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X - 8 SIMONS ET AL.: GPS MEASUREMENTS IN S.E. ASIA TO REFINE SUNDALAND MOTION
timated by using a small velocity residual criteria (3 mm/yr) and a
well-balanced choice out of over 40 sites in both the core and the
boundary zones of Sundaland. Although the GPS vector field presented
here provides good coverage on S.E. Asia, there is a higher
density of GPS points in Thailand and Malaysia. Therefore in a
first pole estimation, 17 sites with similar spacing within Sundaland
were selected. Initially, also all stations were equally weighted to
minimize the effect of any bad fitting site(s) with small velocity
sigmas (eg. permanent GPS sites). Only sites that had horizontal
(normalized) residuals smaller than 3 mm/yr were kept into the
computations. In subsequent steps, additional sites that fit well the
initial pole(s) were added to the pole estimations. This led to a
Sundaland pole based on 28 of the 40+ available GPS velocities.
Almost all the rejected points were located in the boundary zones of
Sundaland with its neighboring plates. In a final step, the computation
was repeated, only this time the selected points were weighted
according to their individual velocity errors. This is also necessary
to get a realistic error estimate on the pole location and rotation
rate. Furthermore, the more accurate velocities should be allowed
to contribute relatively more to the pole estimation. In any case, the
pole location and rotation rate remained stable throughout all the
subsequent software runs.
With the expanded GPS vector field, the Sundaland block rotates
clockwise at a rate of 0.341Æ/Myr about a pole located 48.9ÆS and
85.8ÆE in ITRF-2000. The (3-error on this pole is 2.01Æfor
the long axis of the ellipse, oriented 21ÆNNW and 0.24Æfor the
short one. The uncertainty on the rotation rate is 0.006Æ/Myr. The
ellipse error is obtained using the method described by DeMets et al.
[1994]. The residual velocities of all Sundaland sites with respect to
this pole are shown in Figures 5 in 6. For the 28 sites included in the
pole estimation, the residual velocities are all below 3 mm/yr (circle
radius in Fig. 5), and more than half are smaller than 2 mm/yr. The
excluded sites are almost all located in deforming regions near the
Sundaland boundaries (Sabah province in East-Malaysia, Central
Java, Bali, Sumatra and northwest Thailand/Myanmar).
4.2. Pole comparison and history
Although the initial results of the GEODYSSEA project [Wilson
et al., 1998; Simons et al., 1999] were plagued by not-well
Table 7. Overview of published Sundaland poles.
Reference Reference Sites Pole Rotation parameters
frame used Lat(ÆS) Lat(ÆE)Æ/Myr)
GEODYSSEA
Wilson et al. [1998] ITRF-94 12 31.8Æ 134.0Æ-0.280Æ
Simons et al. [1999] ITRF-96 12 43.0Æ 119.0Æ-0.370Æ
Michel et al. [2001] ITRF-97 10 56.0Æ 77.0Æ-0.339Æ
This paper ITRF-00 28 48.9Æ 85.8Æ-0.341Æ
Others
Sella et al. [2002] ITRF-97 3 38.9Æ 93.1Æ-0.393Æ
Kreemer et al. [2003] NNR 9 47.3Æ 89.8Æ-0.392Æ
Bock et al. [2003] ITRF-00 16 49.8Æ 84.1Æ-0.320Æ
Table 8. Differences in predicted motions w.r.t. previous poles.
Differences horizontal motions (mm/yr)
Indochina Sunda Shelf South Borneo
Reference 15ÆN105ÆE 5ÆN110ÆE -5ÆN115ÆE
North East North East North East
This paper (ref. val.) -8.2 31.8 -10.2 30.5 -12.2 26.6
GEODYSSEA
Wilson et al. [1998] +21.1 -11.9 +21.0 -12.0 +20.8 -12.4
Simons et al. [1999] +15.5 +0.9 +15.0 +0.1 +14.2 -1.3
Michel et al. [2001] -1.7 +1.3 -1.3 +2.2 -0.8 +3.1
Others
Sella et al. [2002] +1.2 +1.4 +0.3 -0.3 -0.5 -2.0
Kreemer et al. [2003] +0.5 +4.6 -0.1 +3.9 -0.4 +3.0
Bock et al. [2003] +0.1 -1.9 +0.2 -1.6 +0.4 -1.2
constrained mapping issues into ITRF-94/96, an adjustment of the
velocity vectors after only 2 measurements campaigns already confirmed
and provided some good insights in the relative motion of
Sundaland with respect to Eurasia [Chamot-Rooke et al., 1998].
