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Ground-motion site effect in the Beijing metropolitan area
Journal Pre-proof Ground-motion site effect in the Beijing metropolitan area
Yanju Peng, Zhenming Wang, Edward W. Woolery, Yuejun Lyu, N. Seth Carpenter, Yi Fang, Shuai Huang PII:
S0013-7952(19)30720-3
DOI:
https://doi.org/10.1016/j.enggeo.2019.105395
Reference:
ENGEO 105395
To appear in:
Engineering Geology
Received date:
18 April 2019
Revised date:
24 October 2019
Accepted date:
7 November 2019
Please cite this article as: Y. Peng, Z. Wang, E.W. Woolery, et al., Ground-motion site effect in the Beijing metropolitan area, Engineering Geology (2019), https://doi.org/ 10.1016/j.enggeo.2019.105395
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© 2019 Published by Elsevier.
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Ground-Motion Site Effect in the Beijing Metropolitan Area Yanju Penga,b, Zhenming Wangb, Edward W. Wooleryb, Yuejun Lyua, N. Seth Carpenterb, Yi Fanga, and Shuai Huanga a Institute of Crustal Dynamics, China Earthquake Administration, Beijing, 100085, China b Kentucky Geological Survey, University of Kentucky, Lexington, KY, 40506-0107, USA
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Abstract
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In the past 500 years, many moderate to strong earthquakes have occurred in or near the Beijing metropolitan area, which is underlain by thick Quaternary and Tertiary sediments. Therefore, the area is susceptible to damage associated with ground-motion site effect, particularly for the large number of high-rise buildings. In order to understand the earthquake site effect potential, particularly in the context of the high-rise buildings, we are undertaking an effort to construct a 3-D shear- velocity model of the Quaternary and Tertiary sediments for the Beijing metropolitan area. In this paper, we present an integrated method of deriving shear-wave velocity profiles at sites across the Beijing metropolitan area using shear-wave velocity and ambient-noise measurements. Our results demonstrate that this method is practical for obtaining shear-wave velocity profiles.
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We performed 1-D site response analyses at six sites in the Beijing metropolitan area. The results show that the site resonant periods vary between 0.1 s at shallow sites in the west and 4.0 s for deep sites in the east. Moreover, this range of resonant periods coincides with the fundamental periods of many buildings between one to 40 stories. In other words, earthquake site effect could generate a double resonance in a significant number of buildings in the Beijing area. Results also show that the average shear-wave velocity of the top 20 or 30 m, or , does not correlate with site resonance, and is therefore not an appropriate parameter to account for site effect in earthquake engineering. Keywords Strong ground motion, Site effect, Predominant site period, High-rise buildings
1. INTRODUCTION At least 10 moderate to strong earthquakes with magnitudes of 6 or greater have occurred in the Beijing metropolitan area in the past 500 yr (Fig. 1a). The largest one is 1
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the M 8 Sanhe-Pinggu earthquake in 1679, which caused severe damage in Beijing: VIII to XI on the Chinese intensity scale. Three city gate towers collapsed, as well as 12,798 buildings. An additional 18,028 buildings were severely damaged, and 485 people were killed or injured (Min et al., 1995). The 1976 Tangshan earthquake (M 7.8) occurred more than 100 km away from Beijing, but still caused significant damage in the metropolitan area, particularly in the eastern part: 462 buildings collapsed, 366 were severely damaged, and 3,100 were moderately damaged (Wang et al., 1999). Many high- intensity anomalies were observed in Xiji- Langfu, Caiyu, and Lisui in the eastern part of the metropolitan area. Twenty-one active Quaternary faults are known in the Beijing metropolitan and surrounding area, including the Huangzhuang-Gaoliying Fault, Shunyi-Liangxiang, Xiadian, and Nankou-Sunhe Faults, all of which are seismogenic and capable of generating earthquakes of magnitude 7 or larger (Fig. 1a).
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Figure 1. Active Quaternary faults and seismicity in Beijing and its vicinity. The area of this study is shown by a box (a), and Quaternary sediment thickness (in meters) of the study area (b). D1 is Kunminghu Depression center, D2 is Machikou Depression center and D3 is Houshayu Depression center.
The thickness of Quaternary sediments (Fig. 1b) was compiled by the Beijing Institute of Hydrogeology and Engineering Geology (1979) from geological and geophysical investigations conducted in the 1970s. As shown in Figure 1b, the Beijing metropolitan area is underlain by Quaternary sediments ranging in thickness from 0 m in the west to 700 m in the northeast. The lithology of the Quaternary sediments is complex, including clay, silt, and silty clays as well as coarser sand s, gravels, and boulders. The shear-wave velocity of the Quaternary sediment ranges from 100 to 500 m/s. The Quaternary sediment is underlain by Tertiary sediment ranging in thickness from 0 to 1,000 m in the study area (Cai et al., 2009). The lithology of the Tertiary units is soft rock including conglomerate, claystone, shale，and mudstone with shear-wave velocity greater than 500 m/s, and that velocity increases with depth (BGI Engineering Consultants Ltd., 2011; Wang and Han, 2012). The depth to the top of bedrock varies between 0 m in the western part of the study area to more than 1,000 m in the central and eastern parts. The bedrock is mainly limestone, shale, conglomerate，and basalt (Beijing Institute of Hydrogeology and Engineering Geology, 1979). 3
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Strong ground motion is known to be significantly modified by near-surface sediments: the so-called site effect or site response. A classic example of site effect is the ground- motion amplification caused by the resonance of the soft lake sediments in Mexico City during the 1985 Michoacan earthquake (M 8.0) (Anderson et al., 1986; Seed et al., 1988; Singh et al., 1988). Although the earthquake occurred more than 300 km away from Mexico City, the amplified motions in the period between 0.5 and 3.0 s caused many buildings to collapse, particularly five- to 16-story buildings on ancient lake sediments underlying a significant portion of Mexico City. Similar site effects were also observed during the 2017 M 8.2 Tehuantepec and M 7.1 Puebla-Morelos earthquakes (Celebi et al., 2018; Sahakian et al., 2018). Site effects have been observed worldwide during strong earthquakes such as the 1989 Loma Prieta earthquake (M 6.9) (Borcherdt and Glassmoyer, 1992; Rubinstein, 2004; Olsen et al., 2008), 1994 Northridge earthquake (M 6.7) (Pitarka, 1996; Hartzell et al., 1996), 2008 Wenchuan, China, earthquake (M 8.0) (Ren et al., 2013), and 2015 Gorkha, Nepal, earthquake (M 7.8) (Asimaki et al., 2017; Barbosa et al., 2017).
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2. SITE EFFECT
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In this paper, we first present a brief review of site effect and previous studies in the Beijing metropolitan area. We describe our method to develop shear-wave velocity profiles of the Quaternary and Tertiary sediments, and provide site response analyses derived using 1-D equivalent linear model at selected sites with well- constrained shear-wave velocity profiles. Finally, we assess both the potential impact on the high-rise buildings by site effect and the adequacy of the “simplified” site classification systems in earthquake engineering.
As shown in Figure 2, site effect (i.e., resonance of a sedimentary basin) is a complex 3-D wave-propagation phenomenon and difficult to quantify because it is influenced by many factors, including earthquake source (i.e., fault rupture), wave propagation through rock (i.e., crust), basin response, and soil nonlinearity. Recent theoretical efforts to quantify site effect have focused on 3-D ground-motion simulations (e.g., Olsen, 2000; Aagaard et al., 2010; Rodgers et al., 2019). Along with significant development of supercomputer, the 3-D ground motion simulation has been advanced greatly. As demonstrated by Rodgers and others (2019), for example, the supercomputer at Lawrence Berkeley National Laboratory could simulate ground motions with frequencies up to 5 Hz and sediment shear-wave velocities down to 500 m/s. Although its applications in earthquake engineering are still limited, the 3-D ground-motion simulation will become a viable approach for quantifying site effect in the co ming years.
Empirically, site effect is quantified by the ratio of ground motion amplitude spectra 4
Journal Pre-proof from a sediment site to a nearby rock site (Fig. 2) (Borcherdt, 1970; Dobry et al., 2000) as 𝐴 (𝑓)
𝑆𝑆𝑅 = 𝐴 𝑆 (𝑓) ,
(1)
𝑅
where 𝐴𝑆 (𝑓) and 𝐴𝑅 (𝑓) are the Fourier spectra of ground acceleration for the horizontal component of an S-wave at a given site on sediment and a nearby site on rock, respectively. This method is justified by the standard source-path-site convolution approximation for wave propagation from an earthquake: 𝐴𝑆 (𝑓) can be expressed as the convolution of source S(f), path P(f), site G(f), and instrument response I(f): (2)
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𝐴𝑆 (𝑓) = 𝑆 (𝑓) × 𝑃(𝑓) × 𝐺 (𝑓) × 𝐼 (𝑓).
