Basin Edge Effect Research Articles

Site Response in the Walnut Creek–Concord Region of San Francisco Bay, California: Ground-Motion Amplification in a Fault-Bounded Basin

ABSTRACT Thirty-seven portable accelerometers were deployed in the eastern San Francisco Bay communities of Walnut Creek and Concord to study site response in a fault-bounded, urban, sedimentary basin. Local earthquakes were recorded for a period of two years from 2017 to 2019 resulting in 101 well-recorded events. Site response is estimated by two methods: the reference site spectral ratio method and a source-site spectral inversion method. The reference site spectral ratio method allows investigation of the variability of site amplification with source azimuth and frequency. The source-site spectral inversion method yields the best least-squares fit to site response for a database of ground-motion records. Both methods show substantial amplification in the Walnut Creek–Concord basin below 2 Hz indicating strong surface-wave development. Greater amplification is seen for sources aligned along the long axis of the basin. Inversion using close-in sources at short distances yields lower amplification at longer periods than the entire data set due to reduced surface-wave generation for steeper angles of incidence. Inversion of site response spectra for shallow shear-wave velocity using a global search algorithm yields VS30 values consistent with generalized mapping results based on geology and topography but with greater variability due to local site variations. 3D finite-element modeling shows greater amplification in the Walnut Creek–Concord basin with a basin-edge effect likely contributing to higher ground motions. Topography is also seen to lead to increased scattering and shadowing effects.

Predicted results of weak and strong ground motions at the target site of the blind prediction exercise as steps-2 and -3, Report for the experiments for the 6th international symposium on effects of surface geology on seismic motion

In this study, we compared observations and predictions submitted by participants for blind prediction experiments for ground motions using aftershocks, foreshock, and mainshock of the 2016 Kumamoto earthquake sequence, Japan, to improve our understanding of the quality of state-of-the-art methods on the reproducibility of the effects of surface geology on seismic motions. In the blind predictions, 1D, 2D, and 3D methods, Green’s function methods, spectral ratio approaches, and other approaches were applied. As for PGA/PGV, acceleration/velocity duration, Fourier spectrum, pseudo-velocity response spectrum, and site amplification factors, the observed values are mostly within the range of average ± σ of all the predictions in the case of weak and strong ground motions. The results of the mean absolute percentage errors for these indices show that the applied methods can predict weak and strong ground motions for the three components in the range of one-half to twice the observations. The average goodness-of-fit (GOF) scores for weak and strong ground motions indicate either a very good fit (6.5–8) or a good fit (4.5–6.5) for the three components. Finally, examples of the categorized methods are quite limited; however, results indicate that the predictions by all the categorized methods can adequately reproduce weak and strong ground motions within either a very good or good fit. Although we could not find a significant difference in the results from the categorized methods, scores by the 2D and 3D methods in the frequency range of 0.5–1 and 1–2 Hz for all the blind predictions are higher than the scores by the other methods. The GOF score for the part after the S-wave by the 2D and 3D methods is higher than that by the 1D method. This supports that the predictions by the 2D and 3D methods due to the accounting of the proper geometry could reproduce the basin-induced and/or basin-transduced surface waves excited by the basin-edge effect more than the 1D method using the earthquake record observed at the reference site.Graphical

Basin edge effect on industrial structures damage pattern at clayey basins

In this numerical study, the 2D dynamic behavior of a clayey basin and its effect on damage pattern over basin edge are investigated. To attain this goal, a fully nonlinear time domain analysis method has been applied. Then, the fragility curves of the considered two typical industrial structures for that certain point are estimated using the acceleration time histories recorded at each surface point. The results show that the use of the damage related parameters in site effect analyses, instead of amplification curves, can yield more realistic estimation of the basin dynamic response. In a distance about 150m from outcrop at the basin edge, the differences between fragility curves increase when increasing the distance from outcrop with respect to the reference rock site. Outside this region and towards the basin center, they tend to occur in rather single curves. Furthermore, to connect the structural damage to the basin edge effect, the earthquake demand value at different points for two typical structures was evaluated. It was seen that the probability of occurrence of damage increases over 250 m from outcrop, while the effect of the basin edge was limited to 150 m in case of the basin edge evaluation by using fragility curves.

