A response-spectra database is compiled of hundreds of seismic records from intermediate-depth earthquakes (earthquakes whose foci are located between 45 to 300 km from the earth's surface) with moment magnitudes of M 4.5-6.7 that occurred in the South Aegean subduction zone. The database consists of high-quality data from both acceleration-sensor and broadband velocity-sensor instruments. The database is much larger than previous databases used in the development of past empirical regressions enabling the determination of various parameters of ground-motion attenuation not previously examined. New variables accounting for the highly complex propagation of seismic waves in the Greek subduction zone are introduced based on the hypocentral depth and the location of the event, as these factors control the effects of the back-arc low-velocity/low-Q mantle wedge on the seismic-wave propagation. The derived results show a strong dependence of the recorded ground motions on both hypocentral depth and distance, which leads to the classification of the dataset into three depth-hypocentral distance categories. Ground motions from in-slab earthquakes, especially with hypocentral depths h > 100 km, are amplified for along-arc stations, an expected effect of channeled waves through the high-velocity slab. The ground motions are also strongly attenuated in the back-arc region, due to the low-Q mantle wedge, which are almost independent of the recording hypocentral distance. In contrast, for shallower in-slab events (60 km < h < 100 km), the corresponding differentiation of seismic motion for along-arc and back-arc stations is observed beyond a specific critical distance range. Moreover, for longer periods, both along-arc amplification and back-arc anelastic-attenuation factors strongly diminish, suggesting that the longer wavelengths of seismic waves are not affected by the complex geophysical structure, resulting in more similar ground motions for both back-arc and along-arc stations. Finally, results for interface events (h < 45 km) occurring along the outer Hellenic arc suggest their wave propagation is not affected by the presence of the low-velocity/low-Q S mantle wedge, but is mainly controlled by the differences of the anelastic attenuation between the Mediterranean and Aegean lithospheres.

On 8 January 2006, an intermediate-depth earthquake occurred at the western part of the Hellenic trench close to the island of Kythera (southern Greece). This is the first intermediate-depth earthquake in the broader Aegean area that has produced such an extensive set of useful recordings, as it was recorded by the main permanent seismological networks and numerous acceleration sensors operating in Greece, as well as by EGELADOS, a large-scale temporary amphibian broadband seismological network deployed in the southern Aegean area. An effort to combine all the available data (broadband velocity and acceleration sensor) was made to study the properties of ground-motion attenuation of this earthquake. The combination of both types of data revealed interesting properties of the earthquake wave field, which would remain hidden if only one type of data was used. Moreover, the data have been used for a validation of existing peak ground-motion empirical prediction relations and the preliminary study of the very inhomogeneous attenuation pattern of the southern Aegean intermediate-depth events at both near-and far-source distances.

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Anelastic attenuation studies have been considered one of the main controlling factors affecting seismic wave  propagation, providing important information on the earth’s structure. In order to investigate the attenuation structure of the southern Aegean subduction area, we employed ~400 intermediate depth earthquakes recorded by temporary and permanent networks. Using the recorded waveforms we have calculated a frequency-independent path attenuation operator t* for both P and S waves. Initially an automated method was employed, where t* was automatically calculated by the slope of the acceleration spectrum produced above the corner frequency, fc. Computations were performed in the 0.2-25Hz frequency band, using only spectra with a signal to noise ratio greater than 3, and above the noise level for at least the range 1-4Hz (for S and P waves, respectively). In the second approach, the selection of the segment of the spectrum was carried out manually for optimum fitting.   No considerable linear trend revealing dependence of t* with distance could be observed on the original data, whereas strong clustering for different focal depth ranges was observed. The spatial variation of the obtained t* values shows that, in general, along arc stations present low values of t*, while bac k-arc stations show much larger values. The observed t* difference becomes more pronounced as the depth of the earthquakes increases, suggesting a significant localized effect of the high  -attenuation (low-Q) mantle wedge, in agreement with independent observation.