Estimation of Seismic Attenuation from Ambient Noise Coda Waves: Application to the Hellenic Subduction Zone

The decay of surface-wave coda in ambient noise cross correlations can facilitate the estimation of seismic attenuation. The coda quality factor (Q(c)) can be measured in longer period bands using ambient noise cross correlations, which is the main advantage of using them over earthquakes. The classic model of Aki and Chouet (1975) has been applied previously to estimate coda Q in the Alps using ambient noise cross correlations. The Alps represents an ideal environment for ambient noise study, because it has very high and near-uniform station density, and the region is away from oceanic noise sources. However, many regions around the world do not have uniform station density and may be seismically more active than the Alps. One such region is the Hellenic subduction zone (HSZ), because it has a high rate of seismicity, sparser station coverage, and is surrounded by seas from outside and within. In this study, we estimate ambient seismic noise Q(c) in 2.5-5, 5-10, and 10-20 s period bands for the HSZ, which did not exist previously. Then, we mitigate the effects of lapse time and window length, distance, azimuth, as well as the number of stacked days on the Q(c). Mapping of Q(c) measurements in such a geographically heterogeneous setting poses additional challenges, which we solve using a novel approach that adaptively selects paths based on their lengths and azimuthal distribution. The major tectonic zones are identified in the resulting Q(c) maps in the form of low Q(c) such as the North Anatolian fault, the Kefalonia transform zone, the Gulf of Corinth, the volcanic centers, and so on. The results also show a good correlation with large topographical features such as the Hellenides and the Thessalian plains, which have also been noticed from Q(c) analysis in other parts of the world.