| Popis: |
We estimated the source parameters of 53 local earthquakes ( 2.0 M L 5.7 ) of the Friuli-Venezia Giulia (Northeastern Italy) area, recorded by the short-period local seismic network of the Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS), in the period 1995–2003. Data were selected on the basis of high quality locations and focal mechanisms. Standard H/V spectral ratios (HVRS) of the three-component stations of the network were performed in order to assess local amplifications, and only stations showing HVRS not exceeding two were considered for the source parameters estimation. Both velocity and acceleration data were used to compute the SH-wave spectra. Observed spectra were corrected for attenuation effects using an independent regional estimate of the quality factor Q and a station dependent estimate of the spectral decay parameter k. Only earthquakes with M L > 3.0 recorded with a sampling rate of 125 cps were used to compute k, thus allowing to visualize a linear trend of the high frequency acceleration spectrum up to 40–50 Hz. SH-wave spectra, corrected for attenuation, showed an ω − 2 shape allowing a good fit with the Brune model. Seismic moments and Brune radii ranged between 1.5 × 1 0 12 and 1.1 × 1 0 17 N m and between 0.1 and 2.7 km respectively. We obtained M o = 1.1 × 1 0 17 N m for the seismic moment of the Kobarid (SLO) main shock, in good agreement with the Harvard CMT solution ( M o = 3.5 × 1 0 17 N m). Brune stress drops were confined to the range from 0.07 to 5.31 MPa, with an average value of 0.73 MPa and seem to be approximately constant over five orders of magnitude of seismic moment. Radiated seismic energy computed from two nearby stations scales with seismic moment according to log E s = 1.30 log M o − 9.06 , and apparent stress values are between 0.02 and 4.26 MPa. The observed scatter of Brune stress drop data allowed to hypothesize a scaling relation M o ∝ f c − 3.43 between seismic moment and corner frequency in order to accommodate both Brune stress drop and apparent stress scalings. No systematic differences are evidenced between stress parameters of earthquakes with different focal mechanisms. As a consequence, a relation of the seismic stress release with the strength of rocks can be hypothesized. A high correlation ( r > 0.9 ) of Brune stress drop is found with both apparent stress and RMS stress drop, according to Δ σ B = 2.0 σ a and Δ σ rms = 2.26 Δ σ B respectively. |
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