VOL. 77, NO. $ JOURNAL OF GEOPHYSICAL RESEARCH FEBRUARY 10, 1972

AlaskanEarthquake of 1964and ChileanEarthquake of 1960: Implicationsfor ArcTectonics

GEORGE PLAFKER

U.S. GeologicalSurvey, Menlo Park, Cali]ornia 94025

The 1'964Alaskan earthquake(M.• • 8.4) involved a segmentof the easternAleutian arc 800gm long; the 1960Chilean earthquake sequence (M, • 8.5) affectedroughly 100 km of the southernPeru-Chile arc. These two major eventsare strikinglysimilar in that (1) seismic- ity wasshallow (<70 km), the earthquakefocal regions and mostof the associatedtectonic deformation being between the oceanictrenches and volcanic chains of the two arcs; (2) regionalvertical displacementswere characterizedby broad asymmetricdownwarps elongate parallel to the arcswith flanking zonesof marked uplift on the seawardsides and minor, pos- sibly local,uplift on the landwardsides; and (3) horizontaldisplacements, where determined by retriangulation,involved systematicshifts in a generallyseaward direction and transverse tensilestrains across the zonesof subsidence.Surface displacements and seismicityfor both events are compatible with dislocationmodels involving predominantly dip-slip movement of 20 metersor more on major complexthrust faults (megathrusts)inclined at averageangles of about 9 ø beneath the eastern Aleutian arc and perhaps 20ø beneath the Peru-Chile arc. The thrust-fault mechanism deduced for both the Alaskan and Chilean earthquakes is broadly consistentwith the conceptthat the sectorsof the Pacific rim in which they occurredare major zonesof convergencealong which the oceanicplates progressively underthrust the less mobile Americaplate. Directionsof convergencebetween lithospheric plates at thesearcs as deducedprimarily from paleomagneticdata are in reasonablygood agreement with the ob- servedearthquake-related deformation; the deducedrates of convergence,however, appear to be too highin the easternAleutian arc and too low in the southernPeru-Chile arc. Despite grosssimilarities in tectonicsetting and the presentstyle of earthquake-relateddeformation, the geologiesof thecontinental margins in the easternAleutian arc and southern Peru-Chile arc differsignificantly. This differencesuggests that Mesozoicand Cenozoicsediments and vol- canicrocks conveyed into the easternAleutian trench have progressivelyaccreted to the Alaskancontinental margin, whereas most or all of the materialcarried into the southernPeru- Chiletrench has disappeared beneath the Chileancontinental margin.

Accordingto current modelsof sea-floor indeed reflect relative underthrusting of the spreadingand plate tectonics, the seismiczones oceanicplates beneaththe arcs. of tectonic arcs delineate major underthrusts The first part of this paper presents data alongwhich plates of lithosphereconverge and that emphasizethe striking similaritiesin the are consumed.The eastern end of the Aleutian regionaltectonic setting and mechanismof the arc in the Gulf of Alaska and the Peru-Chile Alaskan and Chilean earthquakes. Basic data arc alongthe westernmargin of SouthAmerica usedto deducethe parametersfor the causative are suchboundaries, and data on the earth- faults are the residual surface displacements, quake-relateddeformation that occursalong the spatial distribution of seismicity, nodal them is especiallypertinent for the insightit plane solutions,dislocation analyses, and geo- can provideinto the nature of arc tectonics. physicalinformation on crustal structure.The The great1964 Alaskan and 1960 Chilean earth- secondpart of the paper considerssome impli- quake sequences,among the largestseismic cationsof these earthquakemechanisms for the events ever recorded, affected major segments direction and rate of convergencebetween of theseplate boundaries. An evaluationof the lithosphereplates and for the geologicrecord of seismicityand deformationrelated to these late Cenozoicdeformation in thesesame regions. events leads to the conclusionthat they do Speculationsin the secondpart of the paper assume that the models deduced for the mecha- Copyright¸ 1972 by the AmericanGeophysical Union. nisms of the 1964 Alaskan and 1960 Chilean ,

901 GEORGE PLAFKER earthquakesare basicallycorrect, although many by Kanamori [1970], a revised dislocation of the detailsmay be in error. model of the earthquakeby Hastie and Savage The present study is an outgrowth of the [1970], and unpublishedresults of retriangula- writer's work on the tectonic deformation as- tion by the U.S. Coast and Geodetic Survey sociated with these two major seismic events. between the Kodiak group of islandsand the Most of the data on which this paper is based Alaska mainland. are from publicationson the Alaska earthquake REGIONAL TECTONIC SETTINGS OF by Plafker [1969] and on the Chile earthquake by PlaCket and Savage [1970]. Pertinent addi- THE EARTHQUAKES tional information that has become available The 1964 Alaskan and 1960 Chilean earth- since the two papers were written are incor- quakes occurred in segments of tectonically porated. In particular, these data include a active arcs along the margins of major litho- surface-wavesolution for the Alaska earthquake spheric plates (Figure 1). Both earthquakes

Fig. 1. Location of the March 27, 1964, Alaskan and May 21-22, 1960, Chilean earthquakes relative to major tectonic elements around the Pacific margin. Boundaries of lithospheric plates are indicated by double lines for spreadingrises, barbed lines for subductionzones, and single lines for predominantly strike-slip faults. Arrows indicate the relatœvehorizontal component of plate motion as derived from nodal plane solutions. Modified after Isacks et al. [1968], Morgan [1968], and Morgan et al. [1969]. ALASKAN AND CYIILEAN EARTI-IQUAKES 903 were at the extremities of active arcs as defined deformedTertiary sedimentsare exposedalong by the presenceof oceanic deeps, volcanic the coast and on the offshore islands in the chains,and intermediate-to deep-focusseismic- southernpart of the Peru-Chile arc. ity. The setting of the Alaskan earthquakeis at the northeastern end of the Aleutian are, where the are gradually merges eastward into SPATIALDISTRIBUTION OF TIlE EARTHQUAKES a zone of shallow-focusseismic activity and RELATIVE TO MAJOR ARe FEATURES transform faulting. The Chilean earthquake The epicentersof the main shocksand larger focal region is near the southern end of the aftershocks for the 1964 Alaskan and 1960 Peru-Chile are, which appears to terminate Chilean earthquake sequencesare shown in rather abruptly at its junction with the Chile Figures 2 and 3, respectively.Also shown in rise, a probablesite of combinedspreading and these figures are the earthquake-related re- transform faulting [Herton and Hayes, 1969]. gional tectonic displacements,as well as the Although both the Aleutian and Peru-Chile major arc features. It should be noted that arcs are ocean-continenttransitions, they have seismologiccontrol for earthquakesoriginating all the primary features of structural arcs in in southernChile was poor in 1960, and there ocean-islandarc transition zones [Benio#, 1954; may be a large longitudinal bias. As a conse- Gutenberg and Richter, 1954]: (1) a deep quence, some of the scatter in the positionsof oceanic trench or sediment-filledtrench; (2) the epicentersshown in Figure 3 probably re- a subparalleldiscontinuous chain of active and sults from errors in location that could be as dormant andesitie volcanoes; (3) a belt of much as 50 km. Experienceindicates that the active seismicity,or Benioff zone, in which the epicentral locationsare generally biasedtoward lower limit of hypoeentersgenerally deepens the east (C. Lomnitz, written communication, from beneath the trench to beneath the volcanic 1969). chain and continent, (4) parallel zonesof iso- Both the Alaskan earthquake (M • 8.4) static gravity lows generally situated over the and the Chilean earthquake (M • 8.5) involved lower part of the inner wall of the trench with complexmultiple ruptures [Duda, 1963; Wyss gravity highs near the associatedshelf-edge and Brune, 1967]. Seismicmoments determined break; and (5) progressivethickening of the from very-long-periodseismic waves by Kana- crust from the vicinity of the trench beneath mori [1970, and oral communication] indicate the continental margins. that thesetwo• earthquakes are by far the largest Despite their broad similarities,the Aleutian seismic events of the last few decades. With few and Peru-Chile arcs differ from one another in exceptions,the main shocks,the larger after- three important respects.One is that the Aleu- shocks,and (for the Chilean event) the fore- tian are marks an ocean-continent transition shocks have epicenters that lie between the in its eastern part and a true island are that trench and associated volcanic arc; in the traversesoceanic crust in its western part (west Alaskan earthquake, most of them occur be- of 165øW), whereas the Peru-Chile are is an neath lhe continental shelf and slope. Hypo- ocean-continenttransition along its entirelength. central depths for both main shockswere in- The secondis that the deepestearthquake hypo- determinate. Of those larger aftershocks(M •_ centers recorded in the Aleutian are are at 5) for which hypocenterscould be determined, about 170 km, whereasthey extend to depths all were shallower than 40 km in the Alaskan as great as 650 km in parts of the Peru-Chile sequence[Algermissen, 1965], and all but three arc. Maximum recordedhypocentral depths in were shallower than 64 km in the Chilean the regionof the 1960earthquake, however, are sequence[Talley and Cloud, 1962]. The spatial only about 150-200km. And third, the conti- distribution of aftershock epicenters suggests nental shelf and coastal region in the eastern that most, if not all, the faulting was within Aleutian arc are underlain by thick sequences the crust and upper mantle along the conti- of deformed Mesozoic and Cenozoiceugeosyn- nentalmargin. Faulting probably occurred over clinal roc'ksthat, in general,become younger a segmentof the Aleutian arc about 800 km seaward,whereas Mesozoic and oldercrystalline long and 150-250 km wide during the Alaskan rocks with a local thin veneer of relatively un- earthquakeand overa segmentof the Peru-Chile 904 GEORGE PLAFKER

EXPLANATION

64o• Mainshock (M• 8,4) 0 Aftershock (M >5.5) :,.

