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 Aftershock

by Arben Pi tarka and Koj i ro I r ikura

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 veloc- ity. 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 and in the Los Angeles Basin by the two-dimensional finite-difference modeling of the mainshock 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 ob- served 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 mo- tions 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 a wide area from the San Fernando Valley to the Los Angeles Basin. This earthquake generated not only the largest accel- eration ever recorded but also complicated patterns of peak acceleration and peak velocity at sites surrounding the source 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 ep- icenter. Large horizontal accelerations in phases following the direct S wave by more than 4 sec were observed at some sites 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, and probably the regional geological structure effects all have a role even in the low-frequency range. It is enlightening to analyze such effects by simulating them separately.

First, we investigate basin structure effects in the San Fernando Valley and in the Los Angeles Basin by modeling strong motions recorded during an aftershock with magni- tude 4.1, which was located close to the mainshock. We calculate synthetic seismograms from this event using the 2D finite-difference technique proposed by Vidale et al.

(1985) and assuming a single point source. The basin veloc- ity 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 model of the Northridge and Los Angeles areas (Zhao and Kana- mori, 1995).

Next, we apply the same approach for modeling strong motions observed during the mainshock. We model the seis- mic source with two point sources separated in time and in space. This model was derived from previous source process studies, which showed that the faulting process was domi- nated by two subevents (e.g., Wald and Heaton, 1994). Based on the simulated waveforms at sites in a linear array from the San Fernando Valley to the Los Angeles Basin, the contribution to the long-period waveforms from both effects of 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, -69 ; dip, 44; slip, 70°; and the seismic moment, M o = 2.4 × 1022 dyne-cm (D.

$126

Basin Structure Effects on Long-Period Strong Motions from the 1994 Northridge Earthquake and an Aflershock S 127

Zhao, personal comm., 1994). This aftershock was very well recorded at seven stations deployed roughly in a line, passing through the San Fernando Valley'and northern part of the Los Angeles Basin. Five of the stations, installed by SCEC (Southern California Earthquake Center), are broadband STS2 instruments, and two of them are forced-balanced ac- celerometers. Their coordinates are listed in Table 1, and the locations 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 the two-dimensional velocity model adopted in this study and secondly to investigate basin effects on long-period wave propagation. Because of the simple source process, the waveforms of this aftershock are affected mainly by path effects. We expect that wave path effects will be similar for the aftershock and the mainshock because their hypocenters are very close to each other.

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 pro- posed by Vidale and Helmberger (1988). Their basins model is based on the stratigraphy, the velocities, and the densities found by Duke et al. (1971), while the velocities below the basins were taken from Kanamori and Hadley. Using such a 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 and Kanamori (1995) based on tomography P-wave travel-time inversions (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 are calculated using an elastic staggered FDM scheme of fourth- order accuracy (Levander, 1988). Absorbing boundary con- ditions proposed by Stacey (1988) are applied at all bound- aries, except the surface where free-surface-stress conditions are imposed. The source box technique proposed by Alter- man and Karal (1968) was applied for introducing the seis- mic energy into the grid.

We simulated the velocity waveforms at frequencies only up to 1 Hz, because FDM is expensive when applied to modeling wave propagation in huge realistic structures. In this study, one reason that the above frequency limitation is justified is that there is not enough detailed knowledge on the geological structure in and around the Los Angeles Basin and in the San Fernando Valley to have confidence in higher- frequency simulations. The second reason is that by mod-

Table 1 Strong-Motions Station Location Used in

the Aftershock Simulation

Station Latitude Longitude

B E A R 3 4 . 3 5 8 - 1 1 8 . 3 9 6

F I R E 3 4 . 3 0 9 - 1 1 8 . 4 4 6

S F Y P 3 4 . 2 3 7 - 1 1 8 . 4 3 9

N H F S 3 4 . 1 9 9 - 1 1 8 . 3 9 8

L A 0 3 3 4 . 0 9 0 - 1 1 8 . 3 3 9

W V E S 3 4 . 0 0 5 - 1 1 8 . 2 7 9

K L V C 3 3 . 8 3 5 - 1 1 8 . 1 5 9

24t" 00' 34"30'

34" 00'

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

34" 00'

241"00' 241"30' 242'

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

00'

eling aftershock ground motions, we intend to evaluate basin structure effects at low frequencies. This is helpful for dis- tinguishing between the path effects and the source effects observed during the mainshock.

