masterThesis
Estudo da atividade sísmica em Irauçuba-CE entre setembro de 2015 a março de 2016
Fecha
2017-08-18Registro en:
MENEZES, Eduardo Alexandre Santos de. Estudo da atividade sísmica em Irauçuba-CE entre setembro de 2015 a março de 2016. 2017. 75f. Dissertação (Mestrado em Geodinâmica e Geofísica) - Centro de Ciências Exatas e da Terra, Universidade Federal do Rio Grande do Norte, Natal, 2017.
Autor
Menezes, Eduardo Alexandre Santos de
Resumen
The studied region is located in the municipality of Irauçuba, Ceará, in the Borborema Province and it is one of the most active seismic regions of Brazil. The seismic activity of the Northeast of Brazil is well observed by several events that have occurred in the region for many years, with repetitions in some areas that are known as active areas. Many of these events are perceived by population of the areas where they occur, because even if these events do not reach great magnitudes, they may frighten natives. In some cases, they even cause small damages in houses such as small cracks in the walls and displacement of tiles. One example of this kind of seismic activity is the one that occurs in the Irauçuba city, located in the northwestern portion of the Ceará state, 175 km from its capital, Fortaleza. The seismic activity in Irauçuba is known and studied since 1991, when several tremors occurred in the south, in the Juá city, district of Irauçuba. In this region, the biggest tremor reached 4.9 mb of magnitude, surpassed only in Ceará by the tremor in Pacajus in 1980 (5.2 mb). A new intense seismic activity began in September 9, 2015, and it lasted until the beginning of March, 2016, when more than 500 tremors were registered, but this time the highest intensity IV (MM) was observed, with maximum observed magnitude of 3.8 Mb for this period. This master thesis deals with the analysis of collected data in the performed campaign in the period from October, 2015 to the beginning of March, 2016. In this campaign, seven digital seismographic stations were used, of which six were composed of accelerometers in three components, and one was composed of a set of three short period seismometers (one vertical and two horizontal). The data recorded by these stations were analyzed, with the objective of the determination of hypocenters and focal mechanisms. 294 seismic events were analyzed, recorded by at least four stations, using the Compass software. In the determination, the HYPO71 software was used to determine the hypocenters, using a semi-space model with the following parameters: P wave velocity (Vp) of 6.20 km/s and Vp/Vs ratio of 1.69. All events were detected at least by four stations, with time residue less than 0.01 s and horizontal and vertical errors less than 0.1 km. A total of 69 events have a N-NW alignment and revealed an active zone of approximately 2 km of extension and depth varying from 8 to 9 km. For a set of 23 tremors more well located, a fault plan was determined from the hypocenters, and values of a composite focal mechanism were obtained, performed with the aid of the FPFIT software. It was done for a set of data, fixing the fault plan. The parameters of the seismogenic fault were obtained by the combination of the least squares method and the FPFIT software (strike = 45°, dip = 52° e o rake = -151°), confirming the orientation of the fault in the direction N-NW, with normal movement. The epicenters and the focal mechanism do not seem to indicate that there is correlation with so far mapped structures in surface.