TECTONICS, VOL. 7, NO. 5, PAGES 975-989,
SEISMOGENIC
STRIKE-SLIP
DEVELOPME•
FAULTING
OF TIlE NORTH
CHINA
OCTOBER1988
AND THE
BASIN
Wang-PingChen
Departmentof Geology,Universityof Illinois,Urbana
John Nfibelek
Collegeof Oceanography,
OregonStateUniversity,Corvallis
Abstract. The activelysubsidingNorth China sedimentary
basinis associated
with an unusuallyhigh level of seismic
activity. This oil- and gas-producing
basinhasbeenthe siteof
nine large(M > 7), destructiveearthquakes
since1600 A.D.
An analysisof faultingduringtheTangshanearthquake
sequence,
whichincludessomeof thelargestshocksthathave
occurredin thisbasinduringthepast400 years,showedthat
the dominantpatternof deformationduringthissequence
was
associated
with displacement
onright-stepping,
right-lateral
strike-slipfaults. A large amountof subsidence
(~1.0-1.5 m)
occurredin pull-apartregionsbetweenstepsof the northnortheast(NNE) trendingstrike-slipfaults. The directivityof
theP wave radiationpatternandthe distributionof aftershocks
of theBohaiGulf earthquakeof July 18, 1969,alsoindicate
strike-slipfaultingon planesof similarNNE trend. Together
with thefourlargestshocksof theHsingtaisequence
in 1966,
earthquakes
with right-lateralslipon NNE trendingfaults
account for about two-thirds of the seismic moment released in
thisregionfor thepast100 years. The averageregionalstrain
dueto seismicslip hasa dominantcomponentof dextral
simpleshear(on NNE strikingplanes)thatis approximately
twiceas large as north-southextensionor east-west
shortening.Basedon a combination
of publishedfield and
boreholedata,the dominanceof horizontaldextralsimpleshear
over subsidence
seemsto havebegunno laterthanthe midPliocene and the total amount of extension over the entire
basinis only about20-30%. This patternof deformation,the
spatiallyvariableheatflow values,thescattered
locationsof
rapiddeposition
of Quatemarysediments,
andthefluctuating
rate of sedimentation
in spaceand time duringtheNeogenein
thisbasinare difficult to explainby pureextensionand
thermalsubsidence.We proposethattheseobservations,
and
the shapeof the wholeNorth Chinabasin(thatof a "lazy Z"),
area consequence
of pulling-apartassociated
with fight-lateral
movementon theNNE trendingstrike-slipfault systemsand
that the basin as a whole has formed and evolved as a
compositepull-apartbasindueto fight-lateralslipon
numerousfight-stepping
faultssincetheEocene.
INTRODUCTION
TheNorthChinabasinisa large(~2x105
km2)intracratonic
sedimentarybasin(Figure 1) producingoil and gas[e.g.,Li,
1981]. The basementof the basinhasa long geologichistory,
but the mostrecentphaseof majortectonicactivitystartedin
the Eocene[Lee, 1986;Li, 1982]. The post-Eocene
historyof
subsidence
in thebasinhasbeenqualitativelyexplainedby a
two-stageevolutionarymodelthathasbeencommonlyapplied
to intracontinentalbasins [McKenzie, 1978]: A basin is
formedby a brief initial periodof fault controlledrifting,
followedby long-termgradualthermalsubsidence.
According
to this model,the thermalsubsidence
stagein theNorth China
basinbeganin theNeogeneandis continuingtodaylye et al.,
1985]. A recentanalysisby Hellingeret al. [1985], however,
demonstrates
thatthe historyof subsidence
in thisbasin
cannotbe modeledby eithera one-layermodelof crustal
extension,or a two-layermodelof continentalstretching
[HellingerandSclater,1983]. Thereforealtemativetectonic
models must be examined.
Copyright1988
by theAmericanGeophysical
Union.
Papernumber8T0341.
0278-7407/88/008T-0341 $10.00
Unlike manyotheroil-producingbasins,theNorth China
basinhasan extremelyhigh level of seismicactivity. Nine
earthquakes
with magnitudes
(M) greaterthanor equalto 7
haveoccurredwithin thebasinsince1600 A.D. (Figure 1).
Many of theselargeshockswereaccompanied
by foreshockaftershockactivity,containingeventsas largeasM > 6
976
ChenandNfibelek: Strike-SlipFaultingandNorth ChinaBasin
114ø
116ø
118ø
120 ø
I
I
I
I
122 ø
42_ø
40 ø
38 ø
ß:...........
•
/> 2.0 HFU
..';:"•
•.o > • •>
ß'....
i.5>
ß
I.O•
ß
36 ø
•> I.o
Fig.1. Map of theNorth Chinabasinsummarizing
recentandhistoricalseismicity,heatflow, structure,
and
sedimentation
sincethe Quaternary.The presentboundaryof thebasinis shownby thick solidlines,knownfaults
by thinnersolidlines(dashedwhenlesscertain),andcoastline
by dottedcurves.Depocenters
of Quaternary
sediments
areshownasdepressions
with isopachs
(in 100 m) marked.Solidcirclesareepicentralregionsof large
(M > 7) earthquakes
andearthquakesequences
since1600 A.D. (modifiedfrom Figures10 and 12 of Ye et al.
[1985]). Shadedcirclesare epicentersof earthquakes
with 6 < M < 7 since1964. The locationof a thrustevent
on November6, 1983(Mo-2.4 x 1017Nm; Dziewonskiet al. [1984])is alsoplotted.The "beachball" symbols
showlowerhemisphere,
equalareaprojections
of thecentroidalnodalplanesolutionsof earthquakes
(Table 1) with
thecompressional
quadrants
darkened.Sitesof measured
surfaceheatflow (takenfromYe et al. [1985])aremarked:
squares
for > 2.0 HFU (HFU= gcal/cm2s
= 42 mW/m2),diamonds
for 1.5-2.0HFU, hexagons
for 1.0-1.5HFU,
andtriangles
for < 1.0HFU. Noticethathighvalues(> 100mW/m2)occurnextto low values(-50 mW/m2) and
thatthedepocenters
of Quaternarysediments
arenotinterconnected.
(Table 1). Sincethe installationof theWorld-WideStandard
Seismograph
Network(WWSSN)in 1964,thefaultplane
solutionsandaftershockdistributionfor thelargestearthquakes
in thisregionindicateright-lateralstrike-slipfaultingon
north-northeast
(NNE) trendingplanes(Figure1). The only
largeeventwith normalfaultingmechanism
is thelargest
aftershockof theTangshansequence
(July28, 1976). Thus
faultinghascontinued
in thisbasinlongafterthepresumed
initial stageof pre-Neogene
crustalthinning.More
importantly,
thepatternof present-day
faultingreflected
by
earthquake
dataseemsto showa predominance
of strike-slip
local distances,field and macroseismicobservations,
and
geodeticdata. Our resultsshowthatright-lateralslipon faults
trendingapproximately
NNE is thedominantmodeof
deformation
duringthelargestearthquake
sequences.
Consequently,
the samepatternis observedin theregional
straindueto seismicity.We alsodiscuss
otherobservations
in
theNorth Chinabasinwhichcannotbe explainedeasilyby
purecrustal-stretching
modelsof basinformation,andwe
developan alternativeinterpretation
thatth6basinasa whole
canbe viewedas a largecompositepull-apartsystem.
motion over subsidence.
DEFORMATION
In thisstudywe presentresultsof ananalysisof teleseismic
bodywavesfor theBohaiGulf eventof 1969,andwe review
faultingassociated
withtheTangshan
earthquake
sequence
of
1976,aswell as thatof two othermajoreventswhichoccurred
in thisbasinsince1966. We furthersynthesize
theresults
EARTHQUAKE SEQUENCES
from seismic observations recorded at both teleseismic and
ASSOCIATED
WITH
MAJOR
Tangsban
Earthquake
Sequence
On a scaleof 10-100 km, earthquake
faultingandits
relationshipto local subsidence
of theNorthChinabasinis
ChenandNfibelek:
Strike-Slip
Faulting
andNorthChinaBasin
977
TABLE1. A Summary
ofRecent
andHistorical
(since1600)Seismicity
of theNoahChinaBasin
Latitude,
Longitude,
OriginTime, Msor mbor
Number
Date
1
Sept.2,
2
3
June 12,
June 13,
1679
1830
1888
Aug. 1,
1937
øN
øE
40.0
36.4
38.5
117.0
114.2
119.0
35.3
115.4
(35.2)
(115.3)
39.7
118.7
(40.1)
(118.8)
hr:min:s
M
Mo,
Focal Strike,
Dip,Rake,
M1References
Nm Depth,km
1600-1900 A.D., M _>7
8.0
7.5
7.5
degrees References
1
1
1
Heze
4
5
Sept.23, 1945
10:41:05
15:34:23
2
4
Tangshan
6.3
2
4
6
7
8
9
10
March
March
March
March
March
1966
1966
1966
1966
1966
37.35
37.50
37.53
37.68
37.47
114.92
115.08
115.05
115.27
114.88
21:29:14
08:11:36
08:19:46
15:19:04
06:11:59
11
March 27, 1967
38.56
116.61
08:58:20
7,
22,
22,
26,
29,
7.03
(10:41:03)
Hsingtai
6.8
6.7
7.2
6.2
6.
