JOURNAL OF GEOPHYSICAL RESEARCH VOLUME 67, No. 12 NOVEMBER 1962

Circum-PacificFaulting in the Philippines-TaiwanRegion

CLARENCE R. ALLEN

Division o• Geological Sciences California Institute o• Technology, Pasadena

Abstract. Conflicting views of circum-Pacific tectonics have focused on the Philippines- Taiwan region, where there has been neither convincing documentation nor general agreement on the importance of transcurrent (strike-slip) faulting or the possiblesense of regional hori- zontal displacement.Structural and physiographicfeatures of the 1200-km-long Philippine fault zone are fully as spectacularas those of the better-known San Andreas and Alpine faults, and current activity is indicated by many localities in which scarpscut Recent gravels. Pre- dominance of horizontal over vertical displacementsis indicated by linearity of the fault trace, failure of one side to be consistentlyhigher than the other, disregardfor grossphysiography, and scissoring of individual scarps within the zone. Consistent stream offsets on Luzon, Masbate, and Leyte demonstrateunequivocally that the senseof Recent displacementhas been uniformly sinistral (left-handed). The Philippine fault has no obviousgeologic relation- shipto active volcanoes,but the parallelismand proximity of the fault to the Mindanao trench suggesta close causalrelationship. The remarkable Longitudinal Valley of eastern Taiwan representsanother great transcurrent fault parallelto the westernPacific rim, and grounddis- placementsduring historicearthquakes indicate a sinistralsense of displacementhere as well as in the adjacentPhilippines. This study doesnot supportthe hypothesisof counterclockwise rotation of the Pacificbasin, but more important is the further documentationof the predom- inance of transcurrentfaulting in active circum-Pacificorogenic areas. These results reinforce earlier field studiesin Alaska,California, Chile, and New Zealand,as well as emphasizingthe geologicalreasonability of the resultsof seismicfault-plane solutions indicating the world-wide predominance of transcurrent movements.

