Bulletin of the Seismological Society of America, Vol. 86, No. 1B, pp. S126-S137, February 1996

Basin Structure Effects on Long-Period Strong Motions in the San Fernando
Valley and the Los Angeles Basin from the 1994 Northridge Earthquake
and an Aftershockb y A r b e n Pitarka a n d K o j i r o Irikura

Abstract

The strong ground motions recorded within the San Fernando Valley
and in the Los Angeles Basin during the 17 January 1994 Northridge earthquake
show complex waveforms and complicated patterns of peak acceleration and velocity. This complexity persists even at long periods (1 to 10 sec). We investigate basin
structure effects on long-period wave propagation in the San Fernando Valley andin the Los Angeles Basin by the two-dimensional finite-difference modeling of themainshock and one selected aftershock with M -- 4.1.
The aftershock ground motion records in the San Fernando Valley and in the Los
Angeles Basin can be explained by basin structure effects, assuming a simple point
source model. The structural effects and a source model composed of two subevents
can explain the most prominent characteristics of the strong-motion waveforms observed during the mainshock at sites south of the epicenter, while such a combination
is not enough to explain the large strong motions observed in the north. This result
suggests that the rupture propagation effects significantly amplified the ground motions at sites north of the epicenter.

Introduction
Strong motions of the 17 January 1994 Northridge (Mw= 6.7) earthquake and its aftershocks were recorded in awide area from the San Fernando Valley to the Los Angeles
Basin. This earthquake generated not only the largest acceleration ever recorded but also complicated patterns of peakacceleration and peak velocity at sites surrounding thesource area. Large peak velocities were observed at stations
located north of the epicenter, while high peak accelerations
were observed at stations located north and south of the epicenter. Large horizontal accelerations in phases following
the direct S wave by more than 4 sec were observed at somesites within the Los Angeles Basin. These patterns persist
even at low frequencies, less than 1 Hz. Such observations
imply that the local site conditions, the source process, andprobably the regional geological structure effects all have arole even in the low-frequency range. It is enlightening toanalyze such effects by simulating them separately.
First, we investigate basin structure effects in the San
Fernando Valley and in the Los Angeles Basin by modelingstrong motions recorded during an aftershock with magnitude 4.1, which was located close to the mainshock. We
calculate synthetic seismograms from this event using the2D finite-difference technique proposed by Vidale et al.
(1985) and assuming a single point source. The basin velocity model we adopted is derived from a previous study (Vi-dale and Helmberger, 1988), and the structure model below
the basins is derived from a 3D tomographic velocity modelof the Northridge and Los Angeles areas (Zhao and Kanamori, 1995).
Next, we apply the same approach for modeling strong
motions observed during the mainshock. We model the seismic source with two point sources separated in time and inspace. This model was derived from previous source process
studies, which showed that the faulting process was dominated by two subevents (e.g., Wald and Heaton, 1994).
Based on the simulated waveforms at sites in a linear arrayfrom the San Fernando Valley to the Los Angeles Basin, thecontribution to the long-period waveforms from both effectsof local geological structure and the source is analyzed.
Aftershock Simulation
We have simulated the wave propagation in the San
Fernando Valley and in the Los Angeles Basin, recorded
during the aftershock of 21 January 1994, 18:52:44 (M =4.1). The epicenter of this aftershock was located at 34.30 °N, 118.44 ° W at a depth of 11 km. The focal mechanism
determined by the P-wave first-motion data and the moment
tensor inversion method is strike, - 6 9 ; dip, 44; slip, 70°;
and the seismic moment, Mo = 2.4 × 1022 dyne-cm (D.

$126

S 127

Basin Structure Effects on Long-Period Strong Motions from the 1994 Northridge Earthquake and an Aflershock

Zhao, personal comm., 1994). This aftershock was very wellrecorded at seven stations deployed roughly in a line, passing
through the San Fernando Valley'and northern part of theLos Angeles Basin. Five of the stations, installed by SCEC
(Southern California Earthquake Center), are broadband
STS2 instruments, and two of them are forced-balanced accelerometers. Their coordinates are listed in Table 1, and thelocations are shown by solid triangles in Figure 1.
By modeling strong ground motions recorded during
this aftershock, we intend first to check the validity of thetwo-dimensional velocity model adopted in this study andsecondly to investigate basin effects on long-period wave
propagation. Because of the simple source process, thewaveforms of this aftershock are affected mainly by patheffects. We expect that wave path effects will be similar forthe aftershock and the mainshock because their hypocenters
are very close to each other.

Table 1
Strong-Motions Station Location Used in
the Aftershock Simulation
StationLatitudeLongitude

BEAR

34.358- 118.396

FIRE

34.309- 118.446

SFYP

34.237- 118.439

NHFS

34.199- 118.398

LA03

34.090- 118.339

WVES

34.005- 118.279

KLVC

33.835- 118.159

24t" 00'
34"30'

241" 30'242" 00'
34"30'