Hejian
5.6
5.6
5.9
5.2
6.3
5
5
5
5
2
1.0x1019
3.0x1018
1.8x1019
1.4x1018
11
8
9
9
3
208,89,182
208,82,182
26,73,191
30,74,203
5
5
5
5
(202,80,177)
6
Bohai
12
July 18,
1969
38.43
119.47
05:24:49
7.42
3
4.0x1019
2.1x1019
Subevent 1
Subevent 2
6
9
209,87,200
356,80,183
This
Study
Haicheng
20
Feb.4, 1975 40.65 122.80 11:36:088 7.3
May18, 1978 40.71 122.61 12:33:311ø6.0
14
July27, 1976
13
39.63
118.18
6.48
Tangshah
7.83 6.14
Subevent1
Subevent2
19:42:56
3
9
3.0x1019 10
7
288,78,342 8
(106,84,-20) 10
11
3.5x1019
5.8x1019
2.7x1019
Subevent3
12
7
15
185,78,183
225,80,157
(95,41,111)
7
7
7
15
July28, 1976 39.67 118.57 10:45:37 7.13 6.14 12
3.0x1019 8
16
Nov.15, 1976 39.44
2.6x1018 18
322,67,350 7
17
Aug.31, 1976 39.91 118.90 03:25:01 5.3
5.2
4
1.6x1017 6
(253,67,217)7
18
March7, 1977 40.10
5.2
4
1.2x1017 7
(220,65,221) 7
19
May12, 1977 39.28 117.84 11:17:54 5.94 5.64 13
21
Oct. 19, 1982
22
Nov.6,
39.95
117.83
13:53:01 6.84 6.04 13
117.74 00:28:48
119.01
5.0
20:45:59
5.3
3.8x1017 16
(260,50,261) 7
311,69,2
7
14
Heze
1983 35.21 115.17 21:09:51
15
2.4x1017(21)
(142,44,71) 15
Eventnumbers
arein chronological
order.Orientation
ofonlyoneofthenodalplanes
isgiveninparentheses
forthose
events
whose
truefaultplanes
areunknown.
All events
whose
seismic
moments
areknown
havebeenincluded
inexpression
(5).
References:
1,Seismological
Committee
oftheAcademia
Sinica
[1956];
2,Leeetal.[1978];
3,Molnar
andDeng[1984];
4,
International
Seismological
Summary
orBulletin
oftheInternational
Seismological
Center;5,Chung
andCipar[1983];
6,
Ta•ponniar
andMolnar
[1977];
7,Nfibelek
etal.[1987];
8,Cipar
[1979];
9,Xiang
etal.[1980];
10,Shedlock
etal.[1985];11,
State
Seismological
Bureau
[1982];
12,Zhang
etal.[1980];
13,Shedlock
etal.[1987];
14,Peng
etal.[1985];
15,Dziewonski
etal.
[1984].
bestillustrated
by deformation
associated
withtheTangshan
earthquake
sequence
of 1976.Nfibelek
et al. [1987]have
performed
a detailed
analysis
ofP andSHwaves
generated
by
thesixstrongest
shocks
of thissequence.
Theyalsogavea
detailed discussionof severalother seismicstudiesof this
sequence
[e.g.,Butleret al., 1979;KikuchiandKanamori,
1986]. In thissectionwe shallbrieflysummarize
the
deformationof the entiresequence.
The main shockwas associated
with right-lateralslip on a
nearlyverticalfatfittrending
approximately
N30øEovera
lengthof about100km (theTangshan
fault,segments
A and
B in Figure2). Thedetailed
historyof rupture
of themain
shockis complex,
including
a change
in thestrikeof thefault
fromnorth-northeast
to northeast
(Figure2) andthrustfaulting
whichtookplacesouthof theepicenter
duringthelastphase
of therupture[N,Sbelek
et al., 1987].Nevertheless,
the
97 8
ChenandNfibelek: Strike-SlipFaultingandNorthChinaBasin
TANGSHAN
SEQUENCE
118.0'
118.5'
119.0'
I
7'7/3/7
40.0*
M2
e•e
MI
ß ß
76/8/31
28
39.5*
M3
Fig. 2. A detailedmap of faultingduringthe Tangsbanearthquakesequence(takenfrom Nfibeleket al. [ 1987]).
Mappedgeologicalfaults(solidlines)in the areaareplottedtogetherwith the epicentersof 200 aftershocks(circles)
locatedby Shedlocket al. [1987]. For simplicity,not all knownfaultsareplotted. Heavy dashedlinesare our
interpretation
of the grossshapeof fault segments
thatwereactivatedduringtheTangshansequence(Table 1). The
epicentersof the six largestevents(stars)[StateSeismologicalBureau,1982;Shedlocket al., 1987] andthe large
eventin 1945reportedby theInternationalSeismological
Summary(1945) (shadedsquare)are shown. The location
of thelasteventbasedon macroseismic
damandreportedin thecatalogue
of Lee et al. [1978]is shownby a solid
square.Also plottedare the fault planesolutionsof the threesubevents
of themain shock(M1, M2, andM3) and
thefive largestaftershocks
determined
by Nfibeleket al. [ 1987]. Faultplanesolutions
for the numberedeventsin
the northeastern
portionof the aftershockzonedeterminedby Shedlocket al. [1987] from first motionsare shownin
the upperleft comerof thefigure. In theinsertthearrowsneareachsegmentindicatetheinferreddirectionof relative
displacement
(dashedwhenlesscertain).Regionsof subsidence
(up to 1.5 m) [Zhanget al., 1981] are observednear
segments
D andE wherethemainright-lateralfault systemstepstowardtheright (pull-apart).The shadedareas
outline the regionswhere subsidenceis > 0.5 m.
predominantmodeof slip alongthenorth-northeast
trending
Tangshanfaultis right-lateralasevidencedfromearthquake
mechanisms
[Nfibeleket al., 1987], geodeticmeasurements
[Zhanget al., 1981], and surfaceruptures[Guoet al., 1977].
Fifteenhoursafterthe main shock,a largeaftershock(the
Luanxianearthquake)occurredto the northeastof the main
shockon a fault markedas segmentD in Figure2. The
mechanismof this shockis tightlyconstrained
as normal
faultingon an east-westtrendingplane [Nfibeleket al., 1987].
This segmentis alsodelineatedby about10 events
(4 g M 1g 5) whichoccurred
betweenJuly1982andMarch
1983 [Penget al., 1985]. A derailedlocationof some200
aftershockswhich occurredas late as December1979 [Shedlock
et al., 1987] placedabouta dozenepicentersto the eastof
segmentD extendingto near 119*E (Figure2). Shedlocket al.
[1987] connected
theseeventswith thosenearthejunctionof
segments
B andD (Figure2), and theypostulateda westnorthwesttrendingstrike-slipsegmentextendingfrom about
118.5øto 119øE.While the existenceof sucha segmentis
subjectto interpretation,we notethat the momentreleaseof
theLuanxianearthquake
is at leasttwo ordersof magnitude
greaterthanthe combinedmomentof all the smallaftershocks
in thatregion. This,togetherwith the analysisof geodetic
levelingdamobtainedbefore(in 1975) and6 monthsafterthe
Tangsbansequence
[Zhanget al., 1981], whichshowedrelative
subsidence
of up to 1.0 m (Figure2), leadsus to interpret
segmentD as a majorpull-apartfeaturewheretheright-lateral
Tangshanfault stepstowardthe right.
Thereis no directevidencefor a largeamountof slipon
segmentG duringtheTangshansequence,
althoughmany
aftershocks
occurredthere. Two largestaftershocks
thereshow
a mixtureof strike-slipandnormalfaulting(Figure2). The
fault plane solutionsof small aftershocksat the southernend
of G alsoshowlargelystrike-slipfaulting(events1-6, Figure
2). The distribution
of aftershocks
is irregularbut an overall
NNE trendcanbe identified(Figure2). At the northernendof
ChenandN,Sbelek:Strike-SlipFaultingandNorth ChinaBasin
G the seismicity
becomesdeeperandis againcharacterized
by
normalfaulting [Shedlocket al., 1987]. It is conceivablethat
the northernendof G marksanotherpull-apartregion(H in
Figure 2).
Characteristics
of pull-apartrelatedto fight-steppingof
right-lateralfaultsare alsoapparentat the southernendof the
Tangshanfault. Two largeaftershocks
occurrednearthe
intersectionof the Tangshanand the Jiyunhefault (Figure2;
segmentE). Both aftershocks
involvedeitherleft-lateralslip
on the conjugatefault E, or fight-lateralslip on F and/orA.
Field observations
suggest
thatleft-lateralsliptookplace
alongfault segmentE [Li andGuo, 1979]. Either way, a
substantial amount of extension must have occurred between
segments
A andF nearE, asis evidentfrom thereleveling
data. The maximumsubsidence
herereached1.5 m [Zhanget
al., 1981], which is of the same order as that observedat
segmentD.
