Despite the United Arab Emirates (UAE) is characterized by low to moderate local seismicity, the country is within close proximity to be affected by the surrounding seismically-active regions, mainly, the Zagros and the Makran regions.... more
Despite the United Arab Emirates (UAE) is characterized by low to moderate local seismicity, the country is within close proximity to be affected by the surrounding seismically-active regions, mainly, the Zagros and the Makran regions. Appreciable ground shaking levels were felt in many parts of UAE during the past 15 years. A new seismic source model for UAE was specifically developed herein, as a first and significant step towards a critical re-evaluation of the seismic hazard in the UAE. Updated earthquake and focal mechanism catalogs have been incorporated in the definition and characterization of the seismic sources. This was done after unifying magnitudes, seismicity declustering, as well as performing completeness checks. Geological, tectonic, active faults, and crustal thickness information were considered in the delimitation of the source's boundaries. The proposed seismic source model consists of a total of 12 area seismic sources, i.e., 10 shallow-depth (h ≤ 35 km) and 2 intermediate-depth zones (35 < h ≤ 70 km) spanning the potential seismic regions which may contribute to the UAE seismic hazard. This work is the first in which intermediate-depth seismicity is included in a zoning model for the region. Separate earthquake and focal mechanism sub-catalogs were generated after the delimi-tation of each defined seismic source. The seismicity occurrence parameters (b-value, annual rate of earthquakes above Mw 4.0, and the maximum possible magnitude) were computed individually for each source. A predominant stress regime was also assigned to each seismic source based on the inversion of the available focal-mechanism data.
Rayleigh waves often propagate according to complex mode excitation so that the proper identification and separation of specific modes can be quite difficult or, in some cases, just impossible. Furthermore, the analysis of a single... more
Rayleigh waves often propagate according to complex mode excitation so that the proper identification and separation of specific modes can be quite difficult or, in some cases, just impossible. Furthermore, the analysis of a single component (i.e., an inversion procedure based on just one objective function) necessarily prevents solving the problems related to the non-uniqueness of the solution. To overcome these issues and define a holistic analysis of Rayleigh waves, we implemented a procedure to acquire data that are useful to define and efficiently invert the three objective functions defined from the three following ‘‘objects’’: the velocity spectra of the vertical- and radial components and the Rayleigh-wave particle motion (RPM) frequency- offset data. Two possible implementations are presented. In the first case we consider classical multi-offset (and multi-component) data, while in a second possible approach we exploit the data recorded by a single three-component geophone at a fixed offset from the source. Given the simple field procedures, the method could be particularly useful for the unambiguous geotechnical exploration of large areas, where more complex acquisition procedures, based on the joint acquisition of Rayleigh and Love waves, would not be economically viable. After illustrating the different kinds of data acquisition and the data processing, the results of the proposed methodology are illustrated in a case study. Finally, a series of theoretical and practical aspects are discussed to clarify some issues involved in the overall procedure (data acquisition and processing).
Controlled source seismic investigation of crustal structure below ice covers is an emerging technique. We have recently conducted an explosive refraction/wide-angle reflection seismic experiment on the ice cap in east-central Greenland.... more
Controlled source seismic investigation of crustal structure below ice covers is an emerging technique. We have recently conducted an explosive refraction/wide-angle reflection seismic experiment on the ice cap in east-central Greenland. The dataquality is high for all shot points and a full crustal model can be modelled. A crucial challenge for applying the technique is to control the sources. Here, we present data that describe the efficiency of explosive sources in the ice cover. Analysis of the data shows, that the ice cap traps a significant amount of energy, which is observed as a strong ice wave. The ice cap leads to low transmission of energy into the crust such that charges need be larger than in conventional onshore experiments to obtain reliable seismic signals. The strong reflection coefficient at the base of the ice generates strong multiples which may mask for secondary phases. This effect may be crucial for acquisition of reflection seismic profiles on ice caps. Our experience shows that it is essential to use optimum depth for the charges and to seal the boreholes carefully.
