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Fractal fracture scattering origin of S‐wave coda: Spectral evidence from recordings at 2.5 km
Author(s) -
Leary Peter,
Abercrombie Rachel
Publication year - 1994
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/94gl01575
Subject(s) - coda , seismogram , power law , amplitude , pareto distribution , physics , spectral line , geology , seismology , fractal , computational physics , optics , mathematical analysis , statistics , mathematics , astronomy
Local earthquake seismograms recorded at a depth of 2.5 Km in the Cajon Pass borehole near the San Andreas fault, southern California, yield body‐wave and coda‐wave amplitude spectra at frequencies between 10 and 200 Hz without interference from either near‐surface attenuation or surface waves. The coda‐wave spectra resemble the shear‐wave source spectra except that above the corner frequencies f o ≈ 20–30 Hz coda spectra decay by power‐law exponent n ≈ −2.3±0.1 while the source shear‐wave spectra decay by cubic power‐law (mean power‐law exponent n ≈ −3.1±0.1). Assuming a cubic source power‐law spectral decay, the high frequency power‐law enrichment of coda amplitudes relative to source amplitudes implies a power‐law distribution of scatterers that increases with frequency as ≈ f 0.7±0.1 . The distribution of acoustic reflectivity deduced from the Cajon Pass well log has a power‐law density ≈ ν 0.6 at the relevant spatial frequencies ν. The agreement between the temporal and spatial frequency power‐law exponents may be explained by first order scattering in fractal fracture‐heterogeneous material.