BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICAVOL. 66. PP. 385-400. 16 FIGS. MAY 1964

OROGENESIS AND DEEP CRUSTAL STRUCTURE—ADDITIONALEVIDENCE FROM SEISMOLOGY1

BY HUGO BENIOFF

ABSTRACT

Seismic evidence indicates that the principal erogenic structure responsible for eachof the great linear and curvilinear mountain ranges and oceanic trenches is a complexreverse fault. A study of eight regions in which orogenic activity is in progress revealsthat these great faults occur in two basic types, here designated oceanic and marginal.Oceanic faults, situated within the oceanic domain, extend from the surface to depths of550 to 700 km. They exhibit an average dip of 61°. Their elastic strain-rebound charac-teristics show that these faults are composed of two separate mechanical units—a shallowcomponent extending from the ocean bottom to a depth of roughly 60 km, and a deepercomponent extending to the 700 km crustal boundary. The marginal faults situated alongthe continental margins occur in dual and triple forms. The dual faults comprise a shal-low member extending from the surface to a depth of approximately 60 km and an inter-mediate member extending to a depth of 200 to 300 km. The average dip is 33°. Themarginal triple form is similar to the dual down to the 300 km level. At this depth thedip changes abruptly to 60° to form a third component extending down to the 650± kmcrustal boundary. The elastic strain-rebound characteristics of the marginal faults in-dicate that the components of these structures also move as separate units, although inSouth America the two lower elements exhibit some evidence for mechanical coupling.In the continental domain the 300 km level thus represents a tectonic discontinuity notas yet revealed by seismic wave-propagation studies but which is apparently the lowerboundary of the continents. Since the oceanic faults and the deep components of themarginal faults have the same average dip (61°) it may be assumed that both are frac-tures in a single, continuous mechanical structure subject to a single stress system. Thedifferent average dip (33°) of the marginal intermediate fault components suggests thatthey occur in a structure mechanically distinct from the deep oceanic and continentallayer and that they are activated by a different stress system. A hypothesis offered forthe origin of the volcanoes associated with the faults assumes that the source of volcanicenergy is heat produced in the fault rocks by the inelastic components of the repeatedto-and-fro strains involved in the generation of the sequences of earthquakes and after-shocks.

CONTENTS

TEXT PageMexico and Central America 390page Aleutian Arc 390

Introduction 386 New Hebrides region 390Regional seismic sequences—Observations.... 386 Tonga-Kermadec region 391

South America , 386 Philippine Islands 392Kurile-Kamchatka segment 387 Summary of fault measurements 393Bonin-Honshu segment 388 Implications concerning crustal structure andSunda Arc 389 orogeny 393' Except for the paragraph on the origin of vol- P,ri™Pal or,0g.erf m<*h™. reverse fault. 393

canoes and a few minor deletions and additions, this Sha»ow andu intermediate crustal structure

paperwasread under the title, Evidence from seismol- indicated by faults 393ogy on the nature of orogeny, at the Panel on Orogeny Deep crustal structure indicated by faults. . 394of the Cordilleran Section meeting of the Geological Rigidity in deep layer 394Society of America, Tucson, Arizona, April 11-12, Orogenic fault-generating stresses 3951952. Direction of slip 396

385

386 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

PageRelationship between shallow oceanic and

intermediate marginal layers 396Orogenesis and volcanoes 399References cited 400

ILLUSTRATIONS

Figure Page1. Map and composite profiles, South Ameri-

can earthquake sequences ............. 3872. Map and composite profile, Kurile-

Kamchatka earthquake sequences ...... 3883. Map and composite profile, Bonin-Honshu

earthquake sequences ................. 3894. Maps showing present and assumed original

configurations of Bonin-Japan-Kam-chatka arc .......................... 389

5. Map and composite profile, Sunda Arcearthquake sequences ................. 390

6. Map and composite profile, earthquakesequences of Mexico and Central America 390

7. Map and composite profile, Aleutian earth-quake sequences ..................... 391

8. Map and composite profile, New Hebridesearthquake sequences ................. 391

Figure Page9. Map and composite profiles, Tonga-Ker-

madec earthquake sequences 39210. Map and composite profile, Philippine

earthquake sequences 39211. Generalized oceanic and continental deep

crustal sections with orogenic fault types. 39412. Stress relations indicated by fault dips. . . . 39513. Elastic strain-rebound characteristic,

Tonga-Kermadec deep earthquake se-quence 397

14. Elastic strain-rebound characteristic, New-Hebrides shallow earthquake sequence. . 397

15. Elastic strain-rebound characteristics,Tonga-Kermadec shallow earthquakesequence and New Hebrides inter-mediate sequence 398

16. Map and elastic strain-rebound charac-teristics, New-Hebrides-Tonga earth-quake sequences and local crustal struc-ture 398

