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I am to deliver an informative speech about earthquakes in class. However, I was not the first to talk about earthquakes and his discussion of the cause of earthquakes is different from mine.

He mentions that there are multiple causes of earthquakes like a volcanic eruption, release of seismic waves, and tectonic plate movement.

However, from the information I gathered from the United States Geological Survey, it states that the cause is the movement of tectonic plates. The roughness of the plate boundaries cause them to get stuck; thereafter, when the plates move far enough, the boundaries unstick, releasing seismic waves, causing an earthquake.

Is the movement of tectonic plates one of the causes or the volcanic eruptions, etc. are the cause of the sudden slip or the part when the boundaries unstick which cause the earthquake?

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When you look at a table of all the major historic earthquakes they are all related to plate tectonics, either directly or indirectly. These are all of the magnitude 7 to 9 events, and most of the magnitude 4 to 6 events. The linkage with volcanic eruptions is real, but weak. Again, most but not all, of the world's volcanoes are generated by plate tectonic processes, the main exceptions being intra-plate movement over mantle hot spots. As we get down to lesser magnitudes we are getting into greater frequencies. It is roughly a logarithmic relation ship, so for every major earthquake, there are thousands of low to medium level earthquakes and millions of small micro-seismic events - the latter being so small that nobody notices - except for geophysicists. There is a host of processes which give rise to low-level seismicity, such as submarine slope failures, turbidites, sedimentary compaction, thermal contraction, magma redistribution at depth, erosive load shedding, after-shock adjustments, landslides, mudslides, rock-falls, rapid draw-down in reservoirs, mining subsidence, fracking, and probably a load of other stuff that I can't think of right now.

So it's roughly a matter of scale. The USGS is correct that serious earthquakes are all caused by plate tectonics. On the other hand, your other source was right, that some seismicity can be caused in a multitude of ways.

Afterthought: Probably someone will point out very rare exceptions, such as major bolides crashing into Earth and causing shock waves right through the planet. But such are extremely abnormal.

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  • $\begingroup$ I would re-iterate, most major damaging quakes are tectonic slippage, and most quakes caused by other sources tend to be smaller and localized. Sure, if the Yellowstone Caldera is capable of and has caused significant quakes through magma shifts, but large quakes triggered by non-tectonic actions are much rarer. One concern to many is the Oklahoma fracking quake frequency and size seems to be substantially increasing which may change some observations. $\endgroup$ – dlb Aug 23 '16 at 21:15
  • $\begingroup$ One could look at it like this: Plate tectonics is the driving mechanism. But it is not continuous. At a plate interface the inter-plate movement is jammed by friction, so the strain energy just keeps on building up until something has to give, i.e. the earthquake slippage occurs, thereby releasing all the accumulated strain energy at once. Just before that point is reached, almost anything can tip the balance - the proverbial straw that breaks the camel's back. It can be a storm, groundwater lubrication of a fault plane, volcanoes, landslides, tidal stress, or any other nudge. $\endgroup$ – Gordon Stanger Aug 24 '16 at 11:33

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