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ARTICLE
The Cape Mendocino, California, Earthquakes ofApril 1992:
Subduction at the Triple Junction
D. Oppenheimer, G. Beroza, G. Carver, L. Dengler, J. Eaton, L.
Gee, F. Gonzalez, A. Jayko,W. H. Li, M. Lisowski, M. Magee, G.
Marshall, M. Murray, R. McPherson, B. Romanowicz,
K. Satake, R. Simpson, P. Somerville, R. Stein, D. Valentine
The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake
demonstrated thatthe North America-Gorda plate boundary is
seismogenic and illustrated hazards that couldresult from much
larger earthquakes forecast for the Cascadia region. The shock
occurredjust north of the Mendocino Triple Junction and caused
strong ground motion and moderatedamage in the immediate area.
Rupture initiated onshore at a depth of 10.5 kilometers
andpropagated up-dip and seaward. Slip on steep faults in the Gorda
plate generated twomagnitude 6.6 aftershocks on 26 April. The main
shock did not produce surface ruptureon land but caused coastal
uplift and a tsunami. The emerging picture of seismicity
andfaulting at the triple junction suggests that the region is
likely to continue experiencingsignificant seismicity.
On 25 April 1992 at 18:06 (UTC), asurface wave magnitude (Ms)
7.1 earth-quake occurred near the town of Petrolia,California (Fig.
1). The main shock wasfollowed the next day by two MS 6.6
after-shocks at 07:41 and 11:41, located offshoreabout 25 km
west-northwest of Petrolia.These three earthquakes and more
than2000 recorded aftershocks illuminated theconfiguration of the
Mendocino TripleJunction, where the Pacific, North Ameri-ca, and
southernmost Juan de Fuca (Gorda)plates meet. The occurrence of a M
7earthquake is not unusual at the triplejunction; over 60
earthquakes of ModifiedMercalli intensity > VI (1) or M >
5.5have occurred there since 1853 (2). How-ever, this earthquake
sequence may haveprovided the first direct evidence of inter-plate
seismicity and thus impacts regionalhazard assessment. In this
article, we de-scribe geophysical and seismological obser-vations
and discuss implications for seismichazards in the Pacific
Northwest.
Damage estimates ranged from $48 mil-D. Oppenheimer, J. Eaton,
A. Jayko, M. Lisowski, G.Marshall, M. Murray, R. Simpson, and R.
Stein are withthe U.S. Geological Survey, Menlo Park, CA 94025.
G.Beroza and M. Magee are in the Geophysics Depart-ment, Stanford
University, Stanford, CA 94305. G.Carver, L. Dengler, and R.
McPherson are in theDepartment of Geology, Humboldt State
University,Arcata, CA 95521. L. Gee and B. Romanowicz are atthe
University of California, Seismographic Station,ESB 475, Berkeley,
CA 94720. F. Gonzalez is with theNational Oceanic and Atmospheric
Administration'sPacific Marine Environmental Laboratory, Seattle,
WA98115. W. H. Li is with Geological Sciences, AJ-20,University of
Washington, Seattle, WA 98195. K.Satake is in the Department of
Geological Sciences,University of Michigan, Ann Arbor, Ml 48109. P.
Som-erville is with Woodward-Clyde Consultants, Pasade-na, CA
91101. D. Valentine is in the Department ofGeological Sciences,
University of California, SantaBarbara, CA 93106.
lion to $66 million and President Bushdeclared the region a
major disaster area.Much of the damage resulted from the mainshock;
however, fires triggered by the firstlarge aftershock destroyed
most of theScotia shopping district, and both large,off-shore
aftershocks caused additionalstructural damage. The relatively low
inci-dence of injuries and structural damage
Fig. 1. Simplified tectonic map in the vicinity ofthe Cape
Mendocino earthquake sequence.Stars, epicenters of three largest
earthquakes;contours, Modified Mercalli intensities (values,Roman
numerals) of main shock; open circles,strong motion instrument
sites (22) (adjacentnumbers give peak horizontal accelerations
ing). Abbreviations: FT, Fortuna; F, Ferndale; RD,Rio Dell; S,
Scotia; P, Petrolia; H, Honeydew;MF, Mendocino fault; CSZ, seaward
edge ofCascadia subduction zone; and SAF, San An-dreas fault.
