Luca Malagnini

Ground-Motion Scaling in the Apennines (Italy)

Regressions over a data set of broadband seismograms are performed to quantify the attenuation of the ground motion in the Apennines (Italy), in the 0.25–5.0 Hz frequency band. The data set used in this article consists of over 6000 horizontal-component seismograms from 446 events, with magnitude ranging from M w ≃ 2 to M w = 6.0. Waveforms were collected during recent field experiments along the Apennines. Data from two MedNet broadband stations, located in central and southern Apennines, were also used. Seismograms are bandpass-filtered around a set of sampling frequencies, and the logarithms of their peak values are written as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathrm{AMP}(f,{\ }r)=\mathrm{EXC}(f,{\ }r\_{\mathrm{ref}})+\mathrm{SITE}(f)+D(r,{\ }r\_{\mathrm{ref}},{\ }f).\] \end{document} EXC( f, r ref) is the excitation term for the ground motion at the hypocentral distance r ref. SITE( f ) represents the distortion of the seismic spectra induced by the shallow geology at the recording site. D ( r, r ref, f ) includes the effects of the geometrical spreading, g ( r ), and of a frequency-dependent crustal attenuation Q . It is determined as a piecewise linear function, allowing to consider complex behavior of the regional attenuation. A first estimate of D ( r, r ref, f ) is obtained using a coda normalization technique (Aki, 1980; Frankel et al., 1990) and used as a starting model in the inversion of the peak values. Then, by trial and error, the empirical D ( r, r ref, f ) is fitted using a trilinear geometrical spreading, with crossover distances at 30 and 80 km, and the crustal parameter \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[Q(f)=130\left(\frac{f}{f\_{\mathrm{ref}}}\right)^{0.10};{\ }f\_{\mathrm{ref}}=1.0{\ }\mathrm{Hz}\] \end{document} These results suggest a low- Q crust in the entire Apennines in the 0.25–5.0 Hz range, implying that the seismic hazard in the region may be dominated by the local seismicity. The final section is devoted to highlight the limitations of the formula proposed by Console et al. (1988) to estimate duration magnitudes M d in Italy.

Characteristics of the Ground Motion in Northeastern Italy

A large data set of ground-velocity time histories from earthquakes that occurred in Friuli-Venezia Giulia (northeastern Italy) was used to define regional predictive relationships for ground motion, in the 0.25- to 14.0-Hz frequency band. The bulk of the data set was provided by the seismic network run by Centro Ricerche Sismologiche (CRS), a department of the Istituto Nazionale di Oceanografia e Geofisica (OGS). A collection of 17,238 selected recordings from 1753 earthquakes was compiled for the years 1995–1998, with magnitudes ranging from M w ∼1 to 5.6. Ninety-six three-component strong-motion waveforms belonging to the largest events of the 1976–1977 Friuli seismic sequence were also taken from the enea-enel accelerogram database and included in our data set. For the strongest event, which occurred on 6 May 1976 at 20:00 local time, an average local magnitude M L 6.6 was computed by Bonamassa and Rovelli (1986). The inclusion of a large number of acceleration time histories from this earthquake and six others, from magnitudes from M w 5.2 to magnitude M s 6.1 (three of them of M s ∼6.0), extends the validity of the predictive relationships proposed in this study up to the highest magnitude ever recorded in the region. A total of 10,256 vertical-component and 6982 horizontal-component seismograms were simultaneously regressed for excitation and site characteristics, as well as for the crustal propagation, in the hypocentral distance range 20–200 km. Results are given in terms of excitation, attenuation, and specific site for the vertical ground motion, together with a horizontal-to-vertical ratio for each existing horizontal-component seismometer. The regional propagation was modeled in the 0.5- to 14.0-Hz frequency band by using a frequency-dependent piece wise continuous linear (in a log–log space) geometrical spreading function and a frequency-dependent attenuation parameter: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[Q(f)=260(f{/}1.0)^{0.55}\] \end{document} The excitation spectra of larger events were modeled by using the regional propagation, a single-corner frequency Brune spectral model characterized by an effective stress parameter, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[{\Delta}{\sigma}=60{\ }\mathrm{MPa},\] \end{document} and by a regional estimate of the near-surface, distance-independent, network-averaged attenuation parameter \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[{\kappa}_{0}=0.045{\ }\mathrm{sec}\] \end{document} that was estimated from the rolloff of the empirical source spectra obtained from the regressions. Other studies (De Natale et al. , 1987; Cocco and Rovelli, 1989; Singh et al. , 2001) suggested large stress drops (Δ σ ≃ 30–100 MPa,) to explain the high-frequency amplitude levels of the seismic radiation of the largest quakes of the 1976 sequence. Predictions for peak ground acceleration (PGA) and pseudo–spectral velocity (PSV) (5% damping) were computed through the use of the random vibration theory (RVT), with the parameters obtained from the regressions of this study.

