Tectonic Motion in The Pacific Basin

General Comments - Reference Frame

The horizontal components of tectonic motion that have been estimated from the GSFC analysis of SLR data are plotted as white vectors on this map. The blue vectors represent the motions for the sites as given by the NUVEL-1A geologically determined plate motion model. The frame within which these motions are placed is defined mathematically such that, in an integrated sense, there is no uniform net rotation remaining (called a "no net rotation" frame, or NNR frame). One should keep in mind that this is a mathematical idealization and that these motions only approximate the velocity relative to a fixed mantle.

Broad Description

There are six SLR stations plotted on the immense area of this map. We will treat each of these individually.

Simosato, Japan

The results for the motion of the SLR site at Simosato, Japan is of important interest, especially in light of the recent earthquake in nearby Kobe, Japan. Regrettably, the map above doesn't show the SLR result very clearly as it is hidden beneath the cir cle which represents the site, but it is, within this frame, moving south-southwesterly at about 2 mm/yr. If again, we do the thought experiment of visualizing what the vector would be relative to NNR-NUVEL-1A motion for Eurasia (as shown by the blue vect or), we see, by imagining the vector between the tip of the blue vector to the tip of the SLR vector, we get a vector trending northwest at about 30 mm/yr. It can be shown that the Pacific plate and Philippine plate motions have a similar directions in th e region offshore and across the trenches east and south of Japan. What are the implications then? What the SLR results tell us is that Simosato is experiencing a significant amount of strain from the offshore collision of the! Philippine plate with Japan and the Eurasian plate. This strain is being carried through the region surrounding Simosato to a point further inland where it is accumulating. The underlying cause for the earthquek in Kobe is thought to be related to this p rocess.

Shanghai, China

At Shanghai, the motion recovered from the SLR analysis deviates from that expected from the NNR-NUVEL-1A model for Eurasia motion. The uncertainty for this result is rather large which at this time precludes giving a definite tectonic interpretation. The SLR sustem at Shanghai is a neighbor to a VLBI system which has had a very reliable observation campaign.

Yaragadee and Orroral, Australia

There are two SLR sites situated on both coasts of Australia. These are Yaragadee in the west and Orroral in the east. The SLR results for both of these sites exhibit motion that is in very good agreement with that expected from the NNR-NUVEL1A model for Australia motion.

Huahine Island, French Polynesia and Maui Island, Hawaii

The SLR determined motion at Huahine is also in very good agreement with NNR-NUVEL-1. Maui is used as one of the base-sites in the analysis where the reference frame is defined. The north component of motion for the Maui site is adopted at the outset to b e equal to it's Pacific motion given by the NNR-NUVEL-1A model. Additionally, a site in North America is constrained as well (Greenbelt, see the North America map). However, the east/west component of motion at Maui is free to be estimated from the SLR tracking.

Mark Torrence Mark.H.Torrence.1@gsfc.nasa.gov
Raytheon Information Technology and Scientific Services Corporation

Responsible NASA Official: Ben Chao
Original Web Implementation: John W. Robbins

Updated: February 26, 1998
By: Jennifer Wiser Beall

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