SCIENCE * VOL. 261 * 23 JULY 1993
caused by this sequence is primarily theresult of low population
density and thepredominance of small, wood frame struc-tures in the
epicentral area. The sequencecaused 356 reported injuries,
destroyed 202buildings, and caused damage to an addi-tional 906
structures primarily in the townsof Petrolia, Ferndale, Rio Dell,
Scotia, andFortuna (Fig. 1) (3). It also triggered nu-merous
landslides and rock falls and causedwidespread liquifaction in
local river val-leys. Analysis of 1296 surveys in the northcoast
area indicate that the Modified Mer-calli intensity peaked at IX in
the Petroliaregion and decreased in approximately aradial pattern
around the epicenter (Fig. 1).Both of the two large aftershocks
producedpeak intensities of VIII, although the pat-tern was
somewhat different from the mainshock.
Tectonic SettingThe Cape Mendocino earthquakes are aresponse to
ongoing plate motions betweenthe Gorda, North America, and
Pacificplates at the Mendocino Triple Junction.The Gorda plate is
converging on theNorth America plate at about 2.5 to 3cm/year in
the direction N50WE to N550E(4). The seaward edge of Gorda plate
sub-duction is marked by an abrupt change insea-floor topography
and by the westernlimit of the accretionary prism imaged inseismic
reflection profiles (5). Active foldsand thrust faults in
Franciscan Complexand Cenozoic rocks and sediments of theoverriding
North America plate are parallelto the seaward edge of the Cascadia
subduc-tion zone (6).
Rigid plate theory predicts oblique con-vergence of the Gorda
plate with the Pacificplate at 5 cm/year in the direction N1
15VE(4). Translational motion occurs along theeast-west-trending,
vertical, right-lateralMendocino transform fault, whereas
theconvergence results in internal deformationof the Gorda plate.
The attendant Gordaplate seismicity recorded in the 17 yearsbefore
the Cape Mendocino sequence (7,8) (Fig. 2) has been concentrated in
twoparallel zones with a combined thickness ofapproximately 15 km.
In the region of theCape Mendocino earthquake, most seismic-
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ity locates at depths greater than 17 km; thehypocenter zone
dips about 60 eastwardbetween 124.75 and 123.250W, at whichpoint
the dip increases to about 250 (9).Most M > 5 earthquakes within
the Gordaplate exhibit left-lateral motion on
steepnortheast-oriented faults (7, 10) that re-lieve convergence
between the Gorda andPacific plates through slip on
preexistingplanes of weakness inherited at the GordaRidge (11).
The San Andreas fault marks the prin-cipal Pacific-North America
plate bound-ary south of the Mendocino Triple Junc-tion.
Triangulation data and observationsof ground cracks indicate the
fault rupturedas far north as Point Delgada in 1906 (12),but its
location farther north is uncertain.Some studies place it
immediately offshore(13), but others suggest that it merges
withonshore faults at the triple junction (5).Geometry requires
that the Pacific plate isalso in contact with the North
Americaplate along the Mendocino fault above thesubducting Gorda
plate.
Until the Cape Mendocino earthquake,few earthquakes were
recorded with focalmechanisms that indicated slip on the Cas-cadia
subduction zone. However, compari-sons of the age, spreading rates,
physiogra-
phy, and seismicity of the Juan de Fuca-Gorda plate system with
other subductingplates suggest that it does not subduct
aseis-mically but instead is locked and capable ofgenerating major
earthquakes (14). Paleo-seismic evidence of large, late
Holocenesubduction earthquakes is present along thesubduction zone
in submerged and buriedwetlands (15), raised marine terraces
(16),and surface displacement on thrust faultsthat may be
genetically related to largesubduction events (17). Radiocarbon
dat-ing indicates that at least three episodes ofseismicity of
similar age are represented inthe stratigraphy from central
Washingtonto northern California in the last 2000years; the last
episode occurred at about1700 A.D. (17).
ObservationsSeismicity. The hypocenter of the 25 April1992 main
shock was located 4 km east ofPetrolia at a depth of 10.6 km (Fig.
3). A
focal mechanism determined by the inver-sion of teleseismic
mantle Rayleigh wavesand aftershock locations indicate nearlypure
thrust motion on a N10TW-strikingfault plane that dips 130 to the
east-north-east (18) (Table 1). The location of thehypocenter at
the southeast end of theaftershock zone suggests that the fault
rup-tured unilaterally to the west (19). Mostaftershocks
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shock is unknown because of the paucity ofaftershocks. However,
the second after-shock was located within a trend of
smalleraftershocks at depths of 14 to 30 km on asoutheast-striking
plane that dips about 800to the southwest (Fig. 3B, cc'); this
orien-tation is consistent with the focal mecha-nism. The depths
and mechanisms of thetwo large aftershocks provide evidence
thatrupture took place on faults in the Gordaplate, distinct from
the main shock fault.
