GIAC² project

1 Tectonic Settings
2 Reconnoissance Field Trip
3 Monumentation
4 Measurement Planning
5 Future work
6 Site descriptions

1 Tectonic Settings

The Sagaing fault is a major right-lateral strike-slip fault running North-South in Central Myanmar. This fault accomodates all or part of the differential motion between India and Eurasia (more precisely the Sunda platelet) in this area.
In order to study the Sagaing fault and analyze the local deformations and seismic hazards, a local GPS network was installed in the area. This local network is tied to the main GEODYSSEA network, in order to be able to analyze local displacements in the frame of the large scale plate tectonics. This is particularly important around faults where a reference velocity is needed for the interpretation of the local deformations.
This local network is made of three transects across the Sagaing fault at the latitude of Mandalay. One long transect (10 points, 150 km) sample the deformation in between the 2 north points of the GEODYSSEA network. Two shorter transects (4 points, 60 km each) sample the deformation 30 km North and South of the main transect. The displacements between points from one given transect will allow to localize the position and constrain the locking depth of the active fault. The displacements between points along the fault will allow to detect active segments of the fault and measure co-seismic motions, in case of an Earthquake in this area of the fault.

2 Reconnoissance Field Trip

The field trip took place between February 14th and March 2nd (16 days), 1997. During those 2 weeks, 18 sites were installed, half of which required concrete monumentation. The success of the geodetic mission can be attributed to the dedication of everyone in the team despite bad road conditions, car brake down, driller failure, etc...

The reconnoissance and installation team was made of the following individuals :

3 Monumentation

The 18 sites for such a local network don't need to be as cautiously chosen and monumented as the 4 sites of the large scale network. The destruction of any of them represent a much lighter loss for the network. Therefore only one main marker per site is implemented, which does not require large benchmarks or tie measurements. The markers, provided by IFAG (now BKG, Frankfurt, Germany), are 12 cm thread marks made of brass and copper. They are fixed in the rock using a special stone-metal adhesive. The top of the marker is designed in the form of a screw thread protected by a removable cap. During the occupation of a site, the cap is removed and the GPS antenna is screwed on top of the marker, by the mean of a threaded adapter, which enables the antenna to ba attached directely to the monument. In this way, antennas can be relocated with an accuracy of about 0.1 to 0.2 mm, so that eccentricity errors are not a concern. Whenever possible, the markers were sealed in hard bedrock outcrops. Unfortunately, in some areas around Mandalay (mostly on the western part of the fault), bed rock is almost unexistant. In such situations where bedrock could not be found, concrete benchmark were built (dugged into the ground), and the marker was sealed on top of the benchmark. The site locations were chosen so that the final transects are as close as possible to straight lines running perpendicular to the fault. The interval between points is smaller when close to the fault since the maximum relative displacement gradients are expected there. On the main transect, the end points are around 70 km appart from the fault which is expected to be far enough to be representative of long term tectonic motions, unaffected by visco-elastic effects.		marker and antenna binding
		Concrete benchmark sketch map
		Sketch map of the network: black dots for concrete benchmarks (8), grey dot for boulder (1), white dot for bedrock (9)

Complete site descriptions were written for all sites, based on infomations collected on the field. Those description include the site position, precise directions to the site, detailed access informations, and a brief geological description of the environment. At all places, the local responsable person was contacted, informed and given the responsability for long term conservation of the monument.

4 Measurement Planning

The Local network was measured immediately before the main GEODYSSEA network, using the GPS equipment shiped to Myanmar by IFAG for the main campaign. Such agreements have already been carried out in the Philippines or Indonesia and revealed very efficient in saving time money and management. The measurements took place during 21 days, from Tuesday November 3^rd to Tuesday November 24^th. During this time span, an absolute prioruty was given to the GEODYSSEA network (Taunggyi, Mindat, Hpa-an, Launggyo), which must be measured for 5 continuous days from November 19^th 08:00 AM to November 24^th same time. Priorities were then established as following :

1. The 4 GEODYSSEA points
2. The main Transect (points 01 to 10). One good transect measurement should be achieved before moving to the next ones. The Main transect is more important because it contains more points and they span a larger area, far away from the fault trace.
3. The North and South transects (points 11 to 14 and 15 to 18). All those points are equivalent in importance and any modification should provide a minimum of measurements at all points. In case of problemes, it is better to measure two points only 12 hours each, than one for 24 hours and forget the other one. The minimum time span under which the observations are almost useless is about 6 hours.

Since the distances between points are short, only 1 day of continuous measurements on each site is required to achieve millimeter accuracy. Nevertheless, redundant measurements are required in order to realize ties between the different figures of 4 points. Therefore, throughout the campaign, 3 points are hold fixed: Throughout the campaign, 3 points are hold fixed :

• Mandalay polo ground (point 07) is left unattended and operates on its own
• extreme points (01 and 10) don't move to provide a fix figure

the 2 remaining receivers, row from one point to the other.


The local campaign lasted 13 days. The most important baseline in the network being the longest one across the fault,(in order to get the far field displacement), we choose to hold points Kweh-Taing-Taung (KWEH-01) and Ywe-Nge (YWEN-10) fixed. Additionaly, the spare receiver as left acquiring data unattended at Mandalay polo ground (MDPG-07). This receiver could have been requested to replace any other one any time anywhere in case of brakedown (which indeed happpened later).

During the reconnoissance and installation field mission, it was discovered that an old triangulation network existed in the Mandalay area. If the classical geodetic type measurements are still accessible (angle and distance measurements), it is possible to recompute the position of these pillars back in time with a reasonnable accuracy. Such experiences were conducted on the hold french triangulation networks and proved feasable. In such case, it would be very profitable to re-occupy the still existing pillars by modern means (GPS) to infer their present day positions. given the millimitric accuracy of todays methods, and the very long time span elapsed since the first classical measurements (more than 50 years), rates up to a couple of cm/yr could be easely detected. Such a work requires very little effort, since the old geodetic pillars could be surveyed by GPS very easely, during and/or following the GPS network survey. the only serious requirements are :

• enough pillars still exist and can be found
• old classical measurements still exist and are accessible

Another interesting possibility would be to take advantage of the existing railway track (from Monywa to Pin Oo Lwyn) to measure a topographic profile across the Fault at the latitude of the main transect. Such a profile can be measured very easely and accurately by GPS kinematic methods. One receiver is hold fixed at a reference point (eg. Mandalay Polo ground) and a mobile receiver can be set up on the roof of a railway car. Measuring frequently the position of both receivers (eg. every second), one can reconstruct the trajectory of the mobile with respect to the fixed one, with a centimetric accuracy. From this trajectory one can extract the profile (alitude vs. position), usefull for tectonic interpretation of the area. As a bypass product, we would obtain the railway track path with the same centimetric accuracy, which could even be used in case of an earthquake in the area to measure the deformation pattern (if the track is not too much damaged).
Again, such a work requires very little effort, since those kinematic measurements require only 2 geodetic type GPS receivers for one day during and/or following the GPS network survey. the only need is an authorization to use the roof of a railway car during one journey from Monywa to Pyn Oo Lwyn.


6 Site Descriptions