Approximately20 s after the onsetof the main shock,a
subevent
with a largecomponent
of thrustfaultingoccurred
southof the epicenternearsegmentC (Figure2). The
occurrence
of thrustfaultingin an activelysubsiding
basin
appearssurpfisingat first glance. However,we notethatlocal
uplift is known to occurin pull-apartbasinsthat have
developedin association
with majorstrike-slipfaultswherethe
strike-slip(master)fault changesdirectionslightly[e.g.,
Crowell, 1974;Freund,1971]. Alternatively,local
compression
canoccurat left-stepsof subparallel
right-lateral
faults [e.g., Segalland Pollard, 1980; Sharp, 1972]. Both
mechanisms
arepossibleduringtheTangshansequence
[Nfibeleket al., 1987],andtheyareexpressions
of
transpression
in a strike-slipregime[Harland,1971;Sanderson
and Marchini, 1984]. It is alsopossiblethatlocal
compression
canresultfromblockrotationin a systemof
parallelstrike-slipfaults [e.g.,Nur et al., 1986]. In any case,
it is importantto notethat the effectof local compression
is
overwhelmedby thatof pull-apartin theoverallstatic
deformation
duringtheTangshan
sequence
because
releveling
datashowedonly net subsidence
reachinga maximumof over
1 m [Zhanget al., 1981].
Elsewherein thebasin,a moderate-sized
event(November6,
979
fight-lateralstrike-slipmotionon NE to NNE trendingplanes
wasassociated
with two otherlargeearthquake
sequences.
BohaiGulf. The seismic
momentrelease(---6x10
•9Nm) of
the BohaiGulf eventof 1969 is exceededonly by thatof the
main shockof the Tangshansequence.The BohaiGulf event
occurredoffshoreandtheonly availabledataareteleseismic
recordings
andlocationsof epicenters
of thelargeraftershocks
by a regionalseismicnetworkin northeastern
China.
Following the inversionprocedureoutlinedby Nfibelek
[1984], we have determinedthe seismicmoment, the
orientationof the nodalplanes,andthedepthof thisevent
(Figure3 andTable 2) by formallyinvertingthewaveforms
and amplitudesof 16 teleseismic(30* < A < 90*) P waves
recordedby the long-periodinstruments
of the WWSSN. The
recordings
of S waveswentoff-scaleandcouldnotbe analyzed.
Detailsof thedataprocessing
andtheassumptions
on velocity
structure and attenuation are the same as those for the
Tangshansequence[Nfibeleket al., 1987]whosesourcereceiverconfigurationis nearlyidenticalto thatof theBohai
sequence.
For thiseventall the seismograms
havehigh signal-to-noise
ratiosandcovera wide azimuthalrange. The mechanism
of
predominantlystrike-slipfaulting(Figure 3) is well
constrained. The source time function of about 40-s duration
shows two bursts of moment release:
one at the onset of the
signaland the otherabout20 s later (Figure3), indicatingthe
existence of at least two subevents. The two subevents are
sufficientlyseparated
in time thata slightchangein the
orientationof theirnodalplanescanbe resolved(Figure3a).
In orderto differentiate
thefaultplanefromtheauxiliary
nodalplane,we carriedout analyseswith therupture
propagatingin eachof the four possibledirectionsup and
downthe strikesof bothnodalplanesfor the first subevent,
which accounts for two-thirds of the total moment release. We
1983;Mo•-2.4x10
•7Nm) occurred
onthesouthern
edgeof the
basinneartheepicenterof thelarge(M = 7) eventin 1937
(Figure 1 andTable 1). The preliminaryresultof Dziewonski
et al. [1984] showspure thrustfaulting. The P axis of this
eventis consistentwith thoseof the strike-slipevents,
indicatinga regionalcompression
alongeast-northeast
(ENE)
(Figure1). TapponnierandMolnar [1976, 1977]interpreted
the regionalstressfield nearthe North Chinabasinas a result
foundthata propagatingrupturefrom southto northalongthe
NNE trendingnodalplaneis requiredfor the first subeventin
orderto matchtheoveralldistributionof amplitudesandthe
stronginitial swingobservedat stationsto thenortheastof the
epicenter(e.g., COL, LON, andBKS; Figure3a). For
comparison,
Figure3b showsthe nextbestsolutionof a
eastwardpropagating
rupture.Sucha solutionclearlyprovides
a pooreroverallmatchthana northwardpropagating
sourceand
cannotreproducethelargeamplitudesobservedto the northof
theepicenter.Thusthedirectivityof theP wavesstrongly
suggests
thatthe main shockwasassociated
with fight-lateral
slip on theNNE trendingplane. The dataalsoindicatethat the
secondsubeventoccurredapproximately15 km to the
northwestof the firstbut its exactrelativepositioncannotbe
of the continental collision between India and Eurasia.
resolved.
Althoughthe exactcauseof the thrustfaultingeventis not
known,its occurrence
is generallyconsistent
with this
regionalcompression.In contrast,simplecrustalthinning
modelscannoteasilyaccountfor theoccurrence
of thrust
faulting.
The aftershocks
of theBohaiearthquake
formeda band
trendingapproximately
NNE andtheyagainsuggest
thatthe
fault planestrikesin thatdirection(Figure4). The qualityof
the locationof the aftershocks
is probablymarginal.
OtherMajor Earthquakes
On a basin-widescalethenortheast(NE) to NNE trending
fracturesseemto be the dominantstructuraltrendthroughout
thebasin(Figure 1). In additionto the Tangshansequence,
Nonetheless,the trend in the NNE direction is clear and its
length(about50 kin) is at leasttwice the dimensionof its
width. Sincethe overallshapeof the distributionof
aftershocks
of theTangshansequences
determined
by murine
locations[TangshanSeismological
Team, 1981] doesnot
differ significantlyfrom thatof moreelaboratelocations
[Shedlocket al., 1987] in the sameregion,thisNNE trendis
980
ChenandN•belek:Strike-Slip
Faulting
andNorthChina
Basin
BOHAI , July 18, 1969
Mo = 6x1019 N-m
ESK
GDH
COL
KON
NOR
LON
COP
BKS
NORG,DH COL
ESK, • \ N /
KON,, \
,•--:-:• /
COP• "•
\'•, •V'
LON
_
BKS
\/',
IST
FI
RIV
ER
20mm
0
sec
60
0
sec
Source
Time Function
ChenandNlibelek:Strike-SlipFaultingandNorthChinaBasin
9821.
NAI
JER
20mm
,
BOHAI ,
July 18, 1969
Eastward
Propagating Rupture
Fig. 3b. The observedandsyntheticseismograms
generated
for a modelwith an eastwardpropagating
rupture.The
stationsshownare theonesmostsensitiveto theassumeddirectionof rapture. This modelfails to reproducethe
observed
amplitudes
at stations
to thenorthof theepicenter(e.g.,COL, BKS, andNOR), eventhougha slightly
betterfit is foundat RAB to theeastof theepicenter.Modelswithotherdirections
of propagation
(includingno
rupturepropagation)
weretested,buttheydo notmatchtheobservations
in mostazimuths.
unlikelyto be a consequence
of systematic
errorsin locations.
We concludethatthe BohaiGulf eventis primarilyassociated
with right-lateralslip on the nodalplanestrikingNNE.
Hsingtai(Xingtai). The Hsingtaiearthquake
swarmof 1966
producedno surfacebreaks.However,thelocationsof the
largestshocks,as well asthedistribution
of aftershocks
suggest
thattheyoccurredon a sequence
of NNE trending
right-lateral
faultsalongedgesof a graben[e.g.,Chungand
Cipar,1983]. The sumof the scalarseismicmoments
for the
threelargestshocks
of theHsingtaiswarmis about3.2x10•ø
Nm, or approximately
half of themomentof theBohaiGulf
earthquake.
Haicheng. The only otherlargeeventwhoseseismic
momentcanbe determinedis the famousHaichengearthquake
of 1975. Because of numerous foreshocks its occt._•ence was
successfully
predicted.Relocationof aftershocks
definesa
Fig. 3a. (Opposite)Resultof theinversion
of teleseismic
P
wavesfor the BohaiGulf eventof July 18, 1969. The
observed
seismograms
areshownassolidtracesalongwith the
synthetics
(dashed
traces)generated
fromthebestfittingsource
model(Table2) composed
of two pointsources
(subevents).
Verticalbarson theseismograms
indicatethetimewindow
usedin theinversion.Lowerhemisphere,
equalareaprojection
of thenodalplanes(solidcurvesfor thefirstsubevent,
dashed
curvesfor the second)andlocationsof stationsusedare shown
in thecenterof thefigure. Eachsubevent
is consminedto be
a double-couple.
The inferredsource
timefunctionis shownat
thebottomof thefigure. Therupturepropagation
duringthe
first subevent is toward the NNE.
scattered
bandof activitytrendingwest-northwest
(WNW)
[Shedlocket al., 1985], suggesting
that this is the orientation
of thefault plane. Althoughno surfacerupturehasbeenfound
for thisearthquake,somefield informationis availablefrom a
sparsegeodeticnetworksurveyedin 1958andthenagainin
1975 after the main shock [Wu et al., 1976].
This informationis oftenreferredto asevidencefor simple
left-lateralslip on a WNW trendingplane[e.g.,Shedlocket
al., 1985;Wu et al., 1976]. We reexaminedthe datapresented
by Wu et al. [1976] andfoundthemeasurements
difficultto
interpret.Thereare only four trilaterationpointsin the
immediatevicinityof theepicentralarea,onein eachquadrant
of thefault planesolution.The observedshortening
of
0.38 m betweenthe two stationsjust to the northof the
WNW trendingnodalplaneis grosslyinconsistent
with leftlateralslipalongtheplane. On theotherhand,thedatacannot
be explainedby simplefight-lateralslipon the othernodal
planeeither.
It appearsthateitherthedeformation(seismicandaseismic)
in theepicentralareaof thissequence
overthe 17-yearperiodis
quitecomplex,or that thereare internalinconsistencies
in the
trilatemtiondata. Thesepossibilitiesare evidentfrom the large
amountof shortening(0.20 m) reportedfor two stationsjust
to the northof the inferredfault with respectto a station~35
km fartherto the north [Wu et al., 1976, Figure 15].
