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Keywords:

  • coda;
  • energy;
  • energy-moment scaling;
  • moment magnitude;
  • source spectra

SUMMARY

A stable estimate of the earthquake source spectra in the western Alps is obtained using an empirical method based on coda envelope amplitude measurements described by Mayeda et al. for events ranging between MW∼ 1.0 and ∼5.0. Path corrections for consecutive narrow frequency bands ranging between 0.3 and 25.0 Hz were included using a simple 1-D model for five three-component stations of the Regional Seismic network of Northwestern Italy (RSNI). The 1-D assumption performs well, even though the region is characterized by a complex structural setting involving strong lateral variations in the Moho depth.

For frequencies less than 1.0 Hz, we tied our dimensionless, distance-corrected coda amplitudes to an absolute scale in units of dyne cm by using independent moment magnitudes from long-period waveform modelling for three moderate magnitude events in the region. For the higher frequencies, we used small events as empirical Green's functions, with corner frequencies above 25.0 Hz. For each station, the procedure yields frequency-dependent corrections that account for site effects, including those related to fmax, as well as to S-to-coda transfer function effects. After the calibration was completed, the corrections were applied to the entire data set composed of 957 events.

Our findings using the coda-derived source spectra are summarized as follows:

  • (i) 
    we derived stable estimates of seismic moment, M0, (and hence MW) as well as radiated S-wave energy, (ES), from waveforms recorded by as few as one station, for events that were too small to be waveform modelled (i.e. events less than MW∼ 3.5);
  • (ii) 
    the source spectra were used to derive an equivalent local magnitude, ML(coda), that is in excellent agreement with the network averaged values using direct S waves;
  • (iii) 
    scaled energy, inline image, where ER, the radiated seismic energy, is comparable to results from other tectonically active regions (e.g. western USA, Japan) and supports the idea that there is a fundamental difference in rupture dynamics between small and large crustal earthquakes in tectonically active regions.