The 4.5 mb Amargosa earthquake of 30 August 2020, Bahia, Brazil: Implications for the intraplate stress field across the continental margin

Abstract

The 4.5 mb Amargosa earthquake was the largest event to occur in the state of Bahia, NE Brazil, in more than 100 years. It caused intensity VI MM in the epicentral area and was felt up to more than 200 km away. A previously published focal mechanism determination had suggested a normal faulting event at about 10 km depth. However, our detailed analyses of regional and teleseismic records show, in fact, a reverse faulting mechanism and a very shallow depth of about 1 km. The shallow hypocentral depth was consistently determined by modeling of teleseismic P waves, as well as waveform inversion of regional stations for moment tensor solution. Our focal mechanism solution is also consistent with the relative location of the aftershocks which shows the fault plane to be oriented ESE-WNW dipping to the SSW. The P axis is oriented about NNE-SSW, parallel to the coast. In the interior of the continent, the maximum horizontal stresses tend to be roughly E-W oriented. The margin parallel P axis of the Amargosa earthquake is similar to other SHmax estimates from breakout data in the marginal basins and is yet another example of stress rotation due to local effects associated with the continental/oceanic crustal transition.