Continuous monitoring by ambient-seismic noise tomography |

These data are divided into blocks of 30 minutes with overlap between adjacent blocks and multiplied with a cosine-squared taper. This taper has the advantage of restitching the data together without amplification, according to the Pythagorean trigonometric identity. Each 30 minute block is bandpassed (for Hz) and subsampled by a factor of 50 to a sample frequency of Hz. The data is not restitched, instead, the 30 minutes of data are cross-correlated between each station pair, delivering one EGF for each station pair. For all data in the December 2010 recording, this gives 486 EGFs for each station pair.

Figure 6a) shows one such estimate for all stations cross-correlated with station 1, Figure 6b) shows the mean of all estimates for hours, Figure 6c) shows the mean of all estimates for day, and Figure 6 shows the mean of all estimates for a little over days. In each individual estimate there is little signal that exceeds the background correlation fluctuations. However, as we stack more estimates and thus increase the effective recording time we cross-correlate, over arrivals start standing out from the background correlation fluctuations. These arrivals correspond to the surface-wave Green's function between all stations and station 1.

stacks4
Four virtual seismic sources generated by cross correlating recordings of ambient seismic noise; a)
hour of recordings, b)
hours of recording, c)
day of recording, d)
days of recording.
Figure 6. |
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Continuous monitoring by ambient-seismic noise tomography |

2012-05-10