re: #58 Love-Child of Cassandra and Sisyphus

Hey! Just wanted to follow up on your post last night/question about injection well seismic events and structural, geological, earthquakes like the strike/slip Andreas Fault quakes.

So, again, it’s complex question. Remember the Crandall Coal Mine collapse back in ‘07 in Utah?

Killed six initially and three more during a rescue attempt and was reported by seismometers as a 3.9 Richter event. So, a lot more energy released than the rupture near Smiley, TX which was a 3.3.

In any case, these were “point” events- geologically isolated, restricted in area and scope of damage. In this, they were similar to subsurface nuclear detonations. In fact, during the Cold War, we got really good at telling how big a BOOM an N-test was, just by how the ground shook. It allowed us to to say that the NorKs had tested a bomb with a yield of XX kilotons just by the shake from that individual area and event. If you were to look at the damage zone, it would look like a small bullseye circle

Now, a seism along an active geologic fault is a completely different animal.

Dip-slip faults, where Oceanic crust is forced under Continental crust are the worst. The leading edges of the Continental crust is forced down, like a spring, along the entire zone of subduction, which can be hundreds of miles long. The leading top edge of the Continental crust may be several hundred feet lower than the top of the crust 50 miles further away from the Subduction zone. When it snaps (ruptures), it could be a rupture to relieve some of the stress along a small area of the subduction zone with limited damage.

But if the subduction zone has built up enough stress on it’s entire length, it can be a catastrophic 8.9 to 9.4 quake, where the entire stressed Continental crust snaps upward 2-300 feet in a matter of seconds for 800 miles or more. Think Indonesia, where a wall of water 75 feet tall burgerized Sri Lanka across the Indian Ocean, or Japan, where walls of water wiped out Sendai and the nuclear power plants belonging to TEPCO on the east cost of Honshu several hundred miles south.

Or the Cascadia Fault rupture in 1700 which killed thousands in Japan.

But you asked about strike/slip faults like the San Andreas, where a chunk of former Pacific Ocean crust and a separate chunk of Continental crust has fused and instead of being subducted under the North American plate like farther north in Cascadia has been grinding to the NxNW against the NA plate on a rough line from the northern end of the Gulf of California to Point Reyes.

Dip/slip faults like the Cascadia build their stress/energy like a diver on a diving board, maximizing when the diver pushes the board down as far as he/she can. to get maximum height on their release.

Strike/slip faults build their energy horizontally. First, the original bodies to the east and west of the Fault weren’t ruler straight. The San Andreas looks kinda straight, but there a lot of zigs, zags, and jogs in it. If the fault was ruler straight, the only source of tension would be the walls grinding against each other, and the inch or two of differential motion each year would result in a long continuous series of minor quakes as the plates grind past each other.

But the zigs, zags, and jogs allow energy to build up for decades to centuries. When they rupture, it can be really bad, up to an 8 to an 8.2 quake. But a Continental strike/slip fault will never build up the kind of energy the way a Subduction dip/slip fault can. For one thing, the San Andreas is a fault complex, and keeps breaking off chunks on one or the other side the original fault line to form sub blocks, which relieves tension to the entire system. The east side of the Hayward Fault, to the east of the San Andreas, used to be on the west side of the San Andreas, but splintered off and moved more northerly from the San Francisco peninsula, forming the San Francisco Bay.

Hope this helps, and night all.