Carpenter, Philip J.
M.S. (Master of Science)
Department of Geology and Environmental Geosciences
In February, 2010 the town of Lily Lake experienced a magnitude 3.8 earthquake. This was followed by small earthquakes of magnitude 2-3 in the same general area in 2012 (McHenry), 2013 (Campton Hills) and 2015 (Lake of the Hills). Seismicity within this region is poorly understood. The Fox River Valley (here defined as the Fox River drainage basin) area is not historically seismically active, nor does it lie above a mapped fault. This study examines the timing of these earthquakes and the hypothesis they were triggered by pore-pressure increases at depth from large rainfall events several months earlier. A pressure wave from elevated water levels at the surface can propagate through saturated rock, slightly reducing effective stress at seismogenic depths. This could trigger a small earthquake in a system where faults are on the verge of failure. In order to better understand this phenomenon, two steps are needed: relocation with improved constraints on hypocentral depths, and utilizing the diffusion equation for propagation of the diffusive pressure wave (Biot slow wave) to predict pore pressure increases at a depth z, as a function of time. Within this equation hydraulic diffusivity D controls the radial expansion of the pressure wave and t represents the time it takes for the wave to reach a depth z; [special characters omitted]. An apparent positive correlation exists between large rainfall events and these earthquakes. Utilizing time between the peak rainfall month to the earthquake event, with the diffusion equation, hydraulic diffusivity D may be calculated and compared to other researchers' values. Regional hydraulic diffusivity values (D) for this study averaged about 1.08 m²/s which compares favorably with upper crustal values obtained by several researchers (e.g. Shapiro, 1997; Costain and Bollinger, 2008) who estimate D for the bulk upper crust about 1 m²/s, assuming uniform diffusivity. Thus, hydroseismicity may be a viable trigger mechanism for these earthquakes. Upon relocating these events, an interesting geometry appeared. They all align with an approximate strike of N 16°E. A positive correlation also exists between this strike and the focal plane solution for the Feb. 10, 2010 earthquake. The strike of this newly discovered fault is also subparallel to the Waukesha Fault in southern Wisconsin, leading to the interpretation that these hypocenters lie upon a previously unknown fault, herein provisionally named the "Lily Lake Fault," with an approximate strike of N 16°E. Three conclusions from this study: 1. The Fox River (basin and watershed) earthquakes of 2010-2015 may have been hydroseismically triggered by large rainfall events, 2. A northeast-trending fault was discovered extending from Lily Lake to at least McHenry, IL and is subparallel to the Waukesha Fault exposed in southeastern Wisconsin, 3. Rocks in the Precambrian crystalline basement in this area appear to be critically stressed and close to failure. It would not be advisable to inject liquid wastes into these rocks, including supercritical carbon-dioxide.
Benco, James, "Possible hydroseismic triggering for small earthquakes occurring within the Fox River Valley, Northeastern Illinois" (2019). Graduate Research Theses & Dissertations. 4447.
Northern Illinois University
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