Although no large shocks ruptured theMendocino fault during this
sequence,many aftershocks occurred on the eastwardprojection of the
fault (Fig. 3). The after-shock activity was bounded on the
southwhere the distribution of hypocenters isnear vertical and
extends to a depth of 25km (Fig. 3B, bb'). If this marks the
bound-ary between the Gorda and Pacific plates,then the lack of any
aftershocks in thePacific plate suggests that the main
shockrepresented strain release between theGorda and North America
plates. Themapped location of the Mendocino fault inthis region is
uncertain (5), and this east-
Fig. 4. Broad-band velocity records from fourstations (ALE:
epicentral distance = 5185km, azimuth = 90; HRV: A = 4366 km,
azimuth= 690; MAJO: A = 8029 km, azimuth = 3050;ISA: 1 = 745 km,
azimuth = 1320) for the mainshock (top trace) and the first
(middle) andsecond (lower) aftershocks. The amplitudes ofthe
seismograms at ALE, HRV, and MAJO areincreased relative to ISA for
display. The largeamplitudes of the second aftershock relative
tothe other two events in the along-strike azimuth(ISA) is
attributable to rupture directivity.
west trend of seismicity may define theposition of the Mendocino
fault.
Source properties. The mechanism andlocation of the two
aftershocks were simi-lar, but the second aftershock exhibited
astrong variation of amplitude with azimuth(Fig. 4). The seismic
moment of the secondaftershock was approximately twice that ofthe
first, but amplitudes of the P wave forthis event were as much as
10 times as largenear an azimuth of 1300. This variation ismost
easily attributable to enhancement ofthe amplitude in the direction
of rupture,known as directivity (21). Directivity in Pwaves is
surprising because it requires rup-ture velocities that are a large
fraction ofthe P wave velocity. The high amplitudesand strong
high-frequency content associat-
t ] 16.49 x102090/1.59 x 102 C1/s2
U
2s0
Fig. 5. Strong motion recordings and threecomponents of the peak
accelerations fromstations of the California Strong Motion
Instru-mentation Program (CSMIP) at Cape Mendo-cino and Petrolia
(22). Discernible first motiondirections of the P and S waves for
long-periodevent (Ep1 and ES1) are indicated, as is thelarge,
high-frequency pulse at Cape Mendo-cino (E2).
ed with the second aftershock may explainsome of the differences
in the intensitypatterns for the main shock and two after-shocks.
Although the second aftershockhad 25% of the moment of the main
shock,it has larger velocity amplitudes at stationsto the
southeast, such as ISA. The differ-ence in both Modified Mercalli
intensityand broad-band velocity records betweenthe main shock and
the second aftershockwas probably enhanced by rupture propaga-tion
to the west during the main shock asinferred from the location of
the hypo-center at the down-dip end of the ruptureplane.
The strong ground motions of the mainshock and two aftershocks
were recorded on14 instruments at epicentral distances of 5to 130
km (Fig. 1), and the peak accelera-tions were some of the highest
ever record-ed (22). Recordings of the main shock atPetrolia and
Cape Mendocino (Fig. 5) atepicentral distances of 5 and 10 km,
respec-tively, have absolute time, facilitating theanalysis of
rupture evolution on the fault.Modeling of the large, long-period
pulsethat occurred 1 s after the main shockbegan (Fig. 5, Ep1 and
Es1) with generalizedray theory indicates that this pulse
originat-ed from slip that occurred about 5 kmup-dip from the
hypocenter, beneath Petro-lia. This result is consistent with the
arrivaltimes and polarities of the vertical P wavesand horizontal S
waves at both stations. Inaddition, the P wave first motions
wereupward and northwestward at Cape Men-docino. These motions
indicate that thesource was southeast of the station. Thissource
location is consistent with the west-southwest direction of rupture
determinedfrom teleseismic surface waves (19), al-though that study
inferred that the ruptureinitiated offshore. A large,
high-frequencypulse (Fig. 5, E2) followed the long-periodpulse at
Cape Mendocino and exceeded igon both horizontal components. This
pulsewas not discernible at the neighboringPetrolia station; thus,
it cannot be ex-plained simply by source effects but mayrepresent
motions that were generated oramplified locally near the Cape
Mendocinostation.
Table 1. Earthquake parameters.
