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Seismic Quality Factor (Q) of the Mid-continental Crust From Regional Earthquake Seismograms

Vassil K. Davidov
Northern Illinois University, Department of Geology and Environmental Geosciences

In the winter of 1811-1812 three earthquakes with magnitudes between 7.8 and 8.2 occurred in southeastern Missouri, near the town of New Madrid. These earthquakes produced ground subsidence, uplift, soil liquefaction, and destroyed the town of New Madrid. If such shocks occurred today, they could kill thousands of people in nearby cities like St. Louis and Memphis. Also, the tall structures in downtown Chicago could be affected due to vibrations from earthquake wave resonance in lake sediments underlying downtown Chicago.

The seismic quality factor, Q, is a measure of the energy lost per cycle by a seismic wave as it passes through a volume of rock or soil. Q dramatically affects how earthquake waves are attenuated between the New Madrid Seismic Zone and northeastern Illinois. This absorption represents the conversion of seismic energy into heat as the wave passes through the earth. The magnitude of Q is an important index of how the Earth will filter seismic waves coming from natural sources such as earthquakes.

This study shows that apparent Q generally increases with event distance in the midcontinent. Using the Spectral Slope Method, Qp (from P waves) varies from 418 at an epicentral distance of 196 km to 1636 at an epicentral distance of 1176 km. Qs (from S waves) varies from 688 at an epicentral distance of 196 km to 3785 at a distance of 1050 km. The apparent increase in Q with distance probably represents an increase in Q with depth in the midcontinent.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013