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 PhilippinesTaiwan region, where there has been neither convincing documentation nor general agreement
on the importance of transcurrent (strike-slip) faulting or the possiblesenseof regional horizontal 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. Predominanceof horizontal over vertical displacementsis indicated by linearity of the fault trace,
failure of one side to be consistentlyhigher than the other, disregardfor grossphysiography,
and scissoringof individual scarps within the zone. Consistent stream offsets on Luzon,
Masbate, and Leyte demonstrateunequivocallythat the senseof Recent displacementhas
been uniformly sinistral (left-handed). The Philippine fault has no obviousgeologicrelationshipto activevolcanoes,
but the parallelismand proximityof the fault to the Mindanaotrench
suggesta closecausalrelationship.The remarkableLongitudinalValley of eastern Taiwan
represents
anothergreattranscurrent
fault parallelto the westernPacificrim, and grounddisplacementsduringhistoricearthquakesindicatea sinistralsenseof 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 predominance of transcurrentfaulting in active circum-Pacificorogenicareas. These results reinforce
earlier field studiesin Alaska,California,Chile, and New Zealand,as well as emphasizingthe
geologicalreasonabilityof the resultsof seismicfault-planesolutionsindicatingthe world-wide
predominance of transcurrent movements.
placements
in thissameregion.In bothhypotheses,the assumedsenseof displacement
played
The purposeof this study was to investigate
a
part
in
the
interpretation
of
the
general
some of the major faults of the PhilippinesTI-IE [PROBLEM
Taiwan region with the hope of unequivocally
demonstratingtheir extent, their degreeof current activity, and--particularly--their sense
of displacement.Unusual geophysicalsignificanceis attached to this region becausewidely
divergentviewsof circum-Pacificorogenyfocus
here, especiallyconcerningthe questionof the
relative importance of horizontal fault displacements.Even granting the importanceof
horizontal movements, major disagreement
has existed concerning the sense of possible displacement: theories of Benio# [1959]
and St. Amand [1959] implied the probable
existenceof dextral or right-handedfaults paralleling the westernPacificmargin in this region,
whereastheories of Vening Meinesz [1954] and
Biq [1960] supposedsinistralor left-handeddis-
• Contribution 1099, Division of Geological Sciences, California Institute of Technology.
mechanismof circum-Pacificorogeny. Van Bemmelen [1949] and othershave tendedto discount
the importanceof horizontalfault displacements
in this region altogether.
Willis [1937] was apparentlythe first investigator to recognizethe existenceof a throughgoingfault zonein the Philippinescomparableto
the San Andreas fault in California, although
numerousworkers dating back to von Drasche
[1878] had pointed out the existenceof various tectonic lineaments in the archipelago,
mainly on the basisof the allegedalignmentof
islands, volcanoes,and earthquake epicenters.
Repetti [1935], in particular, had proposedthe
'Master fault' along the general trend of the
Philippine fault zone, but his evidence other
than that
from northern
Luzon was based al-
most wholly on inconclusiveseismicdata. The
term 'Philippine fault zone' used by Willis
[1937] seemsto have becomewell establishedin
the literature, notwithstandingthe later use by
4795
4796
CLARENCE
Willis [1944] of 'Visayan rift' for the same
feature.
Von Drasche[1878, p. 3] and othershad particularly pointed out the bifurcation or virgation of tectonic trends in the southern Philippines,the southwesterly
trend of the islandsin
the southwestern
Philippinescontrastingsharply
with the southeasterlytrend of the islandsand
mountain chainsalong the southeasternrim of
the archipelago.The island of Masbate,x with
its two diverging'arms,' has often been cited
as an exampleof the acute junction of the two
opposingtrends.Willis wassoimpressed
by the
linearity of the southeasterlytrending features
between Ragay Gulf and northern Mindanao
that he proposedthe existenceof the Philippine
fault zone along this line; his evidencewas
mittedly basedalmost wholly on grossphysiographicforms.Other investigatorswho followed
Willis have generallyagreedupon the presence
of a fault zone, althoughthere have been disagreementsconcerning
its possibleextensioninto
northern Luzon and its exact location in the cen-
tral part of the islands.Evidencefor its existence
has continuedto be basedmainly on grossphysiography, inasmuchas there is no clear documentation of displacementsalong its trace during historicearthquakes.Only on the island of
Mindanao do publishedgeologicmaps clearly
demonstrate
and delineate the various breaks
that make up the Philippine fault zone [Ranne/t
et al., 1960]. Probably the most accurate maps
of the fault's locationnorth of Mindanao, and
the most cogentargumentsfor its existence,are
those by Irving [1951, 1953], basedin par• on
an analysisof the submarinemorphologyof the
region.
