Geoscience Reference
In-Depth Information
(5) the ellipsoidal axes (a, b) coincides with the X and Z axis, respectively (Leick
2004
, Hofmann-Wellenhof et al.
2008
).
The realization of a geocentric datum can be classified into three hierarchical
levels: global, regional and national. The global realization of a geocentric datum
is represented as the foremost datum, followed by the regional and national geo-
centric datum whereby each realization of the geocentric datum is essentially
consistent with the International Terrestrial Reference System (ITRS). The ITRS is
realized through the International Terrestrial Reference Frame (ITRF), which
represents the best available global geocentric datum. There are several
improvements that have been continuously made in the data analysis strategy to
achieve the optimal solution for the generation of ITRF (Altamimi et al.
2005
).
The ITRF takes into account the spatial and temporal variations of its network
coordinates and their velocities due to the effects of tectonic plate motion, earth
orientation and polar motion (Altamimi et al.
2008
, Janssen
2009
, Johnston and
Morgan
2010
). This is achieved by updating and refining its frame regularly.
Presently, the version of the ITRF is ITRF2008. Additionally, the ITRF is an
indispensable reference that is required to ensure the integrity and inter-operability
of Global Satellite Navigation System (GNSS) (Altamimi et al.
2008
).
The Geocentric Datum of Malaysia (GDM2000) was adopted by the Department
of Survey and Mapping Malaysia (DSMM) to establish a global and homogeneous
coordinate system across the country. The realization of the GDM2000 was based
on the ITRF2000 at epoch 1st January 2000. However, the GDM2000 remains as a
static datum where all site coordinates are fixed or assumed unchanged with time. In
fact, the Earth is actually dynamic and experiences numerous deformation events
such as plate tectonic motions and earthquakes. The tectonic plates move gradually
with the velocity typically varying in many places, up to a few centimetres annually
(Anderson
2012
). Moreover, a strong earthquake will cause a significant land
displacement ranging from decimetre to meters, depending on the site distance
from the epicentre.
Malaysia is situated within the Sundaland block. Previously, the Sundaland
represents a stable tectonic block, moving approximately east with respect to
Eurasia plate at a velocity of 12 ± 3 mm per year (Michel et al.
2001
) as shown in
Fig.
1
. However, since the mega earthquakes in Aceh (2004), Nias (2005) and
Bengkulu (2007) the country has experienced significant land displacements
(Simons et al.
2007
, Chlieh et al.
2007
, Socquet et al.
2006
, Banerjee et al.
2007
).
According to Vigny et al. (
2005
), the 2004 Aceh earthquake had significantly
resulted in land displacements up to 10 cm in magnitude for a radius of 400 km
away from the earthquake epicentre. Figure
2
shows the post-seismic and
co-seismic motions in Peninsular Malaysia from 2004 until 2008 which is the
combination of the aforementioned earthquakes and plate tectonic motion (Omar
and Mohamed
2010
). These studies have indicated that Malaysia is no longer in a
stable region.
These earthquakes and plate tectonic motions dislocate the GPS reference
stations, thus affecting the geocentric datum causing it to be no longer geocentric
(non-geocentric) and does not represent the ''true'' position of the points. The
