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by Ronald D. Andrus, Cedric D. Fairbanks, Jianfeng Zhang, William M. Camp III, ... and late Pleistocene deposits, the Wando Formation, the Ten Mile Hill beds, ...
Bulletin of the Seismological Society of America, Vol. 96, No. 5, pp. 1897–1914, October 2006, doi: 10.1785/0120050144

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina by Ronald D. Andrus, Cedric D. Fairbanks, Jianfeng Zhang, William M. Camp III, Thomas J. Casey, Timothy J. Cleary, and William B. Wright

Abstract Six major geologic units in Charleston, South Carolina, are characterized in terms of shear-wave velocity (VS) in this article. The characterization is based on in situ VS measurements at 91 sites. The six units are man-made fills, Holocene and late Pleistocene deposits, the Wando Formation, the Ten Mile Hill beds, the Penholoway Formation and the Daniel Island beds, and Tertiary deposits. Median VS values for these units in the top 25 m are 145, 111, 189, 176, 285, and 399 m/ sec, respectively. For Tertiary deposits in the depth intervals of 25–55 m, 55–75 m, and 75–100 m, median VS values are 435, 533, and 663 m/sec, respectively. A seismic-response parametric study is conducted assuming several soil/rock models and two input rock outcrop motions with peak accelerations of 0.3g and 0.1g. It is found that Quaternary sections with VS of 190 m/sec (e.g., the Wando Formation) and thicknesses of about 7 m to 15 m exhibit predominant peaks in the accelerationresponse spectra at periods of about 0.25 to 0.4 sec. These predominant peaks match fundamental periods of many existing buildings in the old city district of Charleston. The results suggest that local site conditions contributed to building damage in the 1886 Charleston earthquake.

Introduction Charleston, South Carolina, is the second most seismically active region in the eastern United States, after the New Madrid seismic zone. The 1886 Charleston earthquake damaged numerous buildings and resulted in about 60 deaths (Bollinger, 1977). Recent estimates of earthquake moment magnitude, Mw, for the 1886 event range from 6.9 Ⳳ 0.3 (Bakun and Hopper, 2004) to 7.3 Ⳳ 0.3 (Frankel et al., 2002). Based on paleoliquefaction studies conducted during the past 20 years, Talwani and Schaeffer (2001) estimated a recurrence rate between 500 and 600 years for magnitude 7Ⳮ earthquakes near Charleston and about 2000 years for magnitude 6.0 events near Georgetown and Bluffton, South Carolina. This evidence led the U.S. Geological Survey in 1996 and 2002 to map significantly higher expected groundshaking levels for Charleston than indicated on previous national maps, with the 2002 levels even higher than the 1996 levels (http://geohazards.cr.usgs.gov/eq/). A repeat of the 1886 earthquake, or even a smaller moderate event, could be devastating to Charleston and the surrounding region (e.g., FEMA, 2000; Silva et al., 2003; Wong et al., 2005). Wong et al. (2005) estimated that a future repeat of the 1886 earthquake could result in 900 deaths, more than 44,000 injuries, and a total economic loss of $20 billion in South Carolina alone. Several studies have identified small-strain, shear-wave

velocity (VS) as a primary controlling factor for site response during earthquake ground shaking (e.g., Seed et al., 1976; Idriss, 1990; Borcherdt, 1994; Boore et al., 1994; Joyner et al., 1994; Midroikawa et al., 1994). Seed et al. (1976) and Idriss (1990) observed distinct differences in average response spectral shapes of sites with different subsurface conditions. The differences in spectral shapes result from vertical variations in soil material properties and strongly depend on VS of the near-surface materials. The determinant effect of VS on ground motion has lead to new site coefficients and classification system used in recent building seismic code provisions (Dobry et al., 2000). Because VS is an important engineering property for earthquake ground-shaking prediction, several efforts to compile VS measurements and other geotechnical information from sites in the greater Charleston area have been initiated in recent years (e.g., Silva et al., 2003; Andrus et al., 2003; Zhang, 2004; Zhang et al., 2004; Fairbanks et al., 2004; Chapman et al., 2006). The conference paper by Zhang et al. (2004) presents composite plots of VS profiles and characterizes average VS in the top 30 m for four major surficial geology groups. The data report by Fairbanks et al. (2004) provides electronic files of VS and Cone Penetration Test (CPT) measurements from the Charleston quadrangle. In this article, characteristic VS properties of six major

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near-surface geologic units are developed using the data compiled by Zhang et al. (2004) and Fairbanks et al. (2004). The characteristic VS properties are defined in terms of median, mean, and standard deviation values. They are then used to illustrate the effects of VS and thickness of Quaternary sediments on seismic ground response. Predominant peaks in the acceleration-response spectra that match the typical range of building periods in the old city district of Charleston are identified based on the results of a seismic ground response parametric study.

Data Zhang et al. (2004) and Fairbanks et al. (2004) compiled VS data from 104 test sites in the greater Charleston area. Figure 1 shows the locations of 60 test sites in the Charleston quadrangle plotted on the geologic map by Weems et al. (1997). Many of these sites are located in the old city district of Charleston on the peninsula between the Ashley and Cooper Rivers, near the center of the geologic map. The locations of the other 44 test sites lie outside the quadrangle (see Zhang et al., 2004). Summary information for VS measurements made at the locations shown in Figure 1, including specific project references, are given in Fairbanks et al. (2004). The VS measurements were conducted by various investigators (i.e., Applied Research Associates, Inc.; ConeTec, Inc.; Georgia Institute of Technology; Gregg In Situ, Inc.; RedPath Geophysics; S&ME, Inc.; WPC, Inc.; and U.S. Geological Survey) between 1998 and 2004. Most tests were made by the seismic cone penetration test (SCPT) using the downhole method with typically 1-m-depth measurement intervals. Some tests (11%, 11 of 104) were made by the seismic downhole, the spectral-analysis-of-surface-waves (SASW), the suspension logger, and the seismic refraction/ reflection (SRR) methods. Comparisons of VS values between performing organizations indicated good agreement (Andrus et al., 2003; Zhang et al., 2004). To further assess the quality of the compiled VS values, several SCPT time histories were reviewed. Most of the time histories were of fair to excellent quality. The VS values for these sites were entered directly into a database and assigned to depths corresponding to the centers of the reported measurement intervals. No attempt was made to delete very low and very high VS values, so that any errors in picking shearwave arrival times would be averaged out. The few sites with poor-quality time histories were excluded from the database. Maximum VS measurement depths ranged from less than 10 m to 107 m. Ground-surface elevations at the test sites ranged from 0 m to 12 m above mean sea level. About two-thirds of the sites have ground surface elevations less than 5 m above mean sea level. Sufficient subsurface information was available to infer major geologic units beneath 91 of the 104 test sites. Key information considered in the identification of subsurface geology includes: several 1:24,000 scale geologic maps and