Ground-Motion Scaling in the Region of the 1997 Umbria-Marche Earthquake (Italy)

We used broadband waveforms collected at short hypocentral distances ( r ≤ 40 km) during the Umbria-Marche (Italy) seismic sequence of September–November, 1997, in order to calculate the scaling relationships for the ground motion within the meizoseismal area, in the 0.5–16.0 Hz frequency band. Data were collected by a 10-station portable seismic network deployed by the Istituto Nazionale di Geofisica (Rome, Italy) shortly after the occurrence of the first mainshock of the sequence, on 26 September 1997. Among the thousands of events recorded, we selected 142 earthquakes characterized by good signal-to-noise ratios at all frequencies, and by the absence of multiple shocks within the time window spanned by each recording. The data set of the selected waveforms was made of 2030 horizontal-component seismograms. The logarithm of the peak values of narrow bandpass-filtered versions of the velocity time histories are modeled at each frequency as \[AMP(f,r)=EXC(f,r_{\mathrm{ref}})+SITE(f)+D(r,r_{\mathrm{ref}},f).\] EXC ( f,r ref ) is the excitation term at an arbitrary reference hypocentral distance, r ref ; SITE ( f ) is a site term. The empirical attenuation functional, D ( r,r ref , f ), represents an estimate of the average crustal response for the region, at the hypocentral distance r , at the frequency f . It is modeled by using the following functional form: \[D(r,r_{\mathrm{ref}},f)=\mathrm{log}{\ }g(r)-\mathrm{log}{\ }g(r_{\mathrm{ref}})-\frac{{\pi}f(r-r_{\mathrm{ref}})}{{\beta}Q_{0}(f{/}f_{\mathrm{ref}})^{{\eta}}};{\ }(f_{\mathrm{ref}}=1.0{\ }\mathrm{Hz},{\ }r_{\mathrm{ref}}=10{\ }\mathrm{km}).\] g ( r ) = r -1 is the body-wave geometrical spreading function; β = 3.5 km/sec is the shear-wave velocity in the crust. Due to the constraints applied to the system prior to the regressions, the excitation term represents the expected peak ground motion at the reference distance, as it would be observed at a site representative of the average site response of the network. The random vibration theory (RVT) is used to obtain a theoretical prediction of the attenuation functional. For reproducing D ( r,r ref , f ) we use the crustal attenuation parameter \[Q(f)=130(f{/}f_{\mathrm{ref}})^{0.10}\] obtained by Malagnini et al. (2000) from the analysis of a regional data set representative of the entire Apennines, in the (0.24–5.0 Hz) band. Two parameters are used to predict shapes and levels of the seismic spectra, the stress drop Δσ, and a high-frequency attenuation parameter κ 0 . The values used to reproduce the observed velocity spectra are \[{\Delta}{\sigma}=200{\ }\mathrm{bars};{\ }{\kappa}_{0}=0.04{\ }\mathrm{sec}.\] The indicated stress drop was estimated in this region by Castro et al. (2000), on recordings of the largest shock of the Umbria-Marche sequence.

Ground-Motion Scaling in the Kachchh Basin, India, Deduced from Aftershocks of the 2001 Mw 7.6 Bhuj Earthquake

We studied the excitation, propagation, and site effects in the Kachchh basin of India by using ground-motion recordings from a temporary seismograph network deployed to study aftershocks of the M w 7.6 Bhuj earthquake of 26 January 2001. The Kachchh basin has been proposed as a useful analog region for studying hazard in other earthquake-prone but slowly deforming regions, such as the central United States. The earthquakes we studied ranged in size from about M 2 to M 5.2, and travel paths ranged from a few kilometers to about a hundred kilometers. There was a broad range of focal depths among the aftershocks, so the data were divided into two overlapping subsets to test the sensitivity of the derived propagation and source parameters to focal depth. Parameters we constrained include the source excitation terms (related to stress drop), a frequency-dependent attenuation operator, a geometric spreading function, and an operator to account for site effects. Our results indicate that seismic-wave attenuation in Kachchh crust is very low, similar to other continental intraplate areas such as central and eastern North America. We also estimated seismic moments and stress drops for the earthquakes by fitting single-corner-frequency source-model spectra to the observed spectra, corrected for propagation by using our derived parameters. Stress drops were found to scale with seismic moment and to be rather high overall. By using a stochastic point-source model to estimate mainshock ground motions, we found that the distance decay of expected peak ground motions, assuming a stress drop of 15-20 MPa, compare well with the scant observations for the Bhuj earthquake. Ground-motion predictions for Kachchh, based on Bhuj aftershock data, support the idea that the region may have similar hazard to proposed analog areas in North America.

