Enhancing 0.4-1.0 Hz seismic signals in Green’s functions through judicious selection of time intervals

Ian Vandevert, Aaron K. Anderson, Taro Okamoto, & Toshiro Tanimoto

Submitted August 15, 2018, SCEC Contribution #8690, 2018 SCEC Annual Meeting Poster #112

When we compute cross-correlation of seismic noise, we typically see dominant signals in the microseism frequency band, approximately 0.05-0.3 Hz. Cross-correlations have been done for higher frequency ranges, say up to 2 Hz, but if we do not pay attention to details, the Green’s functions tend to be dominated by the signals below 0.3 Hz. In this study, we attempt to obtain reliable high-frequency signals in cross-correlated waveforms by choosing time intervals when the Los Angeles Basin is filled with high frequency signals; this should lead to an improved shallow seismic velocity structure in the Los Angeles Basin.

We have noted that there are days when the Los Angeles Basin is filled with high frequency noise about 0.4-1.0 Hz in addition to noise in the microseism frequency band. They last a few days each time but they appear only a few times per month. We propose to choose such days, based on time-frequency plots from various stations, and then apply the cross-correlation Green’s function method. We typically find good signals in Green’s functions for frequencies about 0.4-1.0 Hz.

In order to understand the nature of these waves, we numerically computed the kernels by the finite difference method for a pair of stations for the range 0.4-1.0 Hz and examined depth sensitivity of waves to the SCEC CVM models. The kernels indicate high sensitivity to depths shallower than 2 km and suggest that we can improve LA Basin structure through collections of such Green’s functions. Group velocity measurements of the maximum amplitude phases in such Green’s functions show regional variations and seem to match surface geological features.

Key Words
cross-correlation, noise, Green's,Los Angeles Basin, velocity structure

Citation
Vandevert, I., Anderson, A. K., Okamoto, T., & Tanimoto, T. (2018, 08). Enhancing 0.4-1.0 Hz seismic signals in Green’s functions through judicious selection of time intervals. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
Seismology