Correlations between streamflow and intraplate seismicity in the central Virginia, U.S.A., seismic zone: evidence for possible climatic controls

There is no widely accepted explanation for the origin of intraplate earthquakes. The central Virginia seismic zone, like other seismically active intraplate areas, is a spatially isolated area of persistent, diffuse earthquake activity. We suggested earlier that rainfall plays a key role in the generation of intraplate seismicity (“hydroseismicity”). Observed long-period (10–30 years) changes in streamflow (rainfall) are hypothesized to generate intraplate seismicity by diffusion of pore pressure transients from recharge areas of groundwater basins to depth as deep as the brittle-ductile transition. Streamflow and earthquake strain for a 62-year sample from 1925 to 1987 in the central Virginia seismic zone were cumulated, and a least-squares straight-line fit was subtracted to obtain residuals of streamflow and strain. Residual streamflow was differentiated to obtain the rate of change of residual streamflow. We observed common cyclicities with periods of 10–30 years for residual streamflow and strain. From the one-dimensional diffusion equation, we determined the time response of fluid pressure at depths, ψ, in a hydraulically diffusive crust to an impulsive change in fluid pressure at the surface of a groundwater basin. These responses were convolved with the surface streamflow residuals, or, because of results from reservoir-induced seismicity, with the derivatives of these residuals. Root-mean-square values (rms) of the convolutions were computed for ψ = 5, 10, 15 and 20 km and various values of D. For central Virginia, the number of earthquakes, N, within a crustal slice centered on a depth, ψ, was found to be proportional to the rms value of the convolution, suggesting that the number of intraplate earthquakes generated is directly proportional to the magnitude of the rms changes in fluid pore pressure within the crustal slice. These fluctuations in pore pressure, in concert with stress corrosion and hydrolytic weakening, are hypothesized to trigger intraplate earthquakes in a crust stressed by ridge-push. Crosscorrelation of the residual streamflow convolutions with residual strain, provides a measure of similarity between the streamflow convolution and the strain. Optimum values of crustal diffusivity, D, were considered to be those values for which the Crosscorrelation function looks approximately even. That this occurs only over a reasonable range of diffusivities (near D = 50 km2/year) is consistent with the hypothesized causal relationship between streamflow and intraplate seismicity. Finally, others have shown a clear relationship between the 11-year solar cycle and storm track latitudes in the North Atlantic. Storm tracks over the eastern United States and Canada indicate that north of 50 ° N, the average latitude of storm tracks during December, January, and February is about 2.5 ° farther south at sunspot maximum than at sunspot minimum. A correlation with the 11-year solar cycle was discovered by Labitzke (1987). Significantly, we recognize a clear peak near 11 years in the Fourier spectra of actual (not residual) streamflow in the James River in the central Virginia seismic zone.