Earthquake energy budget and coseismic fault temperature from seismological observations

地震是如何发生的？了解地震的本质是地球科学中一个关键的基本问题，对社会有许多影响。地震通常与能量的突然释放有关，这种能量是在数百年到数千年的时间里缓慢积累起来的，在相当极端的条件下，被埋在我们脚下几公里处的断层的摩擦所强烈控制。例如，短短几秒钟内产生的热量足以极大地改变滑动表面附近断层带的性质。此外，有大量的证据表明，地震期间断层的摩擦减弱（即断层强度随着滑动或滑动速率的增加而减弱）。然而，关于震源过程仍有许多悬而未决的问题：不同温度-压力条件下的地震有多相似？控制地面运动强度的地震能量收支是多少？断层弱化的物理机制是什么？地震成像方法、理论断裂力学和破裂动力学模拟的最新进展有助于解决这些问题。现在，来自世界各地的大量免费地震和大地测量数据可以用于地震模型的常规计算，而地震通常被描述为单个时空点。利用最近开发的复杂建模工具以及高性能计算设备提供的高质量3-D地球内部结构图像，系统地建立强大的、更详细的地震模型的时机已经成熟。此外，现在可以通过分析断裂力学计算和数值破裂动力学模拟，从理论上详细地模拟破裂，例如，估计破裂过程中的断层温度，这是量化摩擦的最直接方法。然而，在这些计算和地震观测之间仍然缺乏系统的定量联系。本项目通过涉及地震学和岩石力学的协调努力来解决这些问题，旨在从新的宏观震源模型中估计地震期间的断层温升。我们将使用先进的震源成像方法，为大量选定的全球地震（例如，平均断层长度，宽度，破裂速度和时间历史，应力降，辐射和破裂能）建立一套新的健壮的运动学，静态和动态震源参数。然后将这些解作为输入参数，使用解析和数值破裂动力学计算来估计断层温度。这将使人们更好地理解发生在几微米到几十厘米尺度上的局部断层过程是如何转化为宏观地震学特性的，地震期间能量是如何分配的，以及导致断层减弱的机制是什么。最终，该项目将为地震科学中的许多基本问题提供新的线索，例如不同P-T条件下地震的相似性以及破裂（例如融化）留下的潜在地质记录。更广泛地说，该项目将有利于危险模型和任何依赖准确震源参数的研究，如地震层析成像、活动构造和微地震活动（例如，与水力压裂有关）的研究。

How do earthquakes happen? Understanding the nature of earthquakes is a key fundamental question in Geociences that holds many implications for society. Earthquakes are typically associated with a sudden release of energy that has slowly accumulated over hundreds to thousands of years, being strongly controlled by friction in faults buried several kilometers beneath our feet under quite extreme conditions. For example, the amount of heat produced in just a few seconds is such that it can dramatically change the nature of the fault zone near the sliding surface. Moreover, there is abundant evidence of substantial frictional weakening of faults (i.e., fault strength weakens with increasing slip or slip rate) during earthquakes. However, there are still many open questions related to earthquake source processes: How similar are earthquakes in different temperature-pressure conditions? What is the earthquake's energy budget, which controls the intensity of ground motions? What are the physical mechanisms responsible for fault weakening? Recent progress in seismological imaging methods, theoretical fracture mechanics and rupture dynamics simulations can help solve these questions. Huge volumes of freely available seismic and geodetic data from around the world now allow the routine calculation of earthquake models where earthquakes are typically described as single space-time points. Time is now ripe for systematically building robust, more detailed seismic models bearing information on earthquake's physics by using recently developed sophisticated modelling tools along with high-quality images of the 3-D Earth's interior structure enabled by high performance computing facilities. Moreover, it is now possible to model ruptures theoretically in detail using both analytical fracture mechanics calculations and numerical rupture dynamics simulations, and, for example, estimate the fault temperature during the rupture process, which is the most direct way to quantify friction. However, systematic quantitative links between these calculations and seismological observations are still lacking. This project addresses these issues through a coordinated effort involving seismology and rock mechanics aiming at estimating fault temperature rise during earthquakes from new macroscopic seismic source models. We will use advanced seismic source imaging methods to build a new set of robust kinematic, static and dynamic earthquake source parameters for a large selected set of global earthquakes (e.g., average fault length, width, rupture speed and time history, stress drop, radiated and fracture energy). These solutions will then be used as input parameters to estimate fault temperature using analytical and numerical rupture dynamics calculations. This will lead to an improved understanding of how local fault processes occurring at scales from few microns to tens of centimetres translate into macroscopic seismological properties, how energy is partitioned during earthquakes and which are the mechanisms responsible for fault weakening. Ultimately this project will shed new light on many basic questions in earthquake science such as the similarity of earthquakes in different P-T conditions and the potential geological record left by ruptures (e.g., melt). More broadly, this project will benefit hazard models and any studies relying on accurate earthquake source parameters such as studies in seismic tomography, active tectonics and microseismicity (e.g., associated with hydraulic fracturing).