JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105,NO. B2, PAGES2969-2980,FEBRUARY 10, 2000
Moho depth variation in southern California
from
teleseismic
receiver
functions
Lupei Zliu
Southern(',aliforniaEarthquake(',e•tev, Universityof SouthernCalifornia,Los Angeles
Hiroo
Ka, namori
Seismological
Laboratory,CaliforniaInstituteof Technology,
Pasadena
Abstract. The numberof broadbandthree-component
seismicstationsin southern
Californiahas morethan tripled recently. In this study we usethe teleseismic
receiverfunctiontechniqueto determinethe crustalthicknesses
and 1/•,/1'•ratios
for thesestationsand map out the lateralvariationof Mohodepthundersouthern
California. It is shownthat a receiverfunction can provide a. very good "point"
measurement of crustal thickness under a broadband station and is not sensitive to
crustalP velocity.However,the crustalthickness
estimatedonlyfrom the delay
time of the MohoP-to-S converted
phasetradesoffstronglywith the crustal
ratio.Theambiguity
canbereduced
significantly
byincorporating
thelatermultiple
converted
phases,
namely,the PpPs and PpSs+PsPs. We propose
a stacking
algorithm
whichsumstheamplitudes
of receiver
function
a,tthe predicted
a,rrival
timesof thesephases
by differentcrustalthicknesses
H and V•,/I/• ratios. This
transformsthe time domainreceiverfunctionsdirectlyinto the H-V•,/I,• domain
withoutneedto identifythesephases
and to picktheir arrivaltimes. The best
estimations
of crustalthickness
andF}•/I• ratioarefoundwhenthe threephases
are stackedcoherently.
By stackingreceiver
functions
fromdifferentdista,
ncesand
directions,effectsof lateral structural variation are suppressed,and an average
crustalmodelis obtained.Applyingthis technique
to 84 digitalbroadbandsta,tions
in southernCalifornia revealsthat the Moho depth is 29 km on averageand varies
from 21 to 37 kin. DeeperMohosare foundunderthe easternTransverse
Range,
thePeninsular
Range,andtheSierraNevadaRange.The centralTransverse
Range,
however,
doesnot havea crustalroot. Thin crustsexistin the InnerCalifornia
Borderland
(21-22kin) andthe SaltonTrough(22 kin). TheMohois relatively
fia,t at the average
depthin the westernandcentralMojaveDesertandbecomes
shallowerto the east under the EasternCaliforniaShearZone (ECSZ). Southern
California,
crusthasan average
Vp/l• ra,tioof 1.78,with higherratiosof 1.8to 1.85
in the mountainrangeswith Mesozoic
basement
and lowerratiosin the Mojave
Block except for the ECSZ, where the ratio increases.
1. Introduction
crustal waveguideand controlsthe strong shakingfrom
damagingearthquakesin certain distanceranges.
The Mohorovicicdiscontinuity(Moho), which sepaIn southern California, tremendous efforts have been
rates Earth's crust from the underlying mantle, repmade to determine the Moho depth with a variety of
resentsa major changein seismicvelocities,chemical
geophysicalmethods. These include an early work of
compositions,and theology. The depth of Moho is an
refraction studies using quarry blasts as energy sources
important parameter •o characterizethe overall struc[I•a•amori andHadlevi,1975],seismicverticalreflection
ture of a crust and can often be related to geologyand
experimentsin the Mojave Desert and vicinity [Cheatectonic evolution of the region. Its lateral variation
dle ctal., 1986; Liet al., 1992; Malin st al., 1995], tohas stronginfluenceon seismicwavepropagationin the
toographic imaging of Moho depth variation using Pn
travel timesfrom localearthquakes[Hca•'n,1994;Hcar•
Copyright
2000bytheAmerican
Geophysical
Union.
a•d Cla.qton, 1986; Sung and Jackson,, 1½)92;Magis-
Papernumber1999JB900322.
t•'ale ctal., 1992], teleseismicreceiverfunction studies [Langston,1989;Amm•)nand Zandt, 1993;Zhu and
0148-0227/00/1999JB900322509.00
2969
2970
ZHU
AND
KANAMORI:
MOHO
DEPTH
VARIATION
IN SOUTHERN
CALIFORNIA
37 ø
36 ø
35 ø
34 ø
33 ø
Pacific Ocean
32"
-121"
-120"
-119"
-118"
-117"
-116"
-115"
-114"
Figure 1. Topographicrelief and faults in southernCalifornia. Trianglesrepresentbroadband
three-componentstations usedin this study. Major tectonic provincesare labeled. WTR, western
TransverseRange; CTR, central TransverseRange; ETR, eastern TransverseRange; LA, Los
Angeles Basin.
Kanamori, 1994; Baker et al., 1996; Ichinose et al., the Pn travel tinhedata [Hearu, 1984;Sungand Jackand
1996;Lewiset al., 1999],and the Los AnglesRegional son, 1992]nor the Prop study []•ichards-Dinger
Seismic
Experiment(LARSE) [KohlerandDavis,1997]. Shearer,1997]detectedsucha deepMoho.
