HVSR Curves Research Articles

OpenHVSR: imaging the subsurface 2D/3D elastic properties through multiple HVSR modeling and inversion

OpenHVSR is a computer program developed in the Matlab environment, designed for the simultaneous modeling and inversion of large Horizontal-to-Vertical Spectral Ratio (HVSR or H/V) datasets in order to construct 2D/3D subsurface models (topography included). The program is designed to provide a high level of interactive experience to the user and still to be of intuitive use. It implements several effective and established tools already present in the code ModelHVSR by Herak (2008), and many novel features such as: -confidence evaluation on lateral heterogeneity -evaluation of frequency dependent single parameter impact on the misfit function -relaxation of Vp/Vs bounds to allow for water table inclusion -a new cost function formulation which include a slope dependent term for fast matching of peaks, which greatly enhances convergence in case of low quality HVSR curves inversion -capability for the user of editing the subsurface model at any time during the inversion and capability to test the changes before acceptance. In what follows, we shall present many features of the program and we shall show its capabilities on both simulated and real data. We aim to supply a powerful tool to the scientific and professional community capable of handling large sets of HSVR curves, to retrieve the most from their microtremor data within a reduced amount of time and allowing the experienced scientist the necessary flexibility to integrate into the model their own geological knowledge of the sites under investigation. This is especially desirable now that microtremor testing has become routinely used. After testing the code over different datasets, both simulated and real, we finally decided to make it available in an open source format. The program is available by contacting the authors.

AMPLIFIKASI DAN ATENUASI GELOMBANG SEISMIK DI LAPISAN SEDIMEN PERMUKAAN

Soft sedimentary layer caused the amplification effect which amplified seismic wave. Besides of that, the soft sedimentary layer also have atenuation effect of seismic wave propagation. This research was conductedto evaluated the amplification and attenuation effect in soft sedimentary layer based on theoritical evaluation and 176 microtremor datas of Denpasar city which analyzed using HVSR method. The results of this research show that the value of amplification have a linier relation with the quality factor that have negative correlation with attenuation. The quality factors that calculated from HVSR curves are the quality factor of secondary seismic wave propagation

Unconventional optimized surface wave acquisition and analysis: Comparative tests in a perilagoon area

In order to investigate the sedimentary cover of a vast perilagoon area from the geotechnical point of view and test different possible and unconventional approaches to surface-wave analysis, a series of different acquisitions and analysis procedures are tested, always considering the need for a light equipment and simple field procedures.In one case a joint inversion of the Horizontal-to-Vertical Spectral Ratio (HVSR — passive seismics) and Rayleigh-wave group-velocity spectrum (active seismics) is performed and results eventually compared to simpler single-component inversion of the group-velocity spectrum only (in both cases the group-velocity spectrum is considered according to the Full Velocity Spectrum – FVS – approach).For a second site, active data collected by using a single 3-component geophone are holistically inverted by considering the group-velocity spectra of the radial- and vertical-components of Rayleigh waves, jointly with the Radial-to-Vertical Spectral Ratio (RVSR). A second procedure based on purely-passive data (HVSR curve and phase velocity dispersion curve determined via Miniature Array Analysis of Microtremors) was also considered and results compared.In all cases, results appear extremely consistent providing mutual evidence of the efficiency of the considered methods and showing very homogenous stratigraphic conditions in the investigated area.

Spectral Ratio Estimates for Site Effects on the Horst–Graben System in West Turkey

Recordings of micro- and moderate-size local earthquakes have been used to quantify site effects in the central-west Turkey which contains one of the world’s best examples of a rapid intra-continental extension with its high population and industrial potential. We analyzed 436 earthquakes with local magnitudes ranging between 2.0 and 5.6 using three component digital recordings from 32 stations. Site functions were obtained using two different spectral ratio approaches (horizontal to vertical spectral ratio, HVSR, and standard spectral ratio, SSR). HVSR estimates of transverse and radial S-waves were compared with one another. Epicentral distance, magnitude and back-azimuth dependencies of site functions were also evaluated. In general, HVSR values from transverse and radial S-waves are similar within a factor of 2. The back-azimuth dependencies of transverse S-wave HVSR results are more significant than distance and magnitude dependencies. On the other hand, averaging of transverse and radial S-wave HVSR results eliminates systematic back-azimuth dependencies caused by source radiation effects. Distributions of HVSR estimates along ~N–S linear array, which traversed main grabens in the region with a station spacing of 3–4 km, reflect subsurface geological complexities in the region. The sites located near the basin edges are characterized by broader HVSR curves. Broad HVSR peaks could be attributed to the complexity of wave propagation related to significant 2D/3D velocity variations at the sediment–bedrock interface near the basin edges. The results also show that, even if the site is located on a horst, the presence of weathered zones along the surface could cause moderate frequency dependent site effects. Comparison of HVSR and SSR estimates for the stations on the graben sites showed that SSR estimates give larger values at lower frequencies which could be attributed to lateral variations in regional velocity and attenuation values caused by basin geometry and edge effects.

