Study on Kappa Value in Taiwan Using Borehole and Surface Seismic Array

Since the inception of the borehole seismic array deployed by the Central Weather Bureau (CWB) in Taiwan in recent years, a large quantity of strong-motion records have been accumulated from frequently occurring earthquakes around Taiwan that provide the opportunity to investigate the spectral decay parameter kappa (kappa) and to understand further the site effects of seismic stations. In this study, we used 133 earthquakes recorded at 30 borehole seismic arrays. Each array includes two force-balance accelerometers, one at the surface and the other inside the borehole. Based on the regression analysis between kappa-value and hypocentral distance for each surface-borehole station pairs, the resulting kappa(0) derived from surface stations are higher than results from borehole stations. The estimated kappa(0)-values are found to range from 0.032 to 0.097 s at surface and 0.012 to 0.078 s in borehole. In comparison with a study of site corrections for earthquake magnitude determination, these higher kappa(0)-values associated with negative station corrections can correspond to the effect of soil conditions. To evaluate station corrections of borehole seismic array, we collected records from the cosite stations of Taiwan Strong-Motion Instrumentation Program (TSMIP) and Central Weather Bureau Station Network (CWBSN) and then derived a linear relationship between the kappa(0)-values and station corrections. Finally, both resulting station corrections and kappa(0)-values reveal high coherence in local site conditions.