Origin time* (UTC) Latitude* Longitude* Depth* Centroid Monmentt
Striket Dipt RaketOrigntie* (TC) (North) (West) (kin) deptht mbt
Mst (dyn~cm) Srkt Dp ae(kin)
Main shock 25 April 18:06:05.16 40019.94' 124013.69' 10.6 20 to
25 6.3 7.1 4.45 x 1026 349.70 13.00 105.60First after- 26 April
07:41:39.98 40026.13' 124034.43' 19.3 20 to 25 5.9 6.6 6.35 x 1025
122.30 75.90 175.20shock
Second after- 26 April 1 1:18:25.82 40023.38' 124034.30' 21.7 30
to 35 6.5 6.6 1.20 x 1Qo26 311.50 89.60 181.80shock
*Hypocentral location determined from the local seismic network
of the U.S. Geological Survey (8). tFrom surface-wave inversion
(18, 20). *National EarthquakeInformation Center, Preliminary
Determination of Epicenters.
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ALE (X0 ) HRV (x4)ifV~~lv'n1 I
MAJO( 0) ISA
01+0Time (s) -
--------------------------
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Coseismic displacement. The elastic strainreleased by the main
shock caused signifi-cant horizontal and vertical deformation inthe
epicentral region. The main shock ele-vated about 25 km of the
coast from 3 kmsouth of Punta Gorda to Cape Mendocino(Fig. 6). Many
intertidal organisms inhab-iting rocky reefs perished in the 3
weeksafter the main shock. Maximum uplift was140 + 20 cm at Mussel
Rock and 40 to 50cm at the northernmost reef at Cape Men-docino
(23). The lack of rocky intertidalenvironments farther north
precluded theprecise location of the northern limit of theuplift,
but several near-shore rocks locatedabout 7 km north of Cape
Mendocinoshowed no evidence of uplift.
Coseismic horizontal and vertical sitedisplacements in a
regional geodetic net-work (Fig. 6) were determined from
GlobalPositioning System (GPS) surveys in 1989,1991, and 1 month
after the main shock.The relative positions of most sites near
theepicenter were measured shortly after the17 August 1991 Honeydew
earthquake[body wave magnitude (mb) 6.0], whichoccurred 6 km south
of the Cape Mendo-cino epicenter. The coseismic displace-ments were
determined by comparison of the1989 to 1992 observations, except in
thevicinity of the Honeydew event where the1991 survey was
referenced. All displace-ments were corrected for secular strain
accu-mulation estimated from Geodolite trilater-ation measurements
made between 1981 and1989. A site 13 km northeast of the epicen-ter
had the largest measured coseismic dis-placement, moving 40 2 cm to
the west-southwest and subsiding 16 8 cm.
Our preferred uniform-slip fault model(24), estimated from the
coseismic sitedisplacements and coastal uplift observa-tions,
indicates 2.7 m of nearly pure thrustmotion occurred on a gently
dipping faultplane. This model, chosen from a suite ofacceptable
models (24), is consistent withthe main shock focal mechanism, the
hy-pocenter location, and the distribution ofaftershocks (Fig. 6
and Table 2). The rangeof geodetic moment inferred from the ac-
Table 2. Displacement model.
Parameter Value
Width 16 kmLength 21.5 kmDepth to top edge 6.3 kmLatitude
origin* 40018.08'NLongitude origin* 12411 .80'WDepth at epicenter
9.2 km
locationMomentt 2.79 x 1026 dyn'cmStrike/dip/rake
3500/12.00/940Slip 2.7 m
ceptable models is 2.5 x 1026 to 3.5 x 1026dyn-cm, about 60% of
the main shockseismic moment (Table 1). The model pre-dicts a
maximum uplift along the coast thatis consistent with but somewhat
less thanthe observed uplift. More complex modelsthat use
nonuniform slip to describe therupture may improve these estimates
ofuplift and geodetic moment.
Tsunani. The main shock generated asmall tsunami recorded by
tide gauges alongthe California, southern Oregon, and Ha-waii
coastlines (Fig. 7). The largest tsunamiamplitudes were recorded at
Crescent City,California, where two well-defined packetsof wave
energy were recorded within thefirst 5 hours with maximum positive
heightsof 35 and 53 cm. Neither the precise arrivaltime nor the
polarity of the first wave areclear because of the presence of
backgroundnoise. However, the first packet of waveenergy is
consistent with the predicted trav-el time of 47 min for a wave ray
path thattraversed deep water. The second wavepacket probably
represents coastal trappedwaves, or edge waves, having much
slowervelocities and amplitudes that rapidly de-crease with
distance offshore. Because thetsunami arrival nearly coincided with
lowtide at Crescent City, the wave did notcause any damage. The
tsunami at CrescentCity had an 8-hour duration; wave heightsreached
a maximum 3 to 4 hours after thefirst arrival. Tide gauges also
recorded theinitial arrival and subsequent edge waves atNorth Spit
(Eureka, California) (20 minand 2.5 hours), Arena Cove (35 min
and
3.5 hours), and Point Reyes (65 min and 3hours).
