COHERENCE AND VARIABILITY OF GROUND MOTION OVER 600 m OF THE NONVOLCANIC TREMOR ARRAY SITE AT MOORING, TN

Abstract:

This study examines wave coherency for high frequency P and S waves in the New Madrid Seismic Zone at a site near Mooring, TN. An L-shaped array consisting of 19-seismometers and having arm lengths of 600m located on Holocene fluvial sediments of the Mississippi River was used to examine wave coherency appropriate for many sites throughout the Mississippi embayment and other sediment sites associate with large rivers. Data from local and regional earthquakes within a distance range of 300km show that coherency within the frequency band of 0.5 to 16 Hz degrades with interstation distance across the array according to an empirical exponential model. Vertical component P-waves are coherent over nearly 2 horizontal wavelengths. However, the coherency for horizontal component S-waves degrades more rapidly than the vertical component P waves. S-waves become significantly incoherent at distances of only 0.2 horizontal wavelength and become completely incoherent after 0.5 wavelength. The results of this study show clear and quick decay of wave coherence likely to occur in strong ground motions from nearby earthquakes. The observed incoherence can be a significant factor for the response of structures having foundation lengths of even 100m for a nearby large magnitude earthquake in New Madrid Seismic Zone.