1.1 Background
Taiwan, like many other regions in the western Pacific, is prone to attack from such hazards as landslide, typhoon and earthquake, which may create mass changes and in turn gravity changes. Such gravity changes are usually over a small area, and cannot be sensed by the gravity sensor of a satellite gravity mission such as GRACE, but might be detected by a highly-sensitive, ground-based gravimeter such as SG. In April 2006, two single-sphere observatory SGs (OSGs), serial numbers T48 and T49, were installed at tunnel B of Mt. 18-Peak in Hsinchu City, Taiwan. At the same time, two absolute gravimeters (AGs), serial numbers 224 and 231, were introduced in Taiwan. All the gravimeters belong to Ministry of the Interior (MOI) and setup in tunnels of the national gravity datum service (NGDS).
T48 and T49 were manufactured by GWR and have a nominal sensitivity of one ngal and a stability of 6 μgal year-1 or better (1 ngal = 10-11 m s-2). The HS SG station is now included in the SG network of GGP (Fig. 1-1). The GGP’s objective in the global geodetic observing system (GGOS) is to provide high quality SG data for geodynamic research. Most of the AG measurements collected with the two AGs (FG5 #224 and FG5 #231) are carried out by the laboratory of geodesy and geodynamics (LOGG, http://www.logg.org.tw/, Fig. 1-2). The latitude, longitude and elevation of LOGG are 24.79258 ºN, 120.98554 ºE and 87.6 m, respectively. LOGG is about 8.6 km from the Taiwan Strait, where the average depth is 80 m. Here the ocean tide amplitude and phase change rapidly (Jan et al., 2004).
HS is the closest station to the Tropic of Cancer in GGP and will be most sensitive to gravity change due to the motion of the earth’s inner-core in the summer solstice, making HS the best for testing the universality of free fall (Shiomi, 2006). Real-time
data of typhoons, earthquakes and continuous GPS observations around Taiwan, accessible at the central weather bureau (CWB, http://www.cwb.gov.tw/ ) of Taiwan, are used in connection to SG. The introduction of these gravimeters motivates this study, which will focus on the regional characteristics of Taiwan. Specifically, this study will emphasize gravity datum establishment, geodynamics and atmospheric events.
GPS is an important tool to aid the interpretation of gravity data and it will be also covered in this study. The study will monitor and analyze the mechanisms of gravity changes and GPS changes from 2006 to 2011. The events of typhoons and earthquakes affect gravity changes and may contribute most to gravity and geometric changes in some cases. They will be also investigated using SG, AG and GPS in this dissertation.
Fig. 1-1 Current and planned SG stations in the world, squares represent the new stations, circles the current stations, diamonds the planned stations (Described in the
ggpnews20, 2010)
Fig. 1-2 Layout of NGDS, T48 is installed at B2
1.2 Literature Review
There are many phenomena that cause environmental changes in Taiwan. This study uses gravimeters to monitor selected phenomena. Because of this, we need a gravity datum as the basis for analyzing the mechanisms of such phenomena. SG is highly sensitive to gravity changes due to solid earth tide, ocean tide loading, atmosphere, groundwater, soil moisture, tilt variation and other environmental changes (e.g. Warburton and Goodkind, 1977; Crossley et al., 1995; Dal Moro and Zadro, 1998;
Neumeyer et al., 2004; Boy and Hinderer, 2006). It is necessary to explain the physical significance of SG and to identify the standard operating procedure (SOP) for SG data processing. After removing data noises, including spikes, gaps and steps of SG, we use the parallel observations of AG and SG values for data comparison and calibration (Van Ruymbecke, 1989; Richter et al., 1995; Falk et al., 2001). Currently, the most commonly used technique for calibration of SG record is based on parallel observations of SG and AG (Sato et al., 1996; Francis et al., 1998; Imanish et al., 2002). If AG is not available, the CF of SG is usually determined by comparison between the
theoretical solid earth tide and SG raw measurements (voltage). The nominal drifting rate of SG is 6 μgal year-1 claimed by GWR instrument (Warburton and Brinton 1995), but the drifting rate could vary from one SG to another.
With SG, it is possible identify gravity changes of non-tectonic origins, such as those due to typhoons and earthquakes (Imanishi et al., 2002). The gravity-atmosphere admittances for various atmospheric conditions in typhoons will vary and is a potential application of SG (Kim et al., 2011). The impact of typhoon-generated gravity changes could be large during the developing stage than during the mature and decaying stages of the typhoon (Kim et al., 2011). About 90% of the atmospheric effects were attributed to local atmospheric variations within 50 km of the station (Mukai et al., 1995).
Selected co-seismic gravity perturbations have been detected and analyzed using SG to demonstrate the sensitivity of SG, and the SG results have been compared with those obtained by seismometers (Imanishi et al., 2004; Hwang et al., 2009; Kim et al., 2009;
Nawa et al., 2009). In addition, AG and continuous GPS measurements have been used to study mass transfer and vertical movement due to mountain building (Segall and Davis, 1997; Jacob et al., 2008).
1.3 Outline of dissertation
This dissertation is organized into 9 chapters. Chapter 2 is to present the principles of SG and AG, including an in-depth introduction of instruments, specifications and software. Chapter 3 describes data processing of SG, including filtering, CF, modeling the solid earth tide and ocean tide loading. Chapter 4 describes the effect of ocean tide loading, tidal analysis, and comparison with theoretical solid earth tide. Chapter 5 shows the environment effects on gravity observations. Chapter 6 presents the geological settings around the NGDS, it is the key work for establishing an AG reference and data preparation for the GGP. Chapter 7 shows the result from gravity
observations of the project “AGTO” in southern Taiwan, jointly conducted by France and Taiwan. The motivation of AGTO is to see mass changes due to middle-to-long term tectonic motion from repeated gravimetric and continual GPS measurements. The gravity and vertical trends of 25 AG stations and 313 permanent GPS stations will be presented. Chapter 8 presents an extensive discussion on global atmospheric and local atmospheric effects on gravity. The gravity effects from Typhoon Kalmaegi and Morakot are described in detail in this chapter. Finally, a summary, conclusions and suggestions are presented in the final chapter.