Motion on the Plate BoundaryInterplate main shock. The main
shock faultprojects to the sea floor within 5 km of theseaward edge
of the Cascadia subductionzone (25) (Fig. 3), suggesting that the
mainshock ruptured the Gorda-North Americaplate boundary. In
contrast, the upperboundary of the pre-main shock seismicity,which
is 7 km deeper than the main shockrupture plane (Figs. 2 and 3),
projects tothe surface about 85 km west of the Casca-dia subduction
zone and thus does notappear to define the plate boundary.
Theseismicity gap between the slip plane of themain shock and the
pre-main shock seis-micity is about the same thickness as theGorda
crust and overlying accretionary sed-iments, as determined from
refraction ex-periments 10 km east of the seaward edge ofthe
subduction zone (26). The gap mayreflect a ductile subducted Gorda
crust, andthe inception of seismicity at a depth of 17km may
reflect brittle behavior of theGorda upper mantle (27). Tabor and
Smith(28) reached a similar conclusion from theirobservations of
seismicity and velocitystructure of the Juan de Fuca plate
beneaththe Olympic peninsula of Washington.
However, an inversion for the three-dimensional velocity
structure of the regionindicates that velocities typical of
Gordacrust are evident at depths greater than 15to 20 km (29).
Moreover, modeling of
Fig. 6. Observed and predictedcoseismic displacements for
theCape Mendocino main shock (epi-center located at star). The
vec-tors are horizontal displacementsrelative to a site located
at4109.20'N, 123052.92'W. Ob-served displacements derivedfrom GPS
and Geodolite mea-surements; ellipses enclose re-
40'40' - gions of 95% confidence. Predict-ed displacements are
from amodel of uniform slip on the north-
(~" east-dipping rectangular faultplane, indicated by its
surfaceprojection. Rounded rectanglesshow vertical
displacementsmeasured by GPS that are great-er than their standard
deviations.Contours are elevation changes
40 2O' - in millimeters predicted by themodel. Abbreviations:
CM, CapeMendocino; MR, Mussel Rock;and PG, Punta Gorda. (Inset)
Up-lift measurements and their stan-dard deviations from the
die-off of
51) marine organisms at coastal sites(open circles on map) and
pre-dicted uplift projected alongN1 0W.
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*Southeast corner of fault plane. tAssumes uni-form rigidity of
3 x 10l dyn/cm2.436
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thermal effects on the strength of the sub-ducting oceanic
lithosphere (30) suggeststhat the double seismic layers observed
atdepths of 20 and 30 km (Fig. 2) reflect,respectively, the brittle
upper crust andupper mantle of the Gorda plate; the inter-vening,
relatively aseismic region wouldcorrespond to the ductile lower
crust. Con-sequently, these studies suggest that theCape Mendocino
main shock was an intra-plate event in the North America plate.
Whether the main shock was an inter- orintra-plate event, the
Cape Mendocino mainshock clearly relieved strain resulting from
therelative Gorda-North America plate motion.We note, however, that
the main shockruptured a region of the plate boundary thatdiffers
considerably from the boundary farthernorth, as indicated by the
change in itsorientation from north-northwest to north-west (5),
the relatively narrow width of theplate, the likely presence of
subducted sedi-ments in the region of main shock rupture,and its
younger age (4). Thus, this earth-quake may not be typical of other
Cascadiasubduction zone earthquakes.
Intraplate aftershocks. The location,depth, and focal mechanisms
of the twolarge aftershocks indicate that they rup-tured the Gorda
plate. The seismic dataindicate that right-lateral slip occurred on
avertical, northwest-oriented fault plane forat least the second
event. For most earlierGorda shocks, rupture occurred as
left-lateral slip on a northeast-oriented plane,perhaps because
this orientation may allowreactivation of normal faults formed at
theGorda spreading ridge (11). From a consid-eration of stress
release, either orientation
Fig. 7. Tsunami measurements at 3tide gauge stations along the E
-coasts of California, Oregon, Ha- _' Hwaii, and Johnston Island.
Tidal 3 2 Isignal has been removed from all * F J1data except for
Crescent City, fCalifornia (inset) (note change of IL.scale). Time
of main shock is 0 0hours. The vertical line marks theexpected
tsunami arrival time.Contour lines (dotted) representocean depth in
kilometers, exceptfor the single, unlabeled 500-mbathymetric
contour line nearestthe coast.
reduces north-south compressional stressand down-dip tension in
the Gorda plate,but rupture of the northwest-oriented planemay have
been favored because of the staticstress changes imposed by the
main shockand, perhaps, the first aftershock.