Site characterization through combined analysis of seismic and electrical resistivity data at a site of Dhanbad, Jharkhand, India

We present the seismic site characterization study using joint modelling of Horizontal-to-Vertical Spectral-Ratio (HVSR) and Rayleigh wave-phase velocity-dispersion curves obtained from Multi-channel Simulation with One Receiver (MSOR) in a part of Dhanbad, Jharkhand, India. The joint analysis of these two different but complementary datasets puts stronger constraints on the model parameter search space than one dataset and may help us in finding more unique shear-wave velocity model. The microtremor data from 12 observation points were utilized to iteratively search 1D shear-wave velocity profiles in a predefined model search space. These 1D shear-wave velocity models were interpolated to generate a 2D shear-wave velocity profile of the site using the cubic spline method. Our results show that the high peak amplitude value of HVSR is associated with low peak-period values of HVSR at a distance of ~ 60 m from the southern end of the profile; which may indicate the presence of the Basin Edge Effect. We identified four layers based on significant changes in the shear wave velocities to a depth of ~ 60 m. The major impedance contrasts are located at average depths of ~ 13 m, ~ 40 m and ~ 55 m, respectively. These layers from the surface may indicate the presence of soil, highly weathered rock mass, moderately weathered rock and bedrock, respectively. The depth of engineering solid bedrock (Vs > 600 m/s) is found at the depth of 55 m in the south which gradually decreases to a depth of 40 m in the northern end of the profile. The shear-wave velocity (Vs 30) for this area varies between 293 and 357 m/s; which can be classified as “D-type site”. For validation and comparison of our results, the Electrical Resistivity Tomography (ERT) data were also recorded along the same traverse using Wenner and Schlumberger configurations. Our results show a significant amount of correlation between the 2D shear-wave velocity and resistivity profiles obtained from joint analysis of tremor and ERT data.

The damage mechanism of water tower in Shaanxi Provence during Wenchuan Earthquake

Through the collection, collation and analysis of the data about damage of Water tower in Shaanxi Provence during 12 May 2008 Wenchuan Earthquake, the mechanism of long-period damage of the water tower is given. The analysis shows that, in Hanzhong basin, Ankang basin, and Weihe basin, serious damage should be caused by the long-period effect of basin of large earthquake in the far field. That magnitude of Wenchuan Earthquake is high, three basins are at the direction where the Wenchuan Earthquake wave propagated, and the epicentre is from 400 to 600km determines that the Wenchuan earthquake has the condition producing long-period effect in the three basins. In addition, the basins have very thick deposits, as well as possible basin edge effect may enhance long period damage. The damage of long-period of water tower results from the resonance between water tower and long-period ground motion.

Three-dimensional simulations of strong ground motion in the Sichuan basin during the Wenchuan earthquake

A three-dimensional model of the Sichuan basin was established by incorporating the Quaternary Chengdu plain, the near surface sediment, the crystalline basement, the 1D crustal velocity structure, and the realistic surface topography of the basin. Based on this model, and by using the spectral-element method and a parallel computing technique, the low frequency (0.05–0.5 Hz) wave amplification behavior of the Sichuan basin during the 2008 Wenchuan earthquake was simulated, and the frequency-related amplification characteristics were investigated. The results reveal that the Sichuan basin significantly amplified low frequency ground motions, with a maximum amplification factor of 9.6 located at the Zhuwa depression, which has the deepest Quaternary sediments. Compared to the vertical component, the horizontal component of the ground motion demonstrates greater amplification factors. The coupling of the basin-edge effect, the rupture directivity effect, and the interference between the waves from different subfaults results in strong local ground motions at the basin edge areas, such as Dujiangyan and Mianzhu. For the studied frequency band, the Quaternary Chengdu plain mainly amplifies seismic waves above 0.3 Hz, while the basin basement is sensitive to waves below 0.3 Hz. The Quaternary Chengdu plain significantly amplifies wave amplitudes and changes the distribution features of strong ground motions.