All techniquesof determining Moho depth from seisMostrecently,I•ichards-DingerandShearer[1997]mapped the crustalthicknessvariation in southernCalifor- mic data suffer, to various degrees,the limitation imnia by stackingshort-periodProp phasesrecordedby posedby the trade-off between the crustal velocity and
the regional network. In general, there is agreement the thickness. The trade-off can be very severefor those
amongthesestudiesthat t.he averagecrustalthickness usingMohowide-anglereflectionand refraction(Prop
in southern California is ,-•30 km with thicker crust unand Pn) traveltimesbecausethesewavesusuallytravel
der the easternTransverseRange and thinner crust un- >100 km laterally within the crust. They are much
derthe SaltonTrough. However,therearestill sonhe
dif- more sensitive to lateral velocity variations than to the
ferencesin Moho depth under several areas. For exam- Moho depth variations. Using the differential travel
ple, there hasbeena longdebateon whetherthe Sierra tinhe between Prop and the first P arrival can reduce
Nevada is supportedby a crustal root or by a buoy- the dependencyon the upper crustal velocity, but the
ant upper mantle [Joneset al., 1994]. DifferentMoho result is still strongly influencedby the lower crustal
and Shearer,1997]. In addidepths,rangingfrom 33 to 50 km, wereobtainedfrom velocity[]•ichards-Dinger
different seismicrefraction profiles, or even from inter- tion, pickingthe secondaryProp arrival is not easyand
pretingthe samedata set [Savageet al., 1984]. Sinhi- can sometimesbe ambiguous. For studiesusing local
larly, Kohler and Davis [1997]reporteda 40-km-deep earthquakesas energysourcethe sourcelocation brings
Moho under the San Gabriel Mountains from invertin additional uncertainty in the Moho depth estimaing teleseismic
travel tinheresiduals.However,neither tion. Vertical seismicreflection experiment can reveal
ß
ZHU
AND
KANAMORI:
MOHO
DEPTH
VARIATION
fine-scalevariation of deep crustal structure, provided
that the energysourcesare strongenoughto illuminate
the Moho. However, the cost of such surveysis often
very high, and their spatial coverageis very limited.
An alternative and more effectivetechnique of estimating Moho depth is to use teleseismicreceiverfunctions.
Owing to the large velocity contrast acrossthe discontinuity, part of the incoming teleseismicP wave energy
will convert into SV wave at the Moho. By measuring the time separation bet,ween the direct P arrival
and the conversionphase, the crustal thicknesscan then
be estimated. The estimation provides a good "point"
measurement at the station because of the steep incidence angle of the teleseismic P wave. Furthermore,
since the direct P arrival is used as a referencetime, it
can be shown that
the result
is not sensitive
IN SOUTHERN
,-(t)-
+
2971
Is()l:+
where S*(w) is the complex conjugateof S(w). The
Gaussian-type
low-pass
filtere-w2/4•2is addedto removehigh-frequency
noise. The quantityc•r• (also
called water level) is used to suppress"holes"in the
spectrumS(w), thusstabilizingthe deconvolution.Here
weusethe autocorrelation
•r• of S(t) to normalize
the
water level so that c can be selectedfrom a narrow range
for different sized earthquakes. The 1 + c factor is used
to compensatethe amplitude lossdue t.othe water level.
In single station applicationsthe vertical component
recording is often used as the effective source time
to crustal
P velocity.
Receiverfunction analysisrequiresdigital three-component seismic stations that. are usually not available
in large numbers for a specific region. This used to
be a drawback of the technique compared with other
methodssuchas usingPn and Prop travel times where
good spatial coveragecan be achieved by using large
numbersof local earthquakesand short-period vertical-
CALIFORNIA
Ps
P wave
......
PpPs
PpSs PsPs
s wave
P$
0.08
componentstations. Only recently, the number of digital
stations
in southern
California
has increased
dra-
matically with the start of TriNet project by the Cal-
ifornia Institute of Technology(Caltech), the U.S. GeologicalSurvey(USGS), and the CaliforniaDivisionof
Mines and Geology(CDMG) [Mori et al., 1998]. Cur-
0.07
rently, there are 65 TriNet broadband stationscovering
mostof the southernCalifornia(Figure1) andthe number will reach 150 in the next 2 years. This provides
an unique opportunity to map out the lateral varia- •,• 0.06
tion of Moho depth usingthe receiverfunction method.
In this paper we will first discussthe methodologyof
using teleseismicreceiver functions to estimate Moho
depth and the associatedtincertainties. We will develop
a receiverfunction stacking algorithm to transform the
0.05
time domain waveforms into the depth domain which
gives the best estimations for both the crustal thick-
hessand Vp/• ratio. Then we apply this techniqueto
all availablest,ations and generatea map of Moho depth
variation
in southern
California.
0.04
2. Method
The teleseismicreceiver function representsthe structural responsenear a recording station to the incom-
ing teleseismicP wave. It is obtained by removingthe
source
time
function
from
raw
teleseismic
records
us-
ing deconvolution. Details on the computation can be
foundelsewhere
[Langston,1977;Owenset al., 1984].In
our study we use a modified frequency domain deconvolution which is implemented by dividing the spectrum
/•(w) of teleseismicP waveformby the sourcespectrum
S(•):
0
5
10
15
20
t(s)
Figure 2. Radial receiverfunction a.sa function of ray
parameter p for the Standard Southern California Ve-
locity Model (seeTable 1). The Moho convertedphase
Pa' and the multiples PpPs, PpSs, and P.sPs' are IBbeled, and their ray paths are illustrated at the top.
Other unlabeled phases are the P-to-S conversionsat
the 5.5 km and 16 km intracrustal
model.
discontinuities
in the
2972
ZHU
Table 1.