+ + Uplift Openpattern where /•feffed • ',

Subsidence '4pprox/mate axis $ho•/n

Zero isobose

Horizon•olex,ens•on :::?' str•/'n(ZlO-•]

Reversefault :: )o'• Barbson upper plote .::!.: i: ++++++ + ++ + + ++++,• +++ + "r . ::•:::::. ++ ++ + +.,++ ++•+ + +•. + ,, ,'• .:i:::::ii:: volcano

....i.... '•'...... •::•?:•'. _•ff•-•<• + + + + ./+ + / /.•

.• .' '•:•'.• •:•

•..:•.'• • + +/+ + + + / .%' •++ + + +_+/ + + + + / • • • 0 IOOMlLES ++_ A+ + + + • - +/ • • 0 I00 KILOMETERS Of/

,4 0 20M hMontague Island-,0 • o PattonBoy Fault S.45 E.I componlnt of /k o

'1 horizontaldisplacement ! I,,,..>_ ._ J / --';• Midd•eton0..,. _.

_-• Profile showing vertical (curve) and horizontal (arrows) displacements along A-A' Fig. 2. Tectonic displacementsand seismicityassociated with the 1964Alaskan earthquake relative to the Aleutian trench and volcanic are. Tectonic data, after Plaiker [1969] and un- publishedUSCGS data; epicentersfrom Page [1968]. ALASI•N AND CHILEAN EARTHQUAKES 905

I 76 ø EXPLANATION

Moin shock (M•8 4)

o Aftershock (M>5 5)

+++++l +++++ + + 4-/ + + •- o o+ Uplift •- Open pottern where inferred + .: -F .. --40 ø O + • + + + Subsidence • + + ,.: -f- ..: Approximo/e ox/'s shown + -F

ß . • ++ ++• __ ß , + + ..' Zero Isobase :.,.,.... • + + + + + + Horizontal extension + ..'f..::',. A/umero/ /7?dicotes tensile s/ro/• (XIO-S) hi - + + +

I + • + + Volcano -I- + + + O o + + ,- --F IOO MILES + l I + + Ioo KILOMETERS + + +

+ + ß --44 ø + + + 4/+ + + + i++ + +++ + +4-

-F + A A'JB + + -F -F (•I•__sll:l: Islou:u•I: In

:-:.:..'...... Profile showingverticol displocement o long A-A', B-B'

Fig. 3. Tectonic displacementsand seismicityassociated with the 1960 Chilean earthquake relative to the Peru-Chile trench and volcanic arc. Tectonic displacementsafter Pla/ker and Savage [1970]; epicentersfrom Talley and Clou,d [1962]. arc about 1000 km long and roughly 150 km associatedwith previoushistoric seismicevents. wide during the Chilean earthquakeß As is indicated by Figures 2 and 3, significant tectonic deformation, involving uplift, subsi- EARTHQUAKE-RELATED DEFORMATION dence,and horizontal displacements,affected a Regionalcrustal movements during the 1964 minimum area of 200,000 km• in Alaska and Alaskan and 1960 Chilean earthquakes were 130,000 km" in Chile. Subsidiarysurface fault-. more extensivethan any known to have been ing occurred within the deformed region re- 906 GEORGEt)LAFKER latedto the Alaskanearthquake, but no faults earthquakesshow patterns of regionalvertical are knownto havemoved at the groundsurface displacementsthat are markedlysimilar. These during the Chileanearthquake. Pertinent data involvea centralbroad asymmetric downwarp on surface displacementsassociated with these elongateparallel to the arc with a flankingzone two earthquakesare comparedin Table 1. of marked uplift on the seawardside, and a Vertical displacements. Tectonic land-level zoneof relatively minor uplift on the landward changes relative to the sea were determined side (Figures2 and 3). In both earthquakes, from systematicfield studies of displacedshore- the major deformationextended from the trench lines in coastalregions affected by the 1964 approximately to its associatedvolcanic arc. Alaskanand 1960 Chilean earthquakes.These Marked uplift occurredover the outer coastal data were locallysupplemented by comparison regions,the continentalshelves, and probably of pre- and postquaketide-gage measurements, much or all of the continentalslopes. The ad- depth soundings,and first-order survey level jacent synclinaldownwarps extended roughly lines. In addition, qualitative information on from the coastalregions inland to the vicinity the general offshoreextent and relative amount of the volcanic arcs. of uplift was obtained from analysesof the As is indicatedby the profilesin Figures2 tsunami waves generatedby uplift of the sea and 3, the zones of uplift and subsidenceare floor. Details on the data sources and their separatedby lines of zero land-levelchange estimated precision have been presented in (zero isobases)without any detectableabrupt earlier papers [PlaCket, 1969; PlaCket and offsetssuggestive of surfacefaulting. Maximum Savage, 1970]. measureduplift in the Alaskanearthquake was Both the 1964 Alaskan and 1960 Chilean 11.3 metersat MontagueIsland with uplift of

TABLE 1. Surface Displacement Data for the 1964 Alaskan and 1960 Chilean Earthquake

Parameter 1964 Alaskan 1960 Chilean

Regional Displacements Area of major deformation 140,000+ km2 85,000+ km2 Zone of major uplift Dimensions 950 km x 150 to 250(?) km 850 to 1,050(?) km x 100(?) km Maximum uplift 11.3 meters 5.7 meters Zone of subsidence Dimensions 950 km x 150 to 250 km 800+ km x 75 to 110 km Maximum subsidence 2.3 meters 2.3 meters Zone of minor uplift Dimensions 700+ km x 160 km 150+ km x 60+ km Maximumuplift • meter %1 meter Maximum transverse horizontal displacement %20 meters Insufficient data Maximum transverse tensile strain 3 x 10 -4 1.2 x 10 -4

Fault Displacements

Number and type 2 left-oblique reverse None Average strike Patton Bay fault N37øE Hanning Bay fault N47øE Dip Patton Bay fault 50ø to 85øNW Harming Bay fault 52 ø to 75øNW Length Patton Bay fault 62 km minimum; possibly 450 km Hanning Bay fault 6.0 km Dip-slip component Patton Bay fault 7.9 meters Hanning Bay fault 6.0 meters Strike-slip component Patton Bay fault <0.6 meters Hanning Bay fault <0.15 meters AND CmLEAN EARTHQUAKES 907 about 4 meters extendingto Middleton Island Horizontal displacements.Horizontal dis- at the edge of the continental shelf. In the placementswere determinedfor parts of the Chilean earthquake,the maximum uplift was earthquake-affectedregions by comparisonof 5.7 meters at Isla Guamblin, the most seaward pre-and postquaketriangulation data of the island on the continental shelf. The maximum U.S. Coast and GeodeticSurvey in Alaska, and measuredsubsidence along the axes of subsid- by the Instituto Geogrf•ficoMilitar in Chile. ence shown in Figures 2 and 3 was 2.3 meters After the Alaskan earthquake, revisional tri- in both earthquakes.The extent and amount angulationwas done in 1964 and 1965 over an of uplift inland from the zoneof subsidenceis area of roughly 65,000 square kilometerscen- poorly known for both events. Measured tered aroundthe main shockepicenter [Parkin, changeswere lessthan x/• meter in Alaskaand 1966] and in 1967 acrossShelikof Strait be- about 1 meter in Chile. tween the northwest coast of the Kodiak group The gross similarity in the pattern of re- of islands and the mainland (C. A. Whirten, gional surfacewarping associatedwith the 1964 written communication,1969). Retriangulation Alaskan and 1960 Chilean earthquakesimplies in Chile was done during the period 1966 to that the fault mechanisms for these events were 1968 over a narrow arc extending along the comparable.Furthermore, a geneticrelation to Central Valley on the mainland [Pla/ker and the Aleutian and Peru-Chile arcs is suggested Savage, 1970]. Although it provided critical by the systematicspatial distributionof the data on the strains in a small part of the dis- major zonesof uplift and subsidencebetween placementfield, coveragein Chile was inade- and parallel to the oceanictrenches and vol- quateto establishthe full amountof horizontal canic chains of these arcs. shift. Surface faults associatedwith the 1964 Alas- Approximateareas covered by the retriangu- kan earthquake. The two surface faults on lation surveysare representedin Figures2 and Montague Island associated with the 1964 3 by the distributionof doublearrows show- Alaska earthquake have been describedin de- ing the deduceddirections and relative mag- tail by Pla/ker [1967]. Both are reversefaults nitudes of maximum horizontal tensile strain. that strike N37ø-47øE and dip 52ø-85øNW. Net horizontal displacementbetween the ends The longer of the two faults, the Patton Bay of the arrows is the product of the indicated fault (Figure 2), is 35 km long on land and ex- strainand map lengthof the arrow.In addition, tends southwestward onto the continental shelf the S45øE componentof horizontal displace- at least 27 km and possibly as much as 450 ment in the epicentral area of the Alaskan km. The Hanning Bay fault (not shown in earthquakeis shownby vectorson profileA-A', Figure 2) is a relatively minor break having a Figure 2. total lengthof only 6 km. Both faults have off- Horizontal extensionspresented in Figure 2 setsthat are almost entirely dip-slip, the north- in the epicentralregion of the Alaska earth- west blocksbeing relatively upthrown as much quake were taken from the adjustedtriangula- as 8 meters and both blocks being upthrown tion data [Pla/ker, 1969, Figure 16]. Those in relative to sea level. There is a left-lateral the Kodiak Islands area of Figure 2 and in strike-slipcomponent of lessthan 60 cm on the Chile (Figure 3) were calculatedby J. C. Patton Bay fault and lessthan 15 cm on the Savagefrom changesin the observed(unad- HanningBay fault. The pattern of horizontal justed) anglesusing the methoddescribed by andvertical displacement across the PattonBay Frank [1966]. fault is indicated on profile A-A', Figure 2. The most striking aspectof the earthquake- Geologicrelations across these faults suggest related horizontaldisplacements is the generally that they are not major tectonicboundaries. transverseextension across the segmentsof the Their spatial distributionrelative to the re- zones of subsidencethat were resurveyed.It is gionalzones of warping,the earthquakefocal probablyreasonable to assumethat comparable region,and the main shockepicenter further extension occurred within those parts of the indicatethat they are subsidiaryto the primary zones of subsidencethat were not resurveyed. fault alongwhich the 1964 Alaskaearthquake Transverse tensile strain in Alaska system- occurred. atically increasesfrom the northwestlimits of 908 GEORGE [PLAFKER the surveyed areas toward the axis of subsid- Within the limits of the data, both zonesof ence,indicating relative seawardelastic rebound slight uplift seem to be gentle upwarps. In within this area. Average tensile strain across Alaska, the zone is about 160 km wide with its the subsidedzone in the vicinity of the main axis centered roughly along the axis of the shock epicenteris 1.1 X 10-' and it reachesa Alaska range. Maximum displacementis close maximum of 3 X 10-' slightly seaward from to % meter. In Chile, the zone of slight uplift the axis of subsidence. The maximum indicated just east of Isla Chilo8 appearsto be at least seaward displacementis about 20 meters for 60 km wide, maximumdisplacement is about 1 points on the mainland coast just west of meter, and the axis of uplift lies just west of Montague Island (Figure 2). This measured the line of active volcanoesthat capsthe Andes. displacementmust be somewhat less than the THRUST FAULT MECHANISM OF total horizontal shift, becausethe triangulation THE EARTHQUAKES net was not carried inland to an area of stability. The faults along which the 1964 Alaskan and The retriangulation survey in the vicinity of 1960 Chilean earthquakesoccurred are not ex- the main shockepicenter of the Alaskan earth- posed at the surface on land, nor could they quake was carried seaward into a part of the bc uniquely inferred from the seismologicdata. zone of major uplift that includes Montague For both events, however, the combinedseis- and Middleton Islands (Figure 2). Although mologicdata and surface displacementssuggest the surveys in this area are too poorly con- similar causative mechanismsinvolving shear trolled to permit a detailed analysisof strains, failure on complexmajor thrust faults or mega- the data indicate that the resurveyed segment thruststhat dip from the vicinity of the trenches of the uplifted zone is probably characterized beneath the arcs [Pla/ker, 1969; Pla/ker and by transverse compression.The shortening is Savage, 1970]. The important parametersde- most pronouncedin the vicinity of Montague rived for the inferred earthquake faults are pre- Island, where it is at least partly related to sented in Table 2, and the mechanismsare slip on the two subsidiaryreverse surface faults illustrated by schematicvertical sectionsacross and to local crustal warping in the vicinity of the earthquake-affectedregions in Figure 4. these faults (profile A-A t , Figure 2). Becauseof limitations of the data, the sections The retriangulation data for both earth- are of necessityhighly simplifiedportrayals of quakes suggest oceanward rebound of broad the earthquakemechanisms. An effort has been segmentsof the continental margin that had made to make them reasonablyconsistent with been elasticallycompressed before these events. most of the available data on the earthquakes Such shorteningimplies prequakeregional com- and the sparseinformation on crustalstructure pressionoriented roughly normal to the trend in the eastern Aleutian arc and southern Peru- of the arcs. Chile arc. Inland zones o/minor upli/t. Both the 1964 In both earthquakes,the displacementswere Alaskan and 1960 Chilean earthquakesappear large (of the order of 20 metersor more) and to have been accompaniedby relative minor were predominantly dip-slip. There was a sig- uplift landward from the synclinal downwarp nificant left-lateral componentof movementon (Figures 2 and 3). In both cases,the areal ex- the Alaskan megathrust and a possiblesmall tent of these zones is imperfectly known be- right-lateral componentnear the north end of cause they lie largely inland where precise the Chilean megathrust. The observedearth- pre- and post-earthquakeleveling is required quake-related surface displacementsresulted to detect the regional uplift. The distribution primarily from a relative seawardthrusting and of uplift shownon the figureswas determined uplift of the continentalmargins of Alaska and from observations of shoreline changes where Chile with elastic horizontal extension and at- these zoneslie along the coast. In Alaska, the tenuation (subsidence)behind the thrust blocks. shorelinedata were supplementedby two lines Comparisonof the observedprofiles of ver- of levels that extend into the interior across tical displacementon Figure 4 with the theo- the Alaska Range to Fairbanks. retical displacementsderived from dislocation ALASKAN AND CHILEAN EARTHQUAKES 909

