Strong shaking at fault stepover has rocks--briefly--defying gravity

He wasn't the only one to observe this phenomenon at Ridgecrest and other places where faults have a similar stepover geometry, in which faults are offset with a "step" or gap between them. But now Lozos, a seismologist at California State University, Northridge and his colleagues have an explanation for how these rocks could have been flung from their original resting places.

At the Seismological Society of America's Annual Meeting, Lozos presented evidence that earthquake rupture within this stepover geometry can cause highly localized strong ground acceleration, stronger than the force of gravity.

"You'll see a hole in the ground shaped like the rock, and then the rock is next to it, but there's no drag mark between the hole and the rock," Lozos explained.

The extremely strong shaking within the stepover area and less than a kilometer on each side of the fault is enough to launch rocks, they concluded.

Some earthquakes cause even stronger ground acceleration. Peak ground shaking during the 1994 Northridge, California earthquake reached about 1.8 times the force of gravity, and more than twice the force of gravity during the 2011 Christchurch earthquake in New Zealand.

But shaking during these earthquakes was spread over a much larger area compared to the extremely localized shaking at a stepover, Lozos noted.

When Lozos and Sinan Akçiz, an assistant professor at California State University, Fullerton discussed their Ridgecrest observations, Akçiz shared that he had seen similar rock displacement in a stepover of the 2010 El Mayor-Cucapah earthquake.

To study the phenomenon, Lozos and Akçiz created models of a simple but classic stepover region, where faults are planar and parallel, overlapping some amount along the fault strike, and having some separation perpendicular to the strike. They published their findings earlier this year.

Stepover models like this have been used to study dynamic rupturing, to determine how likely it might be that an earthquake jumps between fault segments, Lozos explained, "but no one was looking at what this geometry was doing to ground motion."

"We wanted to know if it is the stepover geometry itself, if it is the shape of the fault in and of itself and not the particulars of an individual earthquake that led to this happening with rock displacement," he added.

The researchers also tested whether differences in rupture speed might affect this localized strong shaking. For supershear ruptures, which move faster than the speed of sound, the strongest ground acceleration occurs at the point along the fault where the rupture transitions to supershear speed. The stepover geometry doesn't have any effect, they found.

For slower ruptures, geometry does make a difference. The strongest ground acceleration occurs at the end of the first fault.

On a fault with more straightforward geometry, accumulating friction or release of seismic stress eventually causes the rupture to "run out of steam" and taper off, said Lozos. In a stepover, however, the end of a fault segment might not be the end of the fault zone that has been accumulating seismic stress.

In this case, a rupture that reaches the end of a fault segment "isn't running out of energy, it's running out of fault, so the effect is like slamming really hard into a wall," he said.

The study suggests that ground motion predictions and local seismic hazard warnings should account for much stronger ground motion in the area nearest to the fault around stepover and other types of complex fault geometries, the researchers noted.