BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICAVOL. 60. PP. 1337-1866. 10 FIGS. DECEMBER 1949

SEISMIC EVIDENCE FOR THE FAULT ORIGIN OF OCEANIC DEEPS

BY HUGO BENIOFF

CONTENTSPage

Abstract 1837Introduction 1837Determination of strain-rebound increments. 1838Tonga-Kermadec earthquake sequences 1841

PageSouth American earthquake sequences 1846Other deep-focus sequences 1855Tsunamis and the great faults 1855References cited 1856

ILLUSTRATIONSFigure Page1. Strain-rebound increments of the Tonga-

Kermadec shallow sequence (h < 70km.) 1842

2. Strain-rebound increments of the Tonga-Kermadec deep sequence (h = 70 —480km 1843

3. Tonga-Kermadec fault mechanism sug-gested by strain-rebound curve of deepsequence 1843

4. Tonga-Kermadec earthquakes and oceanicdeeps 1844

5. Sketch illustrating proposed origin of thegreat Tonga-Kermadec fault 1845

«geFigure6. Strain-rebound increments of the South

American shallow sequence (h < 70km.) 1852

7. Strain-rebound increments of the SouthAmerican intermediate sequence (h =701-300 km.) 1853

8. Strain-rebound increments of the SouthAmerican deep sequence (h = 550 —660 km.) 1853

9. South American earthquakes and oceanicdeeps 1854

10. Sketch illustrating proposed origin of thegreat South American fault 1855

TABLESTable Page1. Tonga-Kermadec shallow sequence 18392. Tonga-Kermadec deep sequence (h = 70 —

680km.) 18403. South American shallow sequence (h < 70

km.) 1847

Table Page4. South American intermediate sequence (h =

70 - 300 km.) 18495. South American deep sequence (h = 550 —

660 km.) 1851

ABSTRACT

A method is described for determining theelastic-rebound strain increments associated withearthquakes occurring on a particular fault. For agiven sequence of earthquakes, a graph of the ac-cumulated increments so determined plotted againsttime represents the actual fault movement duringthe interval covered by the sequence. The methodalso provides a means for determining whether ornot a chosen sequence of earthquakes representsmovements of a single fault structure. In this wayit becomes possible to discover faults which other-wise may escape detection. Evidence of this kindis offered which indicates that the Tonga-Kermadecand South American sequences of earthquakesoriginate on great faults which dip under the con-tinental masses. The faults are approximately 2500km. and 4500 km. in length respectively. Their

transverse dimensions are approximately 900 km.each. They both extend to a depth of approximately650 km.—more than one tenth of the radius of theearth. The oceanic deeps associated with thesefaults are surface expressions of the downwarpingof their oceanic blocks. The upwarping of their con-tinental blocks have produced islands in the Tonga-Kermadec region and the Andes Mountains inSouth America.

INTRODUCTION

In another paper in preparation the writerdevelops a method for deriving numbers pro-portional to the elastic-rebound strain incre-ments: associated with individual earthquakeson a given fault. The method depends uponthe instrumental earthquake magnitude scale

1837

1838 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

of Richter (1935) as revised by Gutenberg andRichter (1942). In a study of a number of after-shock sequences it has yielded information asto the strain activity of fault rocks during thetime intervals in which aftershocks occurred.The strain characteristics so determined arein close agreement with the creep-recoverycharacteristics of rocks as measured in thelaboratory. It was thus possible to demonstratewith reasonable certainty that aftershocks areproduced by elastic afterworking of the faultrocks. The method has been applied to a studyof sequences of earthquakes, other than after-shocks, originating in a number of active seis-mic regions. If the earthquakes of a givensequence are generated by strain-relief incre-ments on a single fault, a graph of the accumu-lated increments plotted against time repre-sents the actual intermittent motion of thefault blocks. The method thus makes it pos-sible to observe tectonic processes in action.It may serve also to determine whether ornot a given sequence of earthquakes is derivedfrom movements of a single fault structureand so, in effect, to establish the existence of afault which may have otherwise escaped de-tection. The discovery of two great faults fromevidence of this kind is presented.

DETERMINATION OF STRAIN-REBOUNDINCREMENTS

It will be of help to review briefly the pro-cedure for deriving elastic strain-rebound in-crements from earthquake magnitudes. If anaverage elastic strain ei preceding an earth-quake is assumed to be distributed uniformlythroughout an effective volume V of the faultrock, the strain energy stored in the rock isgiven by the equation:

E = foVel. ergs (1)

where /* is the appropriate elastic constant ofthe rock. When the fault slips to produce theearthquake, a fraction p of the stored elasticenergy is converted into seismic waves. Thusif J is the energy of the waves,

ergs (2)

constant and very nearly equal to unity. Henceequation (2) may be written

For a given fault n and V are constants. More-over it is reasonable to assume that p is also

/ = ergs (3)

where C is a constant such that

If the strain is reduced to zero during the earth-quake the stored elastic strain is equal to thestrain rebound,that is

«1 =6Hence

J = CV ergs ..*... .(4)

Taking the square root of equation (4),

/i = Ce (ergs)*....(5)

Thus on a given fault the square root of theenergy of an earthquake is proportional to theelastic-strain rebound increment which generatesit. The fault-rebound displacement * corre-sponding with the strain-rebound increment eis given by

where m is a constant depending upon the sizeand shape of the volume V of strained rock.Hence

kx .(6)

where k—mC. The square root of the earth-quake energy is thus proportional to the re-bound displacement also. If 5 is the accumu-lated sum of the values of 7* for a sequence ofearthquakes derived from a single fault, agraph of 5 versus time represents the fault dis-placement (multiplied by the undeterminedconstant k) or the strain relief (multiplied byC) as a function of time. Values for J* may bederived from Richter's instrumental magni-tude M by means of the following equationwhich is derived by Gutenberg and Richter(1942)1:

log 6.0 + 0.9 M (7)

In the two sequences represented in thisstudy, magnitudes, hypocenters, and origin

1 The constants in equation (7) represent theauthors' latest revised values. The probable erroris ± i magnitude.

TABLE 1.—TONGA-KERMADEC SHALLOW SEQUENCE (h < 70 km.)

Date

1910, June 291913, June 261917, May 11917, June 241917, June 261917, Nov. 161919, Apr. 171919, Apr. 301921, Feb. 271924, Apr. 301924, Aug. 101926, Mar. 161927, July 31929, Aug. 31930, Jan. 141930, Feb. 141930, Sept. 221931, June 91931, June 9

• 1931, Aug. 131931, Nov. 181932, Mar. 81932, Apr. 31932, May 221933, Jan. 271933, Mar. 151933, May 201933, June 181933, July 241934, Jan. 311934, Apr. 241934, May 51935, Feb. 41935, Mar. 291935, Apr. 51936, Mar. 201936, Sept. 61939, Feb. 31940, July 201940, Aug. 111940, Aug. 241941, Aug. 21941, Sept. 161941, Oct. 51942, Nov. 21942, Dec. 221943, June 31943, Sept. 111943, Sept. 141943, Sept. 221943, Oct. 241943, Dec. 271943, Dec. 301946, Nov. 131948, Sept. 9

J.D.

