GPS determined eastward Sundaland motion with respect to Eurasia confirmedby earthquakes slip vectors at Sunda and Philippine trenches

GPS measurements acquired over Southeast Asia in 1994 and 1996 in the frame work of the GEODYSSEA program revealed that a large piece of continental li thosphere comprising the Indochina Peninsula, Sunda shelf and part of Indon esia behaves as a rigid 'Sundaland' platelet. A direct adjustment of veloci ty vectors obtained in a Eurasian frame of reference shows that Sundaland b lock is rotating clockwise with respect to Eurasia around a pole of rotatio n located south of Australia. We present here an additional check of Sundal and motion that uses earthquakes slip vectors at Sunda and Philippine trenc hes. Seven sites of the GEODYSSEA network are close to the trenches and not separated from them by large active faults (two at Sumatra Trench, three a t Java Trench and two at the Philippine Trench). The difference between the vector at the station and the adjacent subducting plate vector defines the relative subduction motion and should thus be aligned with the subduction earthquake slip vectors. We first derive a frame-free solution that minimiz es the upper plate (or Sundaland) motion. When corrected for Australia-Eura sia and Philippines-Eurasia NUVEL1-A motion, the misfit between GPS and sli p vectors azimuths is significant at 95% confidence, indicating that the up per plate does not belong to Eurasia. We then examine the range of solution s compatible with the slip vectors azimuths and conclude that the minimum v elocity of Sundaland is a uniform 7-10 mm/a eastward velocity. However, int roducing the additional constraint of the fit of the GEODYSSEA sites with t he Australian IGS reference ones, or tie with the NTUS Singapore station, l eads to a much narrower range of solutions. We conclude that Sundaland has an eastward velocity of about 10 mm/a on its southern boundary increasing t o 16-18 mm/a on its northern boundary. (C) 1999 Elsevier Science B.V. All r ights reserved.