For
some 30 years, the seismic coda that follows an earthquake has been
explained by the echoes of the earthquake on the inhomogeneities of
the Earth's crust, with echoes on faraway diffractors explaining the
late arrival of the waves. However, a different model, which puts
forward the theory that the Earth's crust is extremely heterogeneous
as a result of its internal deformations, also explains many of the
characteristics of the coda.
Scientists
at the "Laboratoire de physique et modélisation des milieux
condensés" (Laboratory of Physics and Modeling of Condensed
Matter) and the "Laboratoire de géophysique interne et
tectonophysique" (Laboratory of Solid-Earth Geophysics and Tectonophysics)
have analyzed recordings of earthquakes to study the properties of
seismic waves. The measurements were obtained on a network of seismographs
placed near Chilpancingo, Mexico over a period of three months. Three
earthquakes over a magnitude of 4 on the Richter scale produced codas
that lasted longer than several hundred seconds and provided the material
for original analyses.
According
to this model, the coda signals the multiple scattering of seismic
waves. The process of multiple scattering mixes various modes of vibration
of the Earth's crust, and each mode is "equiprobably" present.
The equipartition of the modes of vibration implies that the energy
ratios are constant in time and independent of the earthquake.
Analysis
of the Chilpancingo measurements shows that the energy ratios fluctuate
greatly when the coherent waves arrive, and stabilize during the coda
(the ratios fluctuate by a few percentage points, while the energy
decreases by a factor of 10,000). These ratios are independent of
the various earthquakes, and correspond exactly to what was predicted.
The
demonstration that seismic waves rapidly become scattered brings both
bad news and good news. Bad news, because researchers cannot use standard
techniques for the imaging of the Earth using seismic codas. Good
news, because optical imaging methods have improved greatly and so
these methods, which use the properties of scattered fields, can be
developed in seismology, opening new horizons to explore.