AND
KANAMORI:
MOHO
DEPTH
VARIATION
IN SOUTHERN
CALIFORNIA
OH
The Standard Southern California Velocity
AH- •
Model
AI/• -- 4.3AVp(km),
Layer
Thickness,km
•, km/s
Vp/V•
which means that the uncertainty of H is <0.5 km for
I
5.5
3.18
1.730
2
10.5
3.64
1.731
a 0.1 km/s uncertaintyin Vp. However,the thickness
is
highlydependenton the Vp/V•, asshownby
3
16.0
3.87
1.731
4.50
1.733
4
OH
AH- • An--40.2A,c
function [Langstor•,1977]. For an array of stations
the source time function, which is common to all stations, can be better estimated by stacking all vertical-
(km),
i.e., a 0.1 changein n can lead to about 4 km changein
the crustal thickness. This ambiguity can be reduced
by usingthe later phaseswhich provide additional constraints
componentrecordingsfor eachevent (C. A. Langston,
personalcommunication, 1997). An obviousadvantage
is that
the source time
function
is much
smoother
H = tppss+PsPs
so
adds more information
about
(4)
/• _p2
that its spectrum is lesssingular. One (:an also obtain
[he vertical component of the receiver function which
the structure.
so tha.t both • and H can be estimated [Zhu, 1993;
Zandt ct al., 1995;Zandt and Ammon, 1995].
ture under a station can be derived from the radial reIn real situations, identifying the Moho Ps and the
ceiver function which is dominated by P-to-S converted multiples and measuringtheir arrival times on a single
energy from a seriesof velocity discontinuitiesin the receiver function trace can be very difficult due to backcrust and upper mantle. Becauseof the large velocity ground noise, scatteringsfrom crustal hcterogeneities,
The
first-order
information
about
the crustal
struc-
contrast at the crust-mantle boundary, the Moho P-to-
and P-to-S conversionsfrom other velocity discontinu-
S conversion
(P.s) is often the largestsignalfollowing ities. To increasethe signal/noiseratio (SNR), onecan
the direct P.
An example is shown in Figure 2 for
the Standard SouthernCalifornia Velocity Model (Table 1) [Wald ½t al., 199,5].In this idealizedcaseboth
use multiple events to stack their receiver functions.
the primary converted phase Ps and its crustal multiples PpPs and Pp,S's+P.sPs are clear and have comparable amplitudes. Naming of these phasesfollows the
conventionof Bath and ,S'te•bnson[1966]. Except for
the first arrival, lowercaseletters denote upgoingtravel
paths, uppercaseletters denote downgoingtravel paths,
as illustrated in Figure 2.
The time separation between Ps and P can be used
to estimate crustal thickness,given the averagecrusta.1
velocities,
1.85 -
0.200
(a)
1.80 •
0.150
1.75-
0.100
1.70 -
0.050
1.65 1.60
0.000
1.85 -
1.80
1.75
where p is the ray parameter of the incident wave. An
advantage of this method is that becausethe P-to-$'
conversionpoint is close to the station (usually within
1.70
10 km laterally), the estimationis lessaffectedby lateral velocityvariationsand thus providesa good point
1.60
measurement. One problem is the trade-off between the
thicknessand crustalvelocities.However,sincet?• representsthe differential travel time of 5"with respectto
1.65 -
15
20
25
30
35
40
H (km)
Figure 3. (a) The s(H, •) from stackingthe receiver
functions
in Figure 2 using (5). It reachesthe maxP wavein the crust,the dependence
of H on Vpis not
imum (solid area) when the correctcrustal thickness
asstrongason V.,(or moreprecisely,
onthe V•,/I4 ratio and Vp/V• ratio are usedin the sta.cking.(b) H-• relan).
tions, as givenin (2)-(4), for differentHobo converted
For example,usinga k• of 6.3 km/s and 1/•/1/•,ratio phasesin Figure 2. Each curve representsthe contribution from this convertedphaseto the stacking.
of 1.732 for a 30-kin-thick crust, one gets
ZHU
AND
KANAMORI.
MOHO
DEPTH
VARIATION
Such stacking is usually done in the time domain for
IN SOUTHERN
CALIFORNIA
2973
STS-1 or STS-2 sensorwhich has a fiat velocity response
a clusterof events[e.g., Owenset al., 1984]. Sincewe between 8 Hz to 370 s and 30 Hz to 130 s, respectively.
are mainly interestedin estimatingcrustal thickness,we After the 1994 Northridge earthquake the network is
proposea straightforwardH-to domainstackingdefined nowbeingexpandedaspart of the TriNet project [Mori
as
st el., 1998]. So far nearly45 morebroadbandstations,
where v(t) is the radial receiverfunction, t•, t• and
t3 are the predicted Ps, PpPs, and Pp,S'a'+PsPs arrival times correspondingto crustal thicknessH and
most.with CMG-40 sensor,have been added. There are
also other regional broadband station networks operated by other agents, such as the Anza network of the
Scripps Institution of Oceanography and the Nevada
Test
Site Network
of the Lawrence
Livermore
National
Vp/I/• ratio •, asgivenin (2)-(4). The wi are weight- Laboratory. We also used data collected during a teming factors,and •-}'.wi - 1. The s(H,,c) reachesa porary deployment of broadband stations across the
maximum when all three phasesare stackedcoherently
PeninsularRange[Ichi•o.seet al., 1996]. The locations
with the correctH and ,c (Figure 3.). Advantagesof of stations used in this study are listed in Table 2. They
this algorithm are that (1) large amountsof teleseis- cover the entire southern California region fairly well,
mic waveformscan be convenientlyprocessed;(:2)there
except in the western Transverse Ranges and offshore
is no need to pick arrival times of different conversion
area (Figure 1).
phases;(3.) by stackingreceiverfunctionsfrom differ-
ent distances and directions, effects of lateral structure
variation are suppressedand an averagecrustal model
isobtained;and(4) uncertainties
canbeestimatedfrom
the flatnessof s(H, ,c) at the maximum.Usingthe Taylor expansionof .s(H,,c) at the maximumand omitting
the higher-orderterms,onegetsthe variancesof H and
02s
(•)
Waveforms of 416 teleseismic earthquakes recorded
by these stations are retrieved from data archives at the
SouthernCalifornia EarthquakeCenter (SCEC) and
the Incorporated Research Institutions for Seisinology
(IRIS). The actualnumberof eventsfor individualstation varies from 8 to 2½)4,depending on the length of
recording period and backgroundnoise level of the sta-
tion (Table 2). The eventsare selectedfrom the global
earthquakes between 1993 and 1999 with magnitude
>5.5 and distance range from 300 to 950 to the center
of the network. The abundance of earthquakes within
this distance range makes southern California a favorable place for study using teleseismicwaveforms. The
whererr• is the estimatedvarianceof s(H, ,c)from stack- good epicentral distance and azimuthal coveragehelps
ing.
to average out lateral structural variations.
