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  • 1
    Publication Date: 2006-02-01
    Description: We have analyzed the aftershocks (M (sub L) 〈4.5) following the 1999 Izmit earthquake (M (sub w) 7.4) to infer the frequency-dependent attenuation characteristics of both P and S waves, in the frequency range from 1 to 10 Hz and in the distance range from 10 to 140 km. A linear-predictive model is assumed to describe the spectral amplitudes in terms of attenuation and source contributions. The results show that both P and S waves undergo a strong attenuation along ray paths shorter than 40 km, while the secondary arrivals significantly contribute to the spectral amplitudes over the distance range from 40 to 60 km, as also confirmed by the computation of synthetic seismograms. For longer ray paths, the decrease in attenuation suggests an increase in the propagation efficiency with depth. Finally, the spectral attenuation curves are flattened, or sloped upward at low frequencies in the range from 100 to 140 km, due to the contemporary arrivals of direct waves and postcritical reflections from the Moho. In terms of geometrical spreading and anelastic attenuation, the attenuation in the range from 10 to 40 km is well described by a spreading coefficient n = 1 for both P and S waves, and the quality factors can be approximated by Q (sub S) (f) = 17f (super 0.80) for 1 〈 or = f 〈 or = 10 Hz and Q (sub P) (f) = 56f (super 0.25) for 2.5 〈 or = f 〈 or = 10 Hz. For ray paths in the range from 60 to 80 km, the attenuation weakens but the interaction between seismic waves and propagation medium is more complex. The multilapse time window analysis (MLTWA) is applied to quantify the amount of scattering loss and intrinsic absorption for S waves. The seismic albedo B (sub 0) decreases from 0.5 at 1 Hz to 0.3 at 10 Hz, while the total quality factor Q (sub T) increases from about 56 to 408. The multiple lapse time-window analysis (MLTWA) results provide only an average estimate of the attenuation properties in the range from 10 to 80 km. In fact, by neglecting the variation of attenuation with depth, the MLTWA results underestimate attenuation for distances less than 40 km, and do not capture the significant features caused by the integrated energy of the secondary arrivals observed in the range from 40 to 60 km.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 2
    Publication Date: 2006-02-01
    Description: The attenuation of shear waves propagating in the crust of northwestern Turkey has been investigated in the frequency range 1-10 Hz. A standard spectral inversion scheme is applied to a data set of 245 aftershocks (M (sub L) 〈4.5) of the 1999 Izmit earthquake. The obtained attenuation-with-distance curves have been described in terms of the t* cumulative attenuation parameter and its dependence on frequency and distance investigated. At 1 Hz, Q (super -1) , evaluated by normalizing t* to the travel time, is generally larger than 0.025 for source-to-station distances smaller than 40 km, indicating the presence of a highly attenuating upper crust in the area. Over longer distances, Q (super -1) decreases, suggesting a decrease in the attenuation with depth. By contrast, the normalized t* computed for earthquakes recorded at stations having almost the same distance from the sources do not show a strong dependence on the backazimuth. These results suggest that the decrease of Q (super -1) with depth is more significant than its lateral variations. Regarding its frequency dependence, Q (super -1) almost linearly decreases with frequency. Finally, the near-surface-attenuation parameter k is evaluated at 12 stations and the results discussed in terms of site, event, and propagation contributions. The event contribution is not negligible and shows a significant positive correlation with magnitude. The site term is smaller than 0.020 sec for rock or topographic sites, while it assumes values of 0.036 sec and 0.042 sec for two stations installed over thick soft sedimentary layers.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 3
    Publication Date: 2010-12-01
