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Abstract:
We analyze two large data sets of 10 years (2009-2018) of ordinary earthquakes (OEs) detected in Italy by INGV and low-frequency earthquakes (LFEs) detected in Japan by JMA. We estimate the joint probability density function of seismic moment (M0) and corner frequency (fc) from S-wave displacement spectra. We use unfiltered signals and manually revised S-wave arrival times.We estimate a wide range of moment magnitudes (Mw = 0-6 for OEs, 0.5-2.5 for LFEs) and observe a self-similar scaling between M0 and fc for both OEs and LFEs with a constant stress-drop of ~MPa and ~kPa, respectively. However, OE spectra show a constant corner frequency fc* ~10 Hz for Mw <~ 2.5. We refer fc* to the cut-off frequency of the medium anelastic attenuation, acting as a low-pass filter and producing an apparent corner frequency that does not scale with the earthquake source (M0).Conversely, for the same Mw <~ 2.5, LFEs exhibit fc between 1 and 8 Hz that scale with M0, showing that their low-frequency content is a real source characteristic and not an apparent consequence of anelastic attenuation.Finally, both OEs (for Mw <~ 2.5) and LFEs show a systematic underestimation of local magnitude when compared with moment magnitude, that we analytically explain as a consequence of anelastic attenuation (fc*) for OEs and of low stress-drop (~kPa) for LFEs.Our method allows the use of raw earthquake waveforms to infer robust information both for monitoring purposes and for physical interpretations of the earthquake source in different tectonic settings and at different scales.
