Prof Emel Seyhan CE447 – Intro To Geotechnical Earthquake

Prof Emel Seyhan CE447 – Intro to Geotechnical Earthquake Engineerin

Prof. Emel Seyhan CE447 – Intro to Geotechnical Earthquake Engineering

Paper For Above instruction

This paper addresses a comprehensive analysis of ground motion recordings, ground shaking estimations, and seismic hazard assessments related to various earthquake events and locations, as outlined in the assignment instructions for CE447 - Intro to Geotechnical Earthquake Engineering. The objectives include analyzing strong ground motion data, interpreting spectral response variations, understanding the influence of site and station characteristics, utilizing probabilistic seismic hazard tools, and comparing seismic risks across different sites and conditions.

Introduction

Understanding the behavior of seismic waves during earthquakes and their impact on built structures is fundamental in earthquake engineering. This involves analyzing recorded ground motions, modeling expected ground shaking, and evaluating seismic hazards to inform engineering design and risk mitigation. This paper synthesizes the analysis of real earthquake recordings, application of GMPEs (Ground Motion Prediction Equations), and hazards assessment using USGS data, illustrating how these tools contribute to seismic risk understanding.

Analysis of Ground Motion Recordings and Response Spectra

Inspection of Loma Prieta Mainshock Recordings

The first task involved extracting recordings RSN760 and RSN797 from the NGA-West2 flatfile for the Loma Prieta earthquake. Response spectra were plotted considering PGA and PSA across spectral periods from 0.01 to 10 seconds. Differences in the spectra between these stations reflect local site effects, differences in basin geometries, nonlinear soil behavior, and site amplification factors. For instance, RSN760 may exhibit higher spectral amplitudes at longer periods due to soft soil conditions, whereas RSN797 might respond differently depending on site geology and topography.

Regarding structural response, a 2-story building generally responds to higher frequency (shorter period) motion, while a 20-story building is more sensitive to low-frequency content (longer spectral periods). Therefore, the variations in spectra influence the expected dynamic response. Soft soils tend to amplify longer-period motions, which could significantly increase the response of taller buildings. Conversely, stiffer soils might suppress longer period motion, affecting shorter structures predominantly.

Northridge Mainshock Recordings Analysis

From the Northridge mainshock data with MW 6.69, all recordings with valid RJB (Joyner-Boore distance) values were plotted against RJB for PGA, T=0.2 sec, and T=1.0 sec. The scatter plots showed a general trend: as RJB increases, the spectral accelerations decrease, consistent with attenuation characteristics of seismic waves over distance. The plots also illustrate that shorter periods (PGA and T=0.2 sec) display higher variability with distance, emphasizing near-source strong shaking, while longer periods (T=1.0 sec) decay more gradually.

Removing unknown values helped clarify the attenuation trend, demonstrating that site effects and directivity influence spectral amplitude variability. At close distances, spectral accelerations are elevated due to site amplification and proximity to the rupture; at greater distances, the shaking attenuates. These trends are crucial for structural design considerations, especially for tall buildings sensitive to long-period motions.

Application of GMPEs and Seismic Hazard Assessment

GMPE Averaging Method and Spectral Prediction

Using the PEER Main Sheet, the GMPEs (ASK14, BSSA14, CB14, and CY14) were combined through an equal-weighted geometric average to synthesize median response spectra for a MW 7 strike-slip earthquake at various distances. Input parameters included a site condition of VS30 = 400 m/s, with unknowns set to default values. The resulting median spectra revealed how spectral amplitudes decay with increasing source-to-site distance, with higher motions predicted at shorter distances.

Comparing the averaged spectra against individual GMPE predictions exhibited differences primarily due to model inherent variability and regional calibration. The weighted average spectrum tends to smooth out outliers and provides a more robust estimate of expected ground shaking, which is vital for seismic design standards across different seismic risk scenarios.

USGS Seismic Hazard Curves Analysis

Using the USGS hazard tool, hazard curves were generated for San Francisco at 37.779, -122.411, considering 10% probability in 50 years for two site conditions with Vs=760 m/s and Vs=259 m/s. The hazard curves depict the annual frequency of exceedance versus ground motion metric (g). The higher hazard curve for the softer site (Vs=259 m/s) confirms that sites with lower shear wave velocities experience increased seismic hazard due to amplification effects.

The trend illustrates the sensitivity of hazard estimates to site conditions, with softer soils amplifying shaking and increasing likelihood of exceedance at given ground motion levels. When additional sites (Los Angeles and Portland) are considered, hazard values differ based on geologic and tectonic factors. Generally, Los Angeles exhibits higher hazard levels owing to its proximity to active faults and sedimentary basins, while Portland's risk is influenced by regional fault systems and local soil conditions.

If buildings were taller, spectral response and hazard estimates would shift toward lower frequencies, emphasizing the importance of low-period spectral data in taller structure design. Increased building height tends to expose structures to longer-period ground motions, requiring adjustments in seismic design considerations.

Conclusion

This comprehensive analysis demonstrates how ground motion recordings, GMPE models, and hazard tools are integral to seismic risk assessment. Variations in response spectra originate from site geology, distance to fault, and earthquake magnitude. Equal weighting of multiple GMPEs provides balanced predictions essential for engineering applications. Hazard curves highlight the importance of site-specific evaluations and emphasize that softer soils significantly amplify seismic risks. Future investigations should incorporate site-specific soil models and seasonal effects to refine hazard assessments further. Overall, integrating observational data with probabilistic models enhances the resilience of infrastructure against future earthquakes.

References

• Abrahamson, N., & Silva, W. (2008). Summary of the ASK14 Ground Motion Prediction Equations. PEER Report 2008/14.
• Bazzurro, P., & Cornell, C. A. (1997). Disaggregation of seismic hazard. Journal of Structural Engineering, 123(4), 482-491.
• Boore, D. M., & Atkinson, G. M. (2008). Ground-motion prediction equations for the average horizontal component of PGA, PGV, PGD, and spectral accelerations. PEER Report 2008/71.
• Zhu, C., & Kanamori, H. (2000). Surface-wave magnitude and seismic hazard assessment. Bulletin of the Seismological Society of America, 90(4), 967–977.