Willis apparently consideredthe Philippine
fault to be dominantlytranscurrent (i.e., 'strikeslip,' 'lateral,' or 'wrench') becauseof its similarity to the SanAndreasfault, particularly in its
length, linearity, and generalphysiographicexpression.But the senseof horizontal displacement has been the subject of more nebulous
argument.Willis arguedthat the fault was sinistraI, and the mostoften-quotedevidenceis based
on a single set of horizontal asymmetric slickensidesthat were observedby Willis [1937, p.
39] in the Aroroy mining district on Masbate,
although this particular fault is subsidiaryto
• All Philippine place names used in the text
are indicated on Figures 1 and 2.
R. ALLEN
the main zoneand is apparently at an angle of
at least 30ø to it. Willis further arguedthat the
regionalplan of the islandssuggested
a sinistral
senseof displacement.
Other authorshaveagreed
[Alvir, 1941;King and McKee, 1949].But opinion has not been unanimous; Irving [1951, p.
70] thoughtthat many of the offsetsimplied by
suchargumentswere'excessive,'
andAlvir [1926,
p. 453] had used the same kind of argument
(basedon grossphysiographicfeatures)to indicate dextral displacementalong the fault in
northern
Luzon that is herein considered to be
an integral part of the Philippinefault zone.
Another line of evidencebearing on the sense
of fault displacement
is that givenby fracture
patterns in Philippine mining areas, although
none of the areas described in detail has been
within the fault zoneproper. Leith [1938], Livingston [1939], and Wisser [1939] pointedout
that the pervasiveshearand tensionjoints of
the Baguio mining district demand east-west
compression,
at least in Miocenetime, as does
the generaltrend of the foldedmountainranges
north of Manila. Wisser [1952] cited supporting
evidencefrom local fracture patterns in southeastern Luzon and northeastern Mindanao. Such
east-westcompression
is, of course,compatible
with sinistraI transcurrent displacementon the
Philippinefault zonebut by no meansprovesit.
Nor has there been completeagreementon the
regionalstresspattern; the numerousnorthsouth lineamentsof the central Philippineswere
visualizedby Alcaraz [1947] as tensionalin
origin, which impliesa regionalstresssystem
that would lead to dextral displacementalong
the Philippinefault zone.Ranne/t et al. [1960]
indicated sinistral displacementson individual
breaks of the zone in northeastern Mindanao but
did not state why.
It mightbe hopedthat resultsof seismic
faultplanesolutions
on the numerous
earthquakes
of
the Philippinesregionwould clear up the ambiguity resulting from the limited geologic
studies.In a summary of 35 fault-plane solutions from this region,Ritsema and Veldkamp
[1960] statethat the predominantdisplacements
have indeedbeen transcurrent and sinistral,and
the implied directionof horizontalcompression
is about N 60øE. None of the larger earthquakes
of this group, however, occurredwithin the
Philippine fault zone, and the largest shock
(M -- 8.2, Jan. 24, 1948) was clearly dextral.
CIRCUM-PACIFIC
The epicenterof thisgreatearthquake
wasnear
the west coast of Panay, where there is good
evidence of a major northeast-trending fault
that might reasonablybe consideredconjugate
to the Philippine fault and thus dextral. The
fault-planesolution,however,whichis supported
by the trend of the isoseismaIs[Irving and
Teves, 1948], indicatedfailure on a fault striking more nearly parallel to the Philippinefault
than to the local structure. Furthermore, Aki's
[1960] source-functionsolutionsbased on Rayleigh waves continue to indicate dextral displacements around the entire circum-Pacific
margin. The one Philippine earthquakeincluded
in Aki's solutionsindicatesbreaking on a fault
sub-parallelto the Philippine fault in the northern Central Valley of Luzon, and it is clearly
FAULTING
4797
between the towns of Visares and Leyte. The
zonehere consistsof a number of parallel active
breaks over a width of about 500 meters, and
the extreme linearity of the topographywithin
the fault zone stands in marked contrast to the
topographyon both sides(Figures3-5). Neither
side is consistentlyhigher than the other, and
the centralpart of the zoneis, in general,a strucrurally and topographicallydepressedtrough
that has causedthe term 'rift' to be applied to
this fault in analogy with the similar features
and nomenclature of the San Andreas fault. In-
deed,the generalphysiography
is bestdescribed
in the same words used by Noble [1954, p. 37]
in portraying the San Andreas: 'It has a curiouslydirect courseacrossmountainsand plains,
with little regard for grossphysiographicfeadextral.
tures,yet it influencesprofoundlythe localtopoThus, in brief, it was not clear at the begin- graphic and geologicfeatureswithin it.'
