Rapid Earthquake Source Characterization Using Real-Time-Matched Filters (RT-EQcorrscan): Application to Aotearoa New Zealand Aftershock Sequences

Abstract The early aftershock period can provide information on the mainshock source and on the time-varying hazard during an aftershock sequence. However, early aftershock catalogs are often highly incomplete and inaccurate due to elevated rates of aftershocks and high-background seismic amplitudes from overlapping event waveforms. Template-matching (matched-filter) methods, based on cross-correlation of known earthquake waveforms with continuous data, have been shown to significantly improve the completeness of early aftershock catalogs and can generate automatic high-quality phase-arrival information. Until recently, applying matched-filters in real time was not tractable. By applying and updating recently developed robust and efficient near-real-time open-source matched-filter workflows and tools (RT-EQcorrscan), we provide improved real-time location and magnitude calculation processes to characterize the earthquake source during earthquake rapid responses. Here, we outline the development, testing, and application of our methods and demonstrate the capability of RT-EQcorrscan and new associated workflows to enhance rapid aftershock catalogs and source characterization. We provide examples of these methods as applied to simulated real-time scenarios of recent M 6+ sequences across Aotearoa New Zealand: the M 6.2 Eketahuna, M 6.5 Cook Strait, and M 7.8 Kaikōura earthquakes. Our results show that the distribution of detected and relocated aftershocks, estimated in near-real time, can be used to estimate source fault geometry (strike, dip, length, and width) and scaled magnitude within a few minutes to an hour of a trigger event, depending on source complexity and background seismicity rates (available templates). We also discuss several nuances that distinguish real-time applications of matched-filters from more commonplace retrospective analyses, thus highlighting considerations for applying those real-time tools during a response. Our outputs—both updated plug-ins for RT-EQcorrscan, alongside tools for quantification of fault geometry from aftershock distributions—provide valuable resources for use alongside other source characterization methods during earthquake response.