TABLETable Page1. Depth ranges and average dips of orogenic

faults 393

INTRODUCTION

The physical science of seismology is basedalmost entirely on observations made withseismographs and clocks. The principal ob-served data refer to the origin times of earth-quakes, the depths and geographic distributionof their foci, and the amplitudes, frequencies,and propagation characteristics of seismicwaves. In the past, seismic contributions tothe problem of orogenesis included chieflyinformation as to locations of regions of tectonicactivity and evidence for crustal structureexhibited by the speeds and transmission dis-continuities of seismic waves. Publication ofthe monumental catalog of earthquakes byGutenberg and Richter (1950) has made itpossible to augment these early contributionswith knowledge derived from magnitudes, timesequences, and accurate hyperfocal locations.Thus an investigation (Beniofi, 1949) of theelastic strain-rebound characteristics and re-lated spatial distributions of foci of the seismicsequences of South America and the Tonga-Kermadec region indicated that the orogenicfeatures associated with these two structuresrepresent surface expressions of great faultsextending 650 km in depth and up to 4500 kmin length. These preliminary findings werethus at variance with those older concepts in

which orogenesis is considered a result of sinkingand subsequent rising of weakened portions ofa thin (35 km±) crust floating on a viscous orplastic substratum. In the present discussion,studies of an additional number of orogenicallyactive regions are presented. These includethe Sunda arc, the Kurile-Kamchatka segment,Mexico and Central America, the New Hebri-des, the Philippines, the Bonin-Honshu seg-ment, the Aleutian arc, as well as revisions ofthe South American and Tonga-Kermadecpresentations. Magnitudes, origin times, depths,and geographic locations of the earthquakesand the locations of active volcanoes are takenfrom Gutenberg and Richter (1950).

REGIONAL SEISMIC SEQUENCES—OBSERVATIONS

South America

A revised map with composite profiles ofthe South American earthquake sequences isshown in Figure 1. Epicenters and foci ofshallow, intermediate, and deep earthquakesare represented by circles, circular dots, andtriangular dots respectively. Active volcanoesare represented by stars. The linear distribu-tion of volcanoes exhibited here is characteristicof all the sequences represented in this studyexcept the Philippines. The oceanic trench

REGIONAL SEISMIC SEQUENCES—OBSERVATIONS 387

lying approximately parallel to the line ofvolcanoes is also characteristic. In constructingthe composite profiles of these sequences andof the following ones, the plotted horizontal

layer to a depth of approximately 650 km.In the figure, D' and E' are vertically exag-gerated profiles taken along the lines D and Eof the map to show the spatial relations of the

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FIGURE 1.—MAP AND COMPOSITE PROFILES, SOUTH AMERICAN EARTHQUAKE SEQUENCES

positions of foci represent their perpendiculardistances from the volcano lines taken asreference co-ordinates. The region has beendivided as shown into northern and southernsections AB and BC, for geometric simplicityof the projections A'B' and B'C'. The complexSouth American erogenic fault is a good ex-ample of the type which occurs along the mar-gins of continents. Moreover, as the authorhas pointed out (Benioff, 1949), the strain-rebound characteristics and the distribution offoci indicate that this fault structure is madeup of three components having three distinctlydifferent types of tectonic movements. Thefirst component involves the shallow layer ofthe crust above what may possibly be theMohorovicic discontinuity. Near the fault thislayer is assumed to extend to a depth of ap-proximately 60 km. The second componentdefines an intermediate layer extending fromthe bottom of the shallow layer to a depth of250-300 km. The third component extendsfrom the lower surface of the intermediate

oceanic trench, the mountain range, and theline of volcanoes (indicated by the star).

In the composite sections A'B' and B'C', thevertical and horizontal scales are equal. In thenorthern (Peru-Ecuador) segment, A'B', theintermediate component extends to a depthof approximately 250 km with a dip of 22°.In the southern segment (Chile), B'C', thedip is 23°. Unlike other sequences of this type,South America has no shocks with depthsbetween 300 and 550 km so that the dip of thelower component is not accurately defined.Assuming that there is no horizontal displace-ment between the intermediate componentand the deep component, the fault-zone linescan be extended upward to intersect the inter-mediate-zone lines as shown, in which casethe angles of dip of the deep components ofthe two segments are 47° and 58° respectively.

