Array observations for narrow-band background noises in the Hualien area and their seismological implications

Seismic observations in the Hualien area have been greatly disturbed by narrow-band background noise. Thus, the automatic earthquake trigging system of the Taiwan regional seismic network has been equally affected by the same background signals, the origin of which is still controversial. Herein, array measurements were conducted to account for this seismic noise and to recognize its seismic wave propagation properties. Five temporary arrays were designed to observe this background noise. Each array consisted of 15 three-component seismometers spaced from 100 to 300 meters apart. Observed data shows that a notable band limit background noise, in the frequency range of 1.3 to 1.7 Hz, was identified from its seismic power spectrum with strong separated peaks at 1.3 Hz, 1.4 Hz and 1.7 Hz. The origins and characteristics of the background noises were investigated using frequency-wavenumber analyses and waveform stacking techniques. Analyzed results suggested that the feasible sources of the narrow-band signals were located near the southwest boundary of the survey area. Based on particle motion analysis, the narrow-band seismic signals were identified as major Rayleigh wave type ground motions. In this survey area, microtremors were generated from multiple origins. However, the major signal sources remain stable in the southwest. Those signals were also examined with regard to persistence over time from a permanent seismic station leading to the inference that the source of that background noise was industrial in origin.