Velocity Model and Numerical Simulations
The velocity structure (Fig. 2a and Table 2) along the
profile passing through the San Fernando Valley and the Los
Angeles Basin (which we adopted) is similar to the one proposed by Vidale and Helmberger (1988). Their basins modelis based on the stratigraphy, the velocities, and the densities
found by Duke et al. (1971), while the velocities below thebasins were taken from Kanamori and Hadley. Using sucha model, they explained the main characteristics of the strong
ground motions recorded within the San Fernando Valley
and the Los Angeles Basin during the 1971 San Fernando
earthquake. For the structure below the basins and the upper
crust, we adopted the model recently proposed by Zhao andKanamori (1995) based on tomography P-wave travel-timeinversions (Fig. 2b). The S-wave velocities are computed
assuming Vp/Vs = 3 v2, where Vp and Vs are P- and S-wave
velocities, respectively.
For calculating velocity seismograms from a point
source, we use the technique proposed by Vidale et al.
(1985) and Helmberger and Vidale (1988). The SH waves
are calculated using an elastic finite-difference method(FDM) scheme of second-order accuracy in space (Boore,
1972; Zahradnik and Urban, 1984), and the P-SV waves arecalculated using an elastic staggered FDM scheme of fourthorder accuracy (Levander, 1988). Absorbing boundary conditions proposed by Stacey (1988) are applied at all boundaries, except the surface where free-surface-stress conditions
are imposed. The source box technique proposed by Alterman and Karal (1968) was applied for introducing the seismic energy into the grid.
We simulated the velocity waveforms at frequencies
only up to 1 Hz, because FDM is expensive when applied tomodeling wave propagation in huge realistic structures. In
this study, one reason that the above frequency limitation isjustified is that there is not enough detailed knowledge on
the geological structure in and around the Los Angeles Basinand in the San Fernando Valley to have confidence in higherfrequency simulations. The second reason is that by mod-34" 00'34" 00'241"00'241"30'242' 00'

Figure 1. Map of the San Fernando Valley and theLos Angeles regions. The epicenters of the mainshockand the aftershock are marked by filled stars. The
filled triangles are the locations of the strong-motion
stations used in the aftershock simulation, and thedotted lines represent the basin boundaries. The twodimensional velocity model used in the aftershock
simulation corresponds to the cross section along the
profile A-A'.

eling aftershock ground motions, we intend to evaluate basin
structure effects at low frequencies. This is helpful for distinguishing between the path effects and the source effects
observed during the mainshock.
First, we have tested the attenuation effects due to thebasin soft sediments by comparing point source SH seismograms obtained by using both elastic and anelastic FDM
schemes. The technique proposed by Emmerich and Korn
(1987) was applied for considering the anelastic attenuationof soils. This technique makes possible a correct inclusionof the attenuation into time domain calculations. But it has
the disadvantage of requiring much more memory and computations than the elastic scheme.
We adopted a constant quality factor Q = 50 for thebasin sediments. Both the elastic and the anelastic scheme
were applied for calculating synthetic point source seismo-S 128

A. Pitarka and K. Irikura

|
I
SE
v

T

Y

~r

Y

lr

I

tO

H = l l Km

34.40 N- 118.50W

33.85 NSan Femando Valley

Los Angeles Basin- 118.07W

iiiiiiiiiiii:::::iiiiiiiiiiii::::::i~i~iii::iiiii:iiiii::iiiii:!iiiiiiiiiiiiii::i:iiii• ...-*°°-.*--......
° c e i l...................

• , .....

eoe~

°o

,000 .....

* • Iciclego.°°,,o*l,,°.........................

,,,,,,,oooeoe.e......e,,,..........................

leeeeleeeeeoeoeee6eeeeeeeeoee0e

e e e e e e , o e e ° ~o ...................

;;;;;;;;;;*leeooeoeeeoeoeooooaelee[]

Vp < 5.45 km/sec

L..I

5.45 < Vp < 5.6......

•" ' l.l .e1 1 1 1 1

I: : : ; : ; l l l l l l l :......
1 : 1 = : = : ~ : = 1 1 : 1 : 1 1 1 1 1 , "lllll:lll:" °: : : : : : : : : : : : : : : : : : : : : : : : : : :ee -- oeoell
Q,, eeooolooeeee*~*e eeelaoeeoeeeeoeeeeeeeeee,°*oao..
eeee~eeeeeoeeeeeooeeeooeloee
ooee
~**~e
I o ee e e o e 8 o . o . Q ~ e o e e e e e o ooeoeteeeeeoeeleeeeee~oeeole~
to~lol~leee~eeeeeeeleeeeoeee
eee • eee41 ,4,,oeeeeoeeeelee,ele~lle~eee~e~ele~eeoeeeeleeeee,,, eoee~elle~eeo~oloeeeeae°,~ oeoeoeeeeeeeoeoeeoeeeeeoooeoeeeeoeeeeeeeeeoleeeeeooeeeooeeeeeeeeeeeeeeeeioeoe
eee~eeeeo• • ~eeeeeo,, eeoeggee~eeooooo~oee• eeeeee~ole~eee~eeleeeeeeeee~oleee~e~eeeeee~e~eeeoeei

m

•eeoe

5.60 < Vp < 6.20

• eel•"::":1111,=================================================================================~eeee~ee~e~eoeo~oe~oee~e~e~ee~e~oe~e~ee~eeeeeee~e~e~eeee~eee~eeeee~eeeeeeeee~e~eeeeeeeeeeeeeeeeeeeeeeee~e~ee~eee~eeee~e~eeeee~ee~e~eeeeeeo~eeeee~eeeeeeeee~ee~~e~oee~e~eoe~o~ee~e~e~eoe~eeeeeeeeeeeeeee~ee~eeee~oeee~eeeeeeee~eee~ee~eeeeeee~eee

[ ]

[]
[]
!

10km

6.20 < Vp < 6.40
6.40 < Vp < 6.70
Vp > 6.70

J

No vertical exaggeration

Figure 2. (a) Upper part of the structural cross section of the profile A-A' and thestation locations. The stratigraphy and the velocity model are taken from Vidale andHelmberger (1988). (b) P-wave velocity structure along profile A-A' derived from the3D velocity structure proposed by Zhao and Kanamori (1995). The velocity intervals
are arbitrarily chosen.