Kurile-Kamchatka Segment

Figure 2 shows the erogenic elements of theKurile-Kamchatka segment. The composite

388 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

profile C'D' represents the portion of the map layer in the vicinity of the Japanese islands,bounded by CC and DD. A' and B' are ver- The high shallow layer activity and absence oftically exaggerated profiles taken along the foci deeper than ISO km along the Nanseilines A and B. The Kurile-Kamchatka seg- Shoto branch to the west, the paucity of shal-

50" C'D'

1500700

» VOLCANOES»»'»•• LINE OF VOLCANOES

« • ° M = 6.0 - 6.9US! f • o M = 7.0- 7.7

T R E N C H O M -. 7.75-8.5

FIGURE 2.—MAP AND COMPOSITE PROFILE, KURILE-KAMCHATKA EARTHQUAKE SEQUENCES

ment is thus a marginal erogenic fault complexwith three components similar to the SouthAmerican structure. The intermediate com-ponent extends to a depth of 300 km and dipsunder the continent at an angle of 34°. Thedeep component, with a dip of 58°, appears tohave undergone a horizontal displacement ofapproximately 100 km westward (toward thecontinent) relative to the intermediate com-ponent as indicated by failure of the paralleldashed fault-zone lines to intersect at the 300-km level.

Bonin-Honshu Segment

The region of the Bonin-Honshu segment isshown in Figure 3. The composite profileB'C' represents the portion of the map boundedby BBCC. A' is a vertically exaggerated profiletaken along the line A of the map. This oro-genic complex is actually a part of the Kurile-Kamchatka-Japan structure, but has beenseparated here to avoid interpretation diffi-culties arising from distortion of the shallow

low earthquake activity in this (Bonin-Honshu)segment, the large horizontal displacementsbetween its two lower components, and itsreverse curvature all strongly suggest thatthe whole fault complex from northern Kam-chatka to the southern portion of the Bonin-Honshu segment was once continuous andconvex toward the Pacific over its whole extent,as indicated on the left in Figure 4. The presentform appears to be a result of an approximately90-degree counterclockwise bending about avertical axis in Manchuria, of the two lowercomponents of the original arc relative to theoriginal shallow component now forming theNansei Shoto branch, as shown on the map onthe right in Figure 4. Consequently, in thepresent Bonin-Honshu segment the shallowcomponent is missing, and the intermediatecomponent extends from the surface to adepth of 400 km. It is thus the deepest conti-nental structure observed in this study. Thefault dips westward under the continent at 38°.The deep component is displaced horizontally

REGIONAL SEISMIC SEQUENCES—OBSERVATIONS 389

150°

25°

20°

—i—i—i—i—r1000 KILOMETERS

1 • M = 6.0 - 6.9» • M = 7.0 - 7.7

• M = 7.75 - 8.1

0 SHOCKS NOT INCLUDED IN B'C'

in mill LINE OF V O L C A N O E S

^VOLCANOESFIGURE 3.—MAP AND COMPOSITE PROFILE, BONIN-HONSHU EARTHQUAKE SEQUENCES

IIP* 120* 130* 140* ISO* 160* 170* IIP* 120* 130* 140* ISO* 160* 170

ASSUMED ORIGINAL STRUCTURE PRESENT STRUCTURE

—— LOWER FAULT TRACE LINE OF VOLCANOES j S TRENCHFIGURE 4.—MAPS SHOWING PRESENT AND ASSUMED ORIGINAL CONFIGURATIONS OF

BONIN-JAPAN-KAMCHATKA ARC

eastward, away from the continent, some 200km relative to the intermediate component.This displacement, and the 75-degree dip, thelargest of any studied to date, suggest that theforces which originally reversed the curvatureof the Bonin-Honshu segment also twisted thedeep component fault surface about a hori-zontal axis, as indicated on the right in Figure 4.It appears therefore that the common com-ponent of the couples, responsible for the bendand the twist, acted horizontally eastwardalong the lower boundary of the lower layer.

Sunda Arc

Figure 5 shows the region of the Sunda arc.The portion of the map represented in thecomposite profile B'C' is that bounded by thedashed lines BBCC. This structure is also atriple marginal fault. The intermediate com-ponent dips northward under the continent atan angle of 35°. It has a maximum depth ofapproximately 300 km. The dip of the deepcomponent is 61°. It has been displaced hori-zontally approximately 200 km southward(away from the continent) relative to the

390 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

105 130'

» • o M • 6.0 - 6.9» • O M • 7.0 - 7.7

• O M = 7.75 - 8.2I LSHALLOWL INTERMEDIATEDEEP

• VOLCANOES iHf TRENCH ««i» L I N E OF VOLCANOES 0 SHOCKSTIOT INCLUDED IN B'C'FIGURE 5.—MAP AND COMPOSITE PROFILE, SUNDA ARC EARTHQUAKE SEQUENCES

10'• o M « 6.0 - 6:9• O M • 7.0 - 7.7• O M . 7.75- 8.1

110' 100' 80'

» VOLCANOES JSS TRENCH «ii«»» L I N E OF VOLCANOES 0 SHOCKS NOT INCLUDED IN B'C'FIGURE 6.—MAP AND COMPOSITE PROFILE, EARTHQUAKE SEQUENCES OF MEXICO AND CENTRAL AMERICA

intermediate component, as indicated by thedashed lines in B'C'.