TI-IE [PROBLEM placementsin thissame region. In both hypothe- ses,the assumedsense of displacementplayed The purposeof this study was to investigate a part in the interpretation of the general some of the major faults of the Philippines- mechanismof circum-Pacificorogeny. Van Bem- Taiwan region with the hope of unequivocally melen [1949] and othershave tendedto discount demonstratingtheir extent, their degreeof cur- the importanceof horizontalfault displacements rent activity, and--particularly--their sense in this region altogether. of displacement.Unusual geophysicalsignifi- Willis [1937] was apparentlythe first investi- cance is attached to this region becausewidely gator to recognizethe existenceof a through- divergentviews of circum-Pacificorogeny focus goingfault zonein the Philippinescomparable to here, especiallyconcerning the questionof the the San Andreas fault in California, although relative importance of horizontal fault dis- numerousworkers dating back to von Drasche placements.Even granting the importanceof [1878] had pointed out the existenceof vari- horizontal movements, major disagreement ous tectonic lineaments in the archipelago, has existed concerning the sense of possi- mainly on the basisof the allegedalignment of ble displacement: theories of Benio# [1959] islands, volcanoes,and earthquake epicenters. and St. Amand [1959] implied the probable Repetti [1935], in particular, had proposedthe existenceof dextral or right-handedfaults paral- 'Master fault' along the general trend of the leling the westernPacific margin in this region, Philippine fault zone, but his evidence other whereas theories of Vening Meinesz [1954] and than that from northern Luzon was based al- Biq [1960] supposedsinistral or left-handeddis- most wholly on inconclusiveseismic data. The term 'Philippine fault zone' used by Willis • Contribution 1099, Division of Geological Sci- [1937] seemsto have becomewell establishedin ences, California Institute of Technology. the literature, notwithstandingthe later use by 4795 4796 CLARENCE R. ALLEN Willis [1944] of 'Visayan rift' for the same the main zoneand is apparently at an angle of feature. at least 30ø to it. Willis further arguedthat the Von Drasche [1878, p. 3] and othershad par- regionalplan of the islandssuggested a sinistral ticularly pointed out the bifurcation or virga- senseof displacement.Other authorshave agreed tion of tectonic trends in the southern Philip- [Alvir, 1941; King and McKee, 1949].But opin- pines,the southwesterlytrend of the islandsin ion has not been unanimous; Irving [1951, p. the southwesternPhilippines contrasting sharply 70] thoughtthat many of the offsetsimplied by with the southeasterlytrend of the islands and sucharguments were 'excessive,'and Alvir [1926, mountain chainsalong the southeasternrim of p. 453] had used the same kind of argument the archipelago.The island of Masbate,x with (basedon grossphysiographic features) to indi- its two diverging 'arms,' has often been cited cate dextral displacementalong the fault in as an exampleof the acute junction of the two northern Luzon that is herein considered to be opposingtrends. Willis wasso impressed by the an integral part of the Philippine fault zone. linearity of the southeasterlytrending features Another line of evidencebearing on the sense between Ragay Gulf and northern Mindanao of fault displacementis that given by fracture that he proposedthe existenceof the Philippine patterns in Philippine mining areas, although fault zone along this line; his evidencewas none of the areas described in detail has been mittedly based almost wholly on grossphysio- within the fault zoneproper. Leith [1938], Liv- graphicforms. Other investigatorswho followed ingston [1939], and Wisser [1939] pointed out Willis have generallyagreed upon the presence that the pervasiveshear and tensionjoints of of a fault zone, althoughthere have been dis- the Baguio mining district demand east-west agreementsconcerning its possibleextension into compression,at least in Miocenetime, as does northern Luzon and its exact location in the cen- the generaltrend of the foldedmountain ranges tral part of the islands.Evidence for its existence north of Manila. Wisser [1952] cited supporting has continuedto be basedmainly on grossphysi- evidencefrom local fracture patterns in south- ography, inasmuchas there is no clear docu- eastern Luzon and northeastern Mindanao. Such mentation of displacementsalong its trace dur- east-westcompression is, of course,compatible ing historicearthquakes. Only on the island of with sinistraI transcurrent displacementon the Mindanao do publishedgeologic maps clearly Philippinefault zonebut by no meansproves it. demonstrate and delineate the various breaks Nor has there been completeagreement on the that make up the Philippine fault zone [Ranne/t regionalstress pattern; the numerousnorth- et al., 1960]. Probably the most accurate maps south lineamentsof the central Philippineswere of the fault's locationnorth of Mindanao, and visualizedby Alcaraz [1947] as tensionalin the most cogentarguments for its existence,are origin, which implies a regionalstress system those by Irving [1951, 1953], based in par• on that would lead to dextral displacementalong an analysisof the submarinemorphology of the the Philippinefault zone.Ranne/t et al. [1960] region. indicated sinistral displacementson individual Willis apparently consideredthe Philippine breaks of the zone in northeastern Mindanao but fault to be dominantlytranscurrent (i.e., 'strike- did not state why. slip,' 'lateral,' or 'wrench') becauseof its simi- It mightbe hopedthat resultsof seismicfault- larity to the San Andreasfault, particularly in its planesolutions on the numerousearthquakes of length, linearity, and generalphysiographic ex- the Philippinesregion would clear up the am- pression.But the senseof horizontal displace- biguity resulting from the limited geologic ment has been the subject of more nebulous studies.In a summary of 35 fault-plane solu- argument.Willis arguedthat the fault was sinis- tions from this region, Ritsema and Veldkamp traI, and the most often-quotedevidence is based [1960] statethat the predominantdisplacements on a single set of horizontal asymmetric slick- have indeedbeen transcurrent and sinistral,and ensidesthat were observedby Willis [1937, p. the implied directionof horizontalcompression 39] in the Aroroy mining district on Masbate, is about N 60øE. None of the larger earthquakes although this particular fault is subsidiaryto of this group, however, occurred within the • All Philippine place names used in the text Philippine fault zone, and the largest shock are indicated on Figures 1 and 2. (M -- 8.2, Jan. 24, 1948) was clearly dextral. CIRCUM-PACIFIC FAULTING 4797 The epicenterof this greatearthquake was near between the towns of Visares and Leyte. The the west coast of Panay, where there is good zone here consistsof a number of parallel active evidence of a major northeast-trending fault breaks over a width of about 500 meters, and that might reasonablybe consideredconjugate the extreme linearity of the topographywithin to the Philippine fault and thus dextral. The the fault zone stands in marked contrast to the fault-planesolution, however, which is supported topographyon both sides(Figures 3-5). Neither by the trend of the isoseismaIs[Irving and side is consistentlyhigher than the other, and Teves, 1948], indicated failure on a fault strik- the centralpart of the zoneis, in general,a struc- ing more nearly parallel to the Philippine fault rurally and topographicallydepressed trough than to the local structure. Furthermore, Aki's that has causedthe term 'rift' to be applied to [1960] source-functionsolutions based on Ray- this fault in analogy with the similar features leigh waves continue to indicate dextral dis- and nomenclature of the San Andreas fault. In- placements around the entire circum-Pacific deed,the generalphysiography is best described margin. The one Philippine earthquakeincluded in the same words used by Noble [1954, p. 37] in Aki's solutionsindicates breaking on a fault in portraying the San Andreas: 'It has a curi- sub-parallelto the Philippine fault in the north- ously direct courseacross mountains and plains, ern Central Valley of Luzon, and it is clearly with little regard for gross physiographicfea- dextral. tures,yet it influencesprofoundly the local topo- Thus, in brief, it was not clear at the begin- graphic and geologicfeatures within it.' ning of this study that the Philippinefault zone Both north and south of the central Philip- (1) indeed existed, at least as a throughgoing pinesarea, the fault separatesinto a numberof feature of regionaltectonic importance, (2) was branches. The fault that enters northern Luzon active, and (3) was necessarilya transcurrent at Dingalan Bay is herein consideredto be the and sinistral feature. Furthermore, it was hoped main Philippine fault on the basisof its similar that if the Philippine fault zone was indeed a strike, degree of complexity, and senseof dis- feature representingregional horizontal shear- placement as comparedwith the fault farther ing, then similar features might be observedin south. The alignment is not perfect, however, Taiwan or other adjacent circum-Pacificareas. and possiblereasons for this slight kink in the fault trace are discussedlater in the paper. PHILIPPINE FAULT gONE Along the great southwest-facingescarpment Locatio.nand physiographicexpressio,t. Sev- extendingnorthwest from Dingalan Bay, physi- eral investigators have pointed out that the ographic features of Recent displacementare Philippine fault zone encompassesa wide region nearly as spectacularas on Leyte, althoughboth of interlacingand branchingfractures, so that its Willis [1937, p. 20] and van Bemmelen [1949, exact locationis hard to define.This is probably vol. 1A, p. 34] denied the existenceof faulting true, but certainly the evidenceof relatively along most of this line. Recent fault scarpsare recent displacementsis confinedin many areas particularly evident along the base of the range to a single clearly defined zone, and it is this northeast of Umingan and on the divide between most recent or active break that is primarily Dingalan Bay and the Central Valley (Figure 6). shown in Figures I and 2. Evidence for the As was pointedout by Irving [1951, p. 79], how- fault's location is based mainly on physio- ever, this fault separates into a number of graphic features of Quaternary displacement branches that trend north into the Cordillera that are clearlyvisible on aerial photographsand Central, so that probablyno singlebreak extends in the field. These features include fault and fault- completelyacross Luzon into LingayenGulf, at line scarps,fault troughs and valleys, side-hill least at the presenttime. In Mindanao, a num- ridges,and numerousfault sagsand sag ponds. ber of southward-divergingbreaks have been In general, detailed features of active faulting mapped by Ranne/t et al. [1960], but t.hemost are almostas well displayedalong the Philippine recent and continuousappearing break is that fault as alongthe SanAndreas fault in California, which follows the upper Agusan River and despitethe extreme differencesin climate. crosses the mountains to the south coast at Marl. Probably the best-displayedsegment of the In most of the Agusan River Valley itself, the Philippine fault zone is in northwesternLeyte, surficialrocks are so youngthat it is not surpris- 4798 CLARENCER. ALLEN

