How do large earthquakes start? Illuminating the nucleation phase from seismology and geodesy

项目背景大地震是如何发生的？这个简单问题的答案可能有助于改善早期预警和地震预报。在一个端元模型中，小的震动随机地重新分配应力，这可能引发更多的地震，其中一些会不可预测地发展成大的事件。在这个“级联模型”中，震前阶段的过程是随机的，地震的最终规模取决于持续破裂的随机障碍。级联模型是目前世界各地地震预报系统的基础，提供了有用但有限的预测技能。在另一个端元模型中，无源蠕变扫过断层，在无源滑动遇到粗糙体时产生地震事件，当粗糙体很大或相连时导致大地震。在这种“预滑模型”中，前震可能会遵循可辨别的模式，背叛了无源滑动，如前震的空间迁移或重复的震颤，重新打破凹凸不断加载缓慢滑动。这种模式可能有助于更好地预测即将发生的地震。在俯冲带发生一些大地震之前，如2011年东北（日本）和2014年伊基克（智利）地震，最近的大地测量观测似乎支持预滑动模型，但一些陆上事件发生时没有明显的预滑动。大地震如何开始是巨大的科学和实际interests.Project的目的和MethodsThe项目的目标是获得新的见解如何大地震开始，并确定在何种程度上不同的构造环境地震开始级联前震或地震滑动。该项目将主要利用观测地震学和统计地震学的方法，在空间大地测量学（全球定位系统/全球导航卫星系统）、断层力学和地震地质学的限制条件下进行推断。根据学生的兴趣，可以强调经验，统计或计算工作。地震学包括通过机器学习或模板匹配技术搜索更多的前震和重复地震。GPS/GNSS可以识别预滑或记录其缺失。统计方法可以将构造背景下的前震模式与前滑和级联模型的预测进行比较。地震滑动和地震滑动之间相互作用的地质证据提供了关于准备阶段力学的线索。一个关键的问题是，为了确定地震危险性分析的实际价值，在不导致大地震的情况下，模式出现的频率有多高。

Project BackgroundHow do large earthquakes start? The answer to this simple question might enable improved early warning and earthquake forecasting. In one endmember model, small tremors randomly redistribute stresses that can trigger more quakes, and some of these grow unpredictably into large events. In this 'cascade model', the processes during the pre-earthquake phase are stochastic, and the eventual size of a quake is determined by the random barriers to continued rupture. The cascade model underlies current operational earthquake forecasting systems around the world and provides useful but limited predictive skill. In another endmember model, aseismic creep sweeps across a fault, generating seismic events where the aseismic slip encounters asperities, and leading to large earthquakes when the asperities are big or connected. In this 'pre-slip model', foreshocks may obey discernible patterns that betray the aseismic slip, such as a spatial migration of foreshocks or repeating tremors that re-break asperities continually reloaded by slow slip. Such patterns might help to forecast impending quakes better. Recent geodetic observations prior to some giant earthquakes in subduction zones, such as the 2011 Tohoku (Japan) and the 2014 Iquique (Chile) earthquake, seem to support the pre-slip model, but some onshore events occurred without discernible pre-slip. How large earthquakes start is of immense scientific and practical interest.Project Aims and MethodsThe goal of this project is to gain new insights into how large earthquakes start and to determine the extent to which quakes in different tectonic settings start by cascading foreshocks or aseismic slip. The project will primarily draw on methods from observational earthquake seismology and statistical seismology to make inferences in the context of constraints from space geodesy (GPS/GNSS), fault mechanics and earthquake geology. Depending on the student's interests, empirical, statistical or computational work can be emphasised. Seismology includes searching for more foreshocks and repeaters via machine learning or template-matching techniques. GPS/GNSS can identify pre-slip or document its absence. Statistical approaches can compare foreshock patterns across tectonic settings with predictions from the pre-slip and cascade models. Geological evidence of the interplay between aseismic and seismic slip provide clues about the mechanics of the preparatory phase. A crucial question is how frequently patterns emerge without leading to large earthquakes in order to determine the practical value for seismic hazard analysis.