Aybige Akinci

Attenuation of coda waves in Western Anatolia

Abstract By analyzing the decay of coda wave amplitude, we have determined coda Q,Qc, in Western Anatolia (Turkey). Using the single isotropic scattering model, we analyzed 116 earthquakes which registered at the Gebze station by using five narrow frequency bands centered at 1.5, 3, 6, 8 and 10 Hz. Coda Q values were obtained using different lapse times, between 30 and 190 s, in steps of 10 s. Coda Qc for Western Anatolia depends on frequency and lapse time. For a lapse time of 30 s, the frequency dependence of Qc is Q c (ƒ) = 50.7ƒ 1.01 and for a lapse time of 190 s it is Q c (ƒ) = 183.2ƒ 0.76 . In this region, the exponential value of the Qc frequency dependence is practically constant, between 0.7 and 1.0. The obtained coda Qc values were compared with those estimated in other regions. The coda Qc values for lapse times between 60 and 100 s for Western Anatolia and Southern Spain are practically the same, indicating similar coda wave attenuation patterns in both regions of the Mediterranean Basin.

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 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.

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.

Evaluation of Deep Sediment Velocity Structure in the New Madrid Seismic Zone

Detailed knowledge of the physical properties of the sediments filling the Mississippi Embayment has proven critical to both unravel the tectonic frame-work operating in the region and assess the seismic hazards posed by the New Madrid Seismic Zone. In this article we show that independent geotechnical estimates for P - and S -wave velocities are compatible with a sedimentary model of K-feldspar clasts embeded in water, and we test its validity by modeling receiver functions at a number of broadband stations. By constraining the bulk sediment thicknesses beneath each station from independent reflection profiling estimates, we have been able to recover the depth to the top of the Cretaceous from the receiver function data at individual stations. Our receiver function modeling thus provides confidence in the velocity and density structures extrapolated from in situ geotechnical measurements in the Upper Mississippi Embayment.

Frequency-dependent attenuation of S and coda waves in Erzincan region (Turkey)

Abstract The attenuation structure of the Erzincan region is studied using the single scattering model of the coda wave generation and coda normalization method for S waves. We have determined the seismic quality factors Q s ( f ) (for S waves) and Q c ( f ) (for coda waves) as a function of frequency for the frequency range 1.5–24 Hz. The quality factors were derived for 161 seismograms that were registered in a 7-day period after the 13 March 1992 Erzincan earthquake ( M s = 6.8). Digital recordings from six stations at epicentral distances ranging from 5 to 40 km were used. Frequency-dependent attenuation of S waves according to Q s ( f ) = Q o fn in the crust beneath Erzincan was obtained as Q s ( f ) = 35 f 0.83 by analysing seismograms of 161 local earthquakes which were selected on the basis of good signal to noise ratios. Coda Q values were calculated from the amplitude decay rate of the S-wave coda in seven frequency bands from 1.5 to 24 Hz. In order to investigate a possible lapse time dependence of the estimated coda Q , we have carried out the analysis of each seismogram using four coda lengths which correspond to four different lapse times 20, 30, 40, 50 s measured from the origin time of the earthquake. Q c averages about 46 and 766 at 1.5 Hz and 24 Hz, respectively, at 20 s lapse time and has a frequency dependence of the form Q c = 29 f 1.03 . For large lapse time data, t c = 50 s, Q c ( f ) was found to vary from 84 at 1.5 Hz to 783 at 24 Hz with the degree of the frequency dependence, n = 0.81.

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.

Ground-Motion Scaling in Eastern Sicily, Italy

We describe the characteristics of crustal wave propagation in eastern Sicily by using the background seismicity of the area. We follow the approach de- scribed by Malagnini, Hermann, and Di Bona (2000) and Malagnini et al. (2002). Our data set consists of 106 earthquakes recorded by nine three-component digital seismic stations between 1994 and 2001. We used only crustal events (depths shal- lower than 25 km), with local magnitudes ranging from 1.0 to 4.3, and hypocentral distances from 10 to 130 km. Peak ground velocities from 1311 narrow bandpass-filtered waveforms are mea- sured in thefrequency range 1.0-16.0 Hz,and regressedtodefinecrustalpropagation, excitation, and site characteristics, with respect to a reference station. A subsequent modeling effort is carried out, through the use of random vibration theory (RVT), for obtaining a quantitative evaluation of the apparent geometrical spreading g(r), and of the crustal quality factor Q(f). An attenuation parameter, j0, is also evaluated relative to a reference rock site. The attenuation and source parameters estimated in this study are used through the RVT in order to predict the peak horizontal ground acceleration (PGA), and the 5% damping pseudoacceleration spectra (PSA).

M ≥ 7 earthquake rupture forecast and time‐dependent probability for the sea of Marmara region, Turkey

We forecast time-independent and time-dependent earthquake ruptures in the Marmara region of Turkey for the next 30 years using a new fault segmentation model. We also augment time-dependent Brownian passage time (BPT) probability with static Coulomb stress changes (ΔCFF) from interacting faults. We calculate Mw > 6.5 probability from 26 individual fault sources in the Marmara region. We also consider a multisegment rupture model that allows higher-magnitude ruptures over some segments of the northern branch of the North Anatolian Fault Zone beneath the Marmara Sea. A total of 10 different Mw = 7.0 to Mw = 8.0 multisegment ruptures are combined with the other regional faults at rates that balance the overall moment accumulation. We use Gaussian random distributions to treat parameter uncertainties (e.g., aperiodicity, maximum expected magnitude, slip rate, and consequently mean recurrence time) of the statistical distributions associated with each fault source. We then estimate uncertainties of the 30 year probability values for the next characteristic event obtained from three different models (Poisson, BPT, and BPT + ΔCFF) using a Monte Carlo procedure. The Gerede fault segment located at the eastern end of the Marmara region shows the highest 30 year probability, with a Poisson value of 29% and a time-dependent interaction probability of 48%. We find an aggregated 30 year Poisson probability of M > 7.3 earthquakes at Istanbul of 35%, which increases to 47% if time dependence and stress transfer are considered. We calculate a twofold probability gain (ratio time dependent to time independent) on the southern strands of the North Anatolian Fault Zone.