A Numerical Study on Comparison of 1D and 2D Seismic Responses of a Basin in Turkey

Local site conditions such as seismic bedrock depth, bedrock slope of the edge, geometry and characteristics of soil layers, topographical irregularities, etc. are the most important factors affecting earthquake ground motion in a specific site. The amplitude and frequency content of bedrock motion can be changed by local site effects, and this variation is denoted as an amplification or de-amplification. Among the several factors, basin edge effect plays an important role in the transformation of earthquake waves and increase of the surface motion duration and amplitude. The limited width of the soil layers or the edge geometry at the deep formations cause earthquake wave transformations, thus the amplitude of the surface ground motion may vary depending on its location. For this reason the frequency content of surface ground motion may differ from the calculated surface ground motion by one dimensional dynamic analysis. In this case two dimensional analysis is required. In this study, in order to compare the soil response under different strong ground motion, one and two dimensional dynamic analyses were performed by using the Dinar Basin model in Turkey. The acceleration time histories and absolute acceleration spectra were obtained for pre-selected points on the ground surface. The 2D/1D spectral acceleration ratios were calculated by dividing the absolute acceleration spectra obtained from two dimensional (2D) and one dimensional (1D) dynamic analysis. The variations of the spectral acceleration ratios (2D/1D) with distance from basin edge were evaluated for different period values. The calculated 2D/1D spectral acceleration ratios reached their maximum values at a certain zone (X/D<3) near basin edge for every interested period value. While approaching to center of basin models, especially at the zones after X/D=3 point it can be noticed that 2D/1D spectral acceleration ratios generally converged to 1 regardless of the edge bedrock slope values. The highest average spectral acceleration ratios were calculated when the relevant period values were between T=0.2~0.5 s. They took values varying between 2 and 3 for this period interval. A relationship between the results of 1D and 2D dynamic analyses was established. In addition, the approximate validity range of 1D and 2D dynamic analysis at the basin edges was investigated for the model.

A numerical study on the basin edge effect on soil amplification

In this paper a numerical study on the effects of the basin edge on the dynamic behavior of the model basins are investigated. For this purpose a range of bedrock inclinations at the valley sides from slighter \(10^{\circ }\) and \(20^{\circ }\) to steeper \(30^{\circ }\) and \(40^{\circ }\) are selected. A numerical study using nonlinear code which utilizes appropriate static and dynamic boundary conditions, and includes hysteresis damping formulation based on user defined degradation curves is conducted utilizing two sandy and clayey materials. Using several different real earthquake motions provide opportunity for the assessment of the site response to the variation of the motion intensity. The analyses results are presented in the form of the acceleration and spectral acceleration amplification curves. Also, by conducting 1D analyses along the valley the aggravation curve for every case are evaluated and discussed. It was seen that variation of the bedrock inclination not only affects the peaks of the spectral amplification curves, but also the position of the maximums of the curves on the valley surface are changed. Also, the frequency domain results show that different parts of the valleys are sensitive to different periods. While the lateral parts are sensitive to lower periods, the maximum amplification of the inner parts takes place at higher periods. Based on results the 2D behavior not only is dominant at the latreal parts of the valley, but also affects the behavior of the inner parts. Also, the use of the 1D analyes for the estimation of the 2D behavior remains insufficient. Finally, the results of this research show the important effect of the motion intensity on the 2D behavior of the valley specially on the increase of the resonance period at higher period.

