2012 seismicity quiescence in Taiwan a result of site-effect artifacts

A significant seismic quiescence in Taiwan was reported by Wu and Chiao (2006) before the occurrence of the 1999 M_w 7.6 Chi-Chi earthquake. A similar kind of activity was also observed in the 2012 earthquake catalog reported by Central Weather Bureau (CWB). Seven months in the 2012 catalog seem to have lower monthly seismicity rates than the one standard deviation below the mean (673 i 82) for earthquakes with ML ≥ 2:0. Here, we checked the seismic network in Taiwan and found that some new seismic stations, including the Broadband Array in Taiwan for Seismology (BATS), were incorporated within the Taiwan Central Weather Bureau Seismic Network (CWBSN) since 2012. Most of these BATS stations are located on hard-rock sites, which may affect magnitude estimation because of their site characteristics. To account for the impact of site effect, earthquake catalog data during the period of 1994 to 2014 was collected and station correction was calculated for each station. A strong correlation is found between station corrections determined in this study and geological settings of the region. Stations located on soil sites have high amplifications with negative station corrections. On the other hand, stations located on hard-rock sites have low amplifications with positive station corrections. After applying station correction, the mean seismicity rate is found to be 716 events per month with a standard deviation of 76 events for earthquakes with M_L ≥ 2:0. We conclude that the reason behind the apparent low seismicity in 2012 is due to the installation of new seismic stations on rock sites, which lead to underestimation of ML.