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NEESR-CR: Topographic Effects in Strong Ground Motion - From Physical and Numerical Modeling to Design

Principal Investigator: Adrian Rodriguez-Marek, Washington State University

Topographic effects refer to the modification and amplification of seismic ground motion in the vicinity of topographic features such as hillsides, ridges, and canyons. This well-documented phenomenon has yet to be addressed in design codes. Because tectonics and topography are closely related, most seismically active regions of the world are marked by significant topographic relief. In recent decades, population growth and scarcity of undeveloped metropolitan land have changed urban land use patterns and placed an increasing number of people and infrastructure assets in areas susceptible to topographic effects during earthquakes.

Although it is widely recognized that topographic amplification can elevate seismic risk, there is currently no consensus on how to reliably quantify its effects. Lack of consensus has precluded development of acceptable guidelines on how to account for this phenomenon in practice, thus leaving an important factor contributing to seismic hazard unaccounted for in routine design. Until now, a major impediment towards understanding and realistically modeling topographic effects has been the lack of a statistically significant number of seismic recordings from densely instrumented sites with topographic features. Moreover, while existing theoretical models are generally capable of qualitatively predicting the effects of irregular topographic features on seismic ground motion, there is still significant quantitative disagreement between predictions and observations.

The proposed research addresses this problem with a study of topographic amplification of ground motion that will include a comprehensive and integrated program of experimental simulations, field measurements, empirical data analysis, and numerical modeling. These research methods, applied together in a framework now made possible by NEES, will quickly and substantially advance the understanding of topographic effects. This new understanding will in turn permit the development of data- and analysis-driven guidelines to account for these effects in engineering design, building code provisions, and seismic risk and microzonation studies.

The proposed study will integrate knowledge about topographic effects gained from centrifuge model testing of topographic features (NEES@UCDavis); field data acquired with temporary, locally-dense instrumentation arrays recording frequent and predictable stress-induced mining seismicity in a mountainous region of Utah (nees@UTexas and PASSCAL); rigorous numerical modeling studies; and statistical analyses of the NGA strong ground motion data base. It is envisioned that this work will result in an order-of-magnitude increase in the amount of high quality data on topographic amplification; greater fundamental understanding of this phenomenon; quantification of topographic effects on ground motions; improved attenuation relationships that account for topographic amplification; and widely adopted guidelines and provisions to account for this seismic hazard in practice. Ultimately, this work will allow seismic risk to be more effectively managed in terms of ground motion quantification and site response prediction.

The NEES@UCDavis centrifuge facility will be used to conduct a two-part comprehensive test series on model slopes of varying inclinations. This facility was selected because of several of its unique features, including: (i) wireless data acquisition system supporting dense instrumentation arrays (i.e. > 100 accelerometers); (ii) Biaxial shaking capability; (iii) Hinged plate container to minimize the effects of boundary conditions on the dynamic response of the model; (iv) Three-dimensional visualization capabilities for reviewing and visualizing dynamic response of the model; (v) Bender elements for in-flight testing of shear wave velocity; and (vi) Robot-assisted cone penetrometer test (CPT) tool for assessing construction-related spatial variability (if any) in the model.

nees@UTexas mobile field equipment will be used to set up a temporary (two-week), locally-dense instrumentation array in a mountainous region directly above Deer Creek Coal Mine in central-eastern Utah. 12 Mark Products 1-Hz geophones (3-component, short-period) and VXI/Sercel data acquisition systems will be used for recording the ground motions and associated topographic effects generated by frequent and predictable underground mining-induced seismicity. Twelve additional broadband sensors obtained from the IRIS PASSCAL instrument program will be coupled with the NEES sensors to capture the entire frequency spectrum of interest. The nees@UTexas instrumentation van will also allow for wireless transmission of data and telepresence for project personnel and other interested parties. Additionally, the nees@UTexas Thumper truck will be used as a dynamic source to perform key surface wave testing within the boundaries of the instrumentation array.