    Description: Kyrgyzstan, which is located in the collision zone between the Eurasian and Indo-Australian lithosphere plates, is prone to large earthquakes as shown by its historical seismicity. Hence, an increase in the knowledge and awareness by local authorities and decision makers of the possible consequence of a large earthquake, based on improved seismic hazard assessments and realistic earthquake risk scenarios, is mandatory to mitigate the effects of an earthquake. To this regard, the Central Asia Cross-Border Natural Disaster Prevention (CASCADE) project aims to install a cross-border seismological and strong motion network in Central Asia and to support microzonation activities for the capitals of Kyrgyzstan, Uzbekistan, Kazakhstan, Tajikistan, and Turkmenistan. During the first phase of the project, a temporary seismological network of 19 stations was installed in the city of Bishkek, the capital of Kyrgyzstan. Moreover, single-station noise recordings were collected at nearly 200 sites. In this study, the site amplifications occurring in Bishkek are assessed by analyzing 56 earthquakes extracted from the data streams continuously acquired by the network, as well as from the single-station noise measurements. A broadband amplification (starting at approximately 0.1 and 0.2 Hz), is shown by the standard spectral ratio (SSR) results of the stations located within the basin. The reliability of the observed low-frequency amplification was validated through a time-frequency analysis of denoised seismograms. Discrepancies between horizontal-to-vertical spectral ratio and SSR results are due to the large amplification of the vertical component of ground motion, probably due to the effect of converted waves. The single-station noise results, once their reliability was assessed by their comparison with the earthquake data, have been used to produce the first fundamental resonance frequency map for Bishkek, whose spatial variation shows a good agreement with the presence of an impedance contrast within the Tertiary sedimentary cover.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 4
    Publication Date: 2011-03-22
    Description: Structural health monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures. The usage of recording systems exploiting wireless communication technology is particularly suitable for SHM, especially for rapid response following earthquakes. In this study, both of these issues are combined, and we report on the application of seismic interferometry to SHM using a dataset of seven earthquakes collected using a novel wireless system of accelerometers during the L'Aquila, Italy, seismic sequence in 2009. We show that interferometric analysis allows the estimation of the shear-wave velocity of seismic phases propagating throughout a structure, and, most important for SHM purposes, allows the monitoring of the velocity variations during the aftershock sequence. Moreover, innovatively we apply the S transform to the building response functions retrieved by interferometry to estimate the fundamental resonance frequency and the quality factor Q.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 5
    Publication Date: 2011-02-01
    Description: After the recent Central Italy Earthquake of the 6th April 2009 (Mw = 6.3), the Italian and German engineer and geophysicist Task Force carried out a wide characterization of sites, buildings and damages. In Navelli, a town about 35 km far from epicentre, heavy damage occurred on a reinforced concrete (RC) building that represent an anomalous case of damage, when compared with those occurred in the neighbouring area. In this paper, characterization of the site and damage of the Navelli RC Building is reported and discussed. We performed ambient noise and strong motion measurements, installing one three-directional accelerometer on each floor of the structure and two in free-field, and we have carried out repeated measurements using a couple of three-directional tromometers. In the mean time, a geological survey was carried out and the site response was investigated, with the aid of down-hole measurements. It was thus possible to investigate the structural response and damage taking into account site condition. One of the main results of this work is that repeating analyses using ambient noise measurements show that the main structural frequencies reached after the first damaging shock are constant over time, and then the structural behaviour appears stationary at long term. On the other hand, the strong motion recordings show that the building exhibits a transient non-stationary behaviour as the fundamental frequency changes during each aftershock, then returning to the starting value after each event. ©2010 Springer Science+Business Media B.V.
