Full waveform seismological advances for microseismic monitoring

The observation of microseismicity has raised the interest of the seismological and geoengineering communities in the last decades, and a significant effort has been spent to develop or improve methodologies able to perform seismological analysis for weak events at a local scale. This effort was accompanied by the improvement of monitoring systems, resulting in an increasing number of large microseismicity catalogs. The interest in microseismicity, involving a synergy among different scientific communities, is in part due to their occurrence both in consequence of natural processes in active regions, swarm areas, hydrothermal and volcanic environments, but also in relation to human activities, e.g., in proximity of mining areas, geothermal systems, oil and gas fields, and water reservoirs. The analysis of microseismicity is challenging, because of the low amplitude and high-frequency content of recorded seismic signals. Whereas many techniques only rely on a minor information provided by microseismicity data, full waveform recordings contain a broad information on the physical processes at the microseismic source as well as on the properties of the surrounding media. This paper reviews recent methodological developments in seismology, which exploit full waveform recordings, upon the analysis of their amplitude, frequency, duration, and polarization properties or the direct modeling of their waveforms, to infer microseismic source properties. We specifically review recent advances targeting the problem of the detection of weak microseismic signals, the automated location using full waveforms, the determination of source parameters through focal mechanisms, moment tensor, and finite source inversion, and the classification of microseismicity waveforms and their attributes. A particular care is given to the assessment of the automation potential of these methods, which is a requirement toward the analysis of massive microseismicity data sets. The review discusses the potential and limitations of many developed techniques, and highlights recent promising ideas, which can significantly contribute toward a better understanding of microseismic processes in the next future.