Achievements of strong motion seismology in 20th century and its
future directions
Kojiro Irikura
Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011

ABSTRACT
We have been trying to make a recipe for predicting ground motions from future large earthquakes.
To do that the most important factors are the source characterizations for complex rupture processes
and the Green’s function estimations for realistic geological structures. We attempt to formulate thesource characterizations based on recent results of the waveform source inversion. There are two
important aspects of characterizing the earthquake sources, outer and inner source parameters. The
outer source parameters such as total fault length, width, seismic moment and so on are obtainable
based on geological investigations of capable earthquake faults and seismological studies of source
models. The inner source parameters are parameters related to slip heterogeneity on fault plane fromthe waveform inversion of strong motion records. We find that the areas of large slip known asasperities follow a self-similar scaling as well as the total rupture areas from the statistical analysisof the inverted results. This implies that both parameters, outer and inner ones, are predictable.
Based on the results above mentioned we construct the procedure of characterizing the source for
scenario earthquakes compiling accumulated knowledge about earthquake faults in geomorphology
and seismology. We have examined the procedure comparing between observed records andsynthetic ground motions from the characterized sources based on kinematic models for the recent
large earthquakes. Then, we have confirmed the validity of the procedures for characterizing theearthquake sources and calculating ground motions. In future directions, the validity andapplicability of predicting strong ground motions should be made taking into account source
dynamics physically realizable. For example, surface displacements for the above model should be
estimated from dynamic rupture process, assuming additional source parameters such as critical slip
and strength excess for dynamic source simulation and compared with observed ones fromgeological and geomorphological survey.

INTRODUCTION
Recent large earthquakes, such as the 1995 Kobe (Japan) earthqauke, the 1999 Kacaeli (Turkey)
earthquake, and the 1999 Chi-chi (Taiwan) earthquake occurred on well-mapped active faults.
However, no obvious precursory evidences appeared even near the faults before the events. On theother hand, high-quality ground motion records have been obtained from those recent earthquakes.

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Accumulated data have been providing us very important knowledge about rupture processes ofearthquakes, propagation-path and site effects on ground motion, the relation between ground motion
and damage, and so on. Then, we have learned that earthquake damage resulted from destructive
ground motion generated by rupture propagation effects in asperities as well as geological conditions.
For example, heavy damage in Kobe area larger than in the Awaji area during the 1995 Hyogo-ken
Nanbu earthquake is explained by source effects such as slip heterogeneity and forward rupture
directivity and basin-edge effects due to basin structures. In the Awaji area there was less damage
although surface breaks appeared along the Nojima faults. This means the importance of strong
ground motion prediction from earthquake faults to mitigate disaster for future earthquakes.
Scenario earthquakes for subject areas have gradually been popular to make seismic disasterprevention measures by some governmental agencies and municipalities. However, most of strong
motion estimations in earthquake hazard analysis are still inclined to empirical methods. Peakground acceleration and velocity and response spectrum for earthquake-resistant design are given by
empirical methods as a function of magnitude, fault distance, ground condition. They have not beentaken into account yet developments in seismology, such as source processes in earthquake faults
and 3-D simulation in complex structures.
We made a framework of strong motion prediction as shown in Fig. 1. The strong motion fromthe scenario earthquake is evaluated from source modeling and Green’s function estimation based on
geological and geomorphological survey of active faults, geophysical profiling of undergroundstructures, strong motion recordings, and so on. The results will lead to improvement of the buildingcode and bridge earthquake-resistant design.

RECIPE FOR STRONG MOTION PREDICTION
We attempted to make a recipe to popularize the prediction of ground motions for engineering
purpose based on seismological fruits. There are two important factors for predicting strong groundmotion, one is source characterization based on geological features for active faults and statisticalanalysis of source processes from the waveform inversion of strong motion records, the other is theestimation of the Green’s functions from source to site.
For the former, we know that most of the uncertainties in predicting strong ground motion is tospecify the proper methods of characterizing the source models of future earthquakes. Conventionalsource parameters such as fault length and width and average slip on fault are only available for very
long period motions but not enough for near-source strong motions dominating short period motions
less than 1 sec of engineering interest. Slip heterogeneity on the fault plane plays important role onsuch short period motions. We tried to make source characterization including slip heterogeneity
based on the statistical analysis of the source inversion results.