To test this hypothesis, we modeled thechanges in static stress
(31) imposed by themain shock (Table 2) on three vertical
faultplanes: the east-west-oriented Mendocinofault, the possible
N40E-oriented slipplane of the first large aftershock, and
theN500W-oriented fault of the second largeGorda aftershock. Large
regions of thenorthwest-oriented fault and the Mendo-cino fault
received an increase in right-lateral shear stress greater than 3
bars andequally large increases in normal extensionresulting from
the main shock. Both theincrease in right-lateral shear and the
in-creased extension would bring both ofthese right-lateral faults
closer to failureunder a Coulomb failure criterion for non-zero
coefficients of friction. About 90% ofthe aftershocks within 4 km
of the Men-docino fault and the northwest Gordafault occur where
the modeling predictsthe stress changes should load these
faultstoward failure for coefficients of frictionranging from 0.0
to 0.75. Thus, staticstress changes may have helped
triggeraftershocks along these two faults.
The model also indicates that staticstress changes induced by
the main shockare slightly more favorable for failure on
anortheast-striking, left-lateral fault plane inthe location of the
first large aftershock.primarily because of a decrease in
normalstress. In consideration of the present rela-
tive hypocentral locations of the two largeaftershocks, slip on
the northeast-orientedplane of the first aftershock would haveadded
minor left-lateral shear to the north-west-oriented plane of the
second after-shock but would have greatly decreased thenormal
stress on this plane. This scenarioprovides a simple mechanism in
which thefirst aftershock helped trigger the second,similar to the
scenario proposed for theElmore Ranch-Superstition Hills pair
ofearthquakes (32).
Hazard implications. The Cape Mendo-cino earthquake sequence
provided seismo-logical evidence that the relative motionbetween
the North America and Gordaplates results in significant thrust
earth-quakes. In addition to the large groundmotions generated by
such shocks, they cantrigger equally hazardous aftershock
se-quences offshore in the Gorda plate and onthe Gorda-Pacific
plate boundary. This se-quence illustrates how a shallow
thrustevent, such as the one of moment magni-tude (Mv) 8.5 that is
forecast for the entireCascadia subduction zone (14), could
gen-erate a tsunami of greater amplitude thanthe Cape Mendocino
main shock. Not onlywould this tsunami inundate communitiesalong
much of the Pacific Northwest coastwithin minutes of the main
shock, but itcould persist for 8 hours at some locales.The 25 April
1992 main shock rupturedonly a small part of the plate boundary
andapparently did not trigger slip on any of theHolocene shallow
thrust faults observedonshore in the triple junction region
(17).Thus, given the high level of historicalseismicity and the
emerging picture of manyactive faults, the region is likely to
continueexperiencing significant seismicity.
REFERENCES AND NOTES1. H. 0. Wood and F. Neumann, Bull. Seismol.
Soc.
Am. 21, 277 (1931).2. L. Dengler, G. Carver, R. McPherson,
Calif. Geol.
45, 40 (1992).3. California Office of Emergency Services
Situation
Report, 4 May 1992; American Red Cross Disas-ter Update, 8 May
1992.
4. R. Riddihough, J. Geophys. Res. 89,6980 (1984);C. Nishimura,
D. S. Wilson, R. N. Hey, ibid., p.10283; D. S. Wilson, ibid., in
press.
5. S. H. Clarke, Jr., Am. Assoc. Pet. Geol. Bull. 76,199
(1992).
6. H. M. Kelsey and G. A. Carver, J. Geophys. Res.93, 4797
(1988).
7. R. C. McPherson, thesis, Humboldt State Univer-sity
(1989).
8. Earthquakes for the period August 1974 to De-cember 1984 were
recorded by the Tera Corpo-ration. Since 1983, the seismicity has
been re-corded by local networks operated by the U.S.Geological
Survey and the University of Californiaat Berkeley. All earthquake
locations are basedon a one-dimensional velocity model with
stationcorrections developed from a joint hypocenter-velocity
inversion of travel-time data and locatedwith Hypoinverse [F. W.
Klein, U.S. Geol. Surv.Open File Rep. 89-314 (1989)]. Calibration
explo-sions at Punta Gorda and Cape Mendocino werelocated to the
northwest, 1.5 and 2.7 km, respec-
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tively, at the surface. Locations of the offshoreearthquakes are
not as accurate as those onshorebecause of the network
geometry.
