Presenter: Ching-Yu Cheng
Date: 2017/01/05
Abstract
For accurate seismic hazard evaluation, both the spatial and frequency-dependent variabilities in the amplitudes of earthquake ground motions are needed. Traditional seismic hazard studies have primarily focused on describing the expected shaking from potential future earthquakes based on empirical observations of previous earthquakes. These observations have shown that the amplitudes of seismic waves can be strongly affected by shallow crustal heterogeneities. Ambient noise cross correlations provide a signal rich in these higher frequencies and offer the flexibility of making observations in the absence of earthquakes. These two articles used the same method of studying amplitude variability. The main theoretical background is in Lin et al. [2012]. The observed amplitude variation corrected by wavefront focusing and defocusing can be related to the local amplification variation, attenuation, and internal sources. Bowden et al. [2015] use 5200-geophone array in Long Beach, California to analyze Rayleigh-wavefield amplitude gradients from ambient noise correlations that are processed so that relative amplitudes satisfy the wave equation and are therefore meaningful. The results in this article shows that ambient noise tomography can be used to map site amplification at high frequencies in the range 0.67 to 2.0 Hz. Also, they find a sharp contrast across the Newport-Inglewood Fault, and generally higher amplitudes to the Southwest of the city.
Reference
D. C. Bowden,V. C. Tsai, F. C. Lin (2015), Site amplification, attenuation, and scattering from noise correlation amplitudes across a dense array in Long Beach, CA, Geophysical Research Letters, 42, 1360-1367.
F. C. Lin, V. C. Tsai, and M. H. Ritzwoller (2012), The local amplification of surface waves: A new observable to constrain elastic velocities, density, and anelastic attenuation, Journal of Geophysical Research, 117, B06302, doi:10.1029/2012JB009208.