First, we have tested the attenuation effects due to the basin soft sediments by comparing point source SH seis- mograms obtained by using both elastic and anelastic FDM schemes. The technique proposed by Emmerich and Korn (1987) was applied for considering the anelastic attenuation of soils. This technique makes possible a correct inclusion of the attenuation into time domain calculations. But it has the disadvantage of requiring much more memory and com- putations than the elastic scheme.

We adopted a constant quality factor Q = 50 for the basin sediments. Both the elastic and the anelastic scheme were applied for calculating synthetic point source seismo-

S 128 A. Pitarka and K. Irikura

| I

Y Y lr v T ~r

I

tO H = l l Km

SE

34.40 N 33.85 N

- 118 .50W San Femando Valley Los Angeles Basin - 118 .07W

iiiiiiiiiiii:::::iiiiiiiiiiii::::::i~i~iii::iiiii:iiiii::iiiii:!iiiiiiiiiiiiii::i:iiii [] • . . . - * ° ° - . * - - . . . . . . . . . . . . . . . . . . . . ° ° , , o * l , , ° . . . . . . . . . . . . . . . . . . . . . . . . . o . . . . . . . . . . . . . . . . . . .

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[]

[]

10km ! J

No vertical exaggeration

Vp < 5.45 km/sec

5.45 < Vp < 5.6

5.60 < Vp < 6.20

6.20 < Vp < 6.40

6.40 < Vp < 6.70

Vp > 6.70

Figure 2. (a) Upper part of the structural cross section of the profile A-A' and the station locations. The stratigraphy and the velocity model are taken from Vidale and Helmberger (1988). (b) P-wave velocity structure along profile A-A' derived from the 3D 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 - W a v e V e l o c i t y S - W a v e V e l o c i t y D e n s i t y

( k m / s e c ) ( k i n / s e e ) ( g i n / e r a 3)

1.4 0.6 1.7

2.0 1.1 1.8

2.5 1.4 1.9

3.1 1.8 2.1

4.3 2.5 2.3

grams at stations shown in Figure 1. The bandpass-filtered synthetics are shown in Figure 3.

The attenuation effect is negligible at sites close to the edges of the basins, while it is significant at sites inside the Los Angeles Basin. Love waves generated at the northern edge of the basin are attenuated while propagating laterally within the soft surface layers. It is important to notice that the amplitude of the body waves one time reflected from the surface is not affected as much, probably because of their relatively short propagation path within the sediments. De- spite such effects, for economical reasons, we have per- formed 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 the aftershock is shown in Figure 4. We have used a bell-shaped source time function with duration To = 0.6 sec and a seis- mic moment o f M 0 = 1.8 × 1024 dyne-cm, which is slightly smaller from that found by Zhao (personal comm., 1994). The three components of the simulated velocity seismo- grams are compared with those of the recorded ones.

The agreement between the recorded and the synthetic waveforms is in general good. The simulation reproduces some of the most noticeable features of the recorded ground motions. The comparison between the synthetic and the ob- served seismograms suggests that the surface waves gener- ated at the basin edges and the multiple surface layer reflec- tions 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 in the 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

Without A Uenuation

k ~ 0.010 _ - _ ~ 0.027

0.022 0.017

Q--.so

0.010 0.026

0,022 0.012

~ / ~ 0.047 0.082

n ~ o.o4s

0.071

S ~ 0.047 0.072

nl'ffs _ ~ 0.045 0.062

la03 oo,, 0.049

,,03 o.o,2°°1'

~es _ ~ 0.019 " _ 0.016

,l~/es , ~ 0.019 0.014

klvc , , ~

I" I I I I I I

mOO 1 ~ 24~Q ~ 4 0 . ~

Time(sec)