2For latter, numerical Green’s functions are calculated for ground motions longer than 1sec basedon methods such as Discrete Wave Number Method, Finite Deference Method, and Finite element
method. The short period motions are estimated by the empirical Green functions and stochastic
simulation method for small events. To combine the long and short period motions, introduce theidea of a hybrid method estimating the Green's functions, deterministic and stochastic approaches inlow (< 1 Hz) and high (> 1 Hz) frequency ranges, respectively, to obtain broad-band ground motions(Irikura and Kamae, 1999).
For estimating strong ground motions in a deterministic approach, we need to have two kinds ofsource parameters, outer and inner ones.
Outer Source Parameters
The outer source parameters are total fault length and width, average slip and slip duration,
rupture velocity and so on, which are to characterize the macroscopic pictures of given source faults.
They are inferred, based on geological investigations of capable earthquake faults and seismologicalstudies of source models.
Total fault lengths (L) of scenario earthquakes would be evaluated as one of the long-term seismic
hazard evaluation. Some attempts have been making to estimate segmentation and grouping of active
faults based on branching features of seismic surface ruptures (e.g. Matsuda, 1998; Nakada, 1998).
Such surveys give us strike (φ ) and slip type of every segment consisting of the fault system. Dip
angle (δ ) is inferred from seismic reflection profile.
Fault width (W) cannot be directly determined from the geological survey but mostly from source
modeling for waveform simulations compared with observed records. The saturation of the widthyields for events larger than M6.8, corresponding the thickness of seismogenic zones. The
seismogenic zones are infered from the depth-frequency distribution of small earthquakes (Ito, 1990).
Recent study by Ito (1999) shows that the seismogenic zones seem to have upper cutoff depth aswell as lower cutoff depth derived from the seimic-aseismic boundary in the mid-crust dependent on
regions.
The seismic moment of the capable faults are estimated by the empirical relationship between thesource areas and seismic source(A = LW), then average slips are automatically constrained by theseismic moment and source area (e.g. Somerville et al. 1999).
Inner Source Parameters – Fault Heterogeneity or Roughness –The slip and slip velocity have been found not to be uniform in the source areas, in particular forlarge earthquakes more than 7 as clarified from the waveform inversion of rupture process (e.g.
Wald, 1996). We need to know slip and slip velocity distribution in the source area as well as theaverage slip to estimate strong ground motions. We call here such source parameters inner source
parameters that express fault heterogeneity or roughness. So far slip models have been derived fromlonger period ground motions using the waveform inversion. Direct application of such long-period3source models to strong ground motion estimation is not always available because higher groundmotions than 1 Hz cannot be generated. Nevertheless, we found that the asperity models derived
from the heterogeneous slip distribution using the waveform inversion of longer-period groundmotion recordings are available for estimating broad-band ground motions of engineering interest(e.g. Kamae and Irikura, 1998).
Somerville et al. (1999) analyzed the characteristics of slip models of totally fifteen crustal
earthquakes ranging from about 6 to 7 in moment magnitude (Mw) for use in the prediction of strong
ground motion. They used two approaches, deterministic and stochastic, in characterizing the slip
models. First they define fault asperities in a deterministic manner to quantify the properties ofheterogeneous slip models. The asperities are areas on the fault rupture surface that have large slip
relative to the average slip on the fault. An asperity is defined to enclose fault elements whose slip is
1.5 or more times larger than the average slip in the fault (in detail refer to Somerville et al.,1999).
The number of asperities in the slip models of those events is 2.6 on average. The slip contrast, theaverage slip on the asperities over average slip is about 2. The combined area of asperities on
average occupies about 22% of the total rupture area.
Total rupture area (A), combined area of asperities (Aa), and area of largest asperity (Am) scale in aself-similar manner with increasing seismic moment.
A(km2) =2.23 x 10-15 x Mo2/3(dyne-cm)(1)

