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Spurious velocity changes caused by temporal variations in ambient noise frequency content

Zhongwen Zhan, Victor C. Tsai, & Robert W. Clayton

Published September 2013, SCEC Contribution #1734

Ambient seismic noise cross correlation is now being used to detect temporal variations of seismic velocity, which are typically on the order of 0.1%. At this small level, temporal variations in the properties of noise sources can cause apparent velocity changes. For example, the spatial distribution and frequency content of ambient noise have seasonal variations due to the seasonal hemispherical shift of storms. Here we show that the seasonal variation of noise frequency content causes apparent velocity changes if the stretching method is used to measure time shifts due to the changes in both amplitude and phase spectra caused by waveform stretching. With realistic temporal variations of frequency content in the Los Angeles Basin, our numerical tests produce about 0.05% apparent velocity change, comparable to what Meier et al. (2010) observed in the Los Angeles Basin. We find that the apparent velocity change from waveform stretching depends on time windows and station-pair distances, and hence it is important to test a range of these parameters to diagnose the stretching bias. Better understanding of spatiotemporal noise source properties is critical for more accurate and reliable passive monitoring.

Zhan, Z., Tsai, V. C., & Clayton, R. W. (2013). Spurious velocity changes caused by temporal variations in ambient noise frequency content. Geophysical Journal International, 194(3), 1574-1581. doi: 10.1093/gji/ggt170.