THE HYDROLOGIC CYCLE AND EARTHQUAKES

“Managing water resources by thoroughly understanding the hydrologic cycle at scales ranging from the entire Earth to the smallest of watersheds is one of the greatest responsibilities of humans.” …Thomas C. Winter

Naturally-occurring earthquakes fall into one of two categories: (1) those associated with the dynamics of plate tectonics, or (2) those associated with the dynamics of the hydrologic cycle (“Hydroseismicity”). The latter includes transient changes in the surface that separates the atmosphere from groundwater, and includes the water table, hurricanes, and typhoons. Results from 30 worldwide studies (Costain and Bollinger, 2010) of earthquake-rainfall correlations published during the past 25 years and conducted in both intraplate and plate marginal environments on five continents collectively provide strong support for the Hydroseismicity hypothesis as a viable explanation for intraplate earthquakes via pore-fluid-pressure diffusion, regardless of the host tectonic regime. Their explanation is provided for the most part by Biot’s theory, which predicts a pulse of pure pore-fluid-pressure that persists for weeks or months and is unique to poroelastic materials. For example, an impulsive increase in the water table (from a hurricane, say) results in a Biot “slow wave” of pore-fluid overpressure (pore-fluid pressure above hydrostatic) that is severely attenuated and dispersed over distance; however, the overpressure never (theoretically) goes to zero. Thus, an earthquake triggered at some random intraplate hypocenter can be expected to have been the result of the constructive interference (i.e., a convolution) of overlapping contributions of pore-fluid overpressures from individual transient hydrologic events that reach the hypocenter at the same time but over different crustal paths of different hydraulic diffusivities and traveltimes in a fractured crust of self-organized criticality (SOC). Typhoons can trigger “slow earthquakes”. The correlations between meteorological parameters (rainfall, streamflow, typhoons, and hurricanes) and seismicity now being reported in the literature suggest the need for more local and regional earthquake monitoring networks as well as additional stream gaging stations.