auger hole logs (e.g., Weems and Lemon, 1985, 1993); the 1:250,000 scale geologic map by McCartan et al. (1984); CPT tip, sleeve, and pore pressure measurements; and geologic interpretations provided in the project reports. Representative CPT, VS, and geologic profiles from a selected site in Charleston are presented in Figure 2. CPT tip resistances are corrected to account for the effect of water pressure acting behind the cone tip. The friction ratio (FR) is defined as the sleeve resistance measurement divided by the corrected cone tip resistance (qT). Values of FR are usually much greater (over 1%) in clayey soils than sandy soils. Hydrostatic pore pressures (u0) are assumed equal to the depth below the groundwater table multiplied by the unit weight of water. Pore-water pressure measurements made with the transducer located immediately behind the cone tip are denoted as u2. Values of u2 close to u0 indicate freely draining soil (e.g., sand). Higher u2 values, compared with u0, indicate lower permeable soil (e.g., clay). Thus, the material in Figure 2 at depths of 3–14 m, 18–23 m, and 25– 38 m are clayey soils. Lower VS values indicate softer or looser soil.

VS and Geology The VS data from the top 25 m are grouped into six major geologic units and plotted versus depth in Figure 3. The six geologic units are: (1) man-made fills, (2) Holocene and late Pleistocene deposits, (3) the Wando Formation, (4) the Ten Mile Hill beds, (5) the Penholoway Formation and the Daniel Island beds, and (6) Tertiary deposits. Only VS data measured completely within a unit are plotted in Figure 3, to avoid incorrect VS assignments. Measurements made on unit boundaries or where unit designation is uncertain are not plotted. For example, the VS measurement shown in Figure 2d corresponding to the depth interval of 13.8–14.8 m is not plotted in Figure 3b or c. At least two data points within a unit at a test site are required to be included in the plotting of SCPT and downhole data. For the few SASW and SRR data, average VS values are assigned to the interval center depths. A brief description of the six major units is given subsequently. Man-made fills include artificial fill and phosphate spoil. The artificial fill is less than about 300 years old and consists of sands and clayey sands of diverse origin, ranging from nonengineered material to engineered construction fill. Phosphate spoil is less than about 130 years in age, and is material removed and backfilled during phosphate mining primarily in northwest Charleston. The artificial fill and the phosphate spoil are grouped together because both include nonengineered materials, and available VS measurements are limited. As shown in Figure 3a, reported values of VS from man-made fills range from less than 80 m/sec to over 300 m/ sec, with median value of 145 m/sec. Several different types of Holocene (⬍10,000 years or ⬍10 ka) and late Pleistocene (10 ka to 85 ka) deposits are present in the area (see Weems and Lemon, 1993). Grouping

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

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Figure 1. Geologic map of the Charleston quadrangle (modified from Weems et al., 1997) showing locations of VS test sites.

the Holocene and the late Pleistocene data together seemed the most practical approach, given the subtle differences in their CPT and VS profiles. These deposits include beach to barrier-island quartz sands and tidal-marsh clayey sands and clays. As shown in Figure 3b, reported values of VS from the Holocene and late Pleistocene deposits range from less than 80 m/sec to over 200 m/sec, with median value of 111 m/sec. The Wando Formation is 70 ka to 130 ka in age. Weems and Lemon (1993) identified several facies in this formation,

including two fluvial to estuarine facies, two barrier sands, and two fossiliferous shelf sands. One characteristic that can sometimes be used to infer younger clay deposits from older clay deposits below the groundwater table is the trend of qT measurements. As shown in Figure 2a, qT values in the Holocene to late Pleistocene clay deposit project to a value of 0.3 MPa at the ground surface, in contrast qT values in the clay facie of the Wando Formation project to a value of 1.9 MPa at the ground surface. Also, VS is generally higher in the older Wando Formation (see Fig. 2d). Reported values

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R. D. Andrus, C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Casey, T. J. Cleary, and W. B. Wright

Figure 2.

Representative CPT, VS, and geologic profiles from a selected site in Charleston.

of VS from the Wando are plotted in Figure 3c. They range from 85 m/sec to over 300 m/sec, and have a median value of 189 m/sec. The Ten Mile Hill beds are approximately 200 ka to 240 ka in age. Like the Wando Formation, Weems and Lemon (1993) divided the Ten Mile Hill beds into two fluvial to estuarine facies, two barrier-island sands, and two fossiliferous shelf sands. Plotted in Figure 3d are compiled values of VS. These VS values range from 90 m/sec to over 300 m/sec, with median value of 176 m/sec. The Penholoway Formation and the Daniel Island beds are sands and clayey sands, respectively, with age between 730 ka and 1600 ka. They are often identified in CPT profiles by relatively high qT values, compared with values in overlying and underlying units. As shown in Figure 3e, reported values of VS from the Penholoway Formation and the Daniel Island beds range from about 180 m/sec to over 600 m/sec, with median value of 285 m/sec. Tertiary deposits mainly consist of marine sediments

ranging in age from about 2 Ma to 38 Ma (Weems and Lemon, 1993). They include, from youngest to oldest, the Goose Creek Limestone, the Marks Head Formation, the Edisto Formation, the Chandler Bridge Formation, the Ashley Formation, and the Parkers Ferry Formation. The Ashley and Parkers Ferry formations, along with the older Harleyville Formation are three stiff, impermeable members that form the Cooper Group (locally known as the “Cooper Marl”). The Cooper Marl exists throughout the subsurface in the Charleston area and can be up to 100 m thick. In CPT profiles (see Fig. 2), it is characterized by (1) fairly uniform (constant with depth) qT profiles that project to about 3 MPa at the ground surface, (2) occasionally high (⬎10 MPa) qT values, (3) fairly uniform FR profiles, and (4) consistently high (⬎1 MPa) u2 values. Reported values of VS from Tertiary deposits in the top 25 m are plotted in Figure 3f. Most of these measurements are believed to be from the Ashley Formation. They range from less than 180 m/sec to over 700 m/sec, with median value of 399 m/sec.

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Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

Figure 3. Variation of VS measurements from the top 25 m separated by geology. (a) Man-made fills. (b) Holocene and late Pleistocene deposits. (c) Wando Formation. (d) Ten Mile Hill beds. (e) Penholoway Formation and Daniel Island beds. (f) Tertiary deposits.

