Site response, fault distribution, and boundary characteristics of the central-southern Tanlu Fault Zone, eastern China revealed by seismic horizontal-to-vertical spectral ratio

The observed peak ground accelerations and peak ground velocities (PGVs) of the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake generally followed the median values from ground motion prediction equations with reasonable errors at fault distances ≥ 50 km. However, at smaller distances, the equations significantly underestimated the peak ground motions, and it was eminent for PGVs. A comparison of surface-to-borehole spectral ratios of S-waves during the mainshock and other events revealed that the sites at smaller distances experienced various degrees of nonlinear site response. The two most widely known characteristics of nonlinear site response are the weakening of higher-frequency components and shifting of predominant frequencies to lower ones in comparison with the linear site response. At one of the sites that recorded the largest intensity of 7 in JMA scale of 0–7, the latter nonlinear effect was so dominant that the ground motions around the new predominant frequency got intensified by one order of magnitude in comparison with that during the weak-motions. Two sites, which were closely located, recorded vertical peak ground accelerations exceeding 1 g for the up-going motions. The recordings showed asymmetric waveforms and amplitudes characteristics of the nonlinear site response in extreme vertical ground motions recorded during a few earthquakes in the past. Few sites having lower vertical peak ground accelerations were also suspected of being experienced nonlinear site response on vertical motions. These findings suggest taking a cautious approach to enumerate the reduction in amplification at higher frequencies using the single-station horizontal-to-vertical (H-to-V) spectral ratio technique. However, we found that the H-to-V technique was still useful to detect nonlinearity. Finally, an ad hoc equation was derived to correct the nonlinear site amplification in predicting horizontal PGVs with respect to one of the most widely used attenuation models in Japan. The results indicated that the effect was much stronger for a larger input motion than that for a proportional change in the Vs30 values.

The response of Taipei basin upon earthquake excitation was studied using records of recent earthquakes. The strong-motion database includes records obtained at 32 stations of the Taipei TSMIP network from 83 deep and 142 shallow earthquakes (M＞4.0) that occurred in 1992-2004. The characteristics of frequency-dependent site response were obtained as spectral ratios between the actual earthquake records (horizontal components) and those modelled for a hypothetical Very Hard Rock (VHR) condition. The models for VHR spectra of Taiwan earthquakes had been recently proposed by Sokolov et al. (2005b, 2006). Analysis of site response characteristics and comparison with simple 1D models of the soil column resulted in the following conclusions: (1) The spectral ratios throughout the basin obtained from deep earthquakes (depth ＞35 km) exhibit good agreement with the theoretical ratios calculated using the 1D models constructed using available geological and geotechnical data. (2) The spectral ratios obtained from shallow earthquakes show influence of: (a) surface waves generated when travelling from distant sources to the basin and (b) relatively low-frequency (＜1-2 Hz) waves generated within the basin. (3) Some shallow earthquakes produce extremely high amplification at frequencies 0.3-1 Hz within the basin that may be dangerous for high-rise buildings and highway bridges. (4) The obtained results may be used in probabilistic seismic microzonation of the basin when many possible earthquakes located at various distances are considered. 2D and 3D simulation is necessary to model the seismic influence from particularly large earthquakes.

Site response characteristics have a significant influence on ground motion vibrations. To evaluate the site response effects at King Saud University campus, the horizontal-to-vertical spectral ratio (HVSR) technique has been applied at 45 sites with about 400-m spacing. The recording time was more than 1 h at each site with a sampling rate of 100 Hz. The data were processed and analyzed using Geopsy software to produce measurements of fundamental frequency and minimum site amplification factor. Moreover, shear-wave velocity down to 30-m depth has been acquired at 10 measuring sites in the campus. In addition, geotechnical borehole data were collected from four boreholes within the campus. Generally, in majority of sites the H/V curve for amplitude spectra displayed a clear peak, signifying the presence of a sharp soil–bedrock impedance contrast between alluvial deposits and underlying bedrock. It is indicated that the fundamental frequency ranges between 0.64 and 1.94 Hz. Given the measured resonance frequency of around 1 Hz, buildings located on the alluvium that have 5 stories in height could experience soil-structure resonance, while the amplification factor varies from 1.19 to 11.22 indicating localized zones having high vulnerability index due to the considerable thickness of alluvial deposits. Moreover, shear-wave velocity ranges between 320 m/s and 684 m/s revealing zones of stiff soil to very dense soil/or soft rock which is correlated well with the geotechnical parameters of borehole data. Based on these results, the ground conditions of near-surface sediments in the King Saud University campus indicate weak zones that led to ground subsidence resulting in differential settlements of foundations.