Similarly, assuming the site response at the rock site is unity, 𝐴𝑅 (𝑓) can be expressed:
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𝐴𝑅 (𝑓) = 𝑆 (𝑓) × 𝑃(𝑓) × 𝐼 (𝑓).
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Thus, SSR = G(f).
(3)
Figure 2. Schematic diagram for 3-D wave propagation and site effect (i.e., basin resonance).
Site effects have been explored to some degree in the Beijing metropolitan area. Gao et al. (2002) and Fu et al. (2012) used 3-D finite-difference simulations, and Ding et al. (2004) used the modal summation method. However, none of these studies considered a key component of site response: the near-surface low velocity sediments (< 500 m/s). As shown by Chen et al. (2018), near-surface low velocity sediments are important for study of site effects. Thus, constructing an accurate near-surface shear-wave velocity model is essential for reliably predicting site effect. 5
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3. SHEAR-WAVE VELOCITY PROFILES
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We developed a method to determine shear-wave velocity profiles at sites throughout the study area using downhole and ambient-noise measurements. Investigators have collected downhole shear-wave velocity measurements during the last two decades at more than 800 sites in the Beijing area (Fig. 3). The measurements followed Chinese code GB50021-2001(MHUD, 2009) or ASTM D7400/D7400M (ASTM, 2019) specifications that are specific to seismic-hazard analysis for engineering projects, such as subway stations, railway stations, and high-rise buildings. Soil samples were also taken from boreholes for laboratory tests to determine soil properties such as density and damping parameters. Figure 4 shows examples of shear-wave velocity profiles obtained from the downhole surveys at selected sites. These sites were selected as representative examples of the varying geological conditions across the Beijing metropolitan area.
Figure 3. Distribution of the datasets and selected sites for site-response simulation. Contour lines show the thickness of Quaternary sediment.
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Figure 4. Simplified stratigraphic columns and shear-wave velocities for the six selected sites. These sites are representative of the varying geologic conditions across the Beijing metropolitan area. The penetration depths of the boreholes vary, but none was greater than 100 m because that is the standard engineering drilling limit. Also, according to the building code of China (Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2016), the strata with shear-wave velocity equal to or greater than 500 m/s are defined as rock for engineering purpose. As shown in Figure 1b, the thickness of Quaternary sediments is greater than 100 m in many parts of the Beijing metropolitan area. Thus, the available downhole surveys are insufficient for determining shear-wave velocities of the deeper Quaternary and Tertiary sediments. 7
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𝑉𝑆 4𝐻
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𝑓𝑝 =
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Consequently, microtremor measurements were conducted in summer 2007 for assessing local site effects at more than 600 sites in metropolitan Beijing using three-component Guralp 40T-1 broadband seismometers and Refteck 130B recorders (Chen et al., 2009) (Fig. 3). Chen et al. (2009) performed the horizontal-to- vertical ratio analysis, i.e., the H/V technique of Nakamura (1989), on the microtremor data to obtain the predominant site frequencies. Although the predominant site frequency of ambient noise, which contains the ellipticity of the fundamental Rayleigh wave and other body waves (Bonnefoy-Claudet et al., 2006; Kawase et al., 2011), is fundamentally different from the fundemantal frequency of the resonance of the entire sediment column for the SH-wave, they are coincident (Carpenter et al., 2018; Wang et al., 2019). This coincidence makes it possible to use the predominant frequency of ambient noise, f p , and (4)
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to estimate average shear-wave velocity (VS ) or thickness (H) of the entire sediment column. For example, Bodin et al. (2001) used the predominant site period (reciprocal of the predominant site frequency) to determine the thickness of the deep unlithified sediments of the Mississippi Embayment. We utilized the microtremor data and resulting predominant site frequencies of Chen et al. (2009) and thicknesses of Quaternary and Tertiary sediments (Beijing Institute of Hydrogeology and Engineering Geology, 1979; Ding et al., 2004; Cai et al., 2009) to determine the average shear-wave velocities of entire sediment column using equation (4). There were significant transient noises contained in the microtremor measurements because the Beijing metropolitan area is heavily populated. We reprocessed some microtremor data in order to suppress transient noises and better determine the predominant site frequencies using GEOSPY (http://www.geopsy.org/download.php; last accessed December 2018). Figure 5 shows the horizontal-to- vertical spectral ratios of ambient noise at Site 3, 4, 5, and 6.
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Figure 5. Horizontal-to-vertical spectral ratios of ambient noise at Site 3, 4, 5, and Site 6.
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The procedure for determining shear-wave velocity profile for a shallow site is straightforward. As shown in Figure 3 and 4, the shear-wave velocity profiles at sites 1 and 2 could be directly determined from the downhole surveys: two layers (i.e., surficial materials/artificial fill and gravel) of Quaternary sediments overlying bedrock (limestone) (Fig. 6a, b). The shear-wave velocity of bedrock (limestone) is approximately 1,800 m/s (Fu et al., 2012). For deeper sites, the borehole only penetrated the depth at which the shear-wave velocity reached 500 m/s, and did not reach the bedrock. For example, at site 3 (Figs. 3 and 4), there are three layers—surficial materials/artificial fill, gravel of Quaternary, and conglomerate of Tertiary—with shear-wave velocities of 220, 405, and 590 m/s, respectively, from direct borehole measurements. According to Ding et al. (2004) and Cai et al. (2009), the thickness of the underlying Tertiary sediment is approximately 70 m at this site. The resulting shear-wave velocity profile for site 3 is shown in Figure 6c.
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Figure 6. Shear-wave velocity profiles for sites 1–6, derived from borehole data, sediment thicknesses, and ambient-noise measurements. (a) Site 1. (b) Site 2. (c) Site 3. (d) Site 4. (e) Site 5. (f) Site 6.
The average shear-wave velocity or thickness of the Tertiary conglomerate could also be determined from the predominant site frequency. Figure 7 shows a generalized subsurface model consisting of multiple layers, their associated thicknesses and velocities, and the sediment column’s average velocity and total thickness. For given velocities and thicknesses of the first through the n-1th layers, V S1 , H1 , V S2 , H2 , …, VSn-1 , Hn-1 , and average velocity and thickness of entire sediments, V S, and H, the average shear-wave velocity or thickness for the nth layer can be calculated, 𝑆𝑛
=
𝐻𝑛 𝐻𝑖 𝐻 −∑𝑛−1 1 𝑉𝑆 𝑉 𝑆𝑖
.
(5)
As shown in Figure 5a, the predominant frequency is approximately 1.28 Hz at site 3. Equation 4 yields an average shear-wave velocity of 501 m/s for the entire sediment column with a total thickness of 97.8 m. Equation 5 yields an average shear-wave velocity of Tertiary sediments of 605 m/s. The measured shear-wave velocity for the upper part of the Tertiary sediments, 590 m/s, and calculated velocity, 605 m/s, closely 10
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correspond. This demonstrates that the ambient noise dominant site frequency can be used to estimate the average shear-wave velocity of deeper sediments.
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Figure 7. Generalized model of multiple sublayers with average velocities and thicknesses.
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As shown in Figure 4, the profile at site 4 can be simplified into five layers—surficial materials/artificial fill, silty clay 1, silty clay 2, medium sand/silty clay, and claystone (Tertiary sediment)—with shear-wave velocities of 220, 300, 405, 510, and 620 m/s, respectively. The thickness of the Tertiary sediment is approximately 90 m (Ding et al., 2004; Cai et al., 2009). The resulting shear-wave velocity profile for site 4 is shown in Figure 6d. According to direct borehole measurements (Fig. 4), there are five layers at site 5—surficial materials/artificial fill, gravel/silty clay, fine sand, silty clay, lower part of Quaternary sediments—with shear-wave velocities of 220, 280, 340, 430, and 520 m/s, respectively. According to Ding et al. (2004) and Cai et al. (2009), the total thickness of Quaternary sediments is approximately 150 m, and the thickness of Tertiary sediments is 100 m at site 5. As shown in Figure 5c, the predominant frequency is 0.5 Hz at site 5. Equation 4 yields an average shear-wave velocity of the entire sediment column of 500 m/s at site 5. Accordingly, equation 5 yields a shear-wave velocity of the Tertiary sediment of 674 m/s. The resulting shear-wave velocity profile for site 5 is shown in Figure 6e. The borehole measurement at site 6 (Fig. 4) revealed that the surficial material is nearly 11 m thick with a shear-wave velocity of 203 m/s. The Quaternary sediment at site 6 can be simplified into six layers: 11 m thick with shear-wave velocity of 203 m/s, 13 m thick with velocity of 280 m/s, 19 m thick with a velocity of 340m/s, 19 m thick with velocity of 410 m/s, 28 m thick with velocity of 470 m/s, and the top 5 m of bottom layer with velocity of 520m/s, respectively. Based on the results of Ding et al. (2004) and Cai et al. (2009), total thickness of Quaternary sediment is approximately 210 m and total thickness of Tertiary sediment is 150 m. Thus, the thickness of the bottom layer of Quaternary sediment is about 120 m. As shown in Figure 5d, the predominant 11
Journal Pre-proof frequency at site 6 is 0.34 Hz, which results in an average shear-wave velocity of 490 m/s for the entire sediment column. Equation 5 yields a shear-wave velocity of the Tertiary sediment of 606 m/s. The resulting shear-wave velocity profile for site 6 is shown in Figure 6f.
