Friday, May 04, 2012

GDC: GPS Date Converter

GPS Date Converter (GDC) is a small utility to convert six different date formats, including: (1) decimal year, (2) year, month, day, (3) year, day of year, (4) GPS week, day of week, (5) Julian date, and (6) modified Julian date.

Monday, April 30, 2012

Geological setting of the Saint Lawrence valley

The Saint Lawrence valley region is located in an intra-plate tectonic setting since about 200 MYBP but has been the locus of several major tectonic events in the last approximately 1 GYBP

Tuesday, February 21, 2012

Seismicity in the Saint Lawrence valley

Canada east of the Cordillera, extending north from the US border to the Arctic Ocean, comprises about two-third of the stable craton of the North American plate. Much of this large area appears to be substantially aseismic, although it contains several zones of significant seismicity and a few other regions of lower-level seismicity.

Wednesday, February 08, 2012

The stable North America reference frame

In 2003, a group of scientists was established to determine the stable North America reference frame (SNARF). They aimed to define a regional reference frame that is consistent and stable at the level of sub-mm yr-1 across North America (Henton et al., 2006).

Friday, February 03, 2012

RADAR interferometry

Standard synthetic aperture RADAR interferometry (InSAR), is a method which is applied to phase component of two SAR images acquired from relatively close positions to form the interference patterns between the two images. If the Earth surface deforms between the two acquisitions, the surface displacement with sub-centimetre accuracy can be theoretically mapped in the absence of atmospheric artefacts (Sarti et al., 2006).

Wednesday, February 01, 2012

Geodynamics and the mean sea level

The long term periodic, and particularly, the secular changes in the mean sea level (MSL) are important in geodynamics when only linear movements are considered.

Thursday, January 26, 2012

Precise levelling in Canada

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.

Tuesday, January 24, 2012

Other limitations of geodetic methods

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 horizontal motion

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 problem in the Fourier domain filtering methods

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 problem in the assumption about the existing noise

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

The MRNF GPS network

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).

Problems in the current reference frames

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

The problem in determination of earthquakes in intra-plate regions

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.

Resolution of geodetic methods

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.

Limitation of single-solution methods

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).

The problem in the campaign data for studying geodynamic processes

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.

Ongoing research in the Saint Lawrence valley

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).

Geodynamic processes in North America

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

Common Mode Filtering for Noise Removal

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.