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.
By comparing ground data with model predictions, the researchers were able to link the repeated negative phase to the earthquake stopping process. This led to the discovery of a previously unrecognized feature in near-fault seismic records of large earthquakes: a distinct stopping phase in the ground motion.
Kearse and co-author Yoshihiro Kaneko found that the negative phase represents a systematic signal associated with the termination of rupture, demonstrating that many near-field recordings of strike-slip ruptures contain this coherent stopping phase. This is most commonly observed near the end points of the rupture, linking it directly to the earthquake stopping process.
These findings provide new insight into rupture arrest, which is difficult to observe directly while it is occurring. However, the team found that strong stopping-phase signals are generated most efficiently during abrupt rupture arrest, when a rupture stops suddenly rather than slowing down to a stop more gradually. Their discovery of this radiation of stopping phases results in the most effective way to study this critical stage of natural earthquakes.
The recognition of a stopping phase has important implications for seismic hazard near strike-slip faults. The stopping phase produces long, whiplash-like ground motions that are a particular challenge to engineer against, so hazard models will need to explicitly account for these motions, especially near expected rupture endpoints and internal segment boundaries, where stopping is most likely to occur.
In the future, the researchers plan to continue investigating the global catalog of near-fault observations to gain insight into the dynamic rupture process of large earthquakes.