2,418,85219,94521,35021,40421,40621,54922,06622,07922,74823,90624,00824,59125,06525,82725,99126,02226,24226,50226,50226,56726,66426,77526,801

2,426,85027,10027,14727,21327,24227,27827,46927,55227,56427,83827,89127,89828,24828,41829,29829,83129,85329,86630,20930,25430,27330,66630,716

2,430,87930,97930,98230,99031,02231,08631,08932,13832,804

<t>

32 S202921154292941918|343016*1517162135i14244292018430|20 S1620201516161432202922144214224IS*15415J28i28}14i1916}16 S153035422323242021

X

176 W17417717417317741781724173176178171172417241711751794174176178174179 E1774 W174 W1721741744172173417341741791741774175173417417541731721731781774173}173174173 W174177178417417841771234174

a

7.07.68.07.258.37.57.08.37.26.06.756.06.256.56.256.56.56.56.256.256.06.256.256.256.756.06.06.06.756.256.256.256.256.256.06.256.06.256.06.06.07.17.06.56.96.756.56.97.66.757.06.256.257.58.0

/* X 1012

2.07.116.023.428.05.62.028.03.2.2

1.2.2.4.7.4.7.7.7.4.4.2.4.4.4

1.2.2.2.2

1.2.4.4.4.4.4.2.4.2.4.2.2.2

2.52.0.7

1.61.2.71.67.11.22.0.4.45.616.0

s x w»

2.09.125.128.556.562.164.192.195.395.596.796.997.398.098.499.199.8100.5100.9101.3101.5101.9102.3102.7103.9104.1104.3104.5105.7106.1106.5106.9107.3107.7107.9108.3108.5108.9109.1109.3109.5112.0114.0114.7116.3117.5118.2119.8126.9128.1130.1130.5130.9136.5152.5

1839

1840 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

TABUS 2.—TONGA-KERMADEC DEEP SEQUENCE (h = 70-680 km.)

Date

1907, Apr. 1

1909, Feb. 221910, Apr. 211910, Aug. 211910, Dec. 151911, Apr. 291911, July 191911, Aug. 221912, May IS1913, May 81915, Feb. 261916, July 81918, May 221918, Oct. 41919, Jan. 11919, Aug. 191922, Mar. 111924, Jan. 171924, May 51924, May 261924, Dec. 271927, Apr. 21928, June 171928, Sept. 121930, Apr. 301931, Apr. 41931, May 151931, July 201931, Oct. 181932, May 271932, May 271932, May 271932, May 271932, June 171932, July 211932, Oct. 211933, Jan. 241933, May 211933, June 121933, Sept. 71933, Nov. 81934, Jan. 181934, Feb. 101934, Oct. 111934, Sept. 231934, Nov. 91934, Dec. 121934, Dec. 161935, Jan. 21935, Apr. 201935, July 161935, July 29

J-D.

2,417,66718,36118,78418,90619,02219,15719,23019,27119,53919,89720,55621,05421,73721,87221,96122,19123,12623,80323,91223,93324,14824,974

2,425,41625,49326,09826,43726,47826,54426,63426,85626,85626,85626,85626,87726,91127,03427,09827,24627,26727,32427,38627,45927,48027,72327,735

2,427,75127,78527,78927,80627,91428,00128,014

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18 S18201721272921301720181719191201222121192112021i S31202029142625125J25*25120273021352221130212012313017J S252311713112020J

X

177 W179 W177 W179 W178 W1791 E179 W176 W1781 W1741 W1801801771 W174 W1761 W1781 W180176 VV178 W179 W179 W1771 W1791 W180176 W179J W180173 W180179J E179J EI79i E179J E176 W178 W179 W180180176 W179f W177 W179 W1761 W180177 W174 W178 E1791 W1741 W1791 W1781 W178 VV

if

7.257.57.07.257.06.56.97.36.57.07.257.07.07.07.757.26.757.07.36.256.257.16.256.256.756.756.06.56.757.756.56.255.756.06.56.06.256.06.57.15.56.56.57.36.06.256.506.97.15.756.57.2

/* x io«

3.305.602.003.302.00.701.603.50.70

2.003.302.002.002.009.003.201.202.003.50.42.42

2.50.42.42

1.201.20.20.70

1.209.00.70.40.10.25.70.25.42.25.702.50.10.70.70

3.50.25.42.701.602.50.15.70

3.20

5 X 10"

3.308.9010.9014.2016.2016.9018.5022.0022.7024.7028.0030.0032.0034.0043.0046.2047.4049.4052.9653.3253.7456.2456.6657.0858.2859.4859.6860.3861.5870.5871.2871.6871.7872.0372.7372.9873.4073.6574.3576.8576.9577.6578.3581.8582.1082.5283.2284.8287.3287.4788.1791.37

i

h

400550330600600600200300250200600600380130180300570350560550540400640500180680500805006006006006002001707060010080600805802305408080600530300500580510

DETERMINATION OF STRAIN REBOUND INCREMENTS 1841

TABLE 2.—Continued

Date

1935, Aug. 221935, Sept. 131936, Feb. 111936, Apr. 71936, Nov. 161936, Nov. 261937, Apr. 161937, May 111937, June 201937, Sept. 11938, Mar. 71938, Apr. 151938, Apr. 251938, May 161939, May 221942, Jan. 11942, June 161942, July 71942, Aug. 291943, Mar. 41943, Mar. 241943, Apr. 291943, May 121943, May 291943, June 261943, July 111943, Sept. 281944, Apr. 231944, May 141944, May 251944, July 111944, Aug. 261944, Oct. 111944, Nov. 141944, Nov. 301944, Dec. 11945, Nov. 261946, Aug. 211946, Sept. 261946, Oct. 81946, Nov. 281946, Dec. 171947, July 131948, Jan. 27

J.D.