Empirical Study of Sediment‐Filled Basin Response: The Case of Taipei City

To overcome the limited information in the northern waters of Sulawesi Island, ocean bottom seismometers (OBS) were installed in the northern part of the Makassar Strait and the Sulawesi Sea. OBS data were analysed using passive seismic methods to determine the characteristics of noise and site response in the Makassar Strait and the Celebes Sea. Probabilistic Power Spectral Density (PPSD) from OBS station showed microtremor in the period 0.06 – 0.7 s, secondary microseismic peak in the period 0.9 – 2 s, secondary microseismic peak in the period 4 – 8 s, and hum in the period more than 10 s. The amplitude of the noise power is in the range of New Low Noise Model (NLNM) and New High Noise Model (NHNM) for a period <10 s and slightly above NHNM for a period >10 s for OBS IGGCAS F-type. For OBS IGGCAS G-type, the noise power amplitude is in the period < 0.7 s in the NLNM and NHNM ranges. However, there is a tendency for the noise power amplitude to be flat with an amplitude above NHNM for a period > 0.7 s. The noise power’s amplitude depends on the station’s location, climate, instruments, daily variations, and seasonal variations. Horizontal-to-Vertical Spectral Ratio (HVSR) in the North Makassar Strait shows an H/V curve with an unclear peak in the middle of the strait and multiple peaks in the eastern part of the strait with a frequency range of 1.35 – 4.7 Hz. Between the Makassar Strait and the Celebes Sea, the H/V curve shows a flat peak and multiple peaks with a frequency range of 1.10 – 4.03 Hz. Meanwhile, in the Celebes Sea, the H/V curve shows an unclear peak (unclear peak) and several peaks (multiple peaks) with a frequency range of 0.65 – 6.25 Hz.

Slope site seismic response is different from one of the flat sites, it has been known for many years; and this phenomenon has also attracted great attention in China since the “5.12” Wenchuan earthquake. From 2008, the State Key Laboratory of Geohazard prevention and Geoenvironment protection (SKLGP) has started to complete long term and systematic monitoring of the site amplification for different slopes. This chapter presents monitoring characteristics of slope site response of seven typical sites. A total of four main shocks were monitored by the Zigong (recorded Wenchuan earthquake), Luding (recorded Lushan and Kangding earthquakes) and Shimian (recorded Changning earthquake) monitoring points. Hundreds of aftershocks were observed by our monitoring stations. The Peak ground acceleration (PGA), horizontal to vertical spectral ratio (HVSR), normalized Arias intensity (Ia), standard spectral ratio (SSR) and azimuth information has been extracted from the monitoring data. The monitoring characteristics of the slope seismic response show that the soil site is always affected by stronger amplification than the bedrock site in a small amplitude range (Reference site < 0.076 g). This is due to the high wave impedance between soil and bedrock promoting site ground motion effects. Meanwhile, significant polarization effects have also been observed on the mountain top (hilltop), the normalized Ia and polarization diagram of the HVSR show the same directivity characteristics of ground motion near the mountain tops, marked by strongest amplitudes transverse to the ridge, where the extension direction of the ridge and orientation of geological structures influence the seismic energy redistribution. In addition, the PGA amplification shows nonlinear increasing with the topographic relief and changing geological conditions. Amplification of PGA of the slope in the upper part can be quite high (>10) with respect to a reference site (for weaker motion of an intensity less than V), but is less marked (<4.5) for strong motion conditions (Intensity ≥ VI). This means that the PGA amplification effect has significant variable characteristics under low intensity of earthquake. The HVSR analysis shows that the amplified resonance frequency of most monitoring sites is in the range of 2–4 Hz, and part are in the range of 6–8 Hz, and that multi peak frequency values can be observed in most slope monitoring sites. The complexity of the slope morphology and of the geology conditions has an influence on related results. This paper presents some analyses of the seismic response of a series of monitored slope sites, which show that predicting the slope response is still very difficult, due to the variability of site effects. An even greater challenge is the prediction of the seismic slope failure potential as there is a great lack of recorded strong motion data on sites affected by permanent deformation after an earthquake.

