Abstract
SUMMARY Coda-Q is used to estimate the attenuation and scattering properties of the Earth. So far focus has been on earthquake data at frequencies above 1 Hz, as the high noise level in the first and second microseismic peak, and possibly lower scattering coefficient, hinder stable measurements at lower frequencies. In this work, we measure and map coda-Q in the period bands 2.5–5 s, 5–10 s and 10–20 s in the greater Alpine region using noise cross-correlations between station pairs, based on data from permanent seismic stations and from the temporary AlpArray experiment. The observed coda-Q for short interstation distances is independent of azimuth so there is no indication of influence of the directivity of the incoming noise field on our measurements. In the 2.5–5 s and 5–10 s period bands, our measurements are self-consistent, and we observe stable geographic patterns of low and high coda-Q in the period bands 2.5–5 s and 5–10 s. In the period band 10–20 s, the dispersion of our measurements increases and geographic patterns become speculative. The coda-Q maps show that major features are observed with high resolution, with a very good geographical resolution of for example low coda-Q in the Po Plain. There is a sharp contrast between the Po Plain and the Alps and Apennines where coda-Q is high, with the exception a small area in the Swiss Alps which may be contaminated by the low coda-Q of the Po Plain. The coda of the correlations is too short to make independent measurements at different times within the coda, so we cannot distinguish between intrinsic and scattering Q. Measurements on more severely selected data sets and longer time-series result in identical geographical patterns but lower numerical values. Therefore, high coda-Q values may be overestimated, but the geographic distribution between high and low coda-Q areas is respected. Our results demonstrate that noise correlations are a promising tool for extending coda-Q measurements to frequencies lower than those analysed with earthquake data.
Highlights
Since its inception by Aki & Chouet (1975), the coda quality factor Qc has arguably become one if not the most employed measure of seismic attenuation around the world
R had to be adapted as the threshold for rejecting almost all measurements is different in the three period bands
We use the correlations of ambient noise to extract coda-Q in the period range
Summary
Since its inception by Aki & Chouet (1975), the coda quality factor Qc has arguably become one if not the most employed measure of seismic attenuation around the world (see Herraiz & Espinosa 1987; Fehler et al 1992; Mikesell et al 2012 for reviews). Where the exponent α typically varies between 1 and 2 depending on the type of waves (surface waves or body waves) and scattering model (single-scattering or diffusion).The coda wavefield itself can be shown to obey Gaussian statistics with very good accuracy (Anache-Menier et al 2009; Obermann et al 2014) This statistical property follows logically from the physical interpretation of the coda as a superposition of a large number of scattered waves with random phase, by application of the central limit theorem. G. Stanke & Kino 1984) and by the strong permanent excitation of ambient noise by swell and oceanic wave interactions between 5 s and 15 s period This period band represents an observational gap for the seismic Q, which makes it difficult to pinpoint the dominant physical mechanisms at the origin of seismic attenuation.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
