CONSTRAINS ON THE NEAR-SOURCE MOTIONS OF THE KOS-BODRUM 20 JULY 2017 MW6.6 EARTHQUAKE (original) (raw)
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Characteristics of the strong ground motions from the 6 April 2009 L’Aquila earthquake, Italy
Soil Dynamics and Earthquake Engineering, 2010
An M w 6.25 earthquake occurred on April 6, 2009 at 03:33 a.m. local time, in the Abruzzo region (Central Italy), close to the city of L'Aquila. The earthquake ruptured a North-West (NW)-South-East (SE) oriented normal fault dipping toward the South-West (SW), with the city of L'Aquila lying a few kilometers away on the hanging wall.
Journal of Seismology
A damaging earthquake occurred on 14 July 1993 in Patras, Western Greece. The mainshock (local magnitude 5.1) was followed on the same day by two aftershocks of magnitudes 4.4 M L and 3.6 M L , respectively. The strong motion record of the mainshock is studied, based on the teleseismically determined seismic moment and focal mechanism. The Discrete Wavenumber (DW) and Empirical Green's Function (EGF) methods are used. The main conclusion is that the 1993 Patras mainshock had a complex S-wave group mainly due to structural (path and site) effect. However, some effects of the rupture stopping on the peak ground acceleration (0.2 g in the so-called S3 phase) cannot be ruled out. Two values of the source radius are suggested: R = 1.9 and 3.0 km. The strong motion record better agrees with R = 1.9 km. If the latter is true, the stress drop was of the order of 20 MPa, i.e., higher than often reported for comparable events in Western Greece. Regardless of the true source radius, the ratio of stress drops between the mainshock and aftershocks was about 1-2. The aftershock waveforms indicate significant lateral heterogeneities around Patras. Therefore, the ground-motion predictions of strong events in the area will remain highly non-unique until weak events from an immediate neighbourhood of the particular fault are recorded.
Bulletin of the Seismological Society of America, 2004
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 singlecorner-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.
Soil Dynamics and Earthquake Engineering, 2004
The objective of this study is to investigate the effects of local site conditions in the Adapazarı plain crossing the severely damaged central part of Sakarya during the 17th August earthquake. A two-dimensional model has been used to obtain the free-field motions in the valley and on surface formations. A preliminary geotechnical profile model developed from available but limited soil data was checked using recordings of some specific locations where a strong ground motion accelerometer array was in operation after the main earthquake. The range of validity of the model was assessed and modifications were made to compare with the actual recorded motions. The purpose of building such a model, which was used to study the aftershocks in Adapazarı, is to provide an improved database to be used for the design of structures in the city. This study also attempts to provide greater insight into the local site response phenomena through the use of a two-dimensional nonlinear analysis. Simulated site responses are in global agreement with the recorded data. In general agreement between the amplification functions of the computed and recorded data is satisfactory for the frequencies of engineering interest. Alluvial sites show amplification factors in the range of 4-6 in the frequencies between 0.6 and 2 Hz. A relatively shallow alluvial station, HASTAHANE, experienced considerable amplification for small aftershocks and deamplification for the strongest aftershock in frequencies between 0.5 and 2 Hz. Another station, TOYOTA, with significant deamplification characteristics and a clearly observed peak response frequency shift at the soil resonant frequency may have experienced soil shear failure during the strongest aftershock. Other alluvial stations, SEKER and GENC, show deamplification characteristics at 0.55 and at around 1 Hz. with the peak frequency shifts during the magnitude 5.8 aftershock. In general, site responses are larger in the alluvial basin for all aftershocks except the magnitude 5.8 event.
A comprehensive near-source strong motion database was compiled. The database includes over 2,800 free-field uncorrected peak ground acceleration (PGA) values from 48 worldwide earthquakes and more than 1,300 free-field response spectra from 33 worldwide earthquakes. The database includes the data recorded within 60 km of the causative fault from earthquakes ranging from 4.7 to 7.7 in magnitude. Attenuation models of PGA and response spectra for both the vertical and horizontal components were developed as functions of magnitude, source-to-site distance, type of faulting, and local soil conditions. The study clearly demonstrates the strong dependence of vertical-to-horizontal (V/H) spectral ratio on oscillator period, source-to-site distance, and local soil conditions. V/H shows a weaker and more limited dependence on magnitude and type of faulting. The largest short-period V/H ratios are observed to occur on Holocene Soil at short periods and short distances where they can reach values in excess of 1.5 at 0.1-sec period. The largest long-period V/H ratios are observed to occur on Hard Rock where they can reach values as high as 0.7. We conclude that the standard engineering practice of assigning V/H a value of two-thirds is unconservative at short periods, especially for unconsolidated soil, but conservative at long periods, and should be modified.
