On Short Period Ambient Noise of Northern Taiwan. Ambient Noise Tomography. Probing the Source of Ambient Noise
Date Issued
2009
Date
2009
Author(s)
Chen, Ying-Nien
Abstract
Retrieving Green functions between stations by cross-correlating continuous seismic records has quickly become a popular technique in seismology for its operational simplicity and various advantages over traditional surface wave tomography. We apply this technique to three component continuous seismic data recorded from three networks in northern Taiwan, including Tatun Volcanic Area array, Hsinchu array and northern part of Central Weather Bureau Seismic Network, for the time period from Jan, 2006 to Dec, 2006. For each station pairs, we derive Love waves from T–T (transverse) component cross-correlation functions (CCF), and Rayleigh waves from Z-Z (vertical) and R-R (radial) component CCF respectively. We measure group and phase velocities for the period range from 1 to 5 seconds. With careful data selection, the qualified dispersion curves are used to derive two dimensional (2-D) phase and group velocity maps for both Rayleigh and Love waves with multi-scale inversion technique. The 2D maps are then used to develop a 3-D shallow velocity structure of the northern Taiwan. e also attempt to probe the sources of ambient noise by several approaches: (1) analyzing the relative strength between the causal and acausal CCF; (2) measuring the relative strength of CCF amplitudes with respect to their own annual average as a function of time and azimuth to determine the background energy flow; and (3) computing power spectra density and rms amplitudes as a function of time for representative costal stations. The results show that (1) offshore ocean waves are likely the major ambient noise source, and bathymetry might play a role in the process of energy transfer from ocean to continent; (2) there are clear and similar temporal variations for different component of CCF, implying different component of noises are well mixed during scattering in the shallow crust, and a quasi-diffuse field for ambient noise is achieve; and (3) the atmosphere perturbations might be responsible for the observed temporal variations of CCF.
Subjects
seismic ambient noise
multi-scale parameterization
background energy flow
Type
thesis
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