4. ANALYSIS OF 1-D SITE-EFFECT
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Ideally, the site effect can be directly determined from the ratio of the recorded groundmotion amplitude spectra of the sediment site and nearby rock site (SSR). Strong-motion recordings are limited, however. As briefly discussed earlier, 3-D ground- motion simulation also has limited applications in earthquake engineering. Currently in engineering practice, the SSR is most commonly determined by 1-D model analyses, such as SHAKE-91 (Schnabel et al., 1972; Idriss and Sun, 1992). The 1-D equivalent linear site-response program RSLEIBM (Li, 1989) was used to determine the SSR in this study. Similar to SHAKE-91, RSLEIBM uses an iterative procedure (i.e., equivalent linear) to account for the nonlinear behavior of the soils. The 1-D model for calculating site response consists of several assumed horizontal layers of nonlinear sediment media overlying an elastic half- space. The motion of a soil layer is caused by shear waves vertically propagated from the elastic half-space upward. In addition to the shear-wave velocities and sediment unit thicknesses, densities, damping, and shear modulus and damping reduction relationships are required for 1-D equivalent linear site-response modeling. Our selected sites used laboratory derived densities, shear modulus and damping reduction relationships from prior investigations (e.g., Shi, 2009; BGI Engineering Consultants Ltd., 2011). Figure 8 shows the shear modulus and damping reduction curves for silty clay, fine sand, and gravel.
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Figure 8. Testing results of dynamic shear modulus and damping ratio versus shear strain for silty clay, fine sand and gravel in the study area. The input motions for 1-D analysis were generated by the finite-stochastic source model for two scenario earthquakes. We chose the large historical 1679 Sanhe-Pinggu earthquake (M 8.0) as one scenario (Fig. 1a). A repeat of such event could have significant impacts on the Beijing metropolitan area. A synthetic ground motion was generated for a rock site at 50 km for the 1679 Sanhe-Pinggu scenario earthquake (Gao et al., 2002). A second synthetic ground motion for a rock site at 120 km was also generated for a scenario event similar to the 1976 Tangshan earthquake (M 7.8). Figure 9 shows the synthetic ground acceleration time histories for the two scenario 13
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earthquakes.
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Figure 9. Synthetic acceleration time histories for the M 8.0 Sanhe-Pinggu scenario earthquake (a) and the M 7.8 Tangshan scenario earthquake (b).
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The shear-wave velocity profiles (Fig. 6), nonlinearity curves (Fig. 8), and the synthetic ground motions (Fig. 9), along with other parameters, were used for 1-D site-response analyses at the six sites across the metropolitan area. Figure 10 shows spectral ratios for the six sites corresponding to the two input ground motions. Figure 11 shows response accelerations with 5 percent damping for the six sites corresponding to the two input ground motions.
Figure 10. Spectral ratios of the selected sites for the input synthetic motions from the M 8 Sanhe-Pinggu scenario earthquake (a) and M 7.8 Tangshan scenario earthquake (b).
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Figure 11. Response accelerations with 5 percent damping of the selected sites for the input synthetic motions from the M 8 Sanhe-Pinggu scenario earthquake (a) and the M 7.8 Tangshan scenario earthquake (b).
5. DISCUSSION
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As illustrated in Figure 2, site effect is a complex 3-D wave-propagation problem. In order to evaluate site effect, establishing 3-D sedimentary basin structure, shear-wave velocity structure in particular, is essential. We are undertaking an effort to construct the 3-D shear-velocity structure of Quaternary and Tertiary sediments for the Beijing metropolitan area, using shear-wave velocity measurements from more than 800 boreholes and ambient-noise measurements from more than 600 sites (Fig. 3). In this paper, we presented the method to derive shear-wave velocity profile utilizing shear-wave velocity and ambient-noise measurements and applied it to obtain the shear-wave velocity profiles at sites across the Beijing metropolitan area. Our results suggest that this method is viable for obtaining shear-wave velocity profiles. The 1-D ground-response analyses at the six sites across the metropolitan area (Figs. 10–11) demonstrated that the site resonant periods increase with the thickness of the entire sediment column, from 0.1 to 0.3 s for the shallow sites (about 30 m deep) in the west to 3.0 to 4.0 s for the deeper site (360 m) in the east, whereas the peak amplifications decrease slightly, from about 4 to 3 from west to east. In other words, the site resonant period is more responsive to the sediment properties (i.e., shear-wave velocity and thickness) than the amplification factor. As demonstrated in Mexico City during the 1985 Michoacan earthquake (M 8.0) (e.g., Anderson et al., 1986; Seed et al., 1988), damage to five- to 16-story buildings was caused by double resonances: corresponding site resonance of the lake sediments and building resonance. Such double resonances have also been observed in Los Angeles and San Francisco, Calif. (e.g., Pitaka, 1996; Rubinstein, 2004) and Maysville, Ky. (Woolery et al., 2008). Thus, site resonant period can in some instances be a more important parameter than amplification factor in site effect analyses.
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Journal Pre-proof 5.1 Potential Impact on High-Rise Buildings The height of buildings in Beijing varies greatly, ranging from one to more than 100 stories. According to the Chinese construction code (China Association for Engineering Construction Standardization, 2012), the fundamental period (T) of a steel-reinforced concrete structure can be estimated by equation (6): 𝑇 = (0.05～0.1)𝑁
(6)
where 𝑁 is the number of stories. According to equation (6), the f undamental periods of buildings in the Beijing metropolitan area are in the range of 0.05 to 10 s.
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As shown in Figures 10 and 11, the site resonance period from 1-D site-response analysis varies from 0.1 s in the western part of the metropolitan area to 4.0 s in the eastern part. According to equation 6, these fundamental periods, 0.1 to 4.0 s, correspond to buildings of one to 40 stories. Table 1 lists the distribution of building heights in the metropolitan area (Hu, 2005), and indicates that the majority (83 percent) of the buildings have fewer than 36 stories. In other words, the majority of the buildings in the metropolitan area are subject to potential damage caused by site resonance of Quaternary and Tertiary sediments.