TABLE 2. Parameters for the Inferred Faulting Associated with the 1964 Alaskan and 1960 Chilean Earthquakes

Parameter 1964 Alaskan Earthquake 1960 Chilean Earthquake

Fault length 4800 km 41000 km Averagestrike N40oE(600 km), E-W (200 km) NlOøE Down-dipwidth 175 to 290 km •120 km Averagedip 9ø 20ø(?) Slip 20+ meters 420 meters Strike-slip Left-lateral, possibly as much as Possibly some right-lateral near component 60% to 80%of dip-slip north end Subsidiary Reverse faults that intersect surface None on land; possible submarine faults on land, on the continental shelf, reverse faulting seaward from and possibly on the continental the continental sheIf edge slope

models (Figure 5) show a reasonably close two alternatives, and it is conceivablethat the agreementbetween observationand theory for uplift may result. from some other unknown the zonesof subsidenceand landward parts of factor. the major zones of uplift, which are areas of Data sources •or the Alaskan earthquake essentiallyelastic deformation.Relatively steep model. The crustalstructure depicted in Fig- secondary faulting at Montague Island in the ure 4A for the eastern Aleutian arc is inferred uplifted area of the Alaskan earthquake causes from available refraction seismic data [Shot, the anomalouslysharp peak shown in the ob- 1962; Hales and Asada, 1966] and the spatial served profile [Hastie and Savage, 1970]. Com- distribution of historic earthquake epicenters parable subsidiary faults that do not intersect [Gutenbergand Richter, 1954; Barazangi and the surface onshore could readily account for Dotman, 1969]. Faults presumedto have moved the departuresof observeddisplacements in the during the earthquake and their relative slip seawardparts of the major zonesof uplift from have been derived from (1) the surface dis- those predicted for an ideally elasticmodel. placements[PlaCket, 1967, 1969], (2) disloca- The causeof the zonesof slight uplift inland tion modelsbased on the surfacedisplacements from the zones of subsidence is not clear. J. C. [Stauder and Bollinger, 1966; Savage and Savage [Plafker and Savage,1970] has shown Hastie, 1966; Hastie and Savage, 1970], (3) that introduction of downward curvature in the nodal plane solutions [Stauder and Bollinger, fault plane in the dislocation models has the 1966], and (4) the spatial distribution of the effectof producingslight uplift inland from the earthquake focal region [Page, 1968]. subsidence.However, no single model closely Aftershock distribution and the pattern of duplicatesboth the amplitudes and widths of vertical surface deformation (Figure 2) suggest the deformed zones, and the best models re- that the segmentof the megathrustalong which quire downward curvature markedly steeper the earthquakeoccurred is roughly 800 km long than that shownin Figure 4. Within the resolu- and has a down-dip width ranging from 175 to tion of the hypocentrallocations, such steepen- 290 km. The fault is arcuate,or banana-shaped, ing is not indicatedby the spatial distribution and has a main segmentthat trends about 600 of the aftershocks.An alternative possibilityis km at an averagestrike of N40øW and a shorter that the slight uplift could represent elastic segmentroughly 200 km long that trends gen- recovery of a broad crustal downbuckle. Such erally eastwardfrom the main shock epicenter. a downbuckle could have formed as a 'result of A northwestdip at an angle of between5 ø and end loading of the continental plate due to 15ø is suggestedby body-wave nodal plane gradual convergencebetween it and the Pacific solutions for a number of aftershocks [Stauder plate. The effect would be analogousto end- and Bollinger,1966], and a surface-wavenodal- loading of an elastic slab resting on a semi- plane solutionfor the main shockindicates that infinite viscoussubstratum. Unfortunately, the the dip may be as high as 20ø [Kanamori, geodeticdata are not adequateto evaluatethese 1970]. However, dislocationanalysis of the sur- Axis of Aleutian PATTON BAY Edge of VOICQniC Qrc Trench Continental Shelf SE FAULT

......

...... •...... __PLATE ......

Illll[1111l ......

i A. 200 I00 o IOO 200 DISTANCE PERPENDICULAR TO ZERO ISOBASE (KM)

-8 I--

4

ß 0 :::)•

Axis of Edge of -- Peru-Chile ContinentalShelf Volcanicarc E W Trench I. _. ' ...... , ..-.-_-;.-:';,'z'.';L'%',.-',',,•;'-•;'.-_';.,.'.'0.•. -.•,.';..,..',,• 0 Ii71;"i-ffl 'r?""; '"r:., • .....,-,--•q .... •-%.;¾/:-'._;;;...;;i:;&--.,y,;,'•{,;¾,•%'.-..s:;;-:.._•,...... ,•.•.•..'-;..';- •.. :,.i.. ,'. •'. ','.'. •'.'