2,428,03828,06028,21128,26728,49028,50028,64128,66628,70628,80928,96529,00429,01429,03529,406

2,430,36130,52730,54830,60130,78830,80830,84430,86830,87430,90230,91730,99631,20431,22531,23631,28331,32931,37531,40631,42531,42631,78632,054

2,432,09032,11232,15432,17232,38132,578

*

162018*212118|21*26*26*321933192722*20* S21*2124222324*20211832*30222321*19*181524*2321212425 S2518*211920

X

174 W179 W178 W177 W178 W178 W177 W178 E178 E180178 W179 E176* W179 E179 W180179 E178 W179* E179* W179 W180175 W179* W178 W178* W178 W177* W179* E179* W175* W175* W173* W179* E179 W178* W180177 W179 E178 E174 W177 W175 W178 W

M

6.256.06.756.06.56.57.756.506.257.06.05.755.55.756.06.256.756.756.756.06.256.55.756.56.257.07.16.56.07.26.756.256.756.06.756.47.07.07.06.756.756.56.257.25

/» X 10"

.42

.251.20

.25

.70

.709.00

.70

.402.00

.25

.15

.10

.15

.25

.421.201.201.20

.25

.42

.70

.15

.70

.422.002.50

.70

.253.201.20

.421.20

.251.20.60

2.002.002.001.201.20

.70

.423.30

S X 10"

91.7992.0493.2493.4994.1994.89

103.89104.59104.99106.99107.24107.39107.49107.64107.89128.11129.31130.51131.71131.96132.38133.08133.23133.93134.35136.35138.85139.55139.80143.00144.20144.62145.82146.07147.27147.87149.87151.87153.87155.07156.27156.97157.39160.69

h

10060057010054056040064065012050058040060060063055043057060043053027063055018090

37060064018024080

610180600600100600670290580120600?

times were generously made available to meby Doctors Gutenberg and Richter from themanuscript of their book on Seismicity of theEarth (1949). The shallow-focus magnitudeswere determined jointly by the two authors.The deep-focus magnitudes were determinedby Gutenberg (1945) using the method hedeveloped for this class of earthquake foci.

TONGA-KERMADEC SEQUENCES

The data for the Tonga-Kermadec sequencesare given in Tables 1 and 2. The dates of ori-gin have been converted into Julian days inorder to simplify making of the graphs. In thetables t is the Julian day of origin, <f> is the lati-tude of the epicenter, X is the longitude of the

1842 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

epicenter, h is the depth of focus in kilometers,and M is the magnitude. Figure 1 is a graph ofS versus time for the shallow-earthquake se-quence. Each circular dot represents the strain-

(Griggs, 1939) and other solids (Lutts and Him-melfarb, 1940) subjected to constant compres-sional stress and which is designated elasticcreep, since upon release of the stress the sam-

relief increment of a single earthquake. The pie recovers completely—given enough time.

^

160

140

120

100

80

60

40

20

0

X to'2

B

JMi

i

J.I, ISi 1 l

)I5i i i

•

f

1 i

» *

>

JAN.l i

4

,1921 i .

•

5i i 1

jti i

\N.I,i 1

*"

935i i i

••

i 1

• |i

• |

• f

jAri i

j_ »*

i/l s €

A * 1

A = 8

J.I, IS1 i

i

. 0 - <

.0-

.0-!

45i i i

•

*

3.9

7.6

3.3

JULIAN DAY 2,422,000I

2,426,000I

2,430,000

FIGURE 1.—STRAIN-REBOUND INCREMENTS OF TONGA-KERMADEC SHALLOW SEQUENCE (h < 70 km.)The plotted values are multiplied by the undetermined constant C of equation (5).

relation between the diameter of a dot and themagnitude of the earthquake with which it isassociated is shown in the legend. This curveexhibits a rather large amount of irregularity—a characteristic found in a number of shallowsequences having large earthquakes. Presuma-bly these sequences require greater time inter-vals than the present available maximum of45 years of instrumental observations in orderto show their characteristic trends. Figure 2is a graph of 5 for the deep-focus sequence.This sequence includes all earthquakes of theregion having focal depths greater than 70 km.which is taken to be the lower limit of the shal-low earthquake sequence. The foci in this deepsequence are distributed uniformly to a depthof approximately 650 km. The curve of Figure2 is made up of two phases. The graph of thefirst phase which ends at J.D. 2,426,634 (1931October 18) was drawn from the calculatedvalues of 5 in the equation

In engineering literature, creep of this kind isconsidered a manifestation of strain hardening.The second phase begins with the great earth-quake of 1932 May 26 and continues to thepresent date. The curve for this phase wascalculated from the equation

S2 = -87.7 X 10'2 + 16.1 X 109 / . .(9)

[-257 + 79 log t] X 1012. .(8)

This equation represents a type of creep whichhas been observed in the laboratory for rocks

This equation represents a movement withconstant velocity. The smoothness and sin-gular shapes of the curves representing the twophases provide convincing evidence that thesequence of earthquakes which they representmust have been generated on a single fault. Onthis basis, the origin of the two phases can beexplained by reference to the schematic faultdiagram shown in Figure 3. It is assumed thatthe fault surfaces were subject to stress in thedirections shown by the arrows. The corre-sponding indentations and elevations A, B, C,D represent points or areas over which thefault surfaces were locked. When the stress atany one of these points exceeded the lockingstrength, faulting took place. In this way eachpoint became in turn the focus of one of the

TONGA-KERMADEC SEQUENCES 1843

JAN.1.1915 JAN. t, 1925 JAN. 1,1935 JAN. 1,1945I70x 10 I i i i i I i i i i I i i i i I i i i i 1 i i i i I i i i i I i i i i I i i i i I \

160

150

140

130

120

no100

90

80

70

60

50

40

30

20

10

0 /

/<

'*

if

.

A,X"

k *

I

K]

i = [-2-

t *J.D.- _ i

= -878 X!0'2 + 16.1

= J.D. -2,417,000

>7 •»

" 1,'H

• r<

i

//

79 log t]x

H5.700 DAY

lU

XIO

DAY

f/

•t

/f*

•/

/f/

^

• M = 5.5 - 6.9<> M = 7.0 - 7.6

• M = 7.7 - 7.8i i i

JULIAN DAY 2,422,000 2,426,000 2,430,000 2,434,000

FIGURE 2.—STRAIN-REBOUND INCREMENTS OF TONGA-KERMADEC DEEP SEQUENCE (h = 70 — 480 km.)The plotted values are multiplied by the undetermined constant C of equation (5).

FIGURE 3.—TONGA-KERMADEC FAULT MECHANISM SUGGESTED BY STRAIN-REBOUND CURVE OE THE DEEPSEQUENCE

1844 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

sequence earthquakes. In order to account forthe shape of this first phase it must be assumedthat one point X had a much higher lockingstrength than any of the others. At the tune

rapidly at a uniform rate with no further ac-cumulation of strain. The graph for 82 whichrepresents this movement is therefore a straightline. The great shock of May 26,1932, occurred

DEEP

1000

1000

175 E. 180 J75W. 170

6 ' KM.'SOD

• « M = 5.5 - 6.9• o M = 7.0 - 7.9

o M = 8.0 - 8.3

FIGURE 4.—TONGA-KERMADEC EARTHQUAKES AND OCEANIC DEEPSThe plane of the great fault intersects the earth's surface along the line of oceanic deeps. The slanting

line in B represents the intersection of the fault with a vertical plane perpendicular to the line of oceanicdeeps.

when the sequence began, faulting at the dif-ferent minor points was frequent, and theblock movements were rapid except in thevicinity of the major point X. The obstructionto the movements of the blocks presented bythis point produced strain in the blocks whichincreased with time. As a result of this ac-cumulating strain, the rate of movement of theblocks as well as the frequency of occurrenceof the earthquakes decreased with time asshown by the graph of Si. Finally on May 26,1932, the stress at point X exceeded its lockingstrength, and this major obstruction gave wayand thus generated the largest earthquake ofthe sequence. Thereafter movement of theblocks was resisted only by the minor surfaceirregularities, and consequently they proceeded

at a depth of 600 km. Moreover it produced aseries of aftershocks, seven of which were largeenough to be recorded in Pasadena.