Spectral Shear-Wave Ground-Motion Scaling in Switzerland

Three-component recordings of local and regional earthquakes and explosions are used to assess the spectral characteristics of attenuation, excitation, and duration of ground motion in Switzerland. The data set consists of 292 events in the magnitude range from 2.0 to 5.2, with a total of 2958 waveforms recorded in Switzerland and the German border region from 1984 to 2000. Distance ranges from 5 to 350 km. Stations are located on National Earthquake Hazard Reduction Program 1994 site classes A and B outcrops. Empirical excitation, site, and attenuation terms are derived for the Fourier spectra and peak ground velocities of the ground motion in the frequency range 1-15 Hz by applying an iterative damped least-squares regression. These results are used to calibrate effective theoretical attenuation and excitation models. A grid search through the parameter space is then applied to obtain the quality factor Q ( f ) and a piecewise linear geometrical spreading function G ( r ), allowing complex behavior of attenuation. Optimum results are obtained for Q ( f ) = 270 f 0.50, G ( r ) = r -1.1 for 0-50 km, r -0.6 for 50-70 km, r +0.2 for 70-100 km, and r -0.5 for distances greater than 100 km. The increased amplitudes in the distance range 70-100 km can be explained by the reflection of shear waves at the Moho. This reflected wave energy also leads to an increased duration of ground motion for the same range. The inverted excitation terms are modeled, based on Brunes source spectrum, applying an effective stress parameter of 5-10 bars and a high-frequency roll off (e- πκ fk) κ = 0.015. The small values of κ agree with the mean site condition. A regional site difference between the Alps and the Alpine Foreland site is obtained with a factor of two higher amplitudes in the latter. Manuscript received 25 August 2001.

Regional Ground-Motion Scaling in Central Europe

Regressions of 2700 horizontal-component broadband seismograms from 213 seismic events recorded by the German Regional Seismic Network (67 earthquakes and 146 large mining explosions and rockbursts) are carried out to study the scaling relationships of high-frequency S -wave motion for central Europe. At a set of sampling frequencies, regressions were performed on the logarithms of the peak amplitudes of narrow bandpass-filtered seismograms, as well as on the logarithms of the Fourier components of the velocity spectra. At a fixed frequency f , these values are written as \[\mathrm{AMP}(f,{\ }r)=\mathrm{EXC}(f,{\ }r_{\mathrm{ref}})+\mathrm{SITE}(f)+D(r,{\ }r_{\mathrm{ref}},{\ }f).\] EXC( f, r ref ) is the excitation at an arbitrary reference hypocentral distance, r ref , SITE( f ) is a site term, and D ( r, r ref , f ) describes the crustal attenuation in the region. The crustal propagation term, empirically estimated in the (0.5–16.0 Hz) frequency band and (40–600 km) distance range, is modeled using a complex geometrical spreading function and a frequency-dependent crustal Q . We suggest \[Q(f)=Q_{0}{\cdot}\left(\frac{f}{f_{\mathrm{ref}}}\right)^{{\eta}},{\ }\begin{array}{ccc}Q_{0}&=&400\\{\eta}&=&0.42\\f_{\mathrm{ref}}&=&1.0{\ }\mathrm{Hz}\end{array}\] and a log-log quadrilinear geometrical spreading. A factor exp(-πκ 0 f ) is used to fit the high-frequency roll-off of the inverted excitation terms. Since we deal with two different kinds of sources (explosions-rockbursts and earthquakes), we use \[{\kappa}_{0}=\begin{array}{l}0.08{\ }\mathrm{sec}{\ }(\mathrm{for\ explosions});\\0.05{\ }\mathrm{sec}{\ }(\mathrm{for\ earthquakes}).\end{array}\] The same Brune spectral model, characterized by a stress drop Δσ = 30 bars, is used to fit both earthquakes and explosive excitation terms. A regression on the effective duration of the ground motion following the S -wave onset is also carried out. In central Europe, duration is observed to be almost frequency-independent. This property might be explained in terms of a self-similar distribution of crustal scatterers.