Aa(km2) =5.00 x 10-16 x Mo2/3(dyne-cm)

(2)

2

-16

Am(km ) = 3.64 x 10

2/3

x Mo (dyne-cm)(3)

The validity and applicability of estimating strong ground motion based on the above recipe should
be examined in comparison with the simulated motions and observed records.

SIMULATION OF STRONG GROUND MOTIONFROM THE 1999 KOCAELI EARTHQUAKE
Strong ground motions from the 1999 Kocaeli earthquake were recorded near the causative faults asshown in lower panel of Fig. 2. We characterize the source model with asperities based on the slip
distribution (Upper panel of Fig.2) determined from the inversion of strong motion data by
Sekiguchi and Iwata (2000). The ground motions from the mainshock are estimated using the hybrid
method (Irikura and Kamae, 1999). We assume pure right-lateral strike slip at each asperity. The

size of the subfaults generating the small events is assumed to be 2x2 km2. For simplicity, wetake into account each asperity as a event with uniform stress drop in a finite extent. The best fit
model to observed records is composed of three asperities (three small rectangles in the upper panelof Fig. 2). The stress drop (∆σ) in each asperity is assumed to be 120 bars in Asperity 1, 50 bars in
Asperity 2 and 100 bars in Asperity 3, assuming circular crack (Brune, 1970). Rupture was initiatedat the corner of each asperity. The synthesized motions, acceleration and velocity and the4acceleration response spectra are compared with the observed at SKR (close to the surface rupture
about 40 km east of epicenter, see Fig.2) and GBZ (about 50 km west of epicenter) in Fig. 3 and 4.
At SKR, only EW-component was recorded. Slips are assumed to exist only on those three asperities
and no slips in the background areas. The synthetic velocity motions agree well with the observed
ones in phase, although they seem to be a little less than the observed ones, while the synthetic
acceleration are almost the same amplitude as the observed. Total seismic moment obtained from thesynthetic motions are assumed to be 44% of the observed ones because zero slips are assumed on the
background area. It is why the synthetic velocity motions are a little less.

SIMULATION OF STRONG GROUND MOTIONFROM THE 1999 CHI-CHI EARTHQUAKE

We assume an initial source model consisting of three asperities based on the results of thekinematic waveform inversion (Iwata et al., 2000), a large asperity No.3 in the northern part ofthe causative fault and two smaller asperities No.1 and No.2 in southern part near the epicentral
area as shown in Fig. 5. We used the same hybrid method for simulating ground motion as theabove. We assumed a rake angle of 60 o for the subfaults of the asperities No1 and No.2 in thesouthern part and a rake angle of 45o for those of the asperity No.3 in the northern part. We
assume that the surface layers with total thickness of 3 km do not generate high frequency
motions following the idea of the dynamic rupture process (Dalguer et al., 2001).
The best-fit source model between the observed and synthetic motions is as follows. The
stress drop is assumed to be 100 bars at those three asperities. The rise time for each asperity istaken to be 0.6 sec. The synthetic motions are compared with the observed ones at TCU078, oneof the sites at the southern part near the epicenter in Fig. 6 and at TCU046 one of the sites at thenorth of the Chelumpu fault in Fig. 7. We find that the acceleration and velocity waveform andthe acceleration response spectra of the synthetic motions at both stations show good agreementwith those of the observed ones, although the characteristics of the waveforms are very different
each other at those two stations. The waveforms at TCU078 close to the epicenter is composedof high-frequency and long duration wavelets. It comes from the fact that this site is located in
the backward direction of those three asperities.
The velocity motions synthesized here as well as those observed at TCU045 are composed ofa single large pulse with duration of about 10 sec. This pulse is indicative of the forward rupture
directivity due to a large asperity of No.3, because the rupture propagated from south to north tobreak Asperity No. 3 north of the hypocenter. Similar large pulses in the near-source area wereobserved during the 1992 Landers earthquake in California, USA and the 1995 Hyogo-ken
Nanbu earthquake. The predominant periods of the near-source pulses in forward rupture
direction depends on the asperity size and slip duration (Irikura et al., 1996; Somerville et al,1996).

