Deblurring of frequency-wavenumber images from small-scale seismic arrays

Temporary arrays installed in urban areas for investigating the upper-most geological structure typically comprised of a limited number of stations and are arranged in geometries constrained by environmental boundaries. Therefore, it is expected that the frequency–wavenumber images are significantly blurred by the array transfer function and are corrupted by noise. In this paper, the effect of theRichardson–Lucy regularization method applied to the problem of deblurring frequency–wavenumber images is investigated. The images are computed by analysing data from two small-aperture 2-D arrays, installed with different configurations in a test-site within the town of Potenza (Southern Italy) for near-surface investigations. We show that removing the effects of the array response from the frequency–wavenumber images improve the phase-velocity estimation, reducing the relevant level of uncertainty. Furthermore, the Richardson–Lucy regularization method is effective in reducing the level of noise related to spatial aliasing by eliminating spurious peaks, allowing the maxima related to different seismic sources to be better discriminated

Applying the damage assessment for rapid response approach to the august 24 M6 event of the seismic sequence in central Italy (2016)

Seismic monitoring networks are increasingly being used in urban areas to record and locate earthquakes. Recordings in the proximity of buildings also allow assessing, as a first approximation, the expected building damage. The DARR (Damage Assessment for Rapid Response) method provides local-scale information on expected damage patterns. The potential of this approach is discussed here for the August 24 M6 event of the Central Italy seismic sequence (2016-2017). We focus only on the first event of the sequence because cumulative damage is outside the scope of this study. The earthquake recordings are available from two Italian monitoring networks: the Italian Accelerometric Archive (ITACA) and the OSS (Osservatorio Sismico delle Strutture), which collects data from monitored buildings and bridges in Italy. We selected four target areas (Amatrice, Norcia, Visso and Sulmona) characterized by different epicentral distances and building typologies, that suffered different levels of damage during the M6 event on 24 August 2016. Using recordings either in the free field or in the basement of buildings, the expected relative displacement of building typologies common in the studied areas is calculated with the DARR method. Using predefined damage thresholds from literature, the obtained results allow quantifying the expected damage for dominant building typologies in the surroundings of the recording sites. We investigate and discuss the potential use and applicability of the DARR method in different areas depending on the epicentral distance and building characteristics. The results indicate that the DARR approach is useful for supporting and improving rapid response activities after a seismic event

Seismic input motion determined from a surface-downhole pair of sensors: a constrained deconvolution approach.

We apply a deconvolution approach to the problem of determining the input motion at the base of an instrumented borehole using only a pair of recordings, one at the borehole surface and the other at its bottom. To stabilize the bottom-tosurface spectral ratio, we apply an iterative regularization algorithm that allows us to constrain the solution to be positively defined and to have a finite time duration. Through the analysis of synthetic data, we show that the method is capable of retrieving reliable input motion, suppressing the effect of the negative interference generated by the downgoing waves. Results obtained by applying the methodology to weak earthquakes recorded at the Ataköy (Istanbul) vertical array are also presented and show that removing the effect of the downgoing waves is remarkable, even considering the recording at a depth of 140 m

Empirical site-specific response-spectra correction factors for the Gubbio basin (central Italy)

Providing quantitative microzonation results that can be taken into account in urban land-use plans is a challenging task that requires collaborative efforts between the seismological and engineering communities. In this study, starting from the results obtained by extensive geophysical and seismological investigations, we propose and apply an approach to the Gubbio basin (Italy) that can be easily implemented for cases of moderate-to-low ground motion and that takes into account not only simple 1D, but also more complicated 3D effects. With this method, the sites inside the basin are classified by their fundamental resonance frequencies, estimated from the horizontal-to-vertical spectral ratio applied to noise recordings (HVNSR). The correspondence between estimates of the fundamental frequency from this method and those derived from earthquake recordings was verified at several calibration sites. The amplification factors used to correct the response spectra are computed by the ratio between the response spectra at sites within the basin and the response spectra at a hard-rock site using data from two seismic transects. Empirical amplification functions are then assigned to the fundamental frequencies after applying an interpolation technique. The suitability of the estimated site-specific correction factors for response spectra was verified by computing synthetic response spectra for stations within the basin, starting from the synthetic recording at a nearby rock station, and comparing them with observed ones