Multimethod Characterization of the French-Pyrenean Valley of Bagneres-de-Bigorre for Seismic-Hazard Evaluation: Observations and Models

A narrow rectilinear valley in the French Pyrenees, affected in the past by damaging earthquakes, has been chosen as a test site for soil response characteriza- tion. The main purpose of this initiative was to compare experimental and numerical approaches. A temporary network of 10 stations has been deployed along and across the valley during two years; parallel various experiments have been conducted, in particular ambient noise recording, and seismic profiles with active sources for struc- ture determination at the 10 sites. Classical observables have been measured for site amplification evaluation, such as spectral ratios of horizontal or vertical motions between site and reference stations using direct S waves and S coda, and spectral ratios between horizontal and vertical (H/V) motions at single stations using noise and S-coda records. Vertical shear-velocity profiles at the stations have first been obtained from a joint inversion of Rayleigh wave dispersion curves and ellipticity. They have subsequently been used to model the H/V spectral ratios of noise data from synthetic seismograms, the H/V ratio of S-coda waves based on equipartition theory, and the 3D seismic response of the basin using the spectral element method. General good agreement is found between simulations and observations. The 3D simulation reveals that topography has a much lower contribution to site effects than sedimentary filling, except at the narrow ridge crests. We find clear evidence of a basin edge effect, with an increase of the amplitude of ground motion at some distance from the edge inside the basin and a decrease immediately at the slope foot.

2D Analysis of Earthquake Ground Motion in Haifa Bay, Israel

We study the earthquake response of the Zevulun Valley basin, underlying northern Israel’s largest urban area, with 2D viscoelastic seismic modeling of a detailed geological section. We found that amplification of the horizontal vibrations, the ratio of basin to no-basin response spectra, correlates with basin depth. In the deepest portion of the basin (Qishon graben), long periods (2–5 sec) are amplified by 400%; in the shallowest portion of the basin (Afeq horst), shorter periods (∼0.5 sec) are amplified by 300%–400%. These resonances in the vertical direction through the basin are strong enough that their amplitude overwhelms the amplitude of a previously recognized basin-edge effect. The horizontal/vertical (H/V) Fourier spectral ratios based on 124 ambient noise measurements do not fully coincide with the basin to no-basin Fourier spectral ratios of the simulation, but the resonance frequencies found in both methods are alike. Moreover, the relation between the resonance frequency and the depth of the corresponding seismic reflector in the simulation is almost identical to the empirical frequency-depth relations obtained from measurements. This indicates that the average shear-wave velocity of the sedimentary column in the model is consistent with measurements. To evaluate the necessity of 2D analysis, we performed additional 1D simulations at two locations along the section. For the Qishon graben, 1D analysis underestimates the amplification factor relative to 2D by 25%, whereas for the Afeq horst, 1D and 2D simulations are similar. For a hard layer within the soft Qishon graben fill, we found that when the hard layer is thinner than ∼50 m, its influence on ground motion is small.

Revisiting the Basin-edge Effect at Kobe During the 1995 Hyogo-Ken Nanbu Earthquake

The basin edge effect, i.e., the interference of the direct S wave with the surface wave diffracted off the basin edge has been invoked by many authors to explain the damage distribution during the January 17, 1995 Hyogo-Ken Nanbu (Kobe) earthquake. Here we present the results of numerical experiments obtained with the spectral element method in 2-D geometry. Our results confirm that the amplification of horizontal motion close to the basin edge can be twice as large as the one measured in the center of the basin. This additional amplification is shown to depend strongly on the edge geometry and on frequency, due to physical dispersion of diffracted surface waves. In particular we obtain maximal amplification around 3 Hz, at frequencies critical for buildings.

Modeling of 3D Subsurface Structure and Numerical Simulation of Strong Ground Motion in the Adapazari Basin during the 1999 Kocaeli Earthquake, Turkey

The 1999 Kocaeli (Izmit) Earthquake seriously damaged the downtown area of Adapazari, Turkey. The wavefield of the strong ground motions is considered to have been affected by the deep ground structure. A refraction and reflection seismic survey was made to obtain the velocity structure of the profile in the north–south direction across the basin. A 3D subsurface model is presented based on results of the refraction and reflection survey, on data obtained from a gravity survey, and on observations of microseisms and aftershocks. The proposed model shows deep sediment with a steep slope beneath downtown Adapazari. Strong ground motions in the frequency range of 0.1–1.0 Hz propagating in the subsurface model during the Kocaeli Earthquake also were simulated numerically by the 3D finite-difference method. The simulated results show that ground motions in downtown Adapazari were larger than those in the area between downtown and the source fault because the former were significantly amplified by the basin-edge effect.

