On 16 June 1992, an mLg 2.3 earthquake occurred in southwestern Indiana, near Evansville. This area is part of the Illinois Basin coal belt, an area of active surface mines with numerous strip-mine blasts daily. The co-location of earthquakes and strip-mine blasts enable spectral comparisons without significant concern for differences due to path propagation effects.
Discriminating between the two types of events can be done visually due to the distinctive appearance of the Rg phase in strip-mine blasts and high frequency coda of earthquakes. A strong Rg phase is indicative of shallow source depths. However, earthquakes previously located at shallow depths elsewhere within the Illinois Basin do not exhibit a distinctive Rg phase, indicating either poor control in focal depth determination or a fundamental difference in source mechanism. Visual and spectral examination shows that earthquakes are richer in energy at higher frequencies than strip-mine blasts. Earthquakes have significant energy at 20–30 Hz, while the significant energy content of blasts is closer to 10 Hz. The significant difference compared to previous earthquake-nuclear explosion discriminant studies is that the chemical explosion has reduced high frequency content compared to the earthquake.