TETTERSTO NATURE
A balanced Gross-section of the
1994 Northrid€e earthquake,
southern Galifornia
Thomas L. Davis & Jay S. Namson
Davisand NamsonConsulting
Geologists,
Valencia,
California
91355, USA
Tsr Northridge earthquakeof 17 January 1994t was the latest in
a series of very damaging, thrust-fault-generated earthquakes to
strike California, following the San Fernando2l9?1, Coalinga3
1983, and Whittier Narrowso't 1987 events.Like the last two of
these, the Northridge event occurred along a fault that did not
reach the surface and which had not been detected bv traditional
seismic-hazard methods6'7. Balanced cross-sections"'n, which
flatten and remove the crustal deformation, can be used to identify
and quantify the seismic hazard posed by thrust faults. Here we
present a balanced cross-section through the Northridge portion
of the TransverseRangesfold-and-thrustbeltro,which showsthat
the earthquakeoccurred on what we call the Pico thrust. A crosssection of this sort constructed before the earthquake would have
revealed the fault, although it would not have predicted the
earthquake.Cross-sectionalmodelling of the Pico thrust yields an
t
averageslip rate of 1,4-1,7 mmyr
and a recurrenceinterval of
(M*
Northridge-sized
6.7) earthquakesevery 1,500-1,800 years.
We show that the Pico thrust is the back thrust to the 170-km
Elysian Park thrust3'awhich underliessome of the most denselv
urbanized portions of the Los Angeles basin.
Belts ofcrustal convergence(fold-and-thrust belts), active and
ancient, have been the focus ofmuch study by structural geologistsduring the past 80 yearsrrand developingtechniqucsthat can
lorecast the subsurfacestructure in these belts has considerablc
economic importance for the exploration of oil and gas
reservesn''t.
Thesetechniqueshave focusedon methods ofcrosssection balancing which kinematically restore the crust to an
undeformed state (following the classicalmechanicsprinciple 9f
conservationof mass) to test the validity of the interpretation''.
Balanced cross-sectionconstruction is further constrained by
geometric and kinematic theories and field observations of the
origin of fault-related folds. The geometry of folds is most
important becauseit is a direct result of fault geometry and slip
at depth and is the most commonly available and continuous
data set in fold-and-thrust belts. The balanced cross-sectionand
seismological data lrom the Northridge earthquake presented
here indicate that the present crustal structure, developed ovcr
several million years, and the recent earthquake are the result
of displacementalong a common fault deep below the surface.
The Northridge earthquake occurred at a depth of 18.0knl
on a previouslyunknown thrust fault (Fig. 1). The focal mechanism and aftershocksofthe earthquakeindicatethat the causative
fault dips -40" to the souths and that the upper plate moved
northwards.The causativefault did not ruoture to the surlacera
although the Santa SusanaMountains weie uplifted -380 mm
(rel l)).
Subsurfaceprojection of surface geology't' ancl data from oil
wells show that the earthquake occurred beneath the San Fcrnando Valley synclinoriumwhich lies betweenthe Santa Monica
Mountains and Santa Susana Mountains anticlinoria (Fig.
2a.b\.The northeastlimb of the Santa SusanaMountains anticlinorium consistsof a panel of beds 2 3 km wide, dipping 60"
FlG. 1 Epicentres,focal mechanismsand local or
moment magnitudes M of the Northridge 1,994
earthquake"
and othersignificant
thrustearthquakes
of
southernCalifornia.
Stipplepatternshowsthe locationof
majorthrustrampsin the westernTransverse
Rangesttn
which have beenthe sourceof manyof the significant
thrustearthquakes.
LA,downtownLosAngeles;
X-Y,line
of cross-section
in Fig.2.
E L Y S I A NP A F K T H F U S T {
r gzg -'rY+.'fitr+1987 [,4=5.9
0
I
NATURE. VOL 372 . 10 NOVEIVIBER
1994
50km
-,-
Pacific
Ocean
LETTERSTO NATURE
70' to thc north. Field mapping following the earthquake
showedactivation of a discontinuousset of bedding-planefaults
with slip directions consistent with coseismicgrowth of the
anticlinorium". Integration of deep oil-well data reveals the
enormous vertical dimensions of the anticlinorium which has
more than 6 km of structuralrelief (Fig. 2b).The gently dipping
south limb shows that the anticlinorium is asymmetricto the
north.