The Basin-Edge Effect from Weak Ground Motions Across the Fault-Bounded Edge of the Lower Hutt Valley, New Zealand

Basin-edge effects are investigated in the Lower Hutt Valley, Wellington, New Zealand, using recordings of weak-motion earthquakes. Twelve seismographs were deployed in a closely spaced array along the fault-bounded edge of the valley. Peak ground motions appear to be locally amplified at 2.0-2.5 Hz in the edge-parallel component at around 116-172 m from the fault and in the edge-normal component more than 295 m from the fault. Group velocity observations made of edge-generated Love waves show the fundamental-mode Airy phase frequency at 2.0-2.5 Hz. This frequency corresponds to the fundamental resonance mode of the Holocene sediments as well as the third-mode resonance of the whole basin. Large differential particle motions are observed at neighboring stations less than 50 m apart. The most plausible explanation to account for these observations is constructive interference between edge-generated surface waves and the delayed direct arrival, commonly referred to as the basin-edge effect. Manuscript received 23 October 2001.

Ground-motion amplification in the Santa Monica area: Effects of shallow basin-edge structure

AbstractThe 1994 Northridge earthquake produced ground motions in the northwest portion of the Los Angeles basin that were significantly larger than rock-site motions observed at locations just north of the basin. The Santa Monica area was hit particularly hard, with numerous structures being damaged or destroyed by the strong ground shaking. In this region, the basin-edge geology is controlled by the active strand of the east-west-striking Santa Monica fault, and virtually all of the structural damage occurred at or south of the fault location. We have used 2D finite-difference ground-motion simulations to investigate the effect of the basin-edge structure in amplifying ground response. Constraints on the basin-edge structure come from geologic cross sections, geophysical data, and seismological observations. Our simulations indicate that the shallow basin-edge structure (1 km deep) formed by the active strand of the Santa Monica fault creates a large amplification in motions immediately south of the fault scarp, in very good agreement with mainshock damage patterns, recorded ground motions, and locations of elevated site response. This large amplification results from constructive interference of direct waves with the basin-edge-generated surface waves and is quite similar to the basin-edge effect associated with the 1995 Kobe earthquake. In addition, we find that focusing effects created by the deeper basin structure (3 to 4 km deep) cannot explain the large motions observed immediately south of the fault scarp. This strongly suggests that the deep-basin focusing models proposed by Gao et al. (1996) and Alex and Olsen (1998) are not likely explanations of the observed pattern of ground-motion amplification in the Santa Monica area.

Three-dimensional simulation of the near-fault ground motion for the 1995 Hyogo-Ken Nanbu (Kobe), Japan, earthquake

AbstractThe 17 January 1995 Hyogo-ken Nanbu earthquake is a typical example showing that the ground motions along basin-edge faults can be very destructive. In this study, we simulate the near-fault ground motion from this earthquake based on a kinematic fault model and a simplified 3D velocity structure of the Kobe area. The kinematic earthquake rupture and the wave propagation are modeled using a 3D finite-difference method (FDM). Our simulation identifies the basin-edge effect as an important factor that influenced the ground-motion amplification pattern in the Kobe area. We found that the coupling of the source directivity and basin-edge effects causes impulsive ground motions with extremely high amplitude at periods greater than 1 sec and in a narrow zone offset less than 1 km from the basin edge. The combination of these effects acted to create a fairly continuous band of amplification that extends about 30 km in an elongated zone parallel to the basin-edge boundary. In some areas, localized site effects might have been as important as the abovementioned effects, but they cannot explain the continuity of the extended east-west zone of damage.