Table 2
Velocity Structure for the Cross-Section Shown in Figure 2
P-Wave Velocity

S-Wave Velocity

Density(km/sec)(kin/see)( g i n / e r a 3)

1.4
2.0
2.5
3.1
4.3

0.6
1.1
1.4
1.8
2.5

1.7
1.8
1.9
2.1
2.3

grams at stations shown in Figure 1. The bandpass-filteredsynthetics are shown in Figure 3.
The attenuation effect is negligible at sites close to theedges of the basins, while it is significant at sites inside theLos Angeles Basin. Love waves generated at the northernedge of the basin are attenuated while propagating laterally
within the soft surface layers. It is important to notice thatthe amplitude of the body waves one time reflected from thesurface is not affected as much, probably because of theirrelatively short propagation path within the sediments. Despite such effects, for economical reasons, we have performed only elastic finite-difference modeling. The attenu-

at, on effects found in this section will be considered in our
final interpretations.
The Aftershock Simulation
The simulation of the strong ground motion from theaftershock is shown in Figure 4. We have used a bell-shapedsource time function with duration To = 0.6 sec and a seismic moment o f M 0 = 1.8 × 1024dyne-cm, which is slightly
smaller from that found by Zhao (personal comm., 1994).
The three components of the simulated velocity seismograms are compared with those of the recorded ones.
The agreement between the recorded and the synthetic
waveforms is in general good. The simulation reproducessome of the most noticeable features of the recorded groundmotions. The comparison between the synthetic and the observed seismograms suggests that the surface waves generated at the basin edges and the multiple surface layer reflections gave the main contribution to the observed waveforms.
Their possible constructive interference might be the reason
for the large amplitude of later phases observed at sites inthe Los Angeles Basin and at NHFS in the San Fernando
Valley (see Fig. 4).
This simulation raises two problems that concern theBasin Structure Effects on Long-Period Strong Motions from the 1994 Northridge Earthquake and an Aftershock

S 129

BAND PASS FILTERED 0.05_1.0 Hz
VELOCITY
Q--.soWithoutAUenuationk~_- _ ~

~

/

~

0.010
0.027

0.010
0.026

0.022

0,022

0.017

0.012

0.047
0.082

n ~

o.o4s

S

~

0.047
0.072

nl'ffs_

0.045
0.062

~

0.071
la03

oo,,
0.049

~es

_

~

" _
,

klvc

,

~

I"
I

I

I

I

mOO

1~

24~Q

Time(sec)

I
~

,,03

0.019
0.016

,l~/es

0.011
0.008
CM,~

klvc

o.o,2°°1'
,

~

0.019
0.014

_j~/~~

0.011
0.008

yw
-~a~,~.~A/~-v~

T

I

I

I

I

I

I

I

40.~

0.130

6,00

ldl,O024.110*?0.*~1

40.00

Time(sec)

Figure 3. Transverse SH component of the recorded and the simulated point source
velocity seismograms for the aftershock of 21 January 1994. Left: elastic scheme.
Right: anelastic scheme, Q = 50. The source time function is a bell-shaped function
with time duration TO = 0.8 sec. The moment scaling and the focal mechanism is
discussed in the text. The heavy traces show the data, with the station name and theepicentral distance to the left and the amplitude to the right. The light traces show thesynthetic seismograms.

velocity model. First, even though the attenuation is nottaken into consideration, the amplitudes of later phases coming after the direct S wave in the synthetics at the SFYP
station are relatively small compared with those of the recorded ones, in both radial and transverse components. They
are mainly secondary surface waves and surface layer multiples that are trapped within the soft layers (e.g., Aki and
Lamer, 1970; Bard and Bouchon, 1980a, 1980b, 1985; Ohtsuki and Harumi, 1983; Zahradnik and Hron, 1987; Kawase
and Aki, 1989). The amplitudes of these waves depend on
the geometrical relation between the hypocenter and the basin edge, i.e., the radiation pattern effects from the source,
the velocity contrast between the soft basin sediments andthe hard rock, and the basin shape (Bard and Bouchon,
1980a, 1980b, 1985; Gaffet and Bouchon, 1991; Yamanaka
et al., 1989). An inclined interface between the basin bed-rock and the sediments is favorable to multiples with a larger
amplitude than direct wave (Helmberger and Vidale, 1988;
Frankel and Vidale, 1992; Scott et al., 1994). A smaller
slope of the northern edge of the San Fernando Valley, compared with our model, might create multiples with a larger
amplitude. Other possibilities, for explaining the large amplitude of later phases in the record at SFYP, are the focusingof the seismic energy inside the valley because of the threedimensional basin structure effects and the resonant coupling
to Rayleigh modes due to the heterogeneity within the basin(Levander and Hill, 1985).
Second, on the synthetic radial component at station
LAO3, the amplitude of the later phases is much larger thanin the recordings, while in the transverse component, thesituation is reversed. As clearly seen in Figure 4, the later
phases are mostly Rayleigh waves generated in the San Fer-

S130

A. Pitarka and K. Irikura

BAND PASS FILTERED 0.05_ 1.0 Hz
VELOCITY

0,018_-_:_--~.-.,,,,.v.v-0.040
0.042

~
5.

17.

