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Centro di Ricerche Sismologiche

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Scientific Background

The seismic moment tensor is a symmetric second order-tensor which gives a complete mathematical representation of the seismic source. To analyze the local and near regional seismicity (ML ≥ 3.6) recorded by the North Eastern Italy broad band network, we implemented the long-period fullwave inversion code (mtpackagev1.1) developed by Douglas Dreger of the Berkeley Seismological Laboratory, that use Green’s functions computed using the FKRPROG software (Saikia, 1994) developed by Chandan Saikia of URS.

The inversion frequency range is selected based on the initial event magnitude estimated by the local network. We choose frequency ranges  0.02–0.05 Hz for 3.5<5.0 earthquakes, and 0.01–0.05 Hz for 5.0<6.5 earthquakes. In these frequency ranges, the crustal heterogeneities have only minor influence on the moment tensor inversion results.

The Time-Domain Moment Tensor INVerse Code (TDMT_INVC) has been already applied and discussed in several papers (e.g. Fukuyama and Dreger, 2000; Dreger  and Kaverina (2000), Dreger 2003). It is widely used in many seismological centers (e.g. Berkeley Seismological Lab., INGV, ETH, NIED)  for automatic moment tensor determination.

Several tests, using synthetic and real data, have been performed to check the sensitivity of solutions to the station geometry and to the 1 D structural models representative of the studied area. Our tests (Saraò, 2007) revealed that though the minimum number of stations depends on the epicenter position and on the source radiation pattern a complete station coverage is not required when three component stations are used and even one or two station solution can be effective in many cases. We proved that the best double couple orientation and the Mw determined by this procedure, are quite robust although the resolution depth, depending on the structural model, on station geometry and on the source radiation pattern can be unstable (Saraò, 2007).

After the moment tensor computation, a plot displaying the recorded three component waveforms (solid line) and the associated synthetics (dashed line) is displayed. Moment tensor is shown in equal area lower hemisphere projection. In each plot, the focal mechanism solution is given in terms of strike, dip and rake of the two nodal planes. Also the focal depth, the scalar seismic moment and the double-couple (DC) percentage is given. The moment magnitude (Mw) is calculated from the seismic moment following Hanks and Kanamori (1979).
The source depth is found iteratively by finding the solution that yields the largest variance reduction (VarRed). VarRed is considered as an index of the waveform fit between observed and synthetic seismograms and is equal to the sum of squares of the difference in amplitude normalized by the observed waveforms (100% is best). Another measure that is useful for determining source depth in regions where explosive events are unlikely is the RES/Pdc, the variance divided by the DC percentage.

 

References

  • Dreger, D., and A. Kaverina (2000). Seismic remote sensing for the earthquake source process and near-source strong shaking: A case study of the October 16, 1999 Hector Mine earthquake, Geophys. Res. Lett., 27, 1941-1944.
  • Dreger, D. S. (2003). TDMT_INV: Time Domain Seismic Moment Tensor INVersion. In: W. K. Lee, H. Kanamori, P. C. Jennings, C. Kisslinger (Eds). International Handbook of Earthquake and Engineering Seismology, Volume 81B, 1627.
  • Fukuyama E., Dreger D. S., (2000). Performance test for automated moment tensor determination system by using synthetic waveforms of the future Tokai earthquake. Earth Planet. Space, 52, 383-392.
  • Saikia C.K., (1994). Modified frequency-wavenumber algorithm for regional seismograms using Filon’s Quadrature-Modeling of Lg waves in eastern North America. Geophys. J. Int. 118, 142–158.
  • Saraò, A. (2007). Seismic moment tensor determination at CRS: feasibility study, Open Report, OGS 2007/60-CRS/16. Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Trieste, 43 pp.