I•:•:,,,,,jlllll':ljjj:;jjjj i.',':;:,:"•?),?.fi l•[,,!JJjjj, j',',',',',',I',',1j j j jjjjj j[ ...... JJi JJ JJJJJ•t•-•! 4/$Xkltt...... •.•,',., ,;•.•..:;'• ,;,f•,,,.'.',-,.,,.k_..".•'.',".' r:;.C!'¾.-' ,-•I T .%',:':;'-_::-_'SJ BjjjjjjjjjjjilJlJJlJ!!JJJJJJJJlJJlJJJJJh /%;'•;}:?'¾"':JI00n l•qCfff,,,FFr•,,,t•H-t t! i i i i I I ,,,, ,,,, ,, ,, ,,I I ,,•_,, •• • • • • • • HHfHJq-HeH•H+H4H•. "•J•h' •• ', • • • ', ', ', •,

'""':'• 150 B. IOO o oo 2_O0 DISTANCE PERPENDICULAR TO ZERO ISOBASE (KM)

Fig. 4. Schematic cross sections showing the suggested mechanisms for (A) the 19• Alaskan earthquake and (B) the 1960 Chilean earthquake. Inferred earthquake faults are shown by solid lœnes,possible faults by dashed lines; arrows indicate the senseof movement. Dotted lines are fault boundariesthat probably did not slip. Profiles of vertical displacement above the sections are the same as in Figures 2 and 3. Data sourcesfor the sections are given in the text. ALASKAN AND CI-IILEAN EARTI-IQUAKES 911 -10 displacementfield coveredby retriangulation. The displacementwas predominantlydip-slip with someleft-lateral strike-slipcomponent of I00Km 0•100Km 2 movementin the region extendingsouthwest- ward from the main shockepicenter. The mag- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::: .:.:.:...•:, .:::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::nitude of this strike-slipcomponent, however, ::::::::::::::::::::::::::::::::::::::::::::::::::::0:::::::::::::::::::::::: .....==•======"• cannot be closely determined from the data available. The angular relation of the well-determined auxiliary plane (N66øE) in nodal-planesolu- tions for the main shockand many of the after- I00 Km 0 Km 2 •.' shocks [Stauder and Bollinger, 1966] to the

• L__• •=. strike of the fault plane indicates that the strike-slip componentshould be roughly 60% of the dip-slipover much of the N40øE-trending segmentof the fault plane. In the vicinity of Montague Island, on the other hand, the ad- justedtriangulation data [Parkin, 1966] suggest a left-lateral strike-slipcomponent that is about 80% of the dip-slip component (for a fault strike of N37øE). Readjustmentof the triangu- IOOKm 0 00'••[[iI (•)2_ lation data using a longer base line to better controlscale and orientationof the net [Pla/ker, 1969, Figure 16] reducesthe strike-slip com- ponent to about 70% but otherwise does not Fig. 5. Theoreticalvertical displacementof the significantlychange the over-all picture. free surfaceproduced by dip-slip motion on over- A large left-lateral component of displace- thrust faults 100km wide having di.ps of 15ø, 30ø, ment is not apparent in the triangulation arc and 45ø. The sectionsare in a vertical plane nor- mal to the fault strike, fault length is four times betweenthe Kodiak Island groupand the main- the down-dip width, and slip is assumedto be land or in the surface faults on Montague constant over the entire fault surface. Modified Island. Horizontal tensile strain that is oriented after Savageand Hastie [1966,Figure 2] and J. C. roughly normal to the strike of the megathrust Savage(personal communicatœon, 1970). (Figure 2) in the Kodiak-mainlandarc suggests almost pure dip-slip movement.It is possible, face displacementssuggests it may be as low however, that the Kodiak-mainland triangula- as 3.7ø [Hastieand Savage,1970]. The mega- tion arc is too far inland to accuratelyreflect the thrust dip indicatedin Table 2 and shown in directionof slip on the megathrust.Maximum Figure4A (about9 ø) is consistentwith the vari- measuredleft-lateral strike-slipmovement across ous linesof evidencesuggestive of a low dip the subsidiaryreverse faults on MontagueIsland angle,and it is drawnto approximatelycoincide of only about 60 cm on the Patton Bay fault with the base of the continental crust within and 15 cm on the Hanning Bay fault is less the focal region (as derived from seismicre- than 10% of the dip-slip offset.The absenceof fraction data). The gradual decreasein the large strike-slip componentsmay be because width of the focal region and deformedzones the Hanning Bay fault is so short that it is from northeastto southwestsuggests progres- constrainedfrom moving laterally at both ends sive steepertingof the fault plane toward the and the Patton Bay fault is constrainedat its southwest.Such steepertingis consistentwith northeast end, where it apparently dies out on decreasingdistance between the trench and the island. It is conceivablethat a larger strike- volcanic arc in the same direction. slip componentmay be present on the Patton Maximumslip on the fault planeis probably Bay fault offshore. at least 20 meters,the maximummeasured hori- Data sourcesfor the Chilean earthquake zontal componentof shift over that part of the model. Crustal structure of the southern Peru- 912 GEORGE •PLAFKER Chile are shownin Figure 4B is basedon marine liable information on the slip direction of the geophysicalsurveys [Fisher and Raitt, 1962; upper plate. Hayes, 1966] and a compilationof the spatial It should be noted that the preferred fault distribution of recent recordedearthquakes and model of Pla/ker and Savage [1970, Figure 11, onshore gravity data [Kausel and Lomnitz, model B] differsfrom the presentone (Figure •969]. The configurationof faults that may have 4B) in that the primary fault in the earlier moved during the •960 earthquake and their model intersectsthe sea floor well up on the relative slip is extrapolated from a dislocation continentalslope rather than in the vicinity of model by J. C. Savage [Plafker and Savage, the trench. A model fault that breaks out high 1970], a few poorly constrainednodal-plane on the continentalslope was required by the solutions[Wickens and Hodgson,•967; Hodgson dislocationanalysis to account for the rather and Wickens, 1965; Balakina, 1962], and the suddenincrease in the amount of uplift toward spatial distributionof the •960 seismicsequence the shelf edge.As interpreted herein, however, [Talley and Cloud, 1962]. the inferred fault on the slope is a subsidiary The pattern of aftershocksand surface de- reversefault (perhapsanalogous to the Patton formation suggestthat the segmentof the mega- Bay fault in the Alaskanearthquake) that splays thrust that movedduring the earthquakestrikes off the main fault and breaks through the closeto N10øE and is about 1000 km long. Dis- upper plate to the surface.The megathrustin locationanalysis of the vertical surfacedisplace- the section curves downward from the vicinity ments by J. C. Savage [Plafker and Savage, of the trench with an averagedip of about 20ø 1970] indicates that, for a fault having an in the earthquakefocal region. Although both averagedip of 35ø , the minimumwidth of the models can account for the observed surface fault plane would be at least 60 km (assuming displacements,the model in Figure 4B is more constant slip over the entire fault surface). compatiblewith (1) the distribution of after- Becauseof the large east-westerrors in epi- shockswell out into the trench, (2) the large central locations,the seismicdata do not pro- horizontal strains in the far field, and (3) the vide reliable control on the width of the fault available data on crustal structure beneath the plane. Their generaldistribution, however, sug- inner wall of the trench. geststhat the focal region could extend from IMPLICATIONS FOR ARC TECTONICS the trench inland at least 150 km to the vicinity of the axis of subsidence;if it does,the down- Studies of the 1964 Alaskan and 1960 Chilean dip width of the fault plane is about 120 km. earthquakeshave yielded unique data on the As calculated from dislocation theory, the present style of deformationalong parts of movementon the fault plane that givesthe' best the Aleutian and Peru-Chile arcs. Inasmuch as fit to the vertical surface displacementsand these great seismicevents affected an 800-km nodal-planedata is closeto 20 metersdip-slip. segmentof the Aleutian are and a 1000-km seg- Dislocation analysis of the horizontal strains ment of the Peru-Chile are, they provide a indicates that 20-meter slip would require a samplingthat is truly regionalin extent and model fault having a down-dip width of about one that is probablyapplicable to most of these 120 km (comparableto that shownin Figure 4). two arcs. If it is assumedthat the simplified The horizontalstrains show no indicationof sig- modelsderived for the earthquakesare consistent nificant lateral slip near the centerof the model with the data, the followingsection considers fault. Near the north end of the fault, nodal- some of the implicationsfor are tectonics. plane solutionsfor an aftershockand a fore- Underthrustingas the /undamental dynamic shockand the horizontal displacementdata are drive. Theories of sea-floorspreading [Dietz, compatiblewith a componentof right-lateral 1961; Hess, 1962; Holmes, 1965] and the more slip that is no more than 40% of the dip-slip recent conceptsof plate tectonics [McKenzie componentand probably is considerablyless agd Parker, 1967; Morgan, 1968; LePichon, [Pla/ker and Savage, 1970, p. 1025]. 