Geophysical Journal International, 2017
, M w 6.1 earthquake with 80 per cent of buildings collapsed or severely damaged. The high vulnerability of predominantly ancient buildings and the propensity of site geology to amplify ground motion on the Holocene sediments of the Aterno river valley were unanimously thought as responsible for the huge destruction. To quantify site effects in the damaged zone of Onna and study source scaling over a wide magnitude range (2.0 ≤ M L ≤ 5.4), we have used recordings of 20 stations installed in Onna and other villages around L'Aquila. We analyse more than 1000 seismograms of 202 aftershocks occurring up to source-to-receiver distances of 50 km and infer site and source parameters by means of an inversion procedure. The source spectra inferred from the data inversion confirm the large variability in the high-frequency radiation already found by other authors for L'Aquila earthquakes, with Brune stress drops around 10 MPa at the highest magnitudes of the investigated range and spreading mostly between 0.1 and 1 MPa at smaller magnitudes. Moreover, the inversion of our data yields larger amplitudes of empirical transfer functions in the village of Onna confirming the role of the local geology on damage. The site functions of Onna show a common resonance mode around 2.7 Hz, with amplitudes attaining a factor of 4-5. Moreover, we find that the transfer function amplitude does not decrease below 2 in a large high-frequency band above the site resonant frequency, up to more than 10 Hz. This indicates a further broad-band contribution to the ground motion amplification in Onna. In a simulation of the main shock scenario applying the estimated source scaling to aftershock records, Onna results in the highest accelerations among the villages around L'Aquila. In distinct contrast, transfer functions close to unity in the entire frequency band are found for stations installed on harder rock formations (Mesozoic limestone and Pleistocene siltstone). Their scenario accelerations result in the smallest values, consistently with the lowest macroseismic intensities.
Bulletin of the Seismological Society of America, 2006
What can be learned about absolute site effects on ground motions, with no geotechnical information available, in a very poorly instrumented region? In addition, can reliable source spectra be computed at a temporary deployment? These challenges motivated our current study of aftershocks of the 2001 M w 7.6 Bhuj earthquake, in western India, where we decouple the ambiguity between absolute source radiation and site effects by first computing robust estimates of coda-derived moment-rate spectra of about 200 aftershocks in each of two depth ranges. Crustal attenuation and spreading relationships, based on the same data used here, were determined in an an earlier study. Using our new estimates of source spectra, and our understanding of regional wave propagation, for direct S waves we isolate the absolute site terms for the stations of the temporary deployment. Absolute site terms for each station were determined in an average sense for the three components of the ground motion via an L 1-norm minimization. Results for each site were averaged over wide ranges of azimuths and incidence angles. The Bhuj deployment is characterized by a variable shallow geology, mostly of soft sedimentary units. Vertical site terms in the region were observed to be almost featureless (i.e., flat), with amplifications slightly Ͻ1.0 within wide frequency ranges. As a result, the horizontal-to-vertical (H/V) spectral ratios observed at the deployment mimic the behavior of the corresponding absolute horizontal site terms, and they generally overpredict them. This differs significantly from results for sedimentary rock sites (limestone, dolomite) obtained by Malagnini et al. (2004) in northeastern Italy, where the H/V spectral ratios had little in common with the absolute horizontal site terms. Spectral ratios between the vector sum of the computed horizontal site terms for the temporary deployment with respect to the same quantity computed at the hardest rock station available, BAC1, are seriously biased by its nonflat, nonunitary site response. This indicates that, occasionally, the actual behavior of a rock outcrop may be far from that of an ideal, reference site (Steidl et al., 1996).
Soil Dynamics and Earthquake Engineering, 2004
The objective of this study is to investigate the effects of local site conditions in the Adapazarı plain crossing the severely damaged central part of Sakarya during the 17th August earthquake. A two-dimensional model has been used to obtain the free-field motions in the valley and on surface formations. A preliminary geotechnical profile model developed from available but limited soil data was checked using recordings of some specific locations where a strong ground motion accelerometer array was in operation after the main earthquake. The range of validity of the model was assessed and modifications were made to compare with the actual recorded motions. The purpose of building such a model, which was used to study the aftershocks in Adapazarı, is to provide an improved database to be used for the design of structures in the city. This study also attempts to provide greater insight into the local site response phenomena through the use of a two-dimensional nonlinear analysis. Simulated site responses are in global agreement with the recorded data. In general agreement between the amplification functions of the computed and recorded data is satisfactory for the frequencies of engineering interest. Alluvial sites show amplification factors in the range of 4-6 in the frequencies between 0.6 and 2 Hz. A relatively shallow alluvial station, HASTAHANE, experienced considerable amplification for small aftershocks and deamplification for the strongest aftershock in frequencies between 0.5 and 2 Hz. Another station, TOYOTA, with significant deamplification characteristics and a clearly observed peak response frequency shift at the soil resonant frequency may have experienced soil shear failure during the strongest aftershock. Other alluvial stations, SEKER and GENC, show deamplification characteristics at 0.55 and at around 1 Hz. with the peak frequency shifts during the magnitude 5.8 aftershock. In general, site responses are larger in the alluvial basin for all aftershocks except the magnitude 5.8 event.
2000
Acceleration time series recorded by the Italian Strong Motion Network (RAN) during the October 31, 2002 (Mw=5.8), Molise earthquake, are employed in order to investigate source effects on the ground motion in the epicentral area. We consider two different seismogenic sources: a fault model inferred from inversion of teleseismic, regional and local seismic signals , and a fault model based on seismotectonic data . Both source studies suggest a deep location of the earthquake fault plane (ranging from 6.0 to 20.1 km and from 12.0 to 19.9 km, respectively), however, with considerably different fault lengths (5.2 and 10.5 km, respectively), and widths (14.2 and 8 km, respectively). Due to these differences, only the second model allows for effective horizontal unilateral rupture propagation. Finite fault effects are modelled by the Deterministic-Stochastic-Method (DSM) [Pacor et al., 2005], and the Hybrid Integral-Composite source model (HIC) [Gallovic and Brokesova, 2006]. In both methods k-square slip distributions on the faults are considered. We simulate the October 31, 2002 earthquake considering: 1) Vallée and Di Luccio [2005] faultwith a bilateral rupture propagation, and 2) Basili and Vannoli [2005] fault with unilateral directions of the rupture propagation.