Topographic Effects in Earthquake Ground Motions: Insights Gained from Field Studies of Frequency and Predictable Mining Seismicity
November 9, 2012 4:00 p.m. in ETC 2.136
Professor Brady R. Cox
Department of Civil, Architectural and Environmental Engineering
The University of Texas at Austin
Topographic effects, in the context of earthquake engineering, refer to a commonly recognized phenomenon that causes amplification and frequency alteration in ground motions measured in the vicinity of a topographic feature (hillsides, ridges and canyons) relative to flat ground conditions. Although it is widely recognized that topographic amplification can elevate seismic hazard, 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 building codes. This presentation details experimental work from the first phase (Phase I) of a two-phase field study aimed at investigating topographic effects using frequent, shallow and predictable seismicity induced by underground longwall coal mining. A locally-dense array of ground motion instruments was used to capture over 50 seismic events on steep, irregular topography above a coal mine in central-eastern Utah. Results from processing these ground motions indicate a regular pattern of amplification near the crest of a ridge of approximately 4 times the “base” ground motions within the frequency range of 1 – 2 Hz. The frequency range of topographic amplification is investigated relative to the shape of the 3D feature and its average shear wave velocity.