(7)
3. Study Region and Data
4.
Results
We usea time window of 150 s in length, starting 30 s
Straddling a major transform boundary separating
the North American plate and the Pacific plate, south- before the P on:,et, to cut the P waveform from raw veern California exhibits a wide variety of physiographic locity records. The two horizontal componentsare rofeatures, ranging from low lying valleys to high moun- tated to the radial and tangential directions. The effectain ranges(Figure 1). The regioncan be divided into tive sourcetime function of each earthquake is obtained
several provinces: The Salton Trough, part of which by stackingall vertical componentsrecordedby the netlies below sea level, is the northern extension of the work, after aligningthem at the first P arrival. It is then
Gulf of California rift zone. To the west. of the Salton
used to deconvolve the three-component waveforms of
Trough lies the PeninsularRange, a north-southtrend- each station using (1). We chosea to be `5Hz, which
ing Mesozoic batholith. It is terminated to the north correspondsto a cutoff frequencyof 1.5 Hz; A range of
by the east-westoriented TransverseRangesthat rise water level c from 0.1 to 0.5 is tested to find the optimal
abruptly >3 km at several mountain peaks. In con- c which givesthe best,deconvolutionresult, jndged by
trast to these rugged topographies,the Mojave Block, the presignalnoiselevel and ta.ngential componentambounded by the San Andreas Fault to the south and the plit.udes. The receiver functions of each station are then
Garlock Fault to the north, showsa very smooth land- stackedusing(5), with w• - 0.7, w• - 0.2, w3 - 0.1.
scapewith an averageelevation around 1 kin. Farther These values are chosen to balance the contributions
to the north, large relief resumesin placesof the Sierra from the three phases. Among them, the Ps has the
highest ,5'WR so it. is given a higher xveightthan the
Nevada Range and the Basin and Range province.
Installation of permanent broadband digital seismic other two. We also set w• > w• + w3 because the later
stationsin southernCalifornia began in the late 1980s. two phaseshavesimilarslopesin the H-,c plane(seeFigBy the end of 199,5, 20 such stations were deployed, ure 3). All stackingsare doneusingan averagecrustal
forming the TERRAscope network operated by Caltech P velocityof 6.3 km/s. Crustal thicknessand V•/V•
and USGS. These stations are equippedwith Strekheisen ratio are obtainedfrom the location of s(H,,c) maxi-
2974
ZHU AND KANAMORI'
MOHO
DEPTH
VARIATION
IN SOUTHERN
CALIFORNIA
mum. Their uncertainties
are estimatedusing(6) and crustall/p/• ratio of 1.73 (Figure48). The predicted
(7). In addition, we also bin receiverfunctionsaccording to their ray parameters and stack receiverfunctions
in each bin to producea ray parameterprofile of receiver functions so that the predicted arrival times of
Ps and multiplescan be visuallychecked.
As an example, stacking25? receiverfunctionsfor
station PAS gives a crustal thicknessof 28 km with a
Table
2.
Locations of Broadband
Moho Ps arrival time agreeswith the receiverfunction
profile which showsa strong convertedphaseat 3.4 s
followingthe direct? arrival(Figure4b). This phase
has the expected increaseof amplitude and time delay with ray parameterfor a primary convertedphase.
A similar crustal thicknesswas obtainedby
[1989]for thisstationbut wasconsidered
contradictory
Stations and Number of Receiver Functions
Used in This Study
Station Latitude Longitude Elevation,m
N
te•, s
205
43
92
41
119
38
36
54
112
24
35
253
1'2
71
6'2
116
98
34
'274
46
209
'22
78
104
37
42
31
17
88
221
132
'26l
29-t
39
71
71
2-18
'25
7(;
195
77
1'2
,'44
8
1'2'2
96
264
37
57
50
284
4.8
3.5
3.4
4.0
4.4
2.8
:3.7
3.6
4.1
3.3
4.4
4.4
3.4
3.6
3.7
3.'2
4.2
3.5
3.7
3.9
4.8
4.4
4.2
3.8
:3.9
4.5
4.0
4.0
3.3
:3.9
3.7
3.4
3.7
3.2
4.7
3.6
3.0
:3.6
4.'2
4.4
4.8
:3.1
:3.9
3.8
'2.6
4.4
5.1
'2.8
4.3
4.4
3.9
H, km
Vp/V•
TriNet
BAR
BC3
BKR
BTP
CALB
CIA
CLC
CPP
CWC
DAN
DEV
DGR
DJJ
EDW
FPC
GLA
GPO
GR2
GSC
HEC
ISA
