We could name super-earthquake a hypothetical large scale quake leading to full destructions of human built structures on the all surface of the Earth. No such quakes happened in human history and the only scientifically solid scenario for them seems to be large asteroid impact.
Such event could not result in human extinction itself, as there would be ships, planes, and people on the countryside. But it unequivocally would destroy all technological civilization. To do so it should have intensity around 10 in Mercally scale http://earthquake.usgs.gov/learn/topics/mercalli.php
It would be interesting to assess probability of worldwide earthquakes, which could destroy everything on the earth surface. Plate tectonics as we know it can't produce them. But distribution of largest earthquakes could have long and heavy tail which may include worldwide quakes.
So, how it could happen?
1) Asteroid impact surely could result in worldwide earthquake. I think that 1 mile asteroid is enough to create worldwide earthquake.
2) Change of buoyancy of large land mass may result whole continent uplifting may be in miles. (This is just my conjecture, not proved scientific fact, so the possibility of it needs further assessment.) Smaller scale event of this type happened in 1957 during Gobi-Altay earthquake when the whole mountain ridge moved. https://en.wikipedia.org/wiki/1957_Mongolia_earthquake
3) Unknown processes in mantle sometimes results in large deep earthquakes https://en.wikipedia.org/wiki/2013_Okhotsk_Sea_earthquake
4) Very hypothetical changes in Earth core also may result in worldwide earthquakes. If core somehow collapse, because of change of crystal structure of iron in it or because of possible explosion of (hypothetical) natural uranium nuclear reactor in it. Passing through clouds of dark matter may result in activation of Earth core as it could be heated by annihilation of dark matter particles as was suggested in one recent research. http://www.sciencemag.org/news/2015/02/did-dark-matter-kill-dinosaurs
Such warming of the earth core will result in its expansion and may trigger large deep quakes.
5) Superbomb explosion. Blockbusters bombs in WW2 was used to create miniquakes as its main killing effect, and they explode after they penetrate ground. Large nukes may be used the same way, but super earthquake requires energy which is beyond current power of nukes on several orders of magnitude. Many superbombs may be needed to create superquake.
6) The Earth cracks in the area of oceanic rifts. I read about suggestions that oceanic rifts expand not gradually but in large jumps. This middle oceanic rifts creates new oceanic floor. https://en.wikipedia.org/wiki/Mid-Atlantic_Ridge The evidence for it is large “steps” in ocean floor in the zone of oceanic rifts. Boiling of water trapped into the rift and contacted with magma may also contribute to explosive zip style rapture of the rifts. But this idea may be from fridge science catastrophism so should be taken with caution.
7) Supervolcano explosions. Largescale eruptions like the Kimberlitic tube explosion would also produce earthquake which will be felt on all earthquake, but not uniformly. But they must be much stronger than Krakatoa explosion in 1883. Large explosions of natural explosives at the depth of 100 km like https://en.wikipedia.org/wiki/Trinitrotoluene were suggested as a possible mechanism of Kimberlitic explosions.
1.Superquake surely will come with megatsunami which will result in most damage. Supertsunami may be miles high in some areas and scenarios. Tsunamis may have different ethology, for example resonance may play a role or change of speed of rotation of the Earth.
2.Ground liquefaction https://en.wikipedia.org/wiki/Soil_liquefaction may result in “ground waves”, that is some kind of surface waves on some kind of soils (this is my idea, which should be more researched).
3. Supersonic impact waves and high frequency vibration. Superquake could come with unusual patterns of wavering, which are typically dissipate in soil or not appear. It could be killing sound – more than 160 db, or shock supersonic waves which reflect from the surface, but result in destruction of solid surface by spalling the same way as antitank munitions do it https://en.wikipedia.org/wiki/High-explosive_squash_head
4. Other volcanic events and gases release. Methane deposits in Arctic would be destabilized strong greenhouse methane will erupt on surface. Carbon dioxide will be released from oceans as a result of shaking (the same way as shaking of soda can result in bubbles). Other gases including sulfur and CO2 will be realized by volcanos.
5. Most dams will fall resulting in flooding
6. Nuclear facilities will meltdown. See Seth Baum discussion http://futureoflife.org/2016/07/25/earthquake-existential-risk/#comment-4143
7. Biological weapons will be released from facilities
8. Nuclear warning system will be triggered.
9. All roads and buildings will be destroyed.
10. Large fires will happen.
11. As natural ability of the earth to dissipate the seismic waves will be saturated, the waves will reflect inside the earth several times resulting in very long and repapering quake.
12. The waves (from the surface location event) will focus on the opposite side of the Earth, as it may be happened after Chicxulub asteroid impact which coincide with Deccan traps on opposite side of the Earth and result in comparable destruction where.
13. Large displacement of mass may result into small change of the speed of rotation of the Earth, which would contribute to tsunamis.
14. Secondary quakes will follow, as energy will be realized from tectonic tensions and mountain collapses.
Large, non-global earthquakes also could become precursors for global catastrophes in several occasions. The following podcast by Seth Baum is devoted to this possibility. http://futureoflife.org/2016/07/25/earthquake-existential-risk/#comment-4147
1) Destruction of biological facilities like CDC which has smallpox samples or other viruses
2) Nuclear meltdowns
3) Economical crisis or showering of tech. progress in case of large EQ in San Francisco or other important area.
4) Starting of nuclear war.
5) X-risks prevention groups are disproportionally concentrated in San Francisco and around London. They are more concentrated than possible sources of risks. So in event of devastating EQ in SF our ability to prevent x-risks may be greatly reduced.