All VS values from Tertiary deposits are presented in Figure 4. Values from depths greater than 50 m are based on two seismic downhole tests and two suspension logger tests conducted in four relatively deep boreholes. One deep borehole was part of the Maybank Highway Bridge replacement project, which connects Charleston peninsula to Johns Island across the Stono River. The other three deep boreholes were for the new Cooper River Bridge project along U.S. Highway 17. The plotted VS values are divided into five depth ranges: 0–25 m, 25–55 m, 55–75 m, 75–100 m, and 100–110 m. Median VS values for these depth ranges are noted in Figure 4. It is likely that VS values below the depth of 55 m are from the Parkers Ferry and Harleyville formations. Although individual VS profiles within a geologic unit often exhibit increasing velocity with depth, the aggregated data plotted in Figure 3a–f exhibit little depth dependence as a whole. Therefore, the data are considered directly, without any correction for depth or overburden pressure, in the statistical analysis.

Statistical Analysis Histograms of the VS data grouped by geology are presented in Figures 5 and 6. The histograms suggest that either

normal or log-normal distributions can be used to represent the data. To determine the type of distribution most suitable, the chi-square test (Ang and Tang, 1975) is applied to the data sets. In the chi-square test, the similarity between the considered data and the assumed distribution is evaluated by the total chi-square value (v2), which is defined as: k

v2 ⳱

兺 i⳱1

(ni ⳮ ei)2 , ei

(1)

where k is the number of data intervals, ni is the observed outcomes for the ith bin, and ei is the theoretically expected outcomes for the ith bin based on the assumed distribution. In general, it is necessary to have k ⱖ 5 and ei ⱖ 5. Higher v2 values imply a significant difference between the data and the assumed distribution. Thus, the distribution with the smallest v2 value is the most suitable distribution to represent the data set. Values of v2 for the six units assuming both normal and log-normal distributions are presented in Table 1. Also presented in Table 1 are mean, standard deviation, and median values. The mean based on a normal distribution, known as the arithmetic mean, is calculated by simply averaging the VS values. The mean based on a log-normal distribution,

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Figure 4. Compiled VS measurements from Tertiary deposits.

known as the geometric mean, is calculated by averaging Ln(VS) values. Based on the v2 values, all VS data sets are equally or better represented by the log-normal distribution, except the fill data set, which is better represented by the normal distribution. Because 9 of the 10 data sets are equally or better represented by the log-normal distribution, it is preferred in this study. The probability density function of the log-normal distribution is given by: f(x) ⳱

1 xr冪2p

冤 冢

exp ⳮ

冣冥

1 Ln(x) ⳮ l 2 r

2

for 0 ⬍ x ⬍ ⬁ ,

(2)

where x is the considered variable, and l and r are the two parameters defining the distribution. Here the variable x is VS, and the parameters l and r are mean and standard deviation values of Ln(VS), respectively. The probability density functions of VS for the six units within the top 25 m are generated according to equation (2) and plotted in Figure 5a–f to compare with the histograms. Clearly, the plotted probability density functions match the histograms well, with the possible exception of the fill data set. Geometric mean values of VS in the top 25 m are 141,

108, 190, 178, 309, and 393 m/sec for the man-made fills, the Holocene and late Pleistocene deposits, the Wando Formation, the Ten Mile Hill beds, the Penholoway Formation and the Daniel Island beds, and the Tertiary deposits, respectively. For the Tertiary deposits within depth intervals of 25–55 m, 55–75 m, 75–100 m, and 100–110 m, geometric mean values of VS are 436, 553, 670, and 822 m/sec, respectively. These mean values of VS are similar to median values noted in Figure 4 and Table 1, indicating that the geometric means are not significantly affected by the few extreme values. Note that both mean and median VS values increase with age in the natural sediment deposits, with the exception of the Ten Mile Hill beds. The Ten Mile Hill beds were deposited in an environment similar to the Wando Formation, but 100,000 years earlier. One possible explanation for the lower VS values in the Ten Mile Hill beds is that the corresponding test sites are located closer to the 1886 epicenter and fault rupture than the test sites corresponding to the Wando Formation measurements. Greater liquefaction effects were observed in the Ten Mile Hill beds following the 1886 earthquake (Bollinger, 1977). It is possible that the sediments closer to the epicenter and fault rupture were so disturbed from liquefaction that their aging clocks were reset, thus lowering the VS values in the older Ten Mile Hill beds. Although granular soils may also densify during strong shaking, there is evidence that liquefied soils can settle into a state that is just as loose as the state prior to shaking. For example, there are several well-documented cases histories in California where liquefaction occurred during more than one event (Youd, 1984). The statistical results presented earlier can be used to generate approximate VS profiles for sites where only the geologic profiles are known. When combined with geologic maps and cross sections of the Charleston area (e.g., Weems and Lemon, 1993), the results provide the required information to accurately assess ground-shaking hazard in the area.

Site Response Parametric Study To illustrate the effects VS and thickness of the Quaternary section have on seismic ground response, the dynamic response of several generalized soil/rock models typical of some locations in the Charleston quadrangle are analyzed using hypothetical hard rock basement outcrop motions. Soil/Rock Models Selected generalized soil/rock models considered are illustrated in Figure 7a–7d. The models illustrated in the figures consist of 0 m, 10 m, 20 m, and 30 m of Quaternary sediment, respectively, with mean VS values of 110 m/sec or 190 m/sec. Additional soil/rock models considered in this parametric study, but not illustrated in Figure 7, consist of 7 m, 13 m, 15 m, and 17 m of Quaternary sediment. The

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

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Figure 5. Statistical distribution of VS measurements from the top 25 m separated by geology. (a) Man-made fills. (b) Holocene and late Pleistocene deposits. (c) Wando Formation. (d) Ten Mile Hill beds. (e) Penholoway Formation and Daniel Island beds. (f) Tertiary deposits.

Figure 6. Statistical distribution of VS measurements from depths of 25–55 m (a), 55–75 m (b), and 75–100 m (c). models with mean VS of 110 m/sec for the Quaternary section represent the range in thicknesses of Holocene and late Pleistocene deposits. The models with mean VS of 190 m/ sec for the Quaternary section represent the range in thicknesses of the Wando Formation deposits. Specific engineering properties assumed for the soil/ rock model with mean VS of 190 m/sec in the top 10 m are given in Table 2. A total of 133 soft-soil to soft-rock layers are assumed in all models analyzed.

