| Popis: |
The observation of surface waves from cross-correlations of ambient noise has proven to be an invaluable tool for the recovery of seismic travel times. Such techniques allow the construction of 3D velocity models and subsequent geologic interpretations in regions where tomography would otherwise be difficult due to the paucity of seismicity and short seismic array deployment times. One such model is developed here for offshore southern California using an array of Ocean Bottom Seismometers to better understand the tectonic history of the region. However, for accurate hazard evaluation, characterizations of the spatial variability of ground motion amplitudes are also desired. Unfortunately, the amplitudes from noise cross-correlations are systematically biased by the spatial-, frequency- and temporal variability of noise sources. We demonstrate in this thesis that an ambient-noise based wavefront tracking technique using an array of stations can recover site-amplification and attenuation estimates successfully and robustly. This is applied on two different scales: to a very dense array of more than 5,000 geophones in the city of Long Beach, CA, and in a broader, continental scale to instruments from the USArray. Finally, we discuss the propagation of surface waves in relation to seismic hazard and site characterization. We point out that the amplitude response and peak frequency of ground motion will be different for a given site, depending on the type of wave being considered. This work should be considered by any future seismic hazard evaluation where surface waves may cause damage. |
