An Analysis of Seismic Hazards in Indiana (original) (raw)
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Probabilistic Seismic Hazard Estimates Incorporating Site Effects--An Example from Indiana, U.S.A
Environmental and Engineering Geoscience, 2010
The U.S. Geological Survey (USGS) has published probabilistic earthquake hazard maps for the United States based on current knowledge of past earthquake activity and geological constraints on earthquake potential. These maps for the central and eastern United States assume standard site conditions with Swave velocities of 760 m/s in the top 30 m. For urban and infrastructure planning and long-term budgeting, the public is interested in similar probabilistic seismic hazard maps that take into account near-surface geological materials. We have implemented a probabilistic method for incorporating site effects into the USGS seismic hazard analysis that takes into account the first-order effects of the surface geologic conditions. The thicknesses of sediments, which play a large role in amplification, were derived from a P-wave refraction database with over 13,000 profiles, and a preliminary geology-based velocity model was constructed from available information on S-wave velocities. An interesting feature of the preliminary hazard maps incorporating site effects is the approximate factor of two increases in the 1-Hz spectral acceleration with 2 percent probability of exceedance in 50 years for parts of the greater Indianapolis metropolitan region and surrounding parts of central Indiana. This effect is primarily due to the relatively thick sequence of sediments infilling ancient bedrock topography that has been deposited since the Pleistocene Epoch. As expected, the Late Pleistocene and Holocene depositional systems of the Wabash and Ohio Rivers produce additional amplification in the southwestern part of Indiana. Ground motions decrease, as would be expected, toward the bedrock units in south-central Indiana, where motions are significantly lower than the values on the USGS maps.
Bulletin of the Seismological Society of America, 2011
Evansville, Indiana, is one of the closest large urban areas to both the New Madrid Seismic Zone, where large earthquakes occurred in 1811-1812, and the Wabash Valley Seismic Zone, where there is evidence of several large prehistoric earthquakes in the last 14,000 yr. For this reason, Evansville has been targeted as a priority region for urban seismic-hazard assessment. The probabilistic seismic-hazard methodology used for the Evansville region incorporates new information from recent surficial geologic mapping efforts, as well as information on the depth and properties of near-surface soils and their associated uncertainties. The probabilistic seismichazard calculation applied here follows the method used for the 2008 United States Geological Survey (USGS) national seismic-hazard maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2% probability of exceedance in 50 yr, which are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. Spectral accelerations at 0.2-s period range from 0.6 to 1.5g, values that are much greater than those of the USGS national seismic-hazard map, which assume B/C site conditions with an average shear-wave velocity of 760 m=s in the top 30 m. The presence of an ancient bedrock valley underlying the current Ohio River flood plain strongly affects the spatial pattern of accelerations. For 1.0-s spectral acceleration, ground motions are significantly amplified due to deeper soils within this structure, to a level comparable to that predicted by the national seismic-hazard maps with D site conditions assumed. For PGA and 0.2-s spectral acceleration, ground motions are significantly amplified outside this structure, above the levels predicted by the national seismic-hazard maps with uniform D site conditions assumed.
Seismic-Hazard Maps and Time Histories for the Commonwealth of Kentucky
2008
The ground-motion hazard maps for the three earthquake scenarios, expected earthquakes, probable earthquakes, and maximum credible earthquakes on the free surface in hard rock (shear-wave velocity >1,500 m/s), were derived using the deterministic seismic hazard analysis and the corresponding time histories were developed using the composite source model for each scenario earthquake. The results are based on (1) historical observations, (2) instrumental records, and (3) current understanding of the earthquake source, recurrence, and ground-motion attenuation relationship in the central United States. It is well understood that there are uncertainties in the groundmotion hazard maps because of the uncertainties inherent in parameters such as earthquake location, magnitude, and frequency used in the study. This study emphasizes the earthquakes that would have maximum impacts on humans and structures. The ground-motion parameters, including time histories, are intended for use in the recommended zone (not site-specific) where the structure is assumed to be situated at the top of a bedrock foundation. For sites underlain by soils, and in particular for sites underlain by poorly consolidated soils, it is recommended that site-specific investigations be conducted by qualified professionals in order to determine the possibilities of amplification, liquefaction, slope failure, and other considerations when subjected to the ground motions.
Engineering Geology, 2001
Local soil conditions have a profound in¯uence on the characteristics of ground shaking during an earthquake. Exceptionally deep soil deposits, on the order of 100±1000 m deep, are found in the Upper Mississippi Embayment of the central United States. Shear waves (SH) from earthquakes in the New Madrid seismic zone are expected to be strongly affected by the sharp impedance contrasts at the bedrock/sediment interface, attenuation of seismic waves in the soil column, and the SH-wave velocities of the more poorly consolidated near-surface (#50 m) soils.
The 2018 update of the US National Seismic Hazard Model: Overview of model and implications
Earthquake Spectra, 2019
During 2017–2018, the National Seismic Hazard Model for the conterminous United States was updated as follows: (1) an updated seismicity catalog was incorporated, which includes new earthquakes that occurred from 2013 to 2017; (2) in the central and eastern United States (CEUS), new ground motion models were updated that incorporate updated median estimates, modified assessments of the associated epistemic uncertainties and aleatory variabilities, and new soil amplification factors; (3) in the western United States (WUS), amplified shaking estimates of long-period ground motions at sites overlying deep sedimentary basins in the Los Angeles, San Francisco, Seattle, and Salt Lake City areas were incorporated; and (4) in the conterminous United States, seismic hazard is calculated for 22 periods (from 0.01 to 10 s) and 8 uniform VS30 maps (ranging from 1500 to 150 m/s). We also include a description of updated computer codes and modeling details. Results show increased ground shaking i...
