| 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. An important variable affecting the strength of these vibrations is the seismic quality factor. 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. Higher Q indicates a lower rate of energy loss as the wave passes through the media. This suggests that seismic waves carry the energy further in higher Q regions than these with lower Q. For instance, earthquakes of similar magnitude occurring in San Francisco and New Madrid would generate a much larger area of high shaking intensity in the New Madrid region. This suggests that mid-continental earthquakes could be extremely destructive.;Q dramatically affects how earthquake waves are attenuated between the New Madrid Seismic Zone (NMSZ) and northeastern Illinois. This absorption represents the conversion of seismic energy into heat as the wave passes through the earth. Proposed loss mechanisms include thermo-elastic relaxation, crystal lattice changes, viscous dissipation and interstitial fluid flow, as well as friction along cracks and fractures. The magnitude of Q is an important index of how the Earth will filter seismic waves coming from natural sources, such as earthquakes, or artificial sources, such as nuclear explosions. Since the seismic energy absorption is different for different geological materials, the Q factor is also a useful parameter in describing the composition and physical condition of rocks beneath the Earth's surface.;This study shows that apparent Q generally increases with event distance in the mid-continent. 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 mid-continent. |
