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Zusammenfassung:
We investigate the dependence of event-specific ground-motion residuals in the
Ridgecrest region, California. We focus on the impact of using either local (ML) or moment (Mw) magnitude, for describing the source scaling of a regional ground-motion model. To
analyze homogeneous Mw, we compute the source spectra of about 2000 earthquakes in
the magnitude range 2.5–7.1, by performing a nonparametric spectral decomposition.
Seismic moments and corner frequencies are derived from the best-fit ω−2 source models,
and stress drop is computed assuming standard circular rupture model. The Brune stress
drop varies between 0.62 and 24.63 MPa (with median equal to 3.0 MPa), and values for
Mw > 5 are mostly distributed above the 90th percentile. The median scaled energy for
Mw <5 is −4.57, and the low values obtained for the Mw 6.4 and 7.1 mainshocks (−5 and
−5.2, respectively) agree with previous studies. We calibrate an ad hoc nonparametric ML
scale for the Ridgecrest region. The main differences with the standard ML scale for
California are observed at distances between 30 and 100 km, in which differences up
to 0.4 magnitude units are obtained. Finally, we calibrate ground-motion models for
the Fourier amplitude spectra, considering the ML and Mw scales derived in this study and
the magnitudes extracted from Comprehensive Earthquake Catalog. The analysis of the
residuals shows that ML better describes the interevent variability above 2 Hz. At intermediate
frequencies (between about 3 and 8 Hz), the interevent residuals for the model
based on Mw show a correlation with stress drop: this correlation disappears, when ML is
used. The choice of the magnitude scale has an impact also on the statistical uncertainty of
the median model: for any fixed magnitude value, the epistemic uncertainty is larger for
ML below 1.5 Hz and larger for Mw above 1.5 Hz.
