We have investigated the time series for earthquake strain energy releases and flow volumes for the major rivers that bisect the regions of seismicity in Virginia (Giles County; central Virginia) and Missouri (New Madrid) seismic zones. Our procedure is to integrate with respect to time over data lengths up to 70 years duration and then to subtract a least squares straight-line fit. The resulting residual earthquake and flow volume time series and their spectral densities both exhibit dominant periods in the 20–30 year range.

These common cyclities lend support for an important role of water in intraplate seismogenesis. The fracture permeability of crystalline rocks, caused by a long history of compressional and extensional tectonic episodes, together with the driving potential supplied by long-term cyclical variations in streamflow, can result in the diffusion of fluid pressure transients to focal depths as deep as 20 km. At those depths there is also present a quasi-static, hydrolytic weakening effect of water on asperities present in the fault zones. This combination of mechanical and chemical effects can cause intraplate earthquakes in highly-stressed crustal volumes.

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