Mexico and Central America

Figure 6 shows the dual marginal Acapulco-Guatemala fault complex. The intermediatecomponent extends (under the continent) to adepth of only 220 km with a dip of 39°. Theshocks northwest of the boundary BB areshallow and presumably represent horizontalmovements on the shallow clockwise trans-current San Andreas fault system. There is noevidence for the existence of a deep componentin this region.

Aleutian Arc

Figure 7 refers to the Aleutian arc, also amarginal dual fault. The composite profileB'C' shows only those features bounded by thedashed lines BBCC of the map. The intermedi-ate component dips northward 28° and extendsto a depth of 175 km, thus forming the shal-lowest continental structure treated in thisstudy, with the possible exception of thePhilippines.

New Hebrides Region

Figure 8 shows the region of the New Heb-rides fault. This orogenic structure is also a

REGIONAL SEISMIC SEQUENCES—OBSERVATIONS

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391

500

50

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"N LINE OFVOLCANOES

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170° 180° 170° 160° 150°FIGURE 7.—MAP AND COMPOSITE PROFILE, ALEUTIAN EARTHQUAKE SEQUENCES

1000

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FIGURE 8.—MAP AND COMPOSITE PROFILE, NEW HEBRIDES EARTHQUAKE SEQUENCES

marginal fault principally of the dual type,although the occurrence of two shocks at the350-km level suggests the possibility of a deepercomponent of small activity. The intermediatecomponent extends to a depth of 300 km anddips northeast 42°.

Tonga-Kermadec Region

A revised map and composite profile of theTonga-Kermadec fault complex is shown inFigure 9. In the earlier paper (Benioff, 1949)the author assumed that the structure consisted

of a single continuous fault. However, theexistence of two lines of volcanoes and theasymmetrical distribution of epicenters arebetter explained with two faults, even thoughthe strain-rebound characteristics indicate thatthe two are actuated by a single stress system.The composite profile A'B' represents themap area AABB associated with the Tongasegment. The vertically exaggerated profileC' is taken along the line C of the map. Thecomposite profile D'E' represents the areaDDEE of the map, associated with the Ker-madec segment. The latter extends well into

392 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

A'B'

D'E'

35'

40'

3 K M .• o M = 6.0 • 6.9• O M = 7.0 • 7.7• O M < 7.75 • 8.3

FIGURE 9.—MAP AND COMPOSITE PROFILES, TONGA-KERMADEC EARTHQUAKE SEQUENCES

700KM.

115° 125° 130° 135°

VOLCANOES §H TRENCH

FIGURE 10. — MAP AND COMPOSITE PROFILE, PHILIPPINE EARTHQUAKE SEQUENCES

* • o M = 6.0 - 6.9T • O M = 7.0 - 7.7

• O M = 7.75 - 8.3

the region oi North Island oi New Zealand, dips approximately 64° NW. and extends toThe Tonga-Kermadec structure is the principal a depth oi 550 km.representative oi the oceanic form of erogenicfault complex. The elastic strain-rebound char- Philippine Islandsacteristics of these faults (Benioff, 1949) andtheir composite profiles show that they are of The region of the Philippine Islands (Fig. 10)dual type with shallow components extending is the most disturbed area in this series. Theto a depth of approximately 70 km. The deep volcanoes exhibit no well-defined line. Thecomponent of the Tonga fault dips 59° NW. projection line B'C' was constructed with theand extends from 70 km to 650 km in depth, reference line OX drawn approximately parallelThe deep component of the Kermadec fault to the strike of the trench. The structure is

REGIONAL SEISMIC SEQUENCES—OBSERVATIONS 393

considered here an oceanic fault complex,although the concentration of foci in the depthrange 70-150 km may represent an intermediatecomponent of corresponding thickness and

the mechanism of orogenesis found in the earlierinvestigation of the Tonga and South Americansequences. Thus it appears that in those greatlinear and curvilinear tectonic structures, now

TABLE 1.—DEPTH RANGES AND AVERAGE DIPS OF OROGENIC FAULTS

MARGINAL FAULTS

FAULT

PERU- ECUADOR SEGMENT

CHILE SEGMENT

BONIN-HONSHU SEGMENT

SUNDA

KURIL- KAMCHATKA SEGMENT

ACAPULCO - GUATEMALA

ALEUTIAN

NEW HEBRIDES

INTERMEDIATE COMP.DEPTH RANGE.KM.

70 - 250

70 - 290

0 - 400

70 - 30070 - 300

70 - 220

70- 17570 - 300

A V E R A G E DIPS

DIP

22°

23°

38°

35°34°

39°

28°42°

33°

DEEP COMPONENTDEPTH RANGE.KM.