Fig. 1. Map of northernPhilippines, showing place names mentioned in the text andtraces of active breaks of Philippine fault zone. CIRCUM-PACIFIC FAULTING 4799

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Fig. 2. Map of southernPhilippines, showing place names mentioned in the text and traces of active breaks of Philippine fault zone. 4800 CLARENCE R. ALLEN CIRCUM-PACIFIC FAULTING 4801

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Fig. 5. Aerial view of features of Recent displacement,Philipp•e fault zone, northern Leyte. ing that the fault zonethere lacksobvious physi- throughoutthe Philippines,but this is difficult ographicexpression. to provein detail under tropicalvegetative con- Current activity of the fault is demonstrated ditions and without a better knowledgeof the in areasin which the fault clearly breaksRecent age o.f the underlying rocks. stream gravels. Prominent examples are: (1) Further evidenceof Quaternary displacements The fan of the Agno River in northern Luzon is given by closeddepressions along the fault, is cut by a distinct scarp that passesthrough which certainly cannot survive long in a region the northern part of the town of San Manuel which not only has an annual rainfall exceeding and decreasesin height as it approachesthe 3 meters alongmuch of the fault trace but can present courseof the river. (2) The fan o.f the also claim someof the world's greatest short-pe- Digrnala River is clearly broken for at least 5 riod rainfalls associatedwith frequent typhoons. km along the fault that passes2 km northeast Probably the largest of these depressionsis of Bongabon,Luzon. The southwestside of this occupiedby Lake Mainit in northeasternMin- fault is raised about I meter. Along this same danao.Lake Danao, 12 km northeastof Ormoc, branch of the fault, 40 km farther northwest, Leyte, is clearly fault controlled,as are several Recent gravels of the Digdig River are broken other dry closeddepressions nearby. Lake Da- 0.3 km east of Digdig. (3) On the island of nao's location at the acute junction of two Mindanao, Recent flood-plain deposits of the closelydivergent branchesof the fault (Figure Agusan River are broken for many kilometers 2, inset) appearsto be a goodexample of the along the fault that passes0.5 km west of Tala- mechanismof grabenformation by transcurrent cogon,as waspointed out to.the authorby F. C. movementproposed by Lensen [1958]. A sag Gervasio. Rannel•t et al. [1960] show many pond borderedby fault scarpsis illustrated by branches of the fault in eastern Mindanao break- the small pond 3.7 km southeast of Bitulok, ing both Quaternary and Plio-Pleistocenerocks. Luzon,near the crestof the fault saddlebetween Indeed, judgingfrom the fault's generalphysio- Dingalan Bay and the Central Valley. In addi- graphic expression, Recent deposits are un- tion, a multitude of ponds and swampy areas doubtedly broken along most of its length along the fault in rice-paddyareas of Leyte, 4802 CLARENCE R. ALLEN

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Fig. 6. Aerial view of Recentscarps of Philippinefault zoneeast from near Bitulok, Luzon, towardI)ingalan Bay (background).Photograph taken from the help of W. R. Merrill.