Cipar [1979] synthesized
theP andSH wavesby usinga
simpletrapezoidsourcetimefunctionwith an averageduration
of about7 s. He furtherinferreda bilateralrupturepropagation
alonga WNW trendingplanebasedon theapparentvariation
in theobserveddurationof the signals.The waveformsof P
wavesin the northeastern
quadrantof thefocalsphere,which
982
ChenandNfibelek:Strike-SlipFaultingandNorthChinaBasin
TABLE 2. SourceParameters
of the BohaiGulf EventDeterminedby Inversionof P Waves
Strike,
degrees
Dip,
degrees
Rake,
degrees
Depth,
km
209-•_6
87+3
2005-_17
6+5
Moment, Time Delay, Distance,
1019
Nm
s
Azimuth,
km
degrees
Subevent 1
4.05-_0.2
Subevent 2
356+19
805-_11
183+8
9-•_8
2.1+0.3
19+1
14+11
59-•_38
1. Distanceandazimutharerelativeto theepicenter,andtimedelayto theorigintime.
2. Realisticestimates
of thetrueuncertainties
for focaldepth,strike,dip, andrakeareshownas+10(• of
theformaluncertainties.Estimatesof the uncertainties
for theotherparameters
are shownas+2 (• of the
formal uncertainties[Nfibelek, 1984].
werenot analyzedby Cipar [1979],arecomplex.Figure5
comparestheP wavefor thiseventrecordedat stationsCOL
andVAL. The signalat VAL appearsto be very simple
[Cipar, 1979], but signalsof significantamplitudeare
observed for the first minute of the P wave train at COL and
otherNorth Americanstations.Thesesignalsappearto be as
complicatedasthosefor theTangshanmainshock[Nfibeleket
al., 1987]. ThesecomplicatedP waveformssuggestthat the
deformationassociated
with theeventis quitecomplex.
Sincethedistributionof aftershocks
suggests
left-lateralglip
on WNW planes[Shedlocket al., 1985], we haveassumedthe
solutiongivenby Cipar [1979] in the followingdiscussion.
Becauseof its relativelysmallseismicmomentthe detailsof
faultingfor this eventalonedo not altertheregionalstrain
significantly.
REGIONAL
DEFORMATION
DUE TO SEISMIC
Deng [1984], whoassumedleft-lateralslipfor theBohaiGulf
event,andconsequently,
theirconclusion
is quitedifferent
from ours.
Assumingthata regionconsistsof discreterigidblocks
boundedby faultsand thatthefaultsare stationarywith respect
toa reference
block,theaverage
regional
deformation
(eij=
8ui/3xj),
including
both(irrotational)
strain
andrigidbody
rotation,dueto earthquakefaultingcanbe estimatedfroma
summation
oftheasymmetric
seismic
moment
tensor
M*ijof
eachearthquake
[Jackson
andMcKenzie,1988;Molnar,1983]'
eij= (•M*ij)/ gV
i, j = 1, 2, 3
(1)
wherela= rigidity,V = volumeof theentireseismogenic
region. The asymmetricmomenttensorof a double-couple
sourceis definedby
SLIP
We estimatedthedeformationdueto seismicslipof all
eventswhoseseismicmomentis greaterthanor equal
COL Z Mag.
1500
to 2.4x10 •7 Nm in the basin since 1946. A similar
calculationhasbeenpreviouslycardedoutby Molnarand
18 ø
119 ø
A
an
12o ø
i
1
1145
121 ø
I
an
VAL Z Mag.
750
39 ø
-
+
0
1148
$:•.o ß
0
•o,
ß MI 40 - 59
O MI 30
39
+
BOHAI
GULF
1969
- 1971
Fig. 4. A mapshowingthe distribution
of large
(5.9 > M1> 3.0) aftershocksup to 1971 of the Bohai Gulf
eventof 1969(redrawnfromFigures6a to 6d ofWei et al.
[ 1984]). Theaftershocks
forma bandtrendingnorth-northeast
andsuggestthatthe fault planestrikesin thatdirection.
FEB. 4 1975
Fig. 5. A comparison
of theverticalcomponent,
long-period
seismogram
of theWWSSN for the 1975Haichengearthquake
recordedat COL, which is located to the northeastof the
epicenter,with that recordedat VAL to the northwest.The P
wavesat COL andotherNorthAmericanstationsarecomplex,
lastingat least60 s. The waveformssuggesta complicated
ruptureprocessfor thisevent.
ChenandNfibelek: Strike-SlipFaultingandNorth ChinaBasin
M*i•= Mobinj
(2)
983
Pos•bve Values of the Elements of the Asymmetric Moment Tensor
n East
wherebi are the componentsof the unit slip vector,ni are the
components
of the unit outwardnormalof thefault plane,and
Mo is the scalarseismicmoment[Molnar, 1983].
An interpretation
of eachelementof theasymmetric
momenttensorin termsof faultingis displayedgraphicallyin
Figure6. Noticethatrigidbodyrotationtoij= (1/2)(%- ej.0.
As pointedout by JacksonandMcKenzie[1988], thisrotation
n North
n Up
/
1
Column 1
Column 2
Column 3
ell
e12
///•'--'Row
1bEast
I
L/
e13
is measured relative to a frame of reference which is attached to
the faults. In the simplecaseof a through-going
fault
separatingtwo blocks(Figure6), if the fault remains
Row 2
b North
Row 3
b Up
stationary
withrespect
tooneof theblocks,thento•jrepresents
rotation between the blocks.
Thereis a setof companionexpressions
for the symmetric
moment
tensor
Mij [Gilbert,1970]:
Mij = Mo(binj+ bin.0
(3)
Theaverage
(irrotational)
strain% in a regionisrelated
tothe
sumof the symmetricmomenttensorof individualearthquakes
[Kostrov, 1974]:
% = ZMj / 2gV
(4)
whereeij= (1/2)(%+ ej.0.
In the caseof theNorth Chinabasin(Figure1) andduring
theTangshansequence
(Figure2), NNE trendingfaultsappear
to be thedominantstructure
whichintersects
theboundaryof
the regionof interest. Given the shorttime window involved,
thesefaultsare considered
stationarywith respectto eitherof
the two blocks to the northwest or southeast of the basin.
Thereforethe asymmetric
momenttensoris a meaningful
formulationto detectsimpleshearacrosstheNNE trending
faults in the north China basin.
The North China Basin
The expressions
below are the sumof the asymmetric
momenttensors(in Nm) andtheaverageregionalstrainfor 16
events(13 individualshocksandtheir subevents)in theNorth
China basin since 1946 (Table 1):
-0.65
+0.24
-0.04]
ZM*= -1.32 +0.68 +0.38 x 1020
-0.07
-0.27
(Sa)
-0.03
-1.0
+0.4
-0.1]
[eij]=-2.0
+1.0
+0.6
x10
'7
-0.1
-0.4
-0.0
e31
Fig. 6. A schematicdiagramshowingthe geometryof
deformation(assumedto be dueto a singlefault) associated
with thepositivevaluesof eachelementof theasymmetric
seismic moment tensor of Molnar [1983].
The two eventsin 1945 and 1967 (M = 6.3) presumably
have moments less than 1018Nm and their effects are
negligible. Thusthe only largeeventsince1888 whichis not
accounted
for hereis theeventin 1937(M = 7). Dependingon
thefrequency-magnitude
relationship
andmagnitude-moment
formula,the cumulativeseismicslip from smallearthquakes
below magnitude6.2 couldbe aslargeasthatin (5) [Chenand
Molnar, 1977]. Thereforewe consider(5) asbeing
representative
of thepatternof straindueto seismicslipin the
North Chinabasinfor thepast80-100 years,but the
numericalvaluesare lowerboundsonly.
In map view the dominantcomponentof deformation
represented
by (5) is right-lateralslip(dextralsimpleshear
strain)alongtheNNE direction,for the sizeof e2•is about
twiceaslargeaseithere• or e22,whosepositivevalues
representeast-westandnorth-south
extension,
respectively.A
positiveet2is equivalentto right-lateralslip on an east-west
trendingplane. However,in reality,the smallpositivevalue
of e•2resultsfrom the observation
thatthe fault planesof the
largestsubevents
of strike-slipmechanism
duringthe
TangshanmainshocktrendNNE to NE, insteadof exactly
north-south.
(5b)
givena rigidityof ~3.3x10tøN/m2,anareaof 1100x 900
km2,andthemaximum
depthof faultingof ~20km [Nfibelek
et al., 1987] for this region. The valuesshownin (5) are
largelydeterminedby thenine shockswith Mo )
1.4 x 10t8 Nm (6.2 < • < 7.8). The four smallestshocks
(17-19, and22) contributed
lessthan1% (< 1 x 10 18Nm) of
the totalscalarseismicmomentin (5).
The rateof seismicslip alongNNE trendingplanesis about
2 to 3 mm/yrwhenaveragedoverthepast100 years.The
amountof netrotation(or simpleshear)is criticallydependent
on theidentificationof whichnodalplaneis thetruefault
plane. If equalamountof seismicmomentwasreleasedby
fight-lateralandleft-lateralsliponconjugate
nodalplanes,the
rotationwould exactlycancel. The resultin (5) reflectsthe
observation
thatonlytheHaichengearthquake
of 1975
contributed
a significant
amountof left-lateraldisplacement.
