ALBERT

Description: SUMMARY We analyse the spatial distribution of the intensity data points surveyed after the M w 6.3, 2009 L’Aquila (central Italy) earthquake, with the aim to recognize and quantify finite-fault and directivity effects. The study is based on the analysis of the residuals, evaluated with respect to attenuation-with-distance models, calibrated for L’Aquila earthquake. We apply a non-parametric approach considering both the epicentral and the rupture distance, which accounts for the finite extension of the source. Then, starting from a simplified kinematic rupture model of the L’Aquila fault, we compute four directivity predictors proposed in literature, and assess their correlation with intensity residuals. We derive a so-called Intensity Directivity Factor by the correlation between theoretical predictors and observed residuals that allows us to identify and quantify the intensity data points affected by forward and backward directivity during L’Aquila earthquake. We find that the effects are more pronounced in the forward directivity direction and increments up to 1 MCS intensity unit are expected. Moreover, the directivity predictor that accounts for radiation pattern poorly correlates with residuals. These results show that the spatial distribution of the L’Aquila macroseismic field is affected by source effects and in particular that directivity-induced amplification effects can be recognized. We show that the quasi-unilateral rupture propagation along the fault can explain the high-intensity patterns observed along specific direction at relatively large distance from the instrumental epicentre, in accordance with the seismological source models derived from the analysis of instrumental observations.

Description: SUMMARY Computing the magnitude of an earthquake requires correcting for the propagation effects from the source to the receivers. This is often accomplished by performing numerical simulations using a suitable Earth model. In this work, the energy magnitude M e is considered and its determination is performed using theoretical spectral amplitude decay functions over teleseismic distances based on the global Earth model AK135Q. Since the high frequency part (above the corner frequency) of the source spectrum has to be considered in computing M e , the influence of propagation and site effects may not be negligible and they could bias the single station M e estimations. Therefore, in this study we assess the inter- and intrastation distributions of errors by considering the M e residuals computed for a large data set of earthquakes recorded at teleseismic distances by seismic stations deployed worldwide. To separate the inter- and intrastation contribution of errors, we apply a maximum likelihood approach to the M e residuals. We show that the interstation errors (describing a sort of site effect for a station) are within ±0.2 magnitude units for most stations and their spatial distribution reflects the expected lateral variation affecting the velocity and attenuation of the Earth's structure in the uppermost layers, not accounted for by the 1-D AK135Q model. The variance of the intrastation error distribution (describing the record-to-record component of variability) is larger than the interstation one (0.240 against 0.159), and the spatial distribution of the errors is not random but shows specific patterns depending on the source-to-station paths. The set of coefficients empirically determined may be used in the future to account for the heterogeneities of the real Earth not considered in the theoretical calculations of the spectral amplitude decay functions used to correct the recorded data for propagation effects.

Description: SUMMARY 3-D shear wave velocity images are of particular interest for engineering seismology. To obtain information about the local subsoil structure, we present a one-step inversion procedure based on the computation of high-frequency correlation functions between stations of a small-scale array deployed for recording ambient seismic noise. The calculation of Rayleigh wave phase velocities is based on the frequency-domain SPatial AutoCorrelation technique. Constitutively, a tomographic inversion of the traveltimes estimated for each frequency is performed, allowing the laterally varying 3-D surface wave velocity structure below the array to be retrieved. We test our technique by using simulations of seismic noise for a simple realistic site and by using real-world recordings from a small-scale array performed at the Nauen test site (Germany). The results imply that the cross-sections from passive seismic interferometry provide a clear image of the local structural heterogeneities and the shear wave velocities are satisfactorily reproduced. The velocity structure is also found to be in good agreement with the results of geoelectrical measurements, indicating the potential of the method to be easily applied for deriving the shallow 3-D velocity structure in urban areas and for monitoring purposes.