Masbate,and Luzonprobably are natural closed during the fault's history has been primarily depressions,but theseare hard to verify because horizontal. of possiblecultural modifications. On Leyte, the (1) The extremelinearity, or smoothnessof fault cuts across the flank of the volcanic cone curvature,of the fault trace over hundredsof of Mount Janagdan (Figure 2, inset), which, kilometers is difficult to reconcilewith anything althoughnot listedas active,must be of Quater- but horizontaldisplacement. Similar linearity is nary age in view of the preservedcrater lake at characteristicof other great transcurrentfaults its summit. In central Leyte, the fault trace is (Figure 9) but is unknownon faults with pre- not as clear as it is farther northwest. This is dominantly vertical displacement.To the au- undoubtedlyrelated to the abundanceof Quater- thor, the linearity of the Philippinefault zoneis nary volcanicrocks in the area. Irving [1951] in itself a diagnosticcriterion of dominanthori- has discussedthe submarine morphology along zontal displacement. the Philippinefault, and his map showselongate (2) Along most of the trace of the Philippine closeddepressions along the extendedtrace of fault, neitherside is consistentlyhigher than the the fault between Alabat Island and Dingalan other. The principal exceptionis in northern Bay, in Ragay Gulf, betweenTicao and Mas- Luzon, where the mountains are consistently bate,between Leyte and northeasternMindanao, high northeast of the fault. But in this area, and south of Mindanao. individualRecent scarps at the baseof the range Senseo• dispIaceme•t. The known juxtapo- exhibit varying and opposingvertical displace- sition of grosslydifferent rock types alongparts ments,and the author suspectsthat even here of the Philippine fault zone suggeststhat the the total horizontal component of movement displacementhas been appreciable,and the ex- has far exceededthe vertical. More typical is istenceof fault scarpsin areasof low relief indi- the situation in central Leyte and southern cates that at least some of the componentof Mindanao,where the fault cuts obliquelyacross displacementhas been vertical. But severallines complete mountain ranges without apparent of evidencesuggest that the over-all displacement grosseffect, in a manner similar to that of the CIRCUM-PACIFIC FAULTING 4803 San Andreas and Alpine faults in mountainous drainages nearly at right-angles rather than areas of southern California and New Zealand. follow a stream valley. This last condition is (3) Laboratory and theoretical studies of seldomrealized, inasmuch as erosionof the pul- rock failure, in addition to a multitude of field verized rocks of the fault zone causes most observations,indicate that faults with predomi- streams to follow the fault rather than to cross nantly vertical displacementtend to have dips it. Only where the fault locally climbs out of a significantly less than 90ø . Yet everything that canyon bottom, such as in crossingfrom one is known about the Philippine fault suggeststhat drainagesystem to another, doesit crossnumer- it is essentiallyvertical. The best evidence for ous closelyspaced tributaries; suchis the situa- this is the linear trace of the fault in areas of tion in each of the three areas of Figure 7. Fur- high relief. For example, southeast from the thermore, unless the senseof displacementis crossingof the Bao River in northernLeyte, the such that the offsetsare in the uphill direction, fault trace climbs more than 400 meters in less they might just as well be attributed to prefer- than 3 km with little, if any, changeof trend. ential erosio,nand stream piracy as to actual (4) Individual parallel Recent scarps within fault movement. Thus the three examplesof the fault zone often have opposingsenses of ver- Figure 7 are from areas in which the sinistral tical displacementand in many places tend to offset along the fault has displaced tributary 'scissor,'i.e., to reverse direction of throw at streamsin a direction oppositeto that of the somepoint along the trace. gradient of the main stream; under these cir- Granting the predominanceof horizontal dis- cumstances,the consistentoffsets shown could placement,the senseof displacement--dextralor not reasonablyhave been developedby normal sinistral--is a much more elusive question.The erostonalprocesses and must have been caused author cannot accept the argument of King and by horizontal fault displacements. McKee [1949, p. 1834] and othersthat 'the re- The examplesof Figure 7 representthree of gional plan of the tectonic features' indicates the most diagrammatic localities of stream off- sinistral displacement.The present distribution sets that were found in an examination of aerial of land and water, as well as that of the various photographs,but they are by no meansunique. mountainranges, appears to be far more a func- Other areas of consistent sinistral offsets include tion of vertical than of horizontal movements. (1) the zone of Recent faulting along the base Nor, as was pointed out in the intro.duction,has of the range northeast of Umingan and Lupao, the regional plan suggestedthe same senseof Luzon, (2) the segment of the fault between displacementto all investigators.And the con- Visares and the Banalahang River, northern clusionsbased on the singleset of slickensideson Leyte (Figure 2, inset), which includesthe very Masbate [Willis, 1937] obviouslyneed further sharp 350-meter offsetof the Bao River marked verification. by several spectacular steam-producing hot Stream offsetsat numerouslocalities along the springs,and (3) the segmentof the fault at the fault on Luzon, Masbate, and Leyte (Figure 7) east base of the central range of Leyte, 14 km powerfully and unequivocallydemonstrate that northwest of Abuyog. No areas were found in the senseof Recent displacementis indeedsinis- which one might make a reasonableargument tral. The fact that stream offsets survive at all for dextral offsets. in a regionwhere erostonalprocesses are as rapid Insofar as is presently known, no geologic as in the Philippines is strong evidenceof the units are offset along the fault in such a way as vigorous and continuing activity of the fault. to indicate the total amount of horizontal dis- Consistent and convincing stream offsets have placement. In addition to the lack of detailed beenobserved on all segmentsof the fault except geologicmaps, much of the difficulty lies in the on Mindanao, where most of the fault trace is fact that large segmentsof the fault are under obscuredby swampsand flood-plaindeposits of water, and even the parts that are exposedare the AgusanRiver. largely within very young rocks. But judging Development and preservationof stream off- from the horizontal displacementsof hundreds sets demandsa unique environment: there must of kilometersthat have beenproposed for other be suffcient relief so that streams are moder- faults of similarlength and regionalimportance, ately entrenched, and the fault must cross the possibilityof very large total offset on the 4804 CLARENCE R. ALLEN

{ km ..__. L. •_/

B Gah,t H,ver, }viaspae / - '•-'• ' • ' • , •.km , / 10035'

....•.125øO0 '

N

C East Branch Layog River, \Leyte... __ Fig. 7. Mapsof sinistralstream offsets along Philippine fault.

Philippinefault shouldbe kept in mind during thoughthe tectonicstrain release of one such further geologicmapping in this region. greatshock may be greaterthan the combined Seismicity. If the Philippinefault zoneis in- effect of smaller shocksover a wide region dur- deed active and perhapsthe dominanttectonic ing the precedinghundred years. Thus the truly feature of the archipelago,should one not expect important tectonic-seismicevents may not ap- to find earthquakeepicenters aligned along its pear on an epicentermap that necessarilycov- trace? A glanceat the epicentermaps of the ers only a very li•nited time range. Philippines [Gutenberg and Richter, 1954; As was stated in the introduction,no historic Mi•oza et al., 1960, p. 62] is disappointing; earthquakesalong the Philippinefault areknown epicentersdepict the generaltrend of the archi- to be unequivocallyassociated with actual sur- pelagobut certainlycannot be construedto de- ficial fault displacement.But several historic lineate the Philippinefault zone within it. In shockswere probably related to surfacefaulting, fact, mostof the largestearthquakes of the re- and if the historic record were more complete gion during this period (1907-1959) occurred the Philippinefault zonemight well appear more elsewherein the Philippines.One shouldnote, active than the better-known San Andreas and on the other hand, that the diffusespread of Alpinefaults. For example,the greatearthquake epicentersalso typifies California and New Zea- of August16, 1869,was destructivethroughout land,where active 'master faults' are well-docu- much of the central Philippines and was ap- mented features. Such major faults as the San parently centerednear Masbate.Judging from Andreas are evidently characterizedby great reportsof changesin groundlevel near Ticao earthquakesat only infrequent intervals, al- and extensive 'cracks' in southern Masbatc CIRCUM-PACIFIC FAULTING 4805