,JCS
JRC
LKL
LRL
LU(-I
MLS
MPM
MTP
NEE
OS[
PAS
PFO
PHI_,
PLM
PLS
RPV
RUS
RVR
SBC
SBPX
SCI
SHO
SLA
SNCC
SOT
SVD
SWS
TAB
VCS
VTV
32.680
33.655
35.269
34.683
34.140
33.402
35.816
34.060
36.440
34.637
33.935
33.650
:34.106
34.883
35.082
33.051
35.649
34.118
35.302
34.839
35.663
3:3.087
35.98'2
34.616
:•5.47,9
34.366
34.005
:36.058
35.485
3-l.825
34.61-1
34.148
3:3.611
35.408
33.354
3:1.795
33.743
:•.1.050
33..993
34.441
34.232
32.9,q0
35.900
35.891
33.248
34.416
34.107
32.941
34.382
34.483
34.561
-116.672
-115.453
-116.070
-118.575
-118.628
-118.414
-117.597
-117.809
-118.080
-115.380
-116.577
-117.010
-118.454
-117.991
-117.583
-114.828
-117.662
-118.299
-116.806
-116.335
-118.474
-116.597
-117.808
-117.824
-117.682
-117.366
-117.561
-117.489
-115.553
-114.599
-118.724
-118.171
-116.459
-120.546
-116.863
-117.609
-118.404
-118.080
-117.376
-119.715
-117.235
-118.547
-116.'276
-117.283
-119.524
-118.449
-117.098
-115.796
-117.682
-118.117
-117.330
496
1080
305
1600
276
425
735
235
1553
398
332
609
245
762
88:3
514
735
',346
954
959
817
1259
14,½42
814
1315
1140
'2'29
1853
1582
1:39
706
'257
1'245
360
1660
1181
64
67
232
61
1875
219
373
1190
'2'27
439
574
134
2250
991
812
34.2 + 16
'25.1 + 16
27.3 + 09
28.3 -4- 07
32.0 4- 11
22.0 -4- 17
26.3 4- 10
29.2 4- 07
29.6 -4- 10
28.'2 4- 07
34.0 4- 12
32.8 -4- 1:3
28.0 + 0.9
30.0 + 0.9
30.0 -4- 0.8
27.0 -4- 0.6
30.0 -k 4.1
30.0 + 1.2
29.5 + 0.9
27.8 + 0.6
36.9 + 2.4
34.9 + !.3
31.8 + 0.9
30.'2 + 1.1
31.0 + 0.9
35.7 + 0.8
32.5 4. 0.8
30.8 + 1.0
29.5 4. 0.8
31.3 + 1.3
:31.5 + 0.7
28.0 4. 1.0
29.4 q-24.3 + 1.1
34.0 + 0.8
28.0 + 0.7
'21.5 q-- 0.7
'27.3 + 0.8
30.7 + 0.9
3:3.3 + 1.5
:36.8 4. 1.7
21.8 4- 0.5
29.'2 4- 1.1
27.0 4- 0.5
21.1 4- 0.9
32.'2 4- 1.0
37.7 4- 1.1
21.7 4- 3.0
32.8 4- 1.1
31.0 4- 0.8
30.9 4- 0.9
1.86
4- 0.06
1.84
4- 0.09
1.76
4- 0.04
1.85
4- 0.04
1.83
4- 0.05
1.86
4- 0.05
1.74
4- 0.04
1.83
4- 0.04
1.70
4- 0.05
1.80
4- 0.06
1.73
4- 0.04
1.73
4- 0.05
1.75
4- 0.03
1.72
4- 0.04
1.85
+
1.69
4. 0.07
1.76
4- 0.05
1.84
4- 0.04
0.10
1.76
4- 0.04
1.80
4- 0.04
1.77
4- 0.05
1.76
4- 0.04
1.76
4- 0.04
1.75
4- 0.04
1.79
4- 0.06
1.68
+
1.75
4- 0.05
0.04
1.71
4- 0.03
1.73
4- O.07
1.75
4- 0.06
1.80
4- 0.06
1.84
q- 0.0-t
1.77 4. 0.O3
1.•4
4. 0.05
1.80 4. 0.05
1.83 4. 0.04
1.79
4- 0.06
1.87
4- 0.04
1.81
+
1.86
4- 0.03
1.74
4- 0.07
0.06
1.83
4- 0.05
1.82
4- 0.05
1.79
4- 0.09
1.85
4- 0.05
1.75
4- 0.03
ZHU AND KANAMORI: MOHO DEPTH VARIATION IN SOUTHERN CALIFORNIA
2975
Table 2. (conrimmed)
Station Latitude I,ongit,
ude Elevation,
m
Anzct Network
ASBS
BZN
CRY
FRD
GLAC
KNW
LVA2
RDM
SHUM
SND
WMC
33.621
33.492
33.565
33.495
33.601
33.714
33.352
33.630
33.633
33.552
33.574
-116.466
-116.667
-116.737
-116.602
-116.478
-116.712
-116.561
-116.848
-116.445
-116.613
-116.675
ALPN
BLSY
BLVD
BWLYV
HONY
LGNA
MICA
PINE
32.871
32.911
32.718
32.842
32.902
32.835
32.651
32.831
-116.749
-116.879
-116.260
-116.226
-116.643
-116.415
-116.117
-116.527
LAC
34.389
-116.411
1400
1301
1128
1164
1169
1507
1435
1365
1195
1358
1271
48
61
55
66
51
60
59
55
45
68
67
3.7
4.0
4.2
3.8
3.7
4.2
3.9
4.3
3.7
4.1
4.0
28.7
30.0
31.5
30.3
28.2
30.7
29.6
32.1
28.2
30.8
31.0
4- 0.9
4- 1.0
4- 0.8
4- 0.7
4- 0.7
4- 1.1
4- 0.9
4- 1.1
4- 1.0
4- 1.0
4- 0.8
1.78 4- 0.04
1.80 4- 0.05
1.81 4- 0.04
1.75 4- 0.03
1.79 4- 0.04
1.83 4- 0.05
1.80 4- 0.05
1.80 4- 0.04
1.79 4- 0.04
1.80 4- 0.05
1.79 4- 0.04
Pe•ir•sulc•r Rct•ge BroadbandExperiment
Nevada
820
530
1070
293
888
1727
1004
1071
13
13
12
8
23
24
18
14
4.9
4.9
3.9
3.4
4.4
4.1
3.5
4.5
35.2 4- 0.8
36.0 4- 0.5
27.7 4- 0.6
25.0 4- 0.6
33.2 4- 0.9
30.7 4- 0.8
27.2 4- 1.9
32.0 4- 0.7
1.84 4- 0.04
1.82 4- 0.02
1.85 4- 0.04
1.81 4- 0.03
1.81 4- 0.04
1.81 4- 0.05
1.85 4- 0.04
3.7
30.2
1.75
Test Site Network
793
1
The time delayof the MohoPs with respectto the directP is measured
on receiver
functionprofileat p = 0.06s/kin. The crustalthickness
H andVp/• ratioareestimated
by stackingreceiverfunctionsusing(5). For stationswherethe V•/• ratiosare not
constrained,the ratio is set to the average1.78 to obtain the thickness.