Simeulue Island is an outer island arc off west of the Sumatra Island. The Island is located close to the interface of the subduction zone between Indo-Australian and Eurasian Plates. Seismic activities around the Island included devastating megathrust earthquakes, such as 2004 M W 9.2 Sumatra-Andaman and 2005 M W 8.7 Nias earthquakes. We investigate the site response characteristics using Horizontal-to-Vertical (H/V) Spectral Ratio method to the continuous ambient noise records from eight broadband seismometers. From the calculation, we generally observe strong peak of H/V spectral ratio which is caused by the strong impedance contrast at these area. However, sites BATU, LABU and DEHI show relatively flat H/V spectral ratio curves that caused by lack of sharp impedance contrast beneath the site. We also observe multiple peaks at several sites that may indicate the presence of a more highly weathered soil/clay layer on top of a more compact medium. H/V peak frequencies, which shows fundamental frequencies resonance of soil sites, generally are observed at range about 2.0 - 16.4 Hz. From the observed H/V spectral ratio, we suggest that the strong impedance contrasts may occur on the shallow part of the medium at 2 up to15 m depth.

We investigated the validity of seismic site response characteristics es- timated from microtremors by comparing them with those of earthquake motions. For this purpose we observed microtremors as well as earthquake motions using large (5-km diameter) and small (0.5-km diameter) arrays deployed on soft sedi- ments. Specifically, we examined four estimates from microtremors: relative site amplification factors to incident shear waves, site amplification factors by the Naka- mura method, resonance frequency in horizontal-to-vertical spectral ratios, and horizontal-to-vertical spectral ratios. As a result of the comparisons, we obtained the following conclusions. The relative amplification factors can be inferred from horizontal-component ratios of microtremors to a reference site within a small area of several hundred meters. The horizontal-to-vertical spectral ratios inferred by the Nakamura method partly reflect site amplification factors, but do not agree with site amplification factors. A sharp-peak frequency in the horizontal-to-vertical spectral ratios is possibly the resonance frequency. The horizontal-to-vertical spectral ratios of microtremors either agree with those of earthquake motions at some array sites or are slightly smaller at the other sites.

The Atlantic and Gulf Coastal Plain in the southern and southeastern United States contains extensive Cretaceous and Cenozoic sedimentary sequences of variable thickness. We investigated the difference in response of sites in the Coastal Plain relative to sites outside that region using Fourier spectral ratios from 17 regional earthquakes occurring in 2010–2018 recorded by the Earthscope transportable array and other stations. We used mean coda and Lg spectra for sites outside the Coastal Plain as a reference. We found that Coastal Plain sites experience amplification of low‐frequency ground motions and attenuation at high‐frequencies relative to average site conditions outside the Coastal Plain. The spectral ratios at high frequencies gave estimates of the difference between kappa at Coastal Plain sites and the reference condition. Differential kappa values determined from the coda are correlated with the thickness of the sediment section and agree with previous estimates determined from Lg waves. Averaged estimates of kappa reach ∼120 ms at Gulf coast stations overlying ∼12 km of sediments. Relations between Lg spectral ratio amplitudes versus sediment thickness in successive frequency bins exhibit consistent patterns, which were modeled using piecewise linear functions at frequencies ranging from 0.1 to 2.8 Hz. For sediment thickness greater than ∼0.5 km, the spectral amplitude ratio at frequencies higher than approximately ∼3 Hz is controlled by the value of kappa. The peak frequency and maximum relative amplification at frequencies less than ∼1.0 Hz depend on sediment thickness. At 0.1 Hz, the mean Fourier amplitude ratio (Coastal Plain/reference) is about 2.7 for sediment of 12 km thickness. Analysis of residuals between observed and predicted ground motions suggests that incorporating the amplification and attenuation as functions of sediment thickness may improve ground‐motion prediction models for the Coastal Plain region.