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SCALING RELATIONS BETWEEN ASPERITY AREAS AND SEISMIC MOMENTThe relation between the combined area asperities and seismic moment and that between thearea of the largest asperities and seismic moment are compiled in Fig. 8. Adding fifteen crustal
earthquakes analyzed by Somerville et al.(1999), there were plotted three crustal earthquakes inJapan by Miyakoshi et al.(2000), and the 1995 Kobe, the 1999 Kocaeli, the 1999 Chi-Chi, and2000 Tottori earthquakes by Sekiguchi and Iwata (2000) and by Iwata and Sekiguchi (2000).
The maximum seismic moment analyzed by Somerville et al.(1999) is 75x1025 dyne-cm. Newly
analyzed results shows the self-similar scaling relations by them can be extended to larger
earthquake. We do not know the saturation levels of those relations yet.
The combined area of asperities and the area of the largest asperities obtained from the bestfit models in this study are also shown by squares with cross mark in the same figures. We findthat the results for the 1994 Northridge, 1995 Kobe, 1999 Kcaeli, and 1999 Chi-Chi earthquake
almost follow the self-similar scaring relationship.
CONCLUSION
We summarized the procedure for predicting strong ground motions from future earthquakes
caused by inland active faults as a recipe. The source characterization for the future earthquakes are
made by the statistical analysis of the source inversion using strong motion and teleseismic data.

Adding fifteen crustal earthquakes analyzed by Somerville et al.(1999), more results arecompiled, three crustal earthquakes in Japan by Miyakoshi et al.(2000), and the 1995 Kobe, the1999 Kocaeli, the 1999 Chi-Chi, and 2000 Tottori earthquakes by Sekiguchi and Iwata (2000)
and by Iwata and Sekiguchi (2000). Then, we find that the combined area of asperities and theareas of the largest asperity as well as total rupture area follows the self-similar scaling. The seismic
moment range applicable are extended to 2.75x1027 dyne-cm (Mw 7.6).
The validity and applicability of the source characterization for strong ground prediction are
examined in comparison with the observed records and broad-band simulated motions using the
hybrid mehtod from recent large earthquakes such as the 1995 Hyogo-ken Nanbu (Kobe) earthquake,the 1994 Northridge earthquake, 1999 Kacaeli earthquake, 1999 Chi-Chi earthquake.
In future directions, strong ground motions should be made taking into account source dynamics
physically realizable. Observed surface displacements from geological and geomorphological survey
should be compared with simulated ones taking into account from dynamic rupture process,additional source parameters such as critical slip and strength excess for dynamic theory.
Acknowledgement
This paper was made by cooperation with Tomotaka Iwata, Katsuhiro Kame, Takao Kagawa, Ken
Miyakoshi, and Hiroe Miyake. Figures are arranged by Hiroe Miyake getting analyzed results fromthe co-researchers above mentioned. This study was partially supported by a grant-in-Aid for
Scientific Research, Number 09044079, from the Ministry of Education, Science, Sports, andCulture.