with the 31 km from the Pn study[Hcar• and Clayto•, out with ray parameterfromthoseof MohoPpPs and
1986]andthegravity
data.Heinterpreted
theoriginof PpS.s+PsPssothat their energywill not stackedcoof these
thisphase
asa conversion
fromthebottomofa middle herentlyin s(H,n). However,the presence
crustallow-velocity
zone.Giventhe largeamplitudeof phaseoftensmearsthe s(H, •) maximumand some-
thisphase
andabsence
ofothercomparable
amplitude times causesother local maxima. In the caseof multiple
phases,
webelieve
that the Mohooriginprovides
the peaksin s(H, •), informationon the crustalthickness
and k•o/V•'ratio fromnearbystationsor othersources
simplestinterpretation.
Station PFO is alsoamongthe early TERRAscope can help to resolvethe ambiguity.
Altogether,weobtained65 Vp/1/•ratiomeasurements
stationsand hasa total of 294 receiverfunctions.Crustal
and
71 crustal thickness measurements from a total
thicknessunder the station is estimatedto be 29.4 km
witha crustalV•/V, ratioof 1.76(Figure5a). There-
of 84 stations. The other 13 stations have very com-
so that their receiverfunctions
ceiverfunctionprofilealsoshowsclearMohoPs phase plicatedsite responses
in the shallow
(Figure
5b). Bakeret al. [1996]
modeled
thePFOre- are overwhelmedby P-to-S conversions
ceiverfunctionsin detail and concludedthat.the Moho
crust.
Most of these stations are located in sedimen-
underthis stationis very complicated,with possible tary basinswhere the high-velocitycontrastbetween
abruptMohotopography
orstepoffsets
ofseveral
kilo-
the sediments and basement rocks and laterally varying
coming
fromtheNW withtheSE,it isevident
thatthe
mask the later Moho conversions. The final thickness
that
meters.Indeed,if we comparethe receiverfunctions basingeometrygeneratelargebasinreverberations
P s from the SE is about 0.3 s faster. The 29 km repre- and V•/V• ratio results,alongwith the corresponding
Moho Ps arrival times,are listedin Table 2. The crustal
sentsan averagecrustalthickness
nearthisstation.
Thereareotherapparentlycoherent
phases
afterthe V•/V• ratio rangesfrom 1.68to 1.87with the average
of 1.78. On average,the crustalthicknessof southern
couldbe generated
by P-to-S conversions
fromsome California is 30 km. However, there is a wide range of
Moho Ps in the abovereceiverfunctionprofiles.They
uppermantle
discontinuities,
ortheymightbethemul- values from 21 to 37 kin.
Mohodepthundereachstationwasobtainedby subtiplesof intracrustal
conversions,
as demonstrated
in
tracting
the stationelevationfrom the crustalthickFigure2. In principle,
thesephases
havedifferent
move-
2976
ZHU
1.85
AND
KANAMORI-
/vlOHO
-
DEPTH
VARIATION
_
-
1.80
1.75
-
1.70
1.65
IN SOUTHERN
CALIFORNIA
grid spacing of ,-•50 kin. The average Moho depth is
29 km, varying from 21 to 37 krrl. The Moho is found
to be deeper under the eastern Transverse Range, the
Peninsular Range, and southern Sierra Nevada, while
shallowerMoho exist under the Salton Trough, the offshoreregion, and the Los AngelesBasin. The Moho in
the western and central Mojave Desert is relatively fiat
-
1.60
15
20
25
30
35
40
H (km)
1.85
-
1.80
-
1.75Ps
0.08
1.70
-
1.65
-
1.60
15
25
20
30
35
40
H (km)
0.07
Ps
0.08
0.06
0.07
0.05
0.06
0.04
0
5
10
15
2o
0.05
t(s)
Figure 4. (a) The ,(H, •) for stationPAS.The best
estimate
of thecrustalthickness
is 28kmwitha Vp/¬
ratio of 1.73. The 1;r uncertaintiesare givenby the
ellipse.(b) Receiverfunctionprofileand the predicted
arrivaltimesof Mohoconvertedphasesby the estimated
0.04
crustalthickness
and Vp/V• ratio.
I
0
5
10
15
20
t (s)
hess. The resultswere then combinedusing kriging to
produce a continuous Moho depth variation of south-
Figure 5. (a) The s(H, re)for stationPFO. The best
of thecrustthickness
is 29.4km with a Vp/V•
ern California (Plate 1). It can be seenthat except estimate
ratio of 1.76. The l•r uncertaintiesare given by the
for the western TransverseRange and the Great Val- ellipse.(b) Receiverfunctionprofileand the predicted
ley, the whole region has been sampled fairly well by arrival timesof Moho convertedphasesby the estimated
the receiver function measurementswith an average crustalthickness
and Vp/V• ratio.
ZHU
AND
[xANAlXiORI-MOHO
DEPTH
VARIATION
IN SOUTttERN
CALIFORNIA
2977
37 ø =
36 ø
30
36
•
35 ø
29
,
\
,...
30
-'""
o.
29
28
2
"
29
3'1
29
28
'--'% 32 30 31 3
,•
•
34 ø
•
2
31
33
29
32283
0___- '• --
1"
29
37
35
•'
.323O
%,?
34,,,,,.