'The • 1968; Isacks et al., 1.968] envisagestructural trangulation arc in the Central Valley, how- arcs, like the Aleutian and Peru-Chile arcs, as ever, may be too far inland to provide re- zonesof convergencein which plates of oceanic ALASKAN AND CHILEAN EARTHQUAKES 913 lithosphere progressivelyunderthrust beneath the Alaskan earthquake [PlaCket, 1969, pp. the arcs. According to these concepts, the 60-63]. Although the evidenceis less certain trenchesand the tensionalfaulting on the outer for the Chileanearthquake region, there is some walls of these trenchesresult from downbowing indicationin the literature that at least parts of and other mechanical effects related to under- the southern sector also may have undergone thrusting. The volcanic chains are built of gradualpre-earthquake submergence [Briiggen, hyperfusiblesderived at depth from the under- 1950]. A possiblycomparable phenomenon has thrust plates. The seismicactivity in the dip- been reportedin the Ryukyu arc of southern ping 'Benioff zones'results from shearingalong Japan,where coastal areas uplifted during each and within the downgoingslabs. Such a model of three historic earthquakesat about 120-year implies regionalcompressire forces between the intervals have been affected by gradual sub- trenches and volcanic chains that are directed mergence between earthquakes [Imamura, generallytransverse to the arcs. 1930]. The thrust-fault mechanisms derived for the The evidence for shoreline submergencein 1964 Alaskan and 1960 Chilean earthquakes can Alaskasuggests that, whilethe continentalmar- readily be explained in terms of a 'stick-slip' gin was horizontallyshortened and thickened, model in which underthrusting provides the it was simultaneouslydepressed relative to sea dynamic drive [PlaCket, 1969, Figure 42] as level.Radiocarbon-dated terrestrial plants killed follows: Gradual underthrustingof oceaniccrust by transgressionof the seaindicate submergence beneath the arcs during the time interval be- over part of the earthquake-affectedregion at tween major earthquakesresults in distorttonal a rate that averagedroughly 6 mm/yr for about drag in the upper plates and the accumulation 930 years.The submergenceoccurred during a of elasticstrain energy due to transverseelastic time interval when sea level was at or near its compression.Shear failure alongsegments of the presentstand and in a regionwhere over-all megathrustsat the time of great shallowearth- isostaticdisplacements should be upward in quakes, such as the 1964 Alaskan and 1960 responseto glacialunloading. The probable Chilean events, is accompaniedby elastic re- cause,therefore, was diastrophicdownbuckling bound in the upper plates involving (1) relative or downwarpingof the continentalmargin that seaward displacementand uplift of the con- may reflecta downwardcomponent of motion tinental margins by movementalong the mega- as the oceanicplate underthrustthe arc. thrusts and subsidiaryreverse faults that break Elsasset[1968] has pointedout that body through the upper plates to the surface,and forcesacting on a relativelydense oceanic plate (2) simultaneouselastic horizontal extension and as it sinks beneath an arc would give rise to a vertical attenuation (subsidence)of the crustal downward-directedpull along the continental slabsbehind the upper plates.These movements margin.Alternative driving mechanisms in which adequatelyaccount for the major observedand the upperplate overrides the oceanbasin or in inferred tectonicdisplacements within the earth- which regionalstress is directedhorizontally quake-affectedregions. Dislocation theory indi- couldnot readily accountfor the combination cates that the vertical displacementsover off- of horizontalshortening and regionalsubmer- shore parts of the lower plate are probably genceindicated by the availabledata. For in- negligiblefor thrust faults of low to moderate stance,although elastic buckling of the conti- dip angles (Figure 5). nental margin could produce the observed Gradual transverseelastic compression of the vertical displacements,buckling alone cannot upper plates implies a componentof vertical account for the regional prequake submer- thickening proportionalto the Poisson'sratio gencethat apparentlyoccurred over parts of for the affected segmentof crust. If all other both the upliftedand subsidedzones or for the conditionsremain unchanged,such thickening magnitudeof the horizontaldisplacements that shouldbe reflectedby gradual uplift and shore- accompaniedvertical displacementsin these line emergencein the affectedregions. In fact, zones.Thus, evidencefor dlastrophicdown- however,gradual pre-earthquakesubmergence warpingbetween earthquakes provides a strong occurred over much of the region affected by argumentfor underthrusting,rather than over- 914 GEORGE PLAFKER thrusting, as the dynamic drive in the eastern rim indicated on Figure 1 have been deduced Aleutian arc. Although the data are inconclu- from combinedstudies of sea-floor paleomag- sive for the Chileanearthquake, the similarities netic data, the orientation of major transform in tectonicsetting and earthquakemechanism, faults, onshoredata on the displacementacross togetherwith at least local preearthquakesub- the San Andreas fault, and nodal-planesolu- mergence,favor underthrustingby the Pacific tions of shallow-focusearthquakes [LePichon, plate as the predominant.dynamicdrive. 1968; Isacks et al., 1968; Morgan, 1968; Pit- Tsunamisand arc-relatedearthquakes. Both man et al., 1968; McKenzie and Parker, 1967; the 1964 Alaskanand 1960 Chileanearthquakes Hamilton and Myers, 1966; Morgan et al., generated major tsunamis that propagated 1969]. As summarized by LePichon [1968, across the entire Pacific Ocean basin. In both Table 5] on the basis of the paleomagnetic events, the tsunami source areas correspond data along oceanic ridges and from transform closelyto the zonesof major uplift on the con- faults, convergencebetween blocks in the east- tinental shelf and slopeand clearly result from ern Aleutian arc is at an azimuth of N44øW, suddenupheaval of the sea floor [Takahasi and and in the southern Peru-Chile arc at latitude Hatori, 1961; Van Dom, 1964; Pararas-Caray- 35øS it is at azimuth N79øE. According to annis, 1965; Wilson and T•rum, 1968; Pla/ker, Morgan et al. [1969], convergencein the south- 1969,p. 22; Pla/ker and Savage,1970, p. 1014]. ern Peru-Chile arc during about the last 25 m.y. Isacks et al. [1968, p. 5884] have emphasized has been more nearly at azimuth S76øE. Slip that great tsunamis,such as the 1964 Alaskan vectorsfor nodal-planesolutions of earthquakes and 1960 Chilean tsunamis, are most apt to in these same general areas as presented by accompany earthquakesin those arc regions of Isacks et al. [1968, Figure 3] are in fair agree- the world characterizedby high rates of dip- ment with the computed directions from mag- slip motion. If these tsunami-generativeearth- netic data, except that the azimuths are some- quakes originate by thrusting comparable to what more northerly for both arcs (about that of the 1964 Alaskan and 1960 Chilean N25øW in the eastern Aleutian arc and N70øW events, a necessary consequenceof the thrust- in the southern Peru-Chile arc). ing is that the senseof vertical displacement Within the limitations of the data, the de- will be uplift within the upper plate (Figure 5) duced directionsof plate convergenceare rea- and the resultant wave must be positive; that sonably consistentwith the surface displace- is, initial water motion recorded at tide gages ments associated with the 1964 Alaskan and 1960 is upward. Positive waves were, in fact, gen- Chilean earthquakes. For the Alaskan earth- erated during the Alaskan and Chilean earth- quake, with an average megathrust strike of quakes. Comparable earthquake mechanisms N40øE, the indicated plate convergencedirec- apparently predominate off the Japan coast tions require predominantlydip-slip displace- where, of 24 tsunamisstudied by Hatori [1966, ment with a small component of left-lateral p. 1461], 21 were positive, as would be expected slip. Just such displacementsare indicated by for predominantly upward displacementof the the triangulation data in the earthquake epi- sea fioo r. central regionand along the surfadefaults on The evidenceavailable from tsunamis sup- MontagueIsland, althoughthe magnitudeof the ports the conceptthat thrusting is probably the strike-slip component is not certain. A small dominant style of deformation in offshoreareas left-lateral component,as indicated by the sur- of volcano-tectonic arcs. Furthermore, the face faulting, is closely compatible with limited work done to date indicates that studies LePichon'sdata (slip direction about 85ø to of the sourceparameters of tsunamis,as deter- the fault strike), whereasthe larger left-lateral mined from tide gages,can provide critical data componentindicated by retriangulation in the on the sense and areal distribution of offshore epicentral region correspondsbetter with the vertical displacementfields associatedwith tec- seismologicdata (slip direction about 65ø to tonic earthquakesthat are presently unobtain- the fault strike). able by any other means. For the Chilean earthquake, which had an Slip directionson the rnegathrustsand. the average megathrust strike of N10øE, both directionso/underthrusting. The relative mo- LePichon's data and the seismic data indicate tions of lithosphericplates aroundthe Pacific a slip directionabout 70ø to the fault strike. ALASKAN AND CHILEAN EARTHQUAKES 915 These data are compatiblewith predominantly magneticdata in the Atlantic Ocean,slip on the dip-slip displacementsderived for the mega- megathrustshould be nearly pure dip-slip. thrust, but they also indicate a small right- Correlation between earthquake-relatedver- lateral strike-slipcomponent. Such a strike-slip tical displacements and long-term shoreline componentis apparent in the horizontalsurface changes. Shorelinestudies in the regionaffected displacementsonly near the northernend of the by the 1964 Alaskan and 1960 Chilean earth- deformedregion, althoughit is likely that the quakes indicate that coastsaffected by uplift triangulation net is too far inland for the hori- during the earthquakesbroadly correspondwith zontal surface displacementsto provide ade- coastsof Holoceneemergence. In contrast,how- quate control on the megathrustslip direction. ever, the zonesof earthquake-relatedsubsidence The earthquake-relateddisplacements are, how- may or may not correspondwith areasof long- ever, in close agreement with the data of term shoreline submergence.This is demon- Morgan et al. [1969], which indicate that the strated by the occurrence of the Cook Inlet present spreadingdirection of the East Pacific basin and youthful I•enai-Kodiak Mountains in plate in the vicinity of the Chile rise is nearly the Alaskan zone of earthquake-related sub- perpendicular (about 86ø ) to the fault strike. sidenceand by the graben-like Central Valley Unless the Americas plate (and Chile) are and uplifted Coast Ranges within the Chilean moving in a direction significantly different zone of subsidence(Figure 6). from the westerly directionindicated by palco- The cause of these long4erm vertical dis-

