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Seismic fault zone trapped noise
Author(s) -
Hillers G.,
Campillo M.,
BenZion Y.,
Roux P.
Publication year - 2014
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2014jb011217
Subject(s) - superposition principle , geology , wavefront , noise (video) , ambient noise level , fault (geology) , directivity , seismic wave , seismology , focal mechanism , isotropy , acoustics , optics , physics , computer science , telecommunications , quantum mechanics , artificial intelligence , antenna (radio) , image (mathematics) , sound (geography)
Systematic velocity contrasts across and within fault zones can lead to head and trapped waves that provide direct information on structural units that are important for many aspects of earthquake and fault mechanics. Here we construct trapped waves from the scattered seismic wavefield recorded by a fault zone array. The frequency‐dependent interaction between the ambient wavefield and the fault zone environment is studied using properties of the noise correlation field. A critical frequency f c ≈ 0.5 Hz defines a threshold above which the in‐fault scattered wavefield has increased isotropy and coherency compared to the ambient noise. The increased randomization of in‐fault propagation directions produces a wavefield that is trapped in a waveguide/cavity‐like structure associated with the low‐velocity damage zone. Dense spatial sampling allows the resolution of a near‐field focal spot, which emerges from the superposition of a collapsing, time reversed wavefront. The shape of the focal spot depends on local medium properties, and a focal spot‐based fault normal distribution of wave speeds indicates a ∼50 % velocity reduction consistent with estimates from a far‐field travel time inversion. The arrival time pattern of a synthetic correlation field can be tuned to match properties of an observed pattern, providing a noise‐based imaging tool that can complement analyses of trapped ballistic waves. The results can have wide applicability for investigating the internal properties of fault damage zones, because mechanisms controlling the emergence of trapped noise have less limitations compared to trapped ballistic waves.