-.._.--
•
323..9,,
32
ß
2
34
33 ø
% , 34
32 ø ß
_121 ø
z
,,
ß
-120 ø
20
-119 ø
25
-118 ø
30
-117 ø
35
-116 ø
-115 ø
_114 ø
40
Moho Depth (km)
Plate 1. Moho depth variation in southern California. Numbers represent the Moho depth
in kilometersat each station. They are combinedusingkriging to generatea continuousMoho
depth map. Crossesare stations where the Moho depth could not be determined, due to either
few number of records or complicated site response.
2978
ZHU
AND
KANAMORI:
MOHO
DEPTH
VARIATION
around the averagedepth and becomesshallowerto the
IN SOUTHERN
CALIFORNIA
37"
..
east under the EasternCaliforniaShearZone (ECSZ).
The
is no crustal
root
found
under
the central
Trans-
verseRange where the Moho is only 1 to 2 km deeper
than the average.
Discussion
and
Conclusions
36 ø
35'
As we have shown above, the largest uncertainty of
crustal
thickness
estimation
from
teleseismic
Moho
P-
to-• conversions
is associated
with crustal•/•,/V• ratio.
Unfortunately,crustal P•,/P• ratio is amongthe lea,
st
constrainedparameters from both laboratory and field
measurements. It is thought that the average composition
of the
continental
crust
is close to andesire
34 ø
33 ø
or
diorite [Andersoz•,1989]. Laboratorymeasurements
of
the V•/¬ ratio of dioriteat crustalpressures
rangefrom
1.75 to 1.79 [Carmichael,1982]. Althoughthere have
32'
21"
-120 ø
-119"
-118"
-117"
-116"
-115"
-114"
been many P wave velocity measurements in southern
Figure 6. The estimatedcrustalVp/V• ratios(circles)
California[e.g.,Ka•amori a•d Hadley, 1975;Magistrale
et al., 1992],crustalV• measurements
are very limited.
Hauksso•a•d Haase[1997]invertedsimultaneously
local P and S wave travel time data for the Los Angeles
area. Their averageVp/V• ratio overthe wholeareais
1.75. Using Ps and PpPs arrival times on receiverfunctions,Zaz•dtaz•dAmmoz•[1995]estimatedPoisson's
ratios of different types of continental crust. The global
averageis 0.27whichcorresponds
to V•/•/; ratio of 1.78.
for broadband stations. Their uncertainties are represented by the sizes of the crosses.
CORP) reflectionprofilesconductedin westernMojave
showeda fiat Moho at ,-031km (two-waytravel time,
or TWTT, of 9.8 s) in the north of the surveyand 26
to 29 km in the south [Cheadleet al., 1986]. A reflection profile acrossthe northern slope of San Bernardino
Mountainsshoweda dipping Moho from 31 km (9.8 s
lowerratio (1.732) with large variations.In our study TWTT) in the north to 33 km (10.4 s TWTT) to the
the average•/•/V• ratio overall stationsis 1.78,which south[Li et al., 1992],whichis consistentwith our deepis closeto their global average. We believethat these ening Moho underneath the eastern TransverseRange.
measurementsfrom directly stackingreceiverfunctions
Richards-Diz•ger
az•d•5'hearer
[1997]estimatedcrustal
For the Mesozoic and Cenozoic belts they obtained a
are more
robust than the estimates derived from Ps
thicknessvariation in southern California by stacking
and PpPs arrival times. The later phase, which has Prop arrivals recorded by the Caltech-USGS shorta longerpath through the crust than the primary con- period network. The overall patterns in their result and
version and has one extra reflection on the surface, is ours are quite similar, despite the fact that these two
sensitive to lateral structural variations such as a dip- results are from completely different data sets and techping Moho or surfacetopography.For example,a Moho niques.The Prop techniquerelieson pickingthe Prop
dipping 50 can delay or advancethe PpPs arrival by 2 arrivals correctly and using a backgroundP velocity
to 3 s dependingon updip or downdip propagationof model. The Moho depth trades off with lower crustal
the incoming wave. Our stacking algorithm using re- P velocity. On the other hand, the receiverfunction re-
ceiver functions
from different
directions
and distances
helps to reduce the effect of lateral variation.
sultsmainlydependon the averagecrustalVp/V• ratio.
The good agreementof these two resultssuggeststhat
The estimatedVp/V• ratiosvary from 1.68 to 1.87, our estimatesof crustal Vp/V• ratio are appropriate.
and the spatial variation is coherentin general. Stations Our result coversa larger area than the Prop study,
in the mountain rangeswith Mesozoicbasementtend including the offshoreInner California Borderland and
to havehigher ratios comparedwith the stationsin the the Peninsular Ranges. There are some differencesbe-
MojaveDesert,exceptalongthe ECSZwherethe Vp/V•
ratio increases(Figure 6). There are two placeswhere
the Vp/V• ratio changesrapidly over a short distance
(stationsOSI versusBTP and GR2 versusCALB; see
Table .2 and Figure 6). This couldbe causedby shortwavelengthvariation of crustal P and S velocities. It
might alsobe an artifact due to the uncertaintyof the
Vp/V• estimation.
Our Moho depths agree with previous results from
tween the two. For example, the Prop data showed a
very thin crust (18 km) in the SaltonTrough,compared
with
the 22 km crust
in our result.
The
thick
crust
(33 km) in the Prop resultnearthe easternTransverse
Rangesextendsto the northwestinto the Mojave Block,
whileour resultshowsa deeperMoho (37 km) right under the San Bernardino
Mountains.
Our result also re-
seismicreflection experiments. For example, a seriesof
vealsa deep Moho beneath the PeninsularRange where
no Moho Prop signalswere detected to determine the
Moho depth. Deep Moho beneaththe PeninsularRange
Consortiumfor ContinentalReflectionProfiling (CO-
is supportedby a seismicrefractionstudy [Nava az•d
ZHU AND KANAMORI:
MOHO
DEPTH
VARIATION
IN SOUTHERN
CALIFORNIA
2979
One of the important issuesin regional seismic toBrune, 1982], and teleseismicreceiverfunction stud•nographystudiesis to separatecrustal thicknessvariies [Ichinoseet al., 1996;Lewiset al., 1999].
