- Email: [email protected]
On the rigidity of the North American plate
Tectonophysics, 84 (1982) Tl-T6 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Letter
Tl
section
On the rigidity of the North American plate
WILLIAM J. WEBSTER, Jr. ~eoFhys~cs Branch, NASA Goddard Space Flight Center, Greenbelt,
(Received October 27,198l;
Md. 20771 (U.S.A.)
revised version accepted January 27, 1982)
ABSTRACT Webster, J., Jr., 1982. On the rigidity of the North American plate. Te~~onophye~~s* 84: Tl-T6. Recently reported Very Long Baseline Interferometer (VLBI) observations between Green Bank, West Virginia, Haystack, Massachusetts and Owens Valley, California, show that the linear distance between these sites has not changed for at least three years (i.e., smooth rate of change lesg than 1 cm/yr). This level of stability is logically consistent with the hypothesis that the continental U.S. North American plate is geodetically rigid at least on the 2 cm level. Model studies show that thii result is in accord with present knowledge of the intra-plate stress field and the elastic properties of typical continental lithosphere.
As part of the preparation for the Crustal Dynamics Project (OSTA, 1979), an extensive series of VLBI observations between the National Radio Astronomy Observatory (NRAO) at Green Bank, West Virginia, the Haystack Radio Observatory (HRO) near Westford, Massachusetts and the Owens Valley Radio Observatory (OVRO) near Bishop, California was conducted (Knight, 1980). These measurements were developmental tests of the various parts of a new geodetic VLBI system called MARK-III (Coates et al., 1975). The expected precision of an individual baseline measured with MARK-III is in the vicinity of 3 cm for a California to Massachusetts baseline. Although the quality of the observations reported by Knight (1980) is not as high as with a full MARK-III system at each station, the long time sequence of data has important implications for the extent in distance of geodetic deformation on the North American plate. Allenby (1979) has discussed the tectonic significance of an earlier generation of VLBI observations on a baseline from HRO to OVRO. These observations imply a rate of change of length of no greater than 1 cmfyr. This was a surprising result since the baseline crosses several areas where changes of several centimeters would be expected, 0040-1951/82/000~000/$02.7S
8 1982 Elsevier Scientific Publishing Company
T2
Since the measurements discussed here were made from stations forming a triangle, north---south changes can be considered in addition to a second baseline roughly east-west. The sequence of observations discussed spans the period from September 1976 to May 1978 and includes fourteen measurements on the NRAO-HRO baseline and eight measurements on the NRAO-OVRO baselines. These data include the HRO-OVRO data discussed by Allenby (1979) as well as some additional measurements on this baseline. DISCUSSION
The observations discussed were taken with the MARK-I system during the testing of components of the MARK-III system (Knight, 1980). The antennas and microwave receivers employed were of comparable gains and noise temperatures. During the period of these observations, the performance of the MARK-I system was upgraded continuously. The improvement is most noticeable when the cable calibration system was added at NRA0 (Knight, 1980). Knight reported an improvement in accuracy corresponding to a decrease from * 15 cm to +3 cm in an individual baseline. For the 20 months during which all baselines were observed in common, the largest rate reported by Knight (1980) (Ryan et al., 19’78) is on the HRO-NRA0 baseline, -1.0 cm/yr f 1.2 cm/yr (nine baselines). In Table I, we give the rate determinations reported by Knight (1980) while in Fig. 1, we produce the time trends for each baseline. The longest time span is covered by the HRO-NRA0 data: 70 months. On no baseline do the data contain any evidence for a statistically significant change (Knight, 1980 contains a detailed discussion of this point). As part of the equipment development prior to the observations reported by Knight (1980), Rogers et al. (1978) made high precision observations on a 1.24 km baseline between the HRO 37 meter antenna used by Knight (1980) and on a 10 m antenna to the southeast. Over the period from October 1974 to January 1976, the length of the baseline was constant to f 3 mm and among the individual components (N/E/Z) the largest possible change was ?: 7 mm (2 component). The stability of the HRO site seems assured although the time period of the short baseline observations does not overlap the VLBI observations. In view of the evidence for vertical crustal movements reported by Brown and Oliver (1976), it is important TABLE I Baseline length rates of change based on single experiment solutions (After Knight, 1980) HRO-OVRO HRO-NRA0 OVRO-NRA0
43.7 f 1.9 cm/yr +0.7 + 1.4 cm/yr -1.0 f 1.2 cm/yr
T3 10
. .
