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A Comparison of Local Site Conditions with Passive and Active Surface Wave Methods
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... They achieved a good spatial distribution between V S30 and the geology: larger V S30 values were associated to Pleistocene and Holocene glacial-fluvial deposits (more rigid material); lower V S30 values were present on soft lacustrine and fluvial Quaternary sediments. Martínez-Pagán et al. (2014) Eker et al. (2010) and Kanli et al. (2006) for V S30 values estimated with MASW in Çubuk district (Turkey) and Dinar region (Turkey), respectively: V S30 values grew towards the edges of the alluvial basins where the alluvium deposits are gradually thinner. Kanlı et al. (2006) and Mártinez-Pagán et al., (2014) compared V S30 distribution with the damage distribution associated to the 1995 Dinar earthquake (Turkey) (Ms = 6.1) and 2011 Lorca earthquake (Spain) (Mw = 5.2), respectively. ...
Setúbal is a county located in the Lisbon metropolitan area (Portugal) which has been hit by historical earthquakes that were responsible for high social and economic losses. The 1858 earthquake, with an epicentre located offshore just south of the Setúbal region, is a good example of an earthquake that caused high damage (IX-VIII MM) in this county. The main city of the region is Setúbal itself, an important and populated urban agglomeration with several industrial facilities very close by. The old city centre is mostly settled upon shallow geological units and taking into consideration the historical seismicity, it is important to examine the possible site effects that could occur during future earthquakes affecting the Setúbal region.
The objectives of this study are therefore the characterisation of the seismic behaviour of Setubal's shallow geological formations and the evaluation of potential site effects. Shear wave velocity profiles were computed with the Refraction Microtremor (ReMi) method, spatial distribution of VS30 values was estimated and a map with the VS30 classification of soils according to the EC8 was produced. Spatial distributions of different data are presented and discussed: VS30 values, thickness of the shallow formations, top Pliocene surface, and results from Standard Penetration Tests (NSPT values). Numerical correlations between these parameters were established. Geotechnical characterisation was based on the analysis of 307 logs and 2950 Standard Penetration Tests. A total of 266 refraction microtremor measurements were performed obtaining VS profiles and VS30 values for 43 different sites. It was found that in Setúbal the majority of the Holocene units belong to EC8 class C (180-360 m/s) but, in the areas where the thickness of shallow formations (TSF) is lower, the Holocene units may belong to class B (> 360 m/s). The spatial distribution of VS30 highlighted that the Plio-Pleistocene, Miocene and Mesozoic areas show the best soil conditions corresponding to EC8 classes B and A. However, it was possible to identify a Plio-Pleistocene area with VS30 varying between 232 and 355 m/s, therefore belonging to class C. This fact highlights that site effects studies should not be performed solely based on geological criteria, but it is essential to estimate S wave velocities (VS) in situ.
... Twenty-four surface wave surveys were obtained at 12 strong motion stations to evaluate the seismic character of the sediment profile in the Ankara region. In situ seismic testing results obtained from the active (MASW) and passive (MAM) surface waves were utilized together before the inversion process is termed as combined surface wave method (CSWM) as stated by Eker et al. (2010Eker et al. ( , 2012Eker et al. ( , 2013. All of the surveys were obtained based on a linear array alignment by using twelve 4.5 Hz vertical geophones with 1.5 m spacing for MASW and 10 m spacing for the MAM. ...
Local site conditions may significantly affect the amplitude and frequency of ground motion during earthquakes. Recordings of recent major earthquakes have demonstrated that soil conditions can generate prominent amplification of ground shaking and can be compared with the amplification predicted by numerical simulation. This study analyzes the ground motion data from the December 2007 and March 2008 moderate Bala earthquake series considering the effect of soil conditions on ground shaking in the southern part of Ankara, the capital city of Turkey. Initially, shear wave velocity profiles of the strong ground motion stations were evaluated to define site classifications for each station in Ankara and its surroundings. Strong motion data collected during the Bala earthquake series (M w range 5.6–4.8) were used to develop event-specific attenuation relationships for peak ground acceleration and spectral acceleration at various periods and different site conditions. Site amplification factors were derived from the regression results from the event-specific attenuation relationships for the Bala earthquake series. The implications of these ground motion amplification factors were also discussed and compared using the current seismic design codes. Finally, the response spectra from the recording stations were used to determine site amplification factors for different site conditions to evaluate the variability of the site response in localized regions in Ankara.
... The SPAC (Spatial Autocorrelation) analysis (Okada, 2003) was utilized to construct the dispersion curve. A one-dimensional inversion using a non-linear least square technique has been applied to the phase velocity curves and a one dimensional shear wave velocity structure was obtained (Eker et al., 2010(Eker et al., , 2012. ...
This study mainly concentrates on the determination of site effects for the Plio-Quaternary and especially alluvial soils of the Çubuk district and its close vicinity in Ankara, Turkey. In the study area, particularly in the vicinity of the asymmetric graben that has formed due to the tectonic activities along the NE-SW trending normal faults, a microtremor survey was implemented to identify the local site response under a possible seismic event. The results were compared and correlated by using passive and active surface wave measurements, engineering geological and geotechnical deep borehole logs along with local geology and geological tectonic setting. Regarding the microtremor results, larger fundamental periods were acquired than expected over the area and the effect of the tectonic deformation on the stiffness of the soils was also observed at either the H/V curves or the shear wave velocity profiles. The results have demonstrated that geotechnical information down to a depth of 30 m may not be compared with the H/V parameters. The H/V amplitudes were not always accompanied with the higher periods in the Quaternary sediments. This could be due to the basin and basin edge effects observed particularly at the center and boundary of the basin, respectively. Additionally, the microtremor survey showed that the spectral ratio amplitudes derived by the H/V ratio is debatable and it is not a direct indicator of local soil behavior under a seismic excitation.