9. R. S. Cockerham, Bull. Seismol. Soc. Am. 74, 569(1984); S. R.
Walter, ibid. 76, 583 (1986).
10. T. V. McEvilly, Nature220, 901 (1968); J. P. Eaton,Eos 62,
959 (1981); T. J. Lay, J. W. Given, H.Kanamori, Bull. Seism. Soc.
Am. 72, 439 (1982).
11. D. S. Wilson, J. Geophys. Res. 94, 3065 (1989).12. A. C.
Lawson, Camegie Inst. Washington Publ.
87, 54 (1908); W. Thatcher and M. Lisowski, Eos68,1507
(1987).
13. J. R. Curray and R. D. Nason, Geol. Soc. Am. Bull.78, 413
(1967); L. Seeber, M. Barazangi, A. Now-roozi, Bull. Seismol. Soc.
Am. 60,1669 (1970).
14. T. H. Heaton and H. Kanamori, Bull. Seismol. Soc.Am. 74, 993
(1984); T. H. Heaton and S. H.Hartzell, ibid. 76, 675 (1986).
15. B. F. Atwater, Science 236, 942 (1987); M. E.Danenz and C.
D. Peterson, Tectonics 9, 1 (1990);A. R. Nelson, Quat. Res. 38, 74
(1992); G. S. Vick,thesis, Humboldt State University (1989); D.
W.Valentine, thesis, Humboldt State University (1992).
16. D. Merritts and W. B. Bull, Geology 17, 1020(1989).
17. S. H. Clarke, Jr., and G. A. Carver, Science 255,188
(1992).
18. We inverted for the main shock source parame-ters using the
formalism of B. Romanowicz and T.Monfret [Ann. Geophys. 4, 271
(1986)] and thespherical-Earth model PREM [A. M. Dziewonskiand D.
L. Anderson, Phys. Earth Planet. Inter. 25,297 (1981)] to correct
for propagation effects.Rayleigh waves with periods between 140
and320 s from a global distribution of stations weresampled at 10
frequencies.
19. An analysis of broad-band surface waves indi-cates that the
main shock ruptured to the south-west (azimuth = 2300) with the
largest momentrelease beginning 5 km from and extending to 20km
offshore [C. J. Ammon, A. A. Velasco, T. Lay,Geophys. Res. Lett.
20, 97 (1993)].
20. For the aftershocks, we computed path-depen-dent amplitude
and phase corrections based onanalysis of the main shock. The
approach ofpath-dependent corrections is advantageous foruse of
data at regional distances as well asshorter periods [M. Pasyanos
and B. Romano-wicz, Eos 73, 372 (1992)].
21. H. Benioff, Calif Div. Mines Geol. Bull. 171, 199(1955); A.
Ben Menahem, Bull. Seismol. Soc. Am.51, 401 (1961).
22. A. Shakal et al., Calif. Div. Mines Geol. Rep.OSMS 92-05
(1992).
23. Where colonies of organisms were completelykilled, the
vertical range of the organisms repre-sents a minimum measure of
the uplift. On somereefs, only the upper parts of some colonies
ofplants and sessile animals, such as mussels,died. The vertical
extent of mortality in thesecolonies provided the best basis for
estimates ofthe amount of uplift. The mean and standarddeviation of
the vertical extent of mortality for eachcolony (Fig. 6, inset) is
estimated from multiplemeasurements made by a laser total station
withan instrument precision of 1 mm.
24. We assumed that slip is uniform on a rectangular,planar
fault embedded in an elastic half space.We performed a Monte Carlo
search of the pa-rameters describing the geometry of the faultplane
and, for each geometry, used the horizontaland vertical GPS and
coastal-uplift data to esti-mate the magnitude and rake of the slip
vector.The scatter of the residuals for the best fittingmodels,
including our preferred model (Table 2),is 2.2 times the a priori
observational errors. AnF-ratio test indicates other models with
residualscatter less than 2.6 times the a priori errors donot
differ significantly from the best fitting modelsat the 95%
confidence level.
25. In the projections, we assumed that faults areplanar, the
ocean depth is 2.3 km at the seawardedge of the Cascadia subduction
zone at thelatitude 40'27'N, and the mean elevation of theseismic
network is 0.9 km. Main-shock parame-ters are from Table 1. We
assumed the upper
boundary of the background Gorda seismicitydips 60 east at a
depth of 17.5 km at the coordi-nates of the main shock
hypocenter.
26. G. G. Shor, P. Dehlinger, H. K. Kirk, W. S. French,J.
Geophys. Res. 73, 2175 (1968); S. W. Smith, J.S. Knapp, R. C.