..1111.1.,,,,,,,1,,,,,,,,,,,,,,,,(,,,,,,1,,,,
OVRO v)
5-
NRA0
RESIDUALS
ABOUT 3.324.244.20
0
M
l
I-
-l
ABOUT 3.928.881.68 5I4
M
.
l
. t:::::;~:::M-p+l+
. . ,b
a
-1O.11II11IIIII’IIIIIIIIIII’IIIIIII11II’IIIIIIIIII1 1976 1977 10 11.11111111,,,,,,,,,,,,,,,1,1,11111,11111,1,1,1-
1976
1979
HRO - NRA0 RESIDUALS ABOUT 84&l 29.89 M .
5In B & p
_ -
El 0
-
l
.
.
:;:::m::::+H::i:::::::::::::::;H::::::~ . l
.
-5 -
After Knight, 1960
Fig. 1. Obwrved baseline length residuals. NRAO. After Knight (1980).
a. OVRO-NRAO.
b. OVRO-HRO.
c. HRO-
to note that Rogers et al. (1978) found no evidence for vertical movements on this particular baseline. As .part of the development work for the NRA0 very large antenna array, a radio-linked interferometer using a 25 m antenna at the NRA0 main site and a portable 14 m antenna was constructed (Basart et al., 1970). This system is currently operating between the main NRA0 site and a site at
iii1.6
6z'---#-I-'_'L_
g 2 1.4 1.2
i t
II 95
0x-_-/_=By-.-_------r--
I
101
IiS<
107
I
113
I
119
I
I
125'
I1)III
I1
131
137
14
143
149
DAYOFYEAR1979
Fig. 2. Normalized changes in the baseline components (Bx, By, Bz) for the period April 11 to May 30, 1979. Width of curve is twice the formal probable error. The top curve represents the change in baseline length (1J31). The scale factor is 5 ps = 2 mm. After Klepczyinski et al. (1980).
Huntersville, West Virginia, a distance of 35.2 km on azimuth 220”. Since 1977, the U.S. Naval Observatory has been using the system to investigate the determination of polar motion by radio interferometry (Johnston et al., 1979). As a by-product of this work, baseline lengths were measured during a part of the period covered by the VLBI observations. In Fig. 2, we reproduce a sample time history of the 35 km baseline lengths (Klepczyinski et al., 1980). No time trend is apparent for the periode of the VLBI observations; a rate of 0.8 cm/yr r 1.1 cm/yr was found by a least squares analysis (Johnston and Klepczyinski, pers. commun., 1980). In addition, the area around Green Bank was surveyed for geologic evidence of faulting or other disturbances which might affect the NRA0 results (Webster et al., 1979). No evidence of actual or potential movement was found. To summarize, there is no evidence of change on any of the VLBI baselines to a 2 cm level. Further, there is no evidence for local motions at HRO and NRA0 which might cancel true baseline changes. We are thus left with the same paradox discussed by Allenby (1979). INTERPRETATION
If one accepts the conclusion that the Yellowstone complex is due to a hot spot, the Minster and Jordan (1978) absolute motion models imply a rate of motion of 2.4 cm/yr for the North American plate over the hot spot.