Both north and south of the central Philipning of this study that the Philippinefault zone
(1) indeed existed, at least as a throughgoing pinesarea, the fault separatesinto a numberof
feature of regionaltectonicimportance,(2) was branches. The fault that enters northern Luzon
active, and (3) was necessarilya transcurrent at Dingalan Bay is herein consideredto be the
and sinistralfeature. Furthermore, it was hoped main Philippinefault on the basisof its similar
that if the Philippine fault zone was indeed a strike, degreeof complexity,and senseof disfeature representingregional horizontal shear- placementas comparedwith the fault farther
ing, then similar featuresmight be observedin south. The alignment is not perfect, however,
Taiwan or other adjacent circum-Pacificareas. and possiblereasonsfor 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- extendingnorthwestfrom Dingalan Bay, physieral investigatorshave pointed out that the ographic features of Recent displacementare
Philippine fault zoneencompasses
a wide region nearly as spectacularas on Leyte, althoughboth
of interlacingand branchingfractures,sothat 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 alongthe baseof 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 DingalanBay and the Central Valley (Figure 6).
shown in Figures I and 2. Evidence for the As waspointedout by Irving [1951, p. 79], howfault'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 clearlyvisibleon aerial photographsand Central, sothat probablyno singlebreak extends
in the field. These features include fault and faultcompletelyacrossLuzon into LingayenGulf, at
line scarps,fault troughs and valleys, side-hill least at the presenttime. In Mindanao, a numridges,and numerousfault sagsand sag ponds. ber of southward-divergingbreaks have been
In general, detailed features of active faulting mappedby Ranne/t et al. [1960], but t.hemost
are almostas well displayedalongthe Philippine recent and continuousappearing break is that
fault asalongthe SanAndreasfault in California, which follows the upper Agusan River and
crosses the mountains to the south coast at Marl.
despitethe extreme differencesin climate.
Probably the best-displayedsegmentof the In most of the AgusanRiver Valley itself, the
Philippine fault zone is in northwesternLeyte, surficialrocksare so youngthat it is not surpris-
4798
CLARENCER. ALLEN
Fig. 1. Map of northernPhilippines,
showing
placenamesmentioned
in the text andtracesof
active breaks of Philippine fault zone.
CIRCUM-PACIFIC
FAULTING
4799
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500
Active
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Fig. 2. Map of southernPhilippines,showingplace namesmentioned 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 lacksobviousphysiographicexpression.
Current activity of the fault is demonstrated
in areasin which the fault clearly breaksRecent
stream gravels. Prominent examples are: (1)
The fan of the Agno River in northern Luzon
is cut by a distinct scarp that passesthrough
the northern part of the town of San Manuel
and decreasesin height as it approachesthe
present courseof the river. (2) The fan o.f the
Digrnala River is clearly broken for at least 5
km along the fault that passes2 km northeast
of Bongabon,Luzon. The southwestside of this
fault is raised about I meter. Along this same
branch of the fault, 40 km farther northwest,
Recent gravels of the Digdig River are broken
0.3 km east of Digdig. (3) On the island of
Mindanao, Recent flood-plain deposits of the
Agusan River are broken for many kilometers
along the fault that passes0.5 km west of Tala-
throughoutthe Philippines,but this is difficult
to provein detail undertropicalvegetativeconditions and without a better knowledgeof the
age o.f the underlying rocks.
Further evidenceof Quaternary displacements
is given by closeddepressions
along the fault,
which certainly cannotsurvivelong in a region
which not only has an annual rainfall exceeding
3 metersalongmuch of the fault trace but can
also claim someof the world's greatestshort-period rainfalls associatedwith frequent typhoons.
Probably the largest of these depressionsis
occupiedby Lake Mainit in northeasternMindanao.Lake Danao, 12 km northeastof Ormoc,
Leyte, is clearly fault controlled,as are several
other dry closeddepressions
nearby. Lake Danao's location at the acute junction of two
closelydivergent branchesof the fault (Figure
2, inset) appearsto be a goodexampleof the
mechanismof grabenformationby transcurrent
cogon,as waspointedout to.the authorby F. C. movementproposedby 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 breakthe small pond 3.7 km southeastof 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 addigraphic 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,
CLARENCE
4802
R. ALLEN
....
