Probabilistic Seismic Hazard Estimates Incorporating Site Effects--An Example from Indiana, U.S.A (original) (raw)
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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 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.
GeoHazards, 2021
Site amplification factors in National Building Codes are typically specified as a function of the average shear wave velocity over the first 30 m (Vs30) or site class (A, B, C, D and E) for defined ranges of Vs30 and/or ranges of depth to bedrock. However, a single set of amplification factors may not be representative of site conditions across the country, introducing a bias in seismic hazard and seismic risk analyses. This is exemplified by significant differences in geological settings between East and West coast locations in North America. Western sites are typically characterized by lower impedance contrasts between recent surface deposits and bedrock in comparison to Eastern sites. In North America, site amplification factors have been derived from a combination of field data on ground motions recorded during West Coast earthquakes and numerical models of site responses that are meant to be representative of a wide variety of soil profiles and ground motions. The bias on ampl...
Soil Dynamics and Earthquake Engineering, 2005
Deep deposits of the Mississippi Embayment, overlying the New Madrid Seismic Zone, present unique challenges for the estimation of local site effects on propagated ground motion. Limited information is available on the dynamic properties of these deposits. This paper develops generalized depth dependent modulus degradation and damping curves specific to the embayment deposits. Depth dependent small strain damping is estimated using weak motion recordings during the Enola earthquake, 2001. Modulus degradation and damping curves are further constrained using limited laboratory test data of embayment soils at low confining pressure. At high confining pressures embayment specific data is unavailable and test data from outside the embayment is used in developing the dynamic properties. The representative modulus degradation and damping curves are used in non-linear and equivalent linear one-dimensional site response analyses. A companion paper describes a large-scale probabilistic seismic hazard analysis study in the Mississippi Embayment that integrates non-linear site effects. q
Probabilistic Seismic Hazard Analysis with Local Site Effects
A probabilistic seismic hazard analysis for Bangalore, south India, is presented in this paper with an emphasis on the local site conditions. Analyses were carried out using the earthquake catalogue available over a radius of 350 km around Bangalore city. Earthquake data were analyzed statistically and recurrence relationship has been obtained using Guttenberg-Richter (G-R) relationship. Probabilistic seismic hazard analyses were then carried out for Bangalore region considering known six seismogenic sources. Results of the present investigation were presented in the form of peak ground acceleration and response spectra at bed rock level and considering the local site conditions. From the extensive field investigation using MASW (Multichannel Analysis of Surface Wave ) from 58 locations in Bangalore region it has been observed that the shear wave velocity falls in the range of 0.18 ≤ Vs ≤ 0.36 (Site Class D). Based on this, a response spectrum is generated by considering the Bangalore region under site class D. The hazard curves of mean annual rate of exceedance for peak ground acceleration and spectral acceleration have been generated at rock level and also by taking into consideration the local site condition. Further, the uniform hazard response spectrum with 5% damping for Bangalore has been generated for 10% probability of exceedance in 50 years for bed rock condition and considering local site effects. The peak ground acceleration (PGA) value of 0.121g at bed rock level and a value of 0.35g considering the local site condition for Bangalore region have been observed from the present investigation.
Seismic hazard mapping for administrative purposes
2001
Local soil conditions, roughly summarised by considering a reference soil for each municipality of the Friuli -Venezia Giulia region in NE Italy, are introduced into probabilistic seismic hazard estimates: the subsequent improvement is checked by comparing these new results and the maximum observed intensities in each municipality to investigate if the major differences between probabilistic estimates and actually observed data can be explained by local site effects and/or by the geometry of the seismogenic zones used in the computation. In addition, a comparison between the new probabilistic hazard results, and the standard ones referred to rock is made for the present and the proposed Italian seismic zonation. The results underline the influence of the seismogenic model used, but are not determinant on the role of site effects.
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.
Physics based probabilistic seismic hazard calculations for Southern California
2008
Deterministic source and wave propagation effects such as rupture directivity and basin response can have a significant impact on near-fault ground motion levels, particularly at longer shaking periods. CyberShake, as part of the Southern California Earthquake Center's (SCEC) Community Modeling Environment, is developing a methodology that explicitly incorporates these effects within seismic hazard calculations through the use of physics-based 3D ground motion simulations. To calculate a waveform-based probabilistic hazard curve for a site of interest, we begin with Uniform California Earthquake Rupture Forecast, Version 2 (UCERF2) and identify all ruptures (excluding background seismicity) within 200 km of the site of interest. We convert the UCERF2 rupture definition into multiple rupture variations with differing hypocenter location and slip distribution, which results in about 400,000 rupture variations per site. Strain Green Tensors are calculated for the site of interest using the SCEC Community Velocity Model, Version 4 (CVM4), and then, using reciprocity, we calculate synthetic seismograms for each rupture variation. Peak intensity measures (e.g., spectral acceleration) are then extracted from these synthetics and combined with the original rupture probabilities to produce probabilistic seismic hazard curves for the site. Thus far, we have produced hazard curves for spectral acceleration at a suite of periods ranging from 3 to 10 seconds at about 20 sites in the Los Angeles region, with the ultimate goal being the production of full hazard maps. Our results indicate that the combination of rupture directivity and basin response effects can lead to an increase in the hazard level for some sites, relative to that given by a conventional Ground Motion Prediction Equation (GMPE). Additionally, and perhaps more importantly, we find that the physics-based hazard results are much more sensitive to the assumed magnitude-area relations and magnitude uncertainty estimates used in the definition of the ruptures than is found in the traditional GMPE approach. This reinforces the need for continued development of a better understanding of earthquake source characterization and the constitutive relations that govern the earthquake rupture process.
Bulletin of the New Zealand Society for Earthquake Engineering
This paper reviews concepts and trends in seismic hazard characterization that have emerged in the past decade, and identifies trends and concepts that are anticipated during the coming decade. New methods have been developed for characterizing potential earthquake sources that use geological and geodetic data in conjunction with historical seismicity data. Scaling relationships among earthquake source parameters have been developed to provide a more detailed representation of the earthquake source for ground motion prediction. Improved empirical ground motion models have been derived from a strong motion data set that has grown markedly over the past decade. However, these empirical models have a large degree of uncertainty because the magnitude - distance - soil category parameterization of these models often oversimplifies reality. This reflects the fact that other conditions that are known to have an important influence on strong ground motions, such as near- fault rupture direc...