Coscismicuplift of thc SantaSusanaMountains anticlinorium
and surfacedisruptions characteristicol foldingrTindicate it is
a fault-relatedlold. We interpret the Santa SusanaMountains
anticlinorium to be a fault-propagationlold'n basedon its asymmetric gcometry. In this typc of thrust-related fold. an asymmetric anticline lorms above a propagatingthrust ramp with fault
slip terminating in the synclinal axis in front of the fold (Fig.
0
FlG. 2 a, Balancedcross-section
poftionof the
acrossthe Northridge
Transverse Range fold-and-thrust
belt with focal mechanismuand
aftershockdistribution5from the
Northridge
1994 earthquake
superimposed. lvlain event and after
shocksbelow5 km occurredalong
the Pico thrust. Shalloweraftershocksare the resultof fold growth
(flexural-slip
faults)and propagation
of the fold hingeintothe SantaClara
s y n c l i n o r i uT
mh. i c kb l a c kl i n e ss h o w
faults,wavylinesshow unconformities. b, Balanced cross-section
s h o w i n gt h e S a n t aS u s a n aM o u n tains and SantaMonicalvlountains
anticlinoriaas crustal-scale
faultpropagationfolds above the Pico
and ElysianPark thrusts, respectively.Assumingthat the Northridge
earthquake is the characteristic
event for the Pico thrust, then it
wouldtake -1,300 eventsto make
the present-daySanta Susana
Mountains
anticlinorium.
Location
of
wellsareshown(triangles)
with bore
holes,and long-shortdashedlines
arefold hinges.c, Restoration
of the
cross-section
showsthat it balances
and is thereforea viablesolution13.
The Santa Monica lvlountains
and
S a n t aS u s a n aM o u n t a i n as n t i c l i n o ria are unfolded,and slip on the
ElysianPark and Pico thrusts is
restoredto the pre-growthsetting
(-2-3 Myr ago). High-anglefaults
areolderMioceneand Pliocene-age
normal faults that formed before
regiona
I convergence.
Abbrevrations:
NF, Northridge Hills fault; DF,
Devonshire
fault;FF,Frewfault;SSF,
Santa Susana fault; SMF, Santa
Monicafault; QTu,Upper Pliocene
and Quaternaryrocks;TKu, Lower
Pliocene to Upper Cretaceous
rocks; Mzcs,CatalinaSchist;Mzb,
Mesozoic
age metamorphic
and plutonic rochs: Mzgr. Mesozoicage
granite, A, B and C, form lines
showingLate Cenozoicconvergent
structure in the undifferentiated
crystalline
basement.
168
2h).We refer to the causativefault of the Northridge earthquake
and the Santa Susana Mountains anticlinorium as the Pico
thrust, because the large Pico anticline occurs as a secondary
structure along the northeast limb of the anticlinorium'n.This
interpretation also provides a kinematic explanation of the zone
ol shallow aftershocks as flexural-slip faulting, fold axis migration into the Santa Clara synclinorium,and incipient propagation of the Pico thrust up the synclinal axis (Fig. 2a). These
secondary processesare commonly observed in the front limbs
of fault-propagationfoldsr* and are consistentwith the discorrtinuous surface disruptions observed in the Santa Clara
synclinoriumrarT.
The steepnorth limb and gentle south-dippingback limb of
the anticlinorium requiresa south-dippingthrust, but the back
limb only constrainsthe dip of the fault to a range of 25" 45"
South
X
SANTA MONICA MTNS
ANTICLINORIUM
North
SANFERNANDOSANTASUSANAI\,4TNSsnnrncuRn Y
VALLEY
ANTICLINORIUM
VALLEY
SYNCLINOBIUM
SYNCLINORIUM
r,,r
3.tnriog-eEart"hquake
o o M = 67 1 / 1 7 / 9 4 _
b sourn
S A N T A M O N I C AM T N S
ANTICLINOBIUI\,4
sANFERNAND.
c
U
Z
SANTACLARA
VALLEY
3i r"r[i'^tl,rt
SANTA SUSANA N/TNSa;
VAuLEY
SYNCLINORIUM ANTiCLINORIUM
Z
North
SL
hr",
'4-
'o\
,>,$ry.
F"
NorthridgeEarthquake
. V O L 3 7 2 , 1 0 N O V E M B E1R9 9 4
NATURE
TETTERSTO NATURE
and only generally constrains the fault depth. Integration of
seismologicaldata on the hypocentre, mainshock focal mechanism and distribution of aftershocksestablishesthe exact dip and
depth of the Pico thrust.
