Origin of coda waves: earthquake source resonance

Coda in local earthquake exhibits resonance-like wave behaviour where the coda emerges as long-duration small-amplitude vibration with selective frequency, slow temporal decay, and uniform spatial energy distribution around the earthquake source. Coda is thought to be the incoherent waves scattered from random small-scale heterogeneity in the earth’s lithosphere. Here I show that the coda in local earthquake is primarily attributed to the natural resonance in strong small-scale heterogeneity around the earthquake’s hypocenter through seismic wave field modeling for 1D heterogeneity. The natural resonance is evolved from the low frequency resonance (LFR) and is an emergent phenomenon that occurs in steady state regime. Effects of heterogeneity on the natural resonance are similar to those of LFR.