0.011 0.008 CM,~

klvc _ j ~ / ~ ~ 0.011 - yw - -

~ a ~ , ~ . ~ A / ~ - v ~ 0.008

T I I I I I I

0.130 6,00 ldl,O0 24.110 *?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 T O = 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 the epicentral distance to the left and the amplitude to the right. The light traces show the synthetic seismograms.

velocity model. First, even though the attenuation is not taken into consideration, the amplitudes of later phases com- ing after the direct S wave in the synthetics at the SFYP station are relatively small compared with those of the re- corded ones, in both radial and transverse components. They are mainly secondary surface waves and surface layer mul- tiples that are trapped within the soft layers (e.g., Aki and Lamer, 1970; Bard and Bouchon, 1980a, 1980b, 1985; Oht- suki 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 ba- sin edge, i.e., the radiation pattern effects from the source, the velocity contrast between the soft basin sediments and the 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, com- pared with our model, might create multiples with a larger amplitude. Other possibilities, for explaining the large am- plitude of later phases in the record at SFYP, are the focusing of the seismic energy inside the valley because of the three- dimensional 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 than in the recordings, while in the transverse component, the situation 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

~ __~: 0.043

5 . " ~ ~ :"'"--- 0 . 0 6 7

0.175

1 7 . - ' ~ 0.051

2: oo23 0 , 0 1 8

3~ i ~ 4 ~ = ~ 0.015 0.014

55. _ _ ~ ~ I ~ 0.009 ~ ~ W ~ L 0.007

¢M~ R

I I I I I I

0.00 I1,00 11.00 24.00 ~ ,IGgO Time(sec)

~ 0.003 0.027

~ 0.016 0.040

~ 0.026 0,042

0.033 0.041

0.005

~ 0.024

~ 0.007 0.008

~ 0,003 0.004

U - D I I I I I I

O*O0 &QO 16.00 24.00 .~2.~1 40~0

Time(sec)

~ _ 0.010 _~ _ ~ 0.027

f ~ 0.022 0.017

S~ ~ / ~ 0.047 0.082

0.071

"03 o o , ,

0.049

wves ........ ~ o.o19 _ _ ~ 0.016

k l v c ~ ~ 0.011 0.008

T I I I I I I

8 ~ 0 t&O0 24~0 32.00 40.00

Time(sec) 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 light traces show the synthetic seismograms.

nando Valley, which tunnel across the Santa Monica Moun- tains. Their amplitude is large compared with the recordings. Such a discrepancy was already observed by Vidale and Helmberger (1988) in their simulation of strong ground mo- tion from the 1971 San Fernando earthquake. They con- cluded 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 of the Los Angeles Basin, and their amplitude at sites around station 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, the amplitude of the later phases is slightly smaller compared with the recordings.

In Figure 5, we show the peak velocity comparison be- tween 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 the epicenter. A possible reason for this misfit is that the syn-

thetics at such close stations are very sensitive to the accu- racy of the epicentral location. Another reason might be that the finite-difference technique we use does not handle the near-field terms properly at small horizontal distances. Also, the station location very close to the northern edge of the surface basin layer might have affected the abnormal am- plification.

It is important to notice themost prominent effects of the 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 Ba- sin. Within the San Fernando Valley, at sites south of the epicenter, the constructive interference between the surface waves and the surface multiples increases the amplitude of later 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

0.20

0.15

:~ O.lO _o

~ 0 . o 5 O _

0.00

a. TRAN~ ;VERSE COMPONENT I , , I , ' I I ! ,

• R~cord~d • S~tnthetic

i i • ! . " ! i .............. i:: ................ i -" - ~ " ~ ~ i . . . . . . . T - Z - " - T .............. # . . . . . . . . . . ~ . . . . . .

~," i "4. i . i i i i , i ' i = i , i • i i i ,

-15 0 15 30 45 60 NW Epicentral Distance (kin) SE

0 . 2 0

0.15

-'5 0.10 _o

-~ 0.05 IX .

