This animation shows the simulated ground motion of the Earth’s surface that could occur in the central U.S. region for a magnitude 7.7 strike-slip earthquake on the southern section of the New Madrid seismic zone. The simulation is based on a detailed geologic model of the central U.S. and is intended to show the general character of the long-period ground shaking expected for a large earthquake in the New Madrid seismic zone. It demonstrates the profound focusing effect the Reelfoot rift has on ground shaking produced by earthquakes. Further, the relatively unconsolidated material in the Mississippi embayment amplifies the ground shaking and also traps seismic energy to prolong the duration of ground shaking.
The solid straight line in the middle of the New Madrid seismic zone is the surface projection of the modeled fault, which ruptures in the simulation. The colors are keyed to the peak intensity of ground velocity at the surface. As the seismic waves propagate away from the hypocenter in all directions, we see successive waves of strong ground shaking that begins moving along the Reelfoot rift and tends to be focused northeast toward Paducah, Kentucky, and southwest toward Little Rock, Arkansas. These higher amplitude waves are the more damaging surface waves. These seismic waves, trapped by the softer sedimentary rock in the Rift and Embayment and amplified by the softer sediments above, cause the duration of long-period shaking to be about 30–45 seconds in some areas, including Memphis, Little Rock, and Paducah, Kentucky.