Newly observed negative-phase waveforms in earthquake data have alerted scientists to a previously unrecognized feature of the "big ones" to do with tectonic plates slipping by each other: they create a kind of geological 'whiplash' that happens when a rupturing fault movement abruptly stops.
The discovery came as Kyoto University researchers were looking at strong-motion data close to fault lines, in an effort to better understand earthquake occurrence and behavior.
They noticed that at a certain point the waveforms collected by scientific data during quakes actually veered into a negative phase – an unexpected pattern to be seen in what we know from this plate activity. Those waves appeared near the end points of an earthquake event, revealing unknown intel on how and what happens when tectonic movement stops.
Effectively, the ground near the fault line doesn't just shift in one direction – it overshoots, then jerks back briefly in the opposite direction. It's a bit like the way your car lurches forward on its suspension when you stop it hard on the brakes, and then springs backward, pushing you back in your seat.
"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.
These negative phase waveforms could have something to do with severity of the event felt by us, far above the action – in particular, in large strike-slip earthquakes – as the researchers note that it's particularly difficult to engineer protections against this reverse movements into buildings and the like.
As the US Geological Survey explains: "Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally." Around the ring of fire – which produces around 90% of the world's earthquakes – non-volcanic, tectonic-plate "grinding" or subduction along boundaries like the San Andreas and Queen Charlotte fault lines can result in sizeable strike-slip events. So what if those newly discovered wave movements hold more clues as to why these quakes can be so disastrous on land?
Using observed ground motion with model predictions, the team essentially paired earthquake acceleration with satellite data, then simulated how such a large strike-slip event could play out if there was some sort of obstruction that abruptly stops movement. Through this, they could see that those waves in negative phase were in fact linked to the plate movement reaching its boundary and halting ground motion.
If you live above a fault line, you will certainly know it – or, rather, feel it. I'm currently on top of where the Philippine Sea Plate, which moves northwest, runs into the Eurasian Plate; Taiwan's east coast sits above one of the world's most "active" sites for tectonic bumping and grinding. In 2024, this kind of strike-slip fault activity resulted in a devastating 7.4-magnitude earthquake. So this kind of data illuminating the very specific behaviors of ground movement is integral for improving on already-strict building codes in earthquake-prone places like Taiwan, New Zealand, Chile and Japan.
The researchers also found that strong "stopping-phase signals" differed between whether an earthquake abruptly came to a stop or slowed down to one, opening the door to a new way of understanding how the physics of this movement affect motion above the surface. Because, the team explains, the stopping phase "produces long, whiplash-like ground motions" that present unique challenges for engineers.
And just like in your car, the quicker an earthquake comes to a stop, the more powerful the backward whiplash becomes.
"Here we present systematic near-field observations of ground-motion stopping phases from large strike-slip earthquakes," the team writes in the study. "Analysis of 12 global events shows that transient overshoot in fault-parallel ground surface displacement is a robust diagnostic signature of abrupt termination of rupture propagation."
The researchers now plan to broaden their study to look at global incidence of large earthquakes, in an effort to better understand the behavior of events in their stopping phase. In the case of earthquakes, like those caused by strike-slip faults, size matters, and their stopping signature could help us better safeguard against their impact.
The study was published in the journal Science.
Source: Kyoto University
Fact-checked by Loz Blain