[MasS, 1895], this shock was probably associ- rect relationto Quaternaryvolcanism; about as ated with slippagealong the fault's central seg- manyvolcanoes occur on oneside of the fault ment, particularly inasmuch as the one area as on the other, and no Quaternaryvolcanoes from which extensive cracks were reported is lie squarelywithin the fault zone, except pos- the only area where the active break crossesdry sibly on Leyte. This appearsto be in contrast land in this region. Descriptions of the great to othergreat active transcurrent faults of the Surigao,Mindanao, earthquakeof July 1, 1879 circum-Pacificregion (Figure 9), where(1) the [Centeno y Garcia, 1882], mention extensive fault zonelies parallel to andbetween the trench fissuringand changesof ground level between and the line of active volcanoes(the Atacama Anaoan and Lake Mainit ('Lago Sapongan'), fault of northern Chile [St. Amand and Allen, which is the exact line of the most recent ap- 1960] and possiblyin northernNew Zealand), pearing break on modern aerial photographs. (2) the fault itselfis markedby a seriesof ac- One of the greatest earthquakesin Philippine tive volcaniccones (probably the situationin history is that of June 21, 1893, in eastern partsof Sumatra,southern Chile [St. Areand, Mindanao, which was clearly associatedwith 1961,p. 30], andpossibly the FossaMagna of widespreadsubsidences in the swampsof the Japan),or (3) thereis little regionalvolcanism upper AgusanRiver and with possibleextensive at all (alongthe SanAndreas and southernAl- faulting near the present town of Monkayo pine faults). [MasS, 1910]. And the largest instrumentally The slightkink in the Philippinefault be- recorded earthquake of the Philippines region tweenI)ingalan ]Bay and Alabat Islandin Luzon (M -- 8.3) occurredon April 14, 1924,along the is interestingbecause this is the only departure projectedtrace of the Philippine fault south of from the otherwisevery smoothcurve of the Mindanao, at 6•øN, 126•øE. At Mati, where fault trace. If one visualizes the northeast coast the most recent break of the fault goes out to of Luzon to be fault controlled,as is suggested sea, large-scalesubsidence and possiblefaulting by the deepoffshore trough [Irving, 1951]and were reported at that time [MasS, 1924; Melen- by themorphology of thecoastline in thevicinity dres and Comsti, 1951, p. 42]. of the San IldefonsoPeninsula, then it is at the Tectonic environment. The presencein the junction with this northeast-trendingsystem Philippines region of a great oceanic trench, that the bendin the Philippinefault takes place. abundant active volcanism, numerous serpen- And if the northeast-trendingsystem is dextraI, tinitic intrusions,and earthquakesof shallowto for which there is no evidence except its pos- deepfocus indicate that this regionis truly part sibleconjugate relation to the northwest-trend- of the circum-Pacificorogenic belt, although the ing sinistralsystem, the bendin the Philippine diverse tectonic trends of the archipelago and fault is in the correct sense--in analogy to the the lack of alignment of volcanoesindicate that bend in the San Andreas fault at its junction the region is hardly typical. The gentle curved with the opposingGarlock fault. Indeed, even tracesof the Philippine fault and the Mindanao the anglesbetween the fault systemsare similar trench illustrate the arcuate structure, convex if one is viewed as a rotated mirror image of the toward the Pacific, and the parallelismof these other. Thus, perhaps the best evidencefor two features is remarkable. The axes of the dextral displacementon the northeast-trending trench and the fault are scarcely150 km apart systemis the kink in the Philippinefault, al- for long distanceseast of Mindanao, and one thoughthe absenceof seismicitysuggests that can hardly escapethe conclusionthat, if sinistral the northeast system--like the Garlock--may displacementis dominanton the Philippinefault, be currently inactive. the Mindanao trench must share the same stress FACLTING IN TAIWAN system and likewise exhibit a large component of sinistral transcurrent displacement.This con- Northern Luzon is far closerto Taiwan (For- clusion is supported by the fault-plane solu- mosa) than it is to the southernislands of the tions [Ritserna and Veldkamp, 1960], but fun- Philippines,yet the tectonicrelationship between damental questions are left unanswered as to Luzon and Taiwan is not at all clear. Wilson the origin of this and other trenches. [1954, p. 184] consideredthe volcanicislands The Philippine fault zone has no obviousdi- linking the two areasto be representativeof a 4806 CLARENCE R. ALLEN typical circum-Pacific arc between the Ryukyu Range demand a fault contact along the axis of arc to the north and the main Philippine arc to the Longitudinal Valley [Hsu, 1956], although the south (Figure 1, inset); Taiwan itself could the contact itself is everywhere concealedby thus be visualizedas a cap range or secondary alluvium. The much greater elevationswest of arc at the acute junction of the Luzon and the valley than to the east suggesta significant Ryukyu arcs. Biq [1960] and Ho [1961] con- vertical component of displacement,and most sideredTaiwan to be an integral part of a pri- authors have consideredthe faults underlying mary doublearc that is convextoward the Asian the Longitudinal Valley to be predominantly continentrather than toward the Pacific,with a thrusts [Hsu, 1956, p. 59; Juan, 1958, p. 286; correspondingreversal in the order of the as- Biq, 1960, p. 209], although Biq arguesfor an sociated tectonic features. Juan [1956, 1958] additional component of sinistral displacement argued that Taiwan is a coastal range of the along the shallow thrust planes. Asiatic mainland rather than an island arc of That the LongitudinalValley is an active fault the type representedby either the Ryukyus or feature is indicated by historic great earth- the Philippines.The presentstudy addslittle to quakes,by consistentlyhigh seismicity,and by this particular problem but does indicate that fault scarpsthat cut Quaternary alluvium of the major transcurrentfaulting appears to typify valley floor. Scarps within the valley are not Taiwan as well as other active circum-Pacific nearly as abundant as one might expect from orogenicareas. the high seismicity,because active alluvial fans Central Taiwan is underlainby a surprisingly of rivers draining the Central Mountain Range high and ruggedlongitudinal mountain range literally inundate most of the adjacent tectonic whose crest lies somewhat east of the island's trough. Annual rainfall in higher parts of the center line. The eastern coastline of the island is range exceeds5 meters, and the steepnessand consequentlyextremely precipitousin places, height of the mountain front compare favor- such as along the famous coastal cliffs between ably with the great eastern escarpmentof the Suao and Hunlien? But the central part of the Sierra Nevada in California. As a result, despite east coastis borderedby a coastalrange sepa- the otherwisesubtropical environment, the im- rated from the main backboneof the islandby mense, barren alluvial fans filling the Longi- a conspicuouslongitudinal valley betweenHun- tudinal Valley appear to be far more active lien and Taitung that is alignedwith the coastal than most typical desert fans. Along the Longi- cliffsto the north and south(Figure 8). Indeed, tudinal Valley, therefore, it is only at drainage the consistenttrend of the valley is so striking divides or otherwise protected points that one that it is commonlycalled simply the Longi- can hope to find Quaternary fault scarpspre- tudinal Valley, althoughmany other terms have served. Among the few such localities,pointed beenused in the geologicliterature. This valley, out to the author in the field by T. L. Hsu and 150 km long and 5 to 10 km wide, is remarkable C. S. Wang, are (1) the prominentscarp trend- for its extraordinarylinearity, its negligiblere- ing north from Hunlien to the sea, associated lief as comparedto the mountainson eitherside, with sag ponds along its base near the Hunlien and the very odddrainage pattern in whichonly airport and (2) a distinct linear scarp at least one of the major rivers draining the central 15 km in length that passesalong the east shore range manages to break through the eastern of the fault-pondedTa Po Chih near Chihshang. wall of the Longitudinal Valley which lies Scarps of at least two ages are present along athwart the normal drainage paths. One could the edge of the valley east of Fuyuan, rela- hardly hope to find a more diagrammatic ex- tively protectedhere becauseof their proximity ampleof a structurallycontrolled valley, and its to the drainage divide. All the above-mentioned fault originhas been recognized for many years scarpstrend parallel to the LongitudinalValley, [Omori, 1908,p. 164]. In additionto the gross but they are so scattered and so dissectedthat physiography,the very differentrock types of little information about the over-all sense of the Coastal Range and the Central Mountain displacementcan be gained.In no areas do in- cisedstreams appear to crossthe breaksin such 2 All place names used in this part of the text a way as to indicate or preclude lateral dis- are indicated on Figure 8. placements. CIRCUM-PACIFIC FAULTING 4807

