Hypocenter migration as an explanation for temporal changes in coda Q

Abstract

In the 1993 earthquake swarm near the Norris Lake Community, Georgia, Qc (coda Q) and b value exhibit systematic temporal variations on a scale of months and hours. The long‐term monthly variations correlate with the swarm's growth and decay. Examination of the hypocenters demonstrates that Qc is a strong function of hypocenter and the temporal variations of Qc reflect a systematic migration of hypocenters. The b value appears to vary in this swarm because the method of magnitude determination is dependent on Qc. Throughout the duration of the swarm, the active areas migrated from a high‐gc area eastward to a low‐Qc area. Variations of Qc over periods of hours were explained by a shifting of hypocenters between high‐ and low‐Qc areas. Significant changes in Qc occur for hypocenters that are separated by distances <1 km even though the width of the volume sampled by Qc is >10 km. The nonuniform (fractal) distribution of relative scattering coefficients plays a major role in determining the spatial dependence of Qc. By using forward coda modeling on the distribution of relative scattering coefficients derived for this earthquake swarm, we can reproduce seismic coda with the same spatial dependence on Qc as found in the earthquakes throughout the swarm. The relative locations of earthquakes associated with different Qc values and our ability to model different Qc values from a known distribution of relative scattering coefficients confirm that the migration of hypocenters, and not a temporal change in attenuation properties, determines the temporal variation of Qc observed during the Norris Lake Community earthquake swarm.