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Table 1. Distribution of building heights in the Beijing metropolitan area. Building Height < 36 stories 37–50 stories > 50 stories 83.4
< 3.6
11.2
5.4
3.7–5.0
> 5.0
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5.2 Adequacy of Vs30 and Vs20
The average shear-wave velocity of the top 30 m of soils and rock, Vs30 , has been used in the United States and other countries as an index to account for site effect in earthquake engineering; the NEHRP site classification (Building Seismic Safety Council, 2015). Similarly, the average shear-wave velocity of the top 20 m of soils and rock, Vs20 , has been used in China for this purpose (e.g., Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2016). Table 2 lists the average shear-wave velocities of the top 20 and 30 m for the six sites (Fig. 6). Table 2 also lists the Chinese and NEHRP site classes for the six sites. According to the Chinese site classification, sites 1 through 4 in this study can be classified as II, and sites 5 and 6 can be classified as III. Similarly, according to the NEHRP site classification, sites 1 through 3 can be classified as C, and sites 4 through 6 can be classified as D. As shown in Figures 10 and 11, the site resonant period increases from site 1 to 6. There are only two site classes, II and III, according to the Chinese site classification, and only two classes, C and D, according to the NEHRP site classification, however. In other 16
Journal Pre-proof words, these site classes are not directly related to site resonances. Thus, neither site classification adequately accounts for the earthquake site effect. This is particularly obvious for sites 3 and 4, both of which have the same Chinese site class of II, but significantly different dominant site periods, 0.2–0.4 s vs. 1.0–2.0 s. Consequently, the average shear-wave velocity of the top 20 or 30 m is not an appropriate parameter to uniquely account for site effect. Table 2. Site classifications based on the average shear-wave velocities of the top 20 and 30 m. Site 1
Site 2
Site 3
Site 4
Site 5
Site 6
Vs20 (m/s)
386
358
336
258
243
239
Chinese Site Class
Ⅱ
Ⅱ
Ⅱ
Ⅱ
Ⅲ
Ⅲ
Vs30 (m/s)
438
383
369
271
263
257
NEHRP Site Class
C
C
C
D
D
D
Site Resonant Period (s)
0.1–0.3
0.1–0.3
0.2–0.4
1.0–2.0
2.0–3.0
2.5–3.5
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6. CONCLUSIONS
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We developed an integrated method to derive shear-wave velocity profiles in deep sediments utilizing the downhole and ambient-noise measurements. The results demonstrated that the method is viable. We performed 1-D equivalent linear site response analysis at six sites. The results suggest that site resonant periods vary between 0.1 s at shallow sites in the western part of the metropolitan area and 4.0 s at deep sites in the eastern part. This resonant period range coincides with the fundamental periods of buildings of one to 40 stories in Beijing. In other words, there is a potential damage for the high-rise buildings caused by a double resonance in much of the Beijing metropolitan area. Additionally, we found that the average shear-wave velocity of the top 20 or 30 m, or , although a standard parameter to account for site effect in earthquake engineering, is not an appropriate proxy, because neither exhibited an ability to accurately estimate the site effect (i.e., site resonance), the predominant site period in particular. This is a significant shortcoming for urban areas where the double resonance problem is manifest.
ACKNOWLEDGMENTS This research is partially funded by a grant from the Institute of Crustal Dynamics, China Earthquake Administration (No. ZDJ2017-28) and the National Key Research and Development Program of China (2017YFC1500403). The epicenters and magnitudes in Figure 1 are from the China Earthquake Data Center (http://data.earthquake.cn/sjfw; last accessed December 2018). We appreciated Chen et al. for allowing us to use the microtremor data, and Weijun Wang of the Institute of 17
Journal Pre-proof Earthquake Forecasting, China Earthquake Administration, for providing the data. The process of ambient noise data was carried out using GEOSPY (http://www.geopsy.org/download.php; last accessed December 2018). We appreciated the anonymous reviewers for their constructive comments and suggestions that have helped to greatly improve this manuscript. We thanked Meg Smath of the Kentucky Geological Survey for editorial help.
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REFERENCES Aagaard, B.T., Graves, R.W., Rodgers, A., Brocher, T.M., Simpson, R.W., Dreger, D., Petersson, N.A., Larsen, S.C., Ma, S., and Jachens, R.C., 2010. Ground- motion modeling of Hayward Fault scenario earthquakes, Part II: Simulation of long-period and broadband ground motions: Bulletin of the Seismological Society of America, 100(6), 2945–2977. https://doi.org/10.1785/0120090379. Anderson, J.G., Bodin, P., Brune, J.N., Prince, J., and Singh, S.K., 1986. Strong ground motion from the Michoacán, Mexico, earthquake. Science, 233(4768), 1043– 1049. http://doi.org/ 10.1126/science.233.4768.1043. Asimaki, D., Mohammadi, K., Mason, B., Adams, R.K., and Khadka, D., 2017. Observations and simulations of basin effects in the Kathmandu Valley during the 2015 Gorkha earthquake sequence. Earthquake Spectra, 33(S1), S35–S53. https://doi.org/10.1193/013117EQS022M. ASTM D7400/D7400M-19. 2019. Standard methods for downhole seismic testing, ASTM International, West Conshohocken, PA, DOI:10.1520./D7400_ D7400M-19. Barbosa, A.R., Fahnestock, L.A., Fick, D.R., Gautam, D., Soti, R., Wood, R., Moaveni, B., Stavridis, A., Olsen, M.J., and Rodrigues, H., 2017. Performance of medium-to- high rise reinforced concrete frame buildings with masonry infill in the 2015 Gorkha Nepal earthquake. Earthquake Spectra, 33(S1), S197–S218. https://doi.org/10.1193/051017EQS087M. Beijing Institute of Hydrogeology and Engineering Geology, 1979. The bedrock geological map of Beijing Plain. Geological Publishing House (in Chinese). BGI Engineering Consultants Ltd., 2011. Data set of the physical＆mechanical test for deep stratum in Beijing's plain area (1955–2011) (in Chinese). Bodin, P., Smith, K., Horton, S., and Hwang, H., 2001, Microtremor observations of deep sediment resonance in metropolitan Memphis, Tennessee, Engineering Geology, 62, 159–168. Bonnefoy-Claudet, S., Cornou, C., Bard, P., Cotton, F., Moczo, P., Kristek, J., and Fah, D., 2006. H/V ratio: A tool for site effects evaluation: Results from 1-D noise simulations. Geophysical Journal International, 167, 827–837. https://10.1111/j.1365-246x.2006.03154. Borcherdt, R.D., 1970. Effects of local geology on ground motion near San Francisco Bay. Bulletin of the Seismological Society of America, 60(1), 29–61. Borcherdt, R.D., and Glassmoyer, G., 1992. On the characteristics of local geology and their influence on ground motions generated by the Loma Prieta earthquake in the 18
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San Francisco Bay region. Bulletin of the Seismological Society of America, 82(2), 603–641. Building Seismic Safety Council, 2015. NEHRP recommended seismic provisions for new buildings and other structures, 2015 edition. FEMA P-1050-1, 515 p. Cai, X.M., Luan, Y.B., Guo, G.X., Liang Y.N., 2009. 3D Quaternary geological structure of Beijing Plain. Geology in China, 36(5), 1021–1029 (in Chinese). Carpenter, N.S., Wang, Z., Woolery, E.W., and Rong, M.S., 2018. Estimating site response with recordings from deep boreholes and HVSR: Examples from the Mississippi Embayment of the Central United States. Bulletin of the Seismological Society of America, 108(3A), 1199–1209. http://dio.org/10.1785/0120170156. Celebi, M., Valerie, J.S., Diego, M., and Luis, Q., 2018. The 19 September 2017 M7.1 Puebla-Morelos earthquake: Spectral ratios confirm Mexico City zoning. Bulletin of the Seismological Society of America, 108(6), 3289–3299. http://doi.org/10.1785/0120180100. Chen, G.X., Jiao, Z., Qiang, M.Y., and Gong, W.P., 2018. Three-dimensional site characterization with borehole data—A case study of Suzhou area. Engineering Geology, 234, 65–82. https://doi.org/10.1016/j.enggeo.2017.12.019. Chen, Q.F., Liu, L.B., Wang, W.J., and Rohrbach, E., 2009. Site effects on earthquake ground motion based on microtremor measurements for metropolitan Beijing. Chinese Science Bulletin, 54(2), 280–287. https:/doi.org/10.1007/s11434-008-0422-2. China Association for Engineering Construction Standardization, 2012. Load code for the design of building structures (GB50009-2012). China Architecture and Building Press, Beijing, China (in Chinese). Ding, Z., Chen, Y.T., and Panza, G.F., 2004. Estimation of site effects in Beijing City. Pure and Applied Geophysics, 161, 1107–1123. https://doi.org/10.1007/s00024-003-2495-9. Dobry, R., Borcherdt, R.D., Crouse, C.B., Idriss, I.M., Joyner, W.B., Martin, G.R., Power, M.S., Rinne, E.E., and Seed, R.B., 2000, New site coefficients and site classification system used in recent building seismic code provisions, Earthquake Spectra, 16 (1), 41–67. Fu, C.H., Gao, M.T., and Chen, K., 2012. A study on long-period acceleration response spectrum of ground motion affected by basin structure of Beijing. Acta Seismologica Sinica, 4(3), 374–382 (in Chinese). Gao, M.T., Yu, Y.X., Zhang, X.M., Wu, J., and Ding, Y., 2002. Three-dimensional finite-difference simulations of ground motions in the Beijing area. Earthquake Research in China, 18(4), 356–364 (in Chinese). Hartzell, S., Leeds, A., Frankel, A., and Michael, J., 1996. Site response for urban Los Angeles using aftershocks of the Northridge earthquake. Bulletin of the Seismological Society of America, 86(1), S168–S192. Haskell, N.A., 1960. Crustal reflection of plane SH waves. Journal of Geophysical Research, 65(12), 4147–4150. Hu, S.D., 2005. Analysis on high-rise building development in Beijing in recent decades. 19