i I 120 ø 121• 122•

Huollen

(Ta iko)

ß Taichung

Huolien Korenko)

17 March 1906 Hsiamei (Baishiko) • © Chla-i (Kagi)

•tung Taito)

50 I00 km , i E/eva/ions /n meters

200,500,Contours at 2000m

Fig. 8. Map of Taiwan (Formosa),showing faulting duringhistoric earthquakes. Japanese names in parentheses. 4808 CLARENCE I{. ALLEN The most powerful line of evidenceconcern- by Richter [1958], and the associatedbreaks ing the senseof displacementalong the Longi- are shownin Figure 8. As opposedto the 1951 tudinal Valley comesfrom the major earthquakes earthquakes,both of the earlier shocksare on of 1951, whosegeological effects have been de- the west side of the island,and the parts of the scribedby Hsu [1955, 1962]. The largestshock breaks associatedwith dominant strike slip of this series(M -- 7.3) occurredon November trend N 60-70 E. Thus they might well be vis- 25 and was associated with well-documented ualized as fractures conjugate to the 'master faulting centerednear Yuli. Displacementtook fault' representedby the LongitudinalValley. place along the fault trace for about 40 km The lineartry and continuity of the Longi- from near Shutshutto near Fuli, within and tudinal Valley fault zone emphasizethe appar- parallel to the LongitudinalValley. Strike slip ent truncation of tectonic elements of the was consistentlysinistral, with a maximum dis- Ryukyu arc by Taiwan. The Ryukyu trench placementof 163 cm near Shutshut,and vertical and its southwestwardextension (Figure 1, in- displacementwas intermittent, with the east set) would, if prolonged, butt into the east block relatively raised as much as 130 cm in coast of Taiwan almost oppositethe midpoint some localities [Hsu, 1955]. Most of the fault of the LongitudinalValley. Likewise,the very trace has been obliteratedsince 1951, but three smooth geanticlinal (nonvolcanic) axis of the offset schoolbuildings in Yuli that were neatly Ryukyu are [Hess, 1948] would intersect Tai- transeetedby the fault remain standingin testi- wan at the north endof the valleynear Hualien, mony to the sinistraldisplacement at that time. yet there is little evidenceof an impending An earlier shock of the same series on October major changein trend of the LongitudinalVal- 22 had been centered farther north along the ley fault system.It is difficultto escapethe con- Longitudinal Valley and was associatedwith clusionthat this fault systemand possibleparal- displacementfor about 7 km alongthe baseof lel offshore equivalents represent an actual the older scarp,north of ttualien. Sinistraldis- truncation, and not merely sharp bending,of placementwas 2 meters,and the eastblock was tectonicelements of the Ryukyu arc. relatively raised 1 meter. Thus, from the avail- CONCLUSIONS able evidence, the current activity along the LongitudinalValley appearsto be dominantly The results of this study tend to reinforce transcurrent,as is indeedsuggested and perhaps earlier observations made in northern Chile demandedby the extremelineartry of the valley. [St. Amand and Allen, 1960] which indicated The author seesno reason to supposethat the that great transcurrentfaults parallel to the controllingfaults are thrusts,but insteadvisual- oceanicmargins are typical features of active izes them as essentiallyvertical fractures similar circum-Pacificarcs and are not limitedto unique to thoseof other great transcurrentfaults where or unusual areas. The best documented trans- the attitudes of the fault planes can be more current faults have generallybeen consideredto satisfactorilydetermined. Hsu [1956] points out be the San Andreas fault of California and the that the nearby longitudinalfaults of the Coastal Alpine fault of southernNew Zealand,yet both Range are nearly all east-dippingthrusts, but of these areas are admittedly atypical of the the associationof thrust faults with major verti- circum-Pacificrim in that they lack intermediate cal transcurrent faults has been well documented and deepearthquakes, abundant volcanism, and in many areas of the world. The senseof dis- oceanictrenches. But the Philippinesand, par- placementalong the LongitudinalValley at the ticularly, northern Chile are so truly circum- presenttime is clearlysinistral, which obviously Pacific in their structure that little doubt re- correspondsto the regionaldisplacement along •nains that major transcurrent faults must now the Philippine fault zone farther south. be includedamong the typical tectonicfeatures More widely known than the 1951 shocksare of the circum-Pacific'ring of fire.' This point of two earlier earthquakes in Taiwan that both view was emphasizedby the great Alaskan demonstrated significant componentsof trans- earthquakeof 1958 [Tocher, 1960], which was current displacementof dextral sense.Geologic associatedwith almostpure strike slip in a re- effectsof these shocks,in 1906 and 1935, have gion where most geologistsprobably would not been summarized from the Japaneseliterature have predictedthis type of displacement.Fur- CIRCUM-PACIFIC FAULTING 4809