McPherson, ibid. 98, 8153 (1993).
27. W. P. Chen and P. Molnar, ibid. 88, 4183 (1983);P. Molnar
and P. England, ibid. 95, 4833 (1990).
28. J. J. Tabor and S. W. Smith, Bull. Seismol. Soc.Am. 75, 237
(1985).
29. D. Verdonck and G. Zandt, Lawrence LivermoreNati. Lab. Rep.
UCRL-JC-1 11629(1992).
30. K. Wang and G. Rogers, Eos 73, 504 (1992).31. We estimated
static stress changes caused by
32.
33.
the main shock using a rectangular dislocationsurface (Table 2)
in an elastic half-space [Y.Okada, Bull. Seismol. Soc. Am. 82, 1018
(1992)]to stimulate the earthquake rupture. The results ofthe model
are insensitive to small variations in theassumed fault geometries
or slip distributions,although the maximum values and spatial
detailsof the stress fields are.K. Hudnut et al., Bull. Seismol.
Soc. Am. 79, 282(1989).We thank B. Ellsworth, C. Weaver, D. Wilson,
andD. Merritts for reviews of the manuscript. Themeasurement of
coastal uplift was supported by agrant from the Pacific Gas and
Electric Company.
* RESEARCH ARTICLE
NMR Structure of a Specific DNAComplex of Zn-Containing DNA
Binding Domain of GATA-1James G. Omichinski, G. Marius Clore,*
Olivier Schaad,
Gary Felsenfeld, Cecelia Trainor, Ettore Appella,Stephen J.
Stahl, Angela M. Gronenborn*
The three-dimensional solution structure of a complex between
the DNA binding domain ofthe chicken erythroid transcription factor
GATA-1 and its cognate DNA site has beendetermined with
multidimensional heteronuclear magnetic resonance spectroscopy.
TheDNA binding domain consists of a core which contains a zinc
coordinated by four cysteinesand a carboxyl-terminal tail. The core
is composed of two irregular antiparallel (3 sheets andan a helix,
followed by a long loop that leads into the carboxyl-terminal tail.
The amino-terminal part of the core, including the helix, is
similar in structure, although not in sequence,to the
amino-terminal zinc module of the glucocorticoid receptor DNA
binding domain. In theother regions, the structures of these two
DNA binding domains are entirely different. TheDNA target site in
contact with the protein spans eight base pairs. The helix and the
loopconnecting the two antiparallel P sheets interact with the
major groove of the DNA. Thecarboxyl-terminal tail, which is an
essential determinantof specific binding, wraps around intothe
minor groove. The complex resembles a hand holding a rope with the
palm and fingersrepresenting the protein core and the thumb, the
carboxyl-terminal tail. The specific inter-actions between GATA-1
and DNA in the major groove are mainly hydrophobic in nature,which
accounts for the preponderance of thymines in the target site. A
large number ofinteractions are observed with the phosphate
backbone.
The erythroid-specific transcription factorGATA- 1 is
responsible for the regulation oftranscription of
erythroid-expressed genesJ. G. Omichinski, G. M. Clore, 0. Schaad,
and A. M.Gronenborn are in the Laboratory of Chemical Phys-ics,
Building 5, National Institute of Diabetes andDigestive and Kidney
Diseases, National Institutes ofHealth, Bethesda, MD 20892. G.
Felsenfeld and C.Trainor are in the Laboratory of Molecular
Biology,Building 5, National Institute of Diabetes and Digestiveand
Kidney Diseases, National Institutes of Health,Bethesda, MD 20892.
E. Appella is in the Laboratoryof Cell Biology, Building 37,
National Cancer Institute,National Institutes of Health, Bethesda,
MD 20892. S.J. Stahl is in the Protein Expression Laboratory,
Build-ing 6B, Office of the Director, National Institutes ofHealth,
Bethesda, MD 20892.*To whom correspondence should be addressed.
and is an essential component required forthe generation of the
erythroid lineage (1).GATA-1 binds specifically as a monomer tothe
asymmetric consensus target sequence(T/A)GATA(AIG) found in the
cis-regu-latory elements of all globin genes and mostother
erythroid-specific genes that havebeen examined (2). GATA-1 was the
firstmember of a family of proteins, which nowincludes regulatory
proteins expressed inother cell lineages, characterized by
theirrecognition of the GATA DNA sequenceand by the presence of two
metal bindingregions of the form Cys-X-X-Cys-(X)17-Cys-X-X-Cys
separated by 29 residues (2,3). Mutation and deletion studies
on
SCIENCE * VOL. 261 * 23 JULY 1993
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