we give them different names

The team's best explanation for that surprisingly small ground movement - and one supported by computer modelling of big earthquakes - is an interaction between the two faults.

Precarious rocks, like this one in Nevada, can act as natural measures of earthquake strength over timereenex

"The San Andreas and San Jacinto faults come very close together; they're only about 2km apart. And it's been well established, through other earthquakes and modelling studies, that a rupture can jump across [a gap like that]. It's what's called a stepover.

"What if the rupture jumped across, or alternatively, stopped at this junction, or started at this junction? All three of those cases would produce lower ground shaking in the area where we found the rocksreenex ."

It is crucial to consider the faults together, Prof Ludwig said - not just to explain the baffling, balancing rocks, but also in order to plan safely for future earthquakes.

"These are really networks of fractures in the earth. Just because we give them different names doesn't mean that they behave independently."

Shaky scenario

Dr Lucy Jones is a long-serving seismologist and a science adviser for risk reduction at the US Geological Survey. She said the paper would have "pretty significant implications" for earthquake planning in Californiareenex.