Quantifying Foreshock Anomalies: Insights from Envelope Waveforms

Predicting large earthquakes remains a complex and critical challenge in seismology. This study investigates distinctive seismic precursors by analyzing unique waveform patterns in foreshock sequences. Using the 2011 Mw9.1 Tohoku earthquake as a case study, preceded by a Mw7.3 foreshock, we identified an anomalous sawtooth pattern in the ground velocity envelope following the foreshock. Unlike typical post-earthquake recordings, this pattern is interpreted as evidence of the locked state of the mainshock fault, which suppresses the foreshock’s ability to trigger aftershocks.To quantify these waveform anomalies, we developed the index Q based on the first 45 minutes of waveform recordings. Applying this method to 75 Mw6+ earthquakes recorded globally since 2010, our approach correctly identified 10 out of 11 foreshock sequences that preceded larger earthquakes within 10 days. Only 7 out of 64 remaining earthquakes were misclassified, highlighting the robustness of the method. Our findings suggest that these sawtooth patterns are reliable indicators of impending large earthquakes, offering a novel tool for seismic forecasting. By integrating this method with other geodetic and seismological datasets, we aim to enhance hazard assessment and mitigation strategies, contributing to improved preparedness for future seismic events. References1Lippiello, E., Petrillo, G., Godano, C., Tramelli, A., Papadimitriou, E., & Karakostas, V. (2019). Forecasting of the first hour aftershocks by means of the perceived magnitude. Nature communications, 10(1), 2953.