In contrast,by assumingleft-lateralslip for theBohai event,
MolnarandDeng [1984]obtaineda dominantpatternof
approximately
equalamountof north-south
extension
(ea2)and
east-westcompression
(- e•):
984
ChenandN•belek: Strike-SlipFaultingandNorthChinaBasin
-1.8
-1.0
+0.4]
[eij
] = +0.4
-1.0+0.1
+2.2-0.4
-0.7x10
'7
(6)
Experimenting
withdifferentcombinations
of thedatain
Table 1 shows that our results for horizontal deformation are
practically
independent
of thechoiceof faultplanesfor thedipslipeventsbecause
verylittle strike-slip
displacement
is
associatedwith them. On the other hand, the relative motion
betweenthetopandbottomof theseismogenic
zone(el3and
eaj),whichoccupies
approximately
theupper
two-thirds
of the
crust,is largelycontrolledby theorientationof thedip-slip
faultplanes.Becausesofew largedip-slipeventsareobserved,
crustalthinning(- e33)due to seismicslip is smallwhen
averagedovertheentireregion,despitethatcrustalstretching
couldbe significantlocallyon smallscales.
TheTangsbanSequence
Wehavealsocalculated
thesumofM*ij(inNm)forthe
Tangshan
sequence
alone(a totalof eightevents/subevents):
-0.32
+0.34
+0.02
-0.13
+0.01
that the most noticeable features of the deformation associated
with theTangshansequence
are thoseof a pull-apartsystem
andthatthepatternsof straindueto seismicslipin thepast
100 yearsare similarat small(---100km) andbasin-wide
scales,the highlevel of seismicityimpliesthatpull-apart
structures
play an importantrole in therecentdevelopment
of
theNorth Chinabasin. Moreover,if we extrapolatethismode
of deformationintotheQuaternaryandTertiary,a varietyof
seeminglypuzzlinggeologicaldatacanbe accounted
forby
viewingthe entireNorth Chinabasinas a largecomposite
pull-apartstructure.
The OverallShapeof theBasin
]
-0.60+0.31+0.42 x 102o
-0.12
Reading,1980]in differentareasof theglobeclearly
demonstrate
thatthesebasinsare usuallycompositein nature
andtheytypicallyexhibitseverallevelsof structures
at smaller
length-scales
("infrastructures").
Small-scalepull-apart
structures(basinsand sometimeshorstswithin basins)are
enclosedin largerstructures
andtheselargerbasinsarepartof
evenlargerones.Basedon ratiosbetweenthelengthandwidth
of manypull-apartbasins,Aydin andNur [1982] suggested
a
mechanism
of forminglargepull-apartbasinsby the
successive
coalescing
of smallerbasininfrastructures.
Given
(7)
This sequence
contributed
about50% of thetotalslipin
expression
(5). Nearlythesamepatternof horizontal
deformation
is observedfor thissequence
asthatof (5) where
almostall largeeventsin theNorth Chinabasinin thepast
100yearsareconsidered.
Theonlydifference
is thatthesizeof
e•2is comparable
to theamountof east-west
shortening
and
north-southextensionin the sourceregionof theTangshan
sequence,
a directconsequence
of slipontheNE strikingfault
planeof thesecondsubevent
of theTangshan
mainshock,
whichhasthe highestmomentof all the shocksin the
sequence.
Sinceall thefault planesolutionsand seismicmoments
usedherearedeterminedfromtheanalysisof waveformsand
absoluteamplitudes
of P andSH waves,theprecisionof the
momentsis probablywithin 20% [N•ibelek,1984].
Considering
possiblesystematic
errorsdueto effectsof local
structureand attenuation,the trade-off betweendurationof the
The overallshapeof thebasinis thatof a "lazyZ"
(Figure 1), typicalfor pull-apartbasinsformedalongfightsteppingfaults[e.g.,Mann et al., 1983]. Basedsolelyon the
shapeof thebasin,BurkeandSeng6r[1986] independently
reachedthe sameconclusionasours. We interpretthe
northeastern
(theLiaohefault zone) and southwestern
arms(the
EastTaihangandtheCangXian-Dongningfault systems)
of
theNorth Chinabasinto be analogousto the two master
faultsin a simplepull-apartbasin(Figure 1).
In the caseof theNorth Chinabasin,a singlestrandof
masterfaultsis replacedby a systemof faultsovera zoneof
up to about100 km wide. Large earthquakes
occuron both
sidesof eacharm, aswell asin the interiorof thestepbetween
themasterfault zones. Becausethemechanismof faultingis
unknownfor theeventsin 1830 and 1937 (Figure 1), we have
no directinformationconcerning
thepresent-day
senseof
motionon the masterfault systems.We do note,however,
thata generalright-lateralfault systemtrendingin theNNE
directionthroughthepresentpositionof theNorth China
sourceanddepth(thereforemoment)for shallowevents,and
basin has often been invoked both in the reconstruction of the
uncertaintiesin sourceorientations,we estimatethat the
geologichistoryof easternChina [e.g.,BurkeandSeng6r,
1986;Lu et al., 1983] and in the interpretationof
paleomagnetic
data[e.g.,CourtillotandBesse,1986;Lee et
al., 1987]. Hencethemodelproposedhereis generally
consistent
with theknownlarge-scalegeologichistoryin the
vicinity of the basin.
accuracy
ofthedominant
components
ofY_,M*ij
inthe
horizontalplaneis betterthan50% (seeMolnarandDeng
[1984]foradditional
discussion
onerrors
in Y_,M*ij).
At any
rate, the relativevaluesof e•, e•2, e2•, ande22are unlikely to
be affectedby systematic
errors,andwe conclude
thatfor both
theTangshan
sequence
andtheentirebasinthestraindueto
seismicslipin thepast100 yearsis predominantly
dextral
simpleshearstrainin theNNE directionwithsomenorth-
QuaternarytoRecentDevelopment
of theNorthChinaBasin
southextensionand east-westshortening.
Ye et al. [1985] notedthatthedepocenters
of therapidly
accumulating
Quaternary
sediments
approximately
coincide
STRIKE-SLIP
THE NORTH
FAULTING AND THE EVOLUTION
BASIN
OF
CHINA
Reviewsof the structureand sedimentation
of manypullapartbasins[AydinandNut, 1982;Mannet al., 1983;
withthesitesof highsurface
heatflow (upto 100mW/m2).
We noticethatthedepocenters
arenot interconnected
andthat
low heatflow valuescanoccurnext to high heatflow values
(Figure 1). Theseobservations,
togetherwith thedominant
roleof strike-slipearthquake
faultingandlocalizedregionsof
ChenandNfibelek:Strike-Slip
FaultingandNorthChinaBasin
985
crustalthinningandsubsidence
canbe intensiveon small
scales(a lengthon theorderof 10 km), butthedominant
regionalpatternof deformation
is strike-slip.
An analogous
situationis observed
in theSaltonTrough,a
pull-apartbasinresultingfromstrike-slipmotionalongthe
SanAndreasfault system,wherewithintheoutlineof the
systemof pull-aparts,
heatflow valuesgreaterthan150
mW/m2 areobserved
only50 km or lessfrom"normal"values
largecomposite
pull-apartbasindrivenby motionon strikeslipfault systems
ratherthantheresultof regionalcrustal
extensionandthinning.In a pull-apartregimetheregionsof
intensiveextensionoccuronly at stepsof strike-slipfaults,
and the totalamountof crustalthinningon a regionalscale
resultingfrom thecumulativeeffectof localizedsubsidence
over geologictime is muchsmallerthanthe maximumvalues
of extensionat localpull-aparts.
Variableratesof subsidence
in spaceand time. In a
composite
pull-apartbasintherateof subsidence
and
sedimentation
is stronglycontrolledby the spatial
configurationof the variousscalesof strike-slipfault systems
within the basin,aswell as the historyof theirdisplacement,
includingtheoccurrence
of occasional
largeearthquakes.
One
canenvisionthatat differentpull-apartsites,rapidcrustal
thinningandconsequently
localisostaticcompensation
have
takenplacethroughoutthehistoryof thebasin. At the same
time, long-termthermalsubsidence
of eachindividualpullapartis complicated
by lateralheattransportandcontinued
displacement
alongstrike-slipfault segments[e.g.,Sawyeret
(-50 mW/m2)andextremely
highvalues
(> 200mW/m2)are
al., 1987].
measured less than 10 km from those between 80 and 100
Consequently,
while the mechanismof basinformationby
regionalcrustalextensionandthermalsubsidence
callsfor
gradualchangesin therateof subsidence
andheatflow over
time andspaceon a basin-widescale,a compositepull-apartis
characterized
by continuingfault activityandby ratesof
subsidence
and sedimentation
whichvary from siteto siteand
fluctuatein time at a givenlocationthroughmostof the
historyof basinevolution.Indeed,Hellingeret al. [ 1985]
reportedthatin theNorth Chinabasin,faultingwasevident
duringtheNeogeneandthattherateof subsidence
not only
fluctuatedat differenttimes(Paleogene,Neogene,and
Quaternary),but alsovariedfrom siteto site. Similarly,in
theLos Angelesbasin,whichmay havea pull-apartorigin,
Sawyeret al. [1987]successfully
modeledthefluctuatingrates
of subsidence
in spaceandtimeby dividingthewholebasin
into severalregionsof localizedextension.The amountof
extensionwithin eachregionvariesfrom 20% up to 178%,
while the total extensionof the entirebasinis only about
rapidsubsidence
andtranspression,
suggest
thatthepatternof
Quaternarytectonicactivityin theNorthChinabasincanbe
explained
by thecumulative
effectof severalscalesof pullapartstructures
resultingfromright-lateral
slipona system
of
right-stepping
faultstrendingNNE toNE.
