Site-Response Models for Charleston, South Carolina, and Vicinity Developed from Shallow Geotechnical Investigations

Abstract

The study models the response of near-surface materials in Charleston, South Carolina, and the adjacent area. Geotechnical investigations at 281 locations were made available by local engineering firms. The data used for dynamic siteresponse analysis were derived from shear-wave velocity measurements at 52 locations. Site response was quantified as the ratio of surface motion to hypothetical hard-rock basement outcrop motion. Scenario earthquake motions were developed with the stochastic model. Acceleration response ratios for 5% critical oscillator damping were computed for 12 frequencies ranging from 0.1 to 30 Hz and for peak ground acceleration.Two features determine the general nature of site response in the study area: the impedance contrast between Mesozoic basement and Cretaceous sediments, and the shallow impedance contrast between Quaternary and Tertiary sediments. Average Swave velocities in the Quaternary are relatively uniform and range from 150 to 250 m/sec. They are not strongly correlated with surface geology. The velocities of the immediately underlying Tertiary sediments range from 300 to 500 m/sec. Because of the uniformity of velocity in the Quaternary, depth to the Quaternary-Tertiary contact appears to be the most important variable leading to differences in calculated site response. This surface is irregular, and varies in depth from near surface at inland sites to approximately 30 m at sites near the coast. As a consequence, estimated site response in the frequency band 1-10 Hz varies by as much as a factor of 3. Site response at frequencies less than 1 Hz is dominated by the first few resonant harmonics of the entire sedimentary section, with fundamental frequency near 0.2 Hz.

On the Geologic Structure at the Epicenter of the 1886 Charleston, South Carolina, Earthquake

“…A density contrast of 300 kg=m 3 was Table 2. assumed for the mafic bodies, in line with the density of 2900 kg=m 3 for Connecticut basalt (Carmichael, 1982). The density contrast assumed for the layer representing the coastal plain sediment section was 600 kg=m 3 , reflecting a density of 2000 kg=m 3 (e.g., Chapman et al, 2006).…”

Ground motion studies for microzonation in Iran

“…They believe that the effect of geology on hazard is only slightly Nat Hazards (2011) 59:481-505 501 observed in the PGA map, more so in the 0.3 s spectral acceleration map and significantly more so in the 1 s spectral acceleration map. Chapman et al (2006) also computed 5% damped spectral acceleration for 12 frequencies ranging from 0.1 to 30 Hz. Finally, it is recommended to take into account site-city interaction (Gueguen et al 2000) in the analyses, particularly in dense urban areas, instead of usual methods that estimate soil response on the basis of the free-field conditions.…”

“…One method is to consider the uncertainty in seismic bedrock level by (1) assuming different levels for seismic bedrock on the basis of the geological studies, leading to different representative geotechnical profiles; (2) extending the trend of Vs variation with depth to the different seismic bedrock levels and (3) performing site response analysis for different representative geotechnical profiles and then using the average value of the resulting PGAs as PGA. This method was also successfully used by Chapman et al (2006) for site response models in the vicinity of Charleston, South Carolina. The second method is to (1) specify the soil type using V s(30) obtained from shallow seismic surveys and the site classification cited in codes such as BSSC (2001) or BHRC (2005); (2) obtain an amplification factor on the basis of the soil type as indicated by different references (e.g.…”

“…The procedure applied by IIEES for ground motion microzonation in Iran is similar to the methodology presented by the US Geological Survey (2003) for the San Francisco Bay Region (SFBR), LeBrun et al (2004) for Pointe-à-Pitre, France and Ansal et al (2004) for Adapazari and Gulcuk, Turkey. However, Chapman et al (2006) in Charleston, South Carolina and vicinity, the Japan International Cooperation Agency (JICA) in Tehran and Grasso and Maugeri (2008) in Catania (Italy) used a different procedure based on earthquake scenario for the seismic microzonation. In recent studies, three-dimensional seismic sources are used for microzonation studies (Irsyam et al 2008).…”

Investigation of Seismic Site Amplification for Non-NEHRP Site Conditions: Site Response Study of Columbia, SC

“…Columbia, South Carolina is located along the Fall Line and soil conditions typically consist of shallow soil deposits underlain by hard crystalline rock, causing sharp velocity contrast between these geologic units. Moving into the Atlantic Coastal Plain the Tertiary and Quaternary sediment stack over the hard crystalline rock increases in thickness towards 1000 m. A series of site response studies focused on sites within South Carolina and has provided insight on site response behavior in the CEUS (Lester, 2005, Lester and Chapman, 2005, Chapman and Talwani, 2005, Chapman et al, 2006 This study investigates site amplification resulting from the sharp shear wave velocity contrast by performing a series of site response analyses for Columbia, SC. Shear wave velocity profiles from twenty-nine (29) sites were obtained for this study.…”