Great Earthquakes Along Major Plate Boundaries and the Nature of the Subduction Process

提出了一个为期3年的大地震和俯冲过程研究计划，以及沿转换断层的大地震性质研究计划。根据国家科学基金会现有的赠款，地震学和海洋地球物理数据的综合表明，俯冲带从0到15公里深度的板间推覆边界主要是地震运动的。在更深的大地震中，向滑动的过渡发生在沿板块边界从年轻、脆弱的沉积物过渡到更坚固的岩石的过程中，这种岩石被称为支撑层。这种转换也控制了增生的楔形-弧前盆地的形态。我们建议绘制断裂带的下倾宽度W，因为它可能是控制逆冲事件的最大尺寸和沿俯冲带特定部分的大地震的平均重复时间的关键参数。对W的了解应该使我们能够确定华盛顿、俄勒冈、新西兰和小安的列斯群岛南部附近的板块边界--所有这些地区都是年轻的厚沉积物--是否能够产生大型或重大的逆冲事件。世界各地大地震的地震矩目录正在编制中，将用于研究地震和无地震滑动沿不同板块边界的分布情况。我们将研究俯冲带从地震滑动到非地震滑动的深度转换，以及导致深度从25到70公里变化的因素。准确的震源位置、震源机制和地震活动的节点将被用来搜索沿逆冲边界的主要凹凸体。我们认为，在双层和斜面结构中，从平面几何图形向下倾斜的偏移量可以作为主要的粗糙度。这些特征对构造地质学家来说是众所周知的，但它们对大地震机制的影响似乎还没有被研究过。根据现有赠款和先前赠款开展的工作导致确定智利中部附近的一个地震空隙在1982至2000年间极有可能破裂。1985年，那里发生了一起重大的破坏性事件。拟议的研究通过确定大地震的重复次数和最大规模以及可能发生前兆现象的特定地点，应该具有重大的社会效益。

A 3-year program of research on great earthquakes and the subduction process and on the properties of large shocks along transform faults is proposed. Under an existing grant from NSF a synthesis of data from seismology and marine geophysics indicates that the interplate thrust boundary from depths of 0 to 15 km at subduction zones moves mainly aseismically. The transition to slip in large earthquakes at greater depth occurs at the transition along the plate boundary from young, weak sediments to stronger rocks of what is called the backstop. That transition also controls the morphology of the accretionary wedge- forearc basin. We propose to map the downdip width, W, of rupture zones since it is likely to be a critical parameter governing the maximum size of thrust events and the average repeat time of large shocks along given parts of subduction zones. Knowledge of W should allow us to ascertain if the plate boundaries off Washington, Oregon, New Zealand and the southern Lesser Antilles - all regions of young thick sediments - are capable of generating large or great thrust events. A catalog of seismic moments for large shocks worldwide is being prepared that will be used to study the distribution of seismic and aseismic slip along and among various plate boundaries. We will examine the transition form seismic to aseismic slip at depth in subduction zones and factors that cause that depth to vary from 25 to 70 km. Accurate hypocentral locations, focal mechanisms and knots of seismic activity will be used to search for major asperities along thrust boundaries. We think that offsets downdip from a planar geometry at duplex and ramp structures can act as major asperities. Such features are well known to structural geologists but their effect on the mechanics of large earthquakes does not appear to have been investigated. Work under existing and prior grants led to the identification of a seismic gap off central Chile as having a high probability of rupturing from 1982 to 2000. A large and damaging event occurred there in l985. The proposed research should have major social benefits by the identification of repeat times and maximum sizes of large earthquakes and specific sites where precursory phenomena are likely to occur.