~
~

0.016

__~: 0.043

" ~ ~

-

'

:"'"---

~

~

0.175
0.051

0.033

i ~ 4 ~ = ~

0.005

0.015
0.014

55. _ _ ~ ~ I ~0.009~ ~ W ~ L
I

I

I

0.00 I1,00 11.00 24.00 ~Time(sec)

I

,IGgO

_~ _

f

~

0.027

~

0.022
0.017

S~ ~

/

0.047

~

0.082

0.071
"03

oo,,

0.049

~

0.024

~

0.007
0.008

wves........~

0,003
0.004

klvc

~

0.007
¢M~

R
I

0.010

0.041

0,018

3~

_

0.040

0.026
0,042

0.067

~

0.027

2: oo23

I

0.003

_ _ ~
~

I
&QO

I
16.00

I
24.00

~

0.011
0.008

T

U-D
I
O*O0

o.o19
0.016

Time(sec)I.~2.~1

I
40~0

I

I
8~0

I
t&O0

I
24~0

Time(sec)

I
32.00

I
40.00

Figure 4. Comparison of filtered velocity recordings with finite-difference velocity
seismograms of the 21 January 1994 aftershock, computed for the 2D velocity structure
shown in Figure 2. The heavy traces show the recorded seismograms, with the station
name and the epicentral distance to the left and the amplitude to the right. The lighttraces show the synthetic seismograms.

nando Valley, which tunnel across the Santa Monica Mountains. Their amplitude is large compared with the recordings.
Such a discrepancy was already observed by Vidale andHelmberger (1988) in their simulation of strong ground motion from the 1971 San Fernando earthquake. They concluded that the three-dimensional effects are at the origin of
this problem; the Rayleigh waves generated in San Fernando
Valley are refracted into the slower material in the center ofthe Los Angeles Basin, and their amplitude at sites aroundstation LAO3 might be small. This conclusion is consistent
with our simulation; on the synthetic seismograms at stations
WVES and KLVC, located at the central part of the basin, theamplitude of the later phases is slightly smaller comparedwith the recordings.
In Figure 5, we show the peak velocity comparison between the synthetics and the 0.05 to 1.0 Hz bandpass-filtered
recordings. The agreement is good, except for the radial
component of station SFYP, which is located close to theepicenter. A possible reason for this misfit is that the syn-thetics at such close stations are very sensitive to the accuracy of the epicentral location. Another reason might be thatthe finite-difference technique we use does not handle thenear-field terms properly at small horizontal distances. Also,
the station location very close to the northern edge of thesurface basin layer might have affected the abnormal amplification.
It is important to notice themost prominent effects ofthe two-dimensional velocity structure on the synthetic
waveforms shown in Figure 6. In the San Fernando Valley,surface waves generated at the two edges are trapped within
the basin. A part of the Rayleigh waves are converted into
body waves propagating inside the Santa Monica Mountains
and reconverted into surface waves in the Los Angeles Basin. Within the San Fernando Valley, at sites south of theepicenter, the constructive interference between the surface
waves and the surface multiples increases the amplitude oflater phases.
In the Los Angeles Basin, the phases coming after theBasin Structure Effects on Long-Period Strong Motions from the 1994 Northridge Earthquake and an Afiershock

S 131

a. TRAN~ ;VERSE C O M P O N E N T
0.20

I

,

,

I

,

'

I

I

-|

,

!

• R~cord~d• S~tnthetic

0.15

-I

Y,

:~ O.lO
_o

-

i
~

0.o5

i •!.

"

.............. i::................ i -" - ~ " ~ ~ i

O_

~,"
0.00

i

i

,

i

-15
NW

i
'

0

!

....... T-Z-"-T

i

"4.
=

i

i

.............. # . . . . . . . . . . ~ . . . . . .

i .
,

tO
|,~,

i
•

i

i
i

i

15
30
Epicentral Distance (kin)

i

!

,

45

60
SE

SE
.2

"

I\

"

lO ~
i

b. RADIAL C O M P O N E N T
,

f

0.20

,

I

,

T

•

j

iio

t

H=ll

Km

~Int-e.cTT........... i ........

............................................................

-~ 0.05

I• Recorded,,

0.15-'5 0.10_oI..............................

..............

÷ ..........

~. . . . . . . . . . . . . .

..........

I!.........................

!

j .............

~- ..........

-i--........

........

...........

4........

+ .........

...........

IX.

a-

0.00

,
-15

NW

i

.

'
0

,
15

.

i.
30

Epicentral Distance (km)i", " ' ~ ' ' *
45

|

60

SE

Figure 5. Peak velocity versus distance for theaftershock. The open triangles show the peak velocityof the filtered recordings, and the filled triangles show
the peak velocity of the finite-difference simulation:(a) transverse component; (b) radial component.

direct S wave dominate the waveforms. The first two later
phases arriving soon after the S waves are first and second
surface multiple reflections. The difference between their arrival time and that of the direct S waves is about 3 sec at
stations close to the northern edge and increases up to 5 sec
at stations toward the center of the basin. The later phases
are surface waves generated at the northern edge of the basin. At stations just above the northern edge of the basin,
Love waves have large peak amplitudes. Scott et aL (1994)
found similar results by calculating synthetic point source
S H seismograms and using a tomographic S velocity model.
Their comparison between the synthetic ground motion for
the Los Angeles Basin velocity model and that for its simplified flat layer model clearly shows that the increasing amplitude of the surface layer multiples with distance is causedby the dipping edge of the basin.
The relatively good fit between the synthetics and therecordings for the aftershock suggests that the adopted twodimensional velocity structure is useful for predicting lowfrequency local geology effects. The simulation shows thatfor an earthquake located in the Northridge region, the am-

!
NW

=

=

z

,

~

SE

Figure 6. (a) Transverse component of synthetic
velocity seismograms calculated across the San Fernando Valley and the Los Angeles Basin. The velocity
profile and the location of the stations used in thecomparison between the synthetics and the data areshown adjacent to the traces. (b) Radial componentof synthetic velocity seismograms calculated acrossthe San Fernando Valley and the Los Angeles Basin.
The velocity profile and the location of the stations
used in the comparison between the synthetics andthe data are shown adjacent to the traces.

plifications due to the basin structure are expected to be maximum at sites close to the edges of the San Fernando Valley
and along the northern edge of the Los Angeles Basin.
Main Shock Simulation
Source Parameters
Source modeling with empirical Green's functions andwaveform inversions (Li and Toksoz, 1994; Dreger, 1994;

S 132

A. Pitarka and K. Irikura

S U B E V E N T LOCATION

-10

E
v,

v

.c -15

-20
0

WNW

5

10

15

Distance along Strike (km)ESE

Figure 7. Subevent location in the fault plane. The
filled star shows the hypocenter location of the 1994
Northridge earthquake. Shadowed circles show thesubevent locations. The moment scaling is explainedin the text.