Microtremor measurements of rolling compacted ground

Ground improvement works are commonly required to overcome poor underlying soils in conjunction with infrastructure and housing development. An extensively employed and popular improvement technique is to impart mechanical compaction to the ground in an effort to achieve adequate strength and favourable load-deformation behaviour (stiffness) for the construction of civil infrastructure, including buildings and roads. This paper describes the study of a passive ambient vibration (often also known as microtremors) HVSR based method for assessing compacted ground. Two methods to control this compaction process at a very large site were employed: (a) strictly controlled rolling compaction (b) visually monitored (by naked eye) rolling compaction. The key features of the measured HVSR curves have been studied and analysed to infer useful insights about the compaction achieved by the two methods. Furthermore, the fitting of these measured HVSR curves by trial-and-error forward modelling forms the basis for inferring the shear wave velocity (Vs) profile and layer thicknesses of the compacted ground. It is shown in this paper that the process of analysing and interpreting the HVSR curves, as well as the forward modelling of the HVSR curves reveal useful information about the quality and consistency of the compacted ground.

Shallow geology characterization using Rayleigh and Love wave dispersion curves derived from seismic noise array measurements

Abstract The local geology and shallow S-wave velocity structure of a site are recognized to be key factors for the increase in the damaging potential of seismic waves. Indeed, seismic amplitudes may be amplified in frequency ranges unfavorable for building stock by the presence of soft sedimentary covers over lying hard bedrock. Hence, microzonation activities, which aim at assessing the site response as accurately as possible, have become a fundamental task for the seismic risk reduction of urbanized areas. Methods based on the measurement of seismic noise, which typically are fast, non-invasive, and low cost, have become a very attractive option in microzonation studies. Using observations derived from seismic noise recordings collected by two-dimensional arrays of seismic stations, we present a novel joint inversion scheme for surface wave curves. In particular, the Love wave, the Rayleigh wave dispersion and the HVSR curves are innovatively combined in a joint inversion procedure carried out following a global search approach (i.e., the Genetic Algorithm). The procedure is tested using a data set of seismic noise recordings collected at the Bevagna (Italy) test-site. The results of the novel inversion scheme are compared with the inversion scheme proposed by Parolai et al. (2005), where only Rayleigh wave dispersion and HVSR curves are used, and with a cross-hole survey.

Peculiar earthquake damage on a reinforced concrete building in San Gregorio (L’Aquila, Italy): site effects or building defects?

In San Gregorio (L’Aquila, Italy) a three-story, reinforced concrete (RC) building had the first floor collapsed following the earthquake of April 6, 2009. The remaining two stories fell with a displacement in the horizontal projection of about 70 cm. This unusual behaviour is made more puzzling by the fact that buildings located at a short distance and with similar features had little or no damage reported. To understand the causes of the collapse we performed strong motion recordings, microtremor measurements, a detailed geological survey, a high-resolution geo-electrical tomography, a borehole with a down-hole test. On the building we performed a geometrical survey and laboratory tests on concrete cores. The acceleration and noise recordings have shown a high amplification with uphill-downhill direction. The geological survey has revealed the presence of co-seismic fractures on stiff soils. Geo-electrical tomography has shown an unexpected, strong discontinuity just below the building. Taking advantage of excavations in adjacent lots, we have highlighted rare cataclastic decimetric bands with a very low resistance material incorporated in well-stratified calcarenites. The same soft material has been founded in the borehole down to 17 m from ground level, showing a shear wave velocity that starts at 250 m/s, increases with depth and has an abrupt transition in calcarenites at 1,150 m/s. The surface geophysical measurements in the vicinity of the site have not shown similar situations, with flat HVSR curves as expected for a rock outcrop, except for a lateral extension of the soft zone. The analysis on the quality of the building materials has yielded values higher than average for the age and type of construction, and no special design or construction deficiencies have been observed. A strong, peculiar site effect thus appears to be the most likely cause of the damage observed.

Processing of three-component VSP data — an application from crystalline bedrock in Finland

In recent decades, numerous observations of the connection between sedimentary basins structure and severity and variability of damages during earthquake events have sparked a series of studies for characterization of such subsurface structures. However, these investigations were always accompanied with cost concerns and executive restrictions. Among non-destructive and cost effective approaches which have been developed in seismic response evaluations, usage of microtremor has attracted considerable attentions. In the current study, we have focused on inversion of microtremor acquired in the Qom basin, Iran. As the area is known to be seismically active, our main contribution is the construction of a three-dimensional model of the sedimentary basin in terms of shear wave velocity, which extends and integrates the previous knowledge (limited to the Vs30) down to the bedrock. Additionally, we provided further evidence of several possible faults in the area, one of which has been introduced earlier but poorly investigated so far: the “Qomrud” Fault. The results of a series of previous studies on the area, comprising down-hole surveys, electrical resistivity surveys, surface seismic refractions and surficial bore-hole excavations have been used, either to form a more precise initial model for inversion process or to evaluate the outputs. Additionally, we investigated possibility of obtaining the distribution of the Vs30 in the area directly from inversion of HVSR curves. It revealed that, although for relatively shallow bedrocks (80–100 m) the differences of achieved velocities is lower than 10–15%, in deep sediments, the Vs30 from microtremor is largely overestimated. Furthermore, we discussed the applicability of empirical relationships for estimating bed-rock depth in the investigated basin.