T5
If this motion occurred only at the western end of the triangle of VLBI stations, a rate of 2.4 cm/yr would have been observed on the east-west baselines. This is the maximum to be expected. No change would be expected on the HRO-NRA0 baselines. The simplest interpretation of the VLBI results is that the North American plate is rigid on this level. Yet, relatively large magnitude intra-plate earthquakes do occur and, in the case of the Charleston, South Carolina and New Madrid, Missouri areas, can have body wave magnitudes near 8. At least on an episodic basis, stress must accumulate within the plate. Other lines of evidence suggest that stress is accumulating now, i.e., recent faulting (York and Oliver, 1976), direct measurement of stress (Sbar and Sykes, 1973), microseismicity, fracturing and other structural trends (Sykes, 1978). We are thus led to the same alternatives considered by Allenby (1979). We may ignore the evidence for regional geodetic changes and stress accumulation and thus conclude the plate is rigid; possibly the motions are episodic and we are observing during a quiescent period. Perhaps those motions which do occur are compensated so that the plate only appears to be rigid. Allenby (1979) considered the questions of compensating for episodic motions and was inclined to favor the compensating motions explanation. Here, we will examine the extent to which a nearly rigid plate is consistent with other results. Richardson et al. (1979) have recently modeled the stress field in the lithospheric plates on a global basis. Intra-plate earthquakes, mechanisms, in-situ stress measurements and various stress sensitive features were used to constrain the models. Each plate was assumed to be a cap of 100 km thickness and to have elastic properties typical of an “almost” rigid plate. (Young’s modulus 0.25, Poisson’s rate 7 - 10” dyn/cm2 ). The choice of elastic constants was somewhat arbitrary but the models suggest that their constraints do not mandate a perfectly rigid plate. The elastic constants used by Richardson et al. (1979) together with the stress fields found in the course of modeling will admit the baseline changes of many cm on an intra-plate baseline without influencing the modeling results. Clearly, the VLBI observations will allow a small non-zero rate and still be consistent with the model. It is, however, necessary that the true rate be less than 1 cm/yr over the 3929 km baseline -between the eastern sites and OVRO. We look forward to the results of the MARK-III observations now in progress. If there is any rate above the 0.5 cm/yr level, these observations will detect it. REFERENCES Allenby, R.J., 1979. Implications of very long baseline interferometry North American intra-plate crustal deformation. Tectonophysics, 60: Basset, J.P., Miley, G.K. and Clark, B.G., 1970. Phase measurements meter baseline of 11.3 km. I.E.E.E. Trans. Antennas Propag., AP-18,
measurements on T27-T35. with an interfero375-379.
