Taipei Basin Strong Motion Downhole Array
In 1990, Institute of Earth Sciences (IES) in the Academia Sinica was contracted by the Central Geological Survey (CGS) in the Ministry of Economic Affairs to execute a project entitled “An Integrated Survey of Subsurface Geology and Engineering Environment of the Taipei Basin.” In order to provide basic ground motion data to measure the response of the soft soil deposits at the Taipei Metropolitan Area (TMA), it was decided to establish Taipei Basin Downhole Seismic Network (TBDSN) in this project. The plan is to select about ten downhole sites in the TMA to install downhole accelerometers from surface down to different depths. In the initial phase, four borehole sites were chosen because of accessibility and of variation in subsurface structure. The borehole was drilled at the Wuku Sewage Disposal Plant (WK) and at the Panchiao Water Conservancy Bureau (BS) in 1993, at the Panchiao Vocational Advisory Committee for Retired Servicemen (TF) in 1994, and at the Luchou water pump station (LC) in 1995. All are located in the western parts of the TMA.
The initial phase of the downhole observations in TMA was finished in June 1996. In order to continue providing basic ground motion data to evaluate site response effects in this area, the CGS decided to initiate the second phase of the program in the TMA to continue monitoring earthquakes.The new boreholes are placed at six different sites on the eastern side of TMA. They are at the Chongshin Bridge (HS) in the Sanchong City, at the Songshan Tabacco Plant (SS) in the Xinyi district, at the Minquan Park (MP) in the Songshan district, at the National Taipei Univ. of Science and Technology (TU) in the Daan district, at Bailin Park (BL) in the Shilin district and at Renai Park of Yengho City (YH). The downhole arrays at HS, SS and MP were completed in 1996, 1997 and 1999, respectively. The procedures of installation and hardware of borehole accelerometer are the same as the initial phase. The sensor at the bottom of the boreholes of SS and MP is below the bedrock interface. It should be noted that accelerometers were not installed at three drilled borehole sites of TU, BL and YH. The downhole network operated quite reliably until late 1998, when Taipei’s rapid transit Metro system was constructed and flood damage reduction improvements were carried out by the Panchiao and Lochou Water Resources Agencies, Taipei County Government. Four sites of the boreholes (BS, LC, HS and SS) were removed because of their locations in construction areas.
In February 2002, the CGS concluded an agreement with IES to move six downhole stations to IES. This includes three existing stations (WK, TF, MP) as well as the three drill sites (TU, BL, YH). To ensure continuity in monitoring seismic events in the TMA, IES contribution to the three drilling sites new instruments. Hardware installation procedures were the same as prior downhole arrays and completed in June 2003. The sensor at the bottom of the boreholes of BL and YH is below the bedrock interface.
In 2005, a new projects entitled “Strong Motions, Active Faults, and Earthquakes in the Taipei Metropolitan Area” (SAFE-Taipei) was initiated Academia Sinica. This project planned to add two boreholes as part of downhole seismic array to improve the ability for detecting earthquakes as well as for monitoring region seismic activity at the magnitude 3+ levels induced by faults nearby the TMA. Two additional accelerometer boreholes are at the Research Center for Environmental Changes hosted by IES (ES) and at 228 Park of Sanchong district (SC) were also constructed.
The strong-motion downhole array is a local network distributing TMA. The array is designed to improve resolution for monitoring seismicity at the target fault in the TMA and to provide high quality data for research projects in local, regional and teleseismic seismology. Since 2007 this array has grown from the original 4 boreholes installed in 1993-1995 to 8 boreholes. At present the TMA itself has relatively low seismic activity; however, it does provide IES with an observation tool to serve as a guide for future station installations in the Taiwan area.