[Michel et al., 2001] included the 3rd observation round of the
GEODYSSEA project, and provided a further refined Sundaland
pole vector in ITRF-97.
In the last 2 years, 3 papers have been published which present
a ’new’ Sundaland pole. The Sella et al. [2002] paper does this by
using only 3 IGS sites (BAKO, NTUS, KUNM), of which 2 not yet
have very accurate velocities in ITRF-97 and 1 is not even located on
Sundaland. The second paper is by Kreemer et al. [2003], which has
made use the geodetic solution of Michel et al. [2001], and considered
8 GEODYSSEA sites on Sundaland and added the IGS station
Singapore. A third paper by Bock et al. [2003], presents results on
the Indonesian archipelago, and thereby also defines a Sundaland
(there named Sunda Shelf) motion, based on Indonesian data for 10
sites, IGS stations BAKO/NTUS/TAIW in/near S.E. Asia and 3 IGS
sites located in South-China (SHAO/WUHN/XIAN).
The history of the angular velocities for Sundaland published both
by the GEODYSSEA project and others is given in Table 7.
In order to determine if these other Sundaland poles provide a
better (geodetical) description of the Sundaland motion than Michel
et al. [2001], it first is important to know how much different the
predicted velocities are within Sundaland. For 3 geographically
spread locations within Sundaland, the differences of the predicted
velocities with respect to the updated Sundaland pole presented in
this paper are shown in Table 8. Also included are the previous
Sundaland poles computed during the GEODYSSEA project.
In Table 8, clearly the mapping problems in ITRF-94 and ITRF-
96 can be observed. The Wilson et al. [1998] solution shows almost
constant offsets w.r.t. the solution in this paper which is taken as
reference. Mainly Helmert translations have offset the velocity vectors
that were used to compute the angular velocities. The second
GEODYSSEA solution by Simons et al. [1999] has already a better
mapping, but still the north velocity component generated by the
Sundaland pole presented there is off by about 15 mm/yr. These
offsets were caused by the use of only 5 IGS sites (YAR1, TIDB,
TSKB, TAIW, KIT3) for the mapping into the ITRF. At least 2 of
these sites (TAIW and KIT3) didn’t follow the linear ITRF-94/96
trend during the first 2 GEODYSSEA campaigns, thereby affecting
the mapping. The final geodetic GEODYSSEA paper by Michel
et al. [2001], the Sundaland pole appears to have stabilized quite
well, as the differences with the ’newer’ Sundaland poles are within
3-5 mm/yr. This was achieved by adding one more observation
round, data from additional IGS stations, and mapping the velocity
field into the ITRF-97. The predicted velocities using the 3 Sundaland
poles from the other 3 papers [Sella et al., 2002; Kreemer et al.,
2003; Bock et al., 2003] are within a few mm/yr (roughly the size of
their velocity errors) of the reference values that are obtained with
the Sundaland pole derived in this paper.
Furthermore, also the amount, location and the produced velocity
residuals of sites within Sundaland are an important factor to
determine the quality of the previously published poles. In combination
with the discussion about the final GEODYSSEA and the
’newer’ 3 Sundaland poles given above, the pole of Sella et al.
[2002] is based on the weakest velocity constraints. The Kreemer
et al. [2003] pole for Sundaland has predicted offsets that are similar
to the ones of Michel et al. [2001], but they are larger in the
east component throughout the longitudinal range. Probably the
reference frame (NNR) is not fully compatible with ITRF-2000 for
the Sundaland block, as the majority of the velocity estimates is
identical to Michel et al. [2001]. The Bock et al. [2003] paper underestimates
the east velocity component by 1-2 mm/yr, and the
pole stability appears to depend mostly on the inclusion of IGS sites
in N-S direction up to South-China, as the N-S spread of the local
Indonesian data is only a few hundred kilometers and most of the
Indonesian sites are in/near the Sundaland deformation zones. This
is confirmed by the relatively higher velocity residuals in the east
components up to 1 cm/yr for the Indonesian sites (including the