Are all the faults on Earth active?
All the faults on Earth are not active. Singapore, for example, is sitting on a fault we know little about, except that it has not been active for a very long time.
Faults are the way the Earth’s crust accommodate the stresses due to gravity and convection at its surface. When the stress conditions evolve, faults get deactivated or reactivated.
Major faults are mainly found at the boundaries between tectonic plates, but active faults also exist within tectonic plates. We count seven or eight major tectonic plates depending on how they are defined, and many minor additional plates. The Indo-Australian plate, for instance, is commonly considered as only one tectonic plate subducting under Eurasia. However, the Indian part of that plate subducts more slowly under the Himalayas than the Australian part under Sumatra and Java: this suggests that there could be a diffuse boundary between them. The April 2012 strike-slip earthquakes offshore Sumatra confirmed such an impression.
Another important aspect of fault activity is the return time of earthquakes. Instrumental and historical records span a relatively short period compared to geological times. Faults that do not seem active to us can simply have a greater return time than the period covered by our historical records: they may have ruptured in the past, long before humans were there to witness it. This is why we need to better understand strains at a large scale as well as faulting mechanisms at the surface of the Earth.