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REFERENCES
Dalguer, L. A., K. Irikura, J. D. Riera, and H. C. Chiu (2001), “Fault dynamic rupture simulation of
the hypocenter area of the thrust fault of the 1999 Chi-Chi (Taiwan) earthquake”, Geophys. Res.
Letter, 27, in press.
Irikura and Kamae (1999), “Strong ground motions during the 1948 Fukui earthquake”, Zisin (J.
Seism. Soc. Japan), 52, pp129-150 (in Japanese).
Irikura, K., T. Iwata, H. Sekiguchi, A. Pitarka, and K. Kamae, Lesson from the 1995 Hyogo-ken
Nanbu earthquake: Why were such destructive motions generated to buildings ?, J. Natur. Disas.
Sci., 18, 99-127, 1996.
Ito, K. (1990), “Regional variations of the cutoff depth of seismicity in the crust and their relation toheat flow and large inland-earthquakes”, J. Phys. Earth., 38, pp223-250.
Ito, K. (1999), “Seismogenic layer, reflective lower crust, surface heat flow and large inlandearthquakes”, Tectonophysics, 306, pp423-433.
Iwata, T., H. Sekiguchi, and K. Irikura, 2000, Rupture process of the 1999 Chi-Chi, Taiwan,
earthquake and its near-source strong ground motions, Proc. International Workshop on Annual
Commemoration of Chi-Chi earthquake, Taipei, Taiwan, 1, 36-46.
Kamae, K. and K. Irikura (1998), “Source model of the 1995 Hyogo-ken Nanbu earthquake andsimulation of near-source ground motion”, Bull. Seism. Soc. Am. , 88, pp400-412.
Matsuda, T. (1998), “Present state of long-term prediction of earthquakes based on active fault data
in Japan, Zisin (J. Seism. Soc. Japan), 50, pp23-34 (in Japanese).
Miyakoshi, K., T. Kagawa, H. Sekiguchi, T. Iwata, and K. Irikura, 2000, Source characterization ofinland earthquakes in Japan using source inversion results, Proc. 12WCEE (CD Rom), 3, 1850.
Nakata, T., K. Shimazaki, Y. Suzuki, and E. Tsukuda (1998), “Fault branching and directivity ofrupture propagation”, Journal of Geography, 107, pp512-528 (in Japanese).
Sekiguchi, H., K. Irikura, and T. Iwata, 2000, Fault geometry at the rupture termination of the 1995
Hyogo-ken Nanbu earthquake, Bull. Seism. Soc. Am., 90, 974-1002.
Sekiguchi, H. and T. Iwata, 2000, Rupture process of the 1999 Kocaeli, Turkey, earthquake using
strong motion waveforms, submitted to Bull. Seism.Soc. Am.
Somerville, P., K. Irikura, R. Graves, S. Sawada, D.Wald, N. Abrahamson, Y. Iwasaki, T. Kagawa,N. Smith, and A. Kowada (1998), “Characterizing earthquake slip models for the prediction ofstrong ground motion”, Seismological Research Letters, 70, pp59-80.
Wald, D. J. (1996), “Slip history of the 1995 Kobe, Japan, earthquake determined from strong
motion, teleseismic, and geodetic data”, J. Phys. Earth, 44, pp489-503.

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(1) Long-term forecasting of earthquakes(2) Strong motion observation and(3) Investigation of

waveform inversion of source process

underground structures

Active fault surveys

Strong motion records

Reflection, Refraction profiling

Historical earthquake records

Teleseismic records

Gravity survey

GPS observation

Broad-band records

Boring, P-S velocity logging

Seismic activity monitoring

Earthquake damage records

Macroscopic

Microscopic(Global)(Local)Empirical approach
Theoretical
Stochastic

Rupture directivity
HybridSource modeling

Estimation of Green’s functions(Global source parameters)(Empirical Green’s function)(Local source parameters)(Stochastic Green’s function)(Theoretical Green’s function)(4) Ground motion simulation
for scenario earthquakes(5) Setting seismic design criteria

Ground motion waveform

Building codes

PGA, PGV

BridgeResponse spectra

Dam

Seismic intensity

Nuclear power plants

Validation by historical records of earthquake damage

Fig. 1.

