Stress drop and source scaling of the 2009 April L’Aquila earthquakes

S U M M A R Y The empirical Green’s function (EGF) technique is applied in the frequency domain to 962 broad-band seismograms (3.3 ≤ MW ≤ 6.1) to determine stress drop and source scaling of the 2009 April L’Aquila earthquakes. The station distance varies in the range 100–250 km from the source. Ground motions of several L’Aquila earthquakes are characterized by large azimuthal variations due to source directivity, even at low magnitudes. Thus, the individual-station stress-drop estimates are significantly biased when source directivity is not taken into account properly. To reduce the bias, we use single-station spectral ratios with pairs of earthquakes showing a similar degree of source directivity. The superiority of constant versus varying stress-drop models is assessed through minimization of misfit in a least-mean-square sense. For this analysis, seismograms of 26 earthquakes occurring within 10 km from the hypocentres of the three strongest shocks are used. We find that a source model where stress drop increases with the earthquake size has the minimum misfit: as compared to the best constant stress-drop model the improvement in the fit is of the order of 40 per cent. We also estimate the stress-drop scaling on a larger data set of 64 earthquakes, all of them having an independent estimate of seismic moment and consistent focal mechanism. An earthquake which shows no directivity is chosen as EGF event. This analysis confirms the former trend and yields individual-event stress drops very close to 10 MPa at magnitudes MW > 4.5 that decrease to 1 MPa, on the average, at the smallest magnitudes. A varying stress-drop scaling of L’Aquila earthquakes is consistent with results from other studies using EGF techniques but contrasts with results of authors that used inversion techniques to separate source from site and propagation effects. We find that there is a systematic difference for small events between the results of the two methods, with lower and less scattered values of stress drop resulting from the EGF approach.

[1]  William R. Walter,et al.  Moment, energy, stress drop, and source spectra of western United States earthquakes from regional coda envelopes , 1996 .

[2]  B. Chiou,et al.  Directivity in NGA Earthquake Ground Motions: Analysis Using Isochrone Theory , 2008 .

[3]  François Renard,et al.  Stress Drop during Earthquakes: Effect of Fault Roughness Scaling , 2011 .

[4]  Badie Rowshandel.  Directivity Correction for the Next Generation Attenuation (NGA) Relations , 2010 .

[5]  J. Boatwright,et al.  Rupture Directivity of Moderate Earthquakes in Northern California , 2010 .

[6]  Peter Goldstein,et al.  85.5 SAC2000: Signal processing and analysis tools for seismologists and engineers , 2003 .

[7]  J. Brune.  Tectonic stress and the spectra of seismic shear waves from earthquakes , 1970 .

[8]  Rachel E. Abercrombie,et al.  Can observations of earthquake scaling constrain slip weakening , 2005 .

[9]  E. Tinti,et al.  Complexity of the rupture process during the 2009 L’Aquila, Italy, earthquake , 2012 .

[10]  Luca Malagnini,et al.  Characteristics of the strong ground motions from the 6 April 2009 L’Aquila earthquake, Italy , 2010 .

[11]  Yehuda Ben-Zion,et al.  On Quantification of the Earthquake Source , 2001 .

[12]  Yehuda Ben-Zion,et al.  Potency-magnitude scaling relations for southern California earthquakes with 1.0 , 2002 .

[13]  R. Herrmann,et al.  Characterization of earthquake-induced ground motion from the L’Aquila seismic sequence of 2009, Italy , 2010 .

[14]  Mehmet Çelebi,et al.  Recorded Motions of the 6 April 2009 Mw 6.3 L'Aquila, Italy, Earthquake and Implications for Building Structural Damage: Overview , 2010 .

[15]  A. Rovelli,et al.  Do Strike-Slip Faults of Molise, Central–Southern Italy, Really Release a High Stress? , 2010 .

[16]  Lucia Luzi,et al.  The Mw 6.3, 2009 L’Aquila earthquake: source, path and site effects from spectral analysis of strong motion data , 2009 .

[17]  J. Boatwright,et al.  The Persistence of Directivity in Small Earthquakes , 2007 .

[18]  Alberto Michelini,et al.  The 2009 L'Aquila (central Italy) MW6.3 earthquake: Main shock and aftershocks , 2009 .

[19]  Luca Malagnini,et al.  A new spectral ratio method using narrow band coda envelopes: Evidence for non‐self‐similarity in the Hector Mine sequence , 2007 .

[20]  Giovanna Cultrera,et al.  Separation of source and site effects by generalized inversion technique using the aftershock recordings of the 2009 L’Aquila earthquake , 2011 .

[21]  Eugenio Chioccarelli,et al.  Near‐source seismic demand and pulse‐like records: A discussion for L'Aquila earthquake , 2010 .

[22]  Incorporating Source Rupture Characteristics into Ground-Motion Hazard Analysis Models , 2006 .

[23]  Anthony Lomax,et al.  Rupture history of the 2009 L'Aquila (Italy) earthquake from non‐linear joint inversion of strong motion and GPS data , 2009 .

[24]  V. Keylis-Borok.  On estimation of the displacement in an earthquake source and of source dimensions , 2010 .

[25]  D. Boore,et al.  Predominant-period site classification for response spectra prediction equations in Italy , 2012 .

[26]  David M. Boore,et al.  Short-period P- and S-wave radiation from large earthquakes: Implications for spectral scaling relations , 1986 .

[27]  Yehuda Ben-Zion,et al.  Statistics of Earthquakes in Simple Models of Heterogeneous Faults , 1997 .

[28]  L. Scognamiglio,et al.  Strong evidence for non‐similar earthquake source scaling in central Italy , 2008 .

[29]  V. K. Borok.  On estimation of the displacement in an earthquake source and of source dimensions , 1959 .

[30]  Yehuda Ben-Zion,et al.  Collective behavior of earthquakes and faults: Continuum‐discrete transitions, progressive evolutionary changes, and different dynamic regimes , 2008 .

[31]  Laura Scognamiglio,et al.  Fast Determination of Moment Tensors and Rupture History: What Has Been Learned from the 6 April 2009 L'Aquila Earthquake Sequence , 2010 .

[32]  J. D. Eshelby.  The determination of the elastic field of an ellipsoidal inclusion, and related problems , 1957, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[33]  John G. Anderson,et al.  A MODEL FOR THE SHAPE OF THE FOURIER AMPLITUDE SPECTRUM OF ACCELERATION AT HIGH FREQUENCIES , 1984 .

[34]  F. Di Luccio,et al.  Source complexity of the 6 April 2009 L'Aquila (central Italy) earthquake and its strongest aftershock revealed by elementary seismological analysis , 2009 .

[35]  R. Herrmann,et al.  Regional Moment Tensors of the 2009 L'Aquila Earthquake Sequence , 2011 .

[36]  Yehuda Ben-Zion,et al.  Large earthquake cycles and intermittent criticality on heterogeneous faults due to evolving stress and seismicity , 2003 .

[37]  Olivier Lengliné,et al.  Rupture directivity of microearthquake sequences near Parkfield, California , 2011 .