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Architecture Technology, 36(6), 408–511 (in Chinese). Idriss, I.M., and Sun, J.I.,1992. User’s manual for SHAKE91. Center for Geotechnical Modeling, Department of Civil & Environmental Engineering, University of California, Davis, Calif., 13 p. Kawase, H., Sánchez-Sesma, F.J., and Matsushima, S., 2011. The optimal use of horizontal- to-vertical spectral ratios of earthquake motions for velocity inversions based on diffuse field theory for plane waves. Bulletin of the Seismological Society of America, 101(5), 2001–2004. https://doi.org/10.1785/0120100263. Li, X.J., 1989. A one dimensional equivalent linear program for seismic response analysis. In Theory and Practice of Seismic Zonation. Seismological Press, Beijing, China (in Chinese). Min, Z.Q., Wu, G., Jiang, Z.X., Liu, C.S., Yang, Y.L., 1995. A catalog of strong historical earthquakes in China (from 2300 BC to 1911 AD). Seismology Publishing House, Beijing, China (in Chinese). Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2016. Code for Seismic Design of Buildings (GB50011-2016). China Architecture and Building Press, Beijing, China (in Chinese). Ministry of Housing and Urban-Rural Development of the People’s Republic of China(MHUD). Code for investigation of geotechnical engineering (GB50021-2001). China Architecture and Building Press, Beijing, 2009 (in Chinese). Nakamura, Y., 1989, A method for dynamic characteristics estimation of subsurface using microtremor on ground surface: Quarterly Report of Railway Technical Research Institute, v. 30, no. 1, p. 25–33. Olsen, A.H., Aagaard, B.T., and Heaton, T.H., 2008. Long-period building response to earthquakes in the San Francisco Bay area. Bulletin of the Seismological Society of America, 98(2), 1047–1065. https://doi.org/10.1875/0120060408. Olsen, K.B., 2000. Site amplification in the Los Angeles Basin from three-dimensional modeling of ground motion. Bulletin of the Seismological Society of America, 90(6B), S77–S94. https://doi.org/10.1785/0120000506. Pitarka, A.,1996. Basin structure effects on long period strong motions in the San Fernando Valley and in the Los Angeles Basin from the 1994 Northridge earthquake and an aftershock. Bulletin of the Seismological Society of America, 86 (1B), S126–S137. https://doi.org/10.1117/12.782693. Ren, Y., Wen, R., Yamanaka, H., and Kashima, T., 2013. Site effects by generalized inversion technique using strong motion recordings of the 2008 Wenchuan earthquake. Earthquake Engineering & Engineering Vibration, 12(2), 165–184. https://doi.org/10.1007/s11803-013-0160-6. Rodgers, A.J., Petersson, N.A., Pitarka, A., McCallen, D.B., Sjogreen, B., and Abrahamson, N., 2019. Broadband (0–5 Hz) fully deterministic 3D ground motion simulations of a magnitude 7.0 Hayward Fault earthquake: Comparison with empirical ground motion models and 3D path and site effects from source normalized intensities. Seismological Research Letters (early edition), https://doi.org/10.1785/0220180261. 20
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Rubinstein, J.L., 2004. Evidence for widespread nonlinear strong ground motion in the Mw 6.9 Loma Prieta earthquake. Bulletin of the Seismological Society of America, 94(5), 1595–1608. https://doi.org/10.1785/012004009. Sahakian, V.J., Melgar, D., Quintanar, L., Ramírez-Guzmán, L., Pérez-Campos, X., and Baltay, A., 2018. Ground motions from the 7 and 19 September 2017 Tehuantepec and Puebla-Morelos, Mexico earthquake. Bulletin of the Seismological Society of America, 108(6), 3300–3312. https://doi.org/10.1785/0120180108. Schnabel, P.B., Lysmer, J., and Seed, H.B., 1972. SHAKE: A computer program for earthquake response analysis of horizontally layered sites. Report No. UCB/EERC-72/12, Earthquake Engineering Research Center, University of California, Berkeley, Calif., 102 p. Seed, H.B., Romo, M.P., Sun, J.I., Jaime, A., and Lysmer, J., 1988. The Mexico earthquake of September 19, 1985—Relationships between soil conditions and earthquake ground motions. Earthquake Spectra 4(4), 687–729. https://doi.org/10.1193/1.1585498. Shi, C.H., 2009. A study of the effect of site conditions on seismic ground motion parameters in Beijing area [master’s degree]. Institute of Crustal Dynamics, China Earthquake Administration (in Chinese). Singh, S.K., Lermo, J., Domínguez, T., Ordaz, M., Espinosa, J.M., Mena, E., and Quaas, R., 1988. The Mexico earthquake of September 19, 1985—A study of amplification of seismic waves in the Valley of Mexico with respect to a hill zone site. Earthquake Spectra, 4(4), 653–673, https://doi.org/10.1193/1.1585496. Wang J.H., and Han, X., 2012. Preliminary study on Tertiary engineering geological conditions in Beijing Plain area. Journal of Engineering Geology, 20(5), 682–686 (in Chinese). Wang, S.Y., Wu, G., Shi, Z.L., 1999. A catalog of modern earthquakes in China (1912– 1990). China Science and Technology Press, Beijing, China (in Chinese). Wang, Z.M., Carpenter, N.S., and Woolery, E.W., 2019. Horizontal-to-vertical spectral ratio of S-waves and SH-wave transfer functions at the vertical seismic and strong- motion arrays in the central United States. Journal of Applied Geophysics, 162, 64–71. https://doi.org/10.1016/j.jappgeo.2018.10.017. Woolery, E.W., Lin, T., Wang, Z., and Shi, B., 2008. The role of local soil- induced amplification in the 27 July 1980 northeastern Kentucky earthquake. Environmental & Engineering Geoscience, 14, 267–280.
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Conflict of interest statement
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We declared that we have no conflicts of interest to this work.
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Highlights:
An integrated method was developed to determine shear-wave velocity profiles
1-D site response analyses were performed in the Beijing metropolitan area
The Beijing metropolitan area is susceptible to the ground-motion site effect
Site resonant periods coincide with the fundamental periods of many buildings
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is not an appropriate proxy for quantifying site effect in engineering
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Yanju Peng, Zhenming Wang, Edward W. Woolery, Yuejun Lyu, N. Seth Carpenter, Yi Fang, Shuai Huang PII:
S0013-7952(19)30720-3
DOI:
https://doi.org/10.1016/j.enggeo.2019.105395
Reference:
ENGEO 105395
To appear in:
Engineering Geology
Received date:
18 April 2019
Revised date:
24 October 2019
Accepted date:
7 November 2019
Please cite this article as: Y. Peng, Z. Wang, E.W. Woolery, et al., Ground-motion site effect in the Beijing metropolitan area, Engineering Geology (2019), https://doi.org/ 10.1016/j.enggeo.2019.105395
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
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Ground-Motion Site Effect in the Beijing Metropolitan Area Yanju Penga,b, Zhenming Wangb, Edward W. Wooleryb, Yuejun Lyua, N. Seth Carpenterb, Yi Fanga, and Shuai Huanga a Institute of Crustal Dynamics, China Earthquake Administration, Beijing, 100085, China b Kentucky Geological Survey, University of Kentucky, Lexington, KY, 40506-0107, USA
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Abstract
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In the past 500 years, many moderate to strong earthquakes have occurred in or near the Beijing metropolitan area, which is underlain by thick Quaternary and Tertiary sediments. Therefore, the area is susceptible to damage associated with ground-motion site effect, particularly for the large number of high-rise buildings. In order to understand the earthquake site effect potential, particularly in the context of the high-rise buildings, we are undertaking an effort to construct a 3-D shear- velocity model of the Quaternary and Tertiary sediments for the Beijing metropolitan area. In this paper, we present an integrated method of deriving shear-wave velocity profiles at sites across the Beijing metropolitan area using shear-wave velocity and ambient-noise measurements. Our results demonstrate that this method is practical for obtaining shear-wave velocity profiles.
