Here’s a good question:- why don’t these rocks fall over?
The above is not the only example of precariously-balanced rocks in Nevada or California. However, they are also near one of the world’s most famous earthquake zones, so the question gains a little more momentum.
I know what I’m about to type, and let me say that it sounds perfectly bizarre to my mental ears, yet all the evidence says that it is true, and the San Andreas Fault is a perfect example.
The San Andreas Faultline is a literal tear in the fabric of the rocks of California. It is described as a “Tectonic Fault” (from the Greek for ‘carpenter’). (I’m fascinated by Tectonics, because I was never taught it at school; it was not known of then).
The surface of the planet consists of Tectonic Plates which move. Some move apart (centre of the Atlantic ocean), some move together (India towards Asia) and some, as with the San Andreas Fault, are slip-lines (they move horizontally past each other).
The San Andreas Faultline essentially runs North-South (more accurately, north/northwest-south/south-east) (NNW/SSE). Some of the northern section is off-shore & runs under the sea, but the central & southern sections run through the state of California, including within 35 miles (56km) of Los Angeles (the fault was responsible for the 1906 earthquake that destroyed Los Angeles, plus many others).
The 2 tectonic plates involved are:
The 2 plates are slipping past each other at a rate of about 33 to 37mm (1.3 to 1.5 in) a year. The North American plate is heading SSE (‘down’ on the map), whilst the Pacific plate is heading NNW (‘up’ on the map). As we are talking of rock rather than butter or oil, the plates tend to stick, then suddenly move. It is that sudden movement that causes the earthquakes.
I recall a TV program where they showed a fence built by a farmer, and unfortunately straight across the fault. It was built many years ago & had broken in the middle. The 2 ends were now a big distance apart.
In general, such balancing rocks are not seen within 15km of major faults (for obvious reasons - they fall over). However, 10 years ago Professor Jim Brune and his colleagues found two sizeable collections of such stones just 7-10km from the San Andreas and San Jacinto faults, in the San Bernardino mountains of California. Prof Brune & his team have spent the intervening time cataloguing & measuring all these stones to try to find why on earth they were still standing. Their results are due to be published within Seismological Research Letters.
Using various techniques, their measurements suggest that the rocks should have fallen over during quakes as recent as 1812 and 1857. But did not. Indeed, an age study on a particular rock suggested that it had continued in it’s position for 18,000 years.
The study’s lead author is Prof Lisa Grant Ludwig, from the University of California, Irvine. She says:-
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 rocks.
The southern part of the San Andreas Fault bends to the north. This is called the “Big Bend” and is thought to be responsible for the greater number of earthquakes suffered in the south. It is thought that, in the long-term, seismic activity is gradually shifting from the southern stretch of the San Andreas fault across to the younger San Jacinto fault, and that that may also play a part in these “jumping earthquakes”.
--------- Alex Kemp