HR: 14:05h
AN: S22D-02    [PDF]
TI: Calculation of Waveform-based Differential Times with both Cross-correlation and Bispectrum Methods
AU: * Du, W
EM: dxw@geology.wisc.edu
AF: Department of Geology & Geophysics, University of Wisconsin-Madison, 1215 W Dayton Street, Madison, WI 53706 United States
AU: Thurber, C H
EM: clifft@geology.wisc.edu
AF: Department of Geology & Geophysics, University of Wisconsin-Madison, 1215 W Dayton Street, Madison, WI 53706 United States
AU: Eberhart-Phillips, D
EM: deberhart@usgs.gov
AF: U. S. Geological Survey, 4200 University Dr., Anchorage, AK 99508 United States
AB: Cross-correlation (CC) determined relative time delays, or related differential times, between pairs of seismic events at the same station are often used as input data to improve earthquake relocation results. Researchers generally select those time delays with associated CC coefficients larger than a chosen threshold. When two similar time series are contaminated by correlated noise sources, the relative time delay between them calculated with the CC technique is sometimes not reliable. Noise at a station for different events are expected to be partially correlated due to a combination of constant noise sources with time-varying amplitudes (microseisms, wind or cultural noise) and site response effects. The bispectrum (BS) method performs better with such data by eliminating the effect of correlated Gaussian noise in the third-order spectral domain. In this work, we use both the CC and BS methods to compute the relative time delay between two windowed waveforms of an event pair recorded at the same station. CC is performed only on the band-pass filtered data, while the BS method is applied to both the raw (unfiltered) and filtered waveforms. Because the characteristics of the noise terms in the raw and filtered data are different, the two BS time delay estimates may not always agree with each other. We then use both of them to verify (select or reject) the computed CC time delay, i.e. to check whether the differences between the CC and the two BS estimates are both within a specified limit. The exact verification process for an event pair varies depending on the size of the maximum CC coefficient across all the common stations. This BS verification process can provide quality control over the chosen CC time delays and potentially more differential times for close event pairs. We apply this technique to obtain bispectrum-verified CC differential times for 822 New Zealand earthquakes in the Wellington region. We find that the bispectrum-verified CC time delays provide improved (smaller rms residual and more clustered) earthquake relocation result compared to those selected with the threshold criterion.
DE: 7200 SEISMOLOGY
DE: 7215 Earthquake parameters
DE: 7260 Theory and modeling
DE: 7294 Instruments and techniques
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2003 Fall Meeting