Investigating the Lithospheric Structure of Northern Algeria from Receiver Functions and Surface Wave Tomography Using Earthquake and Ambient Noise data

In this thesis, we have investigated the lithospheric structure beneath the continental part of northern Algeria. We have used data provided by the Algerian Digital Seismic Network (ADSN) to create images (2D maps and cross-sections) of the Moho depth variation and the velocity structure of the crust and upper mantle under the study region. To realize this main objective, this thesis has been divided into two main studies. In the first study, we have used teleseismic P-wave receiver functions jointly inverted with Rayleigh wave dispersion curves obtained from local earthquakes recorded by ADSN broadband stations. The seismic stations are located in different geological settings, including the Tell Atlas, the High Plateaus, and the Saharan Atlas. In the second study, we have applied a different approach by including a different type of data, namely ambient noise. We used the Rayleigh waves generated by ambient noise and recorded by ADSN’s short-period stations, as well as Rayleigh waves obtained from local earthquakes recorded by ADSN’s broadband stations to create a coverage map that allows a 3D imaging of the crustal structure of northern Algeria with an average resolution of about 100 km using the surface wave tomography method. Crustal thickness and the Vp/Vs ratio are first derived by the H–κ stacking method of receiver functions. Then, the joint inversion of receiver functions and Rayleigh wave group velocity dispersion curves have showed the variation of Moho depth in different geological settings. Moho depth clearly increases from north to south and from west to east. The shallowest Moho depths (~20–30 km) have been estimated along the Algerian continental margin and in the Tell Atlas, while the deepest Moho depths are found in the Saharan Atlas and the Aurès mountains (36–44 km), passing through the High Plateaus region where Moho depths vary from 30–36 km. The crustal structure is interpreted by combining the results of both studies. The crust is divided into two main layers throughout the study region. The upper crust has a thickness of around 8 –14 km and an average shear-wave velocity of around 3.0 km/s. A zone of high velocity is also observed in the lower part of the upper crust at a depth of around 10 km under the Mitidja basin in the Tell Atlas. The lower crust has a thickness of about 12–30 km and an average shear-wave velocity of between 3.4–3.8 km/s. In general, the lower crust is thicker than the upper crust, especially in the Saharan Atlas. A thinning of the lower crust is observed approaching the interplate boundary to the north, especially in the western part of the region. Upper mantle shear-wave velocity varies from 4.1 to 4.5 km/s at maximum, and are generally stable below 60 km depth. Two low-velocity zones are clearly observed, particularly in the eastern Tell Atlas and High Plateaus. The first is around 10 km thick in the lower part of the lower crust, and the other lies in the upper mantle at depths of between 40 and 60 km. The obtained results are in accordance with those found previously in the region, particularly those using land gravimetric and seismic data. The combination of all these seismological methods has allowed the integration of a new shear-wave velocity model and 2D crustal structure cross-sections into previous results found in the northern part of Algeria, which is located on a major plate boundary. This provides a better understanding of the geodynamics and tectonics of the western Mediterranean region.

Author

• Melouk, Billel

• Lithospheric structure
• Moho discontinuity
• Receiver Function
• Joint inversion
• Surface wave tomography
• Crustal imaging

Identifier

hdl:10045/150804

OAI identifier oai:identifier

oai:rua.ua.es:10045/150804