... A one-dimensional inversion using a non-linear least square technique has been applied to the phase velocity curves and a one dimensional S-wave velocity structure down to a depth of 30 m was obtained (Eker et al., 2012). Combination of the active and passive surface wave methods was adopted in order to meet the requirements of preserving high resolution at shallow depths while also extending the V S measurements to greater depths as stated by Eker et al. (2010 and2012). ...
One simple way of accounting for site conditions in calculating seismic hazards is to use the shear-wave velocity in the shallow subsurface to classify materials. The average shear-wave velocity to 30 m ( V 30s) has been used to develop site categories that can be used for modifying a calculated ground motion to account for site conditions. We have prepared a site-category map of California by first classifying the geologic units shown on 1:250,000 scale geologic maps. Our classification of geologic units is based on V 30s measured in 556 profiles and geological similarities between units for which we have V s data and the vast majority of units for which we have no data. We then digitized the geologic boundaries from those maps that separated units with different site classifications.
V s data for California shows that several widespread geologic units have ranges of V 30s values that cross the boundaries between NEHRP-UBC site categories. The Franciscan Complex has V 30s values across UBC categories B and C with a mean value near the boundary between those two categories. Older alluvium and late Tertiary bedrock have V 30s values that range from about 300 to about 450 m/sec, across the boundary between categories C and D. To accommodate these units we have created intermediate categories, which we informally call BC and CD. Geologic units that have, or are interpreted to have, V 30s values near the boundary of the UBC categories are placed in these intermediate units.
In testing our map against the available V 30s measurements, we have found that 74% of the measured V 30s values fall within the range assigned to the V s category where they fall on the map. This ratio is quite good considering the inherent problems in plotting site-specific data on a regional map and the variability of physical properties in geologic units. We have also calculated the mean and distribution of V 30s for each of our map units and prepared composite profiles, showing the variation of V s in the upper 100 m from the available V s data. These data show that the map categories that we have defined based on geologic units have different V s properties that can be taken into account in calculating seismic hazards.
High velocity gradients within the shear wave velocity field consistent with drill confirmed bedrock are considered diagnostic of the bedrock surface and were used to map the top of bedrock on all four lines connected at this site. Calculating the shear wave velocity field from surface wave arrivals was accomplished with a high degree of accuracy regardless of cultural noise. Improved resolution on the surface of the bedrock provides insight into the texture of bedrock and permits identification and appraisal of short wavelength variations in the bedrock surface.
The spac (SPatial AutoCorrelation) method was proposed almost 50 years ago by Aki (1957). This method allows a phase-velocity dispersion curve to be obtained from microtremor measurements using an array of stations arranged in a circle. The subsoil structure is subsequently derived from the inversion of that dispersion curve. In this article we show that it is possible to get similar results using microtremors recorded along a line. We use microtremor records obtained by using four broadband seismographs disposed along a line, with different interstation spacings (5, 10, 20, and 40 m). Our data are precessed by using the standard spac procedure, with the exception of the azimuthal average. The final subsoil structure as determined from the inversion of the phase-velocity dispersion curve shows excellent agreement with previous results at the site of our measurements. Our results suggest that the use of the spac method is not restricted to a particular geometry of the array, provided that the basic requirement of stationarity is fulfilled.
Consideration of site conditions is a vital step in analyzing and predicting earthquake ground motion. The importance of amplification by soil conditions has long been recognized, but though many seismic-instrument sites have been characterized by their geologic conditions, there has been no consistent, simple classification applied to all sites. As classification of sites by shear-wave velocity has become more common, the need to go back and provide a simple uniform classification for all stations has become apparent. Within the Pacific Earthquake Engineering Research Center's Next Generation Attenuation equation project, developers of attenuation equations recognized the need to consider site conditions and asked that the California Geological Survey provide site conditions information for all stations that have recorded earthquake ground motion in California. To provide these estimates, we sorted the available shear-wave velocity data by geologic unit, generalized the geologic units, and prepared a map so that we could use the extent of the map units to transfer the velocity characteristics from the sites where they were measured to sites on the same or similar materials. This new map is different from the California Geological Survey "preliminary site-conditions map of California" in that 19 geologically defined categories are used, rather than National Earthquake Hazards Reduction Program categories. Although this map does not yet cover all of California, when completed it may provide a basis for more precise consideration of site conditions in ground-motion calculations.
ABSTRACT The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging,and character- izing the shallow subsurface. Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode,Rayleigh wave acquired,by stepping out a pair of receivers at inter- vals based on calculated ground,roll wavelengths. Inter- ference by coherent,source-generated,noise inhibits the reliability of shear-wave velocities determined,through inversion of the whole wave,field. Among,these nonpla- nar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates,the dis- persion curve and, ultimately, the inverted shear-wave velocity profile is dependent,on frequency,as well as dis- tance from the source. Multichannel,recording,permits,effective identifica- tion and isolation of noise according,to distinctive trace- to-trace coherency,in arrival time and,amplitude. An added,advantage,is the speed,and,redundancy,of the measurement,process. Decomposition,of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency,component,in a unique,and continuous,format. Coherent,noise contamination,can then be examined,and its effects appraised,in both fre- quency,and offset space. Separation of frequency,com- ponents,permits real-time maximization,of the S/N ratio during acquisition and subsequent,processing steps. Linear separation of each ground,roll frequency,com- ponent,allows calculation of phase velocities by simply