Framework of predicting strong ground motion from scenario earthquake.

Fig. 2. Upper: Slip distribution derived from the
waveform inversion by Sekiguchi and Iwata (2000) andcharacterized source model composed of three asperities
for the 1999 Kocaeli earthquake. Lower: Map of thesurface fault trace inferred for the source inversion andstrong motion stations.

Fig. 3. Comparison between the synthetic accelerationand velocity seismograms and their acceleration response
spectra of EW component close to fault normal using the
hybrid method and observed ones at SKR. The
acceleration and velocity seismograms NS component
close to fault parallel are simulated with the same model,
although it failed in recording. This station is locatedclose to the surface rupture about 40 km east of
epicenter.

Fig. 4. Comparison between the synthetic accelerationand velocity seismograms (EW component close to fault
normal and NS component close to fault parallel) andtheir acceleration response spectra of using the hybrid
method and observed ones at GBZ. This station is
located close to the surface rupture about 50 km west of
epicenter.

Fig. 6. Comparison between the synthetic accelerationand velocity seismograms (EW component close to fault
normal, NS component close to fault parallel) and theiracceleration response spectra using the hybrid method
and observed ones at TCU078. This station is locatedclose to the epicenter at the southern part of the causative
fault.

Fig. 5. Slip distribution with slip vectors (Lower) fromthe waveform inversion by Iwata et al. (2000) andcharacterized source model with three asperities (Upper)
for the 1999 Chi-chi earthquake.
Fig. 7. Comparison between the synthetic accelerationand velocity seismograms (EW component close to fault
normal, NS component close to fault parallel) andacceleration response spectra using the hybrid method
and observed ones at TCU046. This station is locatednorthern part of the causative fault.

Combined Area ofAsperities (km2)

10000

1000

Somervill et al. (1999)
Kagoshima(3/26)
Yamaguchi
Tottori (Miyakoshi et al., 2000)
Iwate
Kobe (Sekiguchi et al, 2000)
Kocaeli (Sekiguchi and Iwata, 2000)
Chichi (Iwata and Sekiguchi, 2000)
Tottori (Sekiguchi and Iwata, 2000)

Chichi
Kocaeli
Kobe

100
Northridge

101
1.00E+24

1.00E+25

1.00E+26

1.00E+27

1.00E+28

Moment(dyne-cm)
Fig. 8. Relation between combined area of asperities and seismic moment based on slip distributions estimated by the
waveform inversion by Somervillle et al. (1999), Miyakoshi et al. (2000), Sekiguchi et al. (2000), Sekiguchi and Iwata(2000), and Iwata and Sekiguchi (2000). The individual events analyzed are listed at the left-upper corner panel. Theline is a least-squares fit under the constraint of self-similarity (slope=2/3) by Somervillle et al. (1999). The combined
asperity areas determined from comparison between the synthetic and observed waveforms in this study are plotted bysquares with cross.

Area of Largest
Asperity(km2)

10000

1000

Somervill et al. (1999)
Kagoshima(3/26)
Yamaguchi
Tottori (Miyakoshi et al., 2000)
Iwate
Kobe (Sekiguchi et al, 2000)
Kocaeli (Sekiguchi and Iwata, 2000)
Chichi (Iwata and Sekiguchi, 2000)
Tottori (Sekiguchi and Iwata, 2000)

Chichi
Kocaeli
Kobe

100
Northridge

101
1.00E+24

1.00E+25

1.00E+26

1.00E+27

1.00E+28

Moment(dyne-cm)
Fig. 9. Relation between area of largest asperity and seismic moment based on slip distributions estimated by the
waveform inversion. The data sources are the same a Fig. 8. The line is a least-squares fit under the constraint of selfsimilarity (slope=2/3) by Somervillle et al. (1999). The largest asperity areas determined from comparison between thesynthetic and observed waveforms in this study are plotted by squares with cross.