Earthquakes are the result of strain build-up from
without and erosion from within faults. A generic co-seismic condition includes
merely three angles representing respectively fault geometry, fault strength
and the ratio of fault coupling to lithostatic load. Correspondingly, gravity
fluctuation, bridging effect, and granular material production/distribution
form the earthquake triad. As a dynamic component of the gravity field,
groundwater fluctuation is the nexus among the three intervened components and
plays a pivotal role in regulating major earthquake irregularity: reducing
natural (dry) inter-seismic periods and lowering magnitudes. It may act
mechanical-directly (MD) through super-imposing a seismogenic lateral stress
field thus aiding plate-coupling from without; or mechanical-indirectly (MI) by
enhancing fault fatigue, hence weakening the
fault from within. A minimum requirement for a working earthquake prediction
system is stipulated and implemented into a well-vetted numerical model. This
fatigue mechanism based modeling system is an important supplement to the
canonical frictional theory of tectonic earthquakes. For collisional systems
(e.g., peri-Tibetan Plateau regions), MD mechanism dominates, because the
orographically-induced spatially highly biased precipitation is effectively
channeled into deeper depth by the prevalence of through-cut faults. Droughts
elsewhere also are seismogenic but likely through MI effects. For example,
ENSO, as the dominant player for regional precipitation, has strong influence
on the gravity field over Andes. Major earthquakes, although bearing the same 4
- 7 years occurrence frequency as ENSO, have a significant hiatus, tracing gravity
fluctuations. That granular channels left behind by seamounts foster major earthquakes
further aver the relevance of MI over Andes. Similarly, the stability of the
Cascadia fault is found remotely affected by Californian droughts (2011-15),
which created a 0.15 kPa/km stress gradient along the Pacific range, which also
is the wave guide.