Since the curve for the shallow sequence(Fig. 1) bears no relationship to that of thedeep sequence (Fig. 2) it may be concludedthat there is no effective mechanical couplingbetween the two layers in which these sequenceshave their origins. Thus the boundary whichseparates them at a depth of approximately 70km. must be a surface of profound disconti-nuity in the physical state or composition of therock. The discontinuity in the seismic wavespeed at this boundary appears to be quitesmall (Gutenberg, 1948). The absence of anyterm proportional to the time in equation (8)for Si indicates that although the layer, in

TONGA-KERMADEC SEQUENCES 1845

which this sequence originates, extends to adepth of nearly 700 km., the rock was able tomaintain a stress without appreciable flow for24 years.

Since the uncertainty in the location of thehypocenters in depth and geographical positioncan be indicated on the plane of the figure bya circle of approximately 100-km. radius it

m

A S, > S2 B

FIGURE 5.—SKETCH ILLUSTRATING PROPOSED ORIGIN OF THE GREAT TONGA-KERMADEC FAULT

A map of epicenters of the Tonga-Kermadecsequences is shown in Figure 4A. The neigh-boring oceanic deeps are also indicated. Theepicenters of shallow-focus earthquakes areshown as open circles. while those of deep-focusshocks are represented by solid circles. Figure4B represents a vertical section of the regiontaken perpendicular to the line OX which wasdrawn parallel to the trend of the oceanicdeeps. The hypocenters corresponding to theepicenters of Figure 4A are shown projectedhorizontally on the section. In this drawing thevertical and horizontal scales are equal. Theoceanic deeps are not shown since, to scale,their depth is about equal to the width of aline. Figure 4C represents an average profileof the upper 10 kilometers of the region, withthe vertical scale exaggerated ten fold in orderto show the oceanic deeps. The heavy horizon-tal line at the 70-km. depth represents the dis-continuity between the shallow and deep lay-ers. The heavy horizontal line at the 690 km.depth indicates the discontinuity between theseismically active deep layer and the layerbelow in which presumably secular strains ofthe kind that produce earthquakes do not orcannot exist. The heavy oblique line representsthe average distribution of the projected foci.

may be assumed that if the observational errorswere small enough all the points would lie onor close to the line. Thus we may assume thatthe hypocenters determine an oblique faulthaving a plane surface indicated in the figureby the line, and that this is the fault deter-mined by the strain-rebound curves. If thespread in the points is real they determine afault zone which is still fairly narrow in com-parison with its length. The fault plane cutsthe earth's surface along the steep westernslopes of the oceanic deeps (Fig. 4C). The totallength of the fault as shown on the Map Figure4A is approximately 2500 km. (1500 mi.).The transverse dimension measured along theoblique intersection is approximately 900 km.(540 mi.). This great fault is thus larger thanany previously known active fault. A simplehypothesis as to its origin can be described withthe help of Figure 5. Assume that originally acontinental mass of density 52 was in contactalong the surface mn with an oceanic mass ofdensity Si (Fig. 5A). As a result of secular flowin the rock masses a hydrostatic pressure ghidideveloped at the depth hi within the oceanicmass, and a corresponding pressure gh^Si de-veloped at a depth hi within the continentalmass. The constant g is the acceleration of

1846 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

gravity. Thus at any given point on the con-tact surface mn the two pressures produced adifferential stress, acting perpendicular to theboundary, given by their difference A, where

A =

A possible distribution with respect to depthof the differential stress A is represented on anarbitrary scale in the figure. The direction anddistance of the arrows from the line mn indicatethe direction and intensity of the stress. At thebottom of the deeper layer the differentialstress was greatest and was directed toward thecontinental mass. It decreased linearly towardshallower depths as indicated by the lightdashed line drawn through the arrow heads.At some level near the earth's surface thegreater height fe of the continental mass rela-tive to that of the oceanic mass hi offset itslower density, and the differential pressure waszero as at 0 in the figure. Above this level thehydrostatic pressure in the continental masswas greater than that in the denser oceanicmass, and the differential stress was thereforedirected toward the oceanic mass. It passedthrough a secondary maximum at the levelof the oceanic mass surface and fell to zero atthe continental surface.

Acting over a sufficiently long interval oftime this differential stress pattern produced a.distortion of the two masses which resulted ina configuration such as shown in Figure SB.Near the surface the continental mass flowedover the oceanic mass, while in the depths theoceanic mass pushed under the continentalmass. Thus the originally vertical contact sur-face mn became the inclined surface m'n'.Below the level at which the differential pres-sure was zero the parallel component was di-rected downward on the oceanic side of thecontact, and above this level it was directedupward on the continental side. Thus the parallelstress components transformed the contact intoa fault in which the oceanic block moves down-ward and the continental block moves upwardalong the oblique fault plane. The surface dis-tortions resulting from the faulting stresses areindicated in the figure. The downwarping ofthe oceanic block thus produced the depres-sions which form the oceanic deeps.

SOUTH AMERICAN EARTHQUAKE SEQUENCES

The observed data for the South Americansequences are given hi Tables 3, 4, and S. Agraph of 5 for the shallow-earthquake sequenceis shown in Figure 6. For the period of timeover which observations are available this graphis a straight line. On the basis of a single faultstructure the straight line indicates that thefault block moves at a uniform rate.

Figure 7 is a graph of 5 for the intermediatedepth sequence (h=7'0-300 km.) drawn onsemilog co-ordinates. In this graph 5 takes theform of two intersecting straight lines. If plot-ted on arithmetic co-ordinates these lines wouldbe exponential curves concave upward. Theyrepresent flows which increase exponentiallywith time. If these movements are produced inresponse to constant stresses acting on thefault rocks, they correspond to a type of creepwhich is characteristic of substances main-tained in the annealing range of temperatureabove the strain-hardening limit. Since therate of flow is curvilinear with respect to timeand presumably with respect to stress it isdefined as plastic flow. (A viscous flow islinear in relation to stress. The flow con-tribution in strain is not recovered upon re-lease of stress.) On May 20, 1918, JD 2,421,734,the first phase ended. After an interval of 2Jyears the second phase began on August 13,1920, with a reduced rate. Discontinuities ofthis type have been observed in a number ofother sequences studied by the writer. Theircause remains obscure. They must representsudden changes in stress, friction, or physicalstate of the fault masses. In any case it isdifficult to understand how changes can occurthroughout such large structures in such shortintervals of time.