The 2014 United States National Seismic Hazard Model
Earthquake Spectra, 2015
New seismic hazard maps have been developed for the conterminous United States using the latest data, models, and methods available for assessing earthquake hazard. The hazard models incorporate new information on earthquake rupture behavior observed in recent earthquakes; fault studies that use both geologic and geodetic strain rate data; earthquake catalogs through 2012 that include new assessments of locations and magnitudes; earthquake adaptive smoothing models that more fully account for the spatial clustering of earthquakes; and 22 ground motion models, some of which consider more than double the shaking data applied previously. Alternative input models account for larger earthquakes, more complicated ruptures, and more varied ground shaking estimates than assumed in earlier models. The ground motions, for levels applied in building codes, differ from the previous version by less than ±10% over 60% of the country, but can differ by ±50% in localized areas. The models are incor...
Developing a Map of Geologically Defined Site-Condition Categories for California
Bulletin of the Seismological Society of America, 2006
Consideration of site conditions is a vital step in analyzing and predicting earthquake ground motion. The importance of amplification by soil conditions has long been recognized, but though many seismic-instrument sites have been characterized by their geologic conditions, there has been no consistent, simple classification applied to all sites. As classification of sites by shear-wave velocity has become more common, the need to go back and provide a simple uniform classification for all stations has become apparent. Within the Pacific Earthquake Engineering Research Center's Next Generation Attenuation equation project, developers of attenuation equations recognized the need to consider site conditions and asked that the California Geological Survey provide site conditions information for all stations that have recorded earthquake ground motion in California. To provide these estimates, we sorted the available shear-wave velocity data by geologic unit, generalized the geologic units, and prepared a map so that we could use the extent of the map units to transfer the velocity characteristics from the sites where they were measured to sites on the same or similar materials. This new map is different from the California Geological Survey "preliminary site-conditions map of California" in that 19 geologically defined categories are used, rather than National Earthquake Hazards Reduction Program categories. Although this map does not yet cover all of California, when completed it may provide a basis for more precise consideration of site conditions in ground-motion calculations.
Open File Report, 2000
We estimate site amplification at the location of a proposed bridge near Charleston, South Carolina. Model calculations indicate that amplification at periods of 1 s and longer is likely to be strongly influenced by the effects of a large contrast in shear-wave velocity at a depth of approximately 1 km (3,000 ft). On-site borehole data, regional geological and geophysical information, and data from a geologically similar setting near Memphis, Tennessee allowed us to estimate profiles of shear-wave velocity, shear-wave attenuation, and density from ground level down to metamorphic and igneous rocks that are approximately 3 km (9,500 ft) beneath the site. We modeled amplifications that would be produced at the surface and at the top and bottom of the Cooper Marl. Amplification estimates that are based only on the shallow shear-wave structure, for example in the upper 100 m (300 ft), can severely underestimate long-period amplification at the site. Additional modeling could help determine whether new data should be collected, to resolve remaining uncertainties about likely amplification.
Seismic Risk Assessment and Application in the Central United States
Georisk 2011, 2011
Seismic risk is a somewhat subjective, but important, concept in earthquake engineering and other related decision-making. Another important concept that is closely related to seismic risk is seismic hazard. Although seismic hazard and seismic risk have often been used interchangeably, they are fundamentally different: seismic hazard describes the natural phenomenon or physical property of an earthquake, whereas seismic risk describes the probability of loss or damage that could be caused by a seismic hazard. The distinction between seismic hazard and seismic risk is of practical significance because measures for seismic hazard mitigation may differ from those for seismic risk reduction. Seismic risk assessment is a complicated process and starts with seismic hazard assessment. Although probabilistic seismic hazard analysis (PSHA) is the most widely used method for seismic hazard assessment, recent studies have found that PSHA is not scientifically valid. Use of PSHA will lead to (1) artifact estimates of seismic risk, (2) misleading use of the annual probability of exccedance (i.e., the probability of exceedance in one year) as a frequency (per year), and (3) numerical creation of extremely high ground motion. An alternative approach, which is similar to those used for flood and wind hazard assessments, has been proposed.
Factors influencing soil surface seismic hazard curves
Soil Dynamics and Earthquake Engineering, 2016
Performance-based seismic design of important structures requires design ground motions from probabilistic seismic hazard analysis (PSHA) that incorporate the effects of local site conditions. Seismic hazard curves incorporating site-specific soil conditions can be generated through the convolution of rock hazard curves with statistical models for site-specific ground motion amplification factors (AF). The AF relationships are developed from a series of site response analyses. The goal of this study is to evaluate how the AF relationships and the resulting surface hazard curves are influenced by different approaches in the site response analysis, specifically the time series (TS) vs. random vibration theory (RVT) approaches, and by different levels of shear wave velocity variability introduced in the site response analysis. The results show that the median AF relationships derived from TS and RVT analyses are similar, except at periods near the site period, where RVT analysis may predict larger AF. Including the effect of shear wave velocity variability reduces the median AF and increases the standard deviation associated with the AF relationship (σ lnAF ). Generally, the soil hazard curve derived by the AF relationship with the largest σ lnAF generates the largest ground motions, and this effect is most significant at small annual frequencies of exceedance. The effect of σ lnAF on soil hazard curves is larger than the effect of different median AF relationships. The value of σ lnAF is influenced significantly by the variability in the shear wave velocity and therefore proper specification of this variability is critical when developing soil hazard curves.