• 6

KENAI I COOKINLET _ ALASKA • MTNS •- "• BASIN•RANGE-•' TRENCH / pMI SEcuu,x, c= c,/c, _ c'cuNW I 'I • •----• .'•z• : • .• .• •.•.•" •' 0

• •••'•:•...... '•? 'q•'...::'• I ,• ': I --•----__- KM

_• 2 ß "' 0 ••

L...,_•..,.iCvE ANDESMTNS• TRENCH | • W. cu,, I = ...... c.u. MANTLE-•'•'• •...•/.•::::..••Nk NMg•// •' T

oI I , , 40 Fig. 6. I)iagrammatic geologicsections and generalizedprofiles of earthquake-relatedver- tical surfacedisplacement across the easternAleutian arc (A) and southernPeru-Chile arc (B). Note the lack of coincidencebetween zones of earthquake-relatedsubsidence (profiles) and the senseof late Cenozoicdisplacement of major physiographicfeatures (indicated by large arrows). Downward-directed large arrows indicate regions of subsidenceand relative late Cenozoicstability; upward-directedarrows indicate regionsgenerally characterizedby late Cenozoic uplift. 916 GEORGE ?LAFKER placements is undoubtedly complex and is or subsidenceoffers one promisingmethod of mainly a matter of speculation. The alterna- deducingthe recurrenceinterval of prehistoric tion of positive and either negative or stable vertical displacements,which presumably ac- geomorphicfeatures within the regionsof earth- companythe major tectonic earthquakesresult- quake-related deformation suggeststhe possi- ing from shallowslippage along the arc mega- bility that the geomorphicfeatures are at least thrusts.It shouldbe emphasized,however, that in part due to transverseplastic crustalbuckling this approach cannot be used to evaluate the normal to the arc trend. However, the irregu- seismichazard in such areas, inasmuchas de- larity in half-wavelengthof these geomorphic structiveearthquakes may alsoresult from deep elements and the many local anomalies, such movementon the megathrusts,or shallowpre- as the active submergenceof part of the Alaskan dominantlystrike-slip faulting which would not coastal mountain belt in the area between be reflected in the regional vertical deforma- Kodiak Island and the central Kenai Peninsula tion. [PlaCket, 1969, p. 58], indicate that the long- An attempt has been made to examine the term deformationcannot be due solely to simple record of older movements along shorelines transversebuckling. In addition to any broad affected by the Alaska earthquake on a recon- crustal warping that may occur in these re- naissancebasis [Pla/ker, 1969; PlaCket and gions,there must also be (1) isostaticloading Rubin, 1969]. No comparabledata have yet in basinalareas and unloading,through erosion, been obtainedfor the Chile earthquakeregion, of the structural highs, and (2) isostatic ad- althoughthere is abundantgeomorphic evidence justmentsrelated to magmatic activity within for repeated shoreline displacementsin that the volcanic arcs. region. Reconnaissancestudies of the emergent The geologicrecord indicates that diastrophic terraces and beachesalong the Gulf of Alaska imbricate faulting and warping, such as was Coast indicatethat the long-termvertical move- observedon land at Montague Island after the ments occurred as a series of upward pulses 1964 Alaskan earthquake [PlaCket, 1967], is that were separated by intervals of stability probablyconfined largely to the seawardmargin or even gradual submergence.These upward of the relatively overthrust upper plate. The pulsespresumably represent earthquake-related Montague Island faults were reverse faults in movements similar to the emergencethat af- which both blockswere absolutelyuplifted rela- fected the same coastsduring the 1964 event. tive to sea level. This type of faulting with The best recordof successiveuplifts is pre- related warping,which has clearly affectedsuc- served in a steplike flight of marine terraces at cessive sequencesof Mesozoic and Cenozoic Middleton Island, 80 km off the mainland coast bedded rocks along the Gulf of Alaska margin, nearthe edgeof the continentalshelf. Pulsating could well be the characteristic form of earth- emergenceof the islandis recordedby six gently quake-relateddeformation in offshoreparts of slopingmarine terracesthat are separatedby the upper plates. Burk [1972] has suggested wave-cut cliffs or rises with average beach- that 'uplift faulting' of the Montague Island angleelevations of about4, 14, 23, 30, 40, and type could be an important mechanismby 46 meters. The 4-meter terrace is a marine which continental rise 'fiysch' sequencesare surfacethat was uplifted suddenlyduring the deformed,accretcd to continentalmargins, and 1964 earthquake.Radiocarbon ages for four of ultimately elevated above sea level. the five older terracessuggest that (1) the last Recurrence interval o[ tectonic earthquakes. previousuplift was about 1400 years ago; (2) The widespreadoccurrence of emergentand sudden uplifts have occurred at Middleton submergentshorelines in the regionsaffected Island at intervalsof 500 to 1400 years (aver- by uplift and subsidenceduring the 1964 age 800 years) since the island emergedfrom Alaskan and 1960 Chilean earthquakessuggests the sea about 4500 years ago; and (3) the that the tectonic movements accompanying averagerate of uplift is about I cm/yr, but the these earthquakeswere but one pulsein a long- rate during the past 1400 years has been one- continuingtrend of deformation.If they were, half that amount [PlaCketand Rubin, 1969,and radiometricdating of the shorelinedeposits and unpublisheddata]. If the deformation rate is physiographicfeatures related to suddenuplift constant, one could predict from the dated ALAS• ,•I)CI-•L•,A• EARTHQUAKES 917 terraces on Middleton Island that another tec- quantitative information on past vertical dis- tonic earthquakeinvolving uplift of the island placement rates and, to some extent, the fre- is overdue.Many more data of this kind are quency of major tectonic movements in that needed,however, before prediction,even on a same segment of the Aleutian arc. More and time scale of centuries,becomes practicable. better data of this type, including numerous Nevertheless,the MiddletonIsland data clearly radiometricdates of shoredeposits, should con- demonstratethe potential of relatively inex- tribute towardunderstanding the Quaternaryde- pensivegeomorphic studies for significantlyex- formattonalprocesses and may make it possible tending the historic record of tectonic earth- to determine the relative susceptibilityof spe- quakesin coastalregions of suitableareas. cific localitiesalong the coastto future tectonic Evidence for prehistoricearthquake-related movements such as those that accompaniedthe displacementsin regionsaffected by subsidence 1964 Alaskan and 1960 Chilean earthquakes. is more difficult to obtain, becausethe older Strain accumulationand rates of underthrust- shorelinedeposits are commonlyunder the sea ing. Postulatedrates of crustal convergencein or are buried beneath marine sediments.Never- the eastern Aleutian and southern Peru-Chile theless,the observed modificationsof drowned arcs are difcult to reconcile with the deduced shorelinesin the earthquake-affectedregions slip of about 20 meters or more for the 1964 suggest that the record of tectonic movements Alaskan and 1960 Chilean earthquakesand with in protectedsubmerged localities may be even the available information on recurrence inter- betterthan in areasof uplift, becausereadily vals of great tectonic earthquakes. identifiablesoil, peat, and forest horizonstend The rate of crustal convergencein the eastern to be buriedand preservedbeneath transgres- Aleutian arc, as determinedfrom paleomagnetic sire marine sedimentarydeposits. In such de- data and onshorefault studies, is of the order posits,the recordof submergencecould be de- of 5 to 7 cm/yr [LePichon, 1968; McKenzie ducedby radiometricdating of suitableorganic and Parker, 1967; McKenzie and Morgan, materialfrom soil horizonsthat are submerged 1969]. In the southern Peru-Chile arc, the below high tide level. Recoveryof suitable convergencerate is considerablyless certain; samples,however, would generally require rather paleomagnetic data indicate that it is in the costlydrilling or submarinetechniques. range 7-12 cm/yr. The lower limit is based on As was already noted, much of the coastin the assumptionthat the East Pacific rise and the regionaffected by suddenuplift or sub- mid-Atlantic ridge have half-spreadingrates of sidenceduring the 1964 Alaskan earthquake 5 and 2 cm/yr at the latitude of southern Chile underwenta pronouncedgradual preearthquake [Heirtzler et al., 1968, p. 2131; Pitman et al., submergenceevidenced by drownedterrestrial 1968, p. 2082] and that the relative position of vegetationand aboriginalsites along the shores. these ridgesis constant.The maximum feasible Radiocarbon dates from the drowned shores rate, 12 cm/yr, requires that the East Pacific suggestthat the submergencewas probably plate be migrating eastward relative to the gradual or in small increments over a time trench at 10 cm/yr [Morgan et al., 1969] and intervalof at least930 yearsand perhapsas the Americasplate be migrating westwardrela- much as 1350 years. The duration of this wide- tive to the trench at 2 cm/yr. Studies of the spreadsubmergence provides an approximate magnetic anomalies on the Chile rise indicate measure of the time interval since the last to Morgan et al. [1969] an averagehalf-spread- major tectonicearthquake in the sameregion ing rate of 10 cm/yr during the past 25 m.y. that is in good agreementwith the data from and a rate of about 4.6 cm/yr between24 m.y. uplifted terraces on Middleton Island. and 55 m.y. ago; Herron and Hayes [1969] From the foregoing,it shouldbe apparent interpret the magnetic data as indicating that that invaluabledata on the tectonichistory of the changein spreadingprobably occurreddur- coastalregions in arc environmentscan be ob- ing the past 10 m.y. tained from studiesof the geologicrecord of For the indicatedconvergence rates, the mini- fluctuationsof the land relative to sea level. In mum time required for accumulation of the the area affected by the 1964 Alaskan earth- necessarystrain for 20 meters of thrust would quake, reconnaissancestudies have provided be 285 to 400 years in the easternAleutian arc 918 GEOR(m PLArKER and between 170 and 300 years in the southern historic earthquakes in the region affected by Peru-Chile arc (assuming all the strain is the 1960 Chilean earthquake were small com- elastic). Yet geologic evidence in the region pared with the 1960 event and that they re- affectedby the Alaskanearthquake suggests that leasedonly a fraction of the elasticstrain energy the interval sincethe last previousmajor earth- stored in that segment of the arc. This possi- quake in that region involving regional tec- bility is suggestedindirectly by the relative tonic displacementsof the surface was about sizesof the tsunamisgenerated by historicearth- 930-1400 years [Plafker, 1969; PlaCket and quakesalong the coastof southernSouth Amer- Rubin, 1969]; historic recordsin the Chilean ica. earthquake region suggestthat it was on the Japanese records indicate that, of the tsu- order of a century [Lomnitz, 1970]. Assuming namis that originated along the Chilean coast, that the tectonic strain released had all accumu- the 1960 tsunami was by far the highest and lated since the last major earthquake in these most destructive during the time interval 1500- same regions, the convergencerate appears to 1960 [Takahasi and Hatori, 1961]. If the size be too high by a factor of between2 and 4 for of these tsunamis as recordedin Japan is rea- the Alaskan earthquakeregion and it is too low sonably proportional to the amount of earth- by a factor of roughly 2 or 