T6 Brown, L.D. and Oliver, J.E., 1976. Vertical crustal movements from leveling data and their relation to geologic structure in the eastern United States. Rev. Geophys. Space Phys., 14: 13-35. Coates, R.J., Clark, T.A., Counselman, C.C., Shapiro, I.I., Hinteregger, H.F., Rogers, A.E. and Whitney, A.R., 1975. Very long baseline interferometry for centimeter accuracy geodetic measurements. Tectonophysics, 29: 9-18. Johnston, K.J., Spencer, J.H., Mayer, C.H., Klepczyinski, W.J., Kaplan, G., McCarthy, D. and Westerhout, G., 1979. The NAVOBSY/NRL program for the determination of earth rotation and polar motion. In: D. McCarthy and J. Pilington (Editors), Time and the Earth’s Rotation. IAU Symp. No. 82, Reidel, Dordrecht, pp. 183-186. Klepczyinski, W.J., Kaplan, G.H., McCarthy, D.D., Josties, F.J., Branham, R.L., Johnston, K.J. and Spencer, K.J., 1980. Progress report on the USNO/NRL Green Bank Interferometer Program. In: Radio Interferometry Techniques for Geodesy. NASA Conf. Publ., 2115: 63-70 -NASA, Washington, D.C. Knight, C.A., 1980. Long baseline vector determinations and intercomparisons, In: Radio Interferometry Techniques for Geodesy. NASA Conference Publication, 2115: 23-26 - NASA, Washington, D.C. Minster, J.B. and Jordan, T.H., 1978. Present-day plate motions. J. Geophys. Res., 83: 5331-5364. Office of Space and Terrestrial Applications (OSTA), 1979. Application of space technology to crustal dynamics and earthquake research. NASA Goddard Space Flight Center Tech. Pap., 1464 - NASA, Washington, D.C. Richardson, R.M., Solomon, S.C. and Sleep, N.H., 1979. Tectonic Stress in Plates. Rev. Geophys. Space Phys., 17: 981-1019. Rogers, A.E.E., Knight, C.A., Hinteregger, H.F., Whitney, A.R., Counselman, C.C., Shapiro, I.I., Gourevitch, S.A. and Clark, T.A., 1978. Geodesy by radio interferometry: determination of a 1.24 km baseline vector with 5 mm repeatability. J. Geophys. Res., 83: 325-334. Ryan, J.W., Clark, T.A., Coates, R., Corey, B.E., Cotton, W.D., Counselman, C.C., Hinteregger, H.F., Knight, C.A., Ma, C., Robertson, D.S., Rogers, A.E.E., Shapiro, I.I., Whitney, A.R. and Whittels, J.J., 1978. Precision surveying using radio interferometry. J. Surv. Mapping Div., A.S.C.E., 104: 26-34. Sbar, M.L. and Sykes, L.R., 1973. Contemporary compressive stress and seismicity in eastern North America: an example of intra-plate tectonics. Geol. Sot. Am. Bull., 84: 1861-1882. Sykes, L.R., 1978. Intra-plate seismicity, reactivation of pre-existing zones of weakness, alkaline’magmatism and other tectonism postdating continental fragmentation. Rev. Geophys. Space Phys., 16:. 621-688. Webster, W.J., Jr., Tiedemann, H.A., Lowman, P.D., Jr., Hutton, L.K. and Allenby, R.J., 1979. Tectonic motion site survey of the national radio astronomy observatory, Green Bank, West Virginia. NASA Tech. Mem., 79691. York, J.E. and Oliver, J.E., 1976. Cretaceous and Cenozoic Faulting in eastern North America. Geol. Sot. Am. Bull., 87: 1105-1114.
Letter
Tl
section
On the rigidity of the North American plate
WILLIAM J. WEBSTER, Jr. ~eoFhys~cs Branch, NASA Goddard Space Flight Center, Greenbelt,
(Received October 27,198l;
Md. 20771 (U.S.A.)
revised version accepted January 27, 1982)
ABSTRACT Webster, J., Jr., 1982. On the rigidity of the North American plate. Te~~onophye~~s* 84: Tl-T6. Recently reported Very Long Baseline Interferometer (VLBI) observations between Green Bank, West Virginia, Haystack, Massachusetts and Owens Valley, California, show that the linear distance between these sites has not changed for at least three years (i.e., smooth rate of change lesg than 1 cm/yr). This level of stability is logically consistent with the hypothesis that the continental U.S. North American plate is geodetically rigid at least on the 2 cm level. Model studies show that thii result is in accord with present knowledge of the intra-plate stress field and the elastic properties of typical continental lithosphere.
