Spectral Examination of the 16 June 1992 Earthquake and Quarry Blast Near Evansville, Indiana

Abstract

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.

Seismicity of the Ste. Genevieve Seismic Zone Based on Observations from the EarthScope OIINK Flexible Array

“…Consequently, a critical issue in data processing is to discriminate earthquakes from blasts. The waveform of a blast typically has a low-frequency P wave and a highfrequency coda of surface waves with relatively large amplitudes, known as the R g phase (Hutchenson and Herrmann, 1993). In addition, the origin times of blasts or explosions are chiefly concentrated during local daylight hours.…”

“…This is effectively a form of additional bias that could cause an underestimation of the seismicity rate if not handled carefully. During the 20-day test period, less than 1% of the local events were classified as natural earthquakes based on the discrimination rules in Hutchenson and Herrmann (1993).…”