Earthquake nucleation, rupture physics and predictability

7 problems

• How do earthquakes occur at depths of 300 to 700 km?

  Contested

  Some earthquakes happen hundreds of kilometers down, where rock is so hot and compressed that ordinary cracking and frictional sliding should be impossible. Which process lets rock fail suddenly there is disputed.

• Which dynamic weakening mechanism controls friction during earthquake slip?

  Partially resolved

  When fault walls slide at about a meter per second, friction drops sharply. Several processes could cause this and it is unknown which one dominates deep in real faults.

• Is an earthquake's final magnitude determined in its first seconds?

  Contested

  A small and a giant earthquake may start out looking identical. Whether the first seconds of shaking already reveal how big the quake will get decides how useful early warning can be.

• Why do mature faults slip at low stress without measurable heating?

  Partially resolved

  Rock friction measured in the lab predicts that a sliding fault like the San Andreas should heat the ground around it. No such heat is seen, so the fault must be weaker than lab rocks.

• Do large earthquakes have a detectable preparation phase?

  Contested

  Lab experiments show faults creep slowly before they snap, but it is unclear whether real large earthquakes do this in a way instruments can see. Some big quakes had small earthquakes or slow slip before them, others did not.

• How predictable are large earthquakes beyond clustering statistics?

  Open

  Earthquakes cluster in time, which allows rough probability forecasts. Whether any physics or data can do much better than that is unknown.

• What decides whether a fault slips slowly or in earthquakes?

  Open

  Some faults release stress in slow slip events lasting days to months, with faint tremor instead of shaking. What property of a fault makes its slip slow instead of fast is not established.