Source Spectra and Site Response from S Waves of Intermediate-Depth Vrancea, Romania, Earthquakes

Seismograms from 55 intermediate-depth Vrancea earthquakes (M 4:0– 7:1) recorded at 43 stations of an accelerometric network in Romania are used to derive source spectra and site amplification functions from S waves in the frequency range 0.5–20 Hz with the generalized inversion technique (GIT) (Castro et al., 1990). Attenuation is taken into account using the nonparametric attenuation functions derived by Oth et al. (2008) from the same dataset, and the attenuation-corrected data are then split into source and site contributions. The source spectra follow the ω 2 model (Brune, 1970, 1971) with high corner frequencies and a related Brune stress drop of the order of 100 MPa. The site amplification functions are separately determined for both horizontal and vertical components. Contrary to widespread expectation, the vertical component shows significant amplification effects at high frequencies. The H=Z ratios determined from the GIT results compare well with H=V ratios computed directly from the S-wave window of the accelerograms (Lermo and Chávez-García, 1993). The basic assumption for the determination of site effects from H=V ratios is that the vertical component is not or only little affected by site effects. For Vrancea earthquakes, this assumption is incorrect; consequently, site effects should not be estimated from H=V ratios. The reason for this peculiar fact is the geometry of intermediate-depth seismicity that leads to almost vertical ray paths beneath the stations

S-Wave Attenuation Characteristics beneath the Vrancea Region in Romania: New Insights from the Inversion of Ground-Motion Spectra

The S-wave attenuation characteristics beneath the Vrancea region in Romania are analyzed from the spectra (frequency range 0.5–20 Hz) of more than 850 recordings at 43 accelerometric stations of 55 intermediate-depth earthquakes (M 4:0–7:1) that occurred in the Vrancea seismogenic zone. The method commonly chosen for this type of investigation in the case of crustal earthquakes is the generalized inversion technique (GIT) (e.g., Andrews, 1986; Castro et al., 1990). Yet the Vrancea dataset is entirely different from common crustal datasets. Because of the strong clustering of the hypocenters within a very small focal volume, there are only few crossing ray paths from sources to receivers. As a consequence, inhomogeneities in the attenuation properties are not averaged out, which leads to unphysical results if the standard GIT approach is adopted. The problem is discussed qualitatively by performing tests with synthetic data and solved quantitatively by adapting the GIT technique in view of these peculiarities.With the optimally adapted inversion scheme, it is possible to unravel differences in the attenuation characteristics between two (or more) sets of stations. The results show that the attenuation of seismic waves is roughly comparable in the low frequency range (<4–5 Hz) but stronger by up to an order of magnitude at higher frequencies within the Carpathian mountain arc as compared with the foreland area. Modeling this strongly frequency-dependent lateral variation of seismic attenuation by a significantly lower Q beneath Vrancea (1) provides a very good fit of observed strong-motion characteristics, (2) sheds new light on the distribution of intensities of the previous strong earthquakes, (3) will have strong implications for future hazard assessment, and (4) is fully compatible with structural models from deep seismic sounding, tomography, and teleseismic attenuation

Suitability of short-period sensors for retrieving reliable H/V peaks for frequencies less than 1 Hz

Using three different short-period electromagnetic sensors with resonant frequencies of 1 Hz (Mark L4C-3D), 2 Hz (Mark L-22D), and 4.5 Hz (I/O SM-6), coupled with three digital acquisition system, the PDAS Teledyne Geotech, the REFTEK 72A, and the Earth Data Logger PR6-24 (EDL), the effect of the seismic instruments on the horizontal-to-vertical spectral ratio (H/V) using seismic noise for frequencies less than 1 Hz has been evaluated. For all possible sensors - acquisition system pairs, the background seismic signal and instrumental self-noise power spectral densities have been calculated and compared. The results obtained when coupling the short-period sensors with different acquisition systems show that the performance of the considered instruments at frequencies < 1 Hz strongly depends upon the sensor-acquisition system combination and the gain used, with the best performance obtained for sensors with the lowest resonance frequency. For all acquisition systems, it was possible to retrieve correctly the H/V peak down to 0.1-0.2 Hz by using a high gain and a 1 Hz sensor. In contrast, biased H/V spectral ratios were retrieved when low-gain values were considered. Particular care is required when using 4.5 Hz sensors since they may not even allow the fundamental resonance frequency peak to be reproduce