600 - 650

550 - 650

400 - 550

300 - 700300 - 650

roip47°

58°

75°

61°58°

60°

OCEANIC FAULTS

FAULTMINDANAO

TONGA

KERMADEC

DEPTH RANGE, KM.

70 - 700

70 - 650

70 - 550

AVERAGE DIP J

DIP

60°

58°

64°

61°

having a dip westward of approximately 36°.If this is the correct interpretation, the faultis a marginal one of triple type. The deepcomponent extends to a depth of 700 km witha dip of roughly 60°.

SUMMARY OF FAULT MEASUREMENTS

Depth ranges and dips of the orogenic faultcomponents for each of the regions studied areassembled in Table 1. In the marginal faultsthe average values of the dips for the inter-mediate and lowest components are 33° and60° respectively. In the oceanic faults theaverage dip of the lower component is 61°.The observational precision (approximately±50 km in any direction) is not high enoughto permit any conclusions regarding the dip ofthe shallow components nor to differentiatebetween dip slip and strike slip faulting intheir movements.

IMPLICATIONS CONCERNING CRUSTALSTRUCTURE AND OROGENY

Principal Orogenic Mechanism, Reverse Fault

This extended study of tectonically activeregions supplements the conclusions regarding

seismically active, the principal orogenicmechanism involves a complex reverse faultextending to a depth which varies from 175km to 720 km depending upon the region.

Shallow and Intermediate Crustal StructureIndicated by Faults

The principal features of the faults and thecrustal structure which they define are shownin the generalized section of the crust in Figure11. The continental marginal structure isindicated on the right, and the oceanic type isshown on the left. The shallow continentallayer A, above the Mohorovicic discontinuityM, is approximately 35 km thick as determinedby many investigators from wave-propagationstudies. It is assumed here to be the same asthe layer defined by the shallow earthquakesequences. The strain-rebound characteristicsstudied so far indicate that, in the vicinity ofthe faults, shocks belonging to the shallowsequences occur down to a depth of approxi-mately 60 or 70 km in accordance with theclassification of shallow, Gutenberg andRichter (1950). Accordingly, in the figure theshallow layer is shown thickened to this extent

394 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

in the vicinity of the fault. Presumably thisthickening is a result of drag and plastic flowof the strained fault lip. Thus, in part at least,the roots of mountains may be expressions of

the oceanic structures the fault extends all theway from this level to the 60-km discontinuity,and presumably to the surface, with a constantaverage dip of approximately 61°. Since the

-TRENCH

K

OLCANOES

FOUNTAIN RANGE r-

-o 35

OCEANIC MARGINAL AND CONTINENTAL

NOTE EXAGGERATION OF SURFACE RELIEF.

FIGURE 11.—GENERALIZED OCEANIC AND CONTINENTAL DEEP CRUSTAL SECTIONS WITHOROGENIC FAULT TYPES

this fault-generated distortion. In the triplemarginal faults the discontinuity in angle ofdip at 300 km±, and the marked dissimilaritybetween the strain-rebound characteristics ofthe shocks occurring in the fault componentsabove and below this level, are interpreted asevidence for an additional tectonic disconti-nuity between B and C, as shown in the figure.In the dual structures the lower fault com-ponent C is either absent or inactive. Theintermediate component with the 32-degreedip occurs in marginal structures, but not inoceanic structures; consequently, the layer Bdefined by this component is here assumed tobe a part of the continental structure. If thisassumption is correct, the continents extenddownward to an average depth of 280 km, with175 km and 400 km extreme values.

Deep Crustal Structure Indicated by Faults

In the triple marginal structures the faultdip increases suddenly at the lower boundaryof the intermediate component to an averagevalue of approximately 60° and continuesdown with this value to the 700 ± km level,below which earthquakes do not occur. In

dips of the oceanic faults and the deep com-ponents of the continental faults are essentiallyequal, it is assumed that both types are frac-tures in a single continuous medium (Fig. 11, C)subject to a single stress sytem. The measuredseismic wave speeds near the upper boundariesof the continental layer B and the oceaniclayer C are very nearly equal (8.2± km/secfor compressional waves). This suggests thatthey may be identical in composition. Bullard(1952), on the other hand, cites evidence tothe contrary. However, whether or not they areof identical composition, the layers are sepa-rated by a tectonic discontinuity and are sub-ject to different stress systems.

