Attenuation and Velocity Structure in Spain and Morocco: Distinguishing Between Water, Temperature, and Partial Melt

Abstract:

Temperature, melt fraction, and water content affect seismic velocity and attenuation differently. Both are sensitive to temperature, but velocity is more sensitive to melt fraction and attenuation is thought to be more sensitive to water content. For these reasons, combining attenuation measurements with tomographic imaging of velocity structure can help untangle these fields and better resolve lithospheric structure and physical state. We map variations in attenuation beneath Spain and northern Morocco using teleseismic data generated by more than a dozen teleseismic deep-focus earthquakes recorded on a dense array of stations. For each event, we first estimate the source from the best quality recordings. We then apply an attenuation operator to the source estimate, using a range of t* values, to match the record at each station. We invert for a smooth map of t* from the ensemble of measurements. The spatial patterns in t* correlate very well with the tectonic domains in Spain and Morocco. In particular, areas in Spain that resisted deformation during the Variscan and Alpine orogenies produce very little attenuation. Comparing the attenuation map with seismic velocity structure we find that, in Morocco, some areas with strong low-velocity anomalies and recent volcanism do not cause high attenuation. These observations suggest that water content is a more likely cause for seismic attenuation in the study area than temperature, and that the non-attenuative low-velocity anomalies in Morocco are produced by partial mel.