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We performed 1-D site response analyses at six sites in the Beijing metropolitan area. The results show that the site resonant periods vary between 0.1 s at shallow sites in the west and 4.0 s for deep sites in the east. Moreover, this range of resonant periods coincides with the fundamental periods of many buildings between one to 40 stories. In other words, earthquake site effect could generate a double resonance in a significant number of buildings in the Beijing area. Results also show that the average shear-wave velocity of the top 20 or 30 m, or , does not correlate with site resonance, and is therefore not an appropriate parameter to account for site effect in earthquake engineering. Keywords Strong ground motion, Site effect, Predominant site period, High-rise buildings
1. INTRODUCTION At least 10 moderate to strong earthquakes with magnitudes of 6 or greater have occurred in the Beijing metropolitan area in the past 500 yr (Fig. 1a). The largest one is 1
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the M 8 Sanhe-Pinggu earthquake in 1679, which caused severe damage in Beijing: VIII to XI on the Chinese intensity scale. Three city gate towers collapsed, as well as 12,798 buildings. An additional 18,028 buildings were severely damaged, and 485 people were killed or injured (Min et al., 1995). The 1976 Tangshan earthquake (M 7.8) occurred more than 100 km away from Beijing, but still caused significant damage in the metropolitan area, particularly in the eastern part: 462 buildings collapsed, 366 were severely damaged, and 3,100 were moderately damaged (Wang et al., 1999). Many high- intensity anomalies were observed in Xiji- Langfu, Caiyu, and Lisui in the eastern part of the metropolitan area. Twenty-one active Quaternary faults are known in the Beijing metropolitan and surrounding area, including the Huangzhuang-Gaoliying Fault, Shunyi-Liangxiang, Xiadian, and Nankou-Sunhe Faults, all of which are seismogenic and capable of generating earthquakes of magnitude 7 or larger (Fig. 1a).
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Figure 1. Active Quaternary faults and seismicity in Beijing and its vicinity. The area of this study is shown by a box (a), and Quaternary sediment thickness (in meters) of the study area (b). D1 is Kunminghu Depression center, D2 is Machikou Depression center and D3 is Houshayu Depression center.
The thickness of Quaternary sediments (Fig. 1b) was compiled by the Beijing Institute of Hydrogeology and Engineering Geology (1979) from geological and geophysical investigations conducted in the 1970s. As shown in Figure 1b, the Beijing metropolitan area is underlain by Quaternary sediments ranging in thickness from 0 m in the west to 700 m in the northeast. The lithology of the Quaternary sediments is complex, including clay, silt, and silty clays as well as coarser sand s, gravels, and boulders. The shear-wave velocity of the Quaternary sediment ranges from 100 to 500 m/s. The Quaternary sediment is underlain by Tertiary sediment ranging in thickness from 0 to 1,000 m in the study area (Cai et al., 2009). The lithology of the Tertiary units is soft rock including conglomerate, claystone, shale，and mudstone with shear-wave velocity greater than 500 m/s, and that velocity increases with depth (BGI Engineering Consultants Ltd., 2011; Wang and Han, 2012). The depth to the top of bedrock varies between 0 m in the western part of the study area to more than 1,000 m in the central and eastern parts. The bedrock is mainly limestone, shale, conglomerate，and basalt (Beijing Institute of Hydrogeology and Engineering Geology, 1979). 3
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Strong ground motion is known to be significantly modified by near-surface sediments: the so-called site effect or site response. A classic example of site effect is the ground- motion amplification caused by the resonance of the soft lake sediments in Mexico City during the 1985 Michoacan earthquake (M 8.0) (Anderson et al., 1986; Seed et al., 1988; Singh et al., 1988). Although the earthquake occurred more than 300 km away from Mexico City, the amplified motions in the period between 0.5 and 3.0 s caused many buildings to collapse, particularly five- to 16-story buildings on ancient lake sediments underlying a significant portion of Mexico City. Similar site effects were also observed during the 2017 M 8.2 Tehuantepec and M 7.1 Puebla-Morelos earthquakes (Celebi et al., 2018; Sahakian et al., 2018). Site effects have been observed worldwide during strong earthquakes such as the 1989 Loma Prieta earthquake (M 6.9) (Borcherdt and Glassmoyer, 1992; Rubinstein, 2004; Olsen et al., 2008), 1994 Northridge earthquake (M 6.7) (Pitarka, 1996; Hartzell et al., 1996), 2008 Wenchuan, China, earthquake (M 8.0) (Ren et al., 2013), and 2015 Gorkha, Nepal, earthquake (M 7.8) (Asimaki et al., 2017; Barbosa et al., 2017).
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2. SITE EFFECT
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In this paper, we first present a brief review of site effect and previous studies in the Beijing metropolitan area. We describe our method to develop shear-wave velocity profiles of the Quaternary and Tertiary sediments, and provide site response analyses derived using 1-D equivalent linear model at selected sites with well- constrained shear-wave velocity profiles. Finally, we assess both the potential impact on the high-rise buildings by site effect and the adequacy of the “simplified” site classification systems in earthquake engineering.
As shown in Figure 2, site effect (i.e., resonance of a sedimentary basin) is a complex 3-D wave-propagation phenomenon and difficult to quantify because it is influenced by many factors, including earthquake source (i.e., fault rupture), wave propagation through rock (i.e., crust), basin response, and soil nonlinearity. Recent theoretical efforts to quantify site effect have focused on 3-D ground-motion simulations (e.g., Olsen, 2000; Aagaard et al., 2010; Rodgers et al., 2019). Along with significant development of supercomputer, the 3-D ground motion simulation has been advanced greatly. As demonstrated by Rodgers and others (2019), for example, the supercomputer at Lawrence Berkeley National Laboratory could simulate ground motions with frequencies up to 5 Hz and sediment shear-wave velocities down to 500 m/s. Although its applications in earthquake engineering are still limited, the 3-D ground-motion simulation will become a viable approach for quantifying site effect in the co ming years.
Empirically, site effect is quantified by the ratio of ground motion amplitude spectra 4
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𝑆𝑆𝑅 = 𝐴 𝑆 (𝑓) ,
(1)
𝑅
where 𝐴𝑆 (𝑓) and 𝐴𝑅 (𝑓) are the Fourier spectra of ground acceleration for the horizontal component of an S-wave at a given site on sediment and a nearby site on rock, respectively. This method is justified by the standard source-path-site convolution approximation for wave propagation from an earthquake: 𝐴𝑆 (𝑓) can be expressed as the convolution of source S(f), path P(f), site G(f), and instrument response I(f): (2)
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𝐴𝑆 (𝑓) = 𝑆 (𝑓) × 𝑃(𝑓) × 𝐺 (𝑓) × 𝐼 (𝑓).
Similarly, assuming the site response at the rock site is unity, 𝐴𝑅 (𝑓) can be expressed:
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Thus, SSR = G(f).
(3)
Figure 2. Schematic diagram for 3-D wave propagation and site effect (i.e., basin resonance).
Site effects have been explored to some degree in the Beijing metropolitan area. Gao et al. (2002) and Fu et al. (2012) used 3-D finite-difference simulations, and Ding et al. (2004) used the modal summation method. However, none of these studies considered a key component of site response: the near-surface low velocity sediments (< 500 m/s). As shown by Chen et al. (2018), near-surface low velocity sediments are important for study of site effects. Thus, constructing an accurate near-surface shear-wave velocity model is essential for reliably predicting site effect. 5
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3. SHEAR-WAVE VELOCITY PROFILES
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We developed a method to determine shear-wave velocity profiles at sites throughout the study area using downhole and ambient-noise measurements. Investigators have collected downhole shear-wave velocity measurements during the last two decades at more than 800 sites in the Beijing area (Fig. 3). The measurements followed Chinese code GB50021-2001(MHUD, 2009) or ASTM D7400/D7400M (ASTM, 2019) specifications that are specific to seismic-hazard analysis for engineering projects, such as subway stations, railway stations, and high-rise buildings. Soil samples were also taken from boreholes for laboratory tests to determine soil properties such as density and damping parameters. Figure 4 shows examples of shear-wave velocity profiles obtained from the downhole surveys at selected sites. These sites were selected as representative examples of the varying geological conditions across the Beijing metropolitan area.
Figure 3. Distribution of the datasets and selected sites for site-response simulation. Contour lines show the thickness of Quaternary sediment.
