TITLE: 

TYPE OF REPORT 

Inter- and Intra-plate deformation at North American 

plate boundaries 

Semiannual Status Report 

PRINCIPAL INVESTIGATOR: John Beavan 

PERIOD COVERED: 1 July - 31 December 1986 

GRANTEE INSTITUTION The Trustees of Columbia University in the City 

of New York, Low Memorial Library, New 

York, New York 10027 

GRANTNUMBER: NAGS-799 

#bASA-CR-l80157) INTLEB- AbiC Ih3PP-PLBTE N87- 1 E934 

CEFOPHATICB AT ICE'IEI ACERICAB ELAlE 

EGUWDABIES Semiannual Status, 1 Gul. - 3 I Dec, 198b (Lamont-Lcherty Geolcgical Unclas 

Ckservatcry) 8 F CSCL O8E G3/46 435UO 
1 

Inter- and Intra-plate deformation at North American plate boundaries 

Semi-annual report to Grant NAG5-799 

July 1 - December 31, 1986 

Jolui Beavan 

Laniont-Dolierty Geological Observatory of Columbia University 

Palisades NY 10964 

Objectives 

The project is divided into five sections; 

1. Alaska tectonics and earthquake hazard studies. 

2. Southern California tectonics - block rotation. 

3. Spreading near the Salton Trough. 

4. California plate motion - fault zone kinematics. 

5. Caribbean plate motion investigations. 

Results 

During the current reporting period, we have made progress in objectives 1, 2 and 

5 above. We describe our results below. 

I. Alaska. Results from the first three years of the Alaska campaign were reported by 

Clark et al. (1986). Clark (pen. comm.) has provided us with a subset of the Alaska 

and Pacific baseline results, and we have used the observed motion of Sand Poini?o 

help place constraints on whether the eastern part of the Shumagin seismic gap is or is 

not slipping aseisnlically. Savage and Lisowski (1986) argued recently that the most 

likely interpretation of their 1980-1985 trilateration data was for plate motion to be 

occurring aseismically. If slip is occurring aseismically over the whole down-dip 

width of the plate interface then Sand Point is expected to remain stationary with 

respect to interior Alaska, md to move at a rate consistent with the full North America 

- Pacific plate motion towards sites such as Vandenberg and Hawaii. If the plate 

boundary is locked, then Sand Point should experience northwestward motion of about 

1 cm/yr according to dislocation models (Savage, 1983) of strdin accumulation. A 

map of the Shumagins is shown in Figure 1, predicted crustal motions from various 

models of subduction are shown in Figure 2, and a sumnary of all available observations is given in Table 1. Table 1 also compares the observed and predicted motions, 

where the aseisnlic model predicts zero defomiation. 

Using recent arguments on forearc mechanics (Byme et al., 1986, 1987) we place 

the upper limit of the locked plate interface at -20 km depth, whereas Savage and 

Lisowski (1986) place it at the surface. This leads Savage and Lisowski to substantially overstate their case for aseisnlic subduction. However, the balance of evidence, 
Table 1. Comparison of Observed and Modelled Deformation 

Observed Elaqtic rebound niodels 

1980-86 Figure 2b figule 2a Savage 

Strain, ptMin/yr 

Outer isliids 

Inner islands -0.01 HO.03 -0.02 -0.06 -0.07 

0.03M.05 -0.05 -0.22 -0.30 

Till, pdd/yr -0.03 0.1 

PRS 0.Cl!kko. I6 -0.4 1 -0.36 -0.3 

-0.36 -0.3 

SWShltI -0.06f0.15 -0.08 -0.07 0.2 

-0.07 0.2 

-2.1 -1.2 -1.2 

SDP/SQH -0.24M. I7 -0.37 

SAD -0.14H.25 -0.4 1 

CHN 0.4333.15 -0.08 

Displacement, cm/yr 

NW velocity of SDP 1.0 1.2 -0.W.9 - 

Figure 2b model is locked between 25 and 50 km depth 

Figure 2a model is locked between 20 and 50 km deplh 

Savage md Lisowski's model is locked from the surface to 50 km depth 

Extensioas are positive, tilts down to the NW m positive 

Displacemenw, upwmb or lo the NW are positive 

Emrs quoted are 1 stdard deviation 

SIM-PRC uplift 0.4M.7 

mflF@- .. .. ,. 

P . . . . . . . . . . .. ... ... .. .. ..'_._.. . .: .:. ... ... , _. 

LY I vo- wANnn c-- 200 

164.W 162. 160. 

f 

7 

I 

158. 

54O 

Figure 1. Location of the Shumagin Islands with respect to the trend1 and the volcanic arc. 