A graph of 5 plotted against time for thedeep-focus sequence (k =550—660 km.) is shownin Figure 8. This graph takes the form of in-tersecting straight lines in linear co-ordinates,indicating constant velocity movements of thefault blocks. From the date at which observa-tions began April 28, 1911, until January 17,1922, the block velocity was large, and theearthquakes which determined it were large.After that interval the earthquake frequencyincreased appreciably, while the magnitudes

SOUTH AMERICAN EARTHQUAKE SEQUENCES 1847

TABLE 3.—SOUTH AMERICAN SHALLOW SEQUENCE (h < 70 km.)

Date

1906, Jan. 311906, Aug. 171909, June 81911, Sept. 151913, July 281913, Aug. 61914, Jan. 301917, Feb. IS1917, July 271917, Aug. 311918, Dec. 41920, Jan. 301920, Aug. 31920, Sept. 241922, Jan. 61922, Oct. 111922, Nov. 71922, Nov. 111923, May 41924, Mar. 111924, June 221924, Oct. 18192S, May IS1926, Aug. 121927, Mar. 131927, Aug. 201927, Nov. 141928, Apr. 91928, Apr. 271928, May 141928, July 181928, July 281928, Nov. 201928, Dec. 11929, May 301929, Aug. 151930, Dec. 241931, Mar. 181931, Apr. 31931, May 201931, June 151931, June 291931, Sept. 121932, Sept. 51933, Feb. 231933, May 61933, May 61933, Oct. 21933, Oct. 31933, Oct. 31934, Apr. 31934, Aug. 6

J.D.

2,417,24217,44018,46619,29519,97719,98620,16321,27521,43721,47021,93222,35422,54022,59223,06123,33923,36623,36823,54423,85623,95924,07724,286

2,424,74024,95325,11325,19925,34625,36425,38125,44625,45625,56125,57225,76225,83926,33526,41926,43526,48226,50826,52226,59726,95627,12727,199

2,427,19927,34827,34927,34927,53127,656

«

1 N33 S26J S20 S17 S17 S35 S30 S31 S4 N

26 S3 N

27J S6 N

16! S16 S28 S28! S28J S4 S5! N2! N

26 S23 S6 S5 N

30| S13 S13 S5 SSi S

31 S22! S35 S35 S5 N25 S

32! S9J S

27i S14! S29! S5 N6 S

20 S5! N5f N2 SIf SIf S4 N3i N

X

81J W7270!727474737370747177|70837372!727071f8278J8071!70 W81!82J71!69|69!78797170!726883!66727971!75i7177!81718382| W8180}80f7877f

u

8.68.47.67.37.07.757.67.07.07.37.756.06.756.57.27.47.08.37.06.756.56.757.16.756.07.06.756.96.757.37.06.57.18.06.756.56.07.16.256.256.06.06.256.07.66.56.06.96.256.06.06.0

/* X 10"

55.035.07.13.52.09.57.12.02.03.59.5

.21.2

.73.24.52.0

28.02.01.2

.71.22.51.2

.22.01.21.61.23.52.0

.72.5

16.01.2

.7

.22.5

.4

.4

.2

.2

.4

.27.1

.7

.21.6

.4

.2

.2

.2

5 X 101J

55.090.097.1

100.6102.6112.1119.2121.2123.2126.7136.2136.4137.6138.3141.5146.0148.0176.0178.0179.2179.9181.1183.6184.8185.0187.0188.2189.8191.0194.5196.5197.2199.7215.7216.9219.6217.8220.3220.7221.1221.3221.5221.9222.1229.2229.9230.1231.7232.1232.3232.5232.7

1848 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

TABLE 3.—Continued

Date

1935, June 281935, Aug. 51935, Dec. 241936, May 221936, July 131936, July 261937, March 141937, June 211937, July 81937, Dec. 121937, Dec. 241938, Apr. 171939, Sept. 201940, March 311940, Apr. 81940, May 51940, May 241940, Oct. 61941, July 31941, July 111942, May 141942, Aug. 241942, Oct. 81943, Mar. 51943, Apr. 61943, May 221943, July 51944, Jan. 151944, Oct. 231946, Aug. 21946, Nov. 101947, Nov. 1

J.D.

2,427,98228,02128,16128,31128,36328,37628,60728,70628,72328,88028,89329,01629,52729,72029,72829,75529,774

2,429,90930, 17930,18730,52530,59630,64130,78930,82130,86730,91131,09531,38732,03532,13532,491

*

34 S35 S3 N

32 S24} S24 S24} S8} S3 N

25 Sio| s19 S11! S19 S33! S

7 S10i S22 S31! S5 Ni s

15 S6 N51 N

30i S30J S16 S31J S

}N26! S8} S

10i S

X

73727966707069i80847076!69!75!70|71*807771 W69!82J81!7682f82!72727468}80!70! W77!75

M

6.06.06.756.07.36.756.56.756.06.06.256.56.06.06.06.08.06.756.256.257.98.16.06.757.96.756.757.46.97.57.257.3

/* X 10"

.2

.21.2

.23.51.2.7

1.2.2.2.4.7.2.2.2.2

16.01.2

.4

.413.020.0

.21.2

13.01.21.24.51.65.63.43.5

5 X 10"

232.9233.1234.3234.5238.0239.2239.9241.1241.3241.5241.9242.6242.8243.0243.2243.4259.4260.6261.0261.4274.4294.4294.6295.8308.8310.0311.2315.7317.3322.9326.3329.8

decreased sufficiently to reduce the block veloc-ity to 0.22 of its initial value. There is some in-dication that in January 1946 the initial rateof the blocks was resumed, but more time willbe required to settle this question. It is note-worthy that the discontinuity in this deep se-quence occurred very nearly at the same timethat the intermediate-sequence discontinuityoccurred and that in each the rate decreased.This would indicate that a certain amount ofmechanical coupling exists between the twolayers in which these sequences have theirorigins. On the other hand there is no evidenceof coupling between the shallow and inter-mediate layers. The boundary between themmust represent a profound mechanical discon-tinuity similar to the corresponding boundary

in the Tonga-Kermadec region. Possibly, sincethe shallow sequences generally show greaterstrain-rebound movements than the deeper se-quences, the shallow movements may includemovements coupled with the deeper layersalong with horizontal or other movementswhich are independent of them. On this basis,however, we would expect to observe someeffect of the lower-layer velocity discontinuitieson the surface-layer movements, unless themovements in the two layers are complemen-tary. These effects appear to be lacking in boththe Tonga-Kermadec region and in South Amer-ica. There is some evidence that faulting inthe shallow layers of both these regions isprincipally horizontal. Nevertheless these lay-ers are sufficiently thin in relation to the thick-

SOUTH AMERICAN EARTHQUAKE SEQUENCES 1849

TABLE 4.—SOOTH AMERICAN INTERMEDIATE SEQUENCE (h = 70-300 km.)

