Recent Advances and Challenges of Waveform-Based Seismic Location Methods at Multiple Scales

Source locations provide fundamental information on earthquakes and lay the foundation for seismic monitoring at all scales. Seismic source location as a classical inverse problem has experienced significant methodological progress during the past century. Unlike the conventional traveltime-based location methods that mainly utilize kinematic information, a new category of waveform-based methods, including partial waveform stacking, time reverse imaging, wavefront tomography, and full waveform inversion, adapted from migration or stacking techniques in exploration seismology has emerged. Waveform-based methods have shown promising results in characterizing weak seismic events at multiple scales, especially for abundant microearthquakes induced by hydraulic fracturing in unconventional and geothermal reservoirs or foreshock and aftershock activity potentially preceding tectonic earthquakes. This review presents a comprehensive summary of the current status of waveform-based location methods, through elaboration of the methodological principles, categorization, and connections, as well as illustration of the applications to natural and induced/triggered seismicity, ranging from laboratory acoustic emission to field hydraulic fracturing-induced seismicity, regional tectonic, and volcanic earthquakes. Taking into account recent developments in instrumentation and the increasing availability of more powerful computational resources, we highlight recent accomplishments and prevailing challenges of different waveform-based location methods and what they promise to offer in the near future. Plain language summary Earthquakes are a common physical phenomenon involving ground shaking and rupturing of the surface of the Earth. In addition to the well-known catastrophic tectonic earthquakes, similar vibration sources also appear at various scales in engineering fields, such as acoustic emissions resulting from microcracks in building walls and bridges, rock bursts in mines, microseismic events generated by mining and fluid injection/extraction, and microseisms caused by crustal activity. Seismic information provides a powerful tool for geophysical and engineering surveys. The source location describes the spatial and temporal extent of an earthquake and lays the foundation for seismic monitoring. Seismic location methods have made significant progress over the last century. Specifically, a category of new waveform-based location methods has emerged as a counterpart of conventional traveltime-based inversion. These methods directly utilize the notion of a wavefield and, very similar to an optical lens, aim at spatially focusing a source's emitted energy. Waveform-based methods have provided robust and effective source location results at various scales. We summarize the development history and current state of waveform-based location methods and discuss the advantages and challenges through their applications for seismic source location at multiple scales.