Wow, my 'rehash' articles are doing well. I've got a huge number of new people from Asia and the Pacific Ring of Fire, who are trying out their English skills. As I've said, if you are a bright person, get out of the trap of where you were born, and learn English with a passion. Then the Internet can lift you up.
Although you can't tell this to a person whose house has fallen on him, earthquakes are essential to life on Earth. Actually, they aren't really, but they are intimately linked to plate tectonics, and the properties of quartz and water. As we know, if the Cosmic Creators had fiddled with any one of these physical properties, we wouldn't be here to discuss them
We all know our Plate Tectonics, turn that off and we become a dead planet, like Mars. My readers on the Rim are aware of all the volcanoes, earthquakes, and tsunamis, and they live with them. Every year they may kill 20,000 people out of billions. I think my drive to the cottage has a greater risk of my shuffling off this mortal coil. I'm going down to Guatemala this year to stare at the active volcanoes. Yeah!
So, there are two distinct mechanisms for earthquakes in this world, although my buddies in the busgus only acknowledge this first one. That's where there is a zone of tremendous weakness, and all the strain concentrates there. These fault zones have rock strengths of orders of magnitude weaker than the surrounding rock. If you go to Callie extreme, you can say that these zones 'cause' earthquakes.
But really, they are only responding to the surrounding strain energy. There are countless sub-categories of the 'weak' zones, such as subduction zones, transform faults, thrust zones such as Nepal, etc.
The basic mechanism is that this fault has a finite shear strength (or shear strain limit) and it accumulates the strain of the region, whether it is compression, tension or shear. The regional strain change is huge, perhaps 10-4 (no units) per year, and can be easily measured with GPS. The weak zones take it all, and when the limit is reached, they slip. If we didn't have any quartz or water, the fault zones would just creep and the only people who got hurt would be those that built on a creeping fault zone.
But unfortunately, the quartz-water combination brings on stick-slip, and we have big earthquakes.
-to be continued