This earthquake was triggered by a strike-slip fault, where two large sections of the Earth's crust move horizontally past each other along a vertical fracture. To someone watching, it would appear as if the ground had split along a clear line, with each side being forced in opposite directions.

Previous studies based on seismic recordings suggested that earthquakes like this may involve a pulse-like rupture and slightly curved motion along the fault. However, those conclusions were based on instruments located far from the fault zone, meaning the observations were indirect.

Rare CCTV Footage Captures Fault Movement

In this case, a CCTV camera recorded the fault as it moved, creating a rare opportunity for researchers at Kyoto University to observe the rupture as it happened. (See video link at bottom of article.) This kind of direct visual evidence is extremely unusual in earthquake research.

Frame-by-Frame Analysis Reveals Extreme Speed

The research team used a method called pixel cross-correlation to examine the video frame by frame and measure how the ground shifted. Their findings show that the fault moved sideways by 2.5 meters in just 1.3 seconds, reaching a top speed of 3.2 meters per second.

While this amount of sideways movement is typical for strike-slip earthquakes, the very short duration of the motion stands out as a significant discovery.

"The brief duration of motion confirms a pulse-like rupture, characterized by a concentrated burst of slip propagating along the fault, much like a ripple traveling down a rug when flicked from one end," says corresponding author Jesse Kearse.

Curved Fault Motion Challenges Assumptions

The analysis also revealed that the path of the slip was slightly curved. This matches earlier geological observations from faults around the world and suggests that fault movement is often not perfectly straight, as commonly assumed.

The study highlights the value of using video footage to monitor fault activity, offering a new way to study earthquakes in detail. Observations like these can improve understanding of how earthquakes unfold and help scientists better estimate the shaking that may occur during future large events.

"We did not anticipate that this video record would provide such a rich variety of detailed observations. Such kinematic data is critical for advancing our understanding of earthquake source physics," says Kearse.

Next Steps in Earthquake Research

The researchers plan to build on these findings by using physics-based models to explore what controls fault behavior, using the new data revealed by this analysis.