  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 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) in transient regime and is an emergent
phenomenon that occurs in steady state regime. Its resonance frequency
decreases with increasing heterogeneous scale, impedance contrast, or random
heterogeneous scale and velocity fluctuations; its intensity diminishes with
decreasing impedance contrast or increasing random heterogeneous scale and
velocity fluctuations.
