[Rockhounds] inverse seismometer

[Kreigh Tomaszewski]

*We've all seen them; we've even taken pictures of ourselves pretending to
hold them up or to push them over.*

These are the precariously balanced rocks on a hill or a coastal cliff.
It's as if the gentlest nudge would send them tumbling.

In truth, the disturbance needed to unsettle the blocks is quite
significant, and that got husband and wife geologists Drs Dylan and Anna
Rood wondering about how these great stones could be used to decipher
earthquake history.

Think about it: if a precariously balanced rock has held its position for
10,000 years without tipping over, it means the land around the stone
hasn't experienced shaking above a certain level in all that time.

"The turn of phrase we're trying to coin is that these precariously
balanced rocks, or PBRs, are an 'inverse seismometer'," explains Anna.

"A normal seismometer records an event that *has* happened, whereas our PBR
is still standing there, and so it records an earthquake that *hasn't*
happened.
Specifically, a large earthquake," the Imperial College London, UK,
researcher tells BBC News.

This is really useful information if you want to build a nuclear power
station or waste repository; or maybe a major dam or bridge.

https://www.bbc.com/news/science-environment-54374465