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Figure 4. Simplified stratigraphic columns and shear-wave velocities for the six selected sites. These sites are representative of the varying geologic conditions across the Beijing metropolitan area. The penetration depths of the boreholes vary, but none was greater than 100 m because that is the standard engineering drilling limit. Also, according to the building code of China (Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2016), the strata with shear-wave velocity equal to or greater than 500 m/s are defined as rock for engineering purpose. As shown in Figure 1b, the thickness of Quaternary sediments is greater than 100 m in many parts of the Beijing metropolitan area. Thus, the available downhole surveys are insufficient for determining shear-wave velocities of the deeper Quaternary and Tertiary sediments. 7
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𝑉𝑆 4𝐻
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Consequently, microtremor measurements were conducted in summer 2007 for assessing local site effects at more than 600 sites in metropolitan Beijing using three-component Guralp 40T-1 broadband seismometers and Refteck 130B recorders (Chen et al., 2009) (Fig. 3). Chen et al. (2009) performed the horizontal-to- vertical ratio analysis, i.e., the H/V technique of Nakamura (1989), on the microtremor data to obtain the predominant site frequencies. Although the predominant site frequency of ambient noise, which contains the ellipticity of the fundamental Rayleigh wave and other body waves (Bonnefoy-Claudet et al., 2006; Kawase et al., 2011), is fundamentally different from the fundemantal frequency of the resonance of the entire sediment column for the SH-wave, they are coincident (Carpenter et al., 2018; Wang et al., 2019). This coincidence makes it possible to use the predominant frequency of ambient noise, f p , and (4)
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to estimate average shear-wave velocity (VS ) or thickness (H) of the entire sediment column. For example, Bodin et al. (2001) used the predominant site period (reciprocal of the predominant site frequency) to determine the thickness of the deep unlithified sediments of the Mississippi Embayment. We utilized the microtremor data and resulting predominant site frequencies of Chen et al. (2009) and thicknesses of Quaternary and Tertiary sediments (Beijing Institute of Hydrogeology and Engineering Geology, 1979; Ding et al., 2004; Cai et al., 2009) to determine the average shear-wave velocities of entire sediment column using equation (4). There were significant transient noises contained in the microtremor measurements because the Beijing metropolitan area is heavily populated. We reprocessed some microtremor data in order to suppress transient noises and better determine the predominant site frequencies using GEOSPY (http://www.geopsy.org/download.php; last accessed December 2018). Figure 5 shows the horizontal-to- vertical spectral ratios of ambient noise at Site 3, 4, 5, and 6.
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Figure 5. Horizontal-to-vertical spectral ratios of ambient noise at Site 3, 4, 5, and Site 6.
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The procedure for determining shear-wave velocity profile for a shallow site is straightforward. As shown in Figure 3 and 4, the shear-wave velocity profiles at sites 1 and 2 could be directly determined from the downhole surveys: two layers (i.e., surficial materials/artificial fill and gravel) of Quaternary sediments overlying bedrock (limestone) (Fig. 6a, b). The shear-wave velocity of bedrock (limestone) is approximately 1,800 m/s (Fu et al., 2012). For deeper sites, the borehole only penetrated the depth at which the shear-wave velocity reached 500 m/s, and did not reach the bedrock. For example, at site 3 (Figs. 3 and 4), there are three layers—surficial materials/artificial fill, gravel of Quaternary, and conglomerate of Tertiary—with shear-wave velocities of 220, 405, and 590 m/s, respectively, from direct borehole measurements. According to Ding et al. (2004) and Cai et al. (2009), the thickness of the underlying Tertiary sediment is approximately 70 m at this site. The resulting shear-wave velocity profile for site 3 is shown in Figure 6c.
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Figure 6. Shear-wave velocity profiles for sites 1–6, derived from borehole data, sediment thicknesses, and ambient-noise measurements. (a) Site 1. (b) Site 2. (c) Site 3. (d) Site 4. (e) Site 5. (f) Site 6.
The average shear-wave velocity or thickness of the Tertiary conglomerate could also be determined from the predominant site frequency. Figure 7 shows a generalized subsurface model consisting of multiple layers, their associated thicknesses and velocities, and the sediment column’s average velocity and total thickness. For given velocities and thicknesses of the first through the n-1th layers, V S1 , H1 , V S2 , H2 , …, VSn-1 , Hn-1 , and average velocity and thickness of entire sediments, V S, and H, the average shear-wave velocity or thickness for the nth layer can be calculated, 𝑆𝑛
=
𝐻𝑛 𝐻𝑖 𝐻 −∑𝑛−1 1 𝑉𝑆 𝑉 𝑆𝑖
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As shown in Figure 5a, the predominant frequency is approximately 1.28 Hz at site 3. Equation 4 yields an average shear-wave velocity of 501 m/s for the entire sediment column with a total thickness of 97.8 m. Equation 5 yields an average shear-wave velocity of Tertiary sediments of 605 m/s. The measured shear-wave velocity for the upper part of the Tertiary sediments, 590 m/s, and calculated velocity, 605 m/s, closely 10
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correspond. This demonstrates that the ambient noise dominant site frequency can be used to estimate the average shear-wave velocity of deeper sediments.
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Figure 7. Generalized model of multiple sublayers with average velocities and thicknesses.
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As shown in Figure 4, the profile at site 4 can be simplified into five layers—surficial materials/artificial fill, silty clay 1, silty clay 2, medium sand/silty clay, and claystone (Tertiary sediment)—with shear-wave velocities of 220, 300, 405, 510, and 620 m/s, respectively. The thickness of the Tertiary sediment is approximately 90 m (Ding et al., 2004; Cai et al., 2009). The resulting shear-wave velocity profile for site 4 is shown in Figure 6d. According to direct borehole measurements (Fig. 4), there are five layers at site 5—surficial materials/artificial fill, gravel/silty clay, fine sand, silty clay, lower part of Quaternary sediments—with shear-wave velocities of 220, 280, 340, 430, and 520 m/s, respectively. According to Ding et al. (2004) and Cai et al. (2009), the total thickness of Quaternary sediments is approximately 150 m, and the thickness of Tertiary sediments is 100 m at site 5. As shown in Figure 5c, the predominant frequency is 0.5 Hz at site 5. Equation 4 yields an average shear-wave velocity of the entire sediment column of 500 m/s at site 5. Accordingly, equation 5 yields a shear-wave velocity of the Tertiary sediment of 674 m/s. The resulting shear-wave velocity profile for site 5 is shown in Figure 6e. The borehole measurement at site 6 (Fig. 4) revealed that the surficial material is nearly 11 m thick with a shear-wave velocity of 203 m/s. The Quaternary sediment at site 6 can be simplified into six layers: 11 m thick with shear-wave velocity of 203 m/s, 13 m thick with velocity of 280 m/s, 19 m thick with a velocity of 340m/s, 19 m thick with velocity of 410 m/s, 28 m thick with velocity of 470 m/s, and the top 5 m of bottom layer with velocity of 520m/s, respectively. Based on the results of Ding et al. (2004) and Cai et al. (2009), total thickness of Quaternary sediment is approximately 210 m and total thickness of Tertiary sediment is 150 m. Thus, the thickness of the bottom layer of Quaternary sediment is about 120 m. As shown in Figure 5d, the predominant 11
Journal Pre-proof frequency at site 6 is 0.34 Hz, which results in an average shear-wave velocity of 490 m/s for the entire sediment column. Equation 5 yields a shear-wave velocity of the Tertiary sediment of 606 m/s. The resulting shear-wave velocity profile for site 6 is shown in Figure 6f.
4. ANALYSIS OF 1-D SITE-EFFECT
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Ideally, the site effect can be directly determined from the ratio of the recorded groundmotion amplitude spectra of the sediment site and nearby rock site (SSR). Strong-motion recordings are limited, however. As briefly discussed earlier, 3-D ground- motion simulation also has limited applications in earthquake engineering. Currently in engineering practice, the SSR is most commonly determined by 1-D model analyses, such as SHAKE-91 (Schnabel et al., 1972; Idriss and Sun, 1992). The 1-D equivalent linear site-response program RSLEIBM (Li, 1989) was used to determine the SSR in this study. Similar to SHAKE-91, RSLEIBM uses an iterative procedure (i.e., equivalent linear) to account for the nonlinear behavior of the soils. The 1-D model for calculating site response consists of several assumed horizontal layers of nonlinear sediment media overlying an elastic half- space. The motion of a soil layer is caused by shear waves vertically propagated from the elastic half-space upward. In addition to the shear-wave velocities and sediment unit thicknesses, densities, damping, and shear modulus and damping reduction relationships are required for 1-D equivalent linear site-response modeling. Our selected sites used laboratory derived densities, shear modulus and damping reduction relationships from prior investigations (e.g., Shi, 2009; BGI Engineering Consultants Ltd., 2011). Figure 8 shows the shear modulus and damping reduction curves for silty clay, fine sand, and gravel.
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Figure 8. Testing results of dynamic shear modulus and damping ratio versus shear strain for silty clay, fine sand and gravel in the study area. The input motions for 1-D analysis were generated by the finite-stochastic source model for two scenario earthquakes. We chose the large historical 1679 Sanhe-Pinggu earthquake (M 8.0) as one scenario (Fig. 1a). A repeat of such event could have significant impacts on the Beijing metropolitan area. A synthetic ground motion was generated for a rock site at 50 km for the 1679 Sanhe-Pinggu scenario earthquake (Gao et al., 2002). A second synthetic ground motion for a rock site at 120 km was also generated for a scenario event similar to the 1976 Tangshan earthquake (M 7.8). Figure 9 shows the synthetic ground acceleration time histories for the two scenario 13
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earthquakes.