Our interpretation indicates a pairing ol the Santa Susana
Mountains anticlinorium uplift and the deeply buried Pico
thrust. The lateral extent of the latter can therefore be defined
by the lateral extent of the Santa Susana Mountains anticlinorir"rmwhich is recorded by surface geology and subsurfacewell
data to be 30 40 km in length. More work is neededto elucidate
the detailed geometry of the anticlinorium to define more precisely the regional geomctry and extent of thc seismicallyactive
Pico thrust.
The Pico thrust is shown to be a backthrust off the northdipping Elysian Park thrust ramp. Thc Elysian Park thrust is
rooted in a mid-crustal detachment aI -22 km depth. Midcrustal detachments havc been proposed by numerous
workerst'"''"' rr to underlie the westernTransverseRansesand
rr.
arc a basiccomponentof fold-and-thrustbeltss'e The restored
cross-section(Fig. 2c) shows the structural geometry before folding and faulting, and provides a check that thc cross-sectionis
balanced.
Thc balanccd cross-section provides information about thc
seismicpotcntial of the Pico and ElysianPark thrusts.This information is cspcciallyimportant for the latter thrust becauseit
underliesthc most urbanizcd parts of the Los Angeles basin
(Fig. I ). Prcviousbalancedcross-section
analysisand subsurfacc
mapping of the Elysian Park thrustr'rr indicate that it is over
l70km long and is a fundamental thrust lault of southern
California. Thc Pico thrust has 3.3 km of displacementand the
Efysian Park thrust has I l .tl km of displaccment( Fig. 2b). Comprcssivedcformation probably began 2 3 Myr ago3 within the
TransverscRange fold-and-thrust belt. Initiation of the Santa
SusanaMountains anticlinorium and Pico thrust is recordcdby
delormation of the youngestunit, the SaugusFormation (QTu,
Fig. 2), which is no older than 2.3 Myr (rcL 23). The displacement and 2.3 2.0 Myr age of fault initiation yields an average
'
slip rate of 1.4 1.7mm yr for the Pico thrust. A 2.0 3.0 Myr
(rcf. 3) age of initiation olthe Santa Monica Mountains anticlinorium yields an averageslip rate of 3.9 5.9 mm yr ' for the
Elysian Park thrust.
Geodeticand seismicmodellingsuggestthc Pico thrust movcd
-2.5 m during the Northridge earthquakers':a
which, divided by
our slip rates,yield an averagerepeattime of I,500 1,800years.
This repeat-time estimate applies to the segment ol the Pico
thrust involved in the earthquake,which is - l5 km long. Similar
rccurrencc calculations can be made for the Elysian Park thrust
(Fig. l). Assuming that a Northridge-sizecarthquake is thc
characteristicevcnt for the Elysian Park thrust trend, then our
slip rate and time-of-initiation estimatcs yield average
carthquake repeat times of 420 640 years for any l5 km segment
of thc thrust, or an event every 39 58 ycars along the trend.
This repeat tirnc is not supportedby the 220 ycars of recordcd
history. This discrepancyis probably due to one or more of the
following; our long-term slip ratcs differ from the short-tcrm
rates, somc crustal shortening is taken up aseismically. the
carthquake repeattime is variable,or the charactcristicevent is
cven largcr and lcss frequcnt than that predicted by a
Northridge-sizeearthquakc.
Fold-and-thrust belts are tcctonic systcmswith complicated
f old and fault patterns that dcvclop over a scaleof kilomctres
(rcf. l3). In scismicallyactivebclts,the integrationof earthquakc
data with balanced cross-scctionsprovides a broader view of
scismicrisk than traditional seismic-hazardmethods. Application of the balanccdcross-sectiontechniqucshould be thc first
step in evaluating the seismicrisk in fold-and-thrustbelts and
would have prcdicted the prcsenceof a young south-dipping
thrust beneaththe Santa SusanaMountains and San Fernando
Valley beforc the Northridge earthquake.
tr
. V O L 3 7 2 . 1 0 N O V E M B E1R9 9 4
NATURE
Received23 February;
accepted6 October1994.
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ACKNOWLEDGEMENTS.
We thank E. Duebendorfer
and D. Schwartzfor commentson the
manuscriot.
169