Note that no VS measurements are currently available below a depth of 110 m in the Charleston area. The values of VS below a depth of 100 m given in Table 2 are based on previous approximate models. Wheeler and Cramer (2000) suggested a linearly increasing profile from a depth of 110 m to a depth of 808 m, where VS is 1300 m/sec at 808 m. Silva et al. (2003) assumed VS increases from about 762 m/sec at 152 m to about 914 m/sec at 213 m, and remains constant until a depth of 1219 m. Chapman et al. (2006) assumed a

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R. D. Andrus, C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Casey, T. J. Cleary, and W. B. Wright

Table 1 Statistical Properties of VS Measurements Grouped by Geology Normal Distribution

Geologic Unit

Log-Normal Distribution

No. of VS Values

Degrees of Freedom

Total v2

Mean VS (m/sec)

Standard Deviation of VS (m/sec)

91 238 538 73 88

4 4 4 4 4

13 11 36 8.3 20

152 116 195 184 328

54 45 47 54 126

4 4 4 4 4

33 12 3.0 1.2 4.5

383 443 61 52 4

4 4 4 4 3

280 32 20 4.1 2.3

417 440 564 679 822

242 83 119 118 130

4 4 4 4 3

23 25 11 1.8 2.3

Man-made fills Holocene and late Pleistocene deposits Wando Formation Ten Mile Hill beds Penholoway Formation and Daniel Island beds Tertiary deposits 0–25 m ⬎25–55 m ⬎55–75 m ⬎75–100 m ⬎100–110 m

similar profile, but a smaller constant VS value was used for the depths between 510 m and 830 m. For this study, the deep VS profile is assumed to increase linearly from 800 m/ sec at 100 m to 920 m/sec at 808 m. This profile is placed on top of pre-Cretaceous basement rock, which is represented by a uniform half-space with VS of 3.5 km/sec, as suggested by Chapman et al. (2006). The groundwater table in Charleston is shallow. For the general models, it is assumed to be 1.5 m below the ground surface for mean effective stress (r⬘m) calculations. Also assumed for r⬘m calculations, are coefficients of at-rest earth pressures (K⬘0 ⳱ horizontal effective stress divided by vertical effective stress) of 0.5 in Quaternary sediments and 1.0 in Tertiary and older sediments. Normalized Shear Modulus and Material-Damping Relationships Small-strain, shear-wave velocity is directly related to small-strain, shear modulus (Gmax) by: Gmax ⳱ qVS2 ,

(3)

where q is the mass density of soil (or total unit weight of the soil divided by the acceleration of gravity). At moderate to high strains, the secant shear modulus (G) is used to represent average soil stiffness. It is common practice to normalize G by dividing by Gmax. A plot of the variation of G/Gmax with shear strain (c) is called a normalized shearmodulus reduction curve. Normalized shear-modulus and material-damping ratio curves used to describe nonlinear behavior of the soil/rock model summarized in Table 2 are shown in Figures 8 and 9. They are based on the predictive relationships developed by Zhang et al. (2005) using resonant column and torsional shear-test results for 8 Quaternary and 66 Tertiary and older undisturbed soil specimens from primarily the South Caro-

Standard Deviation of Ln (VS)

Median VS (m/sec)

141 108 190 178 309

0.402 0.379 0.230 0.267 0.334

145 111 189 176 285

393 433 553 670 814

0.312 0.191 0.197 0.169 0.166

399 435 533 663 841

Degrees of Freedom

Total v2

Mean VS (m/sec)

lina Coastal Plain. Variables used to define the relationships for G/Gmax are shear-strain amplitude, confining stress, and plasticity index (PI). The material damping ratio (D) relationships are defined in terms of a polynomial function of G/Gmax plus a minimum damping ratio. The minimum damping ratio depends on confining stress and PI. Values of PI used in this study are based on the general soil type assumed in Table 2. Based on evaluations of laboratory data and analytical studies, Stokoe et al. (1995) suggested that the estimated field r⬘m should be within about Ⳳ50% of the actual values when selecting G/Gmax and D curves for design. Therefore, the approach used in this study is to divide the soil/rock models into several major units. Average values of r⬘m for each major unit are calculated and compared with r⬘m values calculated for each layer within the unit. If the r⬘m value for each layer is within Ⳳ50% of the average value for the major unit, then the average r⬘m is assigned to all layers within the unit. Otherwise, the unit is subdivided and new average r⬘m values are calculated. According to this approach, the generalized soil/rock model can be divided into seven major units. The corresponding average r⬘m values for the seven major units are listed in Table 2. As observed by Zhang et al. (2005), the curves for Quaternary soils generally exhibit more linearity (i.e., G/Gmax values are closer to 1.0 at higher shear-strain levels) than older soils at the same confining pressure. This trend is not observed in Figure 8 because r⬘m is much higher in the Tertiary sediments than in the Quaternary sediments. Specimens from both age groups exhibit significant variations in the G/Gmax and D curves with confining stress, and moderate variations with PI. Input Rock Outcrop Motions Because actual strong ground motion records are currently not available for Charleston, synthetic rock outcrop

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

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Figure 7. Generalized VS profiles of the top 50 m used in site-response parametric study, assuming thickness of Quaternary section is 0 m (a), 10 m (b), 20 m (c), and 30 m (d).

motions were generated using the point-source stochastic model by Boore (2000). The synthetic motions were based on the same scenario earthquake parameters assumed by Chapman et al. (2006). These parameters are Mw 6.4, 6.7, 7.1, and 7.5; epicentral distance, 30 km; focal depth, 10 km; crustal velocity, 3.5 km/sec, crustal density, 2.6 gm/cm3, stress parameter, 100 bars; crustal quality factor, Q ⳱ 680 f 0.36, where f ⳱ frequency in Hz; free-surface factor, 2.0; radiation pattern, 0.55; and component partition factor, 0.707. These parameters are consistent with a source in the area of maximum shaking in 1886, located near Summerville, South Carolina, and a site located in downtown Charleston. The synthetic ground motions for magnitudes of 6.4 and 7.1 are the primarily focus of this study. A magnitude of 7.1 is the middle range value of estimates for the 1886 event. Based on 20 realizations for each magnitude, the mean peak ground accelerations (PGAs) associated with these synthetic motions are 0.17g and 0.30g, respectively (Chapman et al., 2006). The later PGA agrees well with the 0.3–0.4g predicted to have occurred in Charleston during the 1886 earthquake (Silva et al., 2003). The synthetic motions are scaled to provide two input pre-Cretaceous rock outcrop motions with PGAs of 0.10g and 0.30g. A PGA of 0.1g is selected to evaluate the effects of lower-intensity input motion on site re-