As part of the preparation for the Crustal Dynamics Project (OSTA, 1979), an extensive series of VLBI observations between the National Radio Astronomy Observatory (NRAO) at Green Bank, West Virginia, the Haystack Radio Observatory (HRO) near Westford, Massachusetts and the Owens Valley Radio Observatory (OVRO) near Bishop, California was conducted (Knight, 1980). These measurements were developmental tests of the various parts of a new geodetic VLBI system called MARK-III (Coates et al., 1975). The expected precision of an individual baseline measured with MARK-III is in the vicinity of 3 cm for a California to Massachusetts baseline. Although the quality of the observations reported by Knight (1980) is not as high as with a full MARK-III system at each station, the long time sequence of data has important implications for the extent in distance of geodetic deformation on the North American plate. Allenby (1979) has discussed the tectonic significance of an earlier generation of VLBI observations on a baseline from HRO to OVRO. These observations imply a rate of change of length of no greater than 1 cmfyr. This was a surprising result since the baseline crosses several areas where changes of several centimeters would be expected, 0040-1951/82/000~000/$02.7S
8 1982 Elsevier Scientific Publishing Company
T2
Since the measurements discussed here were made from stations forming a triangle, north---south changes can be considered in addition to a second baseline roughly east-west. The sequence of observations discussed spans the period from September 1976 to May 1978 and includes fourteen measurements on the NRAO-HRO baseline and eight measurements on the NRAO-OVRO baselines. These data include the HRO-OVRO data discussed by Allenby (1979) as well as some additional measurements on this baseline. DISCUSSION
The observations discussed were taken with the MARK-I system during the testing of components of the MARK-III system (Knight, 1980). The antennas and microwave receivers employed were of comparable gains and noise temperatures. During the period of these observations, the performance of the MARK-I system was upgraded continuously. The improvement is most noticeable when the cable calibration system was added at NRA0 (Knight, 1980). Knight reported an improvement in accuracy corresponding to a decrease from * 15 cm to +3 cm in an individual baseline. For the 20 months during which all baselines were observed in common, the largest rate reported by Knight (1980) (Ryan et al., 19’78) is on the HRO-NRA0 baseline, -1.0 cm/yr f 1.2 cm/yr (nine baselines). In Table I, we give the rate determinations reported by Knight (1980) while in Fig. 1, we produce the time trends for each baseline. The longest time span is covered by the HRO-NRA0 data: 70 months. On no baseline do the data contain any evidence for a statistically significant change (Knight, 1980 contains a detailed discussion of this point). As part of the equipment development prior to the observations reported by Knight (1980), Rogers et al. (1978) made high precision observations on a 1.24 km baseline between the HRO 37 meter antenna used by Knight (1980) and on a 10 m antenna to the southeast. Over the period from October 1974 to January 1976, the length of the baseline was constant to f 3 mm and among the individual components (N/E/Z) the largest possible change was ?: 7 mm (2 component). The stability of the HRO site seems assured although the time period of the short baseline observations does not overlap the VLBI observations. In view of the evidence for vertical crustal movements reported by Brown and Oliver (1976), it is important TABLE I Baseline length rates of change based on single experiment solutions (After Knight, 1980) HRO-OVRO HRO-NRA0 OVRO-NRA0
43.7 f 1.9 cm/yr +0.7 + 1.4 cm/yr -1.0 f 1.2 cm/yr
T3 10
. .
..1111.1.,,,,,,,1,,,,,,,,,,,,,,,,(,,,,,,1,,,,
OVRO v)
5-
NRA0
RESIDUALS
ABOUT 3.324.244.20
0
M
l
I-
-l
ABOUT 3.928.881.68 5I4
M
.
l
. t:::::;~:::M-p+l+
. . ,b
a
-1O.11II11IIIII’IIIIIIIIIII’IIIIIII11II’IIIIIIIIII1 1976 1977 10 11.11111111,,,,,,,,,,,,,,,1,1,11111,11111,1,1,1-
1976
1979
HRO - NRA0 RESIDUALS ABOUT 84&l 29.89 M .
5In B & p
_ -
El 0
-
l
.
.
:;:::m::::+H::i:::::::::::::::;H::::::~ . l
.
-5 -
After Knight, 1960
Fig. 1. Obwrved baseline length residuals. NRAO. After Knight (1980).
a. OVRO-NRAO.
b. OVRO-HRO.
c. HRO-
to note that Rogers et al. (1978) found no evidence for vertical movements on this particular baseline. As .part of the development work for the NRA0 very large antenna array, a radio-linked interferometer using a 25 m antenna at the NRA0 main site and a portable 14 m antenna was constructed (Basart et al., 1970). This system is currently operating between the main NRA0 site and a site at
iii1.6
6z'---#-I-'_'L_
g 2 1.4 1.2
i t
II 95
0x-_-/_=By-.-_------r--
I
101
IiS<
107
I
113
I
119
I
I
125'
I1)III
I1
131
137
14
143
149
DAYOFYEAR1979
Fig. 2. Normalized changes in the baseline components (Bx, By, Bz) for the period April 11 to May 30, 1979. Width of curve is twice the formal probable error. The top curve represents the change in baseline length (1J31). The scale factor is 5 ps = 2 mm. After Klepczyinski et al. (1980).