241' 00'241" 30'242" 00'

34"30'

34"30'of 15 km, which is close to that found by Thio and Kanamori(1994). Studies of the source process indicate that a single
fault plane ruptured (Wald and Heaton, 1994; Song et al.,1994). A dislocation model determined from strong motions
records near the hypocenter (Wald and Heaton, 1994) showsthat the biggest amount of slip is northwest of the hypocenterat depths between 15 and 18 km, which is consistent withthe subevents found by Thio and Kanamori (1994) from the
waveform inversion of teleseismic body waves. Their solution shows the first subevent at 19 km, the second subeventat 17 km, and the third smaller subevent at about 13 km.
They found a 2-sec time difference between the first two
subevents, while Li and Toksoz (1994) found 2.5 up to 3
sec.
An along strike subevent separation of about 8 km isconsistent with the observation that the two main first arrivals are separated in time by 4 to 5 sec at sites in the southand by less than 3 sec in the north. By assuming a constantrupture velocity of 2.7 km/sec, the time interval between thetwo events should be around 3 sec.
Based on the above information, we model the seismic
source by two point sources located at different depths, as
shown in Figure 7, and assume a 3.0-sec time interval between the two subevents. Their location corresponds to theasperities found by W a l d and Heaton (1994). Such a source
model does not include rupture propagation effects; conse-

Table 3
Strong Motion Stations Used in the Mainshock Simulation

34"0ff34" CO'

241"00'24I* 30'~s4~° tXI'

Figure 8. Map of Northridge and Los Angeles
regions. The epicenters of the mainshock and theaftershock are marked by filled stars. The filled triangles are the locations of the stations used in thesimulation, and the dotted lines represent the basinboundaries. The two-dimensional velocity models
used in the mainshock simulations correspond to thecross sections along the profiles A-A' and B-B'.

StationLatitudeLongitude

CSTF
NHAL
SHRF
SCCF
UCLG
LABA
LAll
ENCF
SMOF

34.56434.39034.15134.10034.06834.00933.92934.14934.011- 118.642- 118.530- 118.463- 118.478- 118.439- 118.361- 118.260- 118.512- 118.490v.

~~ = o ffi ~

~

v,

v ,.~

.

.

v

i

Song et al., 1994; W a l d and Heaton, 1994; Thio and Kanamori, 1994) show that the mainshock is composed of two
or more subevents and that the rupture propagated northward
and updip. Li and Toksoz (1994) found that the rupture
started with a relatively small subevent and released most of
the energy during the second subevent. In terms of energy
released, Song et al. (1994) found an average source depth~J)3( Second Event H=16 K m~( First Event H=18 K m

Figure 9. Upper part of the structural cross sectionof the profile A-A' (shown in Fig. 8) and the station
locations. The stratigraphy and the velocity model aretaken from Vidale and Helmberger (1988). The velocity model below the basins is shown in Figure 2b.

SES 133Basin Structure Effects on Long-Period Strong Motions from the 1994 Northridge Earthquake and an Afiershock

BAND PASS FILTERED 0.05_

1.0 HzVELOCITYl a l ~

~7o

41.

~

,.,,o

8.44

1&74

f~2

L75

!1.10

12.43

~

7.87

9.04

| 5.63

12.04

8.75

4.75

20.70

13.00

117.2912.411

28.72

11,87

4.07

6.67

R
I

I

I

4.73

U-D
I

Time(sec)

I

I

I

I

I

CM~

T

I

I

Time(sec)

I

I

I

I

I

I

I

Vime(sec)

Figure 10. Comparison of filtered velocity records with finite-difference velocity
seismograms of the 17 January 1994 Northridge earthquake, computed for the 2D
velocity structure shown in Figure 9. The heavy traces show the data, with the station
name and the epicentral distance to the left and the amplitude to the right. The lighttraces show the synthetic seismograms. The location of the two subevents is shown in
Figure 7. See text for source time function model.

quently, it is probably most applicable for explaining strong
motion at sites located south of the epicenter where the rupture propagation effect is not significant, as shown by previous studies (Wald and Heaton, 1994). We use the samebell-shaped source time function with a time duration of 1.2
sec and the same focal mechanism for both subevents. A
seismic moment ratio between the first and the second subevent of 0.5 and the focal mechanism found by Thio andKanamori (1994) ( 134/42/114 [strike/dip/slip]) were used in
the forward strong-motion modeling.
Numerical Results
The same technique as the one used for the aftershock
was applied for calculating velocity seismograms at stationsaligned along the A-A' profile (Fig. 8) where the mainshock
ground motions were recorded. We use strong-motion accelerograms from the California Division of Mines and Geology (Shakal et al., 1994), the U.S. Geological Survey (Porcelia et al., 1994), and the Los Angeles Department of Powerand Water (Wald, personal commun.) (see Table 3).