- :.:..•.::
..•.
.
..
..
..
Fig. 6. Aerial view of Recentscarpsof Philippinefault zoneeastfrom near Bitulok, Luzon,
towardI)ingalanBay (background).Photographtakenfrom the helpof W. R. Merrill.
Masbate,and Luzonprobablyare natural closed during the fault's history has been primarily
depressions,
but theseare hardto verify because horizontal.
(1) The extremelinearity, or smoothness
of
of possible
culturalmodifications.
On Leyte,the
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 agein 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 prenot as clear as it is farther northwest. This is
dominantly vertical displacement.To the auundoubtedlyrelatedto the abundanceof Quater- thor, the linearity of the Philippinefault zoneis
nary volcanicrocksin the area. Irving [1951] in itself a diagnosticcriterion of dominanthorihas discussedthe submarine morphology along zontal displacement.
(2) Along most of the trace of the Philippine
the Philippinefault, and his map showselongate
higherthan the
closeddepressions
along the extendedtrace of fault, neithersideis consistently
the fault between Alabat Island and Dingalan other. The principal exceptionis in northern
Bay, in Ragay Gulf, betweenTicao and Mas- Luzon, where the mountainsare consistently
bate,betweenLeyte andnortheasternMindanao, high northeast of the fault. But in this area,
and south of Mindanao.
individualRecentscarpsat the baseof the range
Senseo• dispIaceme•t. The known juxtapo- exhibit varying and opposingvertical displacethat even here
sition of grosslydifferentrock types alongparts ments,and the author suspects
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,wherethe fault cutsobliquelyacross
displacement
hasbeenvertical.But severallines complete mountain ranges without apparent
of evidencesuggestthat the over-all displacement grosseffect,in a manner similar to that of the
CIRCUM-PACIFIC
FAULTING
4803
San Andreasand Alpine faults in mountainous drainagesnearly at right-angles rather than
California and New Zealand.
follow a stream valley. This last conditionis
(3) Laboratory and theoretical studies of seldomrealized,inasmuchas erosionof the pulrock 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
significantlyless than 90ø. Yet everything that
canyon bottom, such as in crossingfrom one
is knownabout the Philippinefault suggests
that
drainagesystemto another,doesit crossnumerit is essentiallyvertical. The best evidencefor ouscloselyspacedtributaries; suchis the situathis is the linear trace of the fault in areas of
tion in each of the three areas of Figure 7. Furhigh 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 preferthan 3 km with little, if any, changeof trend. ential erosio,nand stream piracy as to actual
(4) Individual parallel Recent scarpswithin fault movement. Thus the three examplesof
the fault zoneoften have opposingsensesof ver- Figure 7 are from areas in which the sinistral
tical displacementand in many placestend to offset along the fault has displacedtributary
'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 cirGranting the predominanceof horizontal dis- cumstances,
the consistentoffsetsshowncould
placement,the senseof displacement--dextralor not reasonablyhave been developedby normal
sinistral--is a much more elusive question.The
erostonalprocessesand must have been caused
author cannot accept the argumentof King and by horizontal fault displacements.
McKee [1949, p. 1834] and othersthat 'the reThe examplesof Figure 7 representthree of
gional plan of the tectonic features' indicates the most diagrammatic localities of stream offsinistral 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,appearsto 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 pointedout in the intro.duction,has of the range northeast of Umingan and Lupao,
the regionalplan suggestedthe same senseof Luzon, (2) the segment of the fault between
displacementto all investigators.And the con- Visares and the Banalahang River, northern
clusionsbasedon the singleset of slickensides
on 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 numerouslocalitiesalongthe 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 unequivocallydemonstratethat northwest of Abuyog. No areas were found in
the senseof Recent displacementis indeedsinis- which one might make a reasonableargument
areas of southern
tral. The fact that stream offsets survive at all
for dextral
in a regionwhereerostonalprocesses
are as rapid
as in the Philippines is strong evidenceof the
vigorous and continuing activity of the fault.
Consistent and convincing stream offsets have
beenobservedon all segmentsof the fault except
on Mindanao, where most of the fault trace is
obscuredby swampsand flood-plaindepositsof
the AgusanRiver.
