The precise levelling method is a well-known approach that has been used for more than 200 years (Gareau, 1986) and still provides the most accurate method for determining height differences over short distances. It involves making differential height measurements between two vertical graduated rods, approximately 100 metres apart, using a tripod mounted telescope whose horizontal line of sight is controlled to better than one second of arc by a spirit level vial or a suspended prism.
Thursday, January 26, 2012
Tuesday, January 24, 2012
Since the post “The problem in the campaign data for studying geodynamic processes”, I explained some limitations of geodetic methods for studying geodynamic processes, especially in the North American plate. Further to the listed problems in the sequential posts, there are still some more limitations in geodetic models and methods.
Monday, January 23, 2012
The GIA modest horizontal velocities can provide an important additional constraint to GIA models. GIA models generally predict a pattern of horizontal divergence away from centres of uplift (James and Lambert, 1993; Peltier, 1998, 2002).
Sunday, January 22, 2012
The relatively large seasonal effects, atmospheric signals, and numerous high-frequency components related to site effects make it difficult to observe and quantify the comparatively smaller signals caused by crustal motion (Tiampo et al., 2004).
The coordinate time series of the GPS stations usually show complex non-linear behaviour, and the associated noise is more complex than simple white noise.
Saturday, January 21, 2012
Over the last ten years, the Ministère des Ressources Naturelles et de la Faune (MRNF*) of Quebec, Canada, has been established a network of 26 permanent GPS stations, mostly across the south and south-east of Quebec province, in order to respond to its needs for positioning within its territory (Figure 1).
There are two basic problems in the current reference frames. One is discarding the underlying geophysical processes, especially in the Earth mantle, and the other is instability or evolution of reference frames in time. Both of these problems become crucial when rate of the deformation is very low.
Friday, January 20, 2012
Unlike plate margins where the seismic activity is directly correlated with plate interactions, eastern Canada earthquakes lie in the "stable" interior of the North American Plate. The driving mechanisms of intra-plate earthquakes are more difficult to determine.
While relative velocities across tectonically active regions are typically of the order of 5-50 mm yr-1 (that can be generally resolved with annual GPS campaign data over a three to five year period), relative velocities across intra-plate regions are typically less than 1 mm yr-1.
The contemporary GIA has been measured by several different methods and data sets individually, such as GPS (Milne et al., 2001), combining tide gauge and altimetry records (Braun et al., 2008), satellite altimetry over land (Lee et al., 2008) and absolute gravimetry (Lambert et al., 2001).
Mazzotti et al. (2005) have shown that by given certain assumptions about the pattern of crustal deformation in intra-plate environments, campaign GPS measurements over 6–9 years can provide useful constraints on the recurrence period and maximum magnitude of large earthquakes.
Since the 1960s geologists know that the Earth's surface is segmented into a number of tectonic plates that are in constant motion relative to each other at rates which are typically about several centimetres per year. These plates generally move as rigid blocks, however, at their edges there is a zone (often called a plate boundary) where they rub against each other and deform, causing earthquakes and other geophysical phenomena. Seismic activities are at the highest rate at the plate boundaries (Figure 1).
The interior part of the North American plate is subject to two important geophysical processes: glacial isostatic adjustment (GIA), and the intra-plate tectonic activities. GIA is the most major geophysical process and most of the Canadian landmass is currently experiencing vertical uplift associated with the GIA (except in Maritimes, south of Saskatchewan and Alberta). Plate tectonic is another geophysical phenomenon across the country which is more observed in four areas, including the western Canada subduction zone, the Queen Charlotte transform fault zone, the Yukon crustal deformation region, and eastern Canada region of high seismicity. Henton et al. (2006) have highlighted long term regions of interest for geodetic investigations across Canada including the Saint Lawrence seismic zone in eastern Canada, the active plate boundary above the Cascadia subduction zone along Canada’s west coast, and the active fault margin of the Queen Charlotte Islands as well as GIA-related deformation all over the Canadian landmass.
Sunday, January 15, 2012
The filter was first developed by Wdowinski et al. (1997) for data analysing of five sites of Permanent GPS Geodetic Array (PGGA) in the southern California to re-evaluate the far-filed displacements induced by Landers earthquake, 1992, in the post-processing step as anew spatial filtering technique. They aimed to distinguish between coseismic and short-term post seismic displacements of the PGGA sites affected by the earthquake. The Signal to Noise Ratio (SNR) of the estimated site positions was significantly improved by the filter,and consequently they could successfully resolve the total surface displacement into its coseismic and post-seismic components site-by-site, rather than by cumbersome analysis of relative displacements between pairs of sites. Concept of the filter has been elaborated in Wdowinski et al. (1997) as it was initially used by them for analysis of GPS data for the Landers earthquake, and a concise summary of the filter is given here from the same source with respect to GNSS data analysis.