Depth cootours an in metres. The Shunragh seismic gap extends from just east of the 

Shutnagin Islands themselves, to the vicinity of Saaak Island in he west. Note the locations of Pavlof Volcano and the Inner and Outer Shumagh. The USGS Uilatention network extends from the Outer Sliumagins to the Alaska peninsula. Also shown nre tbe sites 

of NASA VLBl nieimxements (SDP) cmd of sea-level gauges operatcd by Lamont-Dohefly 

d by the National Ocean Survey (SDP). Sladou SAD is no longer operiled beawe of 

repeated storm damage. Station PRS is not operating this year. Site CHN failed after 2 

month due to a (rm) pressure sensor failure. Site SQH failed in October 1986 after 26 

months of low-nlilinteniii~ operilioti. Site7 PRC and SIM conlhiue to operate s of his 

writiug. All sea level sites tiow use Pmosdentific quartz pressure S~~NOIS. Note drit rime 

of the current geodetic nieasumlieuts extend iuto the western part of the gap. 
3 

I I 1 I 1 I 1 I' - - 

- - 

- - 

-- - 

- - 

- - 

- INNER ,OUTER 

I I 1 I' I I I 

- 

0.4 

z 

- - 

INNER ,OUTER - 1 1 I 1 I 1 1 I I - 

- 

a 

e 

v) 

co 

- 0.4 

=t 

8. 

t' 

0.4 

z 

a 

a 

I-0 

v) 

- 0.4 

* 

e. - 

r = 20. 

1 I 1 I I I 

I-1 L J 

PAVLOF SOP SIM TRENCH 

I' I 1 I 1 I I I 

t0 

Y 

Ot 

n 

-2 3 

2- 

z 

u - 

Y 

-I 

[1 

-2 = 

Figure 2 (a,b). Dislocation models showing surface uplift (thin line), and horizontal 

linear strain in the plate convergence direction (heavy line), due to strain accumulation, 

assutning a 7.5 cndyr plate convergence rate (Minster and Jortimm, 1978). llie strongly 

coupled (locked) part of the plate interface is shown by the heavy line in the lower 

frame. The upper and lower limits of the locked zone are taken from seismic and 

forearc mechanics evidence (see text). Ground extension is positive. Table 1 cotnpares the models with strain, tilt and displacement observations. Aseismic subduction 

would predict zero ground displacemnent. 
including the VLBI data, is still that the eastern Shumagins are indeed behaving aseismically. We note that several of the observations have rather large error bars, and 

that a few more years of observations should substantially improve our knowledge of 

the iinportiiiit question of whether the suhluction in this region is aseisinic or whether 

a major earthquake may soon be expected. 

We emphasise that these results place no constraints on deformation in the 

western part of the Shumagin seismic gap. Historical evidence (Davies et al., 1981) is 

strongly in favor of at least one iiiajor earthquake in this region, so that aqeisinic subduction throughout the whole gap is most unlikely. Recent seisnlic studies (Hudnut 

and Taber, 1987) suggest that the character of the gap changes between the eastern and 

western sections. The western section shows a clear double-planed seismic zone in the 

tlowiigoiiig slab, whereas the eastern section shows only a single plane. The douhleplaned zone has been interpreted elsewhere (e.g. Astiz and Kanamori, 1986) as indicative of strong plate coupling, and thus of great earthquake potential. 

2. Block rotation in Soutlierrt California 

Paleomagnetic data suggest that significant rotations have taken place in California. If any VLBI or SLR sites are on rotating blocks then the details of the rotation 

may be very important in interpreting the data. The Ocotillo site is a clear example of 

a station that is located on a rotating block. 

We are tnking advantage of some existing astro-azimuth data to compare shortterm geodetic rotation rates with long-term paleomagnetic rates in the Borrego Badlands area, which the paleomagnetic data suggest is rotating at perhaps 0.5 pracl/yr, 

due to its location between the San Jachto and Clark faults. The Clark Lake basin, 

next to the Badlands (Figure 3) may be similarly rotating. A geodetic network has 

been measured here several times since the late 1950’s, including measurements of 

angles to Polaris. The advantage of the measurements to Polaris is that the geodetic 

network can be referenced to an external franie, so that rigid bocly rotation can bc 

determined. The disadvantage is that the determination is only good to -5 pad standard error, so that many years’ measurements are required to detect the expected rotation rates of 0.3 to 1.0 pacl/yr. We remeasured part of the ’T’ array (Figure 3) in 

Deceniber 1986 using a Wild T-2 theodolite. Initial results suggested measurable rotation, but this was defined by only a few benchmarks and die standarcl error of the survey was several times worse than expected. We plan to repeat and extend these measurements soon using a superior instrument (Wild T-3). 

