Projects studying different aspects of crustal deformation are discussed. The rifting process at the plate boundary in northern Iceland was investigated using combined seismic, tilt, and displacement measurements. Stresses acting on the Alpine fault, New Zealand, were modeled based on observations of regional variation in metamorphism and argon loss. The relationship between the stress pattern shown by intraplate seismicity and possible stresses on plates arising from litohspheric motion over the asthenosphere was investigated. Small earthquakes in Southern California were studied to determine if subregions of uniform strain release exist and if interaction occurs between faults

Johnson, T. L.

Scholz, C. H.

English

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	 STNAIN BUILCRP AND	 N79-25605:f.7ASF, EARTIIQ ► IAKF PRF T)ICTICN AND SFI.FCTICN
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	 IN1^ PotFIC " P miann^^al Proyross Roport
	
NC 00:1( Laao nt-Doherty Geoloyictl Ohs P rva tory)	 13/46 21262
SF-MI-ANNUAL PROGRESS REPORT
to the
National Aeronautics and Space Administration
for
Grant NGR 33-008-146
"Strain Buildup and Release, Earthquake Prediction and
Selection of VLB sites for the Margins of the North Pacific"
June 1979
Principal Inver. -errors: C. H. Scholz
T. L. Johnson
R. Bilham
Lamont-Doherty Geological Observatory
of Columbia University
Palisades, New York 10964
may;^'^^	 `,^ J1
J1.
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Summary
Projects studying different aspects of crustal deformation in several
areas are in progress or near completion. Evidence that the earth's crust
is broken into blocks with dimensions of ten's of kilometers in orogenic
belts was presented in a paper recently appearing in Tectonophysics. Re-
sults of a detailed study of the rifting process at the plate boundary in
northern Iceland using combined seismic, tilt, and displacement measure-
ments, were reported at the Washington, D.0 meeting of the American Geo-
physical Union. Stresses acting on the alpine fault, New Zealand, were
modeled based on obsei-vations of regional variation in metamorphism and
argon loss. A paper detailing the results has been accepted for publica-
tion in the Journal of Geophysical Research. A study of the relation be-
tween the stress pattern shown by intraplate seismicity and possible stresses
on plates arising from lithespheric motion oven the asthenosphere is near-
ing completion. Recently begun projects include a stud y of smaller earth-
quakes located b y the Southern California Seismic Array to determine if
subregions of uniform strain release exist and to study the interaction of
faults in the Los Angeles Basin and installation of two strainmeters of a
new type near the San Andreas in Southern California on the edge of the
Palmdale bulge to help monitor crustal deformation there. Also, two geo-
logists from Columbia Universit y are presently in the field studying the
area around the Quincy Laser Ranging Site for evidence of an y possible re-
cent movement on nearby faulting which could influence the SAFE distances
measurement.
Discussion
Understanding whether active block structures occur in a given area
where strainmeters or tiltmeters are to be installed singly or in arrays
is Important since some of the elastic properties of the resulting sur-
face fragmentation can cause interpretive ambiguity in observed data.
In general, observed strain fields will be incoherent across block boun-
daries compared to strains within integral blocks. Strain magnitude can
be reduced significantly within blocks which also may exhibit large tilts.
The azimuth of observed principal strain axes may differ from the azi-
muth of applied strain axes in places where local strain fields are gener-
ated at block boundaries. Applied strain will normall y
 be intensified at
block boundaries if they are represented b y
 relativel y weak fault zones.
Evidence for continental crustal block structures revealed by geodetic
work during the last centur y
 is presented in a paper, "Strains and tilts
un crustal blocks" by R. G. Bilham and R. J. Beavan which recently
 ap
-peared in Tectonoph_vsics. Blocks with dimensions ranging from 5 to 50 km
appear to be common in regions of intense tectonic activity. There is
some indication that block boundaries may respond viscoelastically or ex-
hibit strain-dependent elastic properties. Such behavior can account for
the slow transmission of tectonic deformation reported in Japan and else-
where. If nonlinear behavior is a characteristic of regions fragmented
by
 crustal blocks, it introduces problems for the interpretation of ob-
served surface strain and tilt. In particular, it will not be oossible
3to apply a site correction factor based on the observed distortion of
seismic or tidal strains to the interpretation of secular strains.
Growth of large-scale ground fissures during rifting of the plate
boundary in northern Iceland was observed using several geophysical
techniques. The study area is the Krafla volcano and the cross-cutting
fissure swarm that extends 70 km to the north and 30 km south. Dis-
placement sensors installed across fissures measured continuous open-
ing of fissures in the floor of the Krafla caldera. Episodic opening
and closure of fissures outside the caldera was found during the mag-
ma migration even` in September 19 7 7. The currently ongoing tectonic
activity in northern Iceland began in December 1975 with seismic activ-
ity, a minor eruption, land subsidence, and fissure motion. Evidence
from S-wave shadows and gravity and leveling surveys indicates the exis-
tence of a magma chamber at a depth of 3 to 7 km below the Krafla cal-
dera. Leveling surveys show variable tilting interpreted as inflation