fault

San Andreas fault

5OO IOOOi Kms 1 ...... Trench

ß • ill tacoma

/ ,•ø'"'

/

Fig. 9. Maps at samescale of Philippine,San Andreas,Atacama, and Alpine faults. 4810 CLARENCE R. ALLEN

thermore,the much-debatedresults of numerous Bonin-Mariana-Palau arc system, following the fault-plane solutionswhich indicate that three- AndesireLine [Hess, 1948], rather than along quarters of the world's earthquakesare caused the Ryukyu-Taiwan-Philippinesystem, and thus by dominantly transcurrent displacements sinistral displacementin the latter area doesnot [Hodgson,1957; Scheidegger,1959] begin to necessarilydestroy the rotation hypothesis.On look more and more geologicallyreasonable. the other hand, there are a variety of reasons The Philippinefault zoneis physiographically for arguingagainst the rotation hypothesis,the and structurally fully as spectacularas the bet- most cogentbeing the failure of the hypothesis ter-knownSan Andreasand Alpine faults, and to account for the convexity of the arcs or the certainly it deservesa place amongthese as one acute cuspsbetween adjacent arcs, as has been of the great regional tectonic features of the pointed out by Benloft [1962] in a recent re- earth (Figure 9). Its current activity is demon- evaluation of the rigid rotation hypothesis.The strated by abundant evidence of Recent and pattern of individual currents underlying the probably historicdisplacements, and consistent circum-Pacificrim appears to be far more com- offset streams indicate that Quaternary dis- plicated than a single rotational cell. For exam- placementshave been predominantlytranscur- ple, demonstratedsinistra• displacementin the rent and sinistral.In the opinionof the author, Philippinesand Taiwan area gives strong sup- the linearity--or very smoothcurvature--of the port to the proposalof Vening Meinesz [1954] fault trace in itself demandsa history of hori- that the Indonesian archipelagois being pro- zontal displacement,particularly if the total dis- truded southeastward toward Australia with placementhas been at all large.The total length accompanyingsinistral shearing along the Phil- of the Philippine fault zone within the archi- ippine flank and dextral shearing along the pelagois more than 1200km, and its remarkable southern flank. This theory is supported by parallelism and proximity to the Mindanao observationsof transcurrent faulting in Sumatra trench suggesta closecausal relationship be- [Reid, 1913; Durham, 1940]. Similar complex- tweenthe two features.If the PhilippineIslands ities probably exist elsewhere (in Japan, for were slightlymore submerged,the trace of the instance), where no clear-cut picture of syste- fault would be largely concealedbeneath the matic transcurrent displacementshas as yet sea, and the author suspectsthat similar evolved. throughgoingfractures of transcurrentcharacter Acknowledgments. This study was carried out probablyunderlie many of the lesswell exposed with funds from the first Grove Karl Gilbert Award island-arcareas. Likewise, the 150-kmlength of in Seismic Geology from the bequest of the late fault exposedin the LongitudinalValley of Tai- Harry O. Wood. The field work was possible only wan presumablyrepresents only a short seg- with the generous cooperation of the Geological Survey of T•iw•n, T•iw•n Natio•nal University, ment of its total length; everything that is Chinese Petroleum Corporation, University of the knownabout this fault indicatesthat it, too, is Philippines, Philippine Bureau of Mines, and nu- a master fault of regionaltectonic importance. merous oil companies in the Philippines. I am Regarding the question of possiblecounter- especially indebted to field colleaguesC. S. Wang, T. L. I-Isu, C. Y. Meng, E. A. Gamus, J. Pilac, and clockwiserotation of the Pacificbasin [Benion, E. V. Tamesis. 1959; St. Amand, 1959], it seemsclear that the sinistraldisplacement that prevailsin the Philip- t•EFERENCES pines-Taiwanregion is not in harmony with the Aki, K., Further study of the mechanism of circum- dextral displacementsthat have been demon- Pacific earthquakes from Rayleigh waves, J. strated in Alaska, California, Chile, and New Geophys.Research, 65, 4165-4172, 1960. Zealand.This point of view has previouslybeen Alcaraz, A. P., The major structural lines of the Philippines, Philippine Geologist, 1(2), 13-17, expressedby Biq [1958], and the principal con- 1947. tribution of the presentstudy is not so much to Alvir, A.D., A theory on the major tectonic struc- refute the rotation hypothesisas to put forth ture of Luzon, Philippine Islands, Proc. Pan concretegeologic evidence for the senseof dis- Pacific Sci. Congr., 3rd Congr., 1, 451-454, 1926. placement on major faults of the Philippines- Alvir, A.D., The Philippine fault zone and its re- lation to Philippine structure, Philippine J. Sci., Taiwan region. Indeed, one might argue that 74, 411-421, 1941. the true edge of the rotating 'disk' is along the Benloft, I-I., Circum-Pacific tectonics, Publs. Do- CIRCUM-PACIFIC FAULTING 4811