sponse. Both ground-motion times series are presented in Figure 10. Analysis Method The site-response analysis is conducted using computer program DEEPSOIL (Hashash and Park, 2002; Park and Hashash, 2004; Hashash, 2005). DEEPSOIL assumes vertically propagating seismic waves and is used because it allows for soil/rock models with over 100 layers. Also, DEEPSOIL can perform response analysis with both equivalent linear-frequency domain and nonlinear time-domain formulations. The equivalent linear formulation is used in this study because at shear strains greater than about 0.1% the nonlinear formulation of DEEPSOIL predicts significantly higher damping values than were observed by Zhang et al. (2005) for South Carolina Coastal Plain sediments (see Fig. 9). The equivalent linear formulation is considered adequate because the ground surface in Charleston is fairly flat, and the computed ground accelerations and shear strains computed in most of the models are ⬍0.4g and ⬍2%, respectively, the approximate limits suggested by Kramer and Paulsen (2004). Computed maximum accelerations for each layer do not exceed 0.31g in any of the models. Computed maximum shear strains for each layer are all less than 1.8%.

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R. D. Andrus, C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Casey, T. J. Cleary, and W. B. Wright

Table 2 Generalized Soil/Rock Model for a Selected Site in Charleston, South Carolina Normalized Shear Modulus, G/Gmax, and Damping, D, Curves Used

Layer Thickness (m)

S-Wave Velocity, VS (m/sec)

Total Unit Weight (kN/m3)

1 2 3

1.0 1.0 1.0

190 190 190

18.2 18.2 18.2

15

SP-SC

Quaternary, plasticity index (PI) ⳱ 15%

4–5* 6–7 8–10

1.0 1.0 1.0

190 190 190

18.2 18.2 18.2

50

SP-SC

Quaternary, PI ⳱ 15%

11–20 21–25 26–30 31–32

1.0 1.0 1.0 3.0

400 400 435 435

18.5 18.5 18.5 18.5

220

CH

Tertiary and older, PI ⳱ 50%

33–37 38–41 42–45

4.0 5.0 6.0

435 530 660

18.5 18.9 18.9

600

CL

Tertiary and older, PI ⳱ 15%

46–49 50–53 54–57

8.0 8.0 8.0

810 815 820

19.6 19.6 19.6

1400

limestone

Tertiary and older, PI ⳱ 15%

58–65 66–75 76–85 86–95

8.0 8.0 8.0 8.0

830 840 850 860

22.5 22.5 22.5 22.5

3800

sand

Tertiary and older, PI ⳱ 0%

96–102 103–110 111–117 118–125 126–133

8.0 8.0 8.0 8.0 8.5

870 880 890 900 910

22.5 22.5 22.5 22.5 22.5

7500

sand

Tertiary and older, PI ⳱ 0%

Half-space

3500

22.5



rock



Layer Number(s)

134

Mean Effective Stress, r⬘m (kPa)

USCS Soil Type†

*Layers 4 and 5 are each 1 m thick with VS ⳱ 190 m/sec. † SP ⳱ poorly graded sand, SC ⳱ clayey sand, CH ⳱ fat clay, CL ⳱ lean clay

Figure 8.

Selected G/Gmax ⳮ log c curves used in site-response study.

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

Figure 9.

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Selected D - log c curves used in site-response study.

Results

Figure 10.

Synthetic input rock outcrop motions for Mw 6.4 and PGA ⳱ 0.1 g (a) and Mw 7.1 and PGA ⳱ 0.3g (b).

Plotted in Figure 11 are calculated peak ground-surface accelerations for the selected soil/rock models shaken by the two input motions and assuming the equivalent linear formulation of DEEPSOIL. PGAs for the model sites shaken by the Mw 7.1 motion are 0.8 to 1.0 times the peak acceleration of the input rock outcrop motion (0.3g), indicating some deamplification of ground motions for most profiles. Note, however, that when larger Mw (say 7.3, or longer duration) motions are used the amount of deamplification is less and amplification can be significant. For the model profiles shaken by the Mw 6.4 motion, PGAs are 1.0 to 1.5 times the peak acceleration of the input rock outcrop motion (0.1g), indicating amplification at all model sites. Again, note that amplification will be more when larger Mw motions are used. In general, these results are consistent with the observations of Idriss (1990), who concluded that PGAs at soft soil sites are likely to be greater than on rock sites at low to moderate acceleration levels (less than about 0.4g). Also note that the calculated PGAs are generally greater for sites having stiffer Quaternary sections (i.e., mean Vs ⳱ 190 m/sec), than softer Quaternary sections (i.e., mean Vs ⳱ 110 m/sec). This finding agrees with the site-response study conducted by Chapman et al. (2006); Chapman et al. observed that ratios of computed PGA to peak rock outcrop acceleration tend to be greater (for a given input motion level) on sites with higher average Vs in the upper 30 m, and these sites tend to be sites where the depth to the Cooper Marl is small. Shown in Figures 12 and 13 are median Fourier spectrum ratios of the computed surface motions to the rock outcrop motions for Mw 7.1 and 6.4, respectively. The ampli-

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R. D. Andrus, C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Casey, T. J. Cleary, and W. B. Wright

Figure 11.

Comparison of rock outcrop PGAs with predicted ground-surface PGAs.

fication occurs in the band of frequencies from about 0.1 Hz to as high as 5 Hz for the spectrum ratios based on the Mw 7.1 motion (see Fig. 12). For the spectrum ratios based on the Mw 6.4 motion, amplification occurs in the band of frequencies from about 0.1 Hz to over 10 Hz (see Fig. 13). All spectrum ratio plots exhibit the first harmonic near 0.25 Hz. The second harmonic occurs between 0.6 Hz and 0.8 Hz, with greatest variability exhibited by the softer Quaternary sediment profiles (see Figs. 12a and 13a). The results are in good agreement with the previous studies by Silva et al. (2003) and Chapman et al. (2006) for the Charleston area. Acceleration-response spectra for a single-degree-offreedom structure at the ground surface determined with the Mw 7.1 rock motion are shown in Figure 14. As seen in Figure 14a, only the response spectra for the model site with the softer Quaternary section having thickness of 10 m exhibits peak spectral acceleration, Sa, values above 0.81g. The period range of this peak is 0.7 to 0.9 sec, and corresponds to the profile with 10 m of Quaternary sediment. In Figure 14b, the response spectra for the model sites with the stiffer Quaternary sections having thicknesses of 10 m and 20 m