The recorded accelerograms were first integrated to velocity and then bandpass-filtered (0.05 to 10.0 sec). The upper part of the velocity structure corresponding to the A-A'
profile is shown in Figure 9. The result of the simulation isshown in Figure 10. Since there is no absolute timing forthe data, the match in timing is done arbitrarily following
the first clear pulse seen in the transverse component.
We find that the two point sources separated in space
and time and with the same focal mechanism can predict
most of the long-period ground motions recorded at sites
within the San Fernando Valley and in the Los Angeles Basin. The fit between the recordings and the synthetics is goodat almost all the stations to the south, especially at the rocksite, the SCCF station. This result proves the validity of oursource model. However, at two stations to the north, NHAL
and CSTF, there is a great difference not only in the waveforms but also in the peak amplitudes. At these stations, thepeak amplitude of the data is almost four times greater thanthe one obtained by the simulations (Fig. 11). Such differences cannot be explained by inaccuracies in the velocity

S 134model, because the aftershock simulation showed its validity
for evaluating site effects even at northern sites. We attributethem to the fact that, comparing with those to the south ofepicenter, the recorded motions to the north of the rupture
are more sensitive to the details of the source process. Waldand Heaton (1994) found a strong effect of the source radiation (radiation pattern and rupture directivity) at stations
located north of the hypocenter.
A seismic moment ratio of 0.5 between the first and thesecond event produced the best fit between the recordings
and the simulations. The total seismic moment needed to fit
the bandpass-filtered seismograms (0.05 to 1.0 Hz) is smaller
than the amount found for this earthquake by other researchers. This is because the velocity records used in the modeling
do not have much of a signal above a 3-sec period to constrain the long-period motions. The assumed source timefunction with duration 1.2 sec, adopted for fitting the velocity records at periods around 1 to 2 sec, is too short to replicate the real Northridge source, which probably lasted morethan 5 sec.
Our source model predicts the two distinct arrivals andthe large amplitudes of later phases observed at sites locatedsouth and southeast of the epicenter. It is interesting fromthe strong ground motion prediction point of view to clarify
the origin of the large-amplitude later phases recorded atstations in the Los Angeles Basin, close to the northern edge.
Li and Toksoz (1994) suggest that this may be caused by
the mainshock subevents having different focal mechanisms.
However, Thio and Kanamori (1994) found only slight differences between the focal mechanisms of the first two subevents of the mainshock. Without ruling out the source contribution, Wald and Heaton (1994) favor that this effectcomes from the site conditions since several aftershocks recorded along the northern edge of the Los Angeles Basinshow later arrivals as well.
Based on the above simulation, we attribute the waveform complexity observed at stations located north of theLos Angeles Basin (SMOF, UCLG, LABA) to a possible constrnctive contribution from both site effects and source processes. At these stations, the direct S wave corresponding to
the second biggest subevent arrives about 4 to 5 sec after
the direct S wave corresponding to the first subevent. Thisis close to the arrival time found for the first surface layer
multiple or surface wave associated to the first subevent. We
have considered separately the local site effects on the
ground motion from the first small subevent and the effectfrom the second subevent. In Figure 12, for stations southof the epicenter, we display the observed records (top trace)and the synthetic records (second trace) along with the separate contributions to the synthetic records from the first subevent (third trace) and the second subevent (bottom trace).
From this figure, it can be seen that at sites close to thenorthern edge of the Los Angeles Basin (e.g., LAI 1, LABA),a double-event source process combined with local site effects might cause large strong ground motions.
We performed another simulation along the profile B-

A. Pitarka and K. Irikura
a. T R A N S V E R S E C O M P O N E N T

'i
A

60

................................

ih,Rec0rded........................................ •...............................................................................................

E
v

o

i

40

k- ............... .k-...............................

..~.

................... ~-..-~-.~+-~-'---.iii............ .~-~ ..................... ~.................... ~.................... k ................... i ..................... ~.....................

..................
~....................
~..................~..................~--''-~.'~ ..................
!.................i ..................~..................~-................

-~ 20................ ~................... ~................... -.............. " 1 i .....................................

~

-40

-30

...................
-..................-~&.

t ................. ~.....................
~...............~ " ~ , - " ~ " ............~ "
!

!

i

I

t

-20

-10

0

10

I
-50

..

N

I

~"~;-"- ..............

i

i

I
20

30

40

Epicentral Distance (kin)50
SEb. RADIAL C O M P O N E N T
,

,

. .

,

,

,

,

,

~

i

~:lecOrdedi.... i---~60.................

E
v

o

i

,, ~ -%~~~ ............., 7~~.................. ~................... r . . - . . ~ . ~ . ! . . . . . . . . . . .
•~~~i ..................................
i

T ................. T ..............
.
.

40
o

..9o

......................
~.......................................
~ ........................................................

-~ 20.................. i .................... ~ ..................... i .................. ~~ ................................ ~ .................
~...................

....... ~;............... ~................ ~.

~ ...........

a.

0

I
-40

.50

N

I
-30

-20

I
-10

0

10

I
20

30

•

40

Epicentral Distance ( k m )50
SE

Figure 11. Peak velocity versus distance for themainshock. The open triangles show the peak velocityof the filtered data, and the filled triangles show thepeak velocity of the finite-difference simulation: (a)transverse component; (b) radial component.