On onescalewe observedthisprocess
duringtheTangshan
sequence.
Therethemaximumamountof subsidence
determined
fromrelevelingin thesmallpull-apartregionsnear
NingheandLaunxianis about1-1.5 m (Figure2; Zhanget al.
[1981]). Thesevaluesare comparable
to themaximum
horizontalslip(2.3 m) observedalongthesurfaceruptureof
the main shock[Guo et al., 1977]. Thereforethe effect of
mW/m2. The thickness
of sediments
is alsohighlyvariable
on thescaleof thelengthof individualfault segments,
but the
averaged
valuesof therateof sedimentation
andheatflow over
theentiretroughandregionsimmediatelysurrounding
it are
notverydifferentfromsouthern
Californiaasa whole[e.g.,
Lachenbruch et al., 1985].
Faultingand theDevelopmentof the
North ChinaBasinDuring theTertiary
Horizontalversusverticalmotion.The predominance
of
dextralsimpleshearalongNNE to NE oververticalmotionin
andaroundtheNorth Chinabasinseemsto havebegunno
laterthanthe lateTertiary. Ding andLu [1983] estimated
about0.1-0.2 mm/yrof relativeuplift/subsidence
sincethe
mid-Pliocene
(about3.4 Ma ago)froma combination
of field
andboreholeobservations
includingmagnetostratigraphy,
the
positionof peneplains,
andthatof riverterraces.Meanwhile,
theyreportedthatoffsetsin drainagepatternsindicate
horizontaldextralsimpleshearwhoserateis about2-10 times
largerthanthatof verticalmotion. Regardless
of theexact
originof theNorthChinabasin,dextralsimpleshear
apparently
hasbeenthedominant
modeof deformation
which
beganno laterthanthemid-Pliocene
andcontinues
today.
Amountof regionalcrustalthinningandinitialsubsidence.
Assuminga uniformcrustalthickness
of about40 km before
theinitiationof riftingin theEocene,Hellingeret al. [1985]
estimateda total extensionof about30% of the initial length
in thebasin. This valueis largerthanthat(-20%) obtained
fromsummingthedisplacement
onfaultsbasedon reflection
surveys[Li, 1982]. However,basedonmodelsof regional
crustalextension,even 30% of total extensionis too small to
explaintheobserved
highheatflow valuesof upto andgreater
than100mW/m2 [Hellingeret al., 1985]. On theotherhand,
20-30% of crustalthinningpredictsa totalsubsidence
which
is toolargefor theNorthChinabasinin thePaleogene
[Hellingeret al., 1985]. Thisdilemmacanbe resolvedif one
acceptstheview thattheNorthChinabasinhasevolvedasa
66%.
Due to the lack of data,we do not havea detailedmap of
fault geometryandsedimen• sections
to documentexactly
wheretheindividualpull-apartor push-upstructures
have
developed
in thepast. We canonly speculate
thatepisodes
of
upliftwhichapparently
occurredduringtheNeogenein thepart
of thebasinwheredataare availableto Hellingeret al. [1985]
couldbe a resultof localtranspression.
However,if the basin
asa wholewasupliftedin the sametime, suchepisodewould
remainunexplained
by anyof thetectonicmodelsputforward
sofar, includingtheoneproposedhere.
Ye et al. [1985] summarized
thepatternof riftingandthe
distribution
of horstsandgrabensin theNorthChinabasin
basedon thedatapublished
by Li [1981]. At leastoneclear
basinstructurewhichin mapview hasa similargeometryto
thatof thepresent
basincanbe identifiedin theOligocene.
That structure
is highlightedin Figure7. This featureis
almostentirelya structural
low withonlya few scattered
horstsin it. Even in the Eocenewhen the basinjust beganto
form,a somewhatsmallerstructure
apparentlyexisted.Again,
its geometry
is verysimilarto thepresentshapeof thebasin
986
ChenandN•ibelek: Strike-SlipFaultingandNorthChinaBasin
althoughthe structurewasin the form of horstsin a larger
basin(Figure7). In both cases,manysmallerstructures
existedelsewherewithin thelimit of thepresentbasinaswell
aswithinthe structurethatwe haveoutlinedin Figure7. The
reportby Li [1981] is not detailedenoughfor usto investigate
thegeometryof thesesmallstructures.The similargeometry
("lazyZ") of structureswithin theNorth Chinabasinat the
Eocene,Oligocene,andthepresent,however,suggests
that
pull-apartstructureplayedan importantrole in theformation
anddevelopment
of thislargesedimentary
basin.
a
DISCUSSION
ActiveDeformationof CompositePull-Aparts
Giventhatsmallerpull-apartscancombineto formlarge
compositebasins[e.g., Aydin andNur, 1982;Mann et al.,
1983,Reading,1980], how do smallstructures
evolvewhile
the basin as a whole continues to deform?
Eocene
b
, . r
The high level of seismicactivity in theNorth Chinabasin
providesan uniquedatasetfor thepatternof activedeformation
within a basinat two lengthscales.The resemblance
of the
patternof straindue to recentseismicslip betweenthe
Tangshansequence
(length~100km) andthatof theentire
basin(•.1000 km) indicatesthat at least someof the
infrastructures
deformin a mannerwhichis geometrically
similar to that of the whole basin. If the deformation
associated
with otherlargeearthquake
sequences
is similarto
thatof theTangshansequence,
thescattered
natureof the
seismicitywould suggestthatthepresent-day
configuration
of
thebasininvolvesthe formationof manysmallerpull-apart
structures
of variousscalesthroughout
thebasin.Presumably,
the interactionof faultson which smallearthquakes
occur
produces
onlysmall,localized
subsidence,
whilethatof major
faultsonwhichlargeearthquakes
takeplaceresultsin sizable
regions
of subsidence.
Thepull-apart
regions
caused
bylarge
faultscanoverprintthesmallerones,producing
subsidence
on
a regionalscale[N•ibeleket al., 1987].
It is worthnotingthatduringtheTangshan
sequence,
there
was considerable scatter in the distribution of aftershocks
Oligocene
C B
(Figure2) whichcannotbeaccounted
forby uncertainties
in
thelocationof theepicenters
alone. By combining
these
locations
with thefaultplanesolutions
[Shedlock
et al.,
1987],somesmall(< 10 km) pull-apartstructures
couldbe
Fig. 7. (Opposite) A map view of two large,basinlike
structures
with an overallshapeof a "lazyZ" withinthe
presentboundaryof the North Chinabasinwhichexisted
duringits evolutionat differentgeologicaltimes. (a) Map
showingthepatternof riftingandgrabendevelopment
during
theEocenelye et al., 1985,Figure8a], basedon Li [1981].
The identifiedlargepull-apartstructure
is outlinedby heavy
dashedlines. (b) The sameasin Figure7a but duringthe
Oligocene[Ye et al., 1985,Figure8b]. The identified
structureis nearlyall filled with sediments.(c) A comparison
of thetwopull-apartstructures
identifiedin Figure7a (solid
curve)andFigure7b (dashedcurve)with thepresentshapeof
thebasin(brokenlines). The geometryof the outlineof the
basinduringtheEocene,Oligocene,andthepresenttimeis
similar. Severalsmallerstructures
whosegeometryis not
known in detail existed within the ones outlined here.
ChenandNfibelek: Strike-SlipFaultingandNorth ChinaBasin
identified[Nfibeleket al., 1987]. Structures
like thesemay be
inducedor reactivatedby thelargeevents.Certainlynotall
smallstructures
areeliminatedby deformation
on a larger
scale. Thusthe mechanismof coalescinginfrastructures
to
form compositebasinsmustbe a dynamicprocesssuchthat
while the basinas a whole is growingin size,the
infrastructures
remainactivewith new structures
beingcreated.
Yet, complexstructures
of variouslengthscalesin a
compositepull-apartbasinare a resultof a singledriving
mechanism:that of motionon the masterstrike-slipfault
systemat the sidesof the basin.
987
Figure 1) suggests
an earlyphaseof left-lateralmotionwhich
changedto right-lateralmotionwith a component
of reverse
faultingin the late Tertiary [Lu et al., 1983, Xu et al., 1982].
A verylarge(M---8.5)earthquake
occurredalongthetraceof
this fault systemnear 35.3øNin 1668 [Seismological
Committeeof the AcademiaSinica, 1956]. Althoughthe
present-day
rateof motionalongthe Tan-Lufault systemis
poorlyknown,offsetsin the drainagepatternindicaterecent
right-lateralmotion [Ding, 1984].
In thecasethatthe Tan-Lu fault zonehasbeenconcurrently
active with the two arms of the North China basin and the rate
Dip-Slip Faulting in theNorth ChinaBasin
of sliphasbeenapproximatelyequal,thencalculations
based
on the Mohr-Coulombfracturecriterion[e.g.,Nur et al.,
1986] predictconsiderable
anticlockwise
blockrotationwithin
Althoughdetailedseismicsectionsof theNorth Chinabasin
are not availablein the openliterature,geologicinterpretations
of reflectionprofilesoftenindicatedip-slipfaulting[e.g.,
Hellingeret al., 1985; Li, 1981, 1982; Tang, 1982].
