Source Parameters of Weak Crustal Earthquakes of the Vrancea Region from Short-period Waveform Inversion

High-frequency records of nine low magnitude shallow earthquakes of the Vrancea (Romania) seismic region are inverted for the seismic moment tensor (MT). An approach is suggested regarding how to obtain at least a rough estimate of the MT when the information on the structure of the crust is poor. Here simple 1-D layered models are used in the Greens function synthesis despite the fact that the structure of the region is undoubtedly very complex. Different 1-D models were used for different source-station paths to approximate lateral variations. Record of a station located on a ray path which crosses a structure differing substantially from a 1-D model may however bias the retrieved MT essentially. Therefore, we did not collectively process all records, but subsets of stations separately. We check the consistency of the MTs resulting from these individual bootstrap solutions, and reject those which differ substantially, assuming that this is due to the oversimplification of the forward modeling. Thereafter, the averaged moment tensor yielded by the consistent subsets is accepted. Moreover, the distribution of the T, N and P axes from the moment tensors used for averaging provides a rough estimate of their reliability.Following this simplistic procedure, we found seven acceptably constrained solutions among nine events processed. Their P axes are compared with the general trend of the stress in the area: three comply with it, the others should be attributed to the complex stress field active in the region.Acknowledgment This research has been made possible by MURST (40% and 60%), by UNESCO-IGCP project 414 Realistic Modelling of Seismic Input for Megacities and Large Urban Areas and NATO SfP 972266. Partial support issued from the Grant Agency of Czech Rep. (Grant 205/02/0383) and from the National Agency for Science, Technology and Innovation of Romania (Grant 6185 GR/2000).