EASTERN USA SEISMICITY AND SELF-ORGANIZED CRITICALITY: Further support for Hydroseismicity, John K. Costain, Department of Geosciences, Virginia Tech, Blacksburg, VA 24061

Seismicity in the eastern United States occurs in a crust assumed to be in a self-organized critical (SOC) state. Small changes in fluid pressure at hypocentral depths are therefore sufficient to trigger earthquakes. The "Hydroseismicity" model assumes that changes in the elevation of the water table play a key role in triggering intraplate seismicity. Streamflow, and by association changes in the elevation of the water table, can be "downward-continued" to hypocentral depths using the diffusion equation and an appropriate value of crustal hydraulic diffusivity, D. One measure of the changes is the standard deviation of the downward-continued flow; high values of the standard deviation imply relatively rapid fluctuations in fluid pore pressure. It is found that the ratio of the number of earthquakes per year to the standard deviation of the downward-continued streamflow is the same for both the central Virginia and the New Madrid, MO, seismic zone, which implies that the number of earthquakes in a seismogenic crust is directly proportional to the standard deviation of the downward-continued streamflow. In addition, the fractal dimension of downward-continued streamflow is found to be equal to twice the seismicity b-value for D = 1.6 m²/sec, consistent with an SOC crust. In addition, the onset of seismicity coincides with the onset of large values of the standard deviation only for a crustal hydraulic diffuvity value of about 1.6 m²/sec. A similar value for D (Rothert et al., 2003) was also recently reported by direct measurement at the German Continental Deep Drilling Site (KTB). Finally, additional data from the Virginia earthquake catalog allow new calculations of cumulated and residual earthquake strain over the period 1920-2000. The trend of residual strain matches the trend of the downward-continued streamflow. All of these observations are consistent with fluid pressure transients playing a key role in triggering earthquakes in an SOC hydraulically diffusive crust. A downward continuation of streamflow in different groundwater basins in Virginia shows the resulting time series to be remarkably similar at hypocentral depths even though the surface flow is quite different. This similarity suggests that one might consider all of the earthquakes in Virginia without regard to geologic or hydrologic setting. Common spectral peaks in the period range of 9-11 years are found in spectra of the entire earthquake time series and the downward continuation of surface streamflow in different hydrologic units.