Evidence of high lateral variations of coda Q from local earthquakes in Western Iberia and its SW offshore area

Abstract

We compute coda-wave attenuation for the Western Iberia and, for the first time, for its SW offshore area. Western Iberia has been well covered by the growing number of permanent seismic stations during the last decade, providing a large dataset. Using this dataset along with offshore seismograms of local earthquakes recorded by temporary ocean-bottom seismometers, and adopting the single backscattering model, we estimate the coda Q at five frequency bands centred at 1, 2, 4, 8 and 16 Hz. Then, we calculate coda Q average values on a geographical grid, 0.5°x0.5° resolution, using regionalization and inversion techniques. We observe that the coda Q at 10 Hz varies from 1057 in the NW continental Iberia to 448 in the South-western offshore area, where there is no crust. Additionally, we find that the area with similar low coda Q value is located in the SE Iberia (Betic range). Therefore, our coda Q grid results exhibit higher values over the older and stable crust and lower values in the offshore margin or active tectonic regions. This suggests that the regional variation in coda Q is less sensitive to crustal thickness than to the tectonic activity in each area and crustal age.Furthermore, we discuss our coda Q results through a comparison with those from other regions in the world, where coda Q has been determined with the same parameters. Our work highlights the importance of coda Q studies as an effective tool to investigate detailed regional differences in seismic attenuation that may be relevant for earthquake seismic hazard and risk assessment studies.