Analysis of seismic attenuation in the earthquake swarm area near Werdau

西波西米亚/沃格特兰的地震群区过去和现在都是密集的地球科学调查的对象。然而，到目前为止，很少有人注意到这个主要活动带周围的其他小群震区以及与莱比锡-雷根斯堡断裂带的连接。在这里提交的项目中，我们建议对Werdau（西萨克森州）附近最北的已知地震群区进行详细分析。目的是揭示两个群震区之间的差异和相似之处以及可能的联系，并最终提高我们对大陆内环境中流体诱发地震活动的理解。为了对流体成像，这些流体被认为是造成地震群的原因，我们将利用局部地震反演Werdau地震群区的三维地震衰减结构。在第一步，我们将应用辐射传递理论来分离本征吸收和散射衰减，分析衰减的频率依赖性，以及反演详细的源光谱和频率依赖性地震仪现场响应因子。为了求解辐射传递的弹性方程，我们将首次采用一种改进的蒙特卡罗算法，该算法能够分别反演不同深度的本禀衰减和散射衰减。为此，我们将扩展现有的主要算法之一，以允许在任意深度依赖速度和衰减模型中计算能量传播。这种算法在这个特殊的项目之外，还可以用于获得可靠的地壳一维衰减模型。在第二步中，我们将对P波和s波进行三维衰减层析成像，以检测流体含量增加的区域。辐射传递理论和衰减层析成像两种技术的结合是另一个技术目标。将首次结合两种方法各自的优势，制定优化的联合反演策略。结果将与现有的速度模型（vP和vP/vS）进行比较，并进行联合解释。这种地下流体流动成像技术及其与流体诱发地震活动的联合解释将在本项目之外发挥重要作用。我们特别提到了地下技术利用和人为流体诱发的地震活动方面的社会相关性，例如在地热发电厂、碳捕获和封存以及水力压裂的背景下。

The earthquake swarm area of West-Bohemia/Vogtland was and is an object of intense geoscientific investigations. However, little attention has been paid so far to other small swarm earthquake areas in the surroundings of this main zone of activity as well as to the connection to the Leipzig-Regensburg fault zone. In the project submitted here, we propose to carry out a detailed analysis of the northernmost known earthquake swarm area near Werdau (western Saxony). The goal is to reveal differences and similarities as well as possible connections between both swarm earthquake areas, and finally, to improve our understanding of fluid induced seismicity in intracontinental settings. To image the fluids, which are believed to be the cause of the earthquake swarms, we will invert for the 3D seismic attenuation structure of the Werdau earthquake swarm area using local earthquakes. In a first step, we will apply radiative transfer theory to separate intrinsic absorption and scattering attenuation, to analyze the frequency dependence of attenuation, as well as to invert for detailed source spectra and for frequency dependent seismometer site response factors. To solve the elastic equation of radiative transfer we will apply, for the first time, an improved Monte-Carlo algorithm, which is able to separately invert for intrinsic and scattering attenuation in different depths. To do so, we will extend one of the leading existing algorithms to allow the computation of energy propagation in arbitrary depth dependent velocity and attenuation models. Such kind of algorithms will be useful beyond this particular project, to obtain reliable 1D attenuation models of the Earth's crust. In a second step, we will conduct a 3D attenuation tomography for P- and S-waves, to detect areas of increased fluid content. The combination of the two techniques of radiative transfer theory and attenuation tomography is another technical goal. The respective strengths of the two methods will be combined for the first time, to develop an optimized joint inversion strategy. Results will be compared with existing velocity models (vP and vP/vS ) and jointly interpreted. Such techniques to image fluid flow in the subsurface and its joint interpretation with fluid induced seismicity will play a major role also beyond this project. Especially, we refer to the social relevance with respect to the technical use of the subsurface and to anthropogenic fluid induced seismicity, e.g. in the context of geothermal power plants, carbon capture and sequestration, as well as hydraulic fracturing.