exhibit Sa values above 0.81g. The ranges of periods corresponding to the major peaks are 0.24–0.42 sec and 0.4 sec/ 0.65–0.91 sec, respectively. These results illustrate the variations in predicted spectral accelerations that can occur, depending on the stiffness and thickness of the Quaternary section. Figure 15 shows acceleration response spectra determined using the Mw 6.4 rock motion. It is interesting to observe a major peak in the response spectra again between 0.7 sec and 0.9 sec for the softer Quaternary section with thickness of 10 m (see Fig. 15a). Of the sites with the stiffer Quaternary section, the highest Sa peak occurs at a period around 0.25 sec and corresponds to the 10-m-thick section (see Fig. 15b). Less pronounced peaks of Sa occur at about 0.4 sec and 0.7–0.9 sec for the 20-m-thick and 30-m-thick sections (see Fig. 15b). Preliminary comparisons between results presented previously based on the equivalent linear formulation and the nonlinear formulation of DEEPSOIL indicate good general agreement. The results are also in general agreement with previous analyses by Andrus et al. (2005) using similar input

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

1909

Figure 12.

Variations of median Fourier amplitude ratio for sites with Quaternary sediment having VS of 110 m/sec (a) and 190 m/sec (b) shaken by synthetic motion for Mw 7.1 and PGA ⳱ 0.3g.

variables and computer program SHAKE (Schnabel et al., 1972; Idriss and Sun, 1992), which is based on an equivalent linear formulation. The results however, are somewhat different from those obtained by Elton and Martin (1989). Elton and Martin (1989) estimated dynamic site periods of about 0.5 sec to 1.0 sec for areas in Charleston with stiffer Quaternary sections, and 1.0 sec to over 2.0 sec for areas with softer Quaternary sections. The differences between estimated site periods may be explained by the improved VS measurements and nonlinear soil properties used in this

study, but not available at the time of Elton and Martin’s (1989) study.

Significance of Results Greater building damage is expected to occur where the fundamental period of the structure coincides with the period of the predominant peak in the acceleration-response spectrum. According to the International Code Council (ICC

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R. D. Andrus, C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Casey, T. J. Cleary, and W. B. Wright

Figure 13.

Variations of median Fourier amplitude ratio for sites with Quaternary sediment having VS of 110 m/sec (a) and 190 m/sec (b) shaken by synthetic motion for Mw 6.4 and PGA ⳱ 0.1g.

2000), the fundamental period of a building (Tbldg), in seconds, can be estimated from: Tbldg ⳱ CT hn3/4 ,

(4)

where CT is the building coefficient, and hn is the height above the base to the highest level of the building in meters. Many of the existing buildings in the old city district of Charleston are the same ones heavily damaged during the 1886 earthquake, only repaired. They are of brick and/or wood construction. For these buildings, the ICC (2000) rec-

ommended value for CT is 0.049. Current zoning regulations for the city restrict the height of buildings (Zoning Ordinances Section 54.306) to preserve the historic skyline of the city. For example, along King Street between Broad Street and Calhoun Street all new buildings are required to be no shorter than 9.1 m and no taller than 16.8 m (M. J. Cain, personal comm., 2005). Assuming this range of heights for hn, the estimated range of Tbldg for many of the historic and recent buildings in the old city district is 0.26 sec to 0.41 sec. Based on Tbldg range of 0.26 sec to 0.41 sec and results

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

1911

Figure 14.

Variations in spectral acceleration for sites with Quaternary sediment having VS of 110 m/sec (a) and 190 m/sec (b) shaken by synthetic motion for Mw 7.1 and PGA ⳱ 0.3g.

of the site-response parametric study presented in the previous section, the matching of Tbldg and period of predominant peak in the response spectrum is predicted to occur where the Quaternary section has mean VS at about 190 m/ sec and thickness between about 7 m and 15 m (see Fig. 12b). These Quaternary sections are typical of the slightly

higher ground in Charleston where surficial sediments are of the Wando Formation. The prediction of greater building damage occurring at sites where surficial sediments are the Wando Formation is supported by observations following the 1886 earthquake. Some of the greatest structural damage observed occurred in the three- and four-story brick masonry

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R. D. Andrus, C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Casey, T. J. Cleary, and W. B. Wright

Figure 15.

Variations in spectral acceleration for sites with Quaternary sediment having VS of 110 m/sec (a) and 190 m/sec (b) shaken by synthetic motion for Mw 6.4 and PGA ⳱ 0.1g.

Conclusions buildings constructed on the Wando Formation in the commercial district of Charleston (e.g., Dutton, 1889; Robinson and Talwani, 1983; Lindbergh, 1986; Peters and Herrman, 1986). Thus, the results suggest that local ground conditions contributed to building damage in the 1886 earthquake.

Using in situ VS measurements from 91 test sites in the greater Charleston area, the stiffness of six major geologic units are characterized. The six units are (1) man-made fills, (2) Holocene and late Pleistocene deposits, (3) the Wando Formation, (4) the Ten Mile Hill beds, (5) the Penholoway

Shear-Wave Velocity and Seismic Response of Near-Surface Sediments in Charleston, South Carolina

Formation and the Daniel Island beds, and (6) Tertiary deposits. Assuming log-normal distributions and no depth dependencies, calculated mean VS values in the top 25 m for the six units are 141, 108, 190, 178, 309, and 393 m/sec, respectively. For the Tertiary deposits in the depth intervals of 25–55 m, 55–75 m and 75–100 m, calculated mean VS values are 436 m/sec, 553 m/sec, and 670 m/sec, respectively. These mean VS values are nearly equal to the median values. They indicate that VS generally increases with age in the natural sediments, with the exception of the Ten Mile Hill beds. One possible explanation for the lower VS values is in the Ten Mile Hill beds is that test sites used to characterize this unit are located closer to the 1886 fault rupture where greater shaking and soil disturbance occurred. The intense ground shaking and liquefaction that occurred likely reset the aging clock of sediments of the Ten Mile Hill beds. A seismic-response parametric study is conducted assuming several generalized soil/rock profiles and two input ground motions. Predominant peaks in the accelerationresponse spectra occur at periods of 0.24–0.42 sec and 0.4 sec/0.65–0.91 sec for the profiles with Quaternary sections having mean VS of 190 m/sec and thicknesses of 10 m and 20 m, respectively, and using a hard rock input motion with peak acceleration of 0.3g. These periods are lower than periods of predominant peaks predicted by Elton and Martin (1989). The improved in situ VS measurements and nonlinear soil properties used in this study may explain the difference in predicted spectral periods. Many of the historic and new buildings in the old city district of Charleston have fundamental periods between about 0.26 sec and 0.41 sec. At locations where the period of the predominant peak in the response spectrum match these building periods, greater damage likely occurred in 1886 and is expected to occur in future earthquakes. These conditions commonly exist in areas where surficial sediments are of the Wando Formation and buildings are three and four stories high. When combined with available 1:24,000 geologic cross sections, the results can be used to develop generalized VS cross sections of the Charleston area. Additional work is needed to better characterize VS of the sediment facies within the six major geologic units.