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Figure 9. Synthetic acceleration time histories for the M 8.0 Sanhe-Pinggu scenario earthquake (a) and the M 7.8 Tangshan scenario earthquake (b).
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The shear-wave velocity profiles (Fig. 6), nonlinearity curves (Fig. 8), and the synthetic ground motions (Fig. 9), along with other parameters, were used for 1-D site-response analyses at the six sites across the metropolitan area. Figure 10 shows spectral ratios for the six sites corresponding to the two input ground motions. Figure 11 shows response accelerations with 5 percent damping for the six sites corresponding to the two input ground motions.
Figure 10. Spectral ratios of the selected sites for the input synthetic motions from the M 8 Sanhe-Pinggu scenario earthquake (a) and M 7.8 Tangshan scenario earthquake (b).
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Figure 11. Response accelerations with 5 percent damping of the selected sites for the input synthetic motions from the M 8 Sanhe-Pinggu scenario earthquake (a) and the M 7.8 Tangshan scenario earthquake (b).
5. DISCUSSION
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As illustrated in Figure 2, site effect is a complex 3-D wave-propagation problem. In order to evaluate site effect, establishing 3-D sedimentary basin structure, shear-wave velocity structure in particular, is essential. We are undertaking an effort to construct the 3-D shear-velocity structure of Quaternary and Tertiary sediments for the Beijing metropolitan area, using shear-wave velocity measurements from more than 800 boreholes and ambient-noise measurements from more than 600 sites (Fig. 3). In this paper, we presented the method to derive shear-wave velocity profile utilizing shear-wave velocity and ambient-noise measurements and applied it to obtain the shear-wave velocity profiles at sites across the Beijing metropolitan area. Our results suggest that this method is viable for obtaining shear-wave velocity profiles. The 1-D ground-response analyses at the six sites across the metropolitan area (Figs. 10–11) demonstrated that the site resonant periods increase with the thickness of the entire sediment column, from 0.1 to 0.3 s for the shallow sites (about 30 m deep) in the west to 3.0 to 4.0 s for the deeper site (360 m) in the east, whereas the peak amplifications decrease slightly, from about 4 to 3 from west to east. In other words, the site resonant period is more responsive to the sediment properties (i.e., shear-wave velocity and thickness) than the amplification factor. As demonstrated in Mexico City during the 1985 Michoacan earthquake (M 8.0) (e.g., Anderson et al., 1986; Seed et al., 1988), damage to five- to 16-story buildings was caused by double resonances: corresponding site resonance of the lake sediments and building resonance. Such double resonances have also been observed in Los Angeles and San Francisco, Calif. (e.g., Pitaka, 1996; Rubinstein, 2004) and Maysville, Ky. (Woolery et al., 2008). Thus, site resonant period can in some instances be a more important parameter than amplification factor in site effect analyses.
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Journal Pre-proof 5.1 Potential Impact on High-Rise Buildings The height of buildings in Beijing varies greatly, ranging from one to more than 100 stories. According to the Chinese construction code (China Association for Engineering Construction Standardization, 2012), the fundamental period (T) of a steel-reinforced concrete structure can be estimated by equation (6): 𝑇 = (0.05～0.1)𝑁
(6)
where 𝑁 is the number of stories. According to equation (6), the f undamental periods of buildings in the Beijing metropolitan area are in the range of 0.05 to 10 s.
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As shown in Figures 10 and 11, the site resonance period from 1-D site-response analysis varies from 0.1 s in the western part of the metropolitan area to 4.0 s in the eastern part. According to equation 6, these fundamental periods, 0.1 to 4.0 s, correspond to buildings of one to 40 stories. Table 1 lists the distribution of building heights in the metropolitan area (Hu, 2005), and indicates that the majority (83 percent) of the buildings have fewer than 36 stories. In other words, the majority of the buildings in the metropolitan area are subject to potential damage caused by site resonance of Quaternary and Tertiary sediments.
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Table 1. Distribution of building heights in the Beijing metropolitan area. Building Height < 36 stories 37–50 stories > 50 stories 83.4
< 3.6
11.2
5.4
3.7–5.0
> 5.0
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5.2 Adequacy of Vs30 and Vs20
The average shear-wave velocity of the top 30 m of soils and rock, Vs30 , has been used in the United States and other countries as an index to account for site effect in earthquake engineering; the NEHRP site classification (Building Seismic Safety Council, 2015). Similarly, the average shear-wave velocity of the top 20 m of soils and rock, Vs20 , has been used in China for this purpose (e.g., Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2016). Table 2 lists the average shear-wave velocities of the top 20 and 30 m for the six sites (Fig. 6). Table 2 also lists the Chinese and NEHRP site classes for the six sites. According to the Chinese site classification, sites 1 through 4 in this study can be classified as II, and sites 5 and 6 can be classified as III. Similarly, according to the NEHRP site classification, sites 1 through 3 can be classified as C, and sites 4 through 6 can be classified as D. As shown in Figures 10 and 11, the site resonant period increases from site 1 to 6. There are only two site classes, II and III, according to the Chinese site classification, and only two classes, C and D, according to the NEHRP site classification, however. In other 16
Journal Pre-proof words, these site classes are not directly related to site resonances. Thus, neither site classification adequately accounts for the earthquake site effect. This is particularly obvious for sites 3 and 4, both of which have the same Chinese site class of II, but significantly different dominant site periods, 0.2–0.4 s vs. 1.0–2.0 s. Consequently, the average shear-wave velocity of the top 20 or 30 m is not an appropriate parameter to uniquely account for site effect. Table 2. Site classifications based on the average shear-wave velocities of the top 20 and 30 m. Site 1
Site 2
Site 3
Site 4
Site 5
Site 6
Vs20 (m/s)
386
358
336
258
243
239
Chinese Site Class
Ⅱ
Ⅱ
Ⅱ
Ⅱ
Ⅲ
Ⅲ
Vs30 (m/s)
438
383
369
271
263
257
NEHRP Site Class
C
C
C
D
D
D
Site Resonant Period (s)
0.1–0.3
0.1–0.3
0.2–0.4
1.0–2.0
2.0–3.0
2.5–3.5
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Site Name
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6. CONCLUSIONS
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We developed an integrated method to derive shear-wave velocity profiles in deep sediments utilizing the downhole and ambient-noise measurements. The results demonstrated that the method is viable. We performed 1-D equivalent linear site response analysis at six sites. The results suggest that site resonant periods vary between 0.1 s at shallow sites in the western part of the metropolitan area and 4.0 s at deep sites in the eastern part. This resonant period range coincides with the fundamental periods of buildings of one to 40 stories in Beijing. In other words, there is a potential damage for the high-rise buildings caused by a double resonance in much of the Beijing metropolitan area. Additionally, we found that the average shear-wave velocity of the top 20 or 30 m, or , although a standard parameter to account for site effect in earthquake engineering, is not an appropriate proxy, because neither exhibited an ability to accurately estimate the site effect (i.e., site resonance), the predominant site period in particular. This is a significant shortcoming for urban areas where the double resonance problem is manifest.
ACKNOWLEDGMENTS This research is partially funded by a grant from the Institute of Crustal Dynamics, China Earthquake Administration (No. ZDJ2017-28) and the National Key Research and Development Program of China (2017YFC1500403). The epicenters and magnitudes in Figure 1 are from the China Earthquake Data Center (http://data.earthquake.cn/sjfw; last accessed December 2018). We appreciated Chen et al. for allowing us to use the microtremor data, and Weijun Wang of the Institute of 17
Journal Pre-proof Earthquake Forecasting, China Earthquake Administration, for providing the data. The process of ambient noise data was carried out using GEOSPY (http://www.geopsy.org/download.php; last accessed December 2018). We appreciated the anonymous reviewers for their constructive comments and suggestions that have helped to greatly improve this manuscript. We thanked Meg Smath of the Kentucky Geological Survey for editorial help.
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Conflict of interest statement
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We declared that we have no conflicts of interest to this work.
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Highlights:
An integrated method was developed to determine shear-wave velocity profiles
1-D site response analyses were performed in the Beijing metropolitan area
The Beijing metropolitan area is susceptible to the ground-motion site effect
Site resonant periods coincide with the fundamental periods of many buildings
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is not an appropriate proxy for quantifying site effect in engineering
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