The Intracratonic Caraibas-Itacarambi Earthquake of December 9, 2007 (4.9 mb), Minas Gerais, Brazil: predominance of compressional stresses in the middle of the San Francisco craton.
Abstract
An earthquake with magnitude 4.9 mb, in the central part of the San Francisco craton, shook the village of Caraibas, Minas Gerais state, on December 9, 2007 at 00:03 (local time). The epicentral area is near the city of Itacarambi. This event was the first to cause a fatal victim in Brazil (a little girl 6 years old). The maximum intensity reached VII Modified Mercalli and the isoseismal of VI MM intensity comprise an area of about 100 square kilometers. Since May 25, 2007, when a 3.5 mb magnitude event was widely felt by the population, this region has been shaken by small earthquakes. A field campaign was taken during October 23-28 to implement a local seismographic network composed by 6 tri-axial broadband stations that is operating until now. A seismic gap was observed some days before the main shock of December 9. Two imminent foreshocks preceded the main shock by some minutes, and 162 aftershocks followed the main event during the first day. The earthquakes with clear onset times for P and S waves were located with Hypo71 using a local velocity model with a Vp/Vs ratio of 1.72, obtained with a composite Wadati diagram. The events show a trend in the NE-SW direction, with very shallow depths, less than about 2 kilometers. The aftershocks were distributed over an area about 3 kilometers long in the NE-SW direction. A composite focal mechanism, determined using P-wave polarities with the clearest waveforms at local stations, shows a reverse faulting mechanism. This solution, consistent with P-wave polarity data for the main shock recorded at regional and teleseismic stations, shows a near horizontal P-axis trending E- W, similar to an earthquake swarm occurred 50 km to the north in 1990. Stress inversion using five different focal mechanisms in this part of the San Francisco craton indicates compressional stresses with EW maximum compression (S1) and a NS intermediate compression (S2).
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2008
- Bibcode:
- 2008AGUSM.S43A..10C
- Keywords:
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- 7212 Earthquake ground motions and engineering seismology;
- 7215 Earthquake source observations (1240);
- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 8164 Stresses: crust and lithosphere