A Local Magnitude Scale from Borehole Recordings with Site Correction of the Surface to Downhole

The Central Weather Bureau of Taiwan deployed 62 borehole seismic stations equipped with accelerometers and broadband seismometers by the end of 2018. Currently, a large number of earthquake records has been accumulated because of the high seismicity rate of Taiwan. These records can be used to detect microseismicity because the background seismic noise level is lower at downhole stations than surface stations. However, the magnitudes of the waveforms recorded by the downhole stations are lower than those of surface stations because of near‐surface site effects. In Taiwan, local magnitude (⁠$ML$⁠) determinations use an attenuation function derived from surface stations, and $ML$ will be underestimated if this attenuation function is used for downhole stations. Records of 14,653 earthquakes were used to determine site amplifications corresponding to site corrections of the surface to downhole. For all available earthquake–station pairs, a total of 5470 amplification factors satisfied condition of the incident angle less than 35°, which are used for further discussion in this work. The results reveal that amplification factors ranging from 1.11 to 6.07 can be used to describe site effects. These amplification factors are strongly correlated with $VS30$ (average shear‐wave velocity for the top 30 m of strata). The amplification factor was used to revise station $ML$ for downhole accelerometer (denoted $MLB$⁠), and revised $MLB$ (⁠$MLBNEW$⁠) was compared with $ML$ from the surface accelerometer (denoted $MLS$⁠). Results show that $MLBNEW$ and $MLS$ are similar. Using the derived site corrections of surface to downhole, the downhole seismograms can be used to directly determine $ML$⁠, especially for the borehole broadband waveforms, enhancing detection capabilities for microearthquakes and improving the Taiwanese earthquake catalog.