High-Resolution Seismic Imaging of Stratigraphic Boundaries for Urban Seismic Hazards Investigations

Stratigraphic boundaries and their associated P-wave and S-wave velocity contrasts strongly affect the distribution of earthquake ground motions and site amplification in urbanized sedimentary basins. A primary goal of our urban seismic imaging is to map seismic reflectors in the stratigraphic section whose high seismic impedance will alter earthquake-generated wavefields and ground motion amplitudes. We present results of seismic hazard investigations conducted in several urbanized basins using high-resolution P- and S-wave seismic imaging methods including basins along the Wasatch Front, Utah, and the Seattle basin, Washington State. A P-wave seismic profile in the Utah basin images subhorizontal strata at depths between 90 and 900 m that dip increasingly steeper with depth in the direction of the Wasatch fault. Spatial Autocorrelation (SPAC) microtremor data image S-wave velocity (Vs) structure to depths of more than 100 m at two sites along the P-wave profile delineating Vs contrasts critical for assessing site amplification. The Seattle fault zone, an actively-deforming region dominated by reverse faulting, forms the southern boundary of the heavily-urbanized Seattle basin. The location and extent, and therefore the effects of the shallow fault zone on earthquake ground motion in this area, is poorly understood. High-resolution P- and S-wave seismic reflection data acquired across the Seattle fault zone image numerous fault locations and deformed shallow strata. Quaternary and late Tertiary deposits in the northern extent of the fault zone in western Seattle dip gently northward, whereas further south these strata are uplifted as much as 200 m and folded by thrust faulting. Approximately 6 km to the east, P-wave seismic data image Tertiary deposits dipping at greater than 45° north toward the northern end of the fault zone. The overlying Quaternary deposits document growth folding and earthquake recurrence through time. Finite-difference modeling confirms that uplifted and folded Tertiary strata focus and amplify earthquake ground motions at the southern edge of the Seattle basin. This focusing may have caused pockets of anomalous building damage documented after earthquakes in 1949, 1965 and 2001. Our studies demonstrate the significant impact shallow (less than 1 km) crustal structures and stratigraphic boundaries can have on earthquake ground-motion variability.