Modelling of ground motion in the Higashinada (Kobe) area for an aftershock of the 1995 January 17 Hyogo-ken Nanbu, Japan, earthquake

We model the ground motion from an aftershock of the 1995 January 17 Hyogo-ken Nanbu (Kobe) earthquake to investigate basin edge effects on wave propagation in Higashinada ward, downtown Kobe. Point-source finite-difference seismograms calculated using a double-couple solution and a 2-D basin structure are compared with the ground-motion velocity seismograms recorded in a small station array deployed at sites within and outside the heavily damage zone in Higashinada ward. The comparison suggests that in the frequency range 0.1–2 Hz that was analysed, the observed spatial amplitude variation of the aftershock ground motion is attributable mainly to the basin edge effects. We found that the basin edge effect, caused by the superposition of the direct S wave and the basin-edge-diffracted waves, amplified the ground motion in a narrow zone that is offset by about 0.7 km from the basin edge.

3‐D simulation of near‐field strong ground motion: Basin edge effect derived from rupture directivity

The heavily damaged area in a narrow belt in Kobe was a very characteristic feature of the 1995 Hyogoken‐nambu (Kobe) earthquake. The processes responsible for the amplification of the strong ground motion there have not yet been clarified. We model the basin edge effect derived from rupture directivity by simulating dynamic rupture for an event with M=7 on a strike‐slip fault buried near a basin. Ground velocity seismograms up to 1 Hz are computed using a 3‐D finite difference method. Patterns of the peak ground velocities show that they are amplified in a very narrow belt along the basin edge, a few kilometers in width, when the fault runs parallel to the edge. This amplification effect also strongly depends on the fault location. However, the large amplitudes are not concentrated in a narrow belt when the whole fault ruptures at once. This suggests that an intense ground shaking in a narrow belt along a basin edge occurs only when rupture propagates parallel to the basin edge with a finite velocity. This appears to have been the case in the Kobe area in which the basin edge effect derived from rupture directivity may have been the cause of the “heavily damaged belt.”

The Cause of the Damage Belt in Kobe: "The Basin-Edge Effect," Constructive Interference of the Direct S-Wave with the Basin-Induced Diffracted/Rayleigh Waves

INTRODUCTION The Hyogo-ken Nambu earthquake of January 17, 1995 caused devastating in Kobe and surrounding areas. Total casualties were more than 5,500 and about one hundred thousand residential houses must be demolished (AIJ, 1995). After the earthquake different institutions sent survey teams to Kobe and unanimously found that the heavily damaged buildings and collapsed residential houses were concentrated in a narrow zone, the so-called damage oriented WSW-ENE across the city of Kobe. This strike of the belt coincides with the strike of the Rokko geological faults and also with the seismogenic fault plan (AIJ, 1995; Kamae and Irikura, 1995; Kikuchi, 1995) to Chuo-ward. Further to the east the belt starts to bend southward and so departs from the trend of the seismogenic fault plane, but it still follows the southern end of the Rokko geological faults. Since no surface breaks alond the Rokko geological faults...

Basin Structure Effects in the Kobe Area Inferred from the Modeling of Ground Motions from Two Aftershocks of the January 17, 1995, Hyogo-ken Nanbu Earthquake.

Ground motion records from aftershocks of the January 17, 1995, Hyogo-ken Nanbu earthquake provide a good opportunity for evaluating the Osaka basin edge effects on wave propagation in the Kobe area. Aftershock ground motion recorded in a small array deployed at sites within and outside the heavily damaged zone, east of Kobe City, show very large amplification at the sedimentary sites located close to the basin edge. The peak velocities at sediment sites were up to 15 times larger than those at the rock site. To model the effect of the basin edge structure and the source radiation in the frequency range 0.1-2.5 Hz, we calculate double-couple point source SH seismograms using two-dimensional finite difference techniques. By comparing the synthetic seismograms with the transverse component of the observed waveforms from two selected aftershocks, we find that the northern edge of the Osaka basin has a fault-like structure, dipping north. The rather complex basin edge structure causes seismic energy to focus at sites close to the edge. The constructive interference of different basin generated waves amplifies the ground motion. The focal area and the amplification due to the basin structure depend on the source location, i.e. the arrival direction of seismic waves. Our results indicate that the basin edge effect is one of the reasons why the zone heavily damaged during the main shock lies along a narrow belt. They also indicate that the thin surface sediments in the Kobe region amplify the ground motion at frequencies around 2 Hz considerably.