Development and preservationof stream offsets demandsa unique environment: there must
Insofar as is presently known, no geologic
units are offsetalong the fault in sucha way as
to indicate
offsets.
the total amount
of horizontal
dis-
placement. In addition to the lack of detailed
geologicmaps, much of the difficulty lies in the
fact that large segmentsof the fault are under
water, and even the parts that are exposedare
largely within very young rocks. But judging
from the horizontal displacementsof hundreds
of kilometersthat have beenproposedfor other
faults of similarlengthand regionalimportance,
ately entrenched, and the fault must cross the possibilityof very large total offset on the
be suffcient
relief so that streams are moder-
4804
CLARENCE R. ALLEN
{
km
..__.
L.
B Gah,t H,ver, }viaspae
'•
'
•_/
/ - '•-'•
•
,
•.km
,
/ 10035'
....•.12
5øO0'
N
C East BranchLayog River, \Leyte...
__
Fig. 7. Mapsof sinistralstreamoffsets
alongPhilippinefault.
Philippinefault shouldbe kept in mind during
further geologicmappingin this region.
Seismicity. If the Philippinefault zoneis indeedactive and perhapsthe dominanttectonic
thoughthe tectonicstrainreleaseof onesuch
greatshockmay be greaterthan the combined
effect of smaller shocksover a wide region dur-
ingthe preceding
hundredyears.Thusthe truly
eventsmay not apfeature of the archipelago,shouldonenot expect important tectonic-seismic
covto find earthquakeepicentersalignedalongits pear on an epicentermap that necessarily
trace? A glanceat the epicentermaps of the ers only a very li•nited time range.
As was stated in the introduction,no historic
Philippines [Gutenberg and Richter, 1954;
alongthe Philippinefaultareknown
Mi•oza et al., 1960, p. 62] is disappointing; earthquakes
with actual surepicenters
depictthe generaltrend of the archi- to be unequivocallyassociated
pelagobut certainlycannotbe construedto de- ficial fault displacement.But severalhistoric
lineate the Philippinefault zone within it. In shockswereprobablyrelatedto 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 zonemightwellappearmore
elsewherein the Philippines.One shouldnote,
on the other hand, that the diffusespreadof
active than the better-known San Andreas and
Alpinefaults.For example,the greatearthquake
of August16, 1869,was destructivethroughout
epicenters
alsotypifiesCaliforniaand New Zealand,whereactive'masterfaults'are well-docu- much of the central Philippines and was apmented features.Such major faults as the San parentlycenterednear Masbate.Judgingfrom
Andreas are evidently characterizedby great reportsof changesin groundlevel near Ticao
earthquakesat only infrequent intervals, al- and extensive 'cracks' in southern Masbatc
CIRCUM-PACIFIC
[MasS, 1895], this shockwas probably associated with slippagealongthe fault's central segment, particularly inasmuch as the one area
from which extensive cracks were reported is
the only area where the active break crossesdry
land in this region. Descriptions of the great
Surigao,Mindanao, earthquakeof July 1, 1879
[Centeno y Garcia, 1882], mention extensive
fissuringand changesof ground level between
Anaoan and Lake Mainit ('Lago Sapongan'),
which is the exact line of the most recent ap-
pearing break on modern aerial photographs.
One of the greatest earthquakesin Philippine
history is that of June 21, 1893, in eastern
Mindanao, which was clearly associatedwith
widespreadsubsidences
in the swampsof the
upper AgusanRiver and with possibleextensive
faulting near the present town of Monkayo
[MasS, 1910]. And the largest instrumentally
recorded earthquake of the Philippines region
(M -- 8.3) occurredon April 14, 1924,alongthe
projectedtrace of the Philippine fault south of
Mindanao, at 6•øN, 126•øE. At Mati, where
FAULTING
4805
rect relationto Quaternaryvolcanism;aboutas
manyvolcanoes
occuron onesideof the fault
as on the other, and no Quaternaryvolcanoes
lie squarelywithin the fault zone,except possibly on Leyte. This appearsto be in contrast
to othergreatactivetranscurrent
faultsof the
circum-Pacific
region(Figure9), where(1) the
fault zoneliesparallelto andbetweenthe trench
and the line of active volcanoes(the Atacama
fault of northern Chile [St. Amand and Allen,
1960] and possiblyin northernNew Zealand),
(2) the fault itselfis markedby a seriesof active volcaniccones(probablythe situationin
partsof Sumatra,southernChile [St. Areand,
1961,p. 30], andpossibly
the FossaMagnaof
Japan),or (3) thereis little regional
volcanism
at all (alongthe SanAndreasandsouthern
Alpine faults).