0 . 0 0

b. RADIAL C O M P O N E N T f , , I , T I I j t

• • Recorded, , ~ I n t - e . c

............................................................ T T ........... i . . . . . . . . ! . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ÷ . . . . . . . . . . ~ . . . . . . . . . . . . . . j . . . . . . . . . . . . . ~ - . . . . . . . . . . - i - - . . . . . . . . + . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . . . . . . . . . . . . . . . . . . .

, i . ' , . i . i ", " ' ~ ' ' *

- 1 5 0 1 5 3 0 4 5 6 0

NW Epicentral Distance (km) SE

Figure 5. Peak velocity versus distance for the aftershock. The open triangles show the peak velocity of 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 ar- rival 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 ba- sin. 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 SH 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 sim- plified flat layer model clearly shows that the increasing am- plitude of the surface layer multiples with distance is caused by the dipping edge of the basin.

The relatively good fit between the synthetics and the recordings for the aftershock suggests that the adopted two- dimensional velocity structure is useful for predicting low- frequency local geology effects. The simulation shows that for an earthquake located in the Northridge region, the am-

Y,

.2 " I \ "

l O ~ i

iio H = l l K m

- |

-I

tO - |,~,

!

S E

a -

N W

I

!

|

!

= = z , ~ S E

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

plifications due to the basin structure are expected to be max- imum 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 and waveform inversions (Li and Toksoz, 1994; Dreger, 1994;

S 132 A. Pitarka and K. Irikura

-10

E v, v

.c -15

-20

SUBEVENT LOCATION

0 5 10 15

WNW 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 the subevent locations. The moment scaling is explained in the text.

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

34"30'

34"0ff

34"30'

34" CO'

241" 00' 24I* 30' ~s4~ ° tXI'

Figure 8. Map of Northridge and Los Angeles regions. The epicenters of the mainshock and the aftershock are marked by filled stars. The filled tri- angles are the locations of the stations used in the simulation, and the dotted lines represent the basin boundaries. The two-dimensional velocity models used in the mainshock simulations correspond to the cross sections along the profiles A-A' and B-B'.

Song et al., 1994; Wald and Heaton, 1994; Thio and Kan- amori, 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

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) shows that the biggest amount of slip is northwest of the hypocenter at depths between 15 and 18 km, which is consistent with the subevents found by Thio and Kanamori (1994) from the waveform inversion of teleseismic body waves. Their solu- tion shows the first subevent at 19 km, the second subevent at 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 is consistent with the observation that the two main first arri-

vals are separated in time by 4 to 5 sec at sites in the south and by less than 3 sec in the north. By assuming a constant rupture velocity of 2.7 km/sec, the time interval between the two 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 be- tween the two subevents. Their location corresponds to the asperities found by Wald and Heaton (1994). Such a source model does not include rupture propagation effects; conse-

Table 3 Strong Motion Stations Used in the Mainshock Simulation

Stat ion Lat i tude Long i tude

CSTF 34.564 - 118.642 NHAL 34.390 - 118.530 SHRF 34.151 - 118.463 SCCF 34.100 - 118.478 UCLG 34.068 - 118.439 LABA 34.009 - 118.361 LAll 33.929 - 118.260 ENCF 34.149 - 118.512 SMOF 34.011 - 118.490

~ ~ = o ffi ~ ~

v . v, v , . ~ . . v

~ J )3( Second Event H=16 K m

i

~( First Event H=18 K m

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

S E

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

BAND PASS FILTERED 0.05 _ 1.0 Hz

VELOCITY

l a l ~ ~7o ~ , . , ,o 41.