RIGIDITY IN DEEP LAYER

Since earthquakes occur down to depths of550-720 km in both marginal and oceanicfaults, it must be assumed that at these depthsthe crustal rocks can maintain elastic shearingstrains of sufficient duration to accumulate thenecessary strain energy to generate earth-quakes. Moreover, the strain-rebound charac-teristics of the deep sequence of South Americaprovide evidence for accumulation of elastic

RIGIDITY IN DEEP LAYER 395

strain without creep or flow over a period of5% years at a depth of 550-650 km (Benioff,1949). The Tonga deep sequence exhibitedelastic (recoverable) creep without flow over a

average angle of 33°. This value falls withinthe range calculated and observed by Hubbert(1951) for faults produced by stress patternsin which the greatest principal stress is a hori-

G"t + 0~»< -H? CT2 > . ' 2

3

FIGURE 12.—STRESS RELATIONS INDICATED BY FAULT DIPS

period of 15 years down to a depth of 650 km(Benioff, 1949). Thus, for time intervals atleast of the order of a decade, the earth's crustin the vicinity of the erogenic faults is a rigidsolid down to the 700± km level. Accordingly,this depth limit is here taken as the lowerboundary of the solid crust. For geologicallylong time intervals the crust may, of course,behave as a viscous or plastic substance atthese (and shallower) depths. The question asto whether or not the crustal structure, definedin this manner by erogenic faults, extends overthe whole earth to regions not now seismicallyactive, is one which cannot be answered bystudies of this kind alone. In this connection,Birch's (1951) discovery, derived from calcula-tions of the rate of change of the bulk moduluswith pressure, of a substantial departure fromphysical or chemical homogeneity in the depthrange 200-800 km may be additional evidencefor the discontinuities and structures derivedfrom these studies.

OROGENIC FAULT-GENERATING STRESSESThe intermediate components are reverse

faults which dip under the continents at an

zontal compression. However, there is reasonto believe that the Coulomb-Mohr theory offracture on which Hubbert bases his reasoningmay not be applicable to triaxial stress-condi-tions of extreme magnitude which must existat the depths of the intermediate and deepfault components. David Griggs has suggested(Personal communication) that under theseextreme conditions the relative value of . theintermediate principal stress affects the anglewhich the plane of fracture makes with thedirection of the greatest or least principalstress. Thus, in Figure 12 the block representsportions of a fault structure subjected to thethree principal stresses u\ (vertical), o-2 (hori-zontal parallel to the fracture) and as (hori-zontal perpendicular to o-2). In order that thedirection of slip be as indicated by the arrows,the relations of the principal stresses must besuch that

<T3

If o-2 is less than the average of <TI and cr3, theangle of dip is less than 45°. If 02 is greater thanthe average of the other two, the angle of dip

396 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

is greater than 45°. On this basis it would ap-pear that both the 33-degree and 61-degreefaults are produced by stress patterns in whichthe greatest principal stress is a horizontalcompression oriented effectively at right anglesto the erogenic axis. The continental marginsare thus moving relatively toward the adjacentoceanic domains. In the 33-degree intermediatecontinental faults the horizontal intermediatestress trz is nearer in value to the vertical stress<TI than to the horizontal stress tr3. On the otherhand, in the 60-degree oceanic faults and thedeep components of the continental faults,<T2 is closer in value to 03. In other words, thehorizontal stress pattern is more nearly sym-metrical in the deep layer than in the inter-mediate layer.

DIRECTION OF SLIP

The direction of slip assumed for all the faultsin which the underside moves down is dictatedby the geometry of the trenches and adjacentuplifts. It refers to the fault-generating condi-tions only. There is no reason to believe thatthese original conditions necessarily remainunaltered in time, and consequently, once thefault structure has been formed, subsequentstress patterns may change. Later movementsmay thus involve horizontal slip or even re-versals of slip direction. Such alterations maybe temporary or permanent. In the latter case,the trench eventually disappears. Perhaps thishas been the history of the Pacific coast ofNorth America between Alaska and LowerCalifornia and of the Himalayan-Indian arc.A reverse slip may thus be the evidence fordecline in the life of an orogenic fault. Instancesof reversed slip have been observed by Ritsema(1952, thesis, Univ. Utrecht) for two deepshocks in the Sunda and Philippine regionsrespectively, and by Koning (1942) for oneshock of the Sunda arc. However, the methodemployed by these authors for measuring thestrike and direction of slip is of questionablereliability. It depends upon correct determina-tion from seismograms of the displacementdirection of the initial ground motion for eachof a rather large number of seismograph sta-tions. Particularly in deep earthquakes, theinitial wave motion is nearly always made upof relatively high-frequency components for

which most seismographs have inadequatemagnification. Moreover, for those criticallysituated stations to which the initial wavestravel in or near the plane of faulting, thedirection of initial displacement is either in-determinate or else too small in relation to theground unrest to be reliably recorded by aninstrument situated outside the epicentralregion. Evidence that the results of this methodshould be accepted with caution is providedby calculations reported by J. H. Hodgson(1952) using a modified form which, neverthe-less, depends solely on correct observationsof the initial displacements. In an applicationto the Ancash, Peru, earthquake of Novem-ber 10, 1946, the method indicated that thefaulting was of the transcurrent type, whereasobservations on the ground reported by Sil-gado (1951) showed clearly that the faultingwas of the dip-slip type in accordance with theknown tectonic character of the region.

