Search

We present a new 3-D anelastic attenuation model for the upper mantle of the southern Aegean Sea subduction area, by inverting whole-path attenuation operators (t*) data, for both P - and S - waves. For the attenuation tomography, we have employed 383 intermediate-depth earthquakes, recorded by two temporary networks between 2002 and 2007, namely the EGELADOS seismic monitoring project installed in the broader southern Aegean Sea area, and the smaller local CYCNET network which operated in the central part of southern Aegean Sea. Frequency-independent t * values were computed for both P and S waves from acceleration amplitude spectra using two approaches. Initially, we estimated t * values from the slope of the acceleration spectra above the corner frequency, using an automated procedure based on the signal-to-noise ratio. A more standard approach was also followed, where t* values were estimated from the manual (visual) selection of the linearly decaying segment of the acceleration spectrum by an analyst. The t* value data set was inverted to determine the 3-D Q -structure of Southern Aegean Sea, similar to traveltime tomography. The inversion results show high P - and S -wave attenuation in the upper mantle of the back-arc area, with significant low QP and QS anomalies in the mantle wedge of the subducting eastern Mediterranean slab under the Aegean, and higher QP and QS values in the outer Hellenic arc. The strongest anomalies are observed beneath and to the north of the volcanic arc, mainly at depths between 60 and 80 km. At larger depths, the lowest QP and QS anomalies are observed in the area of highest deep (100–170 km) microseismicity, in the easternmost part of the Hellenic volcanic arc. P -wave attenuation models from both the automated and manual t* determination approach are in good agreement. On the contrary, the automated S -wave attenuation models show significant but random spatial variability, while the manual S -wave attenuation models show a similar spatial pattern with the P -wave results, with stronger attenuation in the mantle wedge area. The final anelastic attenuation models are in good agreement with previous results for the study area (e.g. 3-D velocity models, Ground Motion Prediction Equations proposed for intermediate-depth events), as well as with attenuation models proposed for similar subduction zones. [ABSTRACT FROM AUTHOR]

We investigate the seismic attenuation along a 160 km profile from the Lower to the High Himalaya in the Garhwal region, India, through the analysis of Lg waveforms from regional earthquakes recorded on 18 broadband seismographs with interstation spacing of 7–10 km. Lateral variability in attenuation is derived through the inversion of 36 two-station measurements using a global optimization scheme. We observe a contrasting attenuation property in the two Himalayan belts: the Lower Himalaya has of 742 ± 235 similar to those observed in the Indian shield, whereas the High Himalaya is characterized by an unusually low value of 30–60. The seismic attenuation is also well correlated with the P-wave teleseismic traveltime residual pattern: faster arrival (approximately 0.2 s) at stations in the Lower Himalaya as compared to azimuthally independent time delay of ∼0.75 s for the High Himalayan stations. The high attenuation and low velocity in the High Himalaya suggests that low viscosity and partial melt in the crust can be correlated to the presence of Miocene leucogranite plutons in this Himalayan belt, a magmatic product of the Indo-Asian collision and presumably evidence of a partial melting event. This shallow low viscosity channel possibly connects the mid-crustal channel beneath Tibet. [ABSTRACT FROM AUTHOR]