An Earthquake Model Based on Fatigue Mechanism—A Tale of Earthquake Triad

Abstract

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.