In a recent paper, H. Honda and A. Masa-tuka (1952) have studied the direction ofmotion of some 145 intermediate and deepearthquakes of Japan using observations of alarge number of Japanese stations. Since thesestations are all in or near the epicentral regionof the shocks, the reliability of first-motiondeterminations is relatively high. Their resultsindicate clearly that for these shocks the dipof the faults and the direction of slip are insubstantial agreement with the assumptionsand findings of the present investigation.

RELATIONSHIP BETWEEN SHALLOW OCEANICAND INTERMEDIATE MARGINAL LAYERS

The continental surface layer of 35 kmthickness in which the shallow marginalcomponents occur is generally reduced to ap-proximately 5 km under the oceans insofar asthe evidence from wave-propagation studiesindicates. On the other hand, the strain-re-bound characteristics of the Tonga-Kermadecsequences show that shallow earthquakes occurto a depth of 60 or 70 km in the oceanic struc-tures. Hence, in the vicinity of the oceanicfaults we must assume some sort of shallowdiscontinuity having approximately the samedepth as the shallow discontinuity of thecontinents. Such a discontinuity is thereforeshown on the left in Figure 11.

SHALLOW OCEANIC AND INTERMEDIATE MARGINAL LAYERS 397

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As indicated in Figure 11, it is assumed that region the intermediate continental layerin general the intermediate layer B terminates extends across the intermediate fault to becomeat the fault. There is some evidence from the the shallow layer of the Tonga-Kermadecstrain-rebound characteristics, however, for oceanic fault. The elastic strain-reboundthe conclusion that in the New Hebrides-Tonga characteristic of the Tonga-Kermadec deep

398 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

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EARTHQUAKE SEQUENCES AND LOCAL CRUSTAL STRUCTURE

components is shown in Figure 13. That for theshallow component of the New Hebrides faultis shown in Figure 14. It is obvious that thesetwo curves are dissimilar and that they must bederived from movements of two distinctmechanical structures. On the other hand, thecharacteristics of the Tonga-Kermadec shallowsequence and the New Hebrides intermediatesequence (Fig. 15) are strikingly similar, except

for a time delay of approximately 2000 days ofthe former relative to the latter, and both areunlike either the New Hebrides shallow charac-teristic or the Tonga deep characteristic. Itappears, therefore, that the Tonga shallow-faultcomponent and the New Hebrides intermediatecomponent are fractures in a single mechanicalstructure subject to a single stress system. Thesituation can best be understood by reference

SHALLOW OCEANIC AND INTERMEDIATE MARGINAL LAYERS 399

to Figure 16 in which the left portion is a mapand section of the New Hebrides-Tonga region.On the right, the four rebound characteristicsare roughly sketched with coinciding time co-ordinates. Counting from the bottom, the firstcurve—the Tonga deep characteristic—refersto the component D in the section E-E'. Thesecond curve, which is of the Tonga shallowcomponent, refers to the component C of thesection. Since these two curves are so very dis-similar, movements in the two involved com-ponents are unrelated, and consequently adiscontinuity must be drawn between C and D.The third curve is the New Hebrides inter-mediate characteristic. It refers to the com-ponent B in the figure. Since the curves for Band C are so nearly alike we must assume thatthese two components are members of a singlestress system, and consequently the inter-mediate layer at B is shown continuous withthe shallow layer at C. Since the reboundcharacteristic of the shallow New Hebridescomponent A (the fourth curve on the right)is unlike that of the deeper component below it,a discontinuity must be drawn between thetwo. The structure deduced here for the Tonga-New Hebrides region may represent a specialconfiguration of this part of the earth only andmay not be valid for other regions in which thestructure outlined in Figure 11 must be con-sidered representative.