The slightkink in the Philippinefault betweenI)ingalan]BayandAlabatIslandin Luzon
is interestingbecausethis is the only departure
from the otherwisevery smoothcurve of the
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
were reportedat that time [MasS, 1924; Melen-
by the deepoffshore
trough[Irving, 1951]and
by themorphology
of thecoastline
in thevicinity
dres and Comsti, 1951, p. 42].
Tectonic environment. The presencein the
Philippines region of a great oceanic trench,
abundant active volcanism, numerous serpentinitic intrusions,and earthquakesof shallowto
deepfocusindicatethat this regionis truly part
of the circum-Pacificorogenicbelt, although the
diverse tectonic trends of the archipelagoand
the lack of alignment of volcanoesindicate that
the region is hardly typical. The gentle curved
tracesof the Philippine fault and the Mindanao
trench illustrate the arcuate structure, convex
toward the Pacific, and the parallelismof these
of the San IldefonsoPeninsula,then it is at the
two features
is remarkable.
The
axes of the
trench and the fault are scarcely150 km apart
for long distanceseast of Mindanao, and one
can hardly escapethe conclusionthat, if sinistral
displacement
is dominanton the Philippinefault,
the Mindanao
trench must share the same stress
system and likewise exhibit a large component
of sinistral transcurrent displacement.This conclusion is supported by the fault-plane solutions [Ritserna and Veldkamp, 1960], but fundamental questionsare left unansweredas to
the origin of this and other trenches.
The Philippine fault zone has no obviousdi-
junction with this northeast-trending
system
that the bendin the Philippinefault takesplace.
And if the northeast-trending
systemis dextraI,
for which there is no evidence except its pos-
sibleconjugaterelationto the northwest-trending sinistralsystem,the bendin the Philippine
fault is in the correct sense--in analogy to the
bend in the San Andreas fault at its junction
with the opposingGarlockfault. Indeed,even
the anglesbetweenthe fault systems
are similar
if one is viewed as a rotated mirror image of the
other. Thus, perhapsthe best evidencefor
dextral displacement
on the northeast-trending
systemis the kink in the Philippinefault, althoughthe absenceof seismicitysuggests
that
the northeast system--like the Garlock--may
be currently inactive.
FACLTING IN TAIWAN
Northern Luzon is far closerto Taiwan (For-
mosa) than it is to the southernislandsof the
Philippines,
yet the tectonicrelationship
between
Luzon and Taiwan is not at all clear. Wilson
[1954, p. 184] considered
the volcanicislands
linking the two areasto be representative
of a
4806
CLARENCE
typical circum-Pacific arc between the Ryukyu
arc to the north and the main Philippine arc to
the south (Figure 1, inset); Taiwan itself could
thus be visualizedas a cap range or secondary
arc at the acute junction of the Luzon and
Ryukyu arcs. Biq [1960] and Ho [1961] con-
sideredTaiwan to be an integral part of a primary doublearc that is convextoward the Asian
continentrather than toward the Pacific,with a
correspondingreversal in the order of the associatedtectonic features. Juan [1956, 1958]
argued that Taiwan is a coastal range of the
Asiatic mainland
rather
than an island arc of
the type representedby either the Ryukyus or
the Philippines.The presentstudy addslittle to
this particular problem but does indicate that
major transcurrentfaulting appearsto typify
Taiwan
as well as other active circum-Pacific
orogenicareas.
Central Taiwan is underlainby a surprisingly
high and ruggedlongitudinalmountain range
whose crest lies somewhat east of the island's
center line. The eastern coastline of the island is
consequentlyextremely precipitousin places,
such as along the famous coastal cliffs between
Suaoand Hunlien? But the central part of the
east coastis borderedby a coastalrangeseparated from the main backboneof the islandby
a conspicuous
longitudinalvalley betweenHunlien and Taitung that is alignedwith the coastal
cliffsto the north and south(Figure8). Indeed,
the consistenttrend of the valley is so striking
that it is commonlycalled simply the Longitudinal Valley, althoughmany other terms have
beenusedin the geologic
literature.This valley,
150 km long and 5 to 10 km wide, is remarkable
for its extraordinarylinearity, its negligiblerelief as compared
to the mountainson eitherside,
and the very odddrainagepattern in whichonly
one of the major rivers draining the central
range manages to break through the eastern
wall of the Longitudinal Valley which lies
athwart the normal drainage paths. One could
hardly hope to find a more diagrammaticexampleof a structurallycontrolledvalley,and its
fault originhasbeenrecognized
for many years
[Omori, 1908,p. 164]. In additionto the gross
physiography,
the very differentrock types of
the Coastal Range and the Central Mountain
2 All place names used in this part of the text
are indicated on Figure 8.
