THE HYDROLOGIC CYCLE AS THE TRIGGER OF INTRAPLATE EARTHQUAKES IN A QUARTZ-RICH CRUST: A 2-STEP MODEL FOR INTRAPLATE EARTHQUAKES

The primary corrosive agent responsible for the strength reduction of silicate minerals is water . Quartz-rich crust has lower values of Poisson's ratio, nu, which depends upon the solid rock and dry or fluid-filled cracks. For Biot's constant equal to zero and for nu<0.25 stress in a fluid under hydrostatic pressure in a crack can be greater than the horizontal confining stress. The fluid stress is then augmented by increases in pore-fluid overpressure from normal groundwater recharge, which can start crack dilation leading to fracturing and the creation of new permeability. Chemical analyses across the Central Virginia Piedmont show high quartz content between 55-92%.

For a gravitationally loaded rock mass, the horizontal stress is proportional to the vertical stress, and the proportionality constant, k=nu/(1-nu), is independent of depth (Terzaghi and Richart , 1952). Sheorey~(1994) developed a more general model and provided a simplified equation for estimating the ratio k. His value of k is constant below about 2 km but increases above 2 km.

It is suggested that the Aug 23, 2011, Virginia mainshock was triggered by a 2-step process: 1) first, pore- fluid pressure diffusion from normal groundwater recharge into a regionally widespread fracture (Mesozoic?) network channeled pore-fluid pressure diffusion to the future hypocenter where the cumulation of Biot slow waves from different surface sources and with different arrival times resulted in the increase of pore-fluid overpressure until the mainshock was triggered in an SOC crust at a depth of 8 km, and 2) aftershocks occurred upward from the mainshock to a depth of about 2 km, preferring to localize in the weaker and pervasive Mesozoic fabric. Implicit in this model is that the zone of aftershocks was neither permeable nor a major fault zone before the mainshock. Focal depths of the Virginia Aug 23 aftershock sequence were between 2 and 8 km (Horton et al., 2012), possibly restricted to the observed upper limit of 2 km by the increase in the in-situ horizontal crustal stress described by Sheorey's equation. If this 2-step model is correct it may explain why it has been difficult to correlate intraplate earthquakes with mapped faults.