好问题!我会制定普遍的想法,最初提出的(我认为)[赛克斯,1978][1]和支持在许多作者大量的后续工作。然而,还有其他的模型。在最一般的意义上:内部断裂是由于旧的,预先存在的弱点,重新激活由于板内部非常接近极度紧张。换句话说,他们是在板的内部弱点。然而,我们不完全理解为什么有些老断层当别人在同一方向不激活。(它可能有很多与孔隙压力和流体运动深度。)有很多我们不完全了解内部地震。另外,请不要使用术语“断层线”。它冒犯了我的地质情感! :) (Faults are 2D surfaces, not lines, thus the nitpicky objection.) Most of the crust is close to being critically stressed (breaking) --- There's a lot of evidence that the continental crust everywhere is close to being critically stressed -- in other words, failing in an earthquake. (e.g. see the various [Townend and Zoback][2] papers.) The crust isn't just critically stressed near the plate boundaries, but also away from them. This may also be true for oceanic crust, but we don't have as much data there. Another way of saying this is that the plates are being pushed along as hard as they can without crumpling. Basically, this is why things like building a dam or deep fluid injection trigger small earthquakes. The crust is already close to failure, so adding load or increasing pore pressure often triggers small fractures. (However, these sort of things can't trigger a major earthquake unless they're on an already active fault. You need continuous slip along a _very_ large surface for a large earthquake.) The plates being critically stressed is actually is what we'd expect. We see deformation at plate boundaries whenever they collide. Therefore, we know that the stresses driving the plates are greater than the "breaking" strength of the crust. Because stresses from a collision at the boundary will be transmitted to the interior, it intuitively makes sense that the interiors should be close to, but just below (otherwise they'd be deforming), the failure threshold. (For the complete argument, see [Zoback and Townend, 2001][3]) Continental crust is full of old zones of weakness --- The continents are very old. Much older than the oceanic crust. The continents are like rafts that keep bumping into each other. Therefore, they have lots of "scars" (i.e. old fault zones). There's a saying in structural geology: "once a weakness, always a weakness". Fault zones are typically much weaker than the crust around them. Once a fault breaks through, it tends to be a permanent weakness that will be reactivated even if it's not in a perfectly favorable orientation. Critically stressed crust + weak zones --> intra-plate earthquakes. But why don't we see activity on all old fault zones? ------ Short answer, we don't really know. It probably has a lot to do with pore pressure and fluid movement. Some old fault zones have intra-plate earthquakes, while others don't, even if they're in the same orientation. Pore pressure plays a _huge_ role in faulting (it make rocks much easier to break). There's a lot of speculation that active intra-plate fault zones may have more fluids at depth, though explaining why is difficult. [1]: http://onlinelibrary.wiley.com/doi/10.1029/RG016i004p00621/full [2]: http://geology.gsapubs.org/content/28/5/399.short [3]: http://www.sciencedirect.com/science/article/pii/S0040195101000919