minion Observatory Ottawa, 20(2), 395-402, Lensen, G. J., A method of graben and horst for- 1959. mation, J. Geol., 66, 579-587, 1958. Benloft, It., Movements on major transcurrent Livingston, C. W., Mechanics of vein formation in faults, chapter 4 in Continental Drift, edited by the northern half of the Baguio district, Eng. S. K. Runcorn, Academic Press, London, 1962. Mining J., 1•0(9), 38-42, and 140(10), 49-51, Biq Chingchang, Is the Pacific basin rotating 1939. counter-clockwise? (In Chinese, English sum- Mas6, M. S., La seismologia en Filipinas, Tipo- mary), Trans. ChineseAssoc. Advance. Sci., 2(1), grafla de Ramirez y Cia., Manila (Observatorio 10-11 and 16-19, 1958. de Manila), 1895. Biq Chingchang, Circumpacific tectonicsin Taiwan, Mas6, M. S., The seismiccenters of Samar, Leyte, Rept. Intern. Geol. Congr., 21st Congr., 18, 203- and eastern Mindanao, Philippine Weather Bur., 214, 1960. Monthly Bull. Aug. 1910, 279-291, 1910. Centeno y Garcia, J., Informe sobre los temblores Mas6, M. S., The Mati earthquake, April 14, 1924, de tierra ocurridos en el mes de Julio de 1879, Philippine Weather Bur., Seismol. Bull. 1924, en el distrito de Surigao, isla de Mindanao, Bol. 1-2, 1924. Comisidn del Mapa Geoldgico de EspaSa, 9, M elendres, M. M., Jr., and F. Comsti, Reconais- 215-221, 1882. sance geology of southeastern Davao, Philippine Durham, J. W., Oeloe Aer fault zone, Sumatra, Geo•logist,5(2), 38-46, 1951. Bull. Am. Assoc.Petrol. Geologists,24, 359-362, Mifioza, W. A., A. T. Ocampo, Jr., and S. Bello- 1940. sillo, Jr., Significant Philippine earthquakes, Gutenberg, B., and C. F. Richter, Seismicity o• 1949-1959, Philippine Weather Bur. Sci. Paper the Earth, 2nd ed., Princeton University Press, 101, 1960. Princeton, N.J., 1954. Noble, L. F., The San Andreas fault zone from Hess, It. It., Major structural features of the west- Soledad Pass to Cajon Pass, California, chapter ern north Pacific, an interpretation of It.O. 5485, 4, pp. 37-48, in Geology o• Southern California, Bathymetric Chart, Korea to New Guinea, Bull. edited by R. It. Jahns, California Div. Mines Geol. Soc. Am., 59, 417-446, 1948. Bull. 170, 1954. Ito, C. S., Geologic relationships and comparison Omori, F., On the Bokusekikaku and Basshisho between Taiwan and the Philippines, Proc. Geol. (Formosa) earthquake of January 11, 1908, Bull. Soc. China, no. 4, 3-31, 1961. Imp. Earthquake Investigation Comm. (Japan), Itodgson, J. It., Nature of faulting in large earth- 2, 156-165, 1908. quakes, Bull. Geol. Soc. Am., 68, 611-643, 1957. Ranneft, T. S. M., R. M. Hopkins, Jr., A. J. Froelich, Itsu, T. L., The earthquakes of Taiwan (in Chi- and J. W. Gwinn, Reconnaissancegeology and nese), Quart. J. Bank o• Taiwan, 7(2), 148-164, oil possibilities of Mindanao, Bull. Am. Assoc. 1955. Petrol. 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E., Statistical analysis of recent Irving, E. M., and J. S. Teves, The Iloilo earth- fault-plane solutions of earthquakes, Bull. Seis- quake of January 25, 1948, Panay Island, Philip- mol. Soc. Am., 49, 337-347, 1959. pine Islands, Philippine Geologist, 2(2), 6-17, St. Amand, P., Circum-Pacific orogeny,Publs. Do- 1948. minion Observatory Ottawa, 20(2), 403-411, Juan, V. C., Physiography and geology of Taiwan, 1959. Proc. Pacific Sci. Congr., Pacific Sci. Assoc.,8th St. Amand, P., Observacionese interpretaciSn de Congr., 2, 281-312, 1956. los terremotos Chilenos de 1960, Univ. Chile, Juan, V. C., Continental rifting and igneousa. ctivi- ComunicacionesEscuela Geologia, 1(2), 1961. ties in the Neogene marginal geosynclinesof Tai- St. Amand, P., and C. R. Allen, Strike-slip faulting wan, Proc. Geol. Soc. China, no. 1, 27-35, 1958. in northern Chile (abstract), Bull. Geol. Soc. King, P. B., and E. M. McKee, Terrain diagrams Am., 71, 1965, 1960. of the Philippine Islands, Bull. Geol. Soc. Am., Tocher, D., The Alaska earthquake of July 10, 60, 1829-1836, 1949. 1958: Movement on the Fairweather fault and Leith, A., The geology of the Baguio gold district, field investigation of southern epicentral region, Philippine (Commonwealth) Dept. Agr. and Bull. Seismol. Soc. Am., 50, 267-292, 1960. Commerce, Tech. Bull. 9, 1938. Van Berninclen, R. W., The Geology o/ Indonesia, 4812 CLARENCE R. ALLEN Government Printing O•ce, The Hague, 1949. Wilson, J. T., The developmentand structureof Vening Meinesz, F. A., Indonesianarchipelago: a the crust, chapter 4 in The Earth as a Planet, geophysicalstudy, Bull. Geol. Soc Am., 65, 143- edited by G. P. Kuiper, University of Chicago 164, 1954. Press, Chicago, Ill., 1954. Von Drasche,R., Fragmentezu einer geologieder Wisser, E., Deformation in the Baguio gold dis- Insel Luzon (Philippihen), Gerold's Sohn, Vi- trict, Philippine Islands (abstract), Bull. Geol. enna, 1878. Soc. Am., 50, 1943-1944, 1939. 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