Date

1906, Sept. 281909, May 171910, Oct. 41914, Feb. 261915, June 61916, Aug. 251918, May 201920, Aug. 131921, Oct. 201922, Mar. 281923, Sept. 21924, July 221925, Jan. 51925, June 71925, June 231926, Mar. 71926, Apr. 281927, Apr. 141927, May 221927, Aug. 11927, Oct. 31927, Nov. 261928, May 261928, Sept. 211929, May 251929, Oct. 191929, Oct. 191930, Sept. 231930, Nov. 241931, Apr. 31931, May 281931, July 111931, July 181932, Jan. 201932, Apr. 261932, June 181932, July 291932, Nov. 11932, Nov. 291932, Dec. 91933, July 231933, July 311933, Aug. 61933, Aug. 91933, Oct. 11933, Oct. 121933, Oct. 251933, Nov. 141933, Dec. 211934, June 111934, June 111934, June 24

J.D.

2,417,48218,44418,94920,19020,65521,10121,73422,61222,98323,14223,66523,98924,15624,30924,32524,58224,63424,98525,02325,09425,15725,21125,393

2,425,50125,75725,90425,90426,24326,30526,43526,49026,53426,54126,72726,82426,87726,91827,01327,04127,05127,27727,28527,29127,29427,34727,358

2,427,37127,39127,39827,60027,60027,613

<t>

2 S20 S22 S18 S18J S21 S281 S20 S18J S21 S16 S2 S14 S3 N05 S24 S32 S21 S24 S21 S241 S231 S15 S81 S23 S23 S26 S2 S27 S201 S81 S221 S12 S25 S20 S19 S24 S32 S15 S15| S151 S11 S151 S7 S23 S23.0 S32 S31 S331 S331 S22.0 S

X

79 W64 W69 W67 W681 W68 W711 W68 W68 W68 W681 W80 W731 W78 W77 W761 W69 W691 W67 W661 W68 W67 W69 W701 W751 W69 W69 W66 W77 W65 W701 W741 W69 W771 W691 W71 W70 W70 W71 W75 W75J W751 W751 W681 W75J W691 W66.7 W691 W69 W641 W641 W68.6 W

11

7.57.17.257.27.67.57.56.57.07.27.06.56.56.756.756.57.07.16.256.56.56.756.256.756.757.56.06.56.256.256.56.256.756.756.56.256.06.06.756.56.06.06.56.256.256.257.06.56.256.06.06.9

/* X 10"

5.62.53.43.27.15.65.6.72.03.22.0.7.7

1.21.2.72.02.5.4.7.71.2.4

1.21.25.6.2.7.4.4.7.4

1.21.2.7.4.2.2

1.2.7.2.2.7.4.4.42.0.7.4.2.2

1.6

S X 10"

5.68.111.514.721.827.433.033.735.738.940.941.642.343.544.745.447.449.950.351.051.752.953.354.555.761.361.562.262.663.063.764.165.366.567.267.667.868.069.269.970.170.371.071.471.872.274.274.975.375.575.777.3

h

15025012013016018080150120901502502001701801501801101402001001801302501501001001501001801201201501007070110100110758080100170120100220110120100100100

1850 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

TABLE 4.—Continued

Date

1934, Oct. 291934, Dec. 41934, Dec. 161934, Dec. 231935, Feb. 131935, Feb. 281935, Mar. 81935, Mar. 261935, Sept. 181935, Sept. 191935, Nov. 21936, Apr. 301936, May 61936, June 221936, July 41936, Nov. 71936, Nov. 291936, Dec. 51937, Mar. 191937, Mar. 291937, May 211937, July 91937, July 191937, Sept. 241937, Oct. 121937, Oct. 271938, Jan. 161938, Feb. 51938, Apr. 241938, June 151938, June 231938, Aug. 41939, Jan. 181939, Apr. 181939, Apr. 251939, May 191939, July 41939, Oct. 51939, Oct. 71940, Feb. 121940, Aug. 71940, Aug. 261940, Sept. 181940, Sept. 291940, Oct. 11940, Oct. 31940, Oct. 41940, Oct. 231940, Oct. 241940, Dec. 221941, Jan. 241941, Apr. 3

J.D.

2,427,74027,77527,78727,79427,84727,86227,87027,88828,06428,06528,10928,28928,29528,34228,35428,48028,50228,508

2,428,61228,62228,67528,72428,73428,80128,81928,83428,91528,93529,01329,06529,07329,11529,29229,37229,37929,40329,44929,54229,54429,67229,84929,868

2,429,89129,90229,90429,90629,90729,92629,92729,98630,01930,088

«

5 S191 S24 S21 S25$ S23 S4 S151 S51 N151 S2 S31 S8 S.22 S18 S23 S22i S20 S29 S15i S21 N16 S11 S221 S25 S341 S6 S41 N23J S31 S301 S24 S29J S27 S121 S18 S21 S22 S181 S261 S22 S111 S23 S35 S30 S21 S22 S2 S35 S151 S3J S221 S

X

78 W691 W68 W68 W69 W67 W80 W73 W76 W70 W79 W65 W75 W68 W70 W67 W67 W701 W70 W71 W771 W72 W761 W70 W681 VV71 W75 W76i W66 W701 W70 W68 W71 W701 W751 W69 W66 W67 W70 W71 W681 W75 W68 W70 W721 W70 W71 W76 W721 W681 W76| W66 W

11

6.256.96.06.56.56.256.06.06.256.56.06.06.06.06.06.06.06.06.06.756.56.07.16.06.56.06.07.06.06.06.56.756.257.46.256.256.756.06.06.56.256.06.56.256.56.257.36.06.757.16.56.5

J* X 10"

.41.6.2.7.7.4.2.2.4.7.2.2.2.2.2.2.2.2.2

1.2.7.2

2.2.2.7.2.2

2.0.2.2.7

1.2.4

4.5.4.4

1.2.2.2.7.4.2.7.4.7.4

3.5.2

1.22.5.7.7

5 X 10"

77.779.379.580.280.981.381.581.782.182.883.083.283.483.683.884.084.284.484.685.886.586.788.989.189.890.090.292.292.492.693.394.594.999.499.8100.2101.4101.6101.8102.5102.9103.1103.8104.2104.9105.3108.8109.0110.2112.7113.4114.1

h

11013015010010020010012080250130200160100140200230100701209018019013011011010016018070702207010015010029024011070110110110110801107514080230120260

TABLE 4.—Continued

Date

1941, Apr. 31941, July 101941, Aug. 141941, Sept. 181941, Oct. 151941, Nov. 101942, June 291942, July 81942, Nov. 61943, Jan. 301943, Feb. 161943, Mar. 141943, Apr. 51943, Nov. 291943, Dec. 11943, Dec. 221944, Feb. 291944, May 91944, July 231944, Dec. 221945, July 91945, Aug. 211945, Sept. 131946, July 261946, Sept. 30

J.D.

