure how the ground surface deforms between earthquakes. But because rock samples or other direct measurements cannot easily be obtained from these depths, we have a poor understanding of how the lower crust behaves and influences the loading of stresses in the upper crust to cause major earthquakes.We propose an inter-disciplinary project with the aim of understanding the earthquake loading cycle (how stresses build through plate motions and are released in earthquakes) along a major European fault, the North Anatolian Fault Zone (NAFZ) in Turkey. The NAFZ is a strike-slip fault comparable in length and slip rate to the San Andreas Fault in California. It crosses a densely populated region of northern Turkey and constitutes a major seismic hazard - over 1000 km of the fault ruptured during 12 large earthquakes in the 20th century. The western end of the NAFZ ruptured in two major earthquakes in 1999 at Izmit on 17 August and Düzce, 87 days later, killing more than 30,000 people. A seismic gap remains south of Istanbul, an urban centre of more than 10 million people, where there is ~60% chance of significant shaking within the next few decades (Parsons et al. 2000).We aim to measure the properties of the fault in the lower crust to set constraints on the earthquake loading cycle along the NAFZ. The project involves (i) a novel high-resolution seismic experiment aimed at resolving the fault zone structure at depth, (ii) geological analysis of an exhumed fault zone representative of the mid to lower crust under the fault, and (iii) analysis of satellite measurements of surface displacement. The results from these studies will be used to build computational models of the earthquake loading cycle. In this project we aim to explain how the movements of the tectonic plates interact with the fault zone and how this is affected by the lower crustal structure. This will ultimately contribute to better assessment of the seismic hazard associated with large fault zone. The resulting synthesis of the geophysical and geological data together with geodynamical modelling will guide future investigations for other major strike-slip fault zones.