OROGENESIS AND VOLCANOES

One of the most remarkable features of theerogenic faults discussed in this paper is thelinear array of volcanoes lying parallel to thefault strike on the uplifted block. Indeed theline of volcanoes has served as the principalreference co-ordinate for drafting the compositeprofiles. Volcanoes are clearly one of themanifestations of orogeny. In an attempt toexplain this close relationship the writer isoffering a hypothesis of the origin of the vol-canoes which at least will direct thinking towardthis interesting problem. By referring to thecomposite profiles shown in Figures 1-9 itwill be noted that in general the volcano linecoincides with the highest elevation of theoverhanging fault block and thus appears tobe situated along the line of maximum bending

of the block. In recent studies the writer(Benioff, 1951) found evidence that the after-shock sequences which follow significantearthquakes are generated by elastic afterworking or creep recovery of the fault rock.During the interval between principal earth-quakes, strains accumulate in the rocks. Aportion of this strain is purely elastic, and aportion is time-dependent. When the faultslips to produce the principal earthquake, thepurely elastic strain only is involved. The time-dependent or creep strain can be released onlyrelatively slowly in accordance with the creep-recovery characteristics of the rocks. It is thistime-dependent elastic recovery which producesthe aftershocks. However, only a small portionof the energy stored in the creep-elastic ele-ment can be converted into seismic waves inaftershocks. The main portion is liberated asheat. A rough calculation of the energy liberatedas heat in a number of aftershock sequencesshows that it averages approximately halfthe amount of the energy liberated as seismicwaves in the principal shock. In the erogenicfaults on which earthquakes occur repeatedly,the fault rock undergoes a cycle of alternatestrain and relief for each earthquake. Thisrepeated to-and-fro bending generates heat inmuch the same way that quick bending of awire warms it. The thermal time constant ofthe great rock masses below the surface is solong that heat generated by these cycles ofstrain escapes very slowly and consequentlyaccumulates over long intervals of time. If therock being heated has components with differ-ent melting points, there comes a time whenone or more will melt. The region of mostintense heat generation should coincide withthe regions of most intense bending—thehighest elevations of the overhanging blockand the lowest depths of the neighboringtrenches. The low incidence of volcanoes inthe trenches may be ascribable to a highermelting point induced by the increased pressureand to the fact that the trench (in the form of asyncline) is not so effective a structure forconcentration of the molten rock as the com-plementary anticline of the uplifted block onthe upper side of the fault.

A rough idea of the magnitude of energyreleased, say per year, by the aftershock se-

400 HUGO BENIOFF—OROGENESIS AND DEEP CRUSTAL STRUCTURE

quences in a region on one side of the fault canbe obtained by taking one fourth of the energyreleased in the same time by seismic waves inthe principal earthquakes. Thus, in the caseof South America, the shallow and intermediateearthquake sequences each liberate approxi-mately 4 X 1023 ergs per year. Thus roughly1024 ergs per year is being released in the faultblocks. The writer has no knowledge of theamount of energy per year required to maintainthe South American system of volcanoes, andconsequently it is not possible to say whetheror not the energy requirements are met on thishypothesis. Moreover there must be a largetime lag between the liberation of heat in thedepths and its appearance in the form of vol-cano output. Thus the present rate of volcanicenergy release should be equated to a phase ofseismic-heat generation which occurred longago, rather than to the present rate.

REFERENCES CITED

Benioff, Hugo, 1949, The fault origin of oceanicdeeps: Geol. Soc. America Bull., v. 60, p.1837-1866., 1951, Earthquakes and rock creep: Seismol.Soc. America, Bull., v. 41, p. 50-62.

Birch, Francis, 1951, Remarks on the structure ofthe mantle and its bearing upon the possibilityof convection currents: Am. Geophys. UnionTrans., v. 32, no. 4, p. 533-534.

Bullard, E. C., 1952, Heat flow through the floorof the eastern North Pacific Ocean: Nature,v. 170, no. 4318, p. 200.

Gutenberg, B., and Richter, C. F., 1950, Seismicityof the Earth: Princeton Univ. Press, p. 1-273.

Hodgson, J. H., Storey, G. S., and Bremner, P. C.,1952, Progress Report on fault plane project(Abstract): Geol. Soc. America Bull., v. 63,p. 1354.

Honda, H., and Masatuka, A., 1952, On the mechan-ism of the earthquakes and the stresses pro-ducing them in Japan and its vicinity, ScienceReports of the Tohoku University Series 5:Geophysics, v. 4, no. 1, p. 42-60.

Hubbert, M. King, 1951, Mechanical Basis forcertain familiar geologic structures: Geol. Soc.America, Bull., v. 62, p. 355-372.

Koning, L. P. G., 1942, On the determination of thefault planes in the hypocenter of the deep focusearthquakes of June 29, 1934 in the Nether-lands East Indies: Kon. Ned. Acad. Wet.Proc., v. 45, p. 636-642.

Silgado, Enrique, 1951, The Ancash, Peru earth-quake of November 10, 1946: Seismol. Soc.America Bull., v. 41, p. 83-100.

CALIFORNIA INSTITUTE or TECHNOLOGY, PASA-DENA, CALIFORNIA

DIVISION OF EARTH SCIENCES, CONTRIBUTION No.597

MANUSCRIPT RECEIVED BY THE SECRETARY OF THESOCIETY, FEBRUARY 9, 1953.