Plate Tectonics: Support for Steady Plate Motion

Plate tectonics has been an evolving theory for several years. Now, a relatively accepted scientific theory, technology has leaned more and more towards finding new and more precise ways of tracking and measuring plate motion. To explain the way plates moves and their velocities, science have used the plate tectonic model NUVEL-1. This model, like earlier ones, comes from three types of data: 1.) Spreading rates estimated from magnetic anomaly spacing, giving the amount of motion; 2.) The azimuths of submarine transform faults, giving the direction of motion; and 3.) The azimuths of slip vectors calculated from earthquakes along a trench or transform faults.

Until recently, we have not been able to fully understand Pacific-North American plate motion. There have been several methods introduced which give us a better understanding of plate velocities and directions of motion. They have also give us some clues that the San Andreas does not take up all of Pacific-North American motion. Among these are lunar laser ranging, satellite laser ranging, NUVEL-1, and Very Long Baseline Interferometry (VLBI).

METHODS

NUVEL-1 is a plate tectonic model utilizing azimuths or trends of transforms, earthquake slip vectors and spreading rates from magnetic anomalies. From these data are extrapolated plate velocities (from magnetic anomaly spacings), and motion directions (from trends of slip vectors from earthquakes). NUVEL-1 data can measure plate motion over millions of years. Other methods of finding plate motions include SLR (Satellite Laser Ranging) and LLR (Lunar Laser Ranging). Both these methods utilize lasers to sight plate motions. But the method that is becoming more widely used is VLBI.

VLBI utilizes radio waves to calculate the data much like the other methods with the exception that VLBI is said to be more accurate.

"In a geodetic VLBI experiment, two to seven radio telescopes simultaneously record radio noise from one quasar at a time and eventually record noise from 5 to 15 quasars. The radio noise signals recorded at different telescopes are correlated to determine delays and delay rates between telescopes. Delays and delay rates are used to determine site locations, the orientation of the Earth, and azimuth's to quasars. (Argus and Gordon, 1990)"

Argus and Gordon further explain the mathematical process to find plate motions.

"A baseline vector B equals the vector difference () between geocentric vectors measured to tow sites. Components of each baseline are defined relative to a priori estimates of geocentric vectors to the two sites, X1 and X2:

L = X2 - X1

T = L * X2

The length component of each baseline is determined by projecting the measured baseline B onto L, and the transverse component is determined by projecting the measured baseline onto T. Here we use the rates of change of the length of the transverse components to estimate relative plate motion. (Argus and Gordon, 1990)"

In some instances, discrete rotation and strain is calculated from VLBI baseline data. These data form ellipses which help model rotation and strain within plates. Kroger et al (1987) explain how the VLBI data is used in this manner.

"Essentially, the VLBI baselines used in the analysis define a planar region in which deformation is parameterized by a set of horizontal displacement gradients and two tilt parameters. The time dependence of these parameters is modeled by a linear expression consisting of an epoch value (of no physical significance) and a rate. The changes in the baseline coordinates, referred to a local east-west-vertical system, allow the values of the displacement gradient rates ant tilt rates to be estimated."

But now, with larger VLBI databases this model can be expanded to include more regions; and strain models for these areas can be found simultaneously. According to figure * we can see the strain model solutions showing plate motion and rotation.

RESULTS

Since VLBI is becoming more widely used because it offers "an accuracy of about 1cm. (Argus and Gordon, 1991)", there has been a search to find close similarities between VLBI data and NUVEL-1 data. With these similarities, if they lie within accepted confidence parameters, evidence can be found of steady plate motion over millions of years. This is evident in respect to NUVEL-1 data measuring motion over spans of millions of years and VLBI data measuring spans of years to thousands of years. If these can be correlated and reasonable similarities found, a case can be made for steady plate motion being constant over a period of several million years.

Once again Argus and Gordon (1990) gives the evidence to make the connection between NUVEL-1 data and VLBI data. Referring to figure &:

"The VLBI Euler Vector is nearly identical to the NUVEL-1 Pacific-North America Euler Vector ( a rotation of 0.783°/m.y. about 48.7°N, 78.2°W, see figure &). The VLBI Euler Vector has a rotation rate only 0.023°/m.y. faster than that of the NUVEL-1, and the VLBI Euler pole lies only 2° from the NUVEL-1 Euler pole. Furthermore, the three-dimensional 95% confidence ellipsoid of the VLBI Euler Vector includes the NUVEL-1 Euler Vector."

From this account it can be seen that it is likely that plate motion has been steady for several million years.

"...Pacific-North America plate motion has been steady or nearly steady over time spans differing in length by a factor of 1,000,000. The agreement between rates averaged over vastly different intervals gives strong support to a key assumption made in kinematic models that combine rates averaged over intervals as long as millions of years with rates averaged over intervals as short as a few years. (Argus and Gordon, 1990)"

DISCUSSION

It seems logical that plate motion has been steady for several million years, or that it should be steady. Evidence from Argus and Gordon (1990) has given a good argument for this. On the other hand, it would also seem likely that plate motion would fluctuate constantly, like a car does traveling great distances, but no evidence or data was found to support this hypothesis.

The San Andreas Fault has been stated before to take up all of the plate motion in the Western U.S. This can't be true. According to Argus and Gordon (1991), "Where the San Andreas fault is long and straight in Central California, most but not all of Pacific-North America motion is taken up by slip along the fault." Also it was recognized in earlier works, like that of Atwater (1970); "...that slip along the San Andreas fault in Central California is too slow to take up all Pacific-North America motion... (Argus and Gordon, 1991)"

Values for San Andreas slip rates hover around 34+\-2 mm/yr. and Pacific-North America motion is around 48+\-1 mm/yr. (from Argus and Gordon, 1991). This is more evidence that the San Andreas fault could not take up all Pacific-North America plate motion.

From the VLBI (and NUVEL-1) data found, it can be seen that there is motion being taken up with North American plate, whether it is around the Sierra-Nevada or further eastward in the Basin and Range. The strain models seen from Kroger et al. (1987) show that there is indeed motion strain being taken up elsewhere.

CONCLUSIONS

From VLBI and NUVEL-1 data it has been proven that plate motion has been steady for the past several million years. In the future ways may be found to closer examine this theory with greater accuracy. The San Andreas Fault is no longer the mystery it once was. Evidence has been given that disproves all plate motion stress being handled by the fault. People like Argus and Gordon, Atwater, Kroger and many others have found and are finding more evidence that support the theory that there is indeed strain being filtered through other parts of the plate in response to the boundary between the Pacific plate and the North American plate.