Space-time variations of seismicity: quantitative assessment and systematic changes before large earthquakes

We discuss recent results aimed at robust identification and quantification of space-time variations of earthquakes, with the ultimate goal of tracking preparation processes of large earthquakes. The first part focuses on progressive localization of seismicity, which corresponds to mechanical evolution of deformation from distributed failures in a rock volume to localized shear zones, culminating in generation of primary slip zones and large earthquakes. We present a methodology for estimation of localization using earthquake catalogs and acoustic emission experimental data, and showcase its applications to tracking localization processes of large failure events. This analysis is performed with declustered catalogs. The second part describes a technique to assess the degree of regional clustering of earthquakes, and justifies the need for declustering in localization and other analyses of seismicity. We demonstrate that events included in the existing short-duration instrumental catalogs are concentrated strongly within a very small fraction of the space-time volume, which is highly amplified by activity associated with the largest recorded events. The earthquakes that are included in instrumental catalogs are unlikely to be fully representative of the long-term behavior of regional seismicity, creating a bias in a range of seismicity analyses. Methodologically, both discussed topics are based on using the Receiver Operating Characteristic (ROC) framework. We demonstrate how this unified framework is adopted for diverse tasks, including assessment of coupled space-time clustering after controlling for space and time marginal inhomogeneities of earthquake rates, and tracking time-dependent transformations of a highly inhomogeneous earthquake space distribution. The examined data include crustal seismicity in California, Alaska and other regions, synthetic catalogs of the ETAS model, and acoustic emission data of laboratory fracturing experiments.