2,430,08830,18630,22130,25630,28330,30930,54030,54930,67030,75530,77230,798

2,430,82031,05831,06031,08131,15031,22031,29531,44731,64631,68931,71232,02832,094

<t>

22J S18i S23 S13} S15J S22 S32 S24 S6 S2 S15 S20 S6* S29J S21 S2i S14i S24 N24 S25 S

24 N10J S33i S19J S13 S

X

66 W70 W66| W721 W74 W67 W71 W70 W77 W80J W72 W69J W76 W68* W69 W77 W704 W754 W664 W70 W764 W75 W70| W704 W76 W

M

7.26.06.07.06.06.256.97.06.756.97.07.26.56.757.256.257.06.06.06.56.56.757.16.757.0

J* X io»

3.2.2.2

2.0.2.41.62.01.21.62.03.2.71.23.4.4

2.0.2.2.7.7

1.22.51.22.0

5 X 10"

117.3117.5117.7119.7119.9120.3121.9123.9125.1126.7128.7131.9132.6133.8137.2137.6139.6139.8140.0140.7141.4142.6145.1146.3148.3

h

2601201801001102001001401301001901501401001001302001002501201001201007070

TABLE 5.—SOUTH AMERICA DEEP SEQUENCE (h = 550-660 km.)

Date

1911, Apr. 281912, Dec. 71915, Apr. 231916 June 211921, Dec. 181922, Jan. 171922, July 101922, Sept. 41926, Feb. 91927, Apr. 61928, Jan. 51928, Aug. 151930, Aug. 41933, Aug. 291934, Jan. 91935, Dec. 141935, Dec. 161935, Dec. 281936, Jan. 141939, Jan. 241939, Dec. 41940, May 11940, Sept. 231940, Sept. 241942, Nov. 301944, June 81946, Aug. 281947, Jan. 291947, Aug. 6

J.D.

2,419,15619,74620,61321,03823,04423,07323,24723,30324,55724,97925,25325,46626, 19427,31027,44828,15228,15528,16728,18329,28929,62329,751

2,429,89629,89730,69431,25032,06132,21532,404

0

0 S298

28J2124

1910J291019i2894

10.9284919i94

29264

6425423 S9*

271026269

X

71 W624686371716269J62706462470469.56370470|70i624637062464 W70463471636372

M

7.17.57.257.57.67.66.756.96.56.06.756.506.506.506.06.96.255.756.96.05.56.256.56.06.56.257.207.256.75

/* X 1012

2.505.603.305.607.107.101.201.60

.70

.251.20

.70

.70

.70

.251.60

.42

.151.60

.25

.10

.42

.70

.25

.70

.423.203.301.20

5 X 10'«

2.508.10

11.4017.0024.1031.2032.4034.0034.7034.9536.1536.8537.5538.2538.5040.1040.5240.6742.2742.5242.6243.0443.7443.9944.6945.1148.3151.6152.81

h

600620650600650650630660660600640620650650630650600650620580600580550600590600580580600

1851

1852 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

ness of the deeper layers that they can takeon the vertical warping produced by the ver-tically oblique faulting of these larger deepstructures even though their own faulting move-

ditional earthquakes occurred for 5$ years.During this interval the strain continued toaccumulate at the initial constant rate so thatwhen the fault did give way it generated two

340 X IQiJAN. 1. 1920 JAN.1,1930 JAN. 1, 1940

i i i i i i i i I i i i i i i i i i I i i i i i i i i i I i i i i i i i

320

300

280

260

240

220

200

£l80o>£, 160

^140

120

100

80

60

40

20

0

V

*

= 67.7 X IO'2 + 15.3

= J.D. -2.416.000

/*

/

\

S «•

i

/

**

/

XIC

DK1

/'*'

,

\9 *) t0O s

*

w^ ^

/••-'

,sfi"

.

•

/

M =M =

4

7

r V^

6.0- 6.4 _7.5-8.1

• M = 8.3 - 8.6I

JULIAN DAY 2.424,000 2,428,000 2,432.000

FIGURE 6.—STRAIN-REBOUND INCREMENTS OF THE SOUTH AMERICAN SHALLOW SEQUENCE (h < 70 km.)The plotted values are multiplied by the undetermined constant C of equation (5).

ment may not conform with those of the deeperfaults.!•'• In Figure 9 the deep-focus sequence con-sists of two groups of epicenters centeredroughly at 5° and 25° latitude south, respec-tively. Each group has the same number ofshocks and in addition the same number ineach indicated magnitude range. Thus althoughthey represent areas of the fault block whichare some 2000 km. apart their movements arevery nearly identical. In the initial phase ofthis sequence (Fig. 8) the fault locked stronglyfollowing the fourth earthquake, and no ad-

great earthquakes which brought the totalelastic-rebound movement to the value de-termined by the initial line. Thus for a periodof SJ years the rock of this fault maintained alinear elastic strain without flow at a depth of650 kilometers. Clearly any remaining doubtsas to the solidity of rocks at such depths mustbe abandoned.

A map of the region of the South Americansequences is shown in Figure 9. Epicenters ofshallow-focus earthquakes are shown as opencircles. Those of intermediate-depth foci areshown as solid circles, and the deep-focus epi-

200

100

80

6050

»

. 30

20

10

8

65

12 JMIN.I, \yc\J (JAW.X IO, 1 , , i , i , , , , 1 , , i , i i , i ,

1

*

<

/I

/

I

t,

t.

/

.

= [-

•J.C

r'

85

X-2,

'

t 14

4I7.C

^

'

log

XX)

/

s] >DA

t

/

, I OU JAN. 1, l ^fU JAN. 1, lyi i i i 1 i i i i 1 i i i i I i i i i 1 i

rr<I03

YS

S*f

A^/

t,= [-69.7 + 5.5 logs] XIO3

t,= J.D.-2.4I7.000 DAYS

^iT

.

-

M

M

1*

»

-- 6.0 - 6.9 -

= 7.0 - 7.6 -I

JULIAN DAY 2,424.000 2,428,000 2.432,000

EIGUBE 7.—STRAIN-REBOUND INCREMENTS OF THE SOUTH AMERICAN INTERMEDIATE SEQUENCE(h = 70 - 300 km.)

The plotted values are multiplied by the undetermined constant C of equation (5).

JAN. I, 1920 JAN. I, 1930 JAN. I, 194060X IQiai I. i i i i I i i i i 1 .1 . i i i I i i i i I i i i i I i i i i I i r i i

50

40

fe 30

V)

20

10p

s

1

/*

/I—

}

/

S,= 2

S

H

•

.0 x

2 = 27.0 x

---i

IO'2

^

t

IO'2

H-— "

+ 1.59 X I09t

-tf• ^^

= J. D. - 2,419,000

+ 7.33 XI09t

• M

• M

• M

- — ••"

DAYS

I

= 5.5 - 6.9

= 7.0 - 7.3

= 7.4 - 7.6 "

IJULIAN DAY 2.422,000 2,426,000 2,430,000

FIGURE 8.—STRAIN-REBOUND INCREMENTS OF THE SOUTH AMERICAN DEEP SEQUENCE (h = 550 — 660 km.)The plotted values are multiplied by the undetermined constant C of equation (5).

