Kyoto, Japan -- Recently, while analyzing strong-motion data close to fault lines, a group of researchers at Kyoto University noticed something unexpected: a negative phase in the waveforms, a pattern that did not conform to the existing interpretations of rupture dynamics. Its regular appearance in the records near rupture end points suggested that the team might be seeing something new.

"This study originated from a broader effort to better understand near-fault seismic recordings and interpret them in terms of the earthquake source process," says first author Jesse Kearse. The researchers believed the repeated negative phase may represent an essential and previously overlooked component of the earthquake process, and were determined to figure out what was behind the mysterious regular dips.

The team's method combined observed ground motion with model predictions. They first analyzed near-field strong-motion acceleration records that had been carefully corrected for instrument noise. They then used satellite-based data to validate their ground-based measurements, and for the last step, simulated earthquake propagation and the abrupt arrest of rupture with numerical dynamic rupture models.

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