5. Caribbean tectonics 

Dr. Bilham, co-investigator 011 this grant, participated in the June 1986 Caribbean 

GPS experiment organised by NASNJPL. Three University Navstar Consortium 

receivers were donated to the project. 
5 
6 

Future Work 

Dr. Bilham recently left Lamont for a professorial position at the University of 

Colorado, so he will no longer serve as co-investigator on this grant. As a result we 

will eliminate our efforts under Objective 5 above, and will redirect our efforts under 

Objectives 3 and 4. A specific data set that we shall study over the next few rnoritlis 

is a suite of triangulation measurements done in the 1880's' 1920's and several tinles 

since then, that extends east from the Sa Francisco Bay area to the California - 

Nevada border. This data set has never ken studied in its entirety, and it may throw 

light on where the "missing 2 cm/yr" of plate motion is taken up in northern California. 

Papers in press 

The following papers resulting from NASA support have been published during 

the past year, or are in press. 

McNutt, S.R. & R.J. Beavan, 1987. Eruptions of Pavlof Volcano, and their possible 

modulation by ocean load and tectonic stresses, in review, J. Geophys. Res. 
”... 

i 

References and Dibliugraphy 

Adz. L. mcl H. Kimimiori, 1986. Ititerplate coupling and the C1iiporitI variation of mechiuiisnis of 

intemiediate-depth earthquakes in Chile, Bull. seism. SOC. Am., 76, 1614-1622. 

Beavan, J., R. Billrani & K. Hurst, 1984. Coherent till signis observed in lhe Sliuniagiti seismic gap; 

detection of lime dependent signals at depth?, J. geopliys. Res., 89, 44784492. 

Beavan, J., K. Hurst, R. Bibam & L. Skngold, 1986. A densely-spaced array of sea-level inoniton for 

the detection of vertical crustal deformation in the Shumagin seisniic gap, Alaska, J. geopliys. Res., 

Byme, D.E., D.M. Davis and L.R. Sykes, 1986. Forearc mechanics and the seismogenic zone, Eos, 67, 

1196 (abstract). 

Byme, D.E., D.M. Davis and L.R. Sykes, 1987. Forearc mechanics md the seismogenic zone, Tectonics, submitted, Feb 1987. 

Clark, T., J. Ryan, C. Ma, E Himwich, D. Gordon ad A. Tallma, 1986. Geodesy by radio interferometry: measurements of vector site motions in the western U.S. and Alaska, EUS, 67, 906 

(ubsrract). 

Davies, J.N., L. Sykes, L. House & K. Jacob, 1981. Shumagin seisniic gap, AliIska peninsula: History of 

91 : B9, 906 7-9080. 

gwat ediqullkes, tectonic setting, and evidence for high seismic potentid, J. geo&ys. Res., 86, 3821- 

39ss. 

Davis, D.M., D.E. Byme. L.R. Sykes and S.T. Kiorpes, 1986. Forearc mechanics mi the outer-arc 

high, EOS, 67, I196 (abstract). 

House, L.S. & K.H. Jacob, 1983. Earthquakes, plate subduction ad stress revetsals in the eastern 

Aleutian arc, J. geophys. Res., 88, 9347-9373. 

Hudnut, K.W. and J.J. Taber, 1987. Transition fmm double to single Wadati-Benioff seiymic zone in 

the Shumagin Islands, Alaska, Geophys. Res. Lett., in pnss. 

Minster, J.B. & T.H. Jordan, 1978. Present-day plate motions, J. geophy. Res., 83, 3331.5354. 

Reyners, M. & K.S. Coles, 1982. Fine structure of the dipping seismic zone anrl subdudion mechanics 

in the Shumdgin Islands, Alaska, J. geophys. Res., 87,356-366. 

Ruff, L. and H. Kaniunori, 1983. Seismic coupling and uncoupling at subduction zones, Tectonopliysics, 99,99-117. 

Savage, J.C., 1983. A dislocation model of strain accumulation and release at a subduction zone, J. 

geopliys. Res., 88,4984496. 

Savage, J.C. ad M. Lisowski, 1986. Strain accumulation in the Shumagin seismic gap, Alaska, J. geopliys. Res., 91:B7, 7447-7454. 

Sykes, L.R., J.B. Kisslinger, L. House, J.N. Davies ad’K.H. Jacob, 1981. Rupture zmm and repeat 

times of great earthquakes along the Alaska-Aleutian arc, 1784-1980, in Earthquake Prediction - An 

Ititernationul Review, eds. D.W. Simpson and P.G. Riclwrds, Maurice Ewing series; 4, AGU, WashiugIon, DC, pp 73-80. 

Taber, J.J. & K.W. Huhut, 1985. A transition Lion1 a single to a double Benioff zone near the Shwnagin Islands, Alayka, (abstract) EOS. Trans. AGU, 66, 958. 

Thalclter, W. & J.B. Rundle, 1984. A viscoelastic couphg model Bw the cyclic deformation due to 

periodically repeated earthquakes at subduction zones, J. geopliys. Res., 89, 7631 -7640.