3 for the Chilean quake-relatedvertical displacementin the source earthquake. area, such data suggestthat the tectonic defor- The reasons for this apparent discrepancy mat.ion associated with the earlier events was between the durations of strain energy accumu- nowhere near as extensive as that which oc- lation for the two earthquakesare not known. curred in 1960. Unfortunately, although a fair The high rate in Alaska could be accountedfor correlation commonly exists between the mag- by one or more of the following: (1) permanent nitude of earthquakes and the size of the tsu- shortening (nonrecoverablestrain) within the nami generated[Iida, 1963; Wilsonand T•rum, deformed region, (2) release of a fraction of 1968], there have been somenotable exceptions the accumulated elastic strain energy during to this rule. For example, Sykes [1971] has smaller earthquakesnot accompaniedby de- emphasizedthat the 1946Aleutian Islandsearth- tectable regional vertical displacements(prob- quake (M • 7.4), which was smallerthan many ably less than a few feet), or (3) imperfect other Alaskan earthquakesof the century, pro- coupling between the underthrusting oceanic duced by far the largest tsunami originatingin plate and the continentalmargin. the Aleutians. As in Alaska, geologicstudies of The apparently low rate in Chile is much tectonically displacedshorelines along the coast more diflqcultto explain.A major sourceof un- of southernChile could prove useful in extend- certainty there is the recurrence interval of ing the extremely limited historic record on large tectonic earthquakesin the same arc seg- recurrence intervals of major tectonic move- ment that was affected in 1960. Although the ments in any given arc segment. historic record of seismicityextends back to the Late Cenozoic deformation within the arcs. 16th century, information on the areas affecled Tectonic displacementssuch as those that by such earthquakes is sketchy at best, and companiedthe 1964 Alaskan and 1960 Chilean little is known about any tectonic deformation earthquakes have undoubtedly recurred over that may have accompaniedthem. •Nevertheless, millions of years, and their effects should be it is certain that destructive earthquakes in recorded in the late Cenozoic structures. As 1835 and 1837 were accompaniedby significant consequenceof such movements,compressional vertical displacementsof shorelinesat the north- shorteningwould be expected to characterize ern and southernends of the region affectedby the seaward parts of the arcs where the mega- tectonic movements in 1960 [Darwin, 1896; thrust is close to the surface and the upper Lomnitz, 1970], and at least two episodesof plate can be deformed by imbricate faulting earthquake-relatedshoreline submergenceare and warping. The effectsof compressionshould recordedin the folklore of Indians living along diminish inland, where there is a thick, strong, this coast (C. Lomnitz, written communication, continental plate well above the level of the 1969). Despite the evidencefor some vertical megathrustand where the horizontalstrains are displacement,it is possiblethat the previous predominantlyelastic and recoverable. ALASKAN AND CI-IILEAN EARTI-IQUAKES 919 For both the 1964 Alaskan and 1960 Chilean volcanicarc are predominantlysteeply dipping earthquakes,the zero isobaseson the inland fractures that bound blocks along which dis- sides of the zones of subsidencelie close to the placementshave been predominantlyvertical volcanicchains (Figure 6). These isobasesde- [Burk, 1965]. lineate the approximate inland limits of re- Data on the style of late Cenozoicdeforma- peatedelastic compression and extensionrelated tion across the region affected by the 1960 to underthrustingat the continentalmargins. Chilean earthquake are difficult to obtain be- Coincidence of these lines with the volcanic arcs cause of the general scarcity of outcrops of suggeststhe possibilitythat the positionof the younger sedimentaryrocks and of geologicin- volcanoesrelative to the trenchesmay not be formation about them. The highly schematic relatedsolely to the depthto a sourceof andesitic structure section acrossthe arc in the vicinity magmas,as was suggestedby Coats [1962], but of 41øSshown on Figure 6 is basedon all avail- alsoto the magnitudeof the transversehorizontal able publishedand unpublishedinformation. In compression.For a givenrate of underthrusting, general, there is little evidencefor strong late both the depth to the zone of meltingwithin Cenozoic compressionaldeformation. Marine the downgoingplate of oceaniclithosphere and strata of probable late Cenozoic age on the the horizontalcomponent of compressionacross offshoreislands have dips of 15ø or less, and the upperplate vary inverselywith megathrust sparker profiles acrossthe narrow Continental dip. Thus, the pronouncedwestward conver- Shelf and slope [Scholl et al., 1968] do not gence between the Aleutian trench and volcanic showany appreciablefolding of the sedimentary arc most probably is related to a progressive sequence. Late Cenozoic rocks in the Central steepeningof dip from 15ø or lessin the region Valley and Andes of Chile are characterizedby of the 1964 earthquake[Stauder and Bollinger, monoclinalflexures, broad warps, and high-angle 1966] to about 45ø in the vicinity of Amchitka faults [Zeil, 1964]. Deformation is thought to Island (E. R. Engdahl,unpublished data, 1971). result mainly from vertical tectonic displace- Whether this westwardconvergence in the arc ments of crustal blocks bounded by predomi- resultsfrom (1) melting of the more steeply nantly northwest- and northeast-trendinglinea- descendingunderthrust plate closer to the ments [Katz, 1970]. However, there are no trench, (2) decreasein the componentof trans- known faults with unequivocal Holocene dis- verse horizontalcompression across the upper placement in the onshore part of the region plate, which thereby permits magma to be affected by the 1960 earthquake. erupted closerto the trench, or (3) a combina- Judgingby the fault modelsderived from the tion of the above factors cannot be ascertained deformationand seismicityassociated with both from available data. the 1964 Alaskan and 1960 Chilean earthquakes Late Cenozoic structures in the eastern Aleu- (Figure 4), the most intensedeformation is to tian arc clearly reflect a marked changefrom be expectedon the inner walls of the trenches predominantly compressionalfeatures on the at or near the emergence of the megathrust continental shelf to predominantly vertical wherethe upper plate is thinnest.Paradoxically, movementsat and near the volcanicarc [Pla[ker, however,most marine reflection profiles across 1969, pp. 50-51]. On Middleton Island near the inner walls of the trenches do not show the edge of the Continentalshelf, marine sedi- evidenceof compressivedeformation, but rather mentaryrocks of late Plioceneand Pleistocene(?) sedimentsthat roughly parallel the slope walls age have been tilted northwestward at an aver- and grabenlikebasins that commonly contain age angle of 28ø , truncated, raised above sea flat-lying acoustically transparent sediments. level, and displacedby active faults. In con- (Since this paper was written, Jotdesdrilling trast, late Cenozoicdeposits along the Aleutian off Kodiak Island has demonstratedthe pres- volcanic arc and in the Cook Inlet basin south- ence of indurated, highly deformed Pleistocene east of the arc are fiat-lying, gently tilted, or sediment on the inner wall of the Aleutian are folded into broad, open structures with trench [Geotimes,vol. 16, no. 10, p. 15, 1971]). flank dips generallyless than 10ø [Burk, 1965, These basins,the apparent absenceof deformed p. 121; Kelley, 1963, pp. 289 and 296]. Late deep-seasediments in the trenches,and other Cenozoicfaults mapped in the vicinity of the marine geophysicaldata have been widely in- 920 GEORGE PLAFKER terpretedas indicatingtransverse crustal tension ized by predominantly vertical movements. and vertical movements in the trenches [Peter These relations strongly suggestthat, in addi- et al., 1965; Shot, 1962; Ewing et al., 1965; tion to the intensity of horizontal stress and yon Huene et al., 1967; yon Huene and Shot, depth to the megathrust, crustal competence 1969; Hayes, 1966; Scholl et al., 1968]. On the must play a critical part in determining the other hand, hyperbolicreflectors, possibly from nature of arc deformation. buckled trench sediments,have been reported Accretion versusnondeposition or erosiono/ on someacoustic profiles in the Aleutian trench continental margins. Sea-floor paleomagnetic off Amchitka Island [Holmes et al., 1970] and data and other related lines of e•idencesuggest off Kodiak Island (R. yon Huene, personalcom- that underthrusting of the easternAleutian arc munication, 1970); similar features were re- has occurred almost continuouslysince late Cre- ported by Ewing et al. [1969] on a profile line taceoustime [Atwater, 1970,Figure 18] and that that crossesthe Peru-Chile trench off Valparaiso. it has probably been continuousin the southern Although the megathrusts or subsidiary re- Peru-Chile arc, at least since the early Tertiary verse faults may intersect the surface on the [Morgan et al., 1969]. During the time span inner walls of the trenches,it is highly unlikely covered by the paleomagnetic data, several that such features could be identified with nor- thousandkilometers of oceaniccrust have sup- mal acoustic profiling techniques.This is be- posedly been consumedbeneath these arcs. As cause,by their very nature, the scarpsof over- was clearly pointed out by Dietz [1963], long- thrust faults are unstable and tend to collapse continuedunderthrusting implies that ensimatic as soonas they form [Pla/tcer, 1967, pp. 16-18]. pelagic sediment carried into the trenches on As a result, repeatedthrusting tends to form a the oceanicplates and any terrigeneousdetritus fault-line scarp that dips, at the angle of repose that is poured into the trenchesmust either be of material making up the upper plate, in a accreted to the inner walls of the trenches or direction oppositethat of the fault plane. Such somehow carried downward beneath them. a slope,if it occurredunder water, could easily Despite the evidence that both the eastern be misinterpreted as marking the trace of a Aleutian and southern Peru-Chile arcs have had normal fault. Conceivably, conjugate sets of large amounts of late Cenozoicunderthrusting reversefaults could bound some of the graben- and that the recent earthquakes in these areas like features on the inner walls of the trenches. suggest closely similar mechanismsof under- Alternative possibilitiesfor developinggraben thrusting at the present time, the geologicrec- in a regionof compressivestress are by warping ords along the continental margins of the two of the upper plates of overthrustsor by exten- arcs differ markedly. Diagrammaticgeologic sec- sion at the heads of large-scale gravitational tions depictingthe differencesbetween these arcs slides. are shownin Figure 6. In brief, the continental In the writer's judgment, none of the marine margin of the easternAleutian arc is underlain geophysicaldata now available permit unam- by thick sequencesof highly deformed eugeo- biguous conclusionsto be drawn about the synclinal rocks of Mesozoic and Cenozoicage nature of the deformation along the inner walls that apparently have successivelyaccreted to of these trenches. Bottom and sub-bottom sam- the continent, whereas in the southern Peru- pling programs,coupled with detailed seismic Chile arc the continentalmargin consistsmainly studies, may eventually