R. ALLEN
Range demand a fault contact alongthe axis of
the Longitudinal Valley [Hsu, 1956], although
the contact itself is everywhere concealedby
alluvium. The much greater elevationswest of
the valley than to the east suggesta significant
vertical componentof displacement,and most
authors have consideredthe faults underlying
the Longitudinal Valley to be predominantly
thrusts [Hsu, 1956, p. 59; Juan, 1958, p. 286;
Biq, 1960, p. 209], although Biq arguesfor an
additional component of sinistral displacement
along the shallowthrust planes.
That the LongitudinalValley is an active fault
feature is indicated by historic great earthquakes,by consistentlyhigh seismicity,and by
fault scarpsthat cut Quaternaryalluvium of the
valley floor. Scarps within the valley are not
nearly as abundant as one might expect from
the high seismicity,becauseactive alluvial fans
of rivers draining the Central Mountain Range
literally inundate most of the adjacent tectonic
trough. Annual rainfall in higher parts of the
range exceeds5 meters, and the steepnessand
height of the mountain front compare favorably with the great eastern escarpmentof the
Sierra Nevada in California.As a result,despite
the otherwisesubtropicalenvironment,the immense, barren alluvial fans filling the Longitudinal Valley appear to be far more active
than most typical desert fans. Along the Longitudinal Valley, therefore, it is only at drainage
divides or otherwise protected points that one
can hope to find Quaternary fault scarpspreserved. Among the few such localities,pointed
out to the author in the field by T. L. Hsu and
C. S. Wang, are (1) the prominentscarptrending north from Hunlien to the sea, associated
with sag ponds along its base near the Hunlien
airport and (2) a distinct linear scarp at least
15 km in length that passesalongthe east shore
of the fault-pondedTa Po Chih near Chihshang.
Scarps of at least two ages are present along
the edge of the valley east of Fuyuan, relatively protectedhere becauseof their proximity
to the drainage divide. All the above-mentioned
scarpstrend parallel to the LongitudinalValley,
but they are so scattered and so dissectedthat
little
information
about the over-all
sense of
displacementcan be gained.In no areas do incisedstreamsappear to crossthe breaksin such
a way as to indicate or preclude lateral displacements.
CIRCUM-PACIFIC
FAULTING
4807
i
120 ø
I
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
Contou
at
200,500,
2000m
Fig. 8. Map of Taiwan (Formosa),showing
faultingduringhistoricearthquakes.
Japanese
names in parentheses.
4808
CLARENCE
I{. ALLEN
The most powerful line of evidenceconcern- by Richter [1958], and the associatedbreaks
are shownin Figure 8. As opposedto the 1951
earthquakes,both of the earlier shocksare on
the west sideof the island,and the parts of the
breaks associatedwith dominant strike slip
trend N 60-70 E. Thus they might well be vis25 and was associated with well-documented
ualized as fractures conjugate to the 'master
faulting centerednear Yuli. Displacementtook fault' represented
by the LongitudinalValley.
place along the fault trace for about 40 km
The lineartry and continuity of the Longifrom near Shutshutto near Fuli, within and tudinal Valley fault zoneemphasizethe apparparallel 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
extension(Figure 1, inplacementof 163 cm near Shutshut,
andvertical and its southwestward
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 obliteratedsince1951,but three smooth geanticlinal (nonvolcanic) axis of the
offsetschoolbuildingsin Yuli that were neatly Ryukyu are [Hess, 1948] would intersect Taitranseetedby the fault remain standingin testi- wan at the north endof the valleynear Hualien,
monyto 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 LongitudinalVal22 had been centered farther north along the ley fault system.It is difficultto escapethe conLongitudinal Valley and was associatedwith clusionthat this fault systemandpossible
paraldisplacement
for 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
placementwas2 meters,and the eastblockwas tectonicelementsof the Ryukyu arc.
relatively raised 1 meter. Thus, from the availCONCLUSIONS
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 fracturessimilar circum-Pacific
arcsandare not limitedto unique
to thoseof other great transcurrentfaults where or unusual areas. The best documented transthe 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 verticircum-Pacificrim in that they lack intermediate
cal transcurrent faults has been well documented
and deepearthquakes,
abundantvolcanism,and
in many areas of the world. The senseof dis- oceanictrenches.But the Philippinesand, parplacementalongthe LongitudinalValley at the ticularly, northern Chile are so truly circum-
ing the senseof displacement
alongthe LongitudinalValley comesfrom the major earthquakes
of 1951,whosegeologicaleffectshave been describedby Hsu [1955, 1962]. The largestshock
of this series(M -- 7.3) occurredon November
presenttime is clearlysinistral,whichobviously
corresponds
to the regionaldisplacementalong
the Philippine fault zonefarther south.