8.44

1&74 ~ L75

12.43 7.87

| 5.63 12.04

8.75 4.75

20.70 13.00

117.29 12.411

28.72 11,87

4.07 6.67

f~2

!1.10

9.04

4.73

R U-D I I I I I I I I I I I I

Time(sec) Time(sec)

CM~ T

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 light traces 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 rup- ture propagation effect is not significant, as shown by pre- vious studies (Wald and Heaton, 1994). We use the same bell-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 sub- event of 0.5 and the focal mechanism found by Thio and Kanamori (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 stations aligned along the A-A' profile (Fig. 8) where the mainshock ground motions were recorded. We use strong-motion ac- celerograms from the California Division of Mines and Ge- ology (Shakal et al., 1994), the U.S. Geological Survey (Por- celia et al., 1994), and the Los Angeles Department of Power and Water (Wald, personal commun.) (see Table 3).

The recorded accelerograms were first integrated to ve- locity and then bandpass-filtered (0.05 to 10.0 sec). The up- per part of the velocity structure corresponding to the A-A' profile is shown in Figure 9. The result of the simulation is shown in Figure 10. Since there is no absolute timing for the 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 Ba- sin. The fit between the recordings and the synthetics is good at almost all the stations to the south, especially at the rock site, the SCCF station. This result proves the validity of our source model. However, at two stations to the north, NHAL and CSTF, there is a great difference not only in the wave- forms but also in the peak amplitudes. At these stations, the peak amplitude of the data is almost four times greater than the one obtained by the simulations (Fig. 11). Such differ- ences cannot be explained by inaccuracies in the velocity

S 134 A. Pitarka and K. Irikura

model, because the aftershock simulation showed its validity for evaluating site effects even at northern sites. We attribute them to the fact that, comparing with those to the south of epicenter, the recorded motions to the north of the rupture are more sensitive to the details of the source process. Wald and Heaton (1994) found a strong effect of the source ra- diation (radiation pattern and rupture directivity) at stations located north of the hypocenter.

A seismic moment ratio of 0.5 between the first and the second 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 research- ers. This is because the velocity records used in the modeling do not have much of a signal above a 3-sec period to con- strain the long-period motions. The assumed source time function with duration 1.2 sec, adopted for fitting the veloc- ity records at periods around 1 to 2 sec, is too short to rep- licate the real Northridge source, which probably lasted more than 5 sec.

Our source model predicts the two distinct arrivals and the large amplitudes of later phases observed at sites located south and southeast of the epicenter. It is interesting from the strong ground motion prediction point of view to clarify the origin of the large-amplitude later phases recorded at stations 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 dif- ferences between the focal mechanisms of the first two sub- events of the mainshock. Without ruling out the source con- tribution, Wald and Heaton (1994) favor that this effect comes from the site conditions since several aftershocks re- corded along the northern edge of the Los Angeles Basin show later arrivals as well.

Based on the above simulation, we attribute the wave- form complexity observed at stations located north of the Los Angeles Basin (SMOF, UCLG, LABA) to a possible con- strnctive contribution from both site effects and source pro- cesses. 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. This is 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 effect from the second subevent. In Figure 12, for stations south of the epicenter, we display the observed records (top trace) and the synthetic records (second trace) along with the sep- arate contributions to the synthetic records from the first sub- event (third trace) and the second subevent (bottom trace).

From this figure, it can be seen that at sites close to the northern edge of the Los Angeles Basin (e.g., LAI 1, LABA), a double-event source process combined with local site ef- fects might cause large strong ground motions.

We performed another simulation along the profile B-

A 60

E o

v 40

-~ 20

a. T R A N S V E R S E C O M P O N E N T

' i i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . h, Rec0rded

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . k- . . . . . . . . . . . . . . . .k- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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

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

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

.............. " 1 i ..................................... t ................. ~ ..................... ~ ............... ~ " ~ , - " ~ " ............ ~ " ~"~;-"- ..............

! ! i i i I I I t I

- 5 0 - 4 0 - 3 0 - 2 0 - 1 0 0 10 2 0 3 0 4 0 5 0

N E p i c e n t r a l D i s t a n c e ( k i n ) S E

~ 6 0

E o

v 4 0

o ..9o

-~ 20 a .

