Coda (seismology)
In seismology, the coda ( Italian : "tail") describes that part of a seismic wave train that follows immediately after the wave front hits and can last from a few minutes to well over an hour, depending on the strength of the source signal and the external conditions of the measurement .
In a seismogram (see example), the impact of the wave front is characterized by an abrupt increase in amplitude above the level of the background noise . While the seismic source signal (e.g. an earthquake or an explosion source ) usually only lasts a few seconds, the wave coda continues many times longer. Later, weaker phases of the wave field, i. H. Wavefronts that reach the measuring point on other beam paths, are superimposed by the wave coda and are hidden in it.
The complex wave coda is created by scattering , i.e. by the interaction of the seismic wave field with small-scale heterogeneities along the path inside the earth. Heterogeneities of this kind exist almost worldwide in different scale lengths. These can e.g. B. caused by small-scale differences in material, fractures and fissures, changes in the stratification or structure of the rock. This mainly happens in the earth's crust . By interacting with such scattering centers, a small portion of the wave energy is deflected in a random manner and its arrival at the measuring point is delayed.
The amplitude of the wave coda decreases exponentially . The decay behavior of the seismic coda can be investigated and allows conclusions to be drawn about the stochastic properties of the medium passed through. The duration of the fading depends primarily on the source energy, i.e. on the magnitude of the event. The distance between the measuring point and the signal source plays a subordinate role. Thus, the decay time of the wave coda to calculate z. B. serve the earthquake strength according to the so-called decay or code magnitude .
literature
- Peter Bohrmann (Ed.): IASPEI New Manual of Seismological Observatory Practice , GeoForschungsZentrum Potsdam 2002
- Thorne Lay & Terry C. Wallace: Modern global seismology , Academic Press, San Diego 1995, ISBN 978-0127328706