Ambient Seismic Noise Rayleigh Wave Tomography across Northern Taiwan

We apply the ambient noise cross-correlation technique to the vertical component of the continuous short-period waveforms recorded by three seismic arrays, including Tatun Volcanic Area (TVA) array, Hsinchu array and stations in the northern portion of Central Weather Bureau Seismic Network (CWBSN). The noise-derived Green’s functions are treated as fundamental mode Rayleigh waves. With adequate data selection and quality control, the group velocity of each traces is analyzed, and the qualified results at about 0.35Hz are used to invert for a tomographic map of short period Rayleigh wave group velocity in northern Taiwan with multi-scale parameterization. The main feature of the tomographic result shows very good fit to the surface geological units. In addition to tomographic study, we demonstrate how the retrieved surface waves are affected by various factors, such as seasonal change, site location, and path azimuth. In particular, we notice an evident path-azimuth-dependent asymmetry between causal and acausal signals in most paths. The asymmetric properties imply the correlated ambient noise in northern Taiwan is mainly contributed from the north. Two unexpected while useful by-products are observed, the waveform polarity reversal and time shift of the stations, both present typical features on the retrieved surface waves. Thus, the ambient noise cross-correlation technique provides an alternative method to examine the station condition and secure the data quality from instrument errors.