Wednesday, September 22, 2021

A tutorial for the Australian Earthquake

 Dear Australia - you are on a craton, which is a continental landmass. As such, the earthquakes follow the rules of Stable Cratons.  Not anything that the USGS has put out.  In my former job, I read everything there was about this, and then came the Oklahoma earthquake sequence.

It may very well be that your geology institutions follow the USGS religiously, like New Zealand.  In that case, no need to read any further, please worship the ground under their feet.

The usgus believes that faults are actually the source of the seismic motion.  Therefore, mapping every single fault actually tells you something.  All their funding is based on this.  On a Stable Continental Region (scr), there are millions of old faults, all of which don't mean anything.  Go, and map all those faults if you wish.

The scr has a lot of stress, which is potential energy.  All scr's have been pummelled, stretched and compressed over a billion years.  In other words, they have had the poop knocked out of them, and yet they have high horizontal stresses, aligned to the general grain of the rock.  That means there are thousands of faults, out of millions, that are stressed at their maximum and ready to let go.

The above report lists that scr's will let go at a very low rate, and the maximum earthquake is most likely an M6.  That's a slip radius of about 3 km, and that size is probably limited by the complexity of fractures.  The next size up is an M7 which has a radius of 30 km and goes right through the crust.  These are the standard earthquakes of the plate edges, or major continental structural zones.

-- to be continued, if I get a kangaroo in the mail.

ps.  no, I'm not continuing for anything.  It's like pounding my head against the oky-brick-wall, except this time I'll be dangling upside-down.

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