HYDROSEISMICITY AND RECENT SMALL EARTHQUAKES OCCURRING WITHIN THE FOX RIVER VALLEY, ILLINOIS

During the month of February, 2010 the town of Lily Lake experienced a magnitude 3.8 earthquake. This was followed by small earthquakes in the same general area in 2012 (McHenry County) and 2015 (Lake of the Hills). Earthquakes occurring in the Fox River Valley and other areas of northern Illinois have traditionally been attributed to unloading from isostatic rebound, erosion rock quarrying, or slow accumulation of continental-scale tectonic stresses. In this study we examine the hypothesis that these earthquakes were triggered by pore-pressure increases at depth (hydroseismicity) from large rainfall events. A pressure wave from elevated water levels at the surface could propagate through saturated pores and fractures, slightly reducing effective stress at seismogenic depths and triggering a small earthquake in a critically stressed system. We utilized Howell’s equation to relate a pore pressure increase at the Earth’s surface (water table rise) to pore pressure increases at a depth Z, as a function of time. Within this equation D represents hydraulic diffusivity (radial expansion of the pore pressure wave), t represents time, and z represents depth

p(t,z,D) = (p₀e^{-(z^2)/(4Dt)}z)/(2π^1/2(Dt)^3/2).

An apparent positive correlation exists between severe rainfall events and these three earthquakes. Specifically, in these three cases a lag of about 210 days occurs from the peak rainfall month to the earthquake event. Utilizing this rainfall-event lag with Howell’s equation, hydraulic diffusivity D was calculated for the upper crust and compared to estimates from other researchers. Parameters from the Lily Lake 2010 earthquake were used to compute a D = 1.14 m²/s which compares favorably with Howell’s estimate of about 1 m²/s uniform diffusivity. This diffusivity value along with the apparent correlation between large rainfall events and earthquakes several months later suggests hydroseismicity and possible cyclicity for these events.