Alaska tectonics and earthquake hazard studies; Southern California tectonics (block rotation); spreading near the Salton Trough; California plate motion (fault zone kinematics); and Caribbean plate motion investigations are examined

Beavan, John

English

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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) I N T L E B -  AbiC Ih3PP-PLBTE N87- 1 E934 
C E F O P H A T I C B  AT ICE'IEI A C E R I C A B  E L A l E  
E G U W D A B I E S  Semiannual S t a t u s ,  1 Gul. - 3 I Dec, 198b (Lamont-Lcherty Geolcgical Unclas  
Ckservatcry) 8 F CSCL O8E G3/46 435UO 
https://ntrs.nasa.gov/search.jsp?R=19870009501 2020-03-20T11:32:10+00:00Z
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 observa- 
tions 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 substan- 
tially 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 
S A D  -0.14H.25 -0.4 1 
CHN 0.4333.15 -0.08 
Displacement, cm/yr 
1.0 1.2 N W  velocity of SDP -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, u p w m b  or lo the N W  are positive 
Emrs quoted are 1 s tdard deviation 
SIM-PRC uplift 0.4M.7 
mflF@- . .  . .  ,. 
P . . . . . . . . . . . .  ... ... .. . .  .. '_._. .  . .: .:. ... ... , _.  
L Y  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 h e  west. Note the loca- 
tions of Pavlof Volcano and the Inner and Outer Shumagh. The USGS Uilatention net- 
work 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 ~ ~ N O I S .  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) 
c o  
- 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 
t- 
0 
Y 
O t  
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). l l i e  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 cotn- 
pares 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 ase- 
ismically. 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 sub- 
duction 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 douhle- 
planed zone has been interpreted elsewhere (e.g. Astiz and Kanamori, 1986) as indica- 
tive 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 Califor- 
nia. 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 short- 
term geodetic rotation rates with long-term paleomagnetic rates in the Borrego Bad- 
lands 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 p a d  stan- 
dard error, so that many years’ measurements are required to detect the expected rota- 
tion 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 rota- 
tion, but this was defined by only a few benchmarks and die standarcl error of the sur- 
vey was several times worse than expected. We plan to repeat and extend these meas- 
urements 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 S a  Francisco Bay area to the California - 
Nevada border. This data set has never k e n  studied in its entirety, and it may throw 
light on where the "missing 2 cm/yr" of plate motion is taken up in northern Califor- 
nia. 
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, Tecton- 
ics, submitted, Feb 1987. 
Clark, T., J. Ryan, C. Ma, E Himwich, D. Gordon a d  A. Tallma, 1986. Geodesy by radio inter- 
ferometry: 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 a d  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, Tectonopliy- 
sics, 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. a d  M. Lisowski, 1986. Strain accumulation in the Shumagin seismic gap, Alaska, J. geo- 
pliys. 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, Washiug- 
Ion, DC, pp 73-80. 
Taber, J.J. & K.W. Huhut, 1985. A transition Lion1 a single to a double Benioff zone near the Shwna- 
gin 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. 


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

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