Frequency-dependent Lg-wave attenuation in northern Morocco

Abstract

Frequency-dependent attenuation (Q−1) in the crust of northern Morocco is estimated from Lg-wave spectral amplitude measurements every quarter octave in the frequency band 0.8 to 8Hz. This study takes advantage of the improved broadband data coverage in the region provided by the deployment of the IberArray seismic network. Earthquake data consist of 71 crustal events with magnitudes 4≤mb≤5.5 recorded on 110 permanent and temporary seismic stations between January 2008 and December 2013 with hypocentral distances between 100 and 900km. 1274 high-quality Lg waveforms provide dense path coverage of northern Morocco, crossing a region with a complex structure and heterogeneous tectonic setting as a result of continuous interactions between the African and Eurasian plates. We use two different methods: the coda normalization (CN) analysis, that allows removal of the source and site effects from the Lg spectra, and the spectral amplitude decay (SAD) method, that simultaneously inverts for source, site, and path attenuation terms. The CN and SAD methods return similar results, indicating that the Lg Q models are robust to differences in the methodologies. Larger errors and no significant frequency dependence are observed for frequencies lower than 1.5Hz. For distances up to 400km and the frequency band 1.5≤ƒ (Hz)≤4.5, the model functions Q(f)=(529−22+23)(f/1.5)0.23±0.06 and Q(f)=(457−7+7)(f/1.5)0.44±0.02 are obtained using the CN and SAD methods, respectively. A change in the frequency dependence is observed above 4.5Hz for both methods which may be related to the influence of the Sn energy on the Lg window. The frequency-dependent Q−1 estimates represent an average attenuation beneath a broad region including the Rif and Tell mountains, the Moroccan and Algerian mesetas, the Atlas Mountains and the Sahara Platform structural domains, and correlate well with areas of moderate seismicity where intermediate Q values have been obtained.