Huntersville, West Virginia, a distance of 35.2 km on azimuth 220”. Since 1977, the U.S. Naval Observatory has been using the system to investigate the determination of polar motion by radio interferometry (Johnston et al., 1979). As a by-product of this work, baseline lengths were measured during a part of the period covered by the VLBI observations. In Fig. 2, we reproduce a sample time history of the 35 km baseline lengths (Klepczyinski et al., 1980). No time trend is apparent for the periode of the VLBI observations; a rate of 0.8 cm/yr r 1.1 cm/yr was found by a least squares analysis (Johnston and Klepczyinski, pers. commun., 1980). In addition, the area around Green Bank was surveyed for geologic evidence of faulting or other disturbances which might affect the NRA0 results (Webster et al., 1979). No evidence of actual or potential movement was found. To summarize, there is no evidence of change on any of the VLBI baselines to a 2 cm level. Further, there is no evidence for local motions at HRO and NRA0 which might cancel true baseline changes. We are thus left with the same paradox discussed by Allenby (1979). INTERPRETATION
If one accepts the conclusion that the Yellowstone complex is due to a hot spot, the Minster and Jordan (1978) absolute motion models imply a rate of motion of 2.4 cm/yr for the North American plate over the hot spot.
T5
If this motion occurred only at the western end of the triangle of VLBI stations, a rate of 2.4 cm/yr would have been observed on the east-west baselines. This is the maximum to be expected. No change would be expected on the HRO-NRA0 baselines. The simplest interpretation of the VLBI results is that the North American plate is rigid on this level. Yet, relatively large magnitude intra-plate earthquakes do occur and, in the case of the Charleston, South Carolina and New Madrid, Missouri areas, can have body wave magnitudes near 8. At least on an episodic basis, stress must accumulate within the plate. Other lines of evidence suggest that stress is accumulating now, i.e., recent faulting (York and Oliver, 1976), direct measurement of stress (Sbar and Sykes, 1973), microseismicity, fracturing and other structural trends (Sykes, 1978). We are thus led to the same alternatives considered by Allenby (1979). We may ignore the evidence for regional geodetic changes and stress accumulation and thus conclude the plate is rigid; possibly the motions are episodic and we are observing during a quiescent period. Perhaps those motions which do occur are compensated so that the plate only appears to be rigid. Allenby (1979) considered the questions of compensating for episodic motions and was inclined to favor the compensating motions explanation. Here, we will examine the extent to which a nearly rigid plate is consistent with other results. Richardson et al. (1979) have recently modeled the stress field in the lithospheric plates on a global basis. Intra-plate earthquakes, mechanisms, in-situ stress measurements and various stress sensitive features were used to constrain the models. Each plate was assumed to be a cap of 100 km thickness and to have elastic properties typical of an “almost” rigid plate. (Young’s modulus 0.25, Poisson’s rate 7 - 10” dyn/cm2 ). The choice of elastic constants was somewhat arbitrary but the models suggest that their constraints do not mandate a perfectly rigid plate. The elastic constants used by Richardson et al. (1979) together with the stress fields found in the course of modeling will admit the baseline changes of many cm on an intra-plate baseline without influencing the modeling results. Clearly, the VLBI observations will allow a small non-zero rate and still be consistent with the model. It is, however, necessary that the true rate be less than 1 cm/yr over the 3929 km baseline -between the eastern sites and OVRO. We look forward to the results of the MARK-III observations now in progress. If there is any rate above the 0.5 cm/yr level, these observations will detect it. REFERENCES Allenby, R.J., 1979. Implications of very long baseline interferometry North American intra-plate crustal deformation. Tectonophysics, 60: Basset, J.P., Miley, G.K. and Clark, B.G., 1970. Phase measurements meter baseline of 11.3 km. I.E.E.E. Trans. Antennas Propag., AP-18,
measurements on T27-T35. with an interfero375-379.