We found a thin crust (22 to 25 km) in the Salton ations from crustal-upper mantle velocity variations.
Trough (Plate 1). This sea level troughis the north- Most travel time topography work ignoredthe crustal
ern extension of the Baja California rift zone where
the crust is believed to have been thinned due to upper mantle magma intrusion and crustal rifting pro-
thickness vari•½tion m'•d used a fiat Moho in the back-
groundvelocitymodels[e.g., Humphce'.qs
and
1990; Zhao and Ka•ctmori, 1992; Zhao et al., 1996;
cesseswith the openingof the Gulf of California [e.g., Haukssonand Haase, 1997]. With the t.eleseismicreParsons and McCarthy, 1996]. Previous results of ceiver function technique, an additional constraint on
crustal thickness in the Salton Trough range from 16
t,o 19 km [Hadley, 1978;Richards-Dingerand Shearer,
1997],21 to 22 km [Hearn, 1984;Magistraleet al., 1992;
Parsonsand McUarthy, 1996],and to 25 km [•'ungand
Jackson,1992]. We do not havestationslocatedalong
the axis of the trough where the crust is expectedto be
the thinnest. Thus the 22 km Moho depth might only
apply to the western edge of the trough.
The thinnest
crust in our result is offshore under the
Inner California Borderland (Plate 1). The Moho is essentially flat at 21 to 22 km over •he whole area. The
transition to 30 km deep Moho in the TransverseRange
occurs rapidly near the coast,line in Santa,Monica a,nd
beneath the Los Angeles Basin. During the LARSE
1994 experiment, both offshore-onshoreseismic refrac-
tion surveyand oceanbottom seismometer(OBS) survey were conducted along two profiles acrossthe Inner California Borderland. The OBS survey recorded
large amplitude Moho Prop reflections that indicate
the crustal structure can be set, either in the form of laterally varying Moho depth from stackingreceiverfunctions or the time delay of P s measured on a receiver
function profile. The latter representsthe travel time
difference between P and S waves within
the crust for a
nearly vertical ray path and is very sensitiveto crustal
thickness. It can be inverted jointly with other travel
time data from local and teleseismic earthquakes for
both velocity and Moho dept,h variations.
In summary, we found that the receiver timtrion technique is an effectiveway of determining Moho depth and
crustalVp/V, ratio. It can providea goodpoint,measurement
under
a broadband
station
and is not sensi-
tive to crustal P velocity. Crustal thicknessestimated
only from the time delay of Moho P s phase trades off
stronglywith crustalVp/V, ratio. The ambiguitycanbe
reducedsignificantlyby incorporatingthe later multiple
converted phases. Applying a new stacking technique
to 84 digital broadband stations in southern California,
a uniform crust of thickness18 to 20 km [ten Brink showsthat the Moho depth is 29 km on average and
et al., 1995]. The refractionstudy showeda layer of varies from 21 to 37 kin. Deeper Mohos are found under
7.4 km/s P velocitywith the bottom interfacedipping the eastern Transverse Range, the Peninsular Range,
from 15 km offshore in the south end of the profile and the Sierra Nevada Range. The central Transverse
to about 30 km inland near the northern Los Angeles Range does not have a crustal root. Thin crusts ex-
Basin [Noris and Ulayton, 1997].
ist in the Inner CaliforniaBorderland(21-22 kin) and
A deepcrustalroot (40 km) underthe San Gabriel the Salton Trough (22 kin). The Moho is relatively
Mountains was inferred from teleseismic P arrival times
flat at the average depth in the western and central
alongthe LARSE profile [Ifohler and Davis, 1997]. In Mojave Desert and becomes shallower to the east uncontrast, our result does not show such a deep Moho.
Moho under the central Transverse Range is relatively
der the ECSZ. Southern California crust has an average
Vp/V, ratio of 1.78, with higherratiosof 1.8 to 1.85 in
shallow(29-30 km) comparedwith the easternTrans- the Mesozoiccoredntountain rangesand lower ratios in
verseRange(37 km) andthe westernTransverse
Range the MojaveBlockexceptfor the ECSZ wherethe Vp/V,
(gl km)(see Plate 1). This configurationis consis- ratio increases.
tent with the lack of Bougueranomaly over the central
and westernTransverseRanges. Sheffelsand McNutt
[1986]suggested
that the Transverse
Rangesare regionally compensatedby a very stiff, elastic plate instead of
a low-density crustal root. Our result is also consistent with the Prop study and the Pn travel time inversionresultsfor this area [Richards-Dinger
and Shearer,
1997; Hearn, 1984; Sungand Jackson,1992]. Teleseismic travel time inversion for crustal thickness variation
Acknowledgment.
We are grateful to the staff at the
SCEC data center and IRIS DMC, who helped retrieving the waveform data. Chuck Ammon provided the teleseismic waveform of LAC. Comments from G. Zandt, C.
Thurber, and an anonymous reviewer have greatly improve
the manuscript. This research was supported by the Southern California Earthquake Center. SCEC is funded by NSF
Cooperative Agreement EAR-8920136 and USGS Cooperative Agreements 14-08-0001-A0899 and 1434-HQ-97AG01718.
This is SCEC contribution 464 and Division of Geological
and Planetary Sciences,Caltech, contribution 8621.
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becauseof steepray path
throughthe upper mantle and crust wherethe P velocity could changerapidly. So it is possiblethat a large
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MOHO DEPTH VARIATION
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(Received March 23, 1999; revised September 6, 1999;
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