Application of fk Analysis and Entropy to Track the Transition from Spatially Coherent to Incoherent Earthquake Coda in Long Beach, California

Luis A. Dominguez Ramirez, Paul M. Davis, & Daniel D. Hollis

Published 2013, SCEC Contribution #1643

Seismic-scattering theories describe high-frequency coda waves as a combination of waves from random scatterers superimposed on direct waves from the source. The direct waves are expected to be spatially coherent whereas the scattered waves, arriving with random phase, will be spatially incoherent. Our objective is to use data from an extreme high-resolution seismic experiment in Long Beach, California, to determine the transition from coherent to incoherent coda. The network, deployed by Nodal Seismic, comprises ~5400 vertical component instruments, spaced every ∼100 m over an area of ~5 × 7 km2. It was deployed for a period of six months with
the primarily target to image the geological structure of the area
for oil exploration. During the deployment, several thousand
earthquakes and microearthquakes were recorded. We examine coda waves from the two largest events that occurred in the
vicinity of the array.We compute frequency–wavenumber diagrams to determine the sources of coda and their evolution in time. Entropy analysis of the propagation of seismic waves
through the array indicates the transition between the coherent
direct body waves and the onset of incoherent coda waves. Our analysis illustrates that after the arrival of the body waves, the seismic coda is initially dominated by a dispersing wave train
composed of spatially coherent body waves, forward scattered
from 1D crustal layering. This is then followed by omni-directional, spatially incoherent coda waves that can be described as scattered waves from 3D random sources. Pioneering work by Aki (1969) first recognized

Citation
Dominguez Ramirez, L. A., Davis, P. M., & Hollis, D. D. (2013). Application of fk Analysis and Entropy to Track the Transition from Spatially Coherent to Incoherent Earthquake Coda in Long Beach, California. Seismological Research Letters, 84(4), 622-628. doi: 10.1785/0220120099.