T6 Brown, L.D. and Oliver, J.E., 1976. Vertical crustal movements from leveling data and their relation to geologic structure in the eastern United States. Rev. Geophys. Space Phys., 14: 13-35. Coates, R.J., Clark, T.A., Counselman, C.C., Shapiro, I.I., Hinteregger, H.F., Rogers, A.E. and Whitney, A.R., 1975. Very long baseline interferometry for centimeter accuracy geodetic measurements. Tectonophysics, 29: 9-18. Johnston, K.J., Spencer, J.H., Mayer, C.H., Klepczyinski, W.J., Kaplan, G., McCarthy, D. and Westerhout, G., 1979. The NAVOBSY/NRL program for the determination of earth rotation and polar motion. In: D. McCarthy and J. Pilington (Editors), Time and the Earth’s Rotation. IAU Symp. No. 82, Reidel, Dordrecht, pp. 183-186. Klepczyinski, W.J., Kaplan, G.H., McCarthy, D.D., Josties, F.J., Branham, R.L., Johnston, K.J. and Spencer, K.J., 1980. Progress report on the USNO/NRL Green Bank Interferometer Program. In: Radio Interferometry Techniques for Geodesy. NASA Conf. Publ., 2115: 63-70 -NASA, Washington, D.C. Knight, C.A., 1980. Long baseline vector determinations and intercomparisons, In: Radio Interferometry Techniques for Geodesy. NASA Conference Publication, 2115: 23-26 - NASA, Washington, D.C. Minster, J.B. and Jordan, T.H., 1978. Present-day plate motions. J. Geophys. Res., 83: 5331-5364. Office of Space and Terrestrial Applications (OSTA), 1979. Application of space technology to crustal dynamics and earthquake research. NASA Goddard Space Flight Center Tech. Pap., 1464 - NASA, Washington, D.C. Richardson, R.M., Solomon, S.C. and Sleep, N.H., 1979. Tectonic Stress in Plates. Rev. Geophys. Space Phys., 17: 981-1019. Rogers, A.E.E., Knight, C.A., Hinteregger, H.F., Whitney, A.R., Counselman, C.C., Shapiro, I.I., Gourevitch, S.A. and Clark, T.A., 1978. Geodesy by radio interferometry: determination of a 1.24 km baseline vector with 5 mm repeatability. J. Geophys. Res., 83: 325-334. Ryan, J.W., Clark, T.A., Coates, R., Corey, B.E., Cotton, W.D., Counselman, C.C., Hinteregger, H.F., Knight, C.A., Ma, C., Robertson, D.S., Rogers, A.E.E., Shapiro, I.I., Whitney, A.R. and Whittels, J.J., 1978. Precision surveying using radio interferometry. J. Surv. Mapping Div., A.S.C.E., 104: 26-34. Sbar, M.L. and Sykes, L.R., 1973. Contemporary compressive stress and seismicity in eastern North America: an example of intra-plate tectonics. Geol. Sot. Am. Bull., 84: 1861-1882. Sykes, L.R., 1978. Intra-plate seismicity, reactivation of pre-existing zones of weakness, alkaline’magmatism and other tectonism postdating continental fragmentation. Rev. Geophys. Space Phys., 16:. 621-688. Webster, W.J., Jr., Tiedemann, H.A., Lowman, P.D., Jr., Hutton, L.K. and Allenby, R.J., 1979. Tectonic motion site survey of the national radio astronomy observatory, Green Bank, West Virginia. NASA Tech. Mem., 79691. York, J.E. and Oliver, J.E., 1976. Cretaceous and Cenozoic Faulting in eastern North America. Geol. Sot. Am. Bull., 87: 1105-1114.