0

. 50

N

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

i

. . . . i--- i , , ~ -%~ ~ i ~: lecOrded

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

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

.................. i .................... ~ ..................... i .................. ~ ....... ~ ; ............... ~ ................ ~. ~ . . . . . . . . . . .

I I I I •

- 4 0 - 3 0 - 2 0 - 1 0 0 10 20 3 0 4 0 5 0

Epicentral Distance ( k m ) S E

Figure 11. Peak velocity versus distance for the mainshock. The open triangles show the peak velocity of the filtered data, and the filled triangles show the peak 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 were recorded. 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 pre- vious mainshock simulation. Again, the synthetics at the northem station (NHAL) have much smaller amplitudes as compared with the observed records (see Fig. 13). The syn- thetic 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 is slightly greater than the observed one. The generation of those surface waves is probably controlled either by a three- dimensional 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 ef- fects at Santa Monica and the factors controlling the ampli- tude 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

6.44

~ 13.74 12.73

17.30

15.63

scc f 13.17

8.75

R i I 000 500 10'00 15'.00 20.00 25100

Time(sec)

1.80

2.50

5.96

5.32

6.75

7.67 ~ 11.90

9.04

7,42

12.04

13.50

15.97

• .~%~ ~

4.30

4.75

eB~Is U-D

, , , , , , 0.00 5.00 1000 15.00 2 0 0 0 25.00

Time(sec)

6.68

7.02

c l l t S T

I

000 ~00 10100 16100 20100 ' 25.00

mime(sec) 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

s m o f ~ 31.0

~' ~ 2 8 3 S ~ 13.4

17.4

encf ^

.~]--,~ ~ . - , - ~ - ~ - ~ - ~ - ~ 21.6

61.8

39.4

R ' ' o' ~' ' i 0.00 5.00 1 00 1 00 20.00 2500

Time(sec)

11.3

24.8

~ 4.3 7.7

10.8

~ 12.5 45.7

U - D

' ' ' ' '. .'o 0.00 5.00 10.00 15.00 2000 25 0

Time(sec)

~ 16.1

15.5

6.1

5.3

~ 5.2

~ 48,4

11.2

T ! !

000 500 1000 1~'00 20.00 25.'00 Time(sec)

Figure 13. Comparison of filtered velocity data with finite-difference velocity seis- mograms 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 used in the simulation is discussed in the text.

S 136 A. Pitarka and K. Irikura

Conclusions

Two-dimensional finite-difference modeling, based on a heterogeneous velocity structure and a source model con- sisting of a double-couple point source, can reproduce the most prominent characteristics of the long-period (1 to 10 sec) strong ground motions recorded in the southern part of the San Fernando Valley and in the Los Angeles Basin dur- ing an aftershock of the January 1994 Northridge earth- quake. We found that both the valley and the basin generate secondary waves that dominate the later arrivals. The mod- eling 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, indicates significant basin effects that can be identified in the observed strong ground motions.

At sites within the San Fernando Valley, the ground motions have been significantly affected by the surface waves excited at the edges of the valley. Basin effects and the multiple event source process have contributed to the amplification of the second main arrival observed at sites close to the northern edge of the Los Angeles Basin. A three- dimensional or more accurate two-dimensional velocity structure may better explain some small inconsistencies be- tween 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 the mainshock recordings and the synthetics at sites north of the epicenter, while a good fit between the synthetic and the recorded waveforms is obtained to the south, suggests that at sites north of the epicenter, the rupture propagation effect dominates over site effects.

Acknowledgments

We would like to thank Adam Edelman for kindly providing us with the strong-motion data of the aftershock; David Wald for providing us with the January 1994 Northridge earthquake strong-motion data recorded at the SHRF, ENCF, and SCCF stations; and Dapeng Zhao for preparing the P- wave velocity models of the upper crust. We are grateful to F. J. Sanchez- Sesma, 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, Sci- ence and Culture, Japan, and the Kajima Foundation's Research Grant.

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Disaster Prevention Research Institute Kyoto University Gokasho, Uji Kyoto 611, Japan

(A.P., K.I.)

Manuscript received 1 January 1995.