Interpretation of microtremor 2D array data using Rayleigh and Love waves: the case study of Bevagna (central Italy)

In the last decades, geophysicists and seismologists have focused their attention on the inversion of empirical surface-waves’ dispersion curves from microtremor measurements for estimating the Swaves velocity structure at a site. This procedure allows a fast and convenient investigation without strong active sources, which are difficult to deploy especially in urban areas. In this study we report on a 2D seismic noise array experiment carried out at Bevagna (Central Italy) near the station BVG of the Italian Accelerometric Network (RAN). The site was investigated within the DPC-INGV S4 Project (2007-2009). The Rayleigh- and Love- waves dispersion characteristics were estimated using different methods. The inversion of the dispersion curves was then performed independently, obtaining two estimations for the S-waves velocity profiles. The results of cross-hole logging near the seismic station are used for a comparison. The shear waves velocity profiles estimated by microtremor analyses range up to 150m depth. The two independent procedures provide consistent shear waves velocity profiles for the shallow part of the model (20-30 m in depth) in agreement with the results of the cross-hole logging. Some problems arise between 30 and 40 m in depth in the profile estimated by surface waves. In this range cross-hole logging evidences an inversion of S-waves velocity. Although the cross-hole logging stops at 40 m of depth, we are confident about the results provided by the Rayleigh-waves analysis below 40-50 m. This case study suggests that greater efforts should be devoted to exploit the potential of a coupled analysis of Rayleigh and Love waves from microtremor array measurements

Source parameters and seismic moment-magnitude scaling for Northwestern Turkey

Abstract The source parameters of 523 aftershocks (0.5 ML 5.9) of the 1999 Kocaeli earthquake are determined by performing a two-step spectral fitting procedure. The source spectrum, corrected for both site and propagation effects, is described in terms of a standard x-square model multiplied by an exponential term of frequency. The latter term is introduced to estimate the high-frequency (f 12 Hz) fall-off of the acceleration source spectra by computing the j parameter. The seismic moments obtained range between 1.05 1014 and 2.41 1017 N m, whereas the Brune stress drops are between 0.002 and 40 MPa. The j value varies between 0.00 and 0.08 sec, indicating a decay of the acceleration level at the higher frequency part of the spectrum greater than that assumed by the x 2 model. Both the stress drop and the j parameter show the tendency of increasing with aftershock magnitude. No evidence of self-similarity breakdown is observed between the source radius and M0. Finally, both the seismic moment and the moment magnitude are compared with the local magnitude to derive new moment–magnitude relationships for the area

Empirical ground-motion prediction equations for Northwestern Turkey using the aftershocks of the 1999 Kocaeli earthquake

We present ground motion models for northwestern Turkey using the aftershocks of the Mw 7.4, 1999 Kocaeli earthquake. We consider 4047 velocity and acceleration records for each component of motion, from 528 earthquakes recorded by stations belonging to regional networks. The ground motion models obtained provide peak ground velocity, peak ground acceleration, and spectral accelerations for 8 different frequencies between 1 and 10 Hz. The analysis of the error distribution shows that the record-to-record component of variance is the largest contribution to the standard deviation of the calibrated ground- motion models. Furthermore, a clear dependence of inter-event error on stress drop is observed. The empirical ground-motion prediction equations, derived for both the larger horizontal and vertical components, are valid in the local magnitude range from 0.5 to 5.9, and for hypocentral distances up to 190 km. Citation: Bindi, D., S. Parolai, H. Grosser, C. Milkereit, and E. Durukal (2007), Empirical ground-motion prediction equations for northwestern Turkey using the aftershocks of the 1999 Kocaeli earthquake