Acknowledgments This research was supported by the U.S. Geological Survey (USGS), Department of the Interior, under Grants 03HQGR0046 and 05HQGR0037. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. John Unger served as the USGS Grants Program Manager. The support of the USGS and the assistance of J. Unger are greatly appreciated. We thank the many individuals who assisted with data collection, in particular, Ethan Cargill formerly with S&ME, Glenn Rix and Paul Mayne of the Georgia Institute of Technology, Timothy Adams of the South Carolina Department of Transportation, and Daniel Balon and Brian Ellis former graduate students at Clemson University. Martin Chapman of Virginia

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Polytechnic Institute and State University provided the hypothetical rock outcrop motions used in this study. The many helpful review comments from Martin Chapman and the anonymous reviewers are also gratefully acknowledged.

References Andrus, R. D., C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Cleary, T. J. Casey, and W. B. Wright (2005). Shear-wave velocity and seismic response of near-surface sediments in the Charleston quadrangle, South Carolina, Final report to the U.S. Geological Survey, Award no. 03HGR0046, Department of Civil Engineering, Clemson University, Clemson, South Carolina. Andrus, R. D., J. Zhang, B. S. Ellis, and C. H. Juang (2003). Guide for estimating the dynamic properties of South Carolina soils for ground response analysis, Report no. FHWA-SC-03-07, South Carolina Department of Transportation, Columbia, South Carolina. Ang, A. H.-S., and W. H. Tang (1975). Probability Concepts in Engineering Planning and Design, Vol. I, Basic Principles, John Wiley & Sons, New York, New York. Bakun, W. H., and M. G. Hopper (2004). Magnitudes and locations of the 1811–1812 New Madrid, Missouri, and the 1886 Charleston, South Carolina, earthquakes, Bull. Seism. Soc. Am. 94, 64–75. Bollinger, G. A. (1977). Reinterpretation of the intensity data for the 1886 Charleston, South Carolina, earthquake, in Studies Related to the Charleston, South Carolina, Earthquake of 1886: A Preliminary Report, D. W. Rankin (Editor), U.S. Geol. Surv. Profess. Pap. 1028, 17–32. Boore, D. M. (2000). SMSIM Fortran program for simulating ground motions from earthquakes: Version 2.0, A revision of OFR 96-80A, OF 00-509, U.S. Geological Survey, Denver, Colorado. Boore, D. M., W. B. Joyner, and T. E. Fumal (1994). Estimation of response spectra and peak acceleration from western North American earthquakes: an interim report, Part 2, U.S. Geol. Surv. Open-File Rept. 94-124. Borcherdt, R. D. (1994). Simplified site classes and empirical amplification factors for site-dependent code provisions, in Proceedings of 1992 NCEER/SEAOC/BSSC Workshop on Site Response during Earthquake and Seismic Code Provisions, 1992, National Center for Earthquake Engineering Research, Special Publication NCEER-94-SP01, Buffalo, New York. Chapman, M. C., J. R. Martin, C. G. Olgun, and J. N. Beale (2006). Site response models for Charleston, South Carolina and vicinity developed from shallow geotechnical investigations, Bull. Seism. Soc. Am. 96, no. 2, 467–489. Dobry, R., R. B. Borcherdt, C. B. Crouse, I. M. Idriss, W. B. Joyner, G. R. Martin, M. S. Power, E. E. Rinne, and R. B. Seed (2000). New site coefficients and site classification system used in recent building seismic code provisions, Earthquake Spectra 16, 41–67. Dutton, C. E. (1889). The Charleston earthquake of August 31, 1886, U.S. Geol. Surv. Ninth Annual Report 1887–1888, Washington, D.C., 203– 528. Elton, D. J., and J. R. Martin, II (1989). Dynamic site periods in Charleston, SC, Earthquake Spectra 5, no. 4, 703–734. Fairbanks, C. D., R. D. Andrus, J. Zhang, W. M. Camp, T. J. Casey, and T. J. Clearly (2004). Electronic files of shear-wave velocity and cone penetration test measurements from the Charleston quadrangle, South Carolina, Data report to the U.S. Geological Survey, USGS Award no. 03HQGR0046, Civil Engineering Dept., Clemson University, Clemson, South Carolina. Federal Emergency Management Agency (FEMA) (2000). HAZUS威99 estimated annualized earthquake losses for the United States, FEMA 366, Federal Emergency Management Agency, Washington, D.C., September.

