CLIMATIC CHANGES, STREAMFLOW, AND LONG-TERM FORECASTING OF INTRAPLATESEISMICITY

Regions of intraplate seismicity in the eastern United States are spat ially isolated areas of persistent, diffuse earthquake activity. There is no widely accepted explanation for the origin of these earthquakes . We have suggested that climate plays a key role in triggering such i ntraplate seismicity. Long-term increases and decreases in rainfall ca use periodic regional and temporal variations in the elevation of the water table that, by pore pressure diffusion, result in small changes in fluid pressure at any given depth in the crust. In a fractured, hyd raulically permeable crust, the depth of penetration of this pore pres sure diffusion can be as deep as the brittle-ductile transition (15-18 km). In a seismogenic crust, stress corrosion and fatigue of rock asp erities might be more important than purely mechanical effects due to small changes in hydrostatic fluid pressure; however, because any chem ical effects are quasi-static, the temporal characteristics of the tri ggering process might ultimately be determined by the mechanical proce ss, resulting in a ''hydraulically induced'' seismicity trigger that a cts somewhere along paths of pore pressure diffusion. We called this m odel of intraplate earthquake generation ''hydroseismicity''. Because streamflow is related to the regional and temporal morphology of the w ater table, we searched for and report here several attempts to find l inks between climatic changes, streamflow, and intraplate seismicity. Our results fall broadly into four categories: (1) temporal correlatio ns of streamflow with the earthquake strain factor; (2) spectral analy ses of flow and seismicity and the identification of common spectral p eaks; (3) numerical modeling to estimate fluctuations in pore pressure at hypocentral depths; and (4) climatic driving mechanisms (e.g., sun spot cycles) that might substantiate a climate-earthquake link. We obs erve a statistically significant peak in the Fourier spectrum of surfa ce streamflow for the seismic zones bisected by the Mississippi River, Illinois, and James River, Virginia, in the period range of 11-13 yea rs that might be associated with sunspot activity. In addition, there is positive correlation between periods of above average values of the standard deviation of streamflow time series and periods of seismicit y in the central Virginia seismic zone. Many aspects of the weather ap pear to be modulated by a 20-year cycle. We observe a similar periodic ity (18-20 years) in seismicity in the central Virginia seismic zone. A good agreement is observed when a streamflow time series is superimp osed on the record of the earthquake strain factor if a value of 50 km (2)/year is assumed for crustal hydraulic diffusivity. In the central Virginia seismic zone, it is found that the number of earthquakes vers us depth, psi, is directly proportional to pressure fluctuations at th e depth psi. In addition, the fractal dimension determined from downwa rd-continued streamflow is approximately the same as the fractal dimen sion of intraplate seismicity. Furthermore, using the Gutenberg-Richte r relation and assuming that the earthquake data sets in the New Madri d and central Virginia seismic zones are complete for all magnitudes m greater than or equal to 2, the ratio of the number of earthquakes oc curring per year in the New Madrid zone to the central Virginia zone i s about 40. The ratio of the standard deviations of downward-continued Mississippi River streamflow (at Thebes, Illinois) to the James River streamflow is also about 40. One interpretation of this common ratio is that the number of intraplate earthquakes generated in a seismogeni c crust is directly proportional to the standard deviation of vertical variations in the elevation of the water table. If the hydroseismicit y hypothesis is correct, then long-term variations in streamflow can b e used to forecast long-term statistical variations in intraplate seis mic activity. Copyright (C) 1996 Elsevier Science Ltd