B' shown in Figure 8. This profile passes through the Santa
Monica site (SMOF) where large-amplitude later phases wererecorded. Our intention was to find an explanation for the
long-period waveforms observed at SMOF.
We used the same source model as we did for the previous mainshock simulation. Again, the synthetics at thenorthem station (NHAL) have much smaller amplitudes ascompared with the observed records (see Fig. 13). The synthetic waveforms at SMOF show that the large-amplitude
later phases are mainly secondary surface waves generated
at the northern edge of the Los Angeles Basin. Their arrival
time predicted by the simple velocity model we use isslightly greater than the observed one. The generation of
those surface waves is probably controlled either by a threedimensional large-scale structure or by a shallow microbasin
structure with the northern edge close to SMOF.
A comparison between two- and three-dimensional
wave propagation simulations may better clarify the site effects at Santa Monica and the factors controlling the amplitude of the surface waves generated close to this site.

Basin Structure Effects on Long-Period Strong Motions from the 1994 Northridge Earthquake and an Afiershock

S 135

BAND PASS FILTERED 0.05_1.0 Hz
VELOCITY
6.70

1.80

5.96

6.44

2.50

5.32

13.74

6.75

12.73

7.67

~

~

11.90
9.04

17.30

7,42

13.50

15.63

12.04

15.97

•.~%~ ~
sccf

13.17

4.30

6.68

8.75

4.75

7.02

R
i

000

I

,

,

,

Time(sec)

0.00

5.00

1000

500 10'00 15'.00 20.00 25100

U-D

eB~Is

,

,

I

15.00

2000

25.00

000

Time(sec)

clltS

T

,

~00

10100 16100

mime(sec)

20100

'
25.00

Figure 12. Comparison of data (heavy top trace) and synthetics (second trace) with
contributions to the synthetics from second subevent (third trace) and first subevent(forth trace) for stations located along the profile A-A'.

BAND PASS FILTERED 0.05 _ 1.0 Hz
VELOCITY
smof

~

31.0

S

encf

~

24.8
13.4

~

16.1
15.5

4.3

17.4

7.7

21.6

10.8

6.1

5.3

^

.~]--,~~ . - , - ~ - ~ - ~ - ~ - ~

61.8

~

R
5.00
'

1o' 00

1~' 00Time(sec)

~

5.2

~

12.5
45.7

39.4

0.00
'

~

11.3

~' ~ 2 8 3

U-D
20.00
'
2500
i

0.00
'

5.00
'

10.00
'

15.00
'

Time(sec)

48,4
11.2

T

'.

2000

.'o

25 0

!

!

000

500 1000 1~'00 20.00 25.'00

Time(sec)

Figure 13. Comparison of filtered velocity data with finite-difference velocity seismograms of the 17 January 1994 Northridge earthquake, computed for the 2D velocity
structure corresponding to the profile B-B' shown in Figure 8. The heavy traces show
the data, with the station name and the epicentral distance to the left and the amplitude
to the right. The light traces show the synthetic seismograms. The source model usedin the simulation is discussed in the text.

S 136

A. Pitarka and K. Irikura

Conclusions

Two-dimensional finite-difference modeling, based ona heterogeneous velocity structure and a source model consisting of a double-couple point source, can reproduce themost prominent characteristics of the long-period (1 to 10sec) strong ground motions recorded in the southern part of
the San Fernando Valley and in the Los Angeles Basin during an aftershock of the January 1994 Northridge earthquake. We found that both the valley and the basin generatesecondary waves that dominate the later arrivals. The modeling of the mainshock using two double-couple point
sources separated in space and in time, with the same focal
mechanism and with a seismic moment ratio of 0.5, indicatessignificant basin effects that can be identified in the observed
strong ground motions.
At sites within the San Fernando Valley, the groundmotions have been significantly affected by the surface
waves excited at the edges of the valley. Basin effects andthe multiple event source process have contributed to theamplification of the second main arrival observed at sites
close to the northern edge of the Los Angeles Basin. A threedimensional or more accurate two-dimensional velocity
structure may better explain some small inconsistencies between the recordings and the synthetics at sites where local
effects seem to be more complex, such as in Santa Monica.
The large difference between the peak amplitudes of themainshock recordings and the synthetics at sites north of theepicenter, while a good fit between the synthetic and therecorded waveforms is obtained to the south, suggests thatat sites north of the epicenter, the rupture propagation effectdominates over site effects.
Acknowledgments
We would like to thank Adam Edelman for kindly providing us with thestrong-motion data of the aftershock; David Wald for providing us with theJanuary 1994 Northridge earthquake strong-motion data recorded at the
SHRF, ENCF, and SCCF stations; and Dapeng Zhao for preparing the Pwave velocity models of the upper crust. We are grateful to F. J. SanchezSesma, H. Takenaka, and D. Suetsugu for their valuable discussions. We
greatly appreciate the constructive review by Keith L. McLanghlin and an
anonymous reviewer. This work was in part supported by the Grant-in-Aid
for Scientific Research No. 05302069 from the Ministry of Education, Science and Culture, Japan, and the Kajima Foundation's Research Grant.

References
Aki, K. and K. L. Lamer (1970). Surface motion of a layered medium
having an irregular interface due to incident plane SH waves, J. Geophys. Res. 75, 933-954.
Alterman, Z. and F. C. Karal (1968). Propagation of elastic waves in layered
media by finite-difference methods, Bull Seism. Soc. Am. 58, 367398.
Bard, P. Y. and M. Bouchon (1980a). The seismic response of sedimentfilled valleys. Part 1. The case of incident SH waves, Bull. Seism. Soc.
Am. 70, 1263-1286.
Bard, P. Y. and M. Bouchon (1980b). The seismic response of sedimentfilled valleys. Part 2. The case of incident P and SV waves, Bull
Seism. Soc. Am. 70, 1921-1941.

