Fault plane orientations of small earthquakes of the 1997 Umbria-Marche (Italy) seismic sequence from P-wave polarities and rise times

A two-step technique has been developed with the aim of retrieving the fault plane orientation of a small-magnitude earthquake. The technique uses a set of non-linear equations, which relate the rise times of first P waves to source parameters (source dimension L, dip &#948; and strike &#966; of the fault) and intrinsic Qp. At the first step of the technique, the inversion of P polarities provides two fault plane orientations for each focal mechanism solution. At the second step, the inversion of rise times is carried out to retrieve L and Qp by fixing &#948; and &#966; to the values of the fault plane orientation inferred at the first step. A robust analysis, based on the random deviates technique, allows us to evaluate if one fault plane exists which systematically better fits data and can be considered the ‘true’ fault plane. A parameter is introduced to quantify the level of resolution of the ‘true’ fault plane. A probabilistic approach, based on the assumption that errors on data are Gaussian distributed, allows us to a posteriori validate or reject the ‘true’ fault plane. The technique has been applied to 47 small earthquakes (1.3 < ML &lt; 3.9) occurred below the town of Sellano, during the 1997 Umbria-Marche (Central Italy) seismic crisis. The strike of the inferred fault planes is generally along the Apennine direction. Fault plane solutions can be mainly subdivided into two groups: a first group of SW-dipping faults and a second group of NE-dipping faults. These results agree with the present day knowledge of the fault systems in the area. An average Qp equal to 354 ± 63 has been estimated, in agreement with previous attenuation studies.