On Short Period Ambient Noise of Taiwan (1) Ambient Noise Tomography (2) Probing Source of Ambient Noise

Retrieving empirical Green functions (EGF) 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; in particular, the derived short period Green’s functions of surface waves are usually inaccessible from seismic record caused by nature earthquakes. We apply this technique to three component continuous seismic data recorded at 88 short period stations in Taiwan. The data are collected from three seismic networks, including the island-wide Central Weather Bureau Seismic Network, and two temporary local seismic arrays, Tatun Volcanic Area array and Hsinchu array, 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. The achieved dense path coverage together with the retrieved short period EGF provide an unprecedented resolving power to the shallow crust structure of Taiwan island. With the qualified dispersion curves, we apply a multi-scale inversion technique to derive two dimensional phase, group velocity maps for both Rayleigh and Love waves, and three dimensional Vs structure of shallow crust. Besides tomographic study, we also attempt to probe the sources of ambient noise by several approaches: (1) analyzing the relative strength between the causal and acausal empirical Green’s functions (EGF); (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 of continuous record for representative costal stations. With the results, we are able to characterize the spatial and temporal variations of CCF and their possible relationship to the atmospheric perturbations and coastal bathymetry.