More widely known than the 1951 shocksare
two earlier earthquakes in Taiwan that both
Pacific in their structure that little doubt re-
•nains that major transcurrent faults must now
be includedamongthe typical tectonicfeatures
of the circum-Pacific
'ring of fire.' This point of
view was emphasizedby the great Alaskan
demonstrated significant componentsof trans- earthquakeof 1958 [Tocher, 1960], which was
current displacementof dextral sense.Geologic associated
with almostpure strike slip in a reeffectsof these shocks,in 1906 and 1935, have gion where most geologists
probably would not
been summarized from the Japaneseliterature have predictedthis type of displacement.Fur-
CIRCUM-PACIFIC
FAULTING
4809
fault
San Andreas
fault
5OO
IOOO
Kms
1
i
..........Trench
ß • ill tacoma
/
,•ø'"'
/
Fig. 9. Maps at samescaleof Philippine,SanAndreas,Atacama,and Alpine faults.
4810
CLARENCE
R. ALLEN
thermore,the much-debatedresultsof numerous Bonin-Mariana-Palau arc system, followingthe
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 sinistraldisplacementin the latter area doesnot
[Hodgson,1957; Scheidegger,1959] begin to necessarilydestroythe rotation hypothesis.On
the other hand, there are a variety of reasons
look more and more geologicallyreasonable.
The Philippinefault zoneis physiographically for arguingagainstthe rotation hypothesis,the
and structurallyfully as spectacularas the bet- most cogentbeing the failure of the hypothesis
ter-knownSan Andreasand Alpine faults, and to accountfor the convexity of the arcs or the
certainly it deservesa place amongtheseas one acute cuspsbetweenadjacent arcs, as has been
of the great regional tectonic features of the pointed out by Benloft [1962] in a recent reearth (Figure 9). Its currentactivity is demon- evaluationof the rigid rotation hypothesis.The
strated by abundant evidence of Recent and pattern of individual currents underlying the
probablyhistoricdisplacements,
and consistent circum-Pacificrim appearsto be far more comoffset streams indicate that Quaternary dis- plicated than a singlerotational cell. For examplacementshave been predominantlytranscur- ple, demonstratedsinistra• displacementin the
rent and sinistral.In the opinionof the author, Philippinesand Taiwan area gives strong supthe linearity--or very smoothcurvature--of the port to the proposalof Vening Meinesz [1954]
fault trace in itself demandsa history of hori- that the Indonesianarchipelagois being prozontal displacement,particularly if the total dis- truded southeastward toward Australia with
placementhasbeenat all large.The total length accompanyingsinistral shearingalong the Philof the Philippine fault zone within the archi- ippine flank and dextral shearing along the
pelagois morethan 1200km, and its remarkable southern flank. This theory is supported by
parallelism and proximity to the Mindanao observationsof transcurrent faulting in Sumatra
trench suggesta closecausalrelationshipbe- [Reid, 1913; Durham, 1940]. Similar complextweenthe 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 systefault would be largely concealedbeneath the matic transcurrent displacementshas as yet
sea, and the author suspectsthat similar
throughgoingfracturesof transcurrentcharacter
probablyunderliemanyof the lesswell exposed
island-arcareas.Likewise,the 150-kmlengthof
fault exposed
in the LongitudinalValley of Taiwan presumablyrepresentsonly a short segment of its total length; everything that is
knownaboutthis fault indicatesthat it, too, is
a master fault of regionaltectonicimportance.
evolved.
Acknowledgments. This study was carried out
with funds from the first Grove Karl
Gilbert
Award
in Seismic Geology from the bequest of the late
Harry O. Wood. The field work was possible only
with the generous cooperation of the Geological
Survey of T•iw•n, T•iw•n Natio•nal University,
ChinesePetroleum Corporation, University of the
Philippines, Philippine Bureau of Mines, and numerous oil companies in the Philippines. I am
especiallyindebted to field colleaguesC. S. Wang,
T. L. I-Isu, C. Y. Meng, E. A. Gamus, J. Pilac, and
Regarding the questionof possiblecounterclockwiserotation of the Pacificbasin [Benion, E. V. Tamesis.
1959; St. Amand, 1959], it seemsclear that the
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FAULTING
4811
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CLARENCE R. ALLEN
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