Reflectionprofilesgenerallycandetectverticaloffsetswell and
thereis little doubtthat dip-slipmotionis presentin the North
Chinabasin. The publishedinterpretations
of reflection
profilesare not inconsistent
with a pull-apartmodelof the
basinbecausedip-slipmotionis expectedwithin a composite
pull-apartbasinandbecausedetectionof strike-slipfaultsand
and theTan-Lu fault system.A combinationof paleomagnetic
datawith a detailedsearchfor accommodating
strike-slipfaults
intersecting
at 400-45øwith eachotherin thisregionmay
provideimportantinformationaboutthe interactionbetween
thesetwo fault systems.In any case,the possibilityof
continuedactivityalongthe Tan-Lu faultpresents
no conflict
to a pull-apartorigin of the North Chinabasinas long as
movementalongthe Tan-Lu systemis right-lateral[e.g.,Lu et
the amount of movement associated with them is difficult.
Furthermore,the assumption
of predominantdip-slipmotion
after mid-Pliocene conflicts field and borehole observations
[Ding andLu, 1983].
We suggestthatsecondary
structures
associated
with strikeslipfaultingshouldbe searched
for in reflectionrecords.For
example,localtranspression
may be common.It mayreveal
itself as secondarythrustfaultsandlocal foldingat the
intersectionof faults, or as the so-called"flower structures."
"(Positive)flower structures"
areupthrownantiforms(or
synforms
for "negativeflowerstructures")
of sediments
boundedby strike-slipfaults[e.g.,Hardinget al., 1983]. The
uplift of thesesediments
is generallyacceptedto be a resultof
localtranspression
[e.g.,Sanderson
andMarchini,1984]. We
suspectthatsomeof themmightoriginatein pull-apart
regionsat stepsof strike-slipfaultswheretherapidly
depositing
sediments
arelaterrotatedandsqueezed
upby
continuedslip on strike-slipfaults.
OtherFault Systems
Near theBasin
The overall outline of the North China basin is somewhat
complicated
by thepresence
of thenearlynorth-south
trending
EastTaihangfault on the westernborderof the southwestern
arm (Figure 1). At thepresentthereis no evidenceof
seismicityrelatedto thisfault or any othernorth-south
trendingfaultswithin thebasin,asseismicityindicatesthat
theNNE trendingfaultsarethedominantactivefault system
in the interior of the basin.
Outside of the basin, on its southeastside, is the NNE
trendingTan-Lu fault zone(Figure1; Li [1981,Figure9]).
This zoneappearsto be a major fault systemwithin theNorth
China cratonsincethe Cretaceous[e.g.,Lu et al., 1983, Xu et
al., 1982]. The zone containsa small amount of Cenozoic
sedimentin-fill [Li, 1981]. Field mappingin the middleand
southernpart of this system(southwardfrom thebottomof
the zone between the southern ann of the North China basin
al., 1983, Xu et al., 1982].
It is importantto note that our modelimpliesthat sincethe
mid-Tertiarythe southernarm of theNorth Chinabasinhas
beena majorright-lateralstrike-slipsystemwestof theTanLu faultzonewhichhasbeenfrequentlyassumed
to be the
only trajectoryof majorhorizontalcrustalmotionin theNorth
China craton since the Mesozoic [cf., Courtillot and Besse,
1986; Lee et al., 1987].
CONCLUSIONS
The unusuallyhigh level of seismicityin the actively
subsidingNorth Chinabasinis largelyassociated
with strikeslipfaulting. We analyzedfaultingassociated
with the
Tangshanearthquake
sequence
of 1976by combiningsource
parameters
determinedfrom teleseismic
P andSH waveswith
the locationof the largestaftershocks[N•belek et al., 1987],
the distribution of small aftershocks[Shedlock et al., 1987],
background
seismicity,field observations,
andgeodeticdatato
delineatethe relationshipbetweenstrike-slipfaultingand
subsidence.We concludethatregionsof normalfaultingand
subsidence
occurat right-stepping
segments
of right-lateral
strike-slipfaultswhile subsidiarythrustfaultingindicateslocal
transpression.
Basedon thedirectivityof teleseismicP
waveforms and the distribution of aftershocks,we conclude
that the Bohai Gulf event of 1969 was also associated with
predominantlyright-lateralstrike-slipmotion.
A summationof the asymmetricmomenttensorsfor the
largesteventsin the basinsince1900 showsthatthe regional
straindueto seismicslip is primarilydextralsimpleshearin
theNNE direction.The localizedrapiddeposition
of
Quaternarysediments
andvariablevaluesof highandlow heat
flow [Ye et al., 1985] canbe explainedasa resultof pullingapartdue to strike-slipmotionon numerousfaultsof different
scalesin the interiorand on the edgesof thebasinstriking
subparallelto the two armsof thebasin.
This processof basinevolutioncanbe extrapolated
backto
988
ChenandNSbelek:
Strike-Slip
Faulting
andNorthChinaBasin
the Tertiaryby combiningthe followingobservations:(1) the
overallshapeof thebasin(a "lazyZ"), whichis typicalof
pull-apartbasinsformedby right-lateralslippingmasterfaults;
(2) horizontaldextralsimpleshearhasdominatedoververtical
motionsincethe mid-Pliocene[Ding andLu, 1983];(3) the
amountof initial subsidence
dueto faultingwassmalland the
total amountof regionalcrustalthinningis insufficientto
accountfor the highheatflow valuesobservedat thepresent
[Hellingeret al., 1985]; (4) the rate of subsidence
at different
locationshasvariednotablywith time sincetheEoceneand
theratehasfluctuatedfrom siteto site[Hellingeret al., 1985];
and(5) basinlikestructures
whicharegeometricallysimilarto
the presentshapeof the basincanalsobe identifiedin the
OligoceneandEocene(Figure7).
Acknowledgments.
We thankK. ShedlockandGouyuDing
for helpfuldiscussions.We alsothankN. L. Grimisonand S.
Marshakfor criticallyreadingthe manuscript.R. V. Sharp
andC. Nicholsonprovidedmanyhelpfulcommentson an
earlierdraftof themanuscript.Thisresearch
wassupported
by
NSF grantsEAR86-07128 andEAR86-18453.
REFERENCES
Aydin,A., andA. Nur, Evolutionof pull-apartbasinsand
theirscaleindependence,
Tectonics,1, 91-105, 1982.
Burke,K., andC. SengOr,Tectonicescapein theevolutionof
the continentalcrust,in ReflectionSeisinology:The
ContinentalCrust,Geodyn.Ser.,vol. 14, editedby M.
BarazangiandL. Brown,pp. 41-53, AGU, Washington,
D.C.,
1986.
Butler,R., G. S. Stewart,and H. Kanamori,The July 27,
1976Tangshan,
Chinaearthquake--A
complexsequence
of
intraplate events,Bull. Seismol.Soc.Am., 69, 207-220,
1979.
Chen,W.-P., andP. Molnar, Seismicmomentsof major
earthquakes
andtheaveragerateof slipin centralAsia,J.
Geophys.Res., 82, 2945-2969, 1977.
Chung,W. Y., andJ. J. Cipar,Sourcemodelingof the
Hsingtai,Chinaearthquakeof March 1966,Phys.Earth
Planet. Inter., 33, 111-125, 1983.
Cipar,J., Sourceprocesses
of theHaicheng,China,earthquake
from observationsof P and S waves, Bull. Seismol.Soc.
Am., 69, 1903-1916, 1979.
Courtillot, V., and J. Besse,Mesozoic and Cenozoic evolution
of the North and South China blocks,Nature, 320, 86-87,
1986.
Crowell, J. C., Origin of the late Cenozoicbasinsin southern
California,in ModernandAncientGeosynclinal
Sedimentation,
SEPM Spec.Pub. 19, editedby R. H. Dott
and R. H. Shaver,pp. 292-303, Tulsa, Okla., 1974.
Ding, G., Active faultsin China,in A Collectionof Papersof
InternationalSymposium
on ContinentalSeismicity
and
EarthquakePrediction(ISCSEP),editedby theOrganizing
Committeeof ISCSEP,pp. 225-242, Seismological
Press,
Beijing, 1984.
Ding, G., and Y. Lu, On the characteristics
of neotectonic
deformationof theNorthChinablock(in Chinese),N.
ChinaEarthquakeSci.,1(2), 1-9, 1983.
Dziewonski,A.M., J. E. Franzen,andJ. H. Woodhouse,
Centroid-moment
tensorsolutionsfor October-December,
1983, Phys.Earth Planet.Inter., 34, 129-136, 1984.
Freund,R., The HopeFault: A strike-slipfaultin New
Zealand, N. Z. Geol. Surv. Bull., 86, 1-49, 1971.
Gilbert,F., Excitationof thenormalmodesof theEarthby
earthquakesources,Geophys.J. R. Astron.Soc.,22,223226, 1970.
Guo,S., Z. Li, S. Chen,X. Chen,X. Chen,Z. Yang,andR.
Li, Discussion
of theregionalstructural
background
andthe
seismogenic
modelof theTangshan
earthquake
(in Chinese),
Sci. Geol. Sin., 4, 305-321, 1977.
Harding,T. P., R. F. Gregory,andL. H. Stephens,
Convergent
wrenchfault andpositiveflowerstructures,
Ardmorebasin,Oklahoma,in Seismic
Expression
of
StructuralStyles,vol. 3, Stud.Geol. Ser.,15, editedby
A. W. Balley, pp. 4.2-13-4.2-17, AAPG, Tulsa, Okla.,
1983.