resolve the issue. The of Paleozoic and Mesozoic crystalline rocks, onshoredata, however,indicate that late Ceno- with only a thin cover of relatively undeformed zoic compressionaldeformation in the eastern younger sediments. Aleutian arc is largely limited to the wedgeedge Along the eastern Aleutian arc, the geologic of the continental margin, which is underlain record of deformation suggeststhat the con- by thick sequencesof eugeosynclinalrocks. In tinental margin was affected by at least three contrast,late Cenozoicstrata depositedon thick, pre-late Cenozoicorogenic episodes correspond- predominantlycrystalline sialic crust in the ing roughly to (1) late Eoceneto earliestOligo- inner part of the Aleutian arc and all of the cene, (2) Late Cretaceousto earliest Tertiary, onshore southern Peru-Chile arc are character- and (3) Middle(?) Jurassicto Early Cretaceous AND CI•n.E•N E•TI•QV•rS 921 times [PlaCket, 1969, pp. 50-53]. The three erately deformedTertiary continentaland neritic earlier orogeniesare indicated by major uncon- marine sedimentary rocks along the western formities that divide the exposedsection into flank of the Coast range and on the offshore four laterally continuoustime-stratigraphic belts islands. consistingof thick, highly deformed eugeosyn- Acousticreflection profiles acrossthe shelf clinal rocksalong the continentalmargin (units and continentalslope suggestthat the base- M•, Ms, C•, and Cs of Figure 6); thinner, rela- ment complexcommonly extends seaward to the tively undeformedcoeval miogeosynclinal rocks baseof the slopeand that it has a discontinuous occur farther inland and partly overlap older mantle of inferred Cenozoic strata less than eugeosynclinalrocks along the Gulf of Alaska 800 meters thick that is only slightly to mod- margin (units Mu and Cu, Figure 6). The three erately deformed [Scholl et al., 1968, 1970]. older eugeosynclinalsequences, which are locally Abuting the inner wall of the trench there is metamorphosedand intruded by granitic plu- an undeformed fill of Cenozoic pelagic and tons of Mesozoicand Tertiary age,form laterally terrigeneoussediment as much as 800 meters continuousfault-bounded outcrop bands along thick. As interpreted by Scholl et al. [1968, much of the margin of the Gulf of Alaska. A 1970], the structural relations and sediment general seaward decreasein the age of these budget suggestthat there is no deformational sequencesand their prevailing structural com- thickeningof trench sedimenton the inner wall plexity suggest that they were successively of the trench related to late Cenozoic under- buckled againt the continentalmargin by rela- thrusting of the continental margin in Chile. tive underthrusting.An abundanceof volcano- Although Ewing et al. [1969] believe they can genic detritus in the fiysch deposits,in conjunc- identify deformed sedimentson the inner wall tion with the distribution of coeval andesitic vol- of the trench on a profile at the latitude of canicsand calc-alkalinegranitic batholiths,sug- Valparaiso, comparablefeatures have not yet gests that volcanic arcs probably existed close been identified elsewhere in the Peru-Chile to the present volcanic chain during these in- trench or in most other circum-Pacific trenches ferred earlier episodesof underthrusting. that have been studied by acousticprofiling. Unlike the eastern Aleutian arc, the southern The apparent absenceof strong late Cenozoic Peru-Chile arc shows no indication of strong compressionaldeformation along the seaward late Cenozoicdeformation in the seaward part part of the southern Peru-Chile arc or of older of the arc or of progressiveaccretion of deep- sequencesof eugeosynclinalrocks accreted to sea sedimentsto the continentalmargin. As was the continental margin are difiqcultto reconcile noted previously,post-Middle Cretaceous defor- with the conceptof long-continuedunderthrust- mation over most of the region between the ing in that segmentof the arc. On the basis of outer coast and the crest of the Andes has been the presently available data, appeal must be characterizedby gentlewarping and block fault- made either to consumptionof sedimentbeneath ing [Zeil, 1964; Katz, 1970]. The coastalmoun- the continentalmargin durirg late Cenozoictime tains in this region are underlain by a base- or to improbably low sedimentationrates in the ment complexof highly deformedmetamorphic trench [Schollet al., 1970]. rocks of Precambrian(?) and Paleozoicage and The apparent presenceof an old crystalline of Paleozoic(?) and Mesozoic igneous rocks continental crust along the inner wall of the [Gonzdlez-Bonorinoand Aguirre, 1970]. A late trench may indicate that continuous under- Cretaceousgranodiorite batholith underliesex- thrusting of the oceanic crust has gradually tensive areas of the Coast ranges immediately eaten into the continentalmargin by a process north of the regionaffected by the 1960 earth- analogousto that postulatedby Page [1970, p. quake,and comparablegranitic batholithsare 686] to account for the juxtaposition of Fran- intermittently exposedalong the west coast of ciscanformation trench depositsand crystalline South America as far north as northern Peru basementalong the Sur fault. This possibility [Instituto de Investigacionesde Geologicas,is indicated by the fact that the present axis 1968]. The crystalline complex in southern of magmaticactivity within the arc lies roughly Chile is locally veneeredby slightly to rood- 100-150 km east of the midlines of the Meso- 922 GEORGE PLAFKER zoic and late Paleozoic calc-alkaline granitic southern Peru-Chile arc. A subordinate left- batholithsof the CoastRanges. Hamilton [1969] lateral strike-slip componentis indicated for has suggestedthat calc-alkalinebatholiths, such the Alaskan event, and there may be a small as the Cretaceous Sierra Nevada batholith of right-lateral componentfor the Chilean event. California, may representthe roots of former The slight uplifts that occurredduring both volcanic arcs, and Dickinson and Hatherton earthquakesin the vicinity of the volcanic arcs [1967] and Hatherton and Dickinson [1969] could be due to transversebuckling of the con- have presenteddata indicatingthat the position tinental plate, but the data are inadequatefor of the volcanic chains tends to bear some con- unambiguousconclusions to be drawn. sistentspatial relation to the underthrustplate. The postulatedthrust-fault mechanismsfor If the calc-alkaline batholiths of the Chilean the 1964 Alaskan and 1960 Chilean earthquakes Coast Ranges representthe roots of volcanic are compatiblewith their tectonic settings in arcs,it would appear that either the Mesozoic active arcs that are being progressivelyunder- and late Paleozoiccontinental margin of Chile thrust by oceanicplates. Within the limitations lay about 100-150 km farther west, or that the of the data, slip directionsare in satisfactory present angle of underthrustingis much less agreementwith thosededuced from nodal-plane than it formerly was. Far more data will be re- solutions and the directions of plate conver- quired on the structure and stratigraphy of genceas interpreted from sea-floorpaleomag- the trenchdeposits and rockson the inner wall netic studies.However, the rates of plate con- of the Peru-Chile trench before this problem vergenceindicated by the paleomagneticdata can be resolved.Nevertheless, it is geologically are apparentlytoo high in the easternAleutian significantthat, althoughseemingly similar arc- arc and too low in the southern Peru-Chile forming processeshave been active along the arc. These discrepanciesin the underthrust continentalmargins of Chile andAlaska through- rates probably reflect the large uncertainties out much of Cenozoictime, the geologyof these that exist in the recurrenceintervals of major regionsdiffers markedly: There is a continental tectonic earthquakes in the same arc segment, marginof accretionin the easternAleutian arc, the averageslip on the megathrustsduring the whereasthe continentalmargin in the southern great earthquakes,and the processby which Peru-Chilearc appearsto be characterizedby elastic strain energy is accumulatedand dis- nonaccretionor perhapseven erosion. sipatedin the arcs.Radiometric dating of dis- placedshorelines in suchregions is a practical CONCLUSIONS techniquefor extendingthe historic record of Surfacedisplacements that accompaniedthe earthquakerecurrence and for determiningthe 1964 Alaskan and 1960 Chilean earthquakes relative susceptibilityof specificarc segments providecritical constraintson the mechanisms to future earthquake-related tectonic move- of thesemajor seismicevents. Measured vertical ments. and horizontaldisplacements indicate that these In both earthquakes,faulting, warping, tsun- earthquakesresulted from releaseof near-hori- ami generation,and aftershockswere largely zontal compressionalelastic strain oriented restricted to the seaward parts of the arcs. roughlynormal or at a largeangle to the trends These observations are consistent with available of the eastern Aleutian and southern Peru-Chile geologicevidence in the easternAleutian arc arcs.The observedsurface changes in the major which suggeststhat significantlate Mesozoic zonesof uplift and subsidence,together with and Cenozoiccompressive deformation in the the spatial distributionof seismicityand the upper plate is largelylimited to the continental nodal-planesolutions, are consistentwith theo- margin and is relativelyminor in thoseparts of retical strains surroundingdislocation models the arc that are underlainby a thick competent for landward-dippingcomplex thrusts. The data crystallinecrust. Gross differences in the geology suggestpredominantly dip-slip movementon of the eastern Aleutian and southern Peru-Chile the order of 20 meters on megathrustsinclined arcssuggest that long-continuedunderthrusting at averageangles of about9 ø beneaththe east- has resultedin accretionof eugeosynclinalsedi- ern Aleutianarc and perhaps20 ø beneaththe ments to the continentalmargin of Alaska, in ALASKAN AND CHILEAN EARTHQUAKES 923 contrastto either nondepositionor underthrust- G. A. MacDonald and H. Kuno, p. 92, AGU, ing of deep-seasediments beneath the crystalline Washington, D.C., 1962. continental margin of southern Chile. Darwin, C., Geological Observations on the Vol- canic Islands and Parts oi South America Acknowledgments.Many of my colleaguesat Visited during the Voyage oi H.M.S. Beagle, the U.S. Geological Survey assistedin collecting 3rd ed., Appleton, New York, 1896. data on the 1964 Alaskan earthquake and in Dickinson, W. R., and T. Hatherton, Andesitic analyzing the data from that earthquake,as well as volcanism and seismicity around the Pacific, the 1960 Chilean event. In particular, I am in- Science,157, 801, 1967. debted to Dr. James C. Savage of the Geological Dietz, R. S.. Continent and ocean basin evolution Survey for making dislocation model studies of by spreading of the sea floor, Nature, 190, 854, the Chilean earthquake data, for calculating 1961. strains from some of the triangulation surveys, Dietz, R. S., Collapsing continental rises' An and for providing some of the data used in Figure actualistic concept of geosvnclines and moun- 4. I am grateful to Drs. William R. Dickinson, tain building, J. Geol., 71, 314, 1963. Konrad B. Krauskopf, and Ben M. Page of Stan- Duda, S. J., Strain release in •he circum-Pacific ford University and to J. C. Savage for their criti- belt, Chile, 1960, J. Geophys. Res., 68, 5531, cal reviews of this manuscript and many helpful 1963. suggestionsfor its improvement. 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