1853

1854 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

centers are represented by triangles. The mag- which were drawn parallel to the neighboringnitude ranges of the earthquakes corresponding deeps. Above each section is drawn an averageto the epicenters are indicated. The oceanic profile of the upper 10 km. of the projecteddeeps are sketched in roughly. In this region region of each section outlined by the dashed

SHALLOW FOCIo M • 6.0 - 7.4O M • 7.5 - 8.1O M = 8.3 - 8.6

1000 I i I I I i I i i I I i i i

85° 80° 75° 70° 65° 60"

- B'

- C'

FIGURE 9.— SOUTH AMERICAN EARTHQUAKES AND OCEANIC DEEPSThe great fault intersects the earth's surface along the curved line of oceanic deeps. Its positions below

the surface are indicated in the three sectional drawings A', B' and C'.

they have depths in excess of 8 km. Unlike theTonga-Kermadec fault, the surfaces of thisfault do not form a single plane. They can bethought of as a plane which has been bent andsomewhat twisted along the long axis and bentalong the short axis near the middle. It wasnecessary to make three sectional projectionsin order to display the three-dimensional dis-tribution of the foci. Thus A', B', and C' arethe three vertical sections taken perpendicularto the lines A, B, and C respectively of the map

parallel lines on the map. In these profiles thevertical scale is 10 times that of the horizontalscale. In this region the observational uncer-tainty in the position of the hypocentersis about100 km. in any spatial direction. It may beassumed therefore that the lines drawn throughthe projected positions of the hypocenters inA', B', and C' (Fig. 9) represent the projec-tions of the fault surface. The hypocenters inthis region are not distributed uniformly withrespect to depth as they are in the Tonga-Ker-

SOUTH AMERICAN EARTHQUAKE SEQUENCES 1855

madec area. North of Lat. 1SJ°S. the inter-mediate-depth foci form a continuous distri-bution over the depth range 70-200 km. Southof Lat. 1S^°S. the continuous distribution ends

approximately 900 km., and it extends to adepth of approximately 690 km.—more thanone tenth of the radius of the earth. The longline of oceanic deeps which parallel the western

OCEANIC DEEP

ANDES

m,

A S2

S4

B

FIGURE 10.—SKETCH ILLUSTRATING PROPOSED ORIGIN OF THE GREAT SOUTH AMERICAN FAULT

at a depth of 300 km. There are no foci in thedepth range 300-600 km. The deep foci occuronly at a depth of 550 to 660 km. The originof this fault may be explained on the basis ofan original configuration such as the one shownat A (Fig. 10) in which the continental mass hadthree layers and the oceanic mass two (or three)layers of different density. Thus 5i was greaterthan 5i, and 53 was greater than 54. The con-stants 61 and 63 may have been equal. In anycase the inequality between 5i and 62 musthave been greater than that between 53 anddt to provide for the shallower slope of the faultin the 70-300 km. depth range. To account forthe bend in the long axis of the fault it may beassumed that the inequality between 5i and52 varied along the length of the fault; it wasgreatest north of Lat. 15j°S. and approximatelyconstant south of that parallel of latitude. Theinequalities in densities may have been due inpart to temperature differences resulting fromdifferent rates of generation of radioactive heatenergy. The total length of the fault is approx-imately 4500 km. (2700 mi.). Its transversedimension which dips under the continent is

coast of the continent was formed by the down-warping of the oceanic block of this great fault,and the upwarping of its continental blockproduced the Andes Mountains.

OTHER DEEP-FOCUS SEQUENCES

A preliminary examination of a number ofother deep-focus earthquake sequences whichare associated with oceanic deeps indicatesthat they may all be generated on great faultssimilar to those described in this paper. Amongthese may be mentioned the Aleutian sequence,another one which extends from Southern Japanto Kamchatka, several sequences in the EastIndies, the Central American sequences, andthe West Indies sequences.

TSUNAMIS AND THE GREAT FAULTS

The characteristic movements of the greatfaults involve fairly large vertical components.It may be assumed, therefore, that a seismicsea wave or tsunami is generated whenever anearthquake focus on one of these faults is suffi-

1856 HUGO BENIOFF—FAULT ORIGIN OF OCEANIC DEEPS

ciently shallow to allow the faulting movementto reach the upper surfaces of the faultblocks where it can effect a sudden verticaldisplacement of the ocean floor. Moreover, theepicenter does not necessarily have to be sit-uated within the areas of the oceanic deeps,since faulting or strain adjustment can bepropagated up to several hundred kilometersfrom the focus. The faulting speed is finite andsomewhat less than that of seismic waves (Ben-ioff, 1938). Consequently the faulting impulsemay arrive at the ocean floor up to severalminutes after the beginning of the earthquake.The time of origin of the tsunami may thus bedelayed with respect to the origin time of theearthquake. The sea waves generated by theupward movement of the oceanic block mustbegin with a rise in the water level, whereasthose generated by the downward movementof the continental block must begin with arecession. Since the area of the ocean floor overthe oceanic block is generally greater than thatover the continental block the initial wave ofrecession is larger than the initial wave ofadvance. To an observer on the oceanic sideof the fault all tsunamis would begin with arecession. On the other hand, to an observeron the continental side, the initial wave ofadvance may be so small as to be unnoticed,

and the tsunami would thus appear to beginwith a recession also. Presumably this accountsfor the fact that most tsunamis are observedto begin with recessions.

REFERENCES CITED

Benioff, Hugo (1938) The determination of the extentof faulting with application to the Long Beachearthquake, Seismol. Soc. Am., Bull., vol. 28,p. 77-84.

Griggs, David 2939) The creep of rocks, Jour. Geol.,vol. 67, p. 25-251.

Gutenberg, Beno (1945) Magnitude determinationfor deep-focus earthquakes, Seismol. Soc. Am.,Bull., vol. 35, p. 117.

(1948) On the layer of relatively low wavevelocity at a depth of about 80 kilometers, Seismol.Soc. Am., Bull., vol. 38, p. 121-167.

and Richter, C. F. (1942) Earthquakemagnitude, intensity, energy, and acceleration,Seismol. Soc. Am., Bull., vol. 32, p. 143-191.

(1949) Seismicity of theEarth, Princeton Univ. Press.

Lutts, C. E., and Himmelfarb, David (1940) Thecreep phenomenon in ropes and cords, Am. Soc.Test. Mat., Pr., vol. 40, p. 1251.

Richter, C. F. (1935) An instrumental magnitudescale, Seismol. Soc. Am., Bull., vol. 25, p. 1-32.

SEISMOLOGICAL LABORATORY, 220 N. SAN RAFAELAVE., PASADENA, CALIF.

MANUSCRIPT RECEIVED BY THE SECRETARY OF THESOCIETY, JULY 8,1949

PUBLICATIONS or THE DIVISION or GEOLOGICALSCIENCES, CALIFORNIA INSTITUTE OF TECH-NOLOGY, PASADENA, CALIF. CONTRIBUTION No.505