Harland,W. B., Tectonictranspression
in Caledonian
Spitsbergen,Geol. Mag., 108, 27-42, 1971.
Hellinger,S. J., and J. G. Sclater,Somecommentson twolayerextensional
modelsfor theevolutionof sedimentary
basins,J. Geophys.Res.,88, 8251-8269, 1983.
Hellinger, S. J., K. M. Shedlock,J. G. Sclater,and H. Ye,
The Cenozoicevolutionof the North China Basin,
Tectonics, 4, 343-358, 1985.
Jackson,
J., andD. McKenzie,The relationship
betweenplate
motionsand seismicmoment tensors,and the ratesof active
deformation
in theMediterranean
andMiddleEast,Geophys.
J. R. Astr. Soc., 93, 45-73, 1988.
Kikuchi,M., andH. Kanamori,Inversionof complexbody
waves,II, Phys.Earth Planet.Inter., 43, 205-222, 1986.
Kostrov,V. V., Seismicmomentandenergyof earthquakes,
and seismicflow of rock,Izv. Acad. Sci. USSR,Phys.
SolidEarth, Engl. Transl., 1, 23-44, 1974.
Lachenbruch,A. H., J. H. Sass,and S. P. Galanis, Jr., Heat
flow in southernmost
Californiaandtheoriginof theSalton
Trough,J. Geophys.Res., 90, 6709-6736, 1985.
Lee, G., J. Besse,and V. Courtillot, Eastern Asia in the
Cretaceous:New paleomagnetic
datafromSouthKoreaand
a newlookat ChineseandJapanese
data,J. Geophys.
Res.,
92, 3580-3596, 1987.
Lee, K. Y., Geologyof thepetroleumandcoaldepositsin the
NorthChinaBasin,easternChina,U.S. Geol.Surv.Open
File Rep., 86-154, 57 pp., 1986.
Lee, W. H. K., F. T. Wu, and S.C. Wang, A catalogof
instrumentally
determined
earthquakes
in China(magnitude
> 6) compiledfrom varioussources,Bull. Seismol.Soc.
Am., 68, 383-398, 1978.
Li, D., Geologicalstructureandhydrocarbon
occurrence
of the
Bohaigulf oil andgasbasin(China),in PetroleumGeology
in China,editedby J. F. Mason,pp. 180-192, PennWell,
Tulsa, Okla., 1981.
Li, D., Tectonic frameworks of the Bohai Gulf and coastal
basins,Acta Oceanol. Sinica, 1, 82-93, 1982.
Li, Z., andS. Guo,On therelationship
betweentheNinghe
6.9 aftershock
andtheTangsbanearthquake
fromthe
viewpointof seismotectonics
(in Chinese),Seismol.Geol.,
1 (4), 27-35, 1979.
Lu, H., H. Yu, Y. Ding, and Q. Zhang,Changingstressfield
ChenandN•ibelek:Strike-SlipFaultingandNorthChinaBasin
in the middlesegmentof the Tan-Lu fault zone,eastern
China, Tectonophysics,
98, 253-270, 1983.
Mann, P., M. R. Hempton,C. Bradley,and K. Burke,
Development
of pull-apartbasins,J. Geol.,91,529-554,
1983.
McKenzie,D., Someremarkson thedevelopmentof
sedimentary
basins,EarthPlanet.Sci.Lett.,40, 25-32,
1978.
Molnar, P., Averageregionalstraindue to slip on numerous
faultsof differentorientations,
J. Geophys.Res.,88, 64306432, 1983.
Molnar, P., andQ. Deng, Faultingassociated
with large
earthquakes
andtheaveragerateof deformation
in centraland
easternAsia, J. Geophys.Res.,89, 6203-6227, 1984.
N•ibelek,J. L., Determinationof earthquakesourceparameters
from inversionof bodywaves,Ph.D. thesis,Mass.Inst.of
Technol.,Cambridge,1984.
Nfibelek,J., W.-P. Chen,andH. Ye, The Tangshanearthquake
sequence
of 1976andits implicationsfor theevolutionof
the North Chinabasin,J. Geophys.Res.,92, 12,61512,628, 1987.
Nur, A., H. Ron, and O. Scotti, Fault mechanicsand the
kinematicsof blockrotations,Geology,14,746-749, 1986.
Peng,K., L. Xie, S. Li, D. M. Boore,W. D. Iwan, andT. L.
Teng,Thenear-source
strong-motion
accelerograms
recorded
by an experimental
arrayin Tangshah,China,Phys.Earth
Planet. Inter., 38, 92-109, 1985.
Reading,H. G., Characteristics
andrecognitionof strike-slip
fault systems,in Sedimentation
in Oblique-SlipMobile
Zones,Int. Assoc.Sedimentol.Spec.Publ., 4, editedby P.
F. Ballanceand H. G. Reading,pp. 7-26, Liege, Belgium,
1980.
Sanderson,D. J., andW. K. Marchini, Transpression,
J.
Struct. Geol., 6, 449-458, 1984.
Sawyer,D. S., A. T. Hsui, and M. N. Toks0z, Extension,
subsidence
and thermalevolutionof theLos Angelesbasin-A two dimensionalmodel, Tectonophysics,
133, 15-32,
1987.
Segall,P., andD. D. Pollard,Mechanicsof discontinuous
faults,J. Geophys.Res.,85, 4337-4350, 1980.
Seismological
Committeeof the AcademiaSinica,
Chronological
Tablesof EarthquakeData of China(in
Chinese),SciencePress,Beijing, 1956.
Sharp,R. V., Tectonicsettingof the SaltonTrough,U.S.
Geol. Surv.Prof. Pap., 787, 3-15, 1972.
Shedlock,K. M., L. M. Jones,and X. Ma, Determinationof
elasticwave velocityandrelativehypocenterlocationsusing
refractedwaves,II, Applicationto theHaicheng,China,
aftershocksequence,
Bull. Seismol.Soc.Am., 75,427439, 1985.
Shedlock, K. M., J. Baranowski, W.-W. Xiao, and X-L. Hu,
989
The Tangshanaftershocksequence,
J. Geophys.Res.,92,
2791-2803,
1987.
StateSeismological
Bureau,TangsbanEarthquakeof 1976 (in
Chinese),SeismologyPublisher,Beijing, 1982.
Tang,Z., Tectonicfeaturesof oil andgasbasinsin easternpart
of China, Am. Assoc.Pet. Geol. Bull., 66, 509-521, 1982.
TangshanSeismological
Team,The Tangshanearthquake
sequence(in Chinese),in ObservationandInvestigationof
TangshanEarthquake,pp. 74-80, Seismology
Publisher,
Beijing, 1981.
Tapponnier,P., andP. Molnar,Slip-linefield theoryandlarge
scalecontinentaltectonics,Nature, 264, 319-324, 1976.
Tapponnier,P., andP. Molnar,ActivefaultingandCenozoic
tectonicsof China, J. Geophys.Res.,82, 2905-2930,
1977.
Wei, G., T. Ji, and B. Li, Characteristics
of magnitude7.4
earthquakesequence
in theBohaiarea(in Chinese),Seismol.
Geol., 6, 21-29, 1984.
Wu, K.-T., M.-S. Yue, H.-Y. Wu, X.-L. Cao, H.-T. Chen,
W.-Q. Huang, K.-Y. Tian, and S.-D. Lu, Certain
characteristics
of Haichengearthquake(M = 7.3) sequence(in
Chinese),Acta. Geophys.Sin.,19, 95-109, 1976. (Chin.
Geophys.,Engl. Transl.,1,289-308, 1978).
Xiang, H., M. Ding, and F. Song,Analysisof tectonic
conditionfor 1978Haichengearthquake
with magnitudeof
6.0 (in Chinese),Seismol.Geol., 2(2), 61-69, 1980.
Xu, Z., Q. Zhang,andM. Zhao,The maincharacteristics
of
thepaleoriftin themiddlesectionof theTancheng-Lujiang
fracturezone(in Chinesewith Englishabstract),Bull. Chin.
Acad. Geol. Sci., 4, 17-40, 1982.
Ye, H., K. M. Shedlock,S. J. Hellinger, and J. G. Sclater,
The North Chinabasin: An exampleof a Cenozoicrifted
intraplatebasin,Tectonics,4, 153-169, 1985.
Zhang, Z.-L., Q.-Z. Li, J.-C. Gu, Y.-M. Jin, M.-Y. Yang,
andW.-Q. Liu, The fractureprocesses
of theTangshan
earthquakeandits mechanicalanalysis(in Chinese),Acta
Seisin. Sinica, 2(2), 111-129, 1980.
Zhang,Z., J. Xie, F. Xu, L. Huang,H. Hu, S. Peng,S.
Ying, andS. Gen,Tangshan
earthquake
andcrustal
deformation,in ObservationandInvestigationof Tangshan
Earthquake(in Chinese),pp. 163-183, Seismology
Publisher,Beijing, 1981.
W.-P. Chen, 245 NHB, 1301 W. Green Street,Department
of Geology,Universityof Illinois, Urbana,IL 61801.
J. N•ibelek,Collegeof Oceanography,
OregonState
University,Corvallis,OR 97331.
(ReceivedMay 2, 1986;
revisedMay 2, 1988;
acceptedMay 3, 1988.)