Compromising polarity and waveform constraints in focal-mechanism solutions; the Mara Rosa 2010 Mw 4 central Brazil earthquake revisited

Abstract

Focal-mechanism determination of weak events recorded in sparse networks is challenging. First-motion polarities are often available at relatively distant stations, and waveforms only at a few near stations can be modeled. A two-step approach of how to combine such data has been suggested recently (Cyclic Scanning of the Polarity Solutions, or CSPS method; Fojtíková and Zahradník, 2014). It starts with creating a suite of first-motion polarity solutions, which is often highly non-unique. The next step consists of repeating full waveform inversion for all polarity solutions. Even few stations may efficiently reduce the non-uniqueness of the polarity solutions. Centroid depth, time, scalar moment and uncertainty estimate of the well-fitting double-couple solutions are obtained. The CSPS method has been extended in this paper by adding a new feature, i.e. repeated inversions using multiple first-motion polarity sets. The polarity sets are created by projecting the stations on focal sphere in several available velocity models, thus accounting for the takeoff angle uncertainty. The multiple polarity sets provide assessment of the CSPS solution stability. These ideas are demonstrated on a comprehensive analysis of a rare event in central Brazil. It is the Mw ∼4 mainshock of the Mara Rosa 2010 earthquake sequence (Barros et al., 2015, Carvalho et al., 2015). We employ polarities at 11 stations (distances < 730 km) and invert full waveforms at two stations (CAN3 and BDFB at distances ∼120 and 240 km), for 0.1–0.2 and 0.05–0.125 Hz, respectively. Six polarity sets reflect the takeoff angle uncertainty. The obtained CSPS results are very stable across all the polarity sets (in terms of depth, Mw, and strike/dip/rake angles). It is found that the Mara Rosa mainshock mechanism deviated from the composite solution of the whole sequence by 38°. The paper also includes a test simulating situations at which just a single waveform is used, and how it negatively affects the solution stability.