Nonlinear seismic site response classification using K-means clustering algorithm: Case study of the September 6, 2018 Mw6.6 Hokkaido Iburi-Tobu earthquake, Japan

The reference spectral noise ratio method to evaluate the seismic response of a site

Seismic site response of sandy soils and seismic soil–structure interaction are investigated using an electrohydraulic earthquake simulator mounted on a centrifuge container at an 80g field. The results of testing uniform and layered loose to medium-dense sand models subjected to 13 simulated earthquakes on the centrifuge are presented. The variation of shear modulus and damping ratio with shear strain amplitude and confining pressure was evaluated and their effects on site response were assessed. The evaluated shear modulus and damping ratio agreed reasonably with laboratory tests and empirical relationships. Site response analysis using the measured shear wave velocity and estimated modulus reduction and damping ratio as input parameters produced good agreement with the measured site response. The effect of soil–structure interaction for structures situated on dry sand is also investigated. These tests have revealed many important insights with regard to the characteristics of seismic site response and seismic soil–structure behaviour. The tests showed that the seismic response of soil deposits, input motions, and overall behaviour of the structure are affected by soil stratification. The results showed that the seismic kinematic soil–structure interaction is not very significant for structures situated on loose sand.

SUMMARY The within-site variability in site response is the randomness in site response at a given site from different earthquakes and is treated as aleatory variability in current seismic hazard/risk analyses. In this study, we investigate the single-station variability in linear site response at K-NET and KiK-net stations in Japan using a large number of earthquake recordings. We found that the standard deviation of the horizontal-to-vertical Fourier spectral ratio at individual sites, that is single-station horizontal-to-vertical spectral ratio (HVSR) sigma σHV,s, approximates the within-site variability in site response quantified using surface-to-borehole spectral ratios (for oscillator frequencies higher than the site fundamental frequency) or empirical ground-motion models. Based on this finding, we then utilize the single-station HVSR sigma as a convenient tool to study the site-response variability at 697 KiK-net and 1169 K-NET sites. Our results show that at certain frequencies, stiff, rough and shallow sites, as well as small and local events tend to have a higher σHV,s. However, when being averaged over different sites, the single-station HVSR sigma, that is σHV, increases gradually with decreasing frequency. In the frequency range of 0.25–25 Hz, σHV is centred at 0.23–0.43 in ln scales (a linear scale factor of 1.26–1.54) with one standard deviation of less than 0.1. σHV is quite stable across different tectonic regions, and we present a constant, as well as earthquake magnitude- and distance-dependent σHV models.

ABSTRACTDamaging ground motions from the 2011 Mw 5.8 Virginia earthquake were likely increased due to site amplification from the unconsolidated sediments of the Atlantic Coastal Plain (ACP), highlighting the need to understand site response on these widespread strata along the coastal regions of the eastern United States. The horizontal-to-vertical spectral ratio (HVSR) method, using either earthquake signals or ambient noise as input, offers an appealing method for measuring site response on laterally extensive sediments, because it requires a single seismometer rather than requiring a nearby bedrock site to compute a horizontal sediment-to-bedrock spectral ratio (SBSR). Although previous studies show mixed results when comparing the two methods, the majority of these studies investigated site responses in confined sedimentary basins that can generate substantial 3D effects or have relatively small reflection coefficients at their base. In contrast, the flat-lying ACP strata and the underlying bedrock reflector should cause 1D resonance effects to dominate site response, with amplification of the fundamental resonance peaks controlled by the strong impedance contrast between the base of the sediments and the underlying bedrock. We compare site-response estimates on the ACP strata derived using the HVSR and SBSR methods from teleseismic signals recorded by regional arrays and observe a close match in the frequencies of the fundamental resonance peak (f0) determined by both methods. We find that correcting the HVSR amplitude using source term information from a bedrock site and multiplying the peak by a factor of 1.2 results in amplitude peaks that, on average, match SBSR results within a factor of 2. We therefore conclude that the HVSR method may successfully estimate regional linear weak-motion site-response amplifications from the ACP, or similar geologic environments, when appropriate region-specific corrections to the amplitude ratios are used.

Estimating site amplification variability in Yangon, Myanmar, from a dense nodal seismic array