GTSM in Taiwan

 

Background

Three GTSM borehole tensor strain instruments were fabricated and deployed in south western Taiwan in 2003 as the initial components of a monitoring network for the Central Geological Survey. This initiative is part of its intensified earthquake-monitoring program following the Chi-Chi Earthquake. The instruments are intended to supplement the deformation observations from the GPS arrays as part of a systematic program of observation of plate boundary tectonics of the area. The array is to be supplemented by an additional set of three sites to provide some coverage of the northwestern industrial region of Taiwan to be deployed in 2005.

The project also included extensive training of CGS staff for data retrieval and analysis, and training of deployment staff from Groundmat Constructions so that future deployments and all data analysis can be performed by Taiwan research interests. Deployment and logistical support by Groundmat has been world class, indicating the future and independent progress of this project will be possible without problems.

All sites are performing well and have already captured aseismic and seismic data on local and regional events.

The Central Geological Survey of Taiwan deployed three Gladwin Tensor Strain Monitors in October 2003, in south western Taiwan in the area of the Tsengwen Reservoir, to the south of the Chi-Chi Earthquake. The instruments installed are intended to supplement the deformation observations from the GPS arrays as part of a systematic program of observation of plate boundary tectonics of the area. The array is to be supplemented by an additional set of three sites in the northwestern industrial Hsinchu region of Taiwan to be deployed in 2005

The CGS initiative is an extremely important component of future earthquake research studies into regional tectonics and the predictability of major earthquakes and places Taiwan marginally ahead of a similar major USA research initiative (The Plate Boundary Observatory) by the National Science Foundation.

The approximate locations of the instruments installed in late 2003 are identified below.

The instruments were installed in expansive grout in 140 mm diameter boreholes approximately 200m deep at levels determined from cores. The surface installation includes a data management and control system, solar and mains power, rain gauges, atmospheric pressure sensor, thermometers, a remote communications system and various instrument 'state of health' monitoring systems.

Tectonic setting (Google map)for the GTSM sites in Western Taiwan installed in October, 2003, for Northern Taiwan installed in May, 2005 and Central Taiwan in March, 2006

 

High Frequency Data
An example of a triggered recording is shown. These triggered data are recorded at 100Hz at the RNT site and show the deformation field associated with the Mw 6.0 event of May 19, 2004. The origin time (identified by CWB EAR) was 2004 5 19 7: 4:12.00. The hypocenter of this event was 22.70 N 121.39 E at 8.68 km, with a centroid depth of 19 km.

High frequency data from RNT for the Mw 6.0 event on May 19,2004. Nineteen minutes of record are shown.

The first arrivals at this site are shown below. This shows the first 90 seconds of the event that lasted for a total of nineteen minutes. The surface wave dispersion effects for the various components are obvious in the record.

Arrivals for the first 90 seconds at RNT for the previous record . Vertical scale approx 10 microstrain.

 

The three prototype installations of GTSM borehole tensor strain systems in Taiwan are fully operational. These were prototype instruments (the first produced since 1992) and have proved highly successful.

 

The data gathered to date are a valuable extension of the current seismic, GPS and hydrology observation networks in the region. They will provide, for long period data, a high-resolution complement for the GPS arrays already installed (Gladwin et al, 2000). The instruments measure strain with a dynamic range of at least 10-4, and are essentially flat to DC, so that they can directly provide displacement data not normally available for analysis. It is expected that these data will contribute to a better understanding of dynamic rupture effects, and earthquake source mechanisms in Taiwan.

The program was enabled under the auspices of the Central Geological Survey of Taiwan under Director Chao-Chung Lin. Project manager for the project was Amigo Wang of Groundmat Constructions, whose teams provided superb drilling and installation support, and are providing ongoing field support needed to ensure uninterrupted data sets. Instruments were fabricated at CSIRO Exploration and Mining Division. Marie Mee (also then of CSIRO) provided preliminary analysis of the data.