1914

R. D. Andrus, C. D. Fairbanks, J. Zhang, W. M. Camp, T. J. Casey, T. J. Cleary, and W. B. Wright

Frankel, A. D., M. D. Petersen, C. S. Mueller, K. M. Haller, R. L. Wheeler, E. V. Leyendecker, R. L. Wesson, S. C. Harmsen, C. H. Cramer, D. M. Perkins, and K. S. Rukstales (2002). Documentation for the 2002 Update of the national seismic hazards maps, U.S. Geol. Surv. Open-File Rept. 02-420 (http://pubs.usgs.gov/of/2002/ofr-02-420/) (last accessed August 2006). Hashash, Y. M. A. (2005). DEEPSOIL version 2.6, tutorial and user manual, University of Illinois, Urbana-Champaign, Illinois. Hashash, Y. M. A., and D. Park (2002). Viscous damping formulation and high frequency motion propagation in nonlinear site response analysis, Soil Dyn. Earthquake Eng. 22, no. 7, 611–624. Idriss, I. M. (1990). Response of soft soil sites during earthquake, in Proceedings of H. Bolton Seed Memorial Symposium, Vol. 2, BiTech Publishers Ltd., Richmond, British Columbia, Canada, 273–289. Idriss, I. M., and J. I. Sun (1992). SHAKE91 — A computer program for conducting equivalent linear seismic response analyses of horizontally layered soil deposits, User Manual, University of California at Davis, Davis, California. International Code Council, Inc. (ICC) (2000). 2000 International Building Code, Falls Church, Virginia, 679 pp. Joyner, W. B., T. E. Fumal, and G. Glassmoyer (1994). Empirical spectral response ratios for strong motion data from the 1989 Loma Prieta, California, earthquake, in Proceedings of 1992 NCEER/SEAOC/BSSC Workshop on Site Response during Earthquake and Seismic Code Provisions, 1992, National Center for Earthquake Engineering Research, Special Publication NCEER-94-SP01, Buffalo, New York. Kramer, S. L., and S. B. Paulsen (2004). Practical use of geotechnical site response models, in NSF/PEER Int. Workshop on Uncertainties in Nonlinear Soil Properties and their Impact on Modeling Dynamic Soil Response. University of California at Berkeley, Berkeley, California, 18–19 March (http://peer.berkeley.edu/lifelines/Workshop304/) (last accessed August 2006). Lindbergh, C. (1986). Post-workshop section, in Earthquake Hazards, Risk, and Mitigation in South Carolina and the Southeastern United States, The Citadel Press, Charleston, South Carolina, 86 pp. McCartan, L., E. M. Lemon, and R. E. Weems (1984). Geologic map of the area between Charleston and Orangeburg, South Carolina, USGS Miscellaneous Investigations Series Map I-1472, scale 1:250,000, U.S. Geological Survey, Reston, Virginia. Midorikawa, S., M. Matsuoka, and K. Sakugawa (1994). Site effects on strong-motion records observed during the 1987 Chiba-Ken-TohoOki, Japan earthquake, in Proceedings of 9th Japan Earthquake Symposium, Vol. 3, Tokyo, Japan, E085–E090. Park, D., and Y. M. A. Hashash (2004). Soil damping formulation in nonlinear time domain site response analysis, J. Earthquake Eng. 8, no. 7, 249–274. Peters, K. E., and R. B. Herrman (Editors) (1986). First-hand observations of the Charleston Earthquake of August 31, 1886, and other earthquake materials: reports of W. J. McGee, Earle Sloan, Gabriel E. Manigault, Simon Newcomb, and others, South Carolina Geological Survey Bulletin 41, Columbia, South Carolina, 116 pp. Robinson, A., and P. Talwani (1983). Building damage at Charleston, South Carolina, associated with the 1886 earthquake, Bull. Seism. Soc. Am. 73, no. 2, 633–652. Schnabel, P. B., J. Lysmer, and H. B. Seed (1972). SHAKE: a computer program for earthquake response analysis of horizontally layered sites, Report Earthquake Engineering Research Center 72-12, Earthquake Engineering Research Institute, Berkeley, California. Seed, H. B., R. Murarka, J. Lysmer, and I. M. Idriss (1976). Relationship between maximum acceleration, maximum velocity, distance from source and local site conditions for moderately strong earthquakes, Bull. Seism. Soc. Am. 66, 1323–1342. Silva, W., I. Wang, T. Siegel, N. Gregor, R. Darragh, and R. Lee (2003). Ground motion and liquefaction simulation of the 1886 Charleston, South Carolina, earthquake, Bull. Seism. Soc. Am. 93, no. 6, 2717– 2736. Stokoe, K. H., II, S. K. Hwang, M. B. Darendeli, and N. J. Lee (1995).

Correlation study of nonlinear dynamic soil properties, Final Report to Westinghouse Savannah River Company, Aiken, South Carolina. Talwani, P., and W. T. Schaeffer (2001). Recurrence rates of large earthquakes in the South Carolina Coastal Plain based on paleoliquefaction data, J. Geophys. Res. 106, 6621–6642. Weems, R. E., and E. M. Lemon, Jr. (1985). Detailed section from auger holes and outcrops in the Cainhoy, Charleston, and Fort Moultrie quadrangles, South Carolina, U.S. Geol. Surv. Open-File Rept. 85378. Weems, R. E., and E. M. Lemon, Jr. (1993). Geology of the Cainhoy, Charleston, Fort Moultrie, and North Charleston Quadrangles, Charleston and Berkley Counties, South Carolina, USGS Miscellaneous Investigation Map I-1935, scale 1:24,000, U.S. Geological Survey, Reston, Virginia. Weems, R. E., E. M. Lemon, Jr., and P. Chirico (1997). Digital geology and topography of the Chaleston quadrangle, Charleston and Berkeley Counties, South Carolina, U.S. Geol. Surv Open-File Rept. 00-0484. Wheeler, R. L., and C. H. Cramer (2000). Preliminary estimate of the amplification of possible earthquake ground motion at a site in Charleston County, South Carolina, U.S. Geol. Surv. Open-File Rept. 000484. Wong, I., J. Bouabid, W. Graf, C. Huyck, A. Porush, W. Silva, T. Siegel, G. Bureau, R. Eguchi, and J. Knight (2005). Potential losses in a repeat of the 1886 Charleston, South Carolina, Earthquake, Earthquake Spectra 21, no. 4, 1157–1184. Youd, T. L. (1984). Recurrence of liquefaction at the same site, in Proceedings of the 8th World Conference on Earthquake Engineering, Vol. 3, 231–238. Zhang, J. (2004). Characterizing the dynamic properties of South Carolina soils for ground motion evaluation, Ph.D. Dissertation, Civil Engineering Department, Clemson University, Clemson, South Carolina. Zhang, J., R. D. Andrus, W. M. Camp, T. J. Casey, and T. J. Cleary (2004). In Situ VS and NEHRP Site Classification in the Greater Charleston Area, in Proceedings of the 11th International Conference on Soil Dynamics & Earthquake Engineering and the 3rd International Conference on Earthquake Geotechnical Engineering, Vol. 1, University of California, Berkeley, Stallion Press, 437–444. Zhang, J., R. D. Andrus, and C. H. Juang (2005). Normalized shear modulus and material damping ratio relationships, J. Geotech. Geoenviron. Eng. 131, no. 4, 453–464.

Department of Civil Engineering Clemson University 110 Lowry Hall Clemson, South Carolina 29634 [email protected] (R.D.A., C.D.F.)

Fugro Consultants LP 6100 Hillcroft Houston, Texas 77081 (J.Z.)

S&ME, Inc. 620 Wando Park Blvd. Mount Pleasant, South Carolina 29464 (W.M.C., T.J. Cleary)

WPC, Inc. 1017 Chuck Dawley Blvd. Mount Pleasant, South Carolina 29464 (T.J. Casey, W.B.W.)

Manuscript received 20 July 2005.