resumo do trabalho apresentado pelo Igor Eufrásio, aluno do IAG, no First Assembly of the Latin-American and Caribbean Seismological Commission - LACSC, em Bogotá, em julho de 2014. EARTH SCIENCES RESEARCH JOURNAL 44 Earth Sci. Res. J. Vol 18 Special Issue (July, 2014) Variations of parameter b in SE Brazil from local earthquakes Igor Oliveira, Marcelo Assumpção, Hans Agurto-Detzel The slope of the Gutenberg-Richter magnitude-frequency relation, known as parameter b, is widely used to characterize earthquake size distribution. Variations from the average b-value = 1 have been interpreted as indicating the degree of fracture density in the crustal volume, or the predominance of water-induced mechanisms (b-values higher than that of natural seismicity). One of the difficulties of comparing b-values between different earthquake sequences is the use of different local magnitude scales based on signal duration. Two intraplate earthquake sequences in SE Brazil (one case of natural seismicity and one case of water-induced earthquakes) were studied in search for differences in b-values by using the same magnitude scheme based on coda amplitudes. Empirical regressions were found for each local station relating coda amplitudes with regional magnitudes in the range mR= 2.5 to 4.0, which were then used to estimate local magnitudes for the smaller events. In May 2012 a 4.0 mb event occurred in the Sao Francisco craton, near the town of Montes Claros. A local network was installed and recorded a set of about 30 events in the following six months, all occurring in the same rupture plane (inverse faulting mechanism on a NNW-SSE oriented fault plane). These events provided a ¬b¬-value of 0.5 +- 0.1. Although this value is unusually low, it is probably reliable because the G-R frequency-magnitude relation is quite linear above the completeness magnitude of 1.0. In 2005, a large sequence of small earthquakes occurred near Bebedouro town, in the Parana intracratonic basin, induced by the drilling of several new wells, which added extra water into a confined fractured aquifer. Thousands of small events, with magnitudes up to 2.9 mR were recorded by a local network. Using events above the completeness magnitude of 1.0, a b-value = 0.84 +- 0.12 was obtained. Although the b-value of the induced events in Bebedouro is “normal” (i.e., close to 1.0), it is nevertheless significantly higher than the b-value of the natural events in Montes Claros. KeyWords: earthquake statistics, induced seismicity, b-value
Posted on: Wed, 20 Aug 2014 12:06:20 +0000
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