Two-dimensional variations of the coda Q in Northern Morocco and their interpretation

Abstract

Abstract

The following study aims at investigating the 2D distribution of the seismic wave attenuation based on the quality factor of the seismic coda waves in the northern part of Morocco. In this regard, the waveforms of 66 local earthquakes have been analyzed. The waveforms were recorded from 26 broadband seismic stations deployed by the Scientific Institute of Rabat during 2008, through the different structural domains. The coda wave seismic quality factor (Q_C) has been extracted through the application of the back-scattering model proposed by Aki and Chouet (1975) using four central frequencies (1.5, 3, 6, and 12 Hz) and through four lapse time windows (30, 40, 50, and 60 s). Additionally, spatial distribution maps of the quality factor of the coda waves, for this region, were generated. The study area, based on the 2D maps of the quality factor, has revealed the existence of several stable anomalies of the Q_C values for all frequencies and for all windows used, Al-Hoceima and Nador regions for instance, and has shown that all territory under study can be divided into five different zones. The comparison of the spatial variations of the seismic wave attenuation, geological, and geophysical characteristics of the Northern Morocco shows that the seismic attenuation correlates with the seismic activity, the position of the fault zones, the heat flow, and the Cenozoic volcanism. Furthermore, the attenuation of the coda waves decreases with the lapse time window increasing (and accordingly with the depth of the seismic wave penetrating). Such behavior in the Q-factor may indicate that the upper part of the lithosphere is more heterogeneous compared to the lower layers. These results can be used not only for scientific research, but also in the practice domain for the calculation of the earthquake source parameters or in the mapping of seismic hazard.