Bard, P. Y. and M. Bouchon (1985). The two-dimensional resonance of
sediment-filled valleys, Bull Seism. Soc. Am. 75, 519-542.
Boore, D. M. (1972). Finite-difference methods for seismic waves, in Methods in Computational Physics, B. A. Bolt (Editor), Vol. 11, Academic
Press, New York, 1-37.
Dreger, D. S. (1994). Empirical Green's function study of the January 17,
1994 Northridge mainshock (Mw = 6.7), submitted to Geophys. Res.
Lett.
Duke, C. M., J. A. Johnson, Y. Kharraz, K. W. Campbell, and N. A. Malpiede (1971). Subsurface site conditions and geology in the San Fernando earthquake area, UCLA-ENG-7206, School of Engineering,
UCLA, Los Angeles, California.
Emmerich, H. and M. Korn (1987). Incorporation of attenuation into timedomain computations of seismic wavefields, Geophysics 52, 12521264.
Frankel A. and J. Vidale (1992). A three-dimensional simulation of seismic
waves in the Santa Clara Valley, California, from a Loma Prieta aftershock, Bull. Seism. Soc. Am. 82, 2045-2074.
Gaffet S. and M. Bouchon (1991). Source location and valley shape effects
on the P-SV near displacement field using boundary integral equationdiscrete wave number representation, Geophys. J. Int. 106, 341-355.
Helmberger, D. V. and J. E. Vidale (1988). Modeling strong motions produced by earthquakes with two-dimensional numerical codes, Bull
Seism. Soc. Am. 78, 109-121.
Kawase, H. and K. Aki (1989). A study on the response of a soft basin forincident S, P and Rayleigh waves with special reference to the long
duration observed in Mexico city, Bull Seism. Soc. Am. 79, 13611382.
Levander, A. R. (1988). Fourth-order finite-difference P-SV seismograms,
Geophysics 53, 1425-1436.
Levander, A. R. and N. R. Hill (1985). P-SV resonances in irregular lowvelocity surface layers, Bull. Seism. Soc. Am. 75, 847-864.
Li, Y. and M. N. Toksoz, (1994). Empirical Green's function analysis ofsource time functions for the mainshock and large aftershocks of theJanuary 17, 1994 Northridge earthquake sequence, 89th SSA Meeting, Pasadena, California, 1994.
Ohtsuki, A. and K. Harumi (1983). Effect of topographies and subsurface
inhomogeneities on seismic SV waves, Earthquake Eng. Struct. Dyn.
11, 444-461.
Porcella, R. L., E. C. Etheredge, R. P. Maley, and A. V. Acosta (1994).
Accelerograms recorded at USGS national strong-motion network
stations during the Ms = 6.6 Northridge, California earthquake ofJanuary 17, 1994, U.S.G.S. Open-File Report 94-141.
Scott, J. S., E. Hauksson, F. Vernon, and A. Edelman (1994). Los Angeles
basin ground motion estimation for aftershocks of the 1994 Northridge Earthquake from a tomographic velocity model, 89th SSA
Meeting, Pasadena, California, 1994.
Shakal, A., M. Huang, R. Darragh, T. Cao, R. Sherburne, P. Malhotra, C.
Cramer, R. Sydnor, V. Graizer, G. Maldonado, C. Peterson, and J.
Wampole (1994). CSMIP strong motion records from the Northridge,
California earthquake of 17 January, 1994. Report No. OSMS 94-07,
California Strong Motion Instrumentation Program.
Song, X., L. Jones, and D. V. Helmberger (1994). Source characteristicsof the January 17, 1994 Northridge California earthquake from regional broadband modeling, 89th SSA meeting, Pasadena, California,
1994.
Stacey, R. (1988). Improved transparent boundary formulations for the elastic-wave equation, Bull. Seism. Soc. Am. 78, 2089-2097.
Thio, H. and H. Kanamori (1994). Source complexity of the 1994 Northridge earthquake, 89th SSA meeting, Pasadena, California, 1994.
Vidale, J. E. and D. V. Helmberger (1987). Path effects in strong motion
seismology, in Methods of Computational Physics, Vol. 11, Bruce
Bolt (Editor) (entitled Seismic Strong Motion Synthetics).
Vidale, J. E. and D. V. Helmberger (1988). Elastic finite-difference modeling of the 1971 San Fernando, California earthquake, Bull Seism.
Soc. Am. 78, 122-141.
Vidaie, J. E., D. V. Helmberger, and R. W. Clayton (1985). Finite-differ-

Basin Structure Effects on Long-Period Strong Motions from the 1994 Northridge Earthquake and an Aftershockence seismograms for SH waves, Bull. Seism. Soc. Am. 75, 17651782.
Wald, J. D. and T. H. Heaton (1994). A dislocation model of the 1994
Northridge, California, earthquake determined from strong groundmotions, U.S.G.S. Open-File Report 94-278.
Yamanaka, H., K. Seo, and T. Samano (1989). Effects of sedimentary layers
on surface-wave propagation, Bull. Seism. Soc. Am. 79, 631-644.
Zahradnik, J. and L. Urban (1984). Effect of a simple mountain range onunderground seismic motion, Geophys. J. R. Astr. Soc. 79, 167-183.
Zahradnik, J. and F. Hron (1987). Seismic ground motion of sedimentary
valleys: example La Molina, Lima, Peru, J. Geophys. 62, 31-37.

S 137

Zhao, D. and H. Kanamori (1995). The 1994 Northridge earthquake: 3-D
crustal structure in the rupture zone and its relation to the aftershock
locations and mechanisms, Geophys. Res. Lett. 22, no. 7, 763-766.
Disaster Prevention Research InstituteKyoto University
Gokasho, Uji
Kyoto 611, Japan(A.P., K.I.)
Manuscript received 1 January 1995.