A Regional Ground Motion Excitation attenuation Model for the San Francisco Region

By using small-to-moderate earthquakes located within ∼200 km of San Francisco, we characterize the scaling of the ground motions for frequencies ranging between 0.25 and 20 Hz, obtaining results for geometric spreading, Q ( f ), and site parameters using the methods of Mayeda et al. (2005) and Malagnini et al. (2004). The results of the analysis show that, throughout the Bay Area, the average regional attenuation of the ground motion can be modeled with a bilinear geometric spreading function with a 30-km crossover distance, coupled to an anelastic function exp(− ϖfr / [capital greek beta]Q ( f ), where: Q ( f ) = 180 f 0.42 . A body-wave geometric spreading, g ( r ) = r −1.0 , is used at short hypocentral distances ( r g ( r ) = r −0.6 fits the attenuation of the spectral amplitudes at hypocentral distances beyond the crossover. The frequency-dependent site effects at twelve of the Berkeley Digital Seismic Network stations were evaluated in an absolute sense using coda-derived source spectra. Our results show the following. (1) The absolute site response for frequencies ranging between 0.3 Hz and 2.0 Hz correlate with independent estimates of the local magnitude residuals ( δM L ) for each of the stations. (2) Moment magnitudes ( M w ) derived from our path and site-corrected spectra are in excellent agreement with those independently derived using full-waveform modeling as well as coda-derived source spectra. (3) We use our weak-motion-based relationships to predict motions regionwide for the Loma Prieta earthquake, well above the maximum magnitude spanned by our data set, on a completely different set of stations. Results compare well with measurements taken at specific National Earthquake Hazards Reduction Program site classes. (4) An empirical, magnitude-dependent scaling was necessary for the Brune stress parameter to match the large-magnitude spectral accelerations and peak ground velocities with our weak-motion-based model. Online material: Tables of peak ground acceleration, peak ground velocity, and pseudo-spectral acceleration at 0.3 sec, 1.0 sec, and 3.0 sec.

High-Frequency Ground Motion in the Erzincan Region, Turkey: Inferences from Small Earthquakes

This study has been supported by Istituto Nazionale di Geofisica e Vulcanologia, INGV, Internal Project: “Attenuazione e leggi di scala nei paesi dell’area Mediterranea” (internally funded). R. B. Herrmann’s participation was supported by INGV and by the Earthquake Engineering Research Centers Program of the National Science Foundation under Award Number EEC-9701785.

Energy radiation from intermediate to large magnitude earthquakes: implications for dynamic fault weakening

Kevin Mayeda was supported under Weston Geophysical subcontract No. GC19762NGD and AFRL contract No. FA8718-06-C-0024. Work by L. Malagnini was performed under the auspices of the Dipartimento della Protezione Civile, under contract S3 – INGV-DPC (2007-2009), project: “Valutazione rapida dei parametri e degli effetti dei forti terremoti in Italia e nel Mediterraneo”.

3D Ground-Motion Estimation in Rome, Italy

Paleoseismic evidence and seismic-hazard analysis suggest that the city of Rome, Italy, has experienced considerable earthquake ground motion since its establishment more than 2000 years ago. Seismic hazards in Rome are mainly associated with two active seismogenic areas: the Alban Hills and the Central Apennines regions, located about 20 km southeast and 80–100 km east of central Rome. Within the twentieth century, M 6.8 and M 5.3 earthquakes in the Apennines and the Alban Hills, respectively, have generated intensities up to Mercalli-Cancani-Sieberg scale (mcs) VII in the city. With a lack of strong-motion records, we have generated a 3D velocity model for Rome, embedded in a 1D regional model, and estimated long-period (<1 Hz) ground motions for such scenarios from finite-difference simulations of viscoelastic wave propagation. We find 1-Hz peak ground velocities (PGVs) and peak ground accelerations (PGAs) of up to 14 cm/sec and 44 cm/sec2, respectively, for a M 5.3 Alban Hills scenario, largest near the northwestern edge of the Tiber River. Our six simulations of a M 7.0 Central Apennine scenario generate 0.5-Hz PGVs in Rome of up to 9 cm/sec, as well as extended duration up to 60 sec. The peak motions are similar to, but the durations much longer than those from previous studies that omitted important wave-guide effects between the source and the city. The results from the two scenarios show that the strongest ground-motion amplification in Rome occurs in the Holocene alluvial areas, with strong basin edge effects in the Tiber River valley. Our results are in agreement with earlier 2D SH - wave results showing amplification of peak velocities by up to a factor of 2 in the alluvial sediments, largest near the contact to the surrounding Plio-Pleistocene formations. Our results suggest that both earthquakes from the Alban Hills and the Central Apennines regions contribute to the seismic hazards in Rome. Although earthquakes from the former area may generate the larger peak motions, seismic waves from the latter region may generate ground motions with extended durations capable of causing significant damage on the built environment.