episodes due to filling of the magma chamber and deflation episodes re-
lated to migration of magma away from the chamber into the fissure swarm.
Two migrations to the south and nine to the north have been observed.
Fissures in the caldera open during inflation episodes and close during
subsidence, as expected. A small left-lateral shear is observed in each
subsidence episode. Also, a significant secular opening of the fissure
accumulates with each cycle. Fissure growth in the rift zone is epi-
sodic. Virtually no fissure movement was observed prior to or after
the September migration event. When fissures in the rift open, fissures
4outside the rift close. The implication of fissure closure is that the
dike intrusion is to some extent forcibly wedging the two crustal blocks
apart. Observations of compressive stress in the crust adjacent to the
rift zone in Iceland, an extensional area in plate tectonic concepts,
could be related to doming of the caldera and forcible dike intrusion.
A paper detailing these observations is in preparation.
The Alpine fault of New Zealand is a major continental transform
fault which has been uplifted on its southeast side 4 to 11 km within
the last 5 m.y. This uplift has exposed the Alpine schists, which have
been metamorphosed from the adjacent Torlesse graywackes. The Alpine
schists increase in metamorphic grade from prehnite-pumpellyite facies
9-12 km from the fault through the chlorite and biotite zones of the
greenschist facies to the garnet-o'_igoclase zone of amphibolite facies
within 4 km of the fault. These metamorphic zone boundaries are subpar-
allel to the fault for 350 km along the strike. The K-Ar and Rb-Sr ages
of the schists increase with distance from the fault: from 4 m.y. within
3 km of the fault to approximately 120 m.y. 20 km from the fault. Field
relations show that the source of heat that produced the argon depletion
aureole was the fault itself. Adopting a friction model for the fault,
and applying the known geological histor y of motion of the Alpine fault,
we show that the metamorphic culmination resulted from frictional heating
during the 360 km right-lateral slip on the Alpine fault during the Meso-
zoic (Rangitata) period of fault motion (140-80 m.y. ago). Fault motion
began again in the Plio-Pleistocene Kaikoura orogeny, with 120 km of
further right-lateral slip ana 4-11 km of uplift. Frictional heating
sduring this episode produce the argon depletion. Quantitative models
for both the metamorphism and argon depletion require that the fric-
tional shear stress acting on the fault during both episodes of fault
motion was at least 1-1.5 kbar. If, alternatively, it is assumed that
all fault motion took place in the last 30 m.y., this does not change
our conclusion with respect to the stress required for the argon deple-
tion.
The epicenter distribution of intraplate earthquakes in North Amer-
ica, to a first approximation, forms a ring of seismicity around the
continent. Most of the seismicit y is concentrated in the
mobile belts which rim the interior platform and shield provinces. In
contrast, the Precambrian shield and interior platform provinces are
virtuall y aseismic. Evidence from sei3mology and heat flow indicates
that ancient shield areas are characterized by thicker lithosphere and
a deeper, less developed or even absent low-velocity zone when compared
to oceans or continental areas of younger age. The "deep crustal roots"
beneath shield areas may influence the basal drag as continental plates
move laterally over the asthenosphere. If the bottom of the astheno-
spheric laver has a constant depth, then the reduction in asthenospheric
thickness under shields will result in enhanced drag under these areas.
In light of this hypothesis, the ring of seismicity encompas p ing North
America may well reflect stress concentrations due to variations in litho-
Epheric thickness and associated increased asthenospheric drag.
A two-dimensional finite element model of a continent moving over the
asthenosphere was constructed and the resulting stress pattern was calcu-
lated. Stresses are enhanced at the shield boundaries. The stress pat-
tern is antisymmetric from th e front to the back of the continent and is
either compressive or tensile at the leading edge, depending on whether
the asthenospheric flow tends to drive the continent or resist its mo-
tion. Deviatoric stresses in the center of the continent are relatively
low.
6
Phanerozoic mobile belts ma y represent pre-existing zones of weak-
ness along which earthquakes preferentially will occur if a regional
stress field is present. Also, the lithosphere tends to thicken at
the continentward boundary of mobile belts where the y contdct stable plat-
form regions, thus deviatoric stresses from basal drag will tend to con-
centrate in circling mobile belts, too. The two effects may work together
to create the ring pattern of seismicity around North America. Two ef-
fects also may be responsible for the aseismtcity of the North American
craton. Because of their age, these areas may be "annealed" presei:tly
and act as strong blocks resistant to faulting. Secondly, the deviatoric
stresses caused by basal drag beneath the continent will be minimal above
the thickened lithosphere.
Study of instantaneous plate motions suggests that, within observa-
tional error, 'hot-spots' may be considered a fixed reference frame with
respect to the translation of lithospheric pl..tes. From observation of
the Yellowstone and Raton hotspot traces, the North American continent
appears to be moving in a westerly direction at a rate of about 2 to 3
cm per year. If resistive drag at the base of the lithosphere beneath
continents is significant, a change from horizontal tensile to horizontal
