Bayesian Focal Mechanism Estimation from P-, S-Wave Amplitudes, and Polarities for a Microearthquake Sequence in Irpinia, Italy

The P-wave peak distribution in azimuth and takeoff angles, corrected for the distance through empirical attenuation laws, can reveal the radiation pattern amplitude of the source (Tarantino et al., 2019). This piece of information, jointly combined with the available polarities and S-wave peak amplitudes, can provide the full focal mechanism, constraining the solution also when only a few seismic station records are available, that is, in the case of microseismicity. We proposed a new technique, named P-, S-wave amplitude, and polarities (P-SAP), designed to compute the focal mechanism by jointly inverting the P-, S-wave amplitude ratios and P-wave polarities in a Bayesian framework and employing the octree strategy (Fang et al., 1996) to explore the space of possible solutions. The outputs are strike, dip, and rake angles of the most likely triplet (principal and auxiliary planes) with the related uncertainties, as well as other multiple solutions, if present. We tested the methodology to synthetic data, and we applied it to a microseismic sequence that occurred in Irpinia region, southern Italy. A background microseismicity occurs in a volume delimited by the faults activated during the 1980 M 6.9 Irpinia earthquake. This faults system is complex and composed of northwest–southeast-striking normal faults along the Apennines chain and an approximately east–west-oriented strike-slip fault, deep-seated in the Potenza area. A network of three-component accelerometers and velocimeters are currently deployed to monitor the area. The major topic of this work is validating the proposed P-SAP methodology for microseismicity studies. We also inferred the optimum stress tensor of the sequence, confirming that the microseismicity is controlled by the regional stress field and can reveal characteristics useful to highlight behaviors of larger-scale seismicity.