    Print ISSN: 1570-761X
    Electronic ISSN: 1573-1456
    Topics: Geosciences
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  • 6
    Publication Date: 2003-10-01
    Description: A local magnitude scale is derived for northwestern Turkey, using data collected by a temporary network installed by the German Task Force for Earth-quakes after the 1999 Izmit earthquake (M (sub w) 7.4) and the permanent Sapanca-Bolu network. We computed Wood-Anderson seismograms for over 5353 arrivals at 27 three-component stations, from 530 earthquakes. The hypocentral distances considered range from 5 to 140 km, with the best represented range being from 5 to 70 km. We inverted the measured amplitudes following both the nonparametric Richter's (1958) and parametric Bakun and Joyner (1984) approaches. These methods yield consistent magnitude values and station corrections. However, the calculated non-parametric distance correction, logA (sub 0) , implies that ground-motion attenuation is higher than what is accounted for by the equation calibrated in central California. In the range 5-62 km, the best fit is provided by -logA (sub 0) = log(R/17)+0.00960(R-17)+2. This equation is obtained constraining the geometrical spreading parameter n to 1. The calculated value of k = 0.00960 confirms that in this range of distance seismic waves could propagate through a low-Q volume. Station corrections, which allow for a significant reduction of M (sub L) residuals, range between + or -0.5 magnitude units, suggesting a strong influence of local site effects on the amplitude of ground motion. In accord with the obtained attenuation and station corrections, the magnitudes of the considered events range from 0.4 to 4.8.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 7
    Publication Date: 2004-06-01
    Description: On 17 August 1999, the M (sub W) 7.4 Izmit earthquake occurred in north-western Turkey. A temporary seismic network was installed to improve the geometry of a previously installed network. In this study, 262 aftershocks of the Izmit earthquake with magnitude M (sub L) ranging between 0.4 and 4.5 were analyzed using digital recordings from 17 stations of these networks. S-wave and P-wave spectral records, corrected for path attenuation function, were inverted by means of the generalized inversion technique (GIT). The GIT site responses were compared with those calculated by horizontal-to-vertical (H/V) ratios applied to both earthquake data and to pre-event noise. In most cases, the H/V of the earthquake data provided site responses with shapes consistent with those obtained using GIT. However, the level of amplification was occasionally found to be different. In general, these discrepancies can be explained either by the amplification affecting the vertical component of the ground motion that we detected by GIT or by the waves having a propagation in almost the vertical direction. Using the H/V ratio applied to noise, for most cases, we obtained not only the peaks for the fundamental resonance frequencies, but site responses with shapes similar to those obtained by GIT. The detected level of amplifications using the H/V ratio showed a tendency not to exceed those calculated by the inversion. Finally, amplifications up to a factor 5 are found for stations placed over complex topography. For these sites, amplifications are not always correctly identified by the H/V technique, using either earthquakes or noise.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 8
    Publication Date: 2010-01-27
    Description: We investigate the crustal structure beneath the western part of the North Anatolian fault zone (NAFZ), an area where at least five damaging earthquakes occurred during the twentieth century. This study is based on local earthquake tomography using the data from aftershocks of the Izmit event (17 August 1999, M 7.4) recorded by stations of permanent and temporary networks. We derive the distribution of V (sub P) , V (sub S) , and the V (sub P) /V (sub S) ratio based on the iterative inversion for both V (sub P) -V (sub S) and V (sub P) -V (sub P) /V (sub S) using the LOTOS code. Innovatively, in this study we perform an inversion for frequency-dependent S-wave attenuation (1/Q (sub S) ). The reliability of the results is assessed through synthetic tests. The distributions of the resulting seismic parameters (V (sub P) , V (sub S) , V (sub P) /V (sub S) , and Q (sub S) ) highlight important geodynamical features in the study area. The low-velocity and high-attenuation patterns mostly correlate with the fracturing zones of the NAFZ. Low velocities are also observed beneath the main sedimentary basins (e.g., Adapazari, Duezce, and Kuzuluk). High-velocity and low-attenuation patterns correlate with blocks presumed to be rigid (Kocaeli, Armutlu, and Almacik Blocks). The rupture traces of the largest earthquakes in this area pass generally in the transition areas between high and low velocities, while moderate and weak seismicity is mostly concentrated in low-velocity areas. Based on these results we propose and discuss the role that the Almacik Block could have played in producing the largest earthquakes in the study area in the twentieth century.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
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  • 9
    Publication Date: 2015-04-08
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
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  • 10
    Publication Date: 2016-12-21
    Description: In this paper, an underground experiment at the Äspö Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Äspö HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Ävrö granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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