7compressive deviatoric stress would be expected from west to east on a
traverse of the continent. Indeed, such a flip in the stress orienta-
tion is suggested by focal mechanism and hydrofracture measurements.
In view of the simple plate-tectonic model proposed, the regional stress
patterns are consistent with basal shear being a resisting drag force
that is concentrated beneath continental areas.
The northern boundary of Canada is approximately parallel to the
absolute pla t e velocity vector, hence, the two-dimensional model presented
may not be applicable in this region. The seismicity of northern Canada
generally is located along the continental margin and available focal
mechanisms do not appear to suggest the presence of an ambient horiaot.tal
stress field. If a regional stress field does exist, it may be radically
influenced by local loading effects of sed_mentation on the continental
shelf. A paper detailing the model and data indicating stress orienta-
tions is nearing completion.
Development and installation of a new type of strainmeter were par-
tially supported by the grant. An attempt was made in 1969 to use carbon
fibers as a length standard in wire-t ype strainseters. It was not success-
ful due to the inherently high creep rate of the fibers when used as a
20,000 fiber composite wire. The reason for the poor stability was that
the polyacronitrile charring process gale rise to graphite crystallite co-
herence lengths of about 10 meters. In any given 10 m long strainmeter
more than half of the crystallites forming the wire would be discontinuous,
adversely affecting the composite strength and creep properties of the
8bundle. Presently carbon fibers can be made from pitch and coherent
lengths are virtually infinite. We have found that good stability can
be obtained a month after installation (10 -7 strain/month) for a 10,000
fiber bundle 40 Meters long under a tension of 1.5 kg (15 Newtons).
At present she fiber is clamped between thin .Lead washers, cemented
with quick-.getting epoxy, and drawn slightly until set. Ide311y the
clamping process evenly distributes the effective stress across the
entire bundle of fibers but it is probable that the stress is mainly
carried by a relatively small fraction of the 10,000 filaments. The
tensioning device employed in the new strainmeter uses a 25 cm long
lever that holds the fiber bundle under about 10 Newtons tension. The
device is a balance that rotates if strain occurs in the ground. Rota-
tion is detected by a symmetrical capacitance transducer. The useful
sensitivity of the system is about 10 -10 strain, above which ambient
ground noise becomes significant. Three of the new strainmeters are
presently in operation; one in Ogdensburg Seismic Obse r;atory in New
Jersey and two recently installed in California near the San Andreas
fault on the edge of the Palmdale bulge.
A study of seismicity in southern California was recently begun.
Locaticns and first-motion data from small to moderate size earthquakes
recorded by the Southern Califonria Seismic Arra y were obtained and are
being used to systematically determine focal mechanisms over a broad
region of southern California including the Mojave desert, Transverse
Ranges, and Los Angeles basin. The substantial expansion if the number
of instruments operating in the area since 1975 allows location and focal
•
hanism determination for most events down to magnitude two. The
9
smaller events are more numerous and core widely distributed spatially
than larger ones previously stud!ed, so a high resolution study of
variations in regional tectonic deformation is possible. The study
was begun with the ideas of seeing if seismic deformation indicated
subregions of uniform stress distribution, how these regions relate
to the different observable geologic regions, and how faults in the
different regions interact at their boundaries. Data from 1975
through 1978 were obtained. Approximately 30 to 40 u^;eful events per
years are anticipated, so on the order of 150 focal mechanisms will
be obtained. Initial interpietat!.on and correction of the raw data
have been performed but final results are not available. Preliminary
results indicate that sharply bounded regions with considerably dif-
ference strain release patterns do exist in Southern California.
Recent studies of the geology of the Sierra Nevada Mountains, par-
tially motivated by proposed con •,truction of the Auburn Dam near Sacra-
mento, California, indicate that older fault zones in the area thought
to be inactive, instead have been th . a site of recent tectonic movement.
Seismic and geologic evidence suggests that a graben-t ype structure is
forming presently along a line from Lake Tahoe to Lake Almanor. The
structures appear to be similar to those of the Basin and Range province
located to the east and may represent an encroachment of the active tec-
tonic regime there into the more stable Sierra block. Although no recent
motion is known on faults near the Quincy Laser Ranging Site, two geo-
logists from Columbia University are restud y ing the Quincy area to con.-
10
firm this in light of present knowledge of fault reactivation in other
parts of the Sierra. Their findings will be submitted as a supplemental
report.
Publications
Bilham, R. G., and R. J. Beavan, 1979, Strains and tilts on crustal
blocks, Tectonophysics, 52, 121-138.
Accepted for Publication
Scholz, C. H., J. Beavan, and T. C. Hanks, 1979, Frictional metamorphism,
argon depletion, and tectonic